Characterization of salicylic acid-mediated modulation of the drought stress responses: Reactive oxygen species, proline, and redox state in Brassica napus

The goal of this study was to characterize hormonal regulation of drought stress-induced responses and salicylic acid (SA)-mediated stress responses, focusing on redox control. The responses of reactive oxygen species (ROS) and proline metabolism, antioxidant activity, and redox status to SA pretreatment and/or drought imposition were interpreted as being linked to those of endogenous hormonal levels and their signaling genes, especially in redox control processes. Drought enhanced the endogenous levels of abscisic acid (ABA) and jasmonic acid (JA) resulting in an increase of ABA/SA and (ABA + JA)/SA. The changes in endogenous hormonal balance were concomitant with an accumulation of ROS and proline, accompanied by a loss of reducing potential [NAD(P)H/NAD(P)(+) and GSH/GSSG]. SA pretreatment scavenged drought-induced O-2(center dot-) accumulation (but not H2O2) and led to an additional proline accumulation with enhanced expression of proline synthesis-related genes (P5CS1, P5CS2 and P5CR) and NADPH oxidase, and a reset of reducing potential with enhanced expression of redox regulating genes (TRXh5 and GRXC9). SA-mediated stress responses coincided with the enhanced expression of NPR1 and PR-1, with an antagonistic depression of ABA- and JA-related genes (NCED3, MYC2, and PDF1.2). These results indicate that the SA-modulated NPR1-dependent signalling pathway and proline synthesis are an integrative process of redox control under drought.