Plant Hormones and Growth Regulators Quiz

Plant Growth Regulators

• Plant hormones are chemicals produced by plants that modify growth and maintenance.
• These hormones are found in various plant cells and tissues, but are most concentrated in growing areas and buds.
• Hormones control cellular activities by sending chemical signals.
• Plant hormones regulate cell division, elongation, and differentiation.
• Plant hormones frequently interact with each other, and their impact overlaps. The effectiveness of a hormone depends on maintaining a stable amount.
• This stability is achieved through a complex interplay of biosynthesis, storage as inactive conjugates, and molecule degradation.
• Six primary plant hormone groups are recognized: auxins, gibberellins, cytokinins, abscisic acid, ethylene, and brassinosteroids.

Classification of PGR

• Growth-promoting hormones increase plant growth (e.g., auxins, gibberellins, cytokinins).
• Growth-inhibiting hormones reduce plant growth (e.g., abscisic acid, ethylene).

Hormone Action

• Apical meristem: Hormones like auxin, cytokinins, and gibberellins impact cell division and elongation. Abscisic acid influences dormancy.
• Flower: Auxins and gibberellins are involved in flower initiation and differentiation.
• Fruit: Auxin and cytokinins affect fruit growth and ripening; ethylene affects ripening.
• Leaf: Gibberellins promote growth, while ethylene contributes to leaf senescence.
• Lateral bud: Auxin inhibits lateral bud growth, but cytokinins encourage it.
• Stem: Auxin and gibberellins promote stem elongation.
• Root: Auxin promotes root initiation and growth, but inhibits growth in some cases. Gibberellins also affect root development.

Auxin

• Auxin was the first plant hormone discovered.
• Key experiments on auxin were conducted by Darwin (1880), Peter Boysen-Jensen (1913), Paál (1919), and Frits Went (1926).
• Coleoptiles are the pointed protective sheaths, present in the emerging shoots of monocots.
• Auxin is synthesized in meristematic regions, apical buds, young leaves, and the embryo after pollination.
• This hormone moves down the stem primarily through the parenchyma cells by polar transport.
• Auxin enters cells as IAAH (passively) or IAA- (active cotransport).
• Transport proteins are present at a cell's basal end, assisting in moving auxin through the plasma membrane to adjacent cells.
• Auxin plays a role in cell elongation by triggering the expansion of cell walls through proton pumps. This increases the concentration of H+ which activates proteins called expansins.
• Expansins disrupt hydrogen bonds, breaking down cellulose bridges in cell walls which leads to expansion when water is absorbed.
• Auxin is important in tropic responses (plant growth response to stimuli), particularly in the uneven elongation of cells on the shaded side of light-exposed plants.
• Auxin triggers production of secondary xylem by stimulating cambium cell division.
• This hormone plays a key role in wound tissue repair.
• Apical dominance is the effect of auxins produced in apical buds inhibiting the growth of lower buds on the stem.
• Auxin promotes lateral bud formation, but this effect diminishes with distance from the plant's shoot tip.

Gibberellins

• Gibberellins were discovered by E. Kurosawa when he observed "foolish seedling disease", caused by the fungus Gibberella fujikuroi.
• Gibberellins are produced in roots, shoot tips, and young leaves, and concentrated in seeds.
• Gibberellins are a family of molecules (more than 125 known) based on a 20-carbon ent-gibberellane structure.
• Gibberellins promote rapid stem elongation and cell division, coordinating with auxins.
• Gibberellins align microtubules in preprophase bands, influencing cellulose expansion in cells.
• Effects of gibberellins include bolting (growth of stems that result in producing flowers). This occurs in biannual plants during the first growing season.
• Gibberellins reverse genetic dwarfism.
• This hormone regulates juvenile-to-adult transitions, floral initiation, and sex determination. Gibberellins also promote fruit set and seed germination.
• Gibberellins are used in commercial applications like fruit production (longer stalks for seedless grapes), barley malting, and sugarcane yield enhancement.
• Gibberellin biosynthesis inhibitors (e.g., ancymidol, paclobutrazol) are commercially used to stop plant growth, resulting in shorter, more compact plants.

Cytokinins

• Cytokinins are synthesized by a condensation of an isopentenyl group with the amino group of adenosine monophosphate.
• Cytokinins are found in actively dividing plant tissues, including seeds, fruits, leaves, root tips, and wound sites.
• Root tips appear to be the core locations for cytokinin production; these hormones are transported throughout the plant via the xylem.
• Localized cytokinins at lower concentrations are needed to release buds out of dormancy.
• Cytokinins regulate cell division in shoots and roots, affecting cell cycle components.
• High zeatin levels are observed in synchronized tobacco cells at the end of the S phase, during mitosis, and in the G₁ phase.
• The auxin/cytokinin ratio influences morphogenesis in plants.

Ethylene

• Ethylene is a gaseous hydrocarbon with effects on root and shoot development.
• It's synthesized from the amino acid methionine through S-adenosylmethionine to 1-aminocyclopropane-1-carboxylic acid (ACC).
• The production of ethylene is limited in some plants; sulphur is conserved and recycled in the Yang cycle.
• Ethylene production is governed by transcriptional regulation, primarily through the enzyme ACC synthase.
• Ethylene is released from the plant to the environment and in some cases it is deactivated through oxidation.
• Ethylene promotes fruit ripening, lateral cell expansion, seed/bud dormancy break.
• Ethylene promotes growth elongation in submerged aquatic species.
• Ethylene influences flowering, root and root hair development in plants.
• Applications for ethylene include hastening fruit ripening of apples and tomatoes, degreening citrus fruits, and synchronizing flowering and fruit set in pineapples.
• Ethylene accelerates abscission of flowers and fruit; and induces fruit thinning in cotton, cherry, and walnut trees.
• Ethylene promotes female expression in cucumbers thereby enhancing self-pollination and yield.

Abscisic Acid (ABA)

• ABA plays a critical role in seed and bud dormancy and responding to water stress.
• ABA is a 15-carbon terpenoid, produced from the terminal section of carotenoids and is released in the process of cleaving the 40-carbon xanthophyll violaxanthin.
• This hormone is degraded by oxidation into phaseic acid, and then further reduced to dihydrophaseic acid.
• ABA is produced in virtually every part of a plant including the root cap and apical bud, and is mostly found in chloroplasts and amyloplasts.
• ABA promotes winter bud scaling in woody plants.
• ABA derivatives called dormins are used in nurseries to maintain dormant conditions in plants.
• ABA promotes dormancy reversal with the help of gibberellins.
• ABA promotes stomata closure when there is a water deficiency in leaves and triggers the activation of potassium ion transport out of guard cells.
• Signal transduction pathways associated with calcium secondary messengers are needed.
• ABA is produced in roots by detecting low water levels in tissues. This causes ABA to upwardly transport into leaves and activates stomata closure.
• ABA helps in reducing transpiration to prevent water loss in times of scarcity.

Brassinosteroids

• Brassinosteroids are triterpene plant steroid hormones produced in pollen, immature seeds, shoots and leaves.
• The most biologically active form is brassinolide.
• Brassinosteroids are synthesized from the sterol campesterol.
• They are deactivated by epimerization of the a-hydroxyl groups or esterification with fatty acids, or glucosylation.
• Brassinosteroids promote stem elongation and ethylene production.
• Brassinosteroids inhibit root growth and development.

Other Plant Hormones (OTHERS)

• Salicylic acid (phenolic): Activates defense genes, resulting in defense against pathogens, triggered by a hypersensitive reaction.
• Oligosaccharins-Short chain sugars found in cell walls, play a vital role in defense against pathogens, and in the regulation of growth, differentiation, and flower development; acting as signaling molecules.
• Systemin: A small peptide that stimulates defense activities as a ligand (signal molecule). It stimulates defense activation by initiating the signal transduction pathway.
• Jasmonates (fatty acid derivatives): Play a part in regulating seed germination, root development, and protein storage. These hormones are produced in response to wound signals; they also regulate signal transduction pathways that result in the development of secondary metabolites like protease inhibitors or volatile compounds.