Abstract
Acclimation to salt stress in plants is regulated by complex signaling pathways involving endogenous phytohormones. The signaling role of salicylic acid (SA) in regulating crosstalk between endogenous plant growth regulators’ levels was investigated in barley (Hordeum vulgare L. ‘Ince’; 2n = 14) leaves and roots under salt stress. Salinity (150 and 300 mM NaCl) markedly reduced leaf relative water content (RWC), growth parameters, and leaf water potential (LWP), but increased proline levels in both vegetative organs. Exogenous SA treatment did not significantly affect salt-induced negative effects on RWC, LWP, and growth parameters but increased the leaf proline content of plants under 150 mM salt stress by 23.1%, suggesting that SA enhances the accumulation of proline, which acts as a compatible solute that helps preserve the leaf’s water status under salt stress. Changes in endogenous phytohormone levels were also investigated to identify agents that may be involved in responses to increased salinity and exogenous SA. Salt stress strongly affected endogenous cytokinin (CK) levels in both vegetative organs, increasing the concentrations of CK free bases, ribosides, and nucleotides. Indole-3-acetic acid (IAA, auxin) levels were largely unaffected by salinity alone, especially in barley leaves, but SA strongly increased IAA levels in leaves at high salt concentration and suppressed salinity-induced reductions in IAA levels in roots. Salt stress also significantly increased abscisic acid (ABA) and ethylene levels; the magnitude of this increase was reduced by treatment with exogenous SA. Both salinity and SA treatment reduced jasmonic acid (JA) levels at 300 mM NaCl but had little effect at 150 mM NaCl, especially in leaves. These results indicate that under high salinity, SA has antagonistic effects on levels of ABA, JA, ethylene, and most CKs, as well as basic morphological and physiological parameters, but has a synergistic effect on IAA, which was well exhibited by principal component analysis (PCA).
Highlights
Abiotic stresses including soil salinity, high temperatures, and a lack of fresh water present severe agricultural challenges
The analysis indicated that the leaf relative water content and leaf water potential (LRWC and LWP; r = 0.968, p < 0.05, Table 3), the lengths of the roots and shoots (RL and SL), and the fresh and dry weights of the roots and shoots (RFW, SFW, RDW, and SDW; range; r = 0.844 to 973, avg. 0.919) were positively and significantly correlated (p < 0.01 or 0.05)
After 16 days under these conditions, pots were randomly divided into six experimental groups: C: untreated control plants, 150: plants treated with 150 mM NaCl to induce moderate salt stress, 300: plants treated with 300 mM NaCl to induce high salt stress, salicylic acid (SA): plants treated with 0.5 mM SA for 24 h (0 mM NaCl), SA150: plants preincubated for 24 h with 0.5 mM SA cultivated for 4 days in 150 mM NaCl, SA300: 0.5 mM SA pre-treatment for 24 h followed by growth in 300 mM NaCl for 4 days
Summary
Abiotic stresses including soil salinity, high temperatures, and a lack of fresh water present severe agricultural challenges. The world’s population is growing steadily, roughly 793 million people receive insufficient nourishment to support an active and healthy lifestyle [1]. To overcome these problems, agriculture and food production systems must adapt to the adverse effects of climate change and become more resilient, productive, and sustainable. Phytohormones are natural plant growth regulators (PGRs) that act as signaling molecules and are present in plants at very low concentrations. They are key regulators of complex root to shoot interactions that control plant growth and development. There is extensive cross-talk between phytohormone signaling pathways, so PGRs of all kinds can have additive, synergistic, or antagonistic effects on metabolic and signaling pathways [2], and can play significant roles in plants’ responses to biotic and abiotic stresses [3]
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