Plant Hormones and Auxin Quiz

Plant Growth Regulators

• Plant hormones are chemicals produced by plants to alter growth and maintenance.
• Plant hormones are concentrated in meristems and buds (dormant shoot meristems).
• They control cellular activities by sending chemical signals to cells.
• These signals activate genes that code for specific enzymes.
• Plant hormones can promote or inhibit cellular activities.
• Typical roles include cell division, elongation, and differentiation.
• Most hormones have multiple effects in plants.
• Plant hormones work at very small concentrations.
• Hormones often work in conjunction with each other, and have overlapping effects.
• The effectiveness of a hormone depends on maintaining a closely regulated pool size.
• A balance of biosynthesis, storage as inactive conjugates, and catabolic degradation of the molecule.
• Six recognized groups of plant hormones: Auxin, Gibberellins, Cytokinins, Abscisic acid, Ethylene, and Brassinosteroids.

Classification of Plant Growth Regulators (PGRs)

• Growth promoting hormones/Growth promoter: Increase plant growth, Examples include auxins, gibberellins, and cytokinins.
• Growth inhibiting hormones/Growth retardant: Inhibit plant growth, Examples include abscisic acid and ethylene.

Hormone Action

• Apical meristem: Cell division (auxin, cytokinins, gibberellins); Elongation (auxin, gibberellins); Dormancy (abscisic acid)
• Flower: Initiation (unknown); Differentiation (auxin, gibberellins)
• Fruit: Growth (auxin, cytokinins); Ripening (ethylene)
• Leaf: Growth (gibberellins); Senescence (ethylene); Inhibition of senescence (cytokinins)
• Lateral bud: Repression (auxin); Release from apical dominance (cytokinins)
• Stem: Elongation (auxin, gibberellins)
• Roots: Initiation (auxin promotes, gibberellins inhibit); Growth (auxin inhibits)

Auxin

• The first hormone discovered in plants.

• Coleoptile is the protective sheath of monocots.

• Early experiments on auxin (Darwin, 1880; Peter Boysen-Jensen, 1913; Paál, 1919; Frits Went, 1926).

• Major sites of auxin synthesis are meristematic regions in actively growing organs (coleoptile apices, root tips, apical buds, germinating seeds). Also in young, rapidly growing leaves and developing inflorescences. Embryo following pollination.

• Auxin always moves down the stem parenchyma cells towards roots (polar transport).

• Auxin enters cells as IAAH passively, or enters as IAA via active cotransport.

• Auxin moves through phloem sieve tubes.

• Auxin promotes elongation and cell enlargement by increasing H+ concentration in cell walls, affecting expansins and cellulose cross-linkages for wall expansion.

• Auxin is involved in tropic responses. Auxins migrate away from light, affecting uneven elongation on shaded sides.

• Auxin stimulates production of secondary xylem to differentiate wound tissue. It stimulates cambium to divide after injury, leading to secondary growth.

• Auxin produced in apical buds inhibits lower bud activation (apical dominance). Cytokinins counter this effect and promote lateral bud development.

Gibberellins

• Discovered due to the 'foolish seedling' disease caused by Gibberella fujikuroi.
• Synthesized in roots, shoot tips, and younger leaves but have highest concentration in seeds.
• An extensive family of molecules (over 125 now known), based on the 20-carbon ent-gibberellane structure.
• Induce dramatic internode elongation in plants (dwarf and rosette species).
• Related biosynthetically to carotenes and other isoprene derivatives.
• Gibberellins work with auxins to promote rapid elongation and division of stem tissue.
• Gibberellins determine microtubule alignment in the preprophase band. This determines the plane of cellulose expansion.
• Effects of gibberellins include bolting of biennials, and reversal of genetic dwarfism
• Commercial applications like fruit production (increasing stalk length of seedless grapes), malting of barley, and increasing sugarcane yields.
• Uses in plant breeding include reduced time to seed production, and promotion of male flowers. Also used to stimulate bolting in biennial rosette crops, such as beets and cabbages.
• Gibberellin biosynthesis inhibitors (ancymidol, paclobutrazol) used to prevent elongation, reduce stem length, and control lodging in roadside plants.

Cytokinins

• A group of phenylurea derivatives of adenine.
• First chemically isolated in 1913.
• Studied using coconut endosperm since the 1940s.
• A potent growth promoter, used in tissue culture and embryo development studies.
• Kinetin (N6-furfuryl adenine) was the first cytokinin molecule discovered.
• Zeatin is the most abundant natural cytokinin.
• Synthesized by condensation of an isopentenyl group with an adenosine monophosphate (AMP) amino group.
• Also form conjugates with sugars and metabolized by oxidation.
• Found in actively dividing tissues of seeds, fruits, leaves, and root tips. Root tips likely produce cytokinins, which are transported through xylem to other parts of the plant.
• Important for cell division in shoots and roots.
• Regulate specific components of the cell cycle.
• Crucial role to promote lateral bud development
• Zeatin level peaks at the end of S phase, mitosis, and G₁ phase in synchronized tobacco cells.
• Auxin:cytokinin ratio regulates morphogenesis in cultured tissues.
• Modifies apical dominance and promotes lateral bud growth. Also induces bud formation in mosses.
• Related to genetic tumors in plants (Overproduction of protonemal filaments in moss).
• Delays leaf senescence & stimulates chlorophyll degration
• Promotes movement of nutrients from one part of the plant to another part (cytokinin-induced nutrient mobilization).
• Promotes chloroplast development, cell expansion in leaves and cotyledons, and regulates stem and root growth.

Ethylene

• Ethylene is a gaseous hydrocarbon, produced in large amounts by tissues undergoing senescence or ripening.
• Synthesized from the amino acid methionine via S-adenosylmethionine (SAM) and 1-aminocyclopropane-1-carboxylic acid (ACC).
• Ethylene biosynthesis controlled by ACC synthase (rate-limiting enzyme). This enzyme is regulated transcriptionally.
• Released into the atmosphere or deactivated by oxidation.
• Formed in all plant organs (roots, stems, leaves, bulbs, tubers, fruits, seeds).
• Found in the tissues undergoing senescence or ripening. The production rate varies based on the plant. Production rate varies from tissue to tissue within the organ.
• Promotes ripening of some fruits, induces lateral cell expansion, and breaks seed and bud dormancy in some species
• Promotes the growth of submerged aquatic plants.
• Also promotes the formation of roots, root hairs and formation of adventitious roots. It leads flowering for the families pineapple and mango.
• Leads changes in sex in developing flowers
• Enhances leaf senescence rate
• Promotes fruit development, particularly in ovaries in the absence of viable seeds. Important in fruit ripening, like apples, tomatoes, and citrus fruits.
• Ethephon (2-chloroethylphosphonic acid or Ethrel) is sprayed for fruit ripening or degreening of citrus.

Abscisic Acid (ABA)

• ABA is a 15-carbon terpenoid. Produced from carotenoid violaxanthin through cleavage.
• Synthesized in almost all cells containing chloroplasts or amyloplasts, with significant amounts in root tips.
• Detected in all major plant organs.
• Degraded by oxidation to phaseic acid and subsequently reduction to dihydrophaseic acid.
• Plays major rolls in seed and bud dormancy, as well as response to water stress
• Promotes seed dormancy and its activities. Active when seeds mature, lowered metabolism and synthesis of proteins needed during seed dehydration.
• Seeds germinate when ABA levels are degraded by some environmental action
• Promotes winter bud scale formation, to prepare for winter dormancy, and used in commercial nurseries to keep materials in dormant conditions.
• ABA is used commercially and can be reversed with gibberellins.
• Promotes stomata closure during leaf water deficit conditions by activating K+ ion transport in guard cells.
• Involved in signal transduction pathways with Calcium secondary messengers. Originates in roots, detects low water level, then travels up to the leaves, to activate stomata closure.

Brassinosteroids

• Triterpene derivatives similar to animal steroid hormones.
• Brassinolide is the most biologically active brassinosteroid.
• Synthesized from sterol campesterol.
• Deactivated by epimerization or esterification with fatty acids.
• Found in pollen, immature seeds, shoots, leaves, promotes shoot elongation and ethylene production.
• Inhibits root growth and development.

Other Plant Growth Regulators

• Salicylic acid: Activates defense genes against pathogens (hypersensitive response).
• Oligosaccharins: Short chain sugars in cell walls that are involved in plant defense and regulate growth, differentiation, and flower development. Signaling molecules.
• Systemin: Small peptide, involved as signal molecule in wound tissue and stimulates defense activities.
• Jasmonates: Fatty acid derivatives play a role in seed germination, root growth, protein storage, and response to wounds. Involved in signal transduction with secondary metabolites (protease inhibitors), which can poison predators.