Antagonistic shifting from abscisic acid- to salicylic acid-mediated sucrose accumulation contributes to drought tolerance in <i>Brassica napus</i>

Abstract

The phytohormone salicylic acid (SA), as an important signaling molecule involved in the regulation of plant stress responses. This study aimed to characterize the hormonal regulation of drought-responsive sugar metabolism, focusing on SA-mediated sucrose modulation with regard to the drought resistance mechanism. The responses of sucrose synthesis, starch degradation, sucrose transport, as well as stress symptom development to SA pretreatment and/or drought imposition were interpreted in relation to the altered endogenous hormonal status and their signaling genes. Drought-induced severe reduction of leaf biomass coincided with the highest endogenous level of abscisic acid (ABA) and expression of SAG12. Under drought-stressed, sugar accumulation was mainly due to the enhanced hexose level with depressed expression of hexokinase gene HXK1 and, in part, to increased sucrose content with the highest expression of ABA-dependent sucrose signaling genes SnRK2.2 and AREB2. In the presence of SA, an additional sucrose accumulation occurred with further enhancement of sucrose phosphate synthase (SPS) activity and starch degradation-related genes BAM1 and AMY3 expression, which coincided with the depression of SnRK2.2 and AREB2. Further, SA-mediated sucrose accumulation was responsible for the induction of phloem sucrose loading with enhanced expression of sucrose transporter genes SUT1 and SUT4. SA-mediated pathogenesis-related protein 2 (PR2) activation reflected a synergistic interaction between SA and sucrose signaling. These results indicate that antagonistic shifting from ABA- to SA-mediated sucrose accumulation is an important process in regulating osmotic potential and leaf senescence.