Plant Hormones: Regulation and Function

Plant Hormones Overview

• In multicellular plants, coordination between cells is essential.
• Hormones are the main way plants communicate, directing cell development and coordinating growth.

Historical Perspective

• Plant hormones have been studied for about 100 years, but the concept remains controversial.
• Charles Darwin's phototropism experiments marked the beginning of plant hormone research.
• Darwin described the effect of light on the movement of canary grass coleoptiles in his 1880 book "The Power of Movement in Plants."
• F.W. Went identified a hormone-like substance that causes plants to grow toward light almost 50 years after Darwin.

Modern Definition of Hormones

• Hormones are natural organic substances that affect physiological processes at low concentrations.
• According to animal physiology, hormones are synthesized in specific tissues, transported via the bloodstream to target tissues, and control physiological responses depending on concentration.

Differences between Plant and Animal Hormones

• Plant hormones, like animal hormones, influence physiological processes at low concentrations.
• The synthesis site of plant hormones isn't as localized as in animals; synthesis is more diffuse.
• "Action at a distance" is not essential for plant hormones.
• It is still debated whether plant hormones act in a concentration-dependent manner.
• Some argue plant cells respond to hormone concentration, while others suggest changing sensitivity of target cells.

Multiplicity of Plant Hormones

• Plant hormones have multiple effects.
• Each plant hormone group impacts various developmental events, and each event can be influenced by multiple hormone groups.

Major Plant Hormone Groups

• The five traditionally recognized plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
• Brassinosteroids, have recently been found to have well-defined functions in normal plant development, making it a sixth group
• Jasmonic and salicylic acids are elicitor molecules which regulate aspects of plant growth, potentially qualifying for hormone status.

Auxins

• Auxin was the first plant hormone discovered, found throughout the plant in meristematic and actively growing regions.
• High auxin concentrations are found in coleoptile apices, root tips, apical buds, and germinating seeds.
• Other regions with high auxin levels include young leaves, developing inflorescences, and post-pollination/fertilization embryos.
• Auxins stimulate cell elongation, root initiation, vascular differentiation, tropic responses, and the development of axillary buds, flowers, and fruits.
• Indole-3-acetic acid (IAA) is the primary natural auxin.
• Naturally occurring indole derivatives, like indole-3-ethanol, indole-3-acetaldehyde, and indole-3-acetonitrile, show auxin activity.
• These derivatives are precursors to IAA, with activity likely due to conversion to IAA in plant tissue.
• IAA is the most widely distributed auxin.
• IBA and 4-chloroindole-3-acetic acid are naturally occurring but less prevalent.
• 2,4-D and NAA are synthetic auxins.
• IBA, 4-chloroIAA, and PAA are structurally similar to IAA, eliciting similar responses, suggesting they are natural hormones.
• It is undetermined whether these are converted to IAA in plant tissues prior to activation.
• Molecules with auxin activity share acidic side chains on an aromatic ring.
• IAA levels depend on tissue type, age, and growth state.
• Vegetative tissue contains 1-100 mg kg-1 fresh weight of IAA (5.7-570 nanomoles).
• Seed IAA levels can reach up to 300 picomoles, supporting rapid seedling growth after germination.

Auxin Deactivation

• Oxidative decarboxylation of IAA can be catalyzed by peroxidase from plant sources.
• Enzymes from different sources and isozymes within species have different requirements.
• Decarboxylative oxidation of IAA has been extensively studied with purified HRP in vitro.
• IAA oxidation deactivates IAA effectively.
• Enzymes catalyze indole ring oxidation into oxindole or decarboxylate acetate side chain without oxidizing the indole ring.
• The major product formed by the oxindole route, 3-methylene oxindole, has not been detected in plants in vivo.
• Indole-3-carboylic acid, indole-3-aldehyde, and indole -3- methanol have been extracted from plants.
• The decarboxylated indole pathway is the most likely normal catabolic pathway in plants.

Auxin Conjugates

• Decarboxylative catabolism was once considered the main IAA degradation pathway.
• However, evidence from Z. mays, peas, and tomatoes suggests peroxidases play a minor role in endogenous IAA pools.
• IAA-amino acid conjugates have been identified as intermediates in nondecarboxylative catabolic pathways that irreversibly deactivate IAA.
• Conjugating IAA with amino acids leads to irreversible deactivation.
• The fate of the IAA-amino acid conjugate varies among species, but usually involves oxidation of the indole ring follow by glucose addition.
• IAA also exists as inactive chemical conjugates in glycoysl esters.
• Active IAA is released via enzymatic hydrolysis.
• IAA conjugates are a source of active hormone for embryos during the first days of germination.

Gibberellins

• Gibberellins belong to the terpinoid family.
• Over 125 gibberellins have been identified, with more being discovered.
• Gibberellins are defined by chemical structure and biological activity.
• All gibberellins are diterpenes based on the 20-carbon ent-gibberellane structure.
• GA3, or gibberellic acid, was one of the first to be characterized.
• GA3 is readily extracted from fungal cultures, making it commercially common and well-studied.
• C19-GAs, particularly GA1 and GA20 are the most active, therefore most important gibberellins in higher plants.
• Many gibberellins appear inactive.
• Many are intermediates for active forms or inactive metabolic products that retain the basic structure.
• Only a few gibberellins are found in any species or organ, and active forms are even less.
• GA1 is the primary gibberellin that regulates stem elongation in higher plants.
• All GAs feature a carboxyl group at C7 which is required for biological activity, with C19-GAs being more active than C20-GA molecules.
• Those GAs with 3-ẞ-hydroxylation, 3-ẞ,13-dihydroxylation are generally more active
• Those with both 3-ẞ-OH and 1,2-unsaturation demonstrate the highest activity.
• Gibberellin biosynthesis primarily occurs in developing seeds and fruits, young leaves of developing apical buds, elongating shoots, and apical root regions.
• Gibberellins are terpenes sharing a core pathway with other hormones and secondary products.
• They are synthesized from isoprene (5-carbon) and mevalonic acid (C6).

Gibberellin Biosynthesis and Catabolism

• Gibberellin synthesis happens from GGPP to GA12-7-aldehyde.
• GA12-7-aldehyde is inactive while serving as a precursor to all other gibberellins.
• In pea plants, the major GA biosynthesis pathway (bold arrows) occurs in seeds and shoots.
• The pathway shown in light arrows occurs only in shoots.
• The asterisk indicates known endogenous forms.
• Anti-gibberellins reduces stem elongation, which results in the productions of plants that are shorter and more compact in form.
• These agents block gibberellin production, and can be reversed via the application of gibberellins
• Gibberellins become deactivated from 2β-hydroxylation.

Cytokinins

• Cytokinins are N6-substituted derivatives of adenine.
• Primarily it stimulates cell division, in combination with auxin.
• Cytokinins influence shoot/root differentiation, lateral bud growth, leaf expansion, chloroplast development, and leaf senescence.
• Zeatin is the most common in plants.
• Kinetin (N6-furfuryl adenine) was the first cytokinin discovered, a synthetic cytokinin isolated from autoclaved DNA.
• Most cytokinin biosynthesis happens in the root, especially in the root tip.
• It is believed that cytokinins are transported through the xylem.
• High Cytokinins are found in developing fruits and seeds.
• The extent to which cytokinins are in the seed suggests it may just function as a sink
• Isopentenyl transferase, plants use ATP to create CK.
• CYP735A which is a CK oxidase.
• Grey arrows = genetic level.
• Direct Activation Pathway (LONLEY GUY).

Cytokinin Regulation

• Cytokinins may be reversibly, irreversibly deactivated by oxidation.
• The processes regulate removal of Cytokinins or through the addition of either glucose.
• CKX catalyses the oxidation of the secondary amine group on the side-chain on adenine
• Glucosylation is a method for retrieval but is resistant to oxidation.

Abscisic Acid (ABA)

• ABA differs, being a single compound.
• It is a 15-carbon sequiterpene derived from mevalonic acid.
• Two possible pathways exist for synthesis of ABA: direct synthesis from a 15 carbon precursor or indirect synthesis via cleavage of a 40 carbon xanthophyll.
• ABA Synthesis- In fungi, direct process, higher plants, indirect.
• In ABA higher plants ABA synthesized from Violaxanthin-Xanthoxin
• In Indirect pathway violaxanthin coverts

ABA synthesis

• It appears to be synthesized in the cytosol of leaf that are mesophyll but due pH gradient, it concentrates in the chloroplast.
• At low pH, ABA exists, that is ABAH it crosses membranes.
• Dissociated for Impermeant.
• Tends to cross into higher PH which causes dissociates.
• ABA is easy to remove.
• Is rapidly metabolized with glucose applications.

Brassinosteroids

• Brassinosteroids are steroid similar to those in animals
• They stimulate shoot elongation and ethylene production and prevent roots
• Active at Micromolar.

Key points of Brassinosteroids

• Brassinolide, isolated in 1979 was the first hormone identified.
• Over 40 versions, isolated from over 60 plant species and their tissue.
• Play a role.
• The mechanism, a-hydroxyl groups are epimerized, esterified with what acids glucolyslation.

Ethylene

• Structure- H2C=CH2
• Gaseous
• Synthesized: Primarily Stress with senescense.
• Produced to wounding, stress via ACC synthase.
• Ethylene stimulates itself during fruit ripening
• Can be oxidized to ethylene or diffused to atompshere