Chapter 39

Dodder Plant as a Hunter

• Dodder (Cuscuta) is a parasitic, nonphotosynthetic flowering plant. It begins its hunt for a host plant with limited provisions.
• Dodder seedling relies on stored nutrients for survival and host searching.
• If a host isn't found within a week, the dodder seedling dies.
• Dodder uses tendrils to coil around the host and haustoria to tap into the host's phloem.
• The amount of coils depends on the host's nutritiousness.
• Initially, it was believed dodder plants just bumped into the host, but it has been discovered that dodder's are attracted to the shade where stems can be found.
• It is now understood that chemicals released by the host attract the dodder.

Plant Signal Transduction Pathways

• Photosynthetic plants also exhibit environmental awareness, utilizing sunlight and soil nutrients, which involves signal transduction pathways that are similar to those in animals.
• Plant responses to environmental stimuli differ from those of animals; plants alter growth and development, while animals commonly respond through movement.
• Plants must adjust to seasonal changes and interact with various organisms, involving complex signal transduction pathways.
• This chapter focuses on internal chemicals (hormones) and how plants perceive and respond to their environments; signal transduction pathways link signal reception to response.

Concept 39.1: Signal Transduction Pathways Link Signal Reception to Response

• Dodder plants respond to environmental signals to enhance survival and reproduction.
• Example: A potato left in a cupboard sprouts shoots from its eyes (axillary buds).

Etiolation

• Etiolation in a sprouting potato refers to morphological adaptations for growing in darkness.
• Characteristics of etiolation:
  • Pale stems and unexpanded leaves
  • Short, stubby roots
• Etiolation is advantageous because:
  • Expanded leaves would hinder soil penetration.
  • Reduced water loss due to unexpanded leaves.
  • Energy is conserved by not producing chlorophyll in darkness.
  • Energy is allocated to stem elongation to break ground before nutrient reserves are exhausted.

De-etiolation (Greening)

• De-etiolation is the process where a plant transitions to a typical plant form when exposed to light.
• Stem elongation slows, leaves expand, roots elongate, and chlorophyll is produced.
• De-etiolation response serves as an example of how a plant cell's reception of a signal, in this case, light, is transduced into a response (greening).
• Mutants provide insights into cell signal processing stages: reception, transduction, and response.

General Model for Signal Transduction Pathways

• A stimulus interacts with a specific receptor protein, triggering sequential activation of relay proteins and production of second messengers.
• Receptor location can be on the cell surface or inside the cell.

Reception

• Signals are detected by receptors, which change shape in response to a specific stimulus.
• The receptor involved in de-etiolation is phytochrome, a photoreceptor located in the cytoplasm.
• Studies on aurea mutant of tomato (reduced phytochrome levels) showed reduced greening when exposed to light.
• Injecting phytochrome into aurea leaf cells restored normal de-etiolation response.

Transduction

• Receptors are sensitive to weak signals; de-etiolation responses are triggered by low light levels.
• Transduction involves second messengers: small molecules and ions that amplify the signal and transfer it from the receptor to other proteins.
• Second messengers in de-etiolation: Calcium ions (Ca^{2+}) and cyclic GMP (cGMP).
• Phytochrome activation leads to the opening of Ca^{2+} channels, increasing cytosolic Ca^{2+} levels.
• Activated phytochrome changes shape, activating guanylyl cyclase, which produces cGMP.
• Both Ca^{2+} and cGMP are needed for a complete de-etiolation response.
• Injection of cGMP into aurea tomato leaf cells induces only a partial de-etiolation response.

Response

• Second messengers regulate cellular activities, often involving increased activity of particular enzymes.
• Two main mechanisms:
  • Transcriptional regulation: Increases or decreases synthesis of mRNA encoding a specific enzyme.
  • Post-translational modification: Activates preexisting enzymes.

Post-Translational Modification

• Involves phosphorylation of specific amino acids, altering protein hydrophobicity and activity.
• Second messengers (cGMP and Ca^{2+}) activate protein kinases.
• Kinase cascades link stimuli to responses at the gene expression level through phosphorylation of transcription factors.

Protein Phosphatases

• Protein phosphatases dephosphorylate specific proteins, turning off signal transduction pathways when the initial signal is no longer present.
• Cellular functioning depends on the balance of activity between protein kinases and protein phosphatases.

Transcriptional Regulation

• Specific transcription factors bind to DNA and control transcription of specific genes.
• Phytochrome-induced de-etiolation involves transcription factors activated by phosphorylation.
• Activation of transcription factors depends on phosphorylation by protein kinases activated by cGMP or Ca^{2+}.
• Developmental changes depend on transcription factors that are activators (increase transcription) or repressors (decrease transcription).

Arabidopsis Mutants

• Some Arabidopsis mutants exhibit light-grown morphology in the dark, excluding the green color due to a defect in the final step of chlorophyll production that requires light.
• These mutants have defects in a repressor that normally inhibits the expression of other genes activated by light.

De-etiolation Proteins

• Proteins involved in de-etiolation include:
  • Enzymes functioning in photosynthesis
  • Enzymes supplying chemical precursors for chlorophyll production
  • Proteins affecting plant hormones that regulate growth
• Auxin and brassinosteroid levels decrease following phytochrome activation, slowing stem elongation.
• Mutant isolation and molecular biology techniques help researchers identify these pathways.

Concept Check 39.1

• Morphological differences between dark- and light-grown plants: Etiolated plants have pale, elongated stems, while de-etiolated plants have short, sturdy stems and expanded green leaves.
• Etiolation helps compete successfully by elongating the stem quickly to reach light before nutrient reserves are exhausted.
• Cycloheximide inhibits protein synthesis, so de-etiolation would be inhibited because new proteins cannot be synthesized.
• Viagra inhibits an enzyme that breaks down cyclic GMP, so applying Viagra to tomato leaf cells might cause a partial de-etiolation of aurea mutant tomato leaves since cGMP is a second messenger in de-etiolation.

Concept 39.2: Plant Hormones Coordinate Growth, Development, and Responses to Stimuli

• A hormone is a signaling molecule produced in low concentrations in one part of an organism's body and transported to other parts, where it triggers responses in target cells and tissues.
• Plant biologists use the term plant growth regulator to describe organic compounds that modify or control specific physiological processes within a plant.
• Plant hormones are active at very low concentrations and have multiple effects depending on site of action, concentration, and developmental stage.
• Plant hormone responses depend on the amounts and relative concentrations of the hormones involved.

Table 39.1: Overview of Plant Hormones

Auxin (IAA)

• Where Produced: Shoot apical meristems, young leaves, and root apical meristems. Developing seeds and fruits contain high levels.
• Major Functions:
  • Stimulates stem elongation (low concentration only)
  • Promotes formation of lateral and adventitious roots
  • Regulates fruit development
  • Enhances apical dominance
  • Functions in phototropism and gravitropism
  • Promotes vascular differentiation
  • Retards leaf abscission

Cytokinins

• Where Produced: Primarily in roots and transported to other organs.
• Major Functions:
  • Regulate cell division in shoots and roots
  • Modify apical dominance and promote lateral bud growth
  • Promote nutrient movement into sink tissues
  • Stimulate seed germination
  • Delay leaf senescence

Gibberellins (GA)

• Where Produced: Meristems of apical buds and roots, young leaves, and developing seeds.
• Major Functions:
  • Stimulate stem elongation, pollen development, pollen tube growth, fruit growth, and seed development and germination
  • Regulate sex determination and the transition from juvenile to adult phases

Abscisic Acid (ABA)

• Where Produced: Almost all plant cells; detected in every major organ and living tissue; transported in phloem or xylem.
• Major Functions:
  • Inhibits growth
  • Promotes stomatal closure during drought stress
  • Promotes seed dormancy and inhibits early germination
  • Promotes leaf senescence
  • Promotes desiccation tolerance

Ethylene

• Where Produced: Most parts of the plant; high concentrations during senescence, leaf abscission, and fruit ripening; stimulated by wounding and stress.
• Major Functions:
  • Promotes ripening of many fruits, leaf abscission, and the triple response in seedlings (inhibition of stem elongation, promotion of lateral expansion, and horizontal growth)
  • Enhances the rate of senescence
  • Promotes root and root hair formation
  • Promotes flowering in the pineapple family

Brassinosteroids

• Where Produced: All plant tissues; different intermediates predominate in different organs; act near the site of synthesis.
• Major Functions:
  • Promote cell expansion and cell division in shoots
  • Promote root growth at low concentrations; inhibit root growth at high concentrations
  • Promote xylem differentiation and inhibit phloem differentiation
  • Promote seed germination and pollen tube elongation

Jasmonates

• Where Produced: Several parts of the plant; travel in the phloem to other parts of the plant.
• Major Functions:
  • Regulate fruit ripening, floral development, pollen production, tendril coiling, root growth, seed germination, and nectar secretion
  • Produced in response to herbivory and pathogen invasion

Strigolactones

• Where Produced: Roots in response to low phosphate conditions or high auxin flow from the shoot.
• Major Functions:
  • Promote seed germination
  • Control of apical dominance
  • Attraction of mycorrhizal fungi to the root

Auxin

• Concept emerged from experiments on stem response to light.
• Tropism: Growth response resulting in plant organs curving toward or away from stimuli.
  • Phototropism: Growth of a shoot toward (positive) or away from (negative) light.
• Charles and Francis Darwin (1880) found coleoptile tip senses light.
• Peter Boysen-Jensen (1913) showed the signal is a mobile chemical substance.
• Auxin stimulates growth as it passes down the coleoptile.
• Auxin: Indoleacetic acid (IAA) promotes elongation of coleoptiles and has multiple functions in flowering plants.
• IAA is produced in shoot tips and transported from cell to cell down the stem (1 cm/hr).
• Polar transport: Unidirectional transport of auxin, independent of gravity, attributable to the polar distribution of auxin transport protein in the cells.
• Auxin Effects:
  • Stimulating cell elongation
  • Regulating plant architecture

Auxin Role in Cell Elongation

• Stimulates cell elongation in young developing shoots. Auxin causes a cell to grow by binding to a membrane receptor in a plasma membrane.
• Effective within a specific concentration range (10^{-8} to 10^{-4} M).
  • High concentrations inhibit elongation by inducing ethylene production.
• Acid Growth Hypothesis:
  • Auxin stimulates plasma membrane's proton (H^{+}) pumps.
  • Lowering pH activates expansins that break cross-links between cellulose microfibrils loosening cell wall.
  • Enhances ion uptake causing osmotic uptake of water and increased turgor.
  • Increased turgor and cell wall plasticity increases cell elongation.
• Rapidly alters gene expression causing cells to produce new proteins and transcription factors within minutes.
• Auxin stimulates sustained growth response.

Auxin Role in Plant Development

• Polar transport controls spatial organization and pattern formation.
• Reduced auxin flow from a branch indicates insufficient productivity.
• Lateral buds are released from dormancy and begin to grow.
• Key role in establishing phyllotaxy (arrangement of leaves on a stem).
• Local peaks in auxin concentration determine leaf primordium formation.
• Polar transport directs leaf vein patterns.
• Activity of the vascular cambium is also under control of auxin transport.
• Reduction in auxin transport at the end of the growing season
• Also regulates organization of the microscopic angiosperm female gametophytes is regulated by an auxin gradient.

Practical Uses for Auxins

• Indolebutyric acid (IBA) is used in vegetative propagation by cuttings.
• Synthetic auxins (2,4-D) are used as herbicides.
• Developing seeds produce auxin, which promotes fruit growth; synthetic auxins induce normal fruit development in greenhouse-grown tomatoes.

Cytokinins

• Discovery was by trial-and-error attempts to enhance growth and development of plant cells in tissue culture.
• Coconut milk and degraded DNA samples stimulated the growth of plant embryos in culture.
• Active ingredients are modified forms of adenine named cytokinins because they stimulate cytokinesis (cell division).

Zeatin

• The most common natural cytokinin, discovered first in maize.
• Cytokinins effects includes:
  • cell division and differentiation
  • apical dominance
  • aging

Control of Cell Division and Differentiation

• Produced in actively growing tissues (roots, embryos, fruits).
• Transported from roots to target tissues in the xylem sap.
• Cytokinins, acting synergistically with auxin, stimulate cell division and influence differentiation pathway.
• Parenchyma tissue in culture grows large but doesn't divide without cytokinins.
• Ratio of cytokinins to auxin controls cell differentiation.
• Callus: Undifferentiated cell mass. When cytokinin levels increase, shoot buds develop from the callus. If auxin levels increase, roots form.

Apical Dominance

• The ability of the apical bud to suppress the development of axillary buds depends on sugar and various plant hormones that include auxin, cytokinins, and strigolactones.
• Cutting off the apical bud increases sugar availability to axillary buds initiating but release.
• Auxin transported down the shoot from the apical bud indirectly inhibits axillary buds from growing. The polar flow of auxin down the shoot triggers of synthesis strigolactones, which directly repress bud growth.
• Mutants that overproduce cytokinins or plants treated with cytokinins are bushier than normal.
• Removing the apical bud wanes auxin and strigolactone levels causes the axillary buds closest to the cut surface to grow most vigorously, and one of these axillary buds will eventually take over as the new apical bud.
• Applying auxin to the cut surface of the shoot tip resuppresses the growth of the lateral buds.

Anti-Aging Effects

• Cytokinins slow the aging of certain plant organs.
• They inhibit protein breakdown, stimulate RNA and protein synthesis, and mobilize nutrients from surrounding tissues.
• Leaves dipped in cytokinin solution stay green longer.

Gibberellins (GAs)

• Discovered in early 1900s when a fungus of the genus Gibberella caused