Mind Map: Topic - "Benefits of Exercise"

Central Idea: Benefits of Exercise

Main Branches:

1. Physical Health

2. Mental Health

3. Emotional Well-being

4. Disease Prevention

5. Improved Sleep

Sub-branches:

1. Physical Health

• Increased strength and endurance

• Weight management

• Improved cardiovascular health

• Enhanced flexibility and balance

2. Mental Health

• Reduced stress and anxiety

• Improved mood and self-esteem

• Increased cognitive function and memory

• Boosted creativity and productivity

3. Emotional Well-being

• Release of endorphins, promoting happiness

• Reduced symptoms of depression

• Enhanced body image and self-confidence

• Improved overall quality of life

4. Disease Prevention

• Lower risk of chronic diseases (e.g., heart disease, diabetes)

• Strengthened immune system

• Decreased risk of certain cancers

• Improved bone density and joint health

5. Improved Sleep

• Better sleep quality and duration

• Reduced insomnia symptoms

• Enhanced daytime alertness and energy levels

• Improved overall sleep patterns

Note: This mind map is a concise representation of the topic "Benefits of Exercise" and can be expanded further with additional details and sub-branches if needed.

Plant Development

Page 2:

• Difference between animal and plant growth and development

  • Plant growth and development occurs throughout the life of the plant

  • Consists of embryonic organs, developing organs, and mature organs

Page 6:

• Development: An overview

  • Plant development involves a series of highly ordered events

  • Events span from fertilization to senescence and death

  • Genetically-programmed events control the progression of a plant through its life cycle

Page 7:

• Growth, development, and differentiation

  • Development refers to all changes that a cell, tissue, organ, or organism goes through during its life cycle

  • Growth involves irreversible, quantitative changes in cell number, size, and/or volume

  • Growth can be measured quantitatively with either fresh weight or dry weight

Page 8:

• Growth, development, and differentiation

  • Cell division and cell enlargement in plants are separate events

  • Differentiation refers to changes in cells, tissues, and organs other than size

  • Differentiation involves qualitative changes in cells, tissues, and organs

Page 9:

• Growth, development, and differentiation

  • Differentiation in plants is reversible and cells can revert to an embryonic form

  • Cells isolated from plants can be induced to de-differentiate, producing a mass of cells called a callus

Page 12:

• Plant meristems

  • Plant growth is restricted to discrete regions of cell division called meristems

  • Two apical meristems: tip of the root and tip of the stem

  • Apical meristems provide primary growth, predominantly vertical

Page 14:

• Plant meristems

  • Root apical meristem (RAM) is a cluster of dividing cells located behind the root cap

  • RAM has a quiescent center where slowly dividing cells produce new root tissues and cells for the root cap

Page 16:

• Plant meristems

  • Shoot apical meristem (SAM) is more complex than the root

  • SAM of dicots is a dome-shaped structure with two regions: tunica and inner corpus layer

  • Tunica has cells that undergo anticlinal divisions, providing surface growth and peripheral stem tissues

  • Inner corpus layer generates most of the shoot

Page 18:

• Plant meristems

  • Leaves develop from nodes on the SAM called primordia

  • The pattern of primordia on the shoot depends on the plant species

  • Primordia development gives rise to the typical flattened shape of leaves

Page 19:

• Plant meristems

  • Tissues derived from apical meristems are called primary tissues

  • Woody plants have a meristem called the vascular cambium that produces secondary tissues, increasing the diameter of the plant

  • Vascular cambium adds secondary xylem and phloem to the plant stem

Page 21:

• Seed development

  • Zygote formation occurs in the maternal organs of the flower

  • Growth and differentiation of the zygote result in an embryo encased in a seed

  • Mature seed maintains the embryo in a dormant state until conditions are favorable for germination

Page 24:

• Seed development

  • Pistil contains female reproductive structures, including the ovary with ovules

  • Megaspore mother cell undergoes meiosis to form an embryo sac with an egg and endosperm

  • Stamens contain male reproductive structures, including microspore mother cells that form pollen grains

Page 27:

• Seed development

  • Pollen grains form a pollen tube when deposited on a pistil

  • Sperm nuclei from the pollen tube fertilize the polar nuclei and the egg through double fertilization

Page 29:

• Seed development

  • Development of a seed is initiated after the formation of a zygote

  • Subsequent cell divisions increase the number of cells in the embryo and the endosperm

  • First division of the zygote establishes the polarity of the embryo

Seed Development

• Embryo of a typical dicot grows through several stages

  • Reaches "heart-shape" stage

  • Apical meristems begin to organize

• Maternal plant allocates nutrients into endosperm or cotyledons during embryonic development

  • Endosperm and cotyledons provide resources for embryo during germination

  • Endospermic seeds retain endosperm at maturity

  • Nonendospermic seeds have enlarged cotyledons and reduced endosperm

• Monocot seeds have endosperm surrounded by aleurone layer

  • Aleurone layer provides enzymes for nutrient mobilization during germination

• Embryo becomes dormant and desiccated at maturation

  • Surrounded by hard seed coat derived from maternal tissue

• Plant hormones play a role in seed development

  • Cytokinin concentrations highest during peak periods of cell division

  • Auxin and gibberellin concentrations increase during cell enlargement

  • Abscisic acid concentrations peak at seed maturity, contribute to dormancy and desiccation tolerance

Seed Germination

• Quiescence and dormancy

  • Quiescence: suspended growth of embryo, resting condition of seed

    • Resumes growth upon exposure to favorable conditions

  • Dormancy: requires special event or "trigger" before growth can resume

    • Fire, scarification, or cold treatment

    • Fast plants seed is quiescent but not dormant

• Imbibition: rehydration of seed in presence of water

  • Driven by matric forces

  • Imbibition pressure helps rupture seed coat

  • Increase in seed metabolism after imbibition

• Some seeds have additional requirements for germination

  • Scarification to break seed coat

  • Oxygen for degradation of germination inhibitors

  • Water to leach inhibitors, hormones, and other compounds from seed coat

• Germination complete when radicle emerges

  • Radicle emergence driven by cell enlargement and imbibition pressures

  • Water and nutrients from media support further growth

From Embryo to Adult

• After radicle emerges, branching lateral roots are produced

• Lateral roots emerge from pericycle inside endodermis of stele

• Shoot axis elongates after radicle emergence

• Epigeal germination: hypocotyl hook emerges and elongates to protect plumule

• Hypogeal germination: cotyledons remain below ground, epicotyl elongates to pull plumule from ground

• Cereal grains have coleoptile and coleorhiza to protect plumule

Factors Affecting Growth

• Shoot height determined by internode elongation

• Different plant species show different patterns of internode elongation

• Internode elongation reduced when gibberellin concentration is low

Senescence and Programmed Cell Death

• Senescence involves specific changes to plant cells, tissues, and organs

• Increased respiration, decreased photosynthesis, catabolism of macromolecules, sugar synthesis via gluconeogenesis

• Helps recover nutrients and molecules from tissues at end of their useful life

Catabolic Reactions

• Metabolism: sum of all chemical reactions within a living organism

• Metabolic pathways: series of chemical reactions catalyzed by enzymes

• Catabolism: breakdown of complex organic molecules into simpler substances

• Catabolic reactions release energy used to drive chemical reactions

• Energy stored in ATP

• Gluconeogenesis: metabolic pathway that generates glucose from non-carbohydrate carbon substrates

Senescence and Programmed Cell Death (Page 53)

• Senescence is a normal, genetic consequence of plant aging.

• Senescence is governed by two groups of senescence-associated genes (SAGs).

  • Group 1 controls catabolic processes and includes enzymes, ubiquitin, and metallothioneins.

  • Group 2 controls the initiation and progression of senescence.

Senescence and Programmed Cell Death (Page 54)

• Senescence is a form of programmed cell death (PCD).

• PCD refers to a proactive, controlled initiation of cell death.

• Examples of PCD in plant growth and development include:

  • Maturation of xylem tracheary elements

  • Formation of aerenchyma during anoxia

  • Development of unisexual flowers

  • Response of some pathogens

PCD (Page 56)

• Wetland plants that produce aerenchyma minimize the stress of low oxygen concentrations in a waterlogged environment.

• Aerenchyma is a tissue characterized by continuous gas spaces that provide a pathway for oxygen transport from shoots to roots.

• Aerenchyma production in roots depends on a genetically controlled program of cell death.

• Programmed cell death (PCD) is an important developmental mechanism in a wide range of plant tissues.

Plant Development: Growth and Development of Cells (Page 57)

• Chapter 17 focuses on the growth and development of plant cells.

Learning Objectives (Page 58)

• Chemical composition of plant cell walls.

• Events associated with cell division in plants.

• Process for cell walls and cells to elongate.

Learning Objectives (Page 59)

• Signals involved in the control of plant development and the crosstalk between signaling pathways.

• Different signal transduction pathways activated by developmental or environmental signals.

Growth and Development of Cells (Page 60)

• The growth and development of plants primarily occur at the cellular level.

• The growth and development of cells are controlled by a complex series of internal and external signals.

Plant Cell Walls (Page 61)

• The outer surface of plant cells consists of a complex extracellular matrix (ECM), mostly comprising the cell wall.

• There are two types of cell walls: primary and secondary.

• The primary cell wall is found around young, actively growing cells.

• The secondary cell wall is deposited around mature cells.

Plant Cell Walls (Page 62)

• The primary cell wall consists of a network of cellulose microfibrils cross-linked with glycans.

• Microfibrils are long, bundled arrays of cellulose molecules linked by hydrogen bonds.

• Microfibril orientation in the primary wall is generally random.

Plant Cell Walls (Page 65)

• Microfibrils are cross-linked by non-cellulosic polysaccharides called glycans.

• Xyloglycans are the glycans found in dicots and many monocots.

• Xyloglycans contain glucose chains with a xylose sugar at carbon-6.

Plant Cell Walls (Page 68)

• The cellulose-glycan complex is embedded in a matrix of pectin and protein.

• Pectin is a heterogeneous mixture of non-cellulosic polysaccharides.

• Pectins cross-link with calcium ions, providing stability to the matrix.

• Pectins are the main component of the middle lamella that cements cell walls of neighboring cells.

Plant Cell Walls (Page 69)

• The matrix of primary cell walls contains extensin, a glycoprotein rich in hydroxyproline.

• Extensin helps lock the wall into shape and contributes to its strength.

• Other cell wall proteins are rich in proline, glycine, and threonine.

Plant Cell Walls (Page 70)

• Cellulose microfibrils are assembled by a plasma-membrane bound cellulose synthase.

• The enzyme is a multimeric complex of six subunits.

• The complex synthesizes cellulose chains and assembles them into microfibrils.

• Cellulose is synthesized from uridine diphosphoglucose.

Cell Division in Plant Cells (Page 72)

• Cell division in plant cells consists of two primary steps: mitosis and cytokinesis.

• Mitosis is the division of the genetic material.

• Cytokinesis is the division of the cytoplasmic contents.

• The events of cell division and the intervening events between cell division comprise the cell cycle.

Cell Division in Plant Cells (Page 73)

• The cell cycle is divided into phases: mitotic (M) phase, G1 phase, S phase, and G2 phase.

• G1, S, and G2 phases collectively are referred to as interphase.

Cell Division in Plant Cells (Page 75)

• The timing of the cell cycle is controlled by enzymes, including cyclin-dependent kinases (CDKs).

• Cyclin is a subunit of CDKs that activates the entire CDK complex.

• Plant cyclins (A, B, D, and H) allow for specificity in the regulation of the cell cycle.

• Phosphatases and CDK inhibitors also play a key role in controlling the cell cycle.

Cell Division in Plant Cells (Page 76)

• Cytokinesis divides the cytoplasm of plant cells by constructing a new membrane and ECM from the inside of the cell outwards.

  • New cell walls begin forming in late M phase.

  • The mitotic spindle fibers disassemble and then reassemble into the phragmoplast.

  • The phragmoplast helps orient secretory vesicles released by the Golgi apparatus.

  • The vesicles fuse to form the cell plate, which separates the parent cell into two daughter cells.

Cell Division in Plant Cells (Page 78)

• While a cell plate is formed, some connections, called plasmodesmata, are maintained between the daughter cells.

  • These plasmodesmata allow a continuity of the symplast to be maintained.

  • Symplastic transport allows for the exchange of molecules between cells without the need to cross membranes.

Cell Walls and Cell Growth (Page 80)

• In order for cells to grow, the connections between xyloglucans and cellulose microfibrils must be loosened, increasing the extensibility of the wall.

• The subsequent expansion of a cell is driven by turgor pressure resulting from the uptake of water.

Cell Walls and Cell Growth (Page 81)

• The relationship between turgor pressure and cell expansion is described by the equation: dV/dt = m(P-Y)

  • "dV/dt" is the change in volume with time.

  • "Y" is the yield threshold for expansion.

  • "m" is the wall extensibility.

  • "P" is the turgor pressure in excess of Y.

Cell Walls and Cell Growth (Page 83)

• Cell wall expansion is stimulated by low pH, a phenomenon termed "acid growth".

Cell Walls and Cell Growth (Page 84)

• Two cell wall proteins, called expansins, stimulate expansion at low pH.

  • Expansins are highly active proteins that act rapidly.

  • These proteins are highly expressed in growing, differentiating tissues, consistent with their role in cell expansion.

Cell Walls and Cell Growth (Page 85)

• Two possible mechanisms have been proposed for the action of expansins.

  • Expansins may hydrolyze the cross-linkages between glycans and other wall components, but there is no direct evidence for this role.

  • Alternately, expansins may weaken the non-covalent bonds which cross-link glycans to the cellulose microfibrils.

Secondary Cell Walls (Page 86)

• When a cell ceases to enlarge and begins to mature, a second cell wall is deposited inside the existing primary cell wall.

• The secondary cell wall is thicker and more rigid than the primary wall and consists of higher proportions of cellulose and lower amounts of glycans and pectins.

Secondary Cell Walls (Page 87)

• Secondary cell walls frequently contain lignin, a polymer that further strengthens the wall.

• The combination of cellulose microfibrils and lignin create the strength and durability seen in woody tissues.

Signalling Pathways Associated with Plant Development (Page 88)

• The various plant hormones regulate a variety of processes in plants, including growth and development.

• The classes of plant hormones include: Auxins, Gibberellins, Cytokinins, Abscisic acid, Ethylene, Brassinosteroids.

Signalling Pathways Associated with Plant Development (Page 89)

• Growth and development can also be modulated by external factors such as light, gravity, moisture, mechanical stimuli, magnetic fields, biotic factors, and anthropogenic compounds.

Signalling Pathways Associated with Plant Development (Page 90)

• In order to influence growth and development, a signal must be perceived and then transduced through the cell.

• There are two mechanisms for signal perception and transduction.

  • The signal binds to a receptor protein on the plasma membrane and initiates a cascade of events that ultimately alter metabolism or gene expression.

  • The signal travels to the nucleus and directly alters gene expression.

The G Protein System (Page 91)

• The G protein system is a common receptor system in cells.

• G protein receptors are guanosine triphosphate (GTP) proteins that bind hormones and other signaling molecules.

• The receptor system consists of the G protein on the cytosolic side of the membrane and the transmembrane G protein coupled receptor (GPCR) protein.

The G Protein System (Page 92)

• The G protein system acts as an on/off switch controlling cellular processes.

• In the absence of a signal, the three subunits of the G protein (a, b, and g) assemble into a single inactive complex.

• The inactive complex is associated with a molecule of GDP.

The G Protein System (Page 93)

• When a ligand, or signal, binds to the GPCR, the G protein binds to the GPCR and the GDP is replaced with a GTP molecule.

• The active G protein dissociates from the GPCR and the Ga subunit dissociates from the G protein.

• The Ga and Gbg are each active effectors.

• The reassembly of the G protein complex returns the system to the inactive state.

Signal Transduction (Page 95)

• After perception of a signal, a series of signal transduction events occur that trigger the cell's response.

• If the signal does not cross the membrane, the signal is relayed within the cell by secondary messengers such as protein kinase enzymes, calcium ions, and phospholipid derivatives.

Protein Kinase Secondary Messengers (Page 96)

• Protein kinases are enzymes that activate other proteins via phosphorylation.

• Weak signals can be amplified if a series of kinases (a protein kinase cascade) act to progressively activate other proteins.

• The action of protein kinases is balanced by phosphatase enzymes.

Phospholipase Secondary Messengers (Page 97)

• Phospholipases create second messengers by hydrolyzing membrane phospholipids.

• There are four different phospholipases: A1 (PLA1), A2 (PLA2), C (PLC), and D (PLD).

Phospholipase Secondary Messengers (Page 99)

• An example of a lipid-based signaling is the inositol triphosphate system.

• When the receptor binds the signal, the signal-receptor complex activates PLC.

• PLC causes the release of inositol triphosphate (IP3) and diacylglycerol (DAG) from phosphatidylinositol bisphosphate (PIP2).

• IP3 activates the calcium channels and the release of internal calcium.

• DAG is converted to phosphatidic acid (PA), and the PA regulates a range of transporters and enzymes.

Calcium Ions as Secondary Messengers (Page 101)

• Divalent calcium ions (Ca2+) regulate numerous processes in plant cells.

• Ca2+ is stored in the ER, vacuole, and mitochondria but when released into the cytosol acts as a secondary messenger.

• Ca2+ ATPases and channels act to regulate Ca2+ concentration in the cytosol.

Page 102:

• Calcium ions act as secondary messengers

• Stimulus such as light or hormones triggers an increase in Ca2+ ions

• Ca2+-ATPase pumps Ca2+ out of the cell

• Ca2+ binds to calmodulin (CaM)

• Ca2+-CaM complex activates target enzymes

Page 103:

• Calmodulin (CaM) is the Ca2+ receptor in cells

• Ca2+-CaM complex activates target proteins

• Target proteins include NAD+ kinases, protein kinases, and Ca2+-ATPases

Page 104:

• Transcription-based signaling involves the binding of the signal to a nuclear receptor

• Specific transcription factors may be involved in inducing or repressing gene expression

Page 105:

• Signaling pathways are interconnected and not isolated

• Crosstalk between pathways occurs when they interact or use common second messengers

• Crosstalk allows for greater coordination of developmental signals

Page 106:

• Topic: Translocation in the phloem and distribution of assimilations

Page 107:

• Topic: Revision - What is A, B, C, and D?

Page 108:

• Xylem and phloem are vascular tissues in plants

• Xylem transports water and minerals

• Phloem transports organic solutes

Page 109:

• Topic: Phloem

Page 110:

• Translocation is the movement of organic solutes through plants

• Solute movement occurs from photosynthetic sites (source) to growing or storage sites (sink)

• Flow in the phloem must be bi-directional to accommodate multiple sinks

Page 111:

• Land colonization by plants led to changes in plant growth and root system development

• Phloem transport separates photosynthesizing regions from areas where sugars are used

• Long-distance transport in the phloem requires a driving force

Page 112:

• Phloem transport redistributes photosynthesis products, organic compounds, and some mineral nutrients

• Redistribution occurs from source to sink

Page 113:

• Water moves up the xylem, while sugars in the phloem move to sinks

• Phloem consists of sieve cells and companion cells

• Sieve cells conduct sugars and amino acids

Page 114:

• Sources are regions that produce more photosynthate than they need

• Sinks are non-photosynthetic organs or organs that don't produce enough photosynthate for their own needs

Page 115:

• Source-sink pathways follow patterns

• Not all sources supply all sinks in a plant

• Proximity and development influence the preference of sources for specific sinks

Page 116:

• Proximity of source to sink is a significant factor in transport

• Upper mature leaves supply photosynthesis products to growing shoot tips and young leaves

• Lower leaves predominantly supply the root system

• Intermediate leaves export in both directions

• Importance of sinks may shift during plant development

• Roots and shoots are major sinks during vegetative growth, while fruits become dominant sinks during reproductive development

Page 117:

• Source leaves preferentially supply sinks with direct vascular connections

• Vascular interconnections can provide alternate pathways for phloem transport in case of interference

Page 118:

• Topic: Exactly what is transported in phloem?

Page 119:

• Phloem sap composition from castor bean includes sugars, amino acids, organic acids, proteins, potassium, chloride, phosphate, and magnesium

Page 120:

• Carbohydrates transported in phloem are nonreducing sugars

• Reducing sugars like glucose, mannose, and fructose are too reactive to be transported in the phloem

Page 121:

• The most common transported sugar in phloem is sucrose

• Sucrose is a reducing sugar that can be combined with other sugars or reduced to an alcohol

• Other mobile sugars transported in phloem contain sucrose bound to varying numbers of galactose units

Page 122:

• Other compounds transported in phloem include water, nitrogen in the form of amino acids and amides, and proteins

Page 123:

• Phloem consists of sieve elements and companion cells

• Sieve elements have no nucleus and conduct sugars and amino acids

• Companion cells provide energy and have cytoplasmic connections with sieve elements

Page 124:

• Image: Phloem sieve plates in a plant

Page 125:

• Phloem transport requires specialized, living cells

• Sieve tube elements form a continuous tube

• Pores in the sieve plate allow transport between sieve tube elements

• Companion cells have a close relationship with sieve tube elements and exchange solutes through plasmodesmata

Page 126:

• Companion cells play a role in transporting photosynthesis products from producing cells to sieve plates

• Companion cells synthesize proteins used in the phloem and provide cellular energy through cellular respiration

• There are three types of companion cells: ordinary companion cells, transfer cells, and intermediary cells

Page 127:

• Ordinary companion cells have chloroplasts and fewer plasmodesmata

• Transfer cells have well-developed thylakoids and fingerlike cell wall ingrowths for better solute transfer

• Both types are specialized for taking up solutes from the apoplast or cell wall space

Page 128:

• Intermediary cells have many plasmodesmata and small vacuoles

• They lack starch grains in chloroplasts and are involved in symplastic transport of sugars from mesophyll cells to sieve elements where no apoplast pathway exists

Page 129: Types of Sieve Elements in Seed Plants

• Sieve tube elements in angiosperms:

  • Some sieve areas have sieve plates

  • Sieve plate pores are open channels

  • P-protein is present in dicots and many monocots

  • Companion cells are sources of ATP and other compounds

• Sieve cells in gymnosperms:

  • No sieve plates; all sieve areas are similar

  • Pores in sieve areas appear blocked with membranes

  • No P-protein

  • Albuminous cells sometimes function as companion cells

Page 130: Protective Mechanisms in Phloem

• Sieve elements are under high internal turgor pressure

• Damage is caused by insects, wind, temperature, and pollution

• Release of pressure causes contents of sieve elements to surge towards the damage site

• Plant could lose sugars if not fixed

Page 131: Protective Mechanisms in Phloem

• P proteins:

  • Occur in different forms depending on plant species and age of cell

  • Seal off damaged sieve elements by plugging up sieve plate pores

• Callose:

  • Long-term solution

  • Made in functioning sieve elements and seals off damaged sieve elements

Page 132: The Mechanism of Phloem Transport

• The Pressure-Flow Model

Page 133: The Pressure-Flow Model

• Translocation moves at 1 meter per hour

• Flow is driven by an osmotically generated pressure gradient between the source and the sink

• Source: Sugars are actively loaded into the sieve element-companion cell complex

• Sink: Sugars are unloaded

Page 134: The Pressure-Flow Model

• In source tissue, energy-driven phloem loading leads to a buildup of sugars

• In sink tissue, phloem unloading leads to lower sugar concentration

• Water movement is driven by pressure gradient, not water potential gradient

Page 135: The Pressure-Flow Model

• Translocation pathway has cross walls

• Water moves in the phloem by bulk flow

• Solutes move at the same rate as water

Page 136: Phloem Loading: Where do the solutes come from?

• Triose phosphate is formed from photosynthesis during the day and moved from chloroplast to cytosol

• At night, triose phosphate and glucose from stored starch are converted to sucrose

• Sucrose moves from mesophyll cell to sieve elements via short distance pathway

Page 137: Phloem Loading: Where do the solutes come from?

• Sugars are transported into the sieve elements and companion cells

• Sugars become more concentrated in sieve elements and companion cells than in mesophyll cells

• Sugars are transported away from the source tissue (export) to the sink tissue (long distance transport)

Page 138: Phloem Loading: Where do the solutes come from?

• Movement can occur via apoplast or symplast

• Apoplastic pathway requires active transport against chemical potential gradient

• Symplastic pathway depends on open plasmodesmata between different cells

Page 139: Symplastic phloem loading

• Depends on plant species

• Requires open plasmodesmata between different cells in the pathway

• Dependent on plant species with intermediary companion cells

Page 140: Symplastic phloem loading

• Sucrose synthesized in mesophyll diffuses into intermediary cells

• Raffinose is synthesized in intermediary cells and can diffuse into sieve element

Page 141: Phloem unloading

• Three steps: sieve element unloading, short distance transport, storage and metabolism

• Unloaded sugars leave the sieve elements of sink tissue

• Sugars are transported to cells in the sink by a short distance pathway

• Sugars are stored or metabolized in sink cells

Page 142: Phloem unloading

• Can occur by symplastic or apoplastic pathways

• Symplastic pathway in young dicot leaves

• Moves through open plasmodesmata

Page 143: Phloem unloading

• Apoplastic pathway has three types

• Involves transport from the sieve element-companion cell complex to successive sink cells

• Movement through the plant cell wall

Page 145: Summary

• Sugars and other organic materials are conducted throughout the plant in the phloem by sieve elements

• Sieve elements have structural adaptations for transport

• Materials are translocated from sources to sinks

Page 146: Summary

• Translocated solutes are mainly carbohydrates

• Sucrose is the most common translocated sugar

• Phloem also contains amino acids, proteins, inorganic ions, and plant hormones

• Average velocity of translocation is 1 meter per hour

Page 147: Plant Hormones I: Auxins

Page 148: Learning Objectives

• Understand the concept of hormones, particularly plant hormones

• Recognize the range of chemical compounds considered auxins

• Know the tryptophan dependent and independent pathways for auxin synthesis

Page 149: Learning Objectives

• Know the roles for auxins in plant growth and development

• Understand how auxins activate auxin responsive genes

• Know how auxins are transported in plants and their relationship to auxin function and plant development

Page 150: Hormones: General Background

• Multicellular organisms require hormones for coordination and control

• Hormones are naturally occurring organic molecules synthesized in discrete organs or tissues

• They are transported from the site of synthesis to the site of action

• Hormones control physiological processes in a concentration-dependent manner

Page 151: Hormones: General Background

• Hormones were initially discovered in animals and later in plants

• Charles Darwin was one of the pioneers in the search for plant hormones

Page 152: Hormones: General Background

• Hormones have specific chemical characteristics

• They are naturally occurring organic molecules

• They are synthesized in discrete organs or tissues

• They are transported from the site of synthesis to the site of action

• They control physiological processes in a concentration-dependent manner

Page 153: Hormones: General Background

• Hormones in plants have the following differences:

  • Hormone synthesis can occur in many cell types simultaneously.

  • Plant hormones may act in the same cells or tissues where they are produced.

  • The relationship between hormone concentration and bioactivity is not always linear.

Page 154: Hormones: General Background

• Plant hormones are signal molecules produced within plants in low concentrations.

• They regulate plant growth, development, and tolerance to stresses.

• Plant hormones play a key role in the interactions between plants and beneficial microbes.

Page 155: Phytohormones (Plant hormones)

• Plant hormones are involved in various processes such as growth, fruit development, seed germination, flowering, abscission, maturity, dormancy, and more.

• Specific hormones mentioned are gibberellin, auxin, cytokinins, ethylene, and abscisic acid.

Page 156: Cytokinin Plant Hormones

• Cytokinins promote cell growth, cell division, cell elongation, and the growth of terminal buds and fruits.

• They also play a role in fruit formation.

Page 156: Auxin Plant Hormones

• Auxins are involved in cell growth, cell differentiation, cell elongation, and the growth of terminal buds and fruits.

• They also stimulate the sprouting of leaves and the breakdown of stored food for germination.

Page 156: Ethylene and Abscisic Acid Plant Hormones

• Ethylene is involved in the ripening of leaves and fruits and causes the dropping of leaves and fruits in excess amounts.

• Abscisic acid is involved in the dormancy of embryos and the dropping of ripened fruits.

Page 157: Auxins: Background

• Auxins were the first plant hormones to be discovered.

• Auxins are considered the "master" hormone due to their importance and numerous functions.

• Auxin synthesis occurs throughout the plant, but primarily in actively growing regions.

Page 158: Auxin Compounds in Plants

• Auxins are not a single compound but a group of structurally similar compounds.

• The most common auxin is indole-3-acetic acid (IAA).

• IAA concentrations in plants are generally from 1 to 100 μg kg FW-1 in leaves, but even higher in seeds.

Page 159: Structure of Indole-3-acetic acid (IAA)

• Indole-3-acetic acid (IAA) is an auxin compound found in plants.

Page 160: Other Natural Auxins

• Other natural auxins include indole-3-ethanol, indole-3-acetaldehyde, indole-3-acetonitrile, indole-3-butyric acid (IBA), 4-chloroIAA, and phenyl acetic acid (PAA).

Page 161: Synthetic Auxins

• Synthetic auxins such as naphthalene acetic acid (NAA) have been created.

• Synthetic auxins can also be manipulated to create herbicides like 2,4-D, 2,4,5-T, and dicamba.

Page 162: Types of Auxin

• Natural auxins occurring in plants are called "Natural auxin" and include indole acetic acid (IAA), indole propionic acid (IPA), indole butyric acid (IBA), and phenyl acetic acid (PAA).

• Synthetic auxins include 2,4-dichloro phenoxy acetic acid (2,4-D), 2,4,5-trichloro phenoxy acetic acid (2,4,5-T), and naphthalene acetic acid (NAA).

Page 163: Auxin Synthesis

• Auxin (IAA) synthesis occurs through two primary pathways in plants: a tryptophan-dependent pathway and a tryptophan-independent pathway.

• The tryptophan-dependent pathway involves the removal of the amino group from tryptophan to produce indole-3-pyruvic acid (IPA), followed by oxidative decarboxylation to generate IAA.

• The tryptophan-independent pathway involves the precursor indole-3-acetonitrile, which is derived from glucobrassicin in some plants.

Page 166: Tryptophan-Independent Synthesis

• The pathway for tryptophan-independent synthesis is not fully described, but it involves indole-3-acetonitrile as the precursor of IAA.

• The source of indole-3-acetonitrile is not known, but it is derived from glucobrassicin in some plants.

Page 168: Biological Functions of Auxins

• The main function of auxins is the promotion of cell elongation.

• Auxins contribute to vertical growth, phototropism, gravitropism, and the growth of shoots.

• The response of cell elongation to auxin increases with concentration but is saturable.

Page 169: Phototropism and Shoot Growth

• Phototropism is caused by an unequal distribution of auxin, with the shaded side containing more auxin and growing longer.

• Auxin stimulates cell elongation in shoots, promoting growth.

• IAA promotes growth in the shoot area of a plant, with higher concentrations promoting more growth.

Page 171: Apical Dominance

• Auxins establish apical dominance over axillary buds on the stem.

• Auxins produced by the apical meristem inhibit the growth of lateral buds.

• Removing the apex of the plant releases the axillary buds from inhibition, allowing their growth.

Page 175: Auxins and Cell Expansion

• Auxins promote cell expansion, and their effect is related to the decrease in pH observed during cell enlargement.

Page 176: Acid Growth Hypothesis

• Auxins promote elongation growth by activating plasma membrane-localized H+-ATPases, resulting in acidification of the intercellular space.

• The reduction in apoplastic pH activates cell wall-loosening enzymes, enabling cellular expansion.

Page 177:

• Auxin activates H+ -ATPase proton pumps in the plasma membrane

  • This causes the secretion of H+ ions into the cell wall

  • The pH of the cell wall falls as low as 4.5

• Expansins, a type of cell wall protein, is activated by the low pH

  • Expansins disrupt the hydrogen bonding between cellulose microfibrils

  • This loosens the laminate structure of the cell wall and increases wall elasticity

• With the cell wall more flexible, an influx of water causes the cell to increase in size

• The reduced rigidity of the cell wall allows the cell to elongate

• The pH is raised back to normal and the expansin-triggered loosening of the wall is reversed

Page 178:

• Diagram showing the process of auxin-induced cell elongation

• Auxin activates H+ pumps and expansins

• Expansins loosen cellulose in the cell wall

Page 179:

• Auxin acts with another protein to stabilize the proton pump membrane

• Increased proton pumping reduces the pH of the cell wall

• Auxin enters the cell and stimulates the expression of the proton pump gene

• Expansins are activated by the reduced pH and disrupt cellulose microfibrils

• The cell wall is loosened to allow cell elongation

Page 180:

• Auxin controls growth and development by changing the activity of AUXIN RESPONSE FACTOR (ARF) proteins

• Auxins induce the transcription of a set of genes called the primary auxin responsive genes

• Primary auxin responsive genes include Small upregulated auxin RNAs (SAUR), AUX/IAA, and Gretchen Hagen3 (GH3)

Page 181:

• At suboptimal levels of auxin, the expression of auxin response genes is repressed by a complex of AUX/IAA and ARF proteins

• When auxin levels rise, auxin binds to the receptor complex SCF TIR1

• This leads to the degradation of AUX/IAA proteins and the release of gene repression

• Further auxin-induced genes are activated, including genes that regulate the cell cycle

Page 182:

• Diagram showing the process of auxin-induced gene expression

• Auxin binds to TIR1 and facilitates the dissociation of AUX/IAA from ARF

• AUX/IAA proteins are degraded, allowing gene expression and cell cycle activation

Page 183:

• Auxin response factor protein (ARF) binds to the DNA in the promoter region of an auxin-responsive gene

• Gene transcription is prevented by the presence of AUX/IAA repressor protein

• When auxin levels are elevated, auxin combines with the nuclear-located auxin receptor TIR1

• This leads to the dissociation of AUX/IAA from ARF and derepression of the gene

• Transcription and translation of auxin-induced proteins occur

Page 184:

• The transport of auxin from sites of synthesis contributes to the control of plant growth and development

• Auxin transport in the shoot is basipetal and occurs through the phloem and cell-to-cell polar transport mechanisms

Page 185:

• Diagram showing polarity in auxin transport in an oat coleoptile segment

• Translocation of radio-labeled IAA is always from the apical end to the basal end of the segment

Page 186:

• Auxin transport in roots occurs in both directions: acropetal and basipetal

• Acropetal stream of auxin travels through the xylem parenchyma to the root tip

• Basipetal stream reverses the direction of auxin flow upwards through the cortical region

• Gradients of auxin created by polar transport contribute to developmental processes

Page 187:

• Diagram showing shoot acropetal auxin transport, root acropetal auxin transport, and root basipetal auxin transport

• Movement of auxin in different directions within the plant

Page 188:

• The mechanism of polar auxin transport involves a carrier-mediated, secondary active transport mechanism

• Inhibitor studies with phytotropins like TIBA, morphactin, and NPA provide evidence for this transport mechanism

Page 189:

• Diagram showing structures of phytotropins TIBA and NPA

Page 190:

• The current chemiosmotic model for polar auxin transport includes a proton motive force as the driving force for transport

• There is an influx carrier at the top of the auxin-transporting cell and an efflux carrier at the base

Page 191:

• Steps involved in the transport of IAA into and out of the cell

• IAA deprotonates in the cell wall space and enters the cell via the influx carrier

• IAAH also enters the cell by simple diffusion

• In the cytoplasm, all of the IAAH deprotonates

• Efflux carriers located at the base of the cell mediate the transport of IAA- out of the cell

Page 192:

• The specific positioning of the efflux carrier ensures polar movement of IAA

Page 193:

• Diagram showing the transport of IAA into and out of the cell

Page 194:

• Polar and lateral auxin transport is controlled by the distribution of auxin efflux carriers in the plasma membrane.

  • Efflux carriers at the basal ends of cells result in downward auxin flow.

  • Efflux carriers at the sides of cells allow for lateral auxin flow.

• In a whole plant, auxin primarily flows downward from shoot tips to root tips, then flows upward for a short distance.

Page 195:

• PIN genes contribute to polar auxin transport and other aspects of plant development.

• Localization of PIN proteins in the cell membrane helps establish the apical-basal axis during embryogenesis.

• Patterns of PIN protein distribution contribute to lateral root initiation and tropic growth responses.

Page 196:

• Plant Hormones II: Gibberellins

Page 197:

• Learning objectives:

  • Recognize the range of chemical compounds considered to be "gibberellins."

  • Recognize the range of compounds, including gibberellins, that are part of the terpene family of secondary compounds.

  • Know the general pathway for the synthesis of terpenes.

Page 198:

• Learning objectives:

  • Understand how gibberellins are synthesized from terpene precursors and where within the cell synthesis occurs.

  • Know the roles for gibberellins, particularly with respect to plant growth and development.

  • Know the gibberellins induced gene expression.

Page 199:

• Gibberellins are part of a diverse group of secondary compounds called terpenes.

• Gibberellins have notable effects on:

  • Stem elongation

  • Seed germination

  • Reproductive processes, such as flower and fruit development

Page 200:

Page 201:

• Arabidopsis dwarf mutant with and without gibberellin

Page 202:

• Grass

Page 203:

• Injecting orchard tree with gibberellin to increase cone production

Page 204:

• Gibberellin increases fruit size (often applied synthetically to make fruit more marketable)

Page 205:

• Gibberellins increasing fruit size

  • GA is used extensively on seedless grape varieties to increase the size and quality of the fruit.

  • Increasing yield in sugarcane

  • Increase inter-nodal distance

Page 206:

• 200 100 50 PPM 0 PPM PPM PPM

Page 207:

• Gibberellins in plants

  • Gibberellins are a large group of compounds with more than 135 members.

  • Only a few of these compounds have biological activity.

  • Most compounds in this group are intermediates in biosynthesis or inactivated gibberellins.

Page 208:

• Gibberellins are diterpenes with the basic ent-gibberellane structure.

• The identification system for gibberellins is based on the order of their discovery.

  • GA1 = the first gibberellic acid discovered in plants

  • GA3 = a natural fungal gibberellic acid

  • GA4 = another bioactive plant gibberellin

Page 209:

• Structure of gibberellins (GA1, GA3, GA4)

Page 210:

• The 19-carbon forms of gibberellins are generally the biologically active forms.

• The presence of an OH group reduces the biological activity of gibberellins.

Page 211:

• Subtle differences in gibberellin structure influence their degree of bioactivity.

• Bioactive gibberellins have a carboxyl group at carbon-7 and may have 3-ß-hydroxylation, 3-ß-1,3-dihydroxylation, 1,2-unsaturation, or both hydroxylation and unsaturation.

Page 212:

• The presence of hydroxyl groups on carbon 3 and carbon 13 increases the biological activity of gibberellins.

• Dihydroxylated gibberellins are the most biologically active compounds.

Page 213:

• There are three principle sites of gibberellin synthesis in plants:

  • Developing seeds and fruits

  • Young leaves of developing apical buds and elongating shoots

  • Root apex

Page 214:

• Gibberellin synthesis by reproductive tissues is well-demonstrated, while evidence for synthesis in vegetative tissues is limited.

• Terpene precursors of gibberellins are derived from chloroplasts and possibly other plastids.

Page 215:

• Terpenes, from which gibberellins are derived, are polymers of isoprene.

• Terpenes are also referred to as isoprenoids.

Page 216:

• Isoprene is phosphorylated to form two basic subunits for terpene synthesis:

  • Iso-pentyl pyrophosphate (IPP)

  • Dimethylallyl pyrophosphate (DMAP)

• IPP and DMAP are synthesized by two pathways:

  • Mevalonic acid (MVA) pathway in the cytoplasm

  • Methyl erythritol phosphate (MEP) pathway in the chloroplast

Page 217:

• Mevalonic acid pathway for IPP/DMAP synthesis in the cytoplasm.

• Synthesis begins with acetyl-CoA condensing to form hydroxymethylglutaryl-CoA.

• Mevalonic acid is phosphorylated to form mevalonic acid pyrophosphate.

Page 218:

• Mevalonic acid pathway for IPP/DMAP synthesis in the cytoplasm.

• Mevalonic acid pyrophosphate is decarboxylated and dephosphorylated to yield IPP.

• IPP is reversibly isomerized to form DMAP.

Page 219:

• Terpene synthesis in the cytoplasmic pathway.

• Mevalonic acid is converted to IPP and DMAP.

Page 220:

• Terpene synthesis in the chloroplastic pathway.

• Pyruvate and glyceraldehyde-3-phosphate condense to form methylerythritol-4-phosphate.

• Methylerythritol-4-phosphate is phosphorylated to form IPP.

• IPP is reversibly isomerized to form DMAP.

Page 221:

• Iso-pentyl pyrophosphate (IPP) serves as the precursor for various compounds, including gibberellins.

• Specific compound synthesis is restricted to either the cytoplasm or the chloroplast.

Terpene Synthesis

Page 223

• Terpenes synthesized in the cytoplasm:

  • Cytokinins

  • Sesquiterpenes (C15)

  • Brassinosteroids

• Intermediate in the cytoplasmic pathway: farnesyl pyrophosphate

Page 224

• Terpenes synthesized in the cytoplasm:

  • Cytokinins (X3)

  • Sesquiterpenes (C15)

  • Brassinosteroids

• Intermediate in the cytoplasmic pathway: farnesyl pyrophosphate

Page 225

• Terpenes synthesized in the chloroplast:

  • Monoterpenes (C10)

  • Gibberellins

  • Abscisic acid

  • Plastoquinones

  • Chlorophyll

• Intermediate in the chloroplastic pathway: geranylgeranyl pyrophosphate (GGPP)

Page 226

• Terpenes synthesized in the chloroplast:

  • Monoterpenes (C10)

  • Gibberellins

  • Chlorophyll (C20)

  • Carotenoids (C40)

  • Plastoquinones

  • Abscisic Acid

• Intermediate in the chloroplastic pathway: geranylgeranyl pyrophosphate (GGPP)

Gibberellin Synthesis

Page 228

• Gibberellins are synthesized from GGPP

• First two cyclization reactions form ent-kaurene

• C19 undergoes three oxidations, converting the methyl group to a carboxyl group

• NADPH-dependent cytochrome P450 monooxygenases synthesize GA12-7-aldehyde

Page 229

• Gibberellins synthesis pathway:

  • GGPP

  • Copalylpyrophosphate

  • ent-Kaurene

  • Kaurenoic acid

  • GA12-7-aldehyde

Page 230

• All other gibberellins are synthesized from GA12-7-aldehyde

• GA12-7-aldehyde is oxidized to GA12 in all plants

• Conversion of GA12 to other bioactive gibberellins varies by species and tissue

• Bioactive gibberellins GA20 and GA1 are synthesized in seeds and shoots

• GA9 is synthesized only in shoots

Page 231

• CPS, GGPP, ent-kaurene, KA, GA12-7-aldehyde, GA13ox, GA, GA2ox, GA20ox, GA3ox are involved in gibberellin synthesis

Growth Retardants

Page 232

• Synthesis of gibberellins can be blocked by antigibberellins

• Examples of antigibberellins: AMO-1618, cycocel, ancymidol, alar

• Antigibberellins have commercial uses in the production of ornamental plants

• Gibberellins themselves have important commercial uses in fruit production and sugarcane yield

Page 233

• Antigibberellins can be applied to potted plants to reduce stem elongation

• Antigibberellins have been used to reduce the need for pruning of vegetation under power lines

Gibberellin Transport

Page 234

• Little is known about the transport of gibberellins in plants

• Gibberellins have been detected in phloem, moving with source-sink patterns

• Gibberellins may be transported in the xylem, either from sites of synthesis in the roots or as a means of redistributing gibberellins via the phloem

Biological Effects of Gibberellins

Page 235

• Gibberellins were discovered through the study of rice plants displaying hyperelongation of the stem due to a fungal gibberellin

• Application of gibberellins can rescue some plants displaying dwarfism, but generally has no effect on normal plants

Page 236

• No information provided

Page 237

• Illustration of internodal elongation in dwarf varieties after treatment with GA

• Illustration of dwarf pea plants without and with GA treatment

Page 238

• Some examples of dwarfism can be rescued by exogenous gibberellins

• Gibberellins enhance stem growth through internode elongation

Page 239

• Additional evidence for the role of gibberellins in internode elongation came from the study of rosette plants

• Application of gibberellins to rosette plants causes hyperelongation of stems, such as that associated with bolting and the shift to reproductive growth

Page 240

• No information provided

Page 241

• Bolting is a survival mechanism in plants, where they produce flowering stems prematurely in response to fluctuating or stressful environmental conditions

Page 242

• Gibberellins promote the mobilization of nutrients stored in the endosperm

• Gibberellins are essential for germination, and some gibberellin mutants require exogenous gibberellins for germination

• Gibberellins complement the role of auxins and brassinosteroids in seed germination

Page 243:

• Biological effects of Aleurone gibberellins

• Role of gibberellins in germination

  • Activation of enzymes

• Seed embryo synthesizes gibberellins

• Gibberellins induce synthesis of amylases and proteases in aleurone layer

• Enzymes break down nutrient reserves in the seed for growth

Page 244:

• Biological effects of Aleurone Endosperm gibberellins

• Gibberellins stimulate synthesis of amylase and protease enzymes

• Enzymes convert inactive B-amylase to active form

• Amylase digests starch to glucose

• Glucose provides energy for the growing embryo

Page 245:

• Aleurone layer, gibberellin, amylase, starch, maltose, glucose, embryo, endosperm

• Water is absorbed

• Embryo synthesizes gibberellins

• Gibberellin stimulates aleurone layer cells to synthesize amylase

• Amylase hydrolyzes starch to form maltose

• Maltose is converted to glucose

• Glucose is transported to the embryo for growth

Page 246:

• Fruit and seed coat, endosperm, gibberellins, enzymes, nutrients, aleurone layer, embryo

• Embryo imbibes water and secretes gibberellins

• Hydrolytic enzymes move into the aleurone layer

• Enzymes digest proteins and starch in the endosperm

• Monomers (amino acids and sugars) are released

• Monomers are used by the embryo to synthesize new cells

Page 247:

• Biological effects of gibberellins

• Incubation of embryoless half-seeds on starch-agar gel

• Gibberellin-stimulated secretion of α-amylase from barley half seeds

• Clear circle or halo indicates digestion of starch by α-amylase

• Gibberellic acid stimulates α-amylase production

Page 248:

• Induction of α-amylase production by gibberellins occurs at the gene level

• Activation of genes leads to an increase in α-amylase mRNA and protein abundance

• Hormonal control of β-amylase biosynthesis by barley aleurone layers

Page 249:

• Plant hormones III: Cytokinins

Page 250:

• Learning objectives for cytokinins

• Synthesis of cytokinins from adenine

• Mechanisms for controlling cytokinin bioactivity

• Transport of cytokinins within plants

Page 251:

• Learning objectives for cytokinins

• Roles of cytokinins in cell division and senescence

• Receptors for cytokinins and signaling pathway activated by cytokinins

Page 252:

• Background on cytokinins

• Cytokinins are derivatives of adenosine monophosphate (AMP)

• Isoprenoid and aromatic cytokinins

• Examples of natural isoprenoid cytokinins

Page 253:

• Background on cytokinins

• Structures of cytokinins

• Adenine is the parent compound of naturally occurring cytokinins

Page 254:

• Background on cytokinins

• Cytokinins influence various traits of plant growth, development, and physiology

Page 255:

• Background on cytokinins

• Benzyladenine (BA) is a natural aromatic cytokinin

• Kinetin is a synthetic derivative of cytokinin

Page 256:

• Sites of cytokinin synthesis

• Cytokinins are transported in the xylem from their primary site of synthesis at the root tips

• High concentrations of cytokinins are found in immature seeds and developing fruits

Page 257:

• Auxin, cytokinins, strigolactones, xylem proteins

• Basipetal and acropetal transport of cytokinins

Page 258:

• Biological effects of cytokinins

• Cytokinins are known for their role in regulating cell division

• Cytokinins stimulate cell division in the presence of auxins

• Cytokinins regulate the progression of the cell cycle

Page 259:

• Biological effects of cytokinins

• Absence of auxin and cytokinin keeps cell cultures in the G1 or G2 phases

• Cytokinins activate cyclin-dependent kinases (CDK) for transition to mitosis

Page 260:

• Biological effects of cytokinins

• Interaction between auxin and cytokinin regulates root and shoot tissue formation in cell culture

• Proper ratio of auxin and cytokinin allows regeneration of callus into intact plant

Page 261:

• Experiment with auxin (NAA) and cytokinin (BA) on callus formation

• Different ratios of auxin and cytokinin result in different tissue formations

Page 262:

• Intermediate, low, and high ratios of auxin to cytokinin for callus formation, shoot formation, and root formation

Page 263:

• Biological effects of cytokinins

• IAA concentration affects root and shoot growth

Page 264:

• The ratio of auxins to cytokinins is important in the morphogenesis of culture systems.

  • High ratio of auxins to cytokinins leads to embryogenesis, callus initiation, and root initiation.

  • Low ratio of auxins to cytokinins is needed for axillary proliferation and shoot proliferation.

• Tissue is removed from a plant and cultivated in dishes on nutrient media.

• Treatment with auxin-to-cytokinin ratios greater than 10:1 causes root development on many replicate plantlets.

• Treatment with auxin-to-cytokinin ratios less than 10:1 induces shoot development on many replicate plantlets.

• Equal proportions of auxin and cytokinin cause the formation of an undifferentiated callus.

Page 265:

• Pith initial stem tissue is placed on culture medium with a balanced auxin-to-cytokinin ratio.

• Callus is moved to a new medium with increased cytokinin concentration.

• Shoots are produced when the auxin concentration is increased.

• Roots are produced when the auxin-to-cytokinin ratio is balanced.

• Callus continues to grow when placed on a medium with low nutrient concentration.

Page 266:

• Witch's broom is a gall that occurs on trees and is caused by various organisms.

• The most frequent cause of witch's broom is the fungus Taphrina betulina.

• Witch's broom occurs when an organism attacks the apical buds of a tree, disrupting the normal growth regulators.

• The infection stimulates the production of cytokinin, which interferes with auxin's ability to regulate shoot growth.

• Cytokinin causes shoots to proliferate wildly, resulting in a shrubby tree within a tree.

Page 269:

• Cytokinins are involved in the formation of crown gall tumors in plants.

• Gall tumors are caused by the bacterium Agrobacterium tumifaciens.

• The bacterium carries a tumor-inducing (ti) plasmid that contains genes responsible for inducing tumor growth.

• The plasmid is transferred to plant cells following A. tumifaciens infection.

Page 270:

• Cytokinins are involved in the formation of crown gall tumors in plants.

• Transferred DNA (t-DNA) from the plasmid is incorporated into the host plant genome.

• The t-DNA contains genes necessary for the synthesis of auxins and cytokinins.

• The third gene causes the plant to produce opines, which are a nutrient for the bacteria.

• A. tumifaciens can genetically reprogram plants and facilitate the transfer of other genes.

Page 273:

• Cytokinins have a role in maintaining the shoot meristem.

• Reducing cytokinin concentrations results in retarded shoot development, dwarf plants, and reduced size of the shoot meristem.

• It also slows the formation of leaf primordia and reduces the number of leaf cells.

Page 276:

• Abscisic acid (ABA) is a hormone that is represented by a single molecule.

• It was named based on the assumption that it was responsible for leaf abscission.

Page 278:

• The primary functions of ABA include preventing precocious germination, initiating and maintaining seed dormancy, stomatal control, and protecting seeds from desiccation.

• Other functions may include induction of storage proteins in seeds, heterophylly, initiation of secondary roots, and flowering and senescence.

Page 279:

• Seed dormancy is a condition that prevents germination under optimal environmental conditions.

• Dormant seeds do not germinate even when soil temperatures and moisture conditions are suitable.

• Delayed germination allows for dispersal and protection from bad weather and competition.

Page 281:

• ABA is the main reason for seed dormancy and inhibits seed germination.

• The ratio of ABA to GA in seeds determines whether the seed remains dormant or germinates.

Page 282:

• ABA is primarily synthesized in roots, shoot apex/leaves, guard cells, phloem parenchyma, xylem parenchyma, and plastids (chloroplasts).

Page 283:

• ABA is highly mobile within plants.

• ABA tends to accumulate in sink tissues, including seeds.

• During periods of water stress, ABA can be stored in roots.

Page 284:

• ABA may also be stored in chloroplasts.

• In the cytosol, the pH favors the protonated form of ABA (ABAH).

• In a chloroplast not undergoing photosynthesis, the stromal pH of 7.5 favors the formation of deprotonated ABA (ABA-), trapping the ABA in the chloroplast.

• During photosynthesis, the stromal pH decreases to a value of 6.5, facilitating the formation of ABAH and the release of ABA to the cytosol.

Page 285:

• ABA is transported from the upper epidermis to the mesophyll cells.

• Under well-watered conditions, the pH of the xylem sap is slightly alkaline (pH 7.2).

• During water stress, the xylem sap becomes acidic, favoring the formation of ABAH.

• ABA favors the dissociation of the undisassociated form of ABA (ABAH) by the mesophyll cells.

• ABA does not easily pass through membranes, so under conditions of water stress, more ABA reaches guard cells.

Page 286:

• ABA binds to receptors at the surface of the plasma membrane of the guard cells.

• The receptors activate several interconnecting pathways which converge to produce a rise in pH in the cytosol and transfer of Ca2+ from the vacuole to the cytosol.

• These changes stimulate the loss of negatively-charged ions (anions) and the loss of K+ from the cell, leading to stomatal closure.

Page 288:

• ABA is metabolized quickly in plants.

• One pathway to control ABA activity involves the formation of a glucose ester.

• The primary pathway involves the oxidation of ABA to phaseic acid (PA) followed by reduction to dihydrophaseic acid (DPA).

Page 290:

• ABA regulates the maturation of the embryo and the germination of seeds.

• ABA levels peak during the maturation stage of embryogenesis.

• ABA imposes dormancy on the embryo, preventing precocious germination.

• ABA induces desiccation of the seed, contributing to seed dormancy.

Page 292:

• ABA has important roles in the response to water stress.

• ABA accumulates in water-stressed leaves and inhibits stomatal opening.

• Some ABA comes from storage pools in the cell apoplast.

• The acidification of the chloroplast caused by water stress allows stored ABA to be released and transported to the guard cells.

• Increased rates of ABA synthesis help maintain stomatal closure during water stress.

Page 294:

• The transport of ABA from roots to shoots is triggered by decreases in soil moisture.

• This feed-forward signal initiates stomatal closure before there is a change in leaf water potential.

• ABA reduces stomatal conductance, helping to preserve water.

Page 295:

• ABA has additional roles in lateral or secondary root development.

• A possible role in the suppression of flower formation.

Page 297:

• Possible ABA receptors include ABAP1 protein, magnesium protoporphyrin-IX chelatase H subunit, flowering-time control protein FCA, and G-protein GCR2.

Page 298:

• The signaling pathway for ABA includes Ca2+ as an important second messenger.

• Transcription factors known as ABA response element binding factors activate ABA-induced genes that promote the synthesis of osmolytes or compatible solutes.

Page 299:

• The ABA-activated protein kinase (AAPK) is a guard cell-specific protein kinase that is activated by ABA.

• AAPK is a positive regulator of ABA-induced stomatal closure through activating plasma membrane anion channels.

Page 302:

• Ethylene has key roles in plant growth and development.

• Ethylene can influence root and shoot growth, but is primarily associated with stress, senescence, and ripening.

• Ethylene can be found in all plant tissues, although the concentration can vary.

Page 303:

• Ethylene is synthesized by a three-step pathway involving methionine, S-adenosylmethionine (SAM), 1-aminocyclopropane-1-carboxylic acid (ACC), and ACC oxidase.

Page 304:

• Sustained synthesis of ethylene requires the recycling of the sulfur-containing by-product of ACC synthesis to methionine via the Yang cycle.

Page 305:

• Methionine ATP cycle

  • Synthetase

  • S adenosyl methionine (Ado-Met)

• 5 Methyl adenosine

• ACC synthase

  • ACC: Amino cyclopropane

  • ACC carboxylate

• Oxidase

• Ethylene (CH2-CH2)

• Co2 + HCN + H2O

Page 306:

• ETHYLENE SYNTHESIS

  • Influenced by:

    • Auxin

    • Wounding

    • Water stress

    • Temperature

    • Inhibitors of RNA and protein synthesis

  • Effects occur at the transcriptional level via ACC synthase.

Page 307:

• FUNCTIONS OF ETHYLENE

  • Involved in the ripening and senescence of fruits.

  • Synthesis of ethylene differs between climacteric and non-climacteric fruit.

    • Climacteric fruit continue to ripen after harvest.

    • Non-climacteric fruit do not.

Page 308:

• FUNCTIONS OF ETH