Plant Development

Chapter 31 Part 1

Impacts, IssuesFoolish Seedlings, Gorgeous Grapes

Gibberellin and other plant hormones control the growth and development of plants – environmental cues influence hormone secretion

31.1 Patterns of Development in Plants

Germination• Process by which a dormant mature embryo

sporophyte in a seed resumes growth• Certain species-specific conditions may be

required to break dormancy• Begins when water activates enzymes in the seed• Ends when the embryo breaks the seed coat

Patterns of Development in Plants

Growth (increase in cell number and size) occurs primarily at meristems

Differentiation results in the formation of tissues and parts in predictable patterns

Patterns of plant development are an outcome of gene expression and environmental influences

Anatomy of a Corn Seed

Fig. 31-2, p. 524

seed coat fused with ovary wall

endosperm cells

cotyledon

coleoptile

plumule (embryonic shoot)

embryo

hypocotyl

radicle (embryonic root)

Early Growth of Corn (Monocot)

Fig. 31-3a, p. 525

Fig. 31-3a, p. 525

coleoptile

branch root

primary root

coleoptile

hypocotyl

radicle

A After a corn grain (seed) germinates, its radicle and coleoptile emerge. The radicle develops into the primary root. The coleoptile grows upward and opens a channel through the soil to the surface, where it stops growing.

Fig. 31-3b, p. 525

Fig. 31-3b, p. 525

primary leaf

coleoptile adventitious (prop) root

branch root

primary root

B The plumule develops into the seedling’s primary shoot, which pushes through the coleoptile and begins photosynthesis. In corn plants, adventitious roots that develop from the stem afford additional support for the rapidly growing plant.

A After a corn grain (seed) germinates, its radicle and coleoptile emerge. The radicle develops into the primary root. The coleoptile grows upward and opens a channel through the soil to the surface, where it stops growing.

B The plumule develops into the seedling’s primary shoot, which pushes through the coleoptile and begins photosynthesis. In corn plants, adventitious roots that develop from the stem afford additional support for the rapidly growing plant.

Fig. 31-3, p. 525

Stepped Art

hypocotyl

radicle

branch root

branch root

primary root

primary root

adventitious (prop) root

primary leaf

coleoptile

coleoptile

coleoptile

Animation: Plant development

Early Growth of a Bean (Eudicot)

Fig. 31-4a, p. 525

Fig. 31-4a, p. 525

seed coat radicle

cotyledons (two)

hypocotyl

primary root

A After a bean seed germinates, its radicle emerges and bends in the shape of a hook. Sunlight causes the hypocotyl to straighten, which pulls the cotyledons up through the soil.

Fig. 31-4b, p. 525

Fig. 31-4b, p. 525

primary leaf

primary leaf

withered cotyledon

branch rootprimary root

root nodule

B Photosynthetic cells in the cotyledons make food for several days, then the seedling’s leaves take over the task. The cotyledons wither and fall off.

Summary: Eudicot Development

Fig. 31-22, p. 535

germinationmature

sporophyte (2n)

zygote in seed (2n)

fertilizationmeiosis in anther

meiosis in ovary

DIPLOID

HAPLOID

microspores (n)

megaspores (n)

eggs (n) sperm (n)

male gametophyte (n)

female gametophyte (n)

31.1 Key Concepts Patterns of Plant Development

Plant development includes seed germination and all events of the life cycle, such as root and shoot development, flowering, fruit formation, and dormancy

These activities have a genetic basis, but are also influenced by environmental factors

31.2 Plant Hormones and Other Signaling Molecules

Plant development depends on cell-to-cell communication – mediated by plant hormones

Plant hormones• Signaling molecules that can stimulate or inhibit

plant development, including growth• Five types: Gibberellins, auxins, abscisic acid,

cytokinins, and ethylene

Gibberellins

Gibberellins induce cell division and elongation in stem tissue, and are involved in germination

Auxins

Auxins promote or inhibit cell division and elongation, depending on the target tissue

Auxin produced in a shoot tip prevents growth of lateral buds (apical dominance)

Auxins also induce fruit development in ovaries, and lateral root formation in roots

Rooting Powder with Auxin

Abscisic Acid

Abscisic acid (ABA) inhibits growth, is part of a stress response that causes stomata to close, and diverts products of photosynthesis from leaves to seeds

Cytokinins

Cytokinins form in roots and travel to shoots, where they induce cell division in apical meristems

Cytokinins also release lateral buds from apical dominance and inhibit leaf aging

Ethylene

Ethylene • The only gaseous hormone• Produced by damaged or aging cells• Induces fruit and leaves to mature and drop• Used to artificially ripen fruit

Major Plant Hormones and Their Effects

Commercial Uses of Plant Hormones

Other Signaling Molecules

Besides hormones, other signaling molecules are involved in plant development• Brassinosteroids• FT protein• Salicylic acid• Systemin• Jasmonates

31.3 Examples of Plant Hormone Effects

Gibberellins and barley seed germination• Barley seed absorbs water• Embryo releases gibberellin• Gibberellin induces transcription of amylase gene • Amylase breaks stored starches into sugars used

by embryo for aerobic respiration

Gibberellins in Barley Seed Germination

Gibberellins in Barley Seed Germination

Gibberellins in Barley Seed Germination

Fig. 31-7a, p. 528

aleurone endosperm embryo

gibberellin

A Absorbed water causes cells of a barley embryo to release gibberellin, which diffuses through the seed into the aleurone layer of the endosperm.

Fig. 31-7b, p. 528

amylase

B Gibberellin triggers cells of the aleurone layer to express the gene for amylase. This enzyme diffuses into the starch-packed middle of the endosperm.

Fig. 31-7c, p. 528

sugars

C The amylase hydrolyzes starch into sugar monomers, which diffuse into the embryo and are used in aerobic respiration. Energy released by the reactions of aerobic respiration fuels meristem cell divisions in the embryo.

Fig. 31-7a, p. 528

Stepped Art

amylaseB Gibberellin triggers cells of the aleurone layer to express the gene for amylase. This enzyme diffuses into the starch-packed middle of the endosperm.

C The amylase hydrolyzes starch into sugar monomers, which diffuse into the embryo and are used in aerobic respiration. Energy released by the reactions of aerobic respiration fuels meristem cell divisions in the embryo.

sugars

aleurone endosperm embryo

gibberellin

A Absorbed water causes cells of a barley embryo to release gibberellin, which diffuses through the seed into the aleurone layer of the endosperm.

Examples of Plant Hormone Effects

Auxin (IAA) plays a critical role in all aspects of plant development• First division of the zygote• Polarity and tissue pattern in the embryo• Formation of plant parts• Differentiation of vascular tissues• Formation of lateral roots• Responses to environmental stimuli

Directional Transport of Auxin

Fig. 31-8, p. 529

auxin

time time

auxin

A A coleoptile stops growing if its tip is removed. A block of agar will absorb auxin from the cut tip.

B Growth of a de-tipped coleoptile will resume when the agar block with absorbed auxin is placed on top of it.

C If the agar block is placed to one side of the shaft, the coleoptile will bend as it grows.

A A coleoptile stops growing if its tip is removed. A block of agar will absorb auxin from the cut tip.

Fig. 31-8, p. 529

time

B Growth of a de-tipped coleoptile will resume when the agar block with absorbed auxin is placed on top of it.

time

C If the agar block is placed to one side of the shaft, the coleoptile will bend as it grows.

Stepped Art

Animation: Auxin’s effects

Examples of Plant Hormone Effects

Jasmonates signal plant defenses• Wounding by herbivores cleaves peptides (such

as systemin) in mesophyll cells• Activated peptides stimulate jasmonate

synthesis, which turns on transcription of several genes

• Some gene products slow growth• Other gene products signal wasps to attack

specific herbivores responsible for damage

Jasmonates in Plant Defenses

31.2-31.3 Key Concepts Mechanisms of Hormone Action

Cell-to-cell communication is essential to development and survival of all multicelled organisms

In plants, such communication occurs by hormones