Plant hormone Study Guide

📖 Core Concepts Plant hormones: tiny signaling molecules made by any plant cell; act at very low concentrations to control growth, development, stress responses, and reproduction. Signal flow: synthesis → storage/conjugation → transport (cytoplasmic streaming, diffusion, phloem, xylem) → perception by specific receptors → transcriptional changes. Hormone classes: Abscisic Acid (ABA) – growth inhibitor, drought‑stress signal, induces dormancy & stomatal closure. Auxins (IAA, synthetic 2,4‑D, NAA, IBA) – cell elongation, apical dominance, adventitious rooting. Cytokinins – cell division, shoot formation, delay senescence; root‑derived, xylem‑transported. Gibberellins (GAs) – seed germination, stem elongation, flowering. Ethylene (C₂H₄) – gaseous hormone; fruit ripening, triple response, flood‑induced elongation. Brassinosteroids (BRs) – steroid hormones; promote elongation, vascular differentiation, stress tolerance. Jasmonates (JA, MeJA, JA‑Ile) – defense against herbivores/pathogens, also affect growth. Salicylic Acid (SA) – systemic acquired resistance, PR‑gene activation. Strigolactones (SLs) – suppress shoot branching, promote mycorrhizal symbiosis. Peptide hormones, Polyamines, Nitric Oxide (NO) – fine‑tune development and stress responses. 📌 Must Remember ABA : GA ratio dictates seed dormancy vs. germination (high ABA = dormancy; high GA = germination). Auxin transport is polar; moves basipetally through phloem, establishing apical dominance. Cytokinin ↔ auxin balance controls shoot vs. root development (CK promotes shoots, auxin promotes roots). Ethylene synthesis: methionine → ACC → ethylene (via ACC oxidase). BR perception: plasma‑membrane receptor BRI1 → phosphorylation cascade (BAK1, BIN2). JA signaling: COI1 receptor → degradation of JAZ repressors → defense gene activation. SA signaling: NPR1 moves to nucleus upon SA accumulation → activates PR genes. Strigolactone effect: inhibits axillary bud outgrowth; secreted by roots to attract mycorrhizal fungi. Key transport routes: auxin & cytokinins → phloem; GAs & ABA → xylem. 🔄 Key Processes ABA‑induced stomatal closure Drought → ↑ ABA in guard cells → ROS & NO production → K⁺/Na⁺ efflux → loss of turgor → stomatal pore closes. Auxin perception (TIR1/AFB pathway) Auxin binds TIR1/AFB → SCF^TIR1 complex ubiquitylates Aux/IAA repressors → proteasomal degradation → ARF transcription factors activate elongation genes. BR signaling cascade BR binds BRI1 → BRI1‑BAK1 heterodimer phosphorylates → inactivates BIN2 kinase → de‑repression of BES1/BZR1 transcription factors → growth‑related gene expression. JA activation Wounding → octadecanoid pathway → JA‑Ile formation → JA‑Ile binds COI1 → JAZ proteins degraded → MYC transcription factors induce defense genes. SA‑mediated systemic acquired resistance Pathogen attack → SA synthesis (PAL or isochorismate) → NPR1 monomerizes, enters nucleus → interacts with TGACG‑binding factors → PR gene transcription. 🔍 Key Comparisons ABA vs. GA – ABA → growth inhibition, dormancy, stomatal closure; GA → growth promotion, germination, stem elongation. Auxin vs. Cytokinin – Auxin → root initiation, apical dominance; Cytokinin → shoot initiation, cell division, delays senescence. Ethylene vs. GA in flooding – Ethylene ↑ → promotes rapid stem elongation; GA ↑ → supports elongation but can be suppressed by ABA. Jasmonate vs. Salicylic Acid – JA → defense against chewing insects & necrotrophs (often antagonistic to SA); SA → defense against biotrophs, systemic resistance. Strigolactone vs. Cytokinin – SL ↓ shoot branching; CK ↑ shoot branching and bud outgrowth. ⚠️ Common Misunderstandings “All hormones are produced in roots.” – Only cytokinins are root‑synthesized; others (auxin, ABA, GA, BR) are made in shoots, meristems, or chloroplasts. “Ethylene is a solid hormone.” – It is a gaseous molecule (C₂H₄) that diffuses out of cells; its low water solubility drives rapid movement. “Higher hormone concentration always means stronger effect.” – Hormone action is stage‑specific; excess can cause inhibition (e.g., too much auxin → root growth arrest). “BRs are the same as steroids in animals.” – They are the only steroid‑based plant hormones but act via distinct receptor kinases, not nuclear steroid receptors. 🧠 Mental Models / Intuition “See‑saw balance” – Visualize auxin ↔ cytokinin as a teeter‑totter controlling root vs. shoot growth. “Lock‑and‑key degradation” – Hormone perception often works by hormone‑bound receptor acting as a “key” that unlocks degradation of repressors (Aux/IAA, JAZ, etc.), releasing transcription factors. “Ratio rule for dormancy” – Remember ABA / GA > 1 = dormancy; ABA / GA < 1 = germination. 🚩 Exceptions & Edge Cases ABA under submergence – While ABA generally promotes drought tolerance, it can interact with ethylene to modulate flood responses, sometimes decreasing its typical stomatal‑closing effect. Cytokinin in stress – High cytokinin can delay leaf senescence, but under severe nutrient deficiency, cytokinin levels drop to favor resource reallocation. Strigolactone exudation – In phosphate‑deficient soils, SL production spikes to enhance mycorrhizal colonization, an adaptive edge case. 📍 When to Use Which Identify dormancy vs. germination problem → examine ABA : GA ratio; manipulate GA (apply GA₃) or reduce ABA. Rooting cuttings → apply synthetic auxins (NAA, IBA). Promote shoot proliferation in tissue culture → use cytokinin‑rich medium (e.g., BAP) with low auxin. Increase stress tolerance → apply brassinosteroid formulations or ABA (drought) / ethylene inhibitors (flooding). Control unwanted branching → treat with strigolactone analogs or reduce cytokinin supply. 👀 Patterns to Recognize High ABA + low GA → seed dormancy, closed stomata, reduced growth. Elevated ethylene + auxin → inhibited root elongation, enhanced “triple response”. JA ↑ + SA ↓ → defense against chewing insects/necrotrophs; opposite pattern signals biotrophic attack. BR signaling activation → concurrent up‑regulation of cell‑wall loosening genes and vascular differentiation markers. 🗂️ Exam Traps “Ethylene is a peptide hormone.” – Wrong; it is a gaseous C₂H₄ molecule. “All hormones travel through phloem.” – Only auxin and cytokinins use phloem; GAs and ABA primarily use xylem. “Higher auxin always promotes root growth.” – Excess auxin can suppress lateral root emergence; optimal concentrations required. “Strigolactones only affect mycorrhizae.” – They also suppress shoot branching; answer choices omitting this are distractors. “ABA only functions in drought.” – It also participates in seed dormancy, stomatal closure via ROS/NO, and interacts with ethylene in flood responses.