Section 2 Plant Phys

Hormones

Chemical signals that control plant growth and development

Diverse group of chemicals that have the ability to control the growth and developmental processes in plants (and animals)

They can be modified amino acids, small organic acids, terpenoids, gases, steroids etc.. They are present in very low concentrations in cells, but control growth and development of plants.

Auxin

Controls many physiological processes of plants

-Shoots always grow towards light

-Roots always grow towards gravity

-Controls proper fruit growth of strawberries

-apical dominance. Shoot apical meristem dominates over axillary buds due to high level of auxin

Major auxin found in plants

Indole-3 acetic acid (IAA)

Auxin synthesis

Synthesized inyoung tissues ike SAM. Then, auxin transports toward roots generating a gradient of auxin from SAM to root (this is known as polar auxin transport).

Morphogen

This gradient of auxin is necessary to maintain the architecture (shape) of the plant. Thus, auxin also considered as a "morphogen".

Polar transport of auxin

Due to two different types of transporter proteins called influx and efflux carriers

Auxin is transported by membrane transporter proteins

Influx carrier

Efflux carrier

Influx carrier

Transports auxin into the cell

Efflux carrier

Transports auxin out of the cell

Auxin is ____ during tropic responses

Redistributed

Phototropic response and auxin

Light changes direction of auxin movement

Cells with more auxin elongate more causing bending of the stem

Light changes the localization ofEfflux carrier (transporter) changing the direction of auxin transport. Blue light is required for this response.

Gravitropic response

Auxin also involve in gravitropic response.

More auxin is transported to cells in lower side causing cells in the upper side of the root to expand.

This is kind of opposite response compared to what is happening in the shoot (phototropism).

Auxin concentration is higher in what region of the root

Columella

Statoliths

Large starch granules that rerceives gravity in roots

Columella cells contain bigger amyloplasts (starch granules) They are also called statoliths.

Statoliths rest on ER membranes of the lower side of the cell due to gravity.

Starch-statolith hypothesis of gravity sensing

Changing the direction of gravity leads to movement of statoliths to a new location (lower ER membrane). This generates a mechanical pressure on ER membrane.

How does auxin control plant growth and development

Genomic responses and non genomic responses

Genomic response for auxin control

Generally, auxin controls the expression of many genes that produces proteins necessary to control growth and development.

Non genomic responses of auxin control

Also, auxin controls the activity of some proteins through post translational modifications to control growth and development

How does auxin control gene expression

High increase of auxin promotes the degradation of the repressor protein.

Degradation of the repressor protein allows the proper interaction of TF leading to gene transcription

Repressors are degraded through Ubiquitin-Proteasome Pathway at high concentration of auxin

Agricultural uses of auxin

Chemicals with auxin activity are being used as selective herbicides.

Auxin is used in tissue culture as it can control cell division, expansion and cell differentiation.

Auxin is also used to produce parthenocarpic (seedless) fruits.

Rooting powder mainly contains auxin

Natural use of auxin as a herbicide?

Cant be used because plants have the means for degrading or metabolizing IAA.

Synthetic auxins cannot be degraded by plants as there are no natural mechanisms to metabolize them. thus when used at higher concentrations they kill dicot plants

Gibberellic acid (GA)

Many physiological responses are regulated by GA

GA controls thestem elongationof plants. Mutants that do not produce enough GA show short stems

GA induces stem elongation of rosette plants

GA is used to enhance the growth of seedless grape

GA induces early reproductive growth of plants

GA promoting seed germination

By controlling the expression of genes that are required for seed germination

Germinating seeds and energy

When seeds germinate until they produces leaves for photosynthesis, germinating seedlings have to rely on glucose produced in the seed. Carbohydrates are stored in seeds as starch (a complex carbohydrate). Starch should be hydrolyzed to glucose to be used as an energy source as well as to produce other essential macromolecules.

How does GA control alpha-amylase gene expression

GA first induces the expression of myb gene to produce myb protein.

Myb protein is a transcriptional factor that interacts with enhancers ofα-amylase gene.

DELLA repressors and high levels of GA

DELLA repressors are degraded through Ubiquitin-Proteasome Pathway at high levels of GA

Developmental processes that ethylene regulates

Fruit ripening

Senescence

Induces root hair formation

In dark, ethylene inhibits hypocotyl elongation, thick hypocotyl and exaggerates apical hook

How does ethylene control developmental processes

Ethylene is a gaseous hormone.

Like other two hormones, ethylene also control the expression of specific set of genes that are required for ethylene induced developmental processes.

Ethylene also uses ubiquitin-proteasome pathway to control gene expression.

However, unlike auxin and GA (in which repressor proteins are degraded), ethylene inhibits the degradation of a transcriptional factors (TFs), which are degraded in the absence of ethylene.

Ethylene activates gene expression by inhibiting the degradation of TFs

When there is no ethylene, TFs necessary for the induction of genes (by ethylene) are continuously degraded. So genes cannot be activated.

When there is ethylene, TFs necessary for the induction of genes (by ethylene) are accumulated as ethylene will inhibits the degradation of TFs.

Gene induction depends on the abundance of the

How TFs are degraded through Ubiquitin-Proteasome Pathways

When there is no ethylene signaling pathway is active and TFs are degraded through ubiquitin Proteasome pathway.

When there is ethylene, ethylene binds with its receptor located in the ER membrane and inhibits the signaling pathway, thus inhibiting the degradation of TFs. TFs now can induce ethylene responsive genes.

Gene induction depends on the

Abundance/activity of the TF or Repressors

How auxin and GA activate gene expression

By degrading repressor proteins

Two major steps of E3 (SCF) mediated protein degradation

1) Ubiquitination of target proteins

2) Degradation of the target proteins

5 steps of E3 mediated protein degradation

1. Ubiquitin undergoes ATP-dependent activation by E1

2. Ubiquitin is transferred to E2

3. Ubiquitinated E2 forms a complex with E3 ligase and the target protein

4. The target protein is ubiquitinated by the E2-E3 complex

5. The target protein is degraded by the 26S proteasome

Specificity of the pathway is regulated by

E3 (SCF) complex

Auxin summary

Molecular glue between Aux/IAA & F-box protein

GA summary

Conformational change in DELLA protein?

Ethylene

Conformational change in EIN3 protein?

LECTURE 7

Three phases of plant life cycle

Embryogenesis

Vegetative phase

Reproductive phase

Embryogenesis

Development of the embryo from the zygote

Vegetative phase

Development of a mature plant from the embryo

Reproductive phase

Development of reproductive structures

Two generations that plant life cycle alternates between

Sporophyte 2n

Gametophyte n

Development of the sporophyte

In flowering plants, sporophyte is easily visible

When a seed germinates, it produces the sporophyte

Sporophyte produces the reproductive structure which is the flower

Process:

Fertilization

Zygote develops into embryo 2n?

Ovary develops into fruit, ovule develops into seed

Germination

Young sporophyte

Flowers with pistil and/or stamen

Pistil has ovary, ovule, megastore mother cell undergoes meiosis

Stamen has microspore mother cells undergoing meiosis

One of the 2n cells in the ovary will become

Megastore mother cell

Cells in the tapetum of the anther will become

Microspore mother cells

The male gametophytes of flowering plants are also referred to as _____.

pollen grains

Development of gametophyte

Microspore mother cell divides by meiosis to produce 4 equal haploid cells, called Microspore. Each microspore divides by mitosis to produce two cells- pollen (male gametophytes)

Development of megaspore

Megaspore mother cell divides by meiosis to produce 4 equal haploid cells, megaspores. 3 of these will degenerate leaving only one megaspore

Meagaposre divides by mitosis to produce 8 nuclei that will end up generating 7 cells that comprise the female gametophyte

Pollination

When pollen comes into contact with stigma of the pistil

Pollen tube

Pollen germinates via pollination and produces a pollen tube, which is guided by tube nucleu to the ovary (towards the egg)

Generative nucleus divides by mitosis to produce two what

Sperm nuclei

One sperm nucleus fuses with egg cell to produce___. Other spern cell fuses with ____ to produce 3n cell

2n zygote

Polar nuclei

Zygote will divide by mitosis to produce ____. 3n cell divides by mitosis to produce ___

Embryo

Endosperm

Establishment of polarity

Zygote will divide by mitosis to produce embryo. 1st cell division is a centric to produce an applicable smaller cell and longer basal cell. This is because of polarity establishment in the zygote probably through auxin distribution

Several distinct development stages that embryogenesis goes through

Embryo sac

Zygote

2-cell

8-cell

16-cell

Early globular

Late globular

Transition

Late heart

Seedling

Apical cell divides to produce ___ while basal cell divides to produce ____ and ____

Embryo proper (globular structure)

Uppermost cell (hypophysis)

Lower suspensor

Hypophysis produces ___ while suspensor ____

Produces the root

Helps absorb nutrients from the endosperm to the growing embryo

Role of auxin in embryo development

During the different stages of embryo development, we can observe maximum auxin concentration in different tissues where we can see higher cell division

What inhibits auxin biosynthesis

Trans-cinnamic acid

Whawt it's he auxin effluent carrier

Pin1

Mutations in auxin related genes cause

Defects in embryo development

How mutations in auxin related genes cause defects in embryo development

When the GNOM gene is mutated, embryo files to develop both shoots and roots

When MONOPTEROUS gene is mutated, embryo families to develop roots

MONOPTEROUS gene encodes a repressor protein involved in auxin response

Apical-basal axis growth is due to

SAM and RAM

Radial axis growth is due to

cambium and cork cambium

Growth in radial axis is due to what cell divisions

Periclinal and anticlinal

Periclinal division

Represents cell division parallel to the periphery of teh stem

Anticlinal division

represents cell division right angle to the periphery of the stem

Majority of plant growth and development occurs through

Post embryonic development

LECTURE 8

Seeds are produced in the ___. After fertilization, ___ turns into the seed while the ___ develops into a fruit

Ovary

Ovule

Ovary

Most of the space of the embryo sac will develop into the ____ while the fertilized egg (zygote) develops into the ___

Endosperm

Embryo

In some dicots, content of the endosperm will do what?

Be absorbed into the growing cotyledon, which replaces the endosperm

Seeds are an adaptation by plants to

The terrestrial environment

Testa

Seed coat

Aleurone layer

Seeds with endosperms contain this

Produces GA

Endospermic

Plant that contains entire endosperm

Non-endospermic

Endosperm absorbed by embryo

Dormancy

If a live seed does not undergo germination when exposed to suitable environmental conditions, it is considered in dormancy

Primary dormancy

Seeds do not germinate even in favorable environment during the dormancy period

Secondary dormancy

Even after dormancy period seeds may not germinate if unfavorable environment conditions are present

Primary dormancy could be due to

Exogenous dormancy

Endogenous dormancy

Exogenous dormancy

Seed coat-imposed dormancy (limits water, gas exchange, inhibitors, mechanical constrain)

Endogenous dormancy

This is due to the germination inhibitory hormone, ABA produced by the embryo

Viviparous

Seeds do not have a dormancy period and germinate on the mother plant

An adaptation to environmental conditions

Can viviparous germination be seen on non-viviparous plants?

Yes, this is due to mutations of genes that are required for dormancy

What hormones regulate dormancy/germiantion of seeds

ABA

GA

ABA promotes

Dormancy

Biosynthesis of ABA

During seed maturation process, water content will drastically reduce. This will enhance ABA biosynthesis. In addition, sensitivity to ABVA can also be increased

GA promotes

Seed germination

Biosynthesis of GA

During seed imbibition cells absorb water, this will enhance GA biosynthesis. In addition, sensitivity to GA can also be increased

Breaking dormancy different requirements

Light

Chilling

After-ripening period

Some chemicals (by inhibiting ABA response)

Mobilization of food storage during seed germination

During seed germination, complex storage food such as starch is converted to simple sugars (glucose) for the initial growth and development

Starch to glucose conversion requires a-amylase enzyme that is produced in the aleurone layer

GA induced starch hydrolysis

Occurs through a-amylase induction in aleurone layer. GA first induces MYB gene and MYB protein functions as a TF to enhance a-amylase gene

Epigeal

pertaining to growing on or germinating above the surface of the earth (depends on the position of cotyledon after germination)

Hypogeal

Growing/germinating below the surface of the ground (depends on position of cotyledons after germination)

What promotes stem growth during seedling growth

Auxin

Acid growth hypothesis

Auxin promotes H+ secretion from cells leading to acidification of cell walls. This will help to increase enzyme activity to degrade cell walls, which leads to cell wall loosening due to turf or pressure causing cell expansion.

LECTURE 9

Reproductive development is regulated by what two major factors

Endogenous

Exogenous