plant biology quiz #2 review

Lecture 13 - Early Development of the Plant Body

selective advantage of seeds

o Stored food

o Protective seed coat

o Facilitation of dispersal

conserved genes

genes with similar DNA sequences in distantly related organisms that regulate key developmental pathways

first stage of embryogenesis

1) The zygote divides within the embryo sac of the ovule

-the first division establishes apical basal polarity

apical-basal polarity

-upper chalazal pole (apical cell) gives rise to most of the mature embryo

-lower micropylar pole (basal cell) produces a suspensor that anchors the embryo

importance of polarity in apical-basal polarity

-Polarity fixes the structural axis of the body (backbone)

-Polarity is a key component of biological pattern formation (different shaped ends)

-if you take a cutting of a mature plant, roots will form on the lower end and a shoot will form on the upper end of the cutting

radial polarity

-embryo is first formed but only contains mass of undifferentiated cells

-internal structure changes to establish radial polarity

structural componenets of radial polarity

-protoderm

-ground meristem

-procambium

protoderm

outer tissue layer. Precursor the epidermis

ground meristem

middle tissue layer. Precursor to the ground tissue. Surrounds the procambium

procambium

inner tissue layer. Precursor to the vascular tissues, xylem and phloem

primary meristems

Together the protoderm, ground meristem, and procambium form the primary meristems

globular stage of embryogenesis

-Embryo is spherical oApical/basal patterns are established

-procambium forms, cotyledons (1st leaves) start to develop

differences in cotyledon development in eudicots and monocots

oAs cotyledon develop in eudicots, the globular embryo gradually forms a two-lobed or heart shape

oMonocot embryos form only one cotyledon and are cylindrical in shape

continued development of the embryo

-Apical-basal patterns continue to develop and embryo is partitioned into:

oShoot apical meristem

oCotyledon(s)

oHypocotyl - stemlike axis below the cotyledon(s)

oRoot apical meristem

torpedo stage

-cotyledon elongate, and primary meristems extend along with them

apical meritstems

Addition of new cells becomes restricted to the apical meristems of the shoot and root

apical meritstems in eudicots vs monocots

-In eudicot embryos, the apical meristem of the shoot arises between the two cotyledons

-In monocot embryos, the apical meristem of the shoot arises on the side of the cotyledon cotyledon and is surrounded by a sheath-like extension from the base of the cotyledon

susponsor

-Supports the development of the early embryo by supplying it with nutrients and hormones, particularly hormones called gibberellins

Arabidopsis mutants

o In some Arabidopsis mutants, mutant genes disrupt the development of the embryo, and this leads to proliferation of suspensor cells

o Some of these suspensor cells acquire characteristics similar those of the developing embryo

-studies tell us that o750+ distinct genes coordinate embryo development in Arabidopsis

mature embryo

-cotyledon

-plumule

-hypoctoyl

-radicle

plumule

the embryonic shoot, which contains a stem (the epicotyl), the shoot meristem, and one or more young leaves

Hypocotyl

the stem below the cotyledon

Radicle

the embryonic root. Sometimes has distinct root-like characteristrics, sometimes is little more than a root meristem region covered with a root cap

Eudicot cotyledons

The embryos of seeds develop large, fleshy cotyledon that nourish the embryo as the seed germinates (pea, sunflower, walnut)

Monocot cotyledon

• Single cotyledon functions as a food storage and photosynthetic organ

• Also performs an absorptive function

monocot embryo

-When fully formed, the embryo of a grass possesses a massive cotyledon called the scutellum

-The radicle and plumule are enclosed by sheath-like protective structures: the coleorhiza (protects the radicle) and the coleoptile (protects the plumule)

the seed coat

• All seeds have an outer coat, the seed coat, or testa

• Develops from the integument(s) of the ovule and provides protection for the embryo

the seed coat in grasses

• In grasses, the outer seed covering is actually the pericarp (ovary wall) - grass seeds are technically fruits

-Micropyle is often visible on the seed coat and is often associated with a scar, called the hilum, which is left on the seed coat after the seed separates from its stalk (funiculus)

seed maturation phase of embryogenesis

-During seed maturation:

o reserves of food (starch, protein, oil) have amassed in the endosperm and/or the cotyledon(s)

o Seed undergoes dessication and loses 90+% of its water to the surrounding environment

o Seed coat hardens, encasing the embryo and stored food in protective armor

dessiccation

-• As a result of desiccation, metabolic activity within the seed decreases to an almost imperceptible level

oAllowing the embryo to remain viable for long periods of time

quiescent phase

• Following desiccation, the seeds of some plants enter a quiescent "resting" phase

• Seeds of other plants go dormant

Requirements for Seed Germination

▪ Water ▪ Oxygen ▪ Temperature ▪ Light (sometimes)

Requirements for Germination: Water

-once seed gains enough water, enzymes are activated for the digestion and utilization of stored food

-Cell enlargement and division are initiated in the embryo

Requirements for Germination: Oxygen

• During the early stages of germination, glucose breakdown may be entirely anaerobic

• As soon as the seed coat is ruptured, the seed switches to an aerobic pathway

Requirements for Germination: Temperature

• Minimum temperature for many species is 0-5C (32 - 41F)

• Maximum temperature is usually 45-48C (113 - 118F)

• Optimum temperature range is usually 25-30C (77 - 86F)

Requirements for Germination: Light

• Some shade-intolerant species will only germinate in high light, ensuring that the seedling will not be shaded

• Some species with very small seeds will only germinate in the presence of light, ensuring that they are not buried too far underground

• Some species will only germinate in darkness, ensuring they are deeply buried in the soil

Quiescent seeds

given favorable external conditions, will germinate on rehydration

Dormant seeds

-will not germinate, even when external conditions are favorable

-Method of ensuring the conditions will be favorable for the growth of the seedling

• Both seed coat and embryo can cause dormancy

Coat-Imposed Dormancy

-mechanisms causing:

-Impermeability of the seed coat to water or oxygen

oRigidity of the seed coat (mechanical constraint) - prevents the embryonic root from penetrating the coat

oCoat contains inhibitors that suppress growth of the embryo

Embryo Dormancy

oAbscisic acid (ABA): promotes dormancy oGibberellic acid (GA): promotes germination through the synthesis of starch Amylase in the endosperm

-dormancy broken when GA outweighs ABA

after-ripening of embryo dormancy

oIn temperate regions, after-ripening can be triggered by low winter temperatures. So, the germination of the seeds is inhibited in the coldest part of winter when the seedling would be unlikely to survive

Dormancy: Primary and Secondary

• The dormancy acquired during seed maturation is called primary dormancy

• Seeds that are no longer dormant, but encounter unfavorable conditions for germination, may reenter a dormant state, called secondary dormancy

Dormancy: Special requirements

• Some seeds must pass through the digestive tract of birds or mammals before they will germinate, resulting in wide dispersal

• Some seeds of desert species germinate only when inhibitors in their coats are washed away by rainfall, ensuring there is sufficient water for the growing seedling

• Some seeds must be mechanically cracked before germination, like they would be if they were caught in the current of a rocky river

• Some seeds must be exposed to smoke before germination, ensuring that all competitors have been cleared away by a forest fire

From Embryo to Adult Plant

• When germination occurs, the first structure to emerge from most seeds is the primary root/taproot

-primary root develops lateral roots

-in monocots, primary root is short-lived and root system develops from adventitous roots

adventitious roots

emerge from nodes on the stem. These adventitious roots then form lateral roots

epigeal germination

-hypocotyl elongates and bends into a hook to protect the meristem

• When the hook reaches the soil surface, it straightens and pulls the cotyledons and plumule up into the air

• Seedling continues to harvest nutrients from the cotyledons, which eventually wither and fall off

hypogeal germination

-the epicotyl is the structure that elongates and forms a hook

• As the epicotyl straightens, the plumule is raised above the soil surface

• The cotyledons remain in the soil, where they eventually decompose

epigeal germination of monocots

-nutrition stored in endosperm

-cotelydon forms hook

• As the cotyledon straightens, it carries the seed coat and enclosed endosperm upwards

• For some time, the cotyledon will continue to harvest nutrients from the endosperm

• The plumule emerges from the sheath-like base of the cotyledon

hypogeal germination of monocots

-the coleorhiza (sheath that protects the radicle) is the first structure to emerge from the seed. The radicle grows quickly, and will soon penetrate the coleorhiza and form the primary root

• The coleoptile (sheath that protects the plumule) emerges. Once it reaches the soil surface, it stops elongating

• The plumule emerges from an opening at the tip of the coleoptile

• The cotyledon remains underground, where it continues to harvest nutrients from the endosperm. Eventually the cotyledon and endosperm decompose

Lecture 14 - Cell and Tissues of the Plant Body

embryogenesis

establishes:

oApical-basal axis, with the shoot apical meristem on one end and the root apical meristem on the other

oRadial pattern of tissue systems within the axis

developmental plasticity

plants can modify their relationship with their environment - like by growing toward light

cells within apical meristems

• Initials - Undifferentiated cells that maintain the meristem as a continuing source of new cells. When initials divide, one sister cell remains in the meristem as an initial. While the other becomes a new body cell, or a:

• Derivative - may divide several times near the meristem before they differentiate into specific cell types in the primary tissues

factors determining developmental pathway

oInternal - the physical position of the cells within the plant body, which affect which genes are expressed

oExternal - environmental factors (day length, light quality and quantity, temperature, and gravity)

three processes of development

growth, morphogenesis, differentiation

first process in development

• Growth - an increase in size, via a combination of cell division and cell enlargement

second process in development

• Morphogenesis - the assumption of a particular shape and form. Via the expansion of tissue, that then differentiates into smaller units

third process in development

• Differentiation - the process by which cells with identical genetic constitutions become different from one another, and from the meristematic cells from which they originated. Via the expression of specific genes (which are not expressed by other types of cells)

-location dependent: final position in the developing organ

permanent tissues

not meristematic

three tissue systems

1) Ground (or fundamental) tissue system

2) Vascular tissue system

3) Dermal tissue system

three tissues in Ground Tissue System

• Parenchyma

• Collenchyma

• Sclerenchyma

Parenchyma cells

-most abundant cell in the plant body. Variable shape and size. Involved in photosynthesis, storage, and secretion

-Commonly occur as continuous masses

-can divide, can become totipotent in correct treatment

-perform photosynthesis, storage, and secretion (require protoplast)

Collenchyma cells

-often occur in strands, typically elongated. Often absent in monocot stems and leaves

-commonly takes the form of strands or continuous cylinders

-soft and flexible, support young growth of stems and leaves

Sclerenchyma cells

-dead cells that lack protoplasts

-continuous masses or small groups

-in primary or secondary plant body

-thick secondary cell walls for strengthening and support

two cell types of Sclerenchyma cells

• Fibers - generally long, slender cells that occur in strands or bundles

• Sclereids- much shorter than fiber cells. Contain a branched cavity. Make up seed coats, nut shells, and fruit pits ("stone cells")

xylem

-water conducting, also minerals dissolved in water

-tracheary elements

-tracheids: in seedles vascular plants, angiosperm, and gymnosperms

-vessel elements

tracheary elements in xylem

oElongated

oHave secondary walls

oNonliving - lack protoplasts

vessel elements of xylem

-specialized conducting cells in angiosperms

-Cell walls contain perforations - areas lacking primary and secondary walls

-Perforated part of the cell wall is called the perforation plate

phloem

-food conducting

-transport of sugars, amino acids, lipids, micronutrients, hormones, and proteins and RNAs that serve as signaling molecules

-Principal conducting cells are the sieve tube elements (living)

Phloem - Sieve tube elements

-sieve tube plate: cluster of pores

-sieve tube: plates arranged end to end

-p protein: plugs pores when wounded

phloem - companion cells

-specialized parenchyma cells

-Deliver substances to the sieve tube cells like ATP and messenger proteins

dermal tissue system

• Epidermis- outer protective covering of the entire primary plant body

-upper walls hahve cuticle

-guard cells: regulate stomata

-trichomes: form outgrowths on epidermis

trichome functions

o reflect solar radiation

o lower leaf temperature

o reduce water loss

o absorb water and minerals (epiphytes)

o defense against herbivores

Lecture 15 - The Root

functions of roots

-anchorage

-absorption

-conduction

-storage

-synthesize: homrones and secondary metabolites

-clonal regeneration: roots of eudicots can produce buds (ex: pando)

-secrete

rhizosphere

the soil that surrounds living plant roots and is influenced by root activity

root systems

-The primary root (taproot) develops from the embryonic radicle and grows straight down, giving rise to lateral roots as it matures

-A root system formed from a strongly developed primary root and its branches is called a taproot system

adventitious roots

Main root system develops from roots that arise from the stem - adventitious roots, which then produce lateral roots

-This is a fibrous root system, in which no one root is more prominent than the others

factors that affect depth and width of root system

oenvironmental parameters (moisture, light, temperature, concentrations of soil nutrients) otype of root system (tap root system generally has greater depth)

-most roots are shallow with long range

fine roots

-fine roots: uptake of nutrients in most shallow areas, heavily infected with mycorrhizal fungi

root to shoot ratio

-the same

-In a mature plant, the total surface area for food production (photosynthesizing surface) and the surface area for water and mineral absorption (root system) will remain more or less the same

-In a developing seedling, root surface area is typically much greater than shoot surface area

how is ratio affected by damage

• If root system is seriously damaged, shoot growth declines due to lack of water, minerals, and hormones produced in the roots

• In turn, root system growth is then reduced due to lack of carbohydrates and shoot-produced hormones

• Increases on either side stimulate growth on the other side

root cap

-Tip of each root is covered by a thimble-like cap made of parenchyma cells

-protects apical meristem

-root growth causes secretion of mucilage that lubricates the root

root cap - border cells

cells on the periphery of the root cap that are eventually released and separate from the root tissue

two regions of apical meristem

• Promeristem - area of least differentiation. Composed of initials and their immediate derivatives

• Quiescent center - inactive middle area of the meristem. Can repopulate meristematic tissue if it is damaged

three growth regions on root

• Region of Cell Division- the area of actively dividing cells - the apical meristem

• Region of Elongation- cells elongate, resulting in an increase in the length of the root

• Region of Maturation- cells differentiate and mature

primary structure

• Epidermis

• Cortex

• Vascular cylinder

epidermis

-absorbing tissue in young roots

-uptake is facilitated by root hairs - tubular outgrowths on epidermal cells

cortex

-represents ground tissue system

-Plastids store starch and do not usually contain chlorophyll

-Contain many air spaces - aerenchyma - parenchyma with large, abundant intercellular spaces

cortex - endodermis

-innermost cortical layer

-casparian strips

endodermis - casparian strips

Characteristic presence of Casparian Strips in the cell walls oBand-like portion of the cell wall reinforced with suberin and lignin - hydrophobic

cortex - exodermis

-casparian strips

-reduce water loss from root to soil

-physical barrier defense

the vascular cylinder - pericycle

-non-vascular

-surrounds vascular tissue

-Contributes to the vascular and cork cambium during secondary growth

the vascular cylinder - xylem and phloem

-core of xylem

-Nestled between the xylem ridges are strands of phloem

lateral roots

• Originate in the pericycle of a root

• As the root primordium (young lateral root) grows, it pushes through the cortex and develops an apical meristem and root cap

arial roots

• Roots produced from aboveground structures

• Prop roots and stilt roots serve support functions

pneumatophores "air roots"

-roots require oxygen, so trees in swamps grow roots out of soil

-grow against gravity

-Epiphytes develop specialized root epidermal layers called velamen that absorb water vapor and photosynthesize

fleshy roots

-food storage

-These roots are fleshy because of the storage parenchyma, which is permeated by vascular tissue

-Upper portion of most fleshy roots develop from the hypocotyl

Ficus aurea

• Monkeys and birds disperse their seeds in the treetops

• Lives as an epiphyte at first, slowly growing long arial roots all the way to the ground as it surrounds the host

• Adaptation for growing in dense forest where competition for light is intense

• Take decades to strangle their hosts. By then, they are strong enough to stand on their own