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In higher plants, autophagy is bulk degradation process in vacuole necessary for survival under nutrient-limited conditions and plays important roles in senescence, development and pathogenic response, etc. Cowpea is one of the most important legume crops in semi-aride region, which is highly tolerant to drought stress. Changes of photoassimilate status by drought stress and/or sink-source balance appeared to affect autophagy and senescence of leaf in cowpea. Accordingly, we focused on roles of sucrose signal in autophagy and amino acid recycling in cowpea. Effects of starvation stress on the expression of autophagy-related genes (ATGs) and amino acid catabolism-related genes in cowpea [Vigna unguiculata (L.) Walp] were examined by Reverse transcription-polymerase chain reaction (RT-PCR) and anti-ATG8i specific antibody. Sucrose starvation stress enhanced the expression levels of VuATG8i, VuATG8c and VuATG4 incowpea seedlings. The expressions of amino acid catabolism related genes, such as asparagine synthase (VuASN1), proline dehydrogenase1 (VuProDH) and branched chain amino acid transaminase (VuBCAT2), are also up-regulated under the sucrose starvation. In contrast, high sucrose condition suppressed autophagy and the expressions of ATGs. These results indicate that sucrose starvation stress stimulates both autophagy and amino acid catabolism by regulation of ATGs and VuBCAT2. It is conceivable that sucrose starvation stress enhances autophagy in cowpea, possibly via branched chain amino acid level regulated by the starvation-induced BCAT.
Heat stress induces expression of a set of thermotolerance-related genes in plants. We focused on rice (Oryza sativa L.) homologs of the gene family that encodes galactinol synthase (OsGolS), which is closely related to the Arabidopsis thaliana galactinol synthase (AtGolS) family whose expression is induced under various stresses. OsGolS1 was highly up-regulated compare to the level of OsGolS2 in re- sponse to heat stress. Interestingly, OsGolS1 was also up-regulated by treatment with the Hsp90 inhibitor, geldanamycin (GDA). Expression profiles of OsGolS1 were correlated to those of OsHsfA2 under the GDA treatments. Treatment with GDA increased expression of OsHsfA2, but marginally increased or did not change OsHsfA1 expression. Notably, gel shift assay indicated that OsHsfA2 binds directly to OsGolS1 promoter region and that OsHsfA1 also binds to the promoter regions of OsHsfA2. Both OsHsfA2 and OsGolS1 were dramatically induced in response to heat stress. Accordingly, galactinol and raffinose contents in rice seedlings increased significantly following the induction of OsGolS1. Pre-treatment of rice seedlings with raffinose or GDA improved their thermotolerance. These results suggest that OsGolS1 plays an important role in response to heat stress, possibly via the transcription cascade of OsHsfA1-OsHsfA2 that leads to galactinol and raffinose accumulation, and that the increased content of these carbohydrates is a