Plant Development Chapter 31 Part 1

Impacts, Issues Foolish 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

seed coat fused with ovary wall endosperm cells cotyledon coleoptile plumule (embryonic shoot) embryo Figure 31.2 Anatomy of a corn seed (Zea mays). During germination, cell divisions resume mainly at apical meristems of the plumule (the embryonic shoot) and radicle (the embryonic root). A plumule consists of an apical meristem and two tiny leaves. In grasses such as corn, the growth of this delicate structure through soil is protected by a sheathlike coleoptile. hypocotyl radicle (embryonic root) Fig. 31-2, p. 524

Early Growth of Corn (Monocot)

Figure 31.3 Early growth of corn (Zea mays), a monocot. Fig. 31-3a, p. 525

coleoptile branch root primary root coleoptile hypocotyl radicle Figure 31.3 Early growth of corn (Zea mays), a monocot. 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. 31-3a, p. 525

Figure 31.3 Early growth of corn (Zea mays), a monocot. Fig. 31-3b, p. 525

adventitious (prop) root primary leaf coleoptile adventitious (prop) root branch root Figure 31.3 Early growth of corn (Zea mays), a monocot. 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. Fig. 31-3b, p. 525

adventitious (prop) root 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. coleoptile primary leaf branch root adventitious (prop) root primary root hypocotyl radicle Figure 31.3 Early growth of corn (Zea mays), a monocot. Stepped Art Fig. 31-3, p. 525

Animation: Plant development

Early Growth of a Bean (Eudicot)

Figure 31.4 Early growth of the common bean plant (Phaseolus vulgaris), a eudicot. Fig. 31-4a, p. 525

A After a bean seed germinates, its radicle emerges seed coat radicle cotyledons (two) hypocotyl Figure 31.4 Early growth of the common bean plant (Phaseolus vulgaris), a eudicot. 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. 31-4a, p. 525

Figure 31.4 Early growth of the common bean plant (Phaseolus vulgaris), a eudicot. Fig. 31-4b, p. 525

primary leaf primary leaf withered cotyledon 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. primary leaf primary leaf withered cotyledon branch root Figure 31.4 Early growth of the common bean plant (Phaseolus vulgaris), a eudicot. primary root root nodule Fig. 31-4b, p. 525

Summary: Eudicot Development

female gametophyte (n) mature sporophyte (2n) germination zygote in seed (2n) meiosis in anther meiosis in ovary DIPLOID fertilization HAPLOID eggs (n) sperm (n) microspores (n) megaspores (n) Figure 31.22 Summary of development in the life cycle of a typical eudicot. male gametophyte (n) female gametophyte (n) Fig. 31-22, p. 535

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

gibberellin aleurone endosperm embryo Figure 31.7 Action of gibberellin in barley seed germination. 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. 31-7a, p. 528

amylase Figure 31.7 Action of gibberellin in barley seed germination. 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. 31-7b, p. 528

sugars Figure 31.7 Action of gibberellin in barley seed germination. 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. 31-7c, 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. 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. 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 Figure 31.7 Action of gibberellin in barley seed germination. Stepped Art Fig. 31-7a, p. 528

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

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. Figure 31.8 A coleoptile lengthens in response to auxin produced in its tip. Auxin moves down from the tip by passing through cells of the coleoptile. The directional movement is driven by different types of active transporters positioned at the top and bottom of the cells’ plasma membranes (right). Fig. 31-8, p. 529

A A coleoptile stops growing if its tip is removed A A coleoptile stops growing if its tip is removed. A block of agar will absorb auxin from the cut tip. 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. Figure 31.8 A coleoptile lengthens in response to auxin produced in its tip. Auxin moves down from the tip by passing through cells of the coleoptile. The directional movement is driven by different types of active transporters positioned at the top and bottom of the cells’ plasma membranes (right). Stepped Art Fig. 31-8, p. 529

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