TABLE 1.
Summary of major plant hormones: biological effects, metabolic precursors, and receptor responses
ET, ethylene; GA, gibberellin; IPP, isopentenyl pyrophosphate; ARR, Arabidopsis response regulator.
| Hormone | Active form | Effects | Precursor | Receptor |
|---|---|---|---|---|
| Auxin | IAA | Cell elongation; apical dominance; tropisms; branching; lateral roots | Tryptophan | AUX/IAA: repressor proteins; TIR1/AFBs: auxin F-box-binding proteins; auxin facilitates binding of AUX/IAA with SCFTIR1/AFB, resulting in degradation of AUX/IAA repressors and altered transcription of auxin-induced genes |
| ET | Ethylene | Flowering/fruit ripening; stress response; seed germination | Methionine | ETR1/ETR2/ERS1/ERS2/EIN4: histidine kinases (dimers); ET represses repressors of ET response, leading to activation of EIN3 and EREBP families of transcription factors |
| JA | JA-Ile | Plant defenses against insect herbivores; root growth inhibition; necrotrophic pathogen responses | α-Linolenic acid | COI1/JAZ co-receptor: COI1 F-box protein; component of the SCFCOI1 E3 ligase complex; targets JAZ proteins for degradation and leads to altered gene expression |
| Cytokinins | trans-Zeatin | Cell division; releases lateral buds from apical dominance; delays senescence; root growth | Adenine | CRE1: similar to histidine kinases; activates histidine kinase activity, initiating a phosphorelay that results in phosphorylation of type B ARRs to induce transcription of type A ARRs |
| Benzoates | SA | Systemic acquired resistance to pathogens; biotrophic pathogen responses | Chorismate | NPR1: transcriptional coactivator; NPR3/ NPR4: CUL3 E3 ligase adaptor proteins |
| ABA | ABA | Stomatal closure; seed maturation, germination, storage, desiccation tolerance; root/shoot growth; leaf senescence | IPP-derived tetraterpene (phytoene) | PYR/PYL/RCAR: START family of ligand-binding proteins; CHLH: cheletase; GTG1/2: G-proteins |
| GAs | GA1, GA4 | Stem elongation; root growth; seed germination; flowering; floral development; fruit growth; stresses | IPP-derived diterpene (geranylgeranyl diphosphate) | GID1: globular protein; GA-GID1 binds DELLA repressors, leading to targeting to the SCF complex and subsequent degradation by the 26 S proteasome |
| Brassinosteroids | Brassinolide | Cell division/elongation in stems/roots; photomorphogenesis; reproductive development; leaf senescence; stress responses | IPP-derived sesquiterpene (farnesyl diphosphate) | BRI1: dual-specificity kinase; dimerizes with BAK1, initiating phosphorelay prevention of phosphorylation of BES1/BZR1 to alter gene expression |
| Strigolactones | Strigol | Branching; leaf senescence; root development; plant-microbe interactions | Carotenoids | D14/DAD2: α/β-hydrolase; putative receptor/co-receptor; initiates SCF-mediated signaling via SCFMAX2 complex |