Table 1 |.
Examples of roles of phytohormones in abiotic stress responses.
| Phytohormone | Ref. |
|---|---|
| Abscisic acid (ABA) | |
| Drought stress in roots induces ABA biosynthesis in shoots via hydraulic signals, and CLE25 peptide-mediated induction of NCED3 expression. | 26,27,32 |
| Osmotic- and salt stress induce the activation of SnRK2-type protein kinases, and SnRK2 activation is mediated by Subgroup B Raf-like kinases. | 34,36,39,42,44 |
| Root hydrotropism requires ABA signaling in the cortex of the elongation zone. | 101 |
| Salt stress inhibits lateral root formation, which depends on ABA synthesis and endodermal ABA signaling. ABA inhibits lateral root formation through interfering with auxin signaling. | 104,116 |
| Salt stress, K+ and SO42− deficiency induce endodermal suberization in an ABA-dependent manner. | 207 |
| Cold stress responses are modulated via SnRK2.6/OST1 phosphorylation of the transcription factor ICE1. | 208 |
| Heat stress tolerance is reduced in mutants deficient in ABA signaling or biosynthesis. | 209 |
| Auxin (IAA) | |
| Salt stress induces halotropism, the preferential growth away from areas of high salinity, which is mediated by auxin redistribution to induce root bending. | 105,106 |
| Hydropatterning, the preferential formation of lateral roots near water, is initiated by auxin signaling, and depends on the auxin response factor ARF7. | 114,115 |
| Drought stress induces the expression of IAA5 and IAA19, two transcriptional repressors of auxin responses. Additionally, iaa mutants have reduced survival during osmotic stress. | 76 |
| Heat stress induces auxin biosynthesis via PIF4, and the stabilization of auxin co-receptors. Auxin signaling via ARFs mediates high temperature-dependent hypocotyl elongation. | 210–212 |
| Brassinosteroid (BR) | |
| Drought stress responses interfere with BR signaling via BR and ABA crosstalk at the level of BES1 and RD26 mediated transcriptional regulation. | 71 |
| Cold acclimation and freezing tolerance involve BR signaling through its effect on COR and CBF gene expression. | 213,214 |
| During Thermomorphogenesis, PIF4 induces BR biosynthesis, whereas the BR activated transcription factor BZR1 functions in a feedforward loop downstream of auxin and PIF4 to further induce PIF4 expression. | 215,216 |
| Cytokinin (CK) | |
| Drought- and salt-stress induce the reduction of CK content and signaling, leading to an increased ABA sensitivity, likely via interaction of SnRK2s with type-A and type-B ARRs. | 74,181,217 |
| The osmotic stress-dependent hydrotropic response depends on the asymmetric distribution of CK signaling in the root tip, which is enhanced at the lower water potential side. | 202 |
| Ethylene (ET) | |
| Salt stress induces the production of ET and ET signaling. ET signaling promotes salt tolerance. | 121 |
| Flooding or submergence adaptation depends on ET production and the function of group VII Ethylene Response Factors (ERFs) in Arabidopsis and other related ERF genes in rice (Sub1A and SNORKEL), likely inducing GA biosynthesis. | 128,129,135 |
| Metal deficiency reduces endodermal suberization in an ET-dependent manner. | 207 |
| Gibberellic acid (GA) | |
| Under drought stress conditions GA signaling interferes with ABA signaling via DELLA protein interactions with the ABA-regulated TF ABF2. | 218 |
| Salt stress reduces the levels of bioactive GAs, likely via ABA signaling. della-quadruple mutants are hypersensitive to salt stress. | 121 |
| Cold stress responses are mediated via DELLA accumulation and interactions with GRF-type TFs. | 219 |
| Heat stress induces GA biosynthesis and the degradation of DELLAs in a COP1-dependent manner to regulate hypocotyl elongation. | 220 |
| Water submergence triggers GA production to induce internode elongation in rice. | 135–137 |
| Jasmonic acid (JA) | |
| Cold stress induces the production of JA. JAZ degradation in response to cold releases ICE1 and ICE2 from JAZ-mediated repression. | 221 |
| Heat stress promotes the accumulation of the JA receptor COI1 to enhance downstream JA responses. | 185 |
| Strigolactone (SL) | |
| Drought- and salt stress responses are positively modulated by SL via ABA-dependent and ABA-independent pathways. | 179,222 |
Abbreviations:
CLE25, CLAVATA3/ENHANCER OF SHOOT REGENERATION-RELATED 25
NCED3, NINE-CIS-EPOXYCAROTENOID DIOXIGENASE 3
SnRK2, SUCROSE NONFERMENTING 1-RELATED PROTEIN KINASE
ARF, AUXIN RESPONSE FACTOR
ICE1, INDUCER OF CBF EXPRESSION 1
IAA5 and IAA9, INDOLE-3-ACETIC ACID INDUCIBLE 5 and 9
PIF4, PHYTOCHROME-INTERACTING FACTOR4
BRL3, BRI1-LIKE 3
BES1, BRI1-EMS-SUPPRESSOR 1
RD26, RESPONSIVE TO DESSICATION 26
COR, COLD REGULATED
CBF, C-REPEAT BINDING FACTOR
ARR, ARABIDOPSIS RESPONSE REGULATOR
ERF, ETHYLENE RESPONSE FACTOR
DELLA, plant-specific GRAS family proteins functioning as repressors of the GA signaling pathway
ABF, ABRE-BINDING FACTOR
GRF, GROWTH REGULATORY FACTOR
COP1, CONSTITUTIVE PHOTOMORPHOGENETIC 1
JAZ, JASMONATE-ZIM-DOMAIN PROTEIN
COI1, CORONATINE-INSENSITIVE 1