Plant Hormones Quiz

Plant Growth Regulators

• Plant hormones are chemicals produced by plants that alter growth patterns and maintenance.
• Plant hormones are concentrated in meristems and buds (dormant shoot meristems).
• They control cell activities by sending chemical signals or messages to cells.
• They can inhibit or promote cellular activities, and regulate cell division, elongation, and differentiation.
• Most hormones have multiple effects in plants.
• Plant hormones work in very small concentrations.
• Hormones often work together and have overlapping effects.
• The effectiveness of a hormone depends on maintaining a closely regulated pool size, accomplished by a balance of biosynthesis, storage as inactive conjugates, and catabolic degradation of the molecule.
• Six recognized groups of plant hormones are Auxin, Gibberellins, Cytokinins, Abscisic acid, Ethylene, and Brassinosteroids.

Auxin

• Auxin is the first plant hormone to be discovered.
• It's produced in meristematic regions and actively growing organs, such as coleoptile apices, root tips, and apical buds of growing stems and germinating seeds.
• It is also found in young rapidly growing leaves and developing inflorescence.
• Auxin always moves down the stem parenchyma cells towards the roots.
• It becomes negatively charged using proton pumps.
• Auxin enters cells as IAAH passively or as IAA via active cotransport.
• It moves through phloem sieve tubes to root tissue.
• It's a crucial component in mediating cell elongation.
• It also plays a vital role in tropic responses, where it migrates away from light.
• It stimulates the production of secondary growth by stimulating cambium cells to divide and secondary xylem to differentiate.
• Wound tissue repair is initiated by auxin when portions of vascular bundles are damaged.
• Auxin produced in apical buds inhibits the activation of buds lower on the stems, known as apical dominance. This effect lessens with distance from the shoot tip.

Gibberellins

• Gibberellins were discovered when someone determined a fungus caused abnormal rice seedling growth, called "foolish seedling" disease.
• This fungus secreted a chemical that caused the rice plants to grew abnormally tall and then collapse
• The fungus is Gibberella fujikuroi, from which the hormone name is derived.
• Gibberellins are produced in roots, shoot tips, and younger leaves but have the highest concentration in seeds.
• They induce dramatic internode elongation.
• Gibberellins work with auxins to promote rapid elongation and division of stem tissue.
• Gibberellins influence floral initiation and sex determination.
• Gibberellins promote fruit set and seed germination.
• Gibberellins are involved in bolting of biennials and reversal of genetic dwarfism.
• Gibberellins have many commercial applications for fruit production, improving the malting of barley, increasing sugarcane yields, and other uses in plant breeding.

Cytokinins

• Cytokinins are a group of phenyl urea derivatives of adenine.
• They were first chemically isolated in 1913.
• Cytokinins were studied using coconut endosperm in the 1940s.
• Kinetin is the first molecule discovered with cytokinin activity, although it's a synthetic cytokinin originally isolated from autoclaved DNA.
• Zeatin is the most abundant natural cytokinin.
• Cytokinins are synthesized by condensation of an isopentenyl group with the amino group of adenosine monophosphate (AMP).
• Cytokinins are found in actively dividing tissues of seeds, fruits, leaves, root tips, and wound tissue sites.
• Studies indicate that root tips are the likely location for cytokinin production and that cytokinins are transported through xylem to the rest of the plant.
• Localized cytokinins are needed to release buds from dormancy.
• Cytokinins regulate cell division in shoots and roots.
• Zeatin levels peak in synchronized culture tobacco cells at the end of S phase, mitosis, and G1 phase.
• The auxin:cytokinin ratio regulates morphogenesis in cultured tissues.
• Cytokinins modify apical dominance, promote lateral bud growth, induce bud formation in mosses, delay leaf senescence, promote nutrient movement into leaves, promote chloroplast development, and promote cell expansion in leaves and cotyledons.

Ethylene

• Ethylene is a gaseous hydrocarbon with significant effects on root and shoot development.
• It's synthesized from the amino acid methionine.
• Ethylene biosynthesis is controlled by transcriptional regulation and the rate-limiting enzyme, ACC synthase.
• It readily releases ethylene slowly by a chemical reaction.
• It speeds up fruit ripening in apples and tomatoes, degreens citrus, and synchronizes flowering and fruit set in pineapples.
• Ethylene is produced in large amounts by tissues undergoing senescence or ripening.
• Ethylene is formed in all plant organs (roots, stems, leaves, bulbs, tubers, fruits, seeds).
• The rate of production varies from tissue to tissue and with the stage of development of an organ.
• Ethylene promotes the ripening of some fruits, induces lateral cell expansion, breaks seed and bud dormancy in some species (cereals, potatoes), and promotes elongation growth of submerged aquatic species.
• It also induces root and root hair formation, induces flowering, may change the sex of developing flowers, and enhances the rate of leaf senescence.
• Ethephon, a commercial derivative of ethylene, hastens fruit ripening.

Abscisic Acid (ABA)

• Abscisic acid (ABA) plays major roles in seed and bud dormancy and responses to water stress.
• It's a 15-carbon terpenoid compound derived from the terminal portion of carotenoids.
• ABA is synthesized by cleavage from a 40-carbon xanthophyll violaxanthin.
• ABA is degraded by oxidation to phaseic acid and subsequent reduction to dihydrophaseic acid.
• ABA is detected in every major organ or living tissue from the root cap to the apical bud.
• ABA is synthesized in almost all cells that contain chloroplasts or amyloplasts.
• ABA promotes seed dormancy activities.
• ABA levels are high when seeds mature, promoting lowered metabolism and synthesis of proteins needed to withstand dehydration.
• Seeds germinate when ABA is degraded by environmental action.
• ABA promotes winter bud scale formation on woody plants in preparation for winter dormancy.
• ABA promotes stomata closure during leaf water deficit conditions by activating K+ ion transport out guard cells, involving signal transduction pathways with calcium secondary messengers.
• In roots, ABA detects low water level in root tissues and activates stomatal closure.
• ABA is produced in roots in response to low soil water potential and other stress situations.

Brassinosteroids

• Brassinosteroids are triterpene derivatives with a chemical structure similar to animal steroid hormones.
• Brassinolide is the most biologically active brassinosteroid.
• Brassinosteroids are synthesized from the sterol campesterol.
• They are deactivated by epimerization of the α-hydroxyl groups on the A ring and subsequent esterification with fatty acids or by glucosylation.
• Brassinosteroids are produced in pollen, immature seeds, shoots, and leaves.
• They promote shoot elongation, ethylene production, and inhibit root growth and development.

Others

• Salicylic acid (phenolic) activates defense genes for resistance against pathogen invaders (hypersensitive response).
• Oligosaccharins (short-chain sugars in cell walls) play roles in defense against pathogens; help regulate growth, differentiation, and flower development; and are signaling molecules.
• Systemin (small peptide in wound tissue) stimulates defense activities, acting as a signal molecule that activates signal transduction pathways including jasmonates.
• Jasmonates (fatty acid derivatives) play roles in seed germination, root growth, and protein storage in response to signal molecules produced in wound areas. They lead to the production of secondary metabolites.