Characterization of salicylic acid- and abscisic acid-mediated photosynthesis, Ca2+ and H2O2 accumulation in two distinct phases of drought stress intensity in Brassica napus

Abstract

Drought modifies the endogenous hormonal status, which is involved in the regulation of stress responses and tolerance. This study aimed to characterize the drought-responsive hormone-mediated regulation of stress responses based on the time-course of drought intensity. Alterations in H2O2, Ca2+ and their signaling genes with decreasing leaf water potential (Ψw) were interpreted as being linked to the endogenous hormonal level. Drought responses in relation to SA and ABA balance represented two distinct phases. The early phase, during the first 6 days when the Ψw was greater than -0.78 MPa, was characterized by a rapid decrease in photosynthetic activity with a progressive accumulation of H2O2 and Ca2+ in the strong correlation with the increases in both SA and ABA level. The late phase, days 6–14, was distinguished by ABA-mediated stress symptomatic responses (e.g., the maintenance of the depressed photosynthesis with severe leaf wilting) with highly enhanced expression of ABA-related genes (NCED3 and MYC2) and concomitant depression in the SA level and SA-related genes (ICS1 and NPR1). The regression analysis revealed that both SA and ABA levels as being positively correlated with Ca2+ and H2O2 were closely related to the depression of photosynthesis for the early phase (days 0–6), whereas the correlations were significant only with ABA for the whole period of drought (days 0–14). These results indicate that SA-stimulated H2O2 accumulation and SA responses during the early drought phase are part of upstream H2O2-stimulated ABA accumulation, which causes ABA signaling and responses, leading to severe drought symptoms during the late phase.