Albia Dugger Miami Dade College Cecie Starr Christine Evers Lisa Starr Chapter 27 Plant Reproduction and Development (Sections )

27.6 Patterns of Development in Plants An embryonic plant with shoot and root apical meristems formed as part of the seed As the seed matures and dries, the embryo enters dormancy

Fig. 27.9, p. 436 radicle (embryonic root) seed coat fused with ovary wall endosperm cells cotyledon coleoptile plumule (embryonic shoot) embryo hypocotyl Patterns of Development in Plants

Germination The embryo resumes development after germination Water seeps into a seed and activates enzymes that break down stored starches into sugars The seed coat ruptures and oxygen diffuses into the seed Meristem cells divide, the embryo grows, and the embryonic root breaks out of the seed coat germination Resumption of growth after dormancy

Triggers for Germination Germination requirements are evolutionary adaptations to life in a particular environment, and maximize a seedling’s chance of survival Triggers differ by species, and have a genetic basis: Some seed coats must be physically broken Some seeds require freezing, or exposure to light Some seeds require exposure to burning

Plant Development Sporophyte tissues and organs develop in characteristic patterns with genetic and environmental components Patterns of early growth (increase in cell number and size) vary by species Cell division occurs primarily at meristems – behind meristems, cells differentiate and form specialized tissues

Early Growth of a Monocot (Corn)

Fig , p. 436 Early Growth of a Monocot (Corn)

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. hypocotyl primary root branch root coleoptile Fig a, p. 436 coleoptile radicle Early Growth of a Monocot (Corn)

Fig b, p. 436 Early Growth of a Monocot (Corn)

Fig b, p. 436 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. branch root adventitious (prop) root primary leaf coleoptile primary root Early Growth of a Monocot (Corn)

Early Growth of a Eudicot (Bean)

Fig a, p. 437 Early Growth of a Eudicot (Bean)

Fig a, p. 437 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. seed coat primary root radicle cotyledons (two) hypocotyl Early Growth of a Eudicot (Bean)

Fig b, p. 437 Early Growth of a Eudicot (Bean)

Fig b, p. 437 B Photosynthetic cells in the cotyledons make food for several days. Then, the seedling’s leaves take over the task and the cotyledons wither and fall off. withered cotyledon primary leaf primary root branch root root nodule Early Growth of a Eudicot (Bean)

Key Concepts Growth and Development Plant development includes seed germination and other events of the life cycle, such as root and shoot development, flowering, fruit formation, and dormancy These events have a genetic basis, and are influenced by the environment

ANIMATION: Plant Growth To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

27.7 Plant Hormones and Other Signaling Molecules Plant development depends on cell-to-cell communication, which is mediated by plant hormones Environmental cues such as availability of water, length of night, temperature, and gravity influence plants by triggering the production and dispersal of hormones hormone Signaling molecule that is released into the body by one type of cell and alters the activity of other cells

Plant Hormones Plant hormones stimulate or inhibit development When a plant hormone binds to a target cell, it may modify gene expression, change solute concentrations, affect enzyme activity, or activate another molecule in cytoplasm Five types of plant hormones interact in plant development: gibberellins, auxins, abscisic acid, cytokinins, and ethylene

Gibberellins gibberellin Plant hormone that induces cell division and stem elongation Helps seeds break dormancy Role in flowering in some species

Auxins Auxins produced in apical meristems result in elongation of shoots, cell differentiation in vascular cambium, fruit development, and lateral root formation in roots Apical dominance: Auxin produced in a shoot tip prevents growth of lateral buds along a lengthening stem auxin Plant hormone that stimulates cell division and elongation

Effects of Auxin

Fig a, p. 439 Effects of Auxin

Fig a, p. 439 A A coleoptile stops growing after its auxin-producing tip has been removed. A block of agar that absorbs auxin from a cut tip can stimulate a de-tipped coleoptile to resume growth. time Effects of Auxin

Fig b, p. 439 Effects of Auxin

Fig b, p. 439 B If an auxin-containing agar block is placed to one side of a cut tip, the coleoptile will continue to grow, but bend as it does. time Effects of Auxin

Abscisic Acid Abscisic acid inhibits growth, diverts photosynthetic products from leaves to seeds, inhibits seed germination in some species, and can cause stomata to close abscisic acid (ABA) Plant hormone that stimulates stomata to close in response to water stress Induces dormancy in buds and seeds

Cytokinins Cytokinins form in roots and travel via xylem to shoots, where they induce cell divisions in apical meristems cytokinin Plant hormone that promotes cell division Releases lateral buds from apical dominance Inhibits aging in leaves

Ethylene Ethylene gas is produced by damaged cells, in autumn in deciduous plants, and near the end of the life cycle as part of a plant’s normal process of aging Ethylene is widely used to artificially ripen fruit that has been harvested while still green ethylene Gaseous plant hormone that inhibits cell division in stems and roots Promotes fruit ripening

Major Plant Hormones and Their Effects

Commercial Uses of Plant Hormones

Other Signaling Molecules Brassinosteroids stimulate cell division and elongation Jasmonates helps inhibit germination and root growth FT protein is part of a signaling pathway in flower formation Salicylic acid helps plants resist attacks by pathogens Systemin helps transcription of genes for insect toxins

ANIMATION: Cell Shapes To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

27.8 Adjusting the Direction and Rate of Growth Plants respond to environmental stimuli by adjusting the growth of roots and shoots These responses (tropisms) are typically mediated by hormones

Responses to Gravity When a seed germinates, its primary root always grows downward, and its primary shoot always grows upward In plants, a shift in dense starch grains (statoliths) to the lowest part of the cell causes auxin to be redistributed to the downward-facing side of roots and shoots gravitropism Plant growth in a direction influenced by gravity

Gravitropism Seedlings rotated 90° adjust by redistributing auxin Auxin transport inhibitors prevent seedlings from adjusting direction of growth

Gravity, Statoliths, and Auxin Ten minutes after root A was rotated 90°, statoliths are already settling to the new “bottom” of the cells

Fig a, p. 440 statoliths Gravity, Statoliths, and Auxin

Fig b, p. 440 Gravity, Statoliths, and Auxin

Responses to Light Phototropism orients plant parts to maximize light reception for photosynthesis – phototropins absorb blue light and control auxin production phototropism Change in the direction of cell movement or growth in response to a light source solar tracking Plant parts change position in response to the sun’s changing angle through the day

Phototropism

Responses to Contact We see thigmotropism when a vine’s tendril touches an object and curls around it Mechanical stress, such as by wind exposure, inhibits stem lengthening in a response related to thigmotropism thigmotropism Directional growth of a plant in response to contact with a solid object Involves calcium ions and products of at least five genes

Effect of Mechanical Stress

Animation: Gravitropism

27.9 Sensing Recurring Environmental Changes Seasonal shifts in night length, temperature, and light trigger seasonal shifts in plant development Most organisms have a biological clock that governs the timing of rhythmic cycles of activity biological clock Internal time-measuring mechanism by which individuals adjust their activities seasonally, daily, or both in response to environmental cues

Biological Clocks A bean plant holds its leaves horizontally during the day but folds them close to its stem at night – these rhythmic leaf movements are an example of a circadian rhythm Similar mechanisms cause flowers of some plants to open only at certain times of day circadian rhythm A biological activity that is repeated about every 24 hours

Rhythmic Leaf Movements Despite being kept in the dark for 24 hours, the leaves of this bean plant kept on folding and unfolding at sunrise (6 a.m.) and sunset (6 p.m.)

Setting the Clock Sunlight resets biological clocks in plants by activating and inactivating photoreceptors called phytochromes Active phytochromes activate genes that control important processes such as germination and flowering phytochrome A light-sensitive pigment that helps set plant circadian rhythms based on length of night

Phytochromes Red light activates phytochromes; far-red light inactivates them

Fig , p. 442 Pfr reverts to Pr in darkness activatedinactive gene expression Pfr red light Pr far-red 730 nm red 660 nm far-red light Phytochromes

When to Flower? Different species of plants flower at different times of the year – in these plants, flowering is photoperiodic Long-day plants flower when the hours of darkness fall below a critical value; short-day plants flower only when the hours of darkness are greater than a critical value photoperiodism Biological response to seasonal changes in the relative lengths of day and night

Control of Flowering In response to night length, companion cells in leaf phloem transcribe more or less of the Flowering locus T (FT) gene Cells export FT protein into sieve tubes, where it migrates from leaves to shoot tips FT protein interacts with a transcription factor to transcribe floral identity genes in cells differentiating behind the meristem

Photoperiodism

Fig a, p. 443 Length of night (hours of darkness) onset of dormancy dormancy short-day plant flowering long-day plant flowering dormancy JANUARY FEBRUARY MARCH APRIL seed germination or renewed growth; short-day plant flowering MAY OCTOBER NOVEMBER DECEMBER JUNE JULY AUGUST SEPTEMBER A Photoperiodism

Experiment: Photoperiodism

Fig b,c, p. 443 C A pulse of far-red light, which inactivates phytochrome, cancels the effect of the red flash, and short-day plants flower. Blue bars indicate night length; yellow bars, day length. B A flash of red light interrupting a long night activates phytochrome. It causes plants to respond as if the night were short, and long-day plants flower. Time being measured (hours)...flowers...does not flower Short-Day Plant: critical night length Long-Day Plant:...flowers...does not flower B C Experiment: Photoperiodism

Vernalization Some plants flower only after exposure to cold winter temperatures In these plants, the FT gene is silenced by a repressor which stops being produced after a period of cold weather vernalization Stimulation of flowering in spring by low temperature in winter

Vernalization Local effect of cold on dormant buds of lilac Only buds exposed to the low outside temperatures resumed growth and flowered in springtime

Key Concepts Responses to Environmental Cues Plants respond to environmental cues, including gravity, sunlight, and seasonal shifts in night length and temperatures, by altering patterns of growth Cyclic patterns of growth are responses to seasons and other recurring environmental patterns

27.10 Plant Defenses Plants protect themselves from predators in several ways: Thorns or nasty-tasting chemicals directly deter herbivores Damage to a leaf stimulate synthesis of jasmonates, which stimulates production of certain genes products: Some products slow growth temporarily Some products release chemicals that attract wasps that parasitize insect herbivores

Jasmonates in Plant Defense

Fig a, p. 444 Jasmonates in Plant Defense

Fig b, p. 444 Jasmonates in Plant Defense

Fig c, p. 444 Jasmonates in Plant Defense

Fig d, p. 444 Jasmonates in Plant Defense

Systemic Acquired Resistance The presence of a virus, bacteria, or fungus in one plant part increases pathogen resistance in the entire plant Affected tissue releases molecular signals that cause cells in other plant parts to produce compounds (hydrogen peroxide, salicylic acid, jasmonates) that strengthen resistance systemic acquired resistance In plants, a long-term, systemic resistance to pathogens

Senescence Dropping of leaves (abscission) may be induced by any stress; it also occurs in the normal life cycle of flowering plants, as part of senescence abscission Process by which plant parts are shed in response to seasonal change, drought, injury, or nutrient deficiency senescence Phase in a life cycle from maturity until death

Hormones and Abscission Hormones mediate abscission in the normal life cycle Example: A deciduous fruit tree In early summer, leaves and fruits produce auxin that maintains growth As growing season ends, auxin production declines, nutrients are routed to stems and roots Ethylene signals cells in abscission zones to drop leaves and fruit

Abscission in a Horse Chestnut Tree

Fig a, p. 445 Abscission in a Horse Chestnut Tree

Fig b, p. 445 Abscission in a Horse Chestnut Tree

Plight of the Honeybee (revisited) Bees in hives affected by colony collapse disorder have large amounts of ribosomal RNA fragments in their guts The problem may be picorna-like viruses, which hijack their hosts’ protein synthesis machinery Bees that can’t make proteins can’t defend themselves against infections, and are vulnerable to starvation