Abstract
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.
Highlights
Recent studies revealed that higher plants have various pathways of protein degradation systems such as ubiqutin-proteasome, autophagy, and vacuolar processing enzyme cascade [1,2]
Previous studies with Arabidopsis and soybean revealed that the upregulation of ATG8s and autophagy-related genes (ATGs)-4 mRNA were accompanied with induction of autophagy in response to sucrose starvation stress [9, 15,17]
It has been reported that low energy stress on Arabidopsis plant transcriptionally and translationally activates heterodimer bZIP trasnription factors, bZIP1 and bZIP53, which regulate the expression of ASN1, ProDH and BCAT2, via the promoter regions containing G-box, C-box and ACT motifs [20]
Summary
Recent studies revealed that higher plants have various pathways of protein degradation systems such as ubiqutin-proteasome, autophagy, and vacuolar processing enzyme cascade [1,2]. Molecular genetic and biochemical studies with autophagy-deficient mutants of budding yeast have unveiled a set of novel genes, autophagy related genes (ATGs) [3]. Molecular genetic studies using mutants of autophagyrelated genes (ATGs) unveiled that autophagy plays essential roles in growth, senescence, nutrient translocation and stress responses of higher plants [2,3]. It has been shown that the conserved TOR and SnRK signaling pathways in higher plants play important roles in growth, metabolisms and stress responses including autophagy in response to nutrient and enegy levels [7]
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