Salicylic acid and formation of plant adaptive responses to abiotic stressors: role of signaling network components
Salicylic acid and formation of plant adaptive responses to abiotic stressors: role of signaling network components Salicylic acid (SA) is one of the key phytohormones. A lot of signaling pathways are activated under its influence. SA takes part in regulation of physiological processes such as seed germination, flowering, synthesis of other hormones, photosynthesis, respiration, transpiration, thermogenesis, responses to infection with pathogens, as well as in adaptation to action of various stressors. Numerous data show an increase in endogenous SA content in plants at the early stages of stress response. Under SA influence, there is antioxidant system activation, as well as synthesis of stress proteins (heat shock proteins, dehydrins, PR- (pathogenesis related) proteins), and accumulation of multifunctional low-molecular-weight protective compounds (proline, glycine betaine, sugars, secondary metabolites). Also, SA has an ability to induce stomatal closure in plants. We discussed the role of NPR1 protein, which is considered a key transcriptional regulator of SA signaling. We noted that the interaction of SA with this protein occurs in at least two ways: through direct binding and through SA-induced changes in cell redox homeostasis, leading to the NPR1 transition from an oligomer state to a monomer form. For NPR1 monomerization in the cytosol, there must be a sufficient pool of reducing agents, but their nature is not fully understood. At a low SA concentration in cells, NPR1 protein forms an oligomer and remains in the cytosol (See Fig. 1). Wherein proteins NPR3 and NPR4 bind residual NPR1 in the nucleus and limit its functional activity. With an increase in the SA concentration, NPR1 transforms into a monomer state and penetrates into the nucleus. In addition to the redox-dependent monomerization of NPR1, the increased SA concentration blocks the activity of NPR3 and NPR4, which in the absence of SA act as repressors of transcription regulated by the TGACG-binding factor in SA-sensitive promoters. Thus, blocking NPR3 and NPR4 leads to the activation of the expression of SA-induced genes. However, the list of SA-binding proteins is not limited to NPR. Data on the participation in SA binding of methylesterase, carbonic anhydrase, several isoenzymes of glutathione S-transferase, chloroplast thioredoxin-ra1, some enzymes of the Krebs cycle, and a number of other proteins, including those with unknown functions, have been obtained. In this regard, the hypothesis of multinodular (decentralized) SA perception by a plant cell is discussed. In addition to specific proteins, calcium ions, reactive oxygen species (ROS), nitric oxide and hydrogen sulfide are involved in SA signaling (See Fig. 2). However, the relationships between most of these mediators during SA signal transduction have not been sufficiently studied. We found out that calcium ions in signaling chains are located both below the SA (involved in the transduction of its signals) and above (involved in the induction of SA synthesis). We considered the participation of cationic channels of the CNGC type (cyclic nucleotide-gated calcium channels) in the activation of SA synthesis in plant cells under an action of stressors. The entry of calcium through them into the cytosol leads to the activation of calmodulin, and then protein kinases dependent on calmodulin and calcium. These calcium sensor proteins modulate the activity of transcription factors CBP60g and CAMTA3/SR1, which are positive regulators of the expression of isochorismate synthase gene and other genes that provide the process of SA synthesis. On the other hand, treatment of plants with exogenous SA or an increase in its content in cells due to stress-induced enhancement of synthesis leads to an increase in the concentration of cytosolic calcium, which is necessary for transduction of the SA signal into the genetic apparatus and the realization of its physiological effects. SA can also be involved in the implementation of action of signaling mediators such as ROS and nitric oxide. In this case, however, not all of their physiological effects are realized with the participation of SA. In particular, our work provides examples of the successful induction of salt tolerance of salicylate-deficient transformants of Arabidopsis NahG by the action of hydrogen peroxide and the nitric oxide donor sodium nitroprusside. On the other hand, ROS, nitric oxide and hydrogen sulfide can act as intermediaries in the implementation of the SA action. This review analyzes new information on the effect of SA on functioning of enzymatic systems that generate ROS (NADPH oxidase, extracellular peroxidase), hydrogen sulfide (L- and D-cysteine desulfhydrases) and that synthesize nitric oxide by the reductive and oxidative pathways. We considered the effects of enhancing stress-protective effect of SA on plants when combined with donors of signaling molecules-gasotransmitters. We noted that the low cost and environmental safety of SA as a natural compound should contribute to the expansion of the scope of its practical application, primarily as a growth regulator of stress-protective action. The paper contains 2 Figures and 94 References. The Authors declare no conflict of interest.