Plant Growth and Development 26 Plant Growth and Development

Chapter 26 Plant Growth and Development Key Concepts 26.1 Plants Develop in Response to the Environment 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action 26.3 Other Plant Hormones Have Diverse Effects on Plant Development 26.4 Photoreceptors Initiate Developmental Responses to Light

Chapter 26 Opening Question What changes in their growth patterns made the new strains of cereal crops produced by the Green Revolution so successful?

Concept 26.1 Plants Develop in Response to the Environment Plants have several characteristics that distinguish them from animals: Meristems—permanent collections of stem cells that allow growth throughout their lifetime. Post-embryonic organ formation—plants can initiate development of new organs throughout their lifetimes.

Concept 26.1 Plants Develop in Response to the Environment Differential growth—resources can be allocated for beneficial growth patterns: for example, more leaves to harvest sunlight or more roots to obtain water and nutrients.

Concept 26.1 Plants Develop in Response to the Environment Factors involved in regulating plant growth and development: Environmental cues (e.g., day length) Receptors to sense environmental cues (e.g., photoreceptors) Hormones—chemical signals The plant’s genome, which encodes regulatory proteins and enzymes

Concept 26.1 Plants Develop in Response to the Environment Seeds are dormant—development of the embryo is stopped. Seeds maintain dormancy by: Exclusion of water or O2 by an impermeable seed coat Mechanical restraint of the embryo by a tough seed coat Chemical inhibition of germination

Concept 26.1 Plants Develop in Response to the Environment Photodormancy: some seeds require a period of light or dark to germinate Thermodormancy: some seeds require a period of high or low temperature to germinate

Concept 26.1 Plants Develop in Response to the Environment Advantages of dormancy: Ensures survival during unfavorable conditions Results in germination when conditions are most favorable Helps seeds survive long-distance dispersal, allowing plants to colonize new territory

Concept 26.1 Plants Develop in Response to the Environment Seed dormancy can be broken by various means: Passage through an animal’s digestive system may damage the seed coat Burial in soil (darkness) Germination inhibitors may be washed away by rain

Concept 26.1 Plants Develop in Response to the Environment Germination—seeds begin to grow, or sprout Imbibition—seeds take up water if seed coat is permeable Enzymes are activated, RNA and proteins are synthesized, cellular respiration increases, and other metabolic pathways start up.

Concept 26.1 Plants Develop in Response to the Environment Embryo grows using food stored in the cotyledons or endosperm. Germination is completed when the radicle (embryonic root) emerges. The plant is then called a seedling.

Concept 26.1 Plants Develop in Response to the Environment Early shoot development: Monocots—growing shoot is protected by a sheath of cells, the coleoptile. Eudicots—growing shoot is protected by the cotyledons.

Figure 26.1 Patterns of Early Shoot Development Figure 26.1 Patterns of Early Shoot Development (A) In grasses and some other monocots, growing shoots are protected by a coleoptile until they reach the soil surface. (B) In most eudicots, the growing point of the shoot is protected within the cotyledons.

Concept 26.1 Plants Develop in Response to the Environment Many environmental cues influence plant growth. Responses are initiated and maintained by hormones and photoreceptors. These regulators act through signal transduction pathways.

Concept 26.1 Plants Develop in Response to the Environment Hormones—chemical signals that act at very low concentrations at sites often far from where they are produced Each hormone plays multiple roles and interactions can be complex. Photoreceptors—proteins with associated pigments that absorb light.

Table 26.1

Concept 26.1 Plants Develop in Response to the Environment Arabidopsis thaliana has been a model organism to understand signal transduction. Genetic screen—a large collection of mutated plants are created by chemical mutagens or insertion of transposons Phenotypes with characteristics influenced by the pathway of interest are selected and their genome compared with wild-type plants.

Figure 26.2 A Genetic Screen Figure 26.2 A Genetic Screen Genetics of the model plant Arabidopsis thaliana can be used to identify the steps of a signal transduction pathway. If a mutant strain does not respond to a hormone (in this case, ethylene), the corresponding wild-type gene must be essential for the pathway (in this case, ethylene response). This method has been instrumental to scientists in understanding plant growth regulation.

Concept 26.1 Plants Develop in Response to the Environment Mutant Arabidopsis plants with altered developmental patterns have provided information about plant hormones and the mechanisms of hormone and photoreceptor action.

First plant hormones to be identified: Gibberellins Auxin Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action First plant hormones to be identified: Gibberellins Auxin Mutant plants that do not make these hormones display dwarfism; supplying them with the hormones results in normal growth. Actions of plant hormones are not unique and specific.

Figure 26.3 Hormones Reverse a Mutant Phenotype Figure 26.3 Hormones Reverse a Mutant Phenotype (A) The two mutant dwarf tomato plants in this photograph were the same size when the one on the right was treated with gibberellins. (B) The short phenotype of this mutant Arabidopsis was reversed in the plant on the right by supplying auxin.

Table 26.2 (Part 1)

Table 26.2 (Part 2)

Gibberellins have several roles in plant growth and development: Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Gibberellins have several roles in plant growth and development: Stem elongation Fruit growth Seed germination—they trigger hydrolysis of stored food molecules.

Figure 26.4 Gibberellins and Seed Germination Figure 26.4 Gibberellins and Seed Germination During seed germination in cereal crops, gibberellins trigger a cascade of events that result in the conversion of starch and protein stores in the endosperm into monomers that can be used by the developing embryo.

Gibberellins are sprayed on seedless grapes to get larger fruit. Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Gibberellins are sprayed on seedless grapes to get larger fruit.

Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Auxin was discovered in the context of phototropism—response to light—stems bend toward a light source. Auxin is made in shoot apex and diffuses down the shoot in one direction (polar or unidirectional transport).

Polar transport results from four processes: Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Polar transport results from four processes: Diffusion of auxin across a cell membrane Membrane protein asymmetry—carrier proteins for auxin are located only in the basal end of the cell Proton pumps move H+ out of cell; sets up a chemiosmotic gradient to drive transport of auxin

Ionization of a weak acid Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Ionization of a weak acid Indole-3-acetic acid (auxin or HA) ionizes in solution: pH determines which direction is favored.

Figure 26.5 Polar Transport of Auxin Figure 26.5 Polar Transport of Auxin Proton pumps set up a chemiosmotic gradient that drives the uptake of non-ionized auxin (HA) and the efflux of ionized auxin (auxin anion; A–) through the basally placed auxin anion efflux carriers, leading to a net movement of auxin in a basal direction.

Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Auxin can also be transported laterally in a stem; results in directional growth (e.g., bending towards a light). Auxin efflux carriers move from the base of the cell to one side. Auxin concentration increases on the shaded side and cell elongation on one side causes the stem to bend.

Figure 26.6 Plants Respond to Light and Gravity Figure 26.6 Plants Respond to Light and Gravity Phototropism (A) and gravitropism (B) occur in shoot apices in response to a redistribution of auxin.

The same type of bending also occurs in eudicots: Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action The same type of bending also occurs in eudicots:

Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Auxins also have a role in gravitropism. Negative gravitropism—upward gravitropic response of shoots Positive gravitropism—downward gravitropic response of roots

Auxin has many roles in plant growth and development: Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Auxin has many roles in plant growth and development: Root initiation—shoot cuttings of many species will develop roots if cut surface is dipped into an auxin solution. Leaf abscission (detachment of old leaves from the stem) is inhibited. Timing of leaf fall is determined by decrease in the movement of auxin through the petiole.

Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Auxin helps maintain apical dominance—apical bud inhibits growth of axillary buds. Fruit development: treatment of unfertilized ovaries with auxin or gibberellins causes parthenocarpy—fruit formation without fertilization. Parthenocarpic fruits form spontaneously in seedless grapes, bananas, some cucumbers.

Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Cell expansion: for plant cells to expand, the cell wall structure must modified. Acid growth hypothesis—H+ is pumped into cell wall; lower pH activates expansins, which catalyze changes that loosen the cell wall Auxin increases synthesis of proton pumps and guides their insertion into the plasma membrane.

Figure 26.7 Auxin and Cell Expansion Figure 26.7 Auxin and Cell Expansion The plant cell wall is an extensive network of cross-linked polymers. Auxin induces loosening of the cell wall by activating proton pumps that reduce pH in the cell wall.

Auxins and gibberellins work by similar mechanisms. Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Auxins and gibberellins work by similar mechanisms. Genetic screens were used to study the mechanisms using insensitive mutants (not affected by added hormones): Excessively tall plants—the hormone response is always “on” Dwarf plants—the hormone response is always “off”

Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action The actions of both hormones are similar. They remove a repressor from a transcription factor that stimulates expression of growth- promoting genes.

Figure 26.8 Gibberellins and Auxin Have Similar Signal Transduction Pathways (Part 1) Figure 26.8 Gibberellins and Auxin Have Similar Signal Transduction Pathways Both hormones act to stimulate gene transcription by inactivating a repressor protein.

Figure 26.8 Gibberellins and Auxin Have Similar Signal Transduction Pathways (Part 2) Figure 26.8 Gibberellins and Auxin Have Similar Signal Transduction Pathways Both hormones act to stimulate gene transcription by inactivating a repressor protein.

Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Both mutations are in the gene that encodes the repressor protein. In excessively tall plants, the mutation is in the repressor region that binds to the transcription complex—the growth-promoting genes are always “on.” In dwarf plants a different region of the repressor is mutated; the repressor is always bound to the transcription factor, so the gene is always “off.”

Concept 26.2 Gibberellins and Auxin Have Diverse Effects but a Similar Mechanism of Action Gibberellins or auxin bind to a receptor, which then binds to the repressor, which stimulates polyubiquitination of the repressor, targeting it for breakdown in the proteasome. Receptors contain a region called an F-box that facilitates protein–protein interactions necessary for protein breakdown. Plant genomes have hundreds of F-box- containing proteins.

Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development Ethylene gas—promotes leaf abscission and senescence; speeds ripening of fruit. Ethylene also causes an increase in its own production. Once fruit ripening begins, more and more ethylene forms.

Ethylene gas is used to ripen stored fruit quickly. Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development Ethylene gas is used to ripen stored fruit quickly.

Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development The apical hook of eudicot seedlings is maintained by asymmetrical production of ethylene, which inhibits elongation of cells on the inner surface.

Plants treated with ethylene show this “triple response.” Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development After seedling breaks through soil, ethylene production stops, cells on inner surface elongate, and the hook unfolds. Ethylene inhibits stem elongation in general, promotes lateral swelling of stems, and decreases sensitivity of stems to gravitropic stimulation. Plants treated with ethylene show this “triple response.”

Cytokinins have several effects, often interacting with auxin: Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development Cytokinins have several effects, often interacting with auxin: Induce proliferation of cultured plant cells In cell cultures, high cytokinin-to-auxin ratio promotes formation of shoots; low ratio promotes formation of roots Cause some light-requiring seeds to germinate even if kept in darkness

Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development Usually inhibit elongation of stems; also cause lateral swelling of stems and roots (e.g., radishes) Stimulate axillary buds to grow into branches; auxin-to-cytokinin ratio controls extent of branching

Cytokinins delay senescence of leaves: Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development Cytokinins delay senescence of leaves:

Target protein—acts as a transcription factor to regulate the response Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development The cytokinin signaling pathway includes proteins similar to those in two-component systems in bacteria: Receptor—acts as a protein kinase, phosphorylating itself and a target protein Target protein—acts as a transcription factor to regulate the response

Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development Genetic screens using Arabidopsis have identified the receptor (AHK, Arabidopsis histidine kinase) and the target protein (ARR, Arabidopsis response regulator). A third protein (AHP, Arabidopsis histidine phosphotransfer protein) transfers phosphates from receptor to target protein.

Figure 26.9 The Cytokinin Signal Transduction Pathway (Part 1) Figure 26.9 The Cytokinin Signal Transduction Pathway Plant cells respond to cytokinins through a two-component signal transduction pathway involving a receptor and a target protein.

Figure 26.9 The Cytokinin Signal Transduction Pathway (Part 2) Figure 26.9 The Cytokinin Signal Transduction Pathway Plant cells respond to cytokinins through a two-component signal transduction pathway involving a receptor and a target protein.

Brassinosteroids also have diverse effects: Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development Brassinosteroids also have diverse effects: Enhance cell elongation and cell division in shoots Promote xylem differentiation Promote growth of pollen tubes Promote seed germination Promote apical dominance and leaf senescence

Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development A defect in the brassinosteroid signaling pathway results in stunted growth in Arabidopsis mutants.

Abscisic acid—actions involve inhibition of other hormones: Concept 26.3 Other Plant Hormones Have Diverse Effects on Plant Development Abscisic acid—actions involve inhibition of other hormones: Prevents seed germination while seeds are still on parent plant Promotes seed dormancy; inhibits initiation of germination events by gibberellins Mediates responses to environmental stresses and pathogens (e.g., closure of stomata to prevent water loss)

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Photomorphogenesis—developmental events that are controlled by light. Light influences seed germination, phototropism, shoot elongation, initiation of flowering, and other events. Plants respond to light quality (wavelengths) and quantity (intensity and duration of exposure).

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light In the action spectrum for phototropism of coleoptiles, blue light was found to be the most effective at inducing the coleoptile to curve. A blue light receptor called phototropin was identified using blue-light-insensitive Arabidopsis mutants.

Figure 26.10 Action Spectrum for Phototropism Figure 26.10 Action Spectrum for Phototropism The absorption spectrum for phototropin (A) is similar to the action spectrum for the bending of a coleoptile toward light (B). After 90 minutes, only the coleoptiles exposed to blue light bend.

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Light absorption by phototropin starts a signal transduction cascade that results in stimulation of cell elongation by auxin. Phototropin also participates with another blue- light receptor, zeaxanthin, in the light-induced opening of stomata.

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Cryptochromes—blue-light receptors located in the nucleus; affect seedling development and flowering Mechanism of action is unknown.

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Phytochromes: red-light (650–680 nm) receptors Lettuce seedlings germinate only in response to red light. Red light responses are reversible by far-red light (710–740 nm).

Figure 26.11 Photomorphogenesis and Red Light (Part 1) Figure 26.11 Photomorphogenesis and Red Light Lettuce seeds will germinate if exposed to a brief period of light. The action spectrum for germination indicated that red light was most effective in promoting it, but far-red light would reverse the effect if presented right after the red light. Harry Borthwick and his colleagues asked what the effect of repeated alternating flashes of red and far-red light would be. In each case, the final exposure determined the germination response. This observation led to the conclusion that a single, photo-reversible molecule was involved. That molecule turned out to be phytochrome.a [a H. Borthwick et al. 1952. Proceedings of the National Academy of Sciences USA 38: 662-666.]

Figure 26.11 Photomorphogenesis and Red Light (Part 2) Figure 26.11 Photomorphogenesis and Red Light Lettuce seeds will germinate if exposed to a brief period of light. The action spectrum for germination indicated that red light was most effective in promoting it, but far-red light would reverse the effect if presented right after the red light. Harry Borthwick and his colleagues asked what the effect of repeated alternating flashes of red and far-red light would be. In each case, the final exposure determined the germination response. This observation led to the conclusion that a single, photo-reversible molecule was involved. That molecule turned out to be phytochrome.a [a H. Borthwick et al. 1952. Proceedings of the National Academy of Sciences USA 38: 662-666.]

Figure 26.11 Photomorphogenesis and Red Light (Part 3) Figure 26.11 Photomorphogenesis and Red Light Lettuce seeds will germinate if exposed to a brief period of light. The action spectrum for germination indicated that red light was most effective in promoting it, but far-red light would reverse the effect if presented right after the red light. Harry Borthwick and his colleagues asked what the effect of repeated alternating flashes of red and far-red light would be. In each case, the final exposure determined the germination response. This observation led to the conclusion that a single, photo-reversible molecule was involved. That molecule turned out to be phytochrome.a [a H. Borthwick et al. 1952. Proceedings of the National Academy of Sciences USA 38: 662-666.]

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Phytochrome exists in two interconvertible isoforms:

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Ratio of red to far-red light determines phytochrome-mediated responses. During daylight, ratio is about 1.2:1—the Pfr isoform predominates. In shade, ratio may be as low as 0.13:1—Pr isoform predominates. Shade-intolerant species respond by stimulating stem cell elongation, thus growing taller to escape the shade.

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Phytochrome is a protein with two subunits, each has a pigment called a chromophore. When Pr absorbs red light, the chromophore changes shape, leading to the Pfr form. This exposes the two regions of the phytochrome, which both affect transcriptional activity.

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Exposure of a nuclear localization signal sequence: Pfr moves from cytosol to nucleus and binds to transcription factors to stimulate expression of genes involved in photomorphogenesis. Exposure of a protein kinase domain: Pfr phosphorylates itself and other proteins, which changes activity of other transcription factors.

Figure 26.12 Phytochrome Stimulates Gene Transcription Figure 26.12 Phytochrome Stimulates Gene Transcription Phytochrome is composed of two subunits, each containing a protein chain and a chromophore. When the chromophore absorbs red light, phytochrome is converted into the Pfr isoform, which activates transcription of phytochrome-responsive genes.

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light In Arabidopsis, a gene family encodes five different phytochromes, each functioning in different photomorphogenic responses. Phytochrome affects 2,500 genes (10% of the genome) by increasing or decreasing their expression.

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Gene interactions: When Pfr is formed at seed germination, genes for gibberellin synthesis are activated, and genes for gibberellin breakdown are repressed. Gibberellins accumulate, and seed food reserves are mobilized for embryo growth.

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Timing and duration of biological activities in living organisms are governed by a biological clock—an oscillator that alternates between two states at roughly 12-hour intervals. This results in circadian rhythms (e.g., the opening of stomata during the day and closing at night).

Concept 26.4 Photoreceptors Initiate Developmental Responses to Light Circadian rhythms can be reset, or entrained by changing light–dark cycles. Phytochrome is likely involved: at sundown phytochrome is mostly Pfr. Through the night, Pfr is converted to Pr. At daylight, it rapidly converts to Pfr. However long the night, the clock is reset at dawn every day; the clock adjusts to changes in day length over the course of the year.

Answer to Opening Question In semi-dwarf wheat, the mutant allele (Rht, “reduced height”) is involved in signal transduction in response to gibberellin. Rht mutants also put a greater proportion of their photosynthate into making seeds than wild-type plants do (higher harvest index). Most wheat varieties today have an Rht mutation—short stems do not topple over with heavy grain load.

Answer to Opening Question Rht mutants are insensitive to gibberellin—the repressor is always bound to the transcription factor, keeping the gibberellin response “off.”

Answer to Opening Question In semi-dwarf rice, the mutant allele is involved with gibberellin synthesis. The gene sd-1 (“semi-dwarf”) encodes an enzyme that catalyzes one of the last steps in gibberellin synthesis.

Figure 26.13 Semi-Dwarf Rice Figure 26.13 Semi-Dwarf Rice A short variety of rice (left) produces higher yields of grain than its taller counterpart (right). The semi-dwarf rice produces less than normal amounts of gibberellins.