To address roles of bZIP transcription factors on regulation of amino acid catabolism under autophagy-induced plant cells, we examined the effect of nutrient starvation on the expression of low energy stress-related transcription factor homologs, GmbZIP53A and GmbZIP53B, and amino acid catabolism-related genes in soybean (Glycine max (L.) Merr.). Sucrose starvation treatment significantly enhanced the expressions of GmbZIP53A, but not GmbZIP53B asparagine synthase (GmASN1), proline dehydrogenase1 (GmProDH), and branched chain amino acid transaminase 3 (GmBCAT3). GmbZIP53-related immunoreactive signals were upregulated under severe starvation with sucrose starvation and protease inhibitors, while 3% sucrose and sucrose starvation had no or marginal effects on the signal. Profiles of induction of GmASN1, GmProDH and GmBCAT3 under various nutrient conditions were consistent with the profiles of GmbZIP53 protein levels but not with those of GmbZIP mRNA levels. These results indicate that GmbZIP53 proteins levels are regulated by posttranslational mechanism in response to severe starvation stress and that the increased protein of GmbZIP53 under severe starvation accelerates transcriptional induction of GmASN1, GmProDH, and GmBCAT3. Furthermore, it is conceivable that decrease of branched chain amino acid level by the BCAT-mediated degradation eventually enhances autophagy under severe starvation. 1. Introduction The perception and management of nutrient and energy levels in organisms are crucial for survival by adjusting metabolism to available resources. Recent studies revealed that sugar signals in higher plants activate various biological modules such as sugar sensor, transcription factors, sugar transporters, and metabolic enzymes of sugar and amino acids [1, 2]. In higher plants, sugar deprivation and/or low energy stress by decreased photosynthesis have appeared to induce protein degradation via autophagy and amino acid metabolism, leading to translocation of nutrients and senescence [3, 4]. Recently accumulated studies on bZIP type transcription factors of Arabidopsis concerning nutrient signal and amino acid metabolism have unveiled that a set of bZIP transcription factors, bZIP1 and bZIP53, classified to S-type subgroup among the bZIP superfamily, are master regulatory components in transcriptional induction of amino acid catabolism-related enzymes involved in low energy stress, sucrose starvation, and senescence-induced nutrient translocation [5–7]. Sucrose starvation and/or low energy stress activate bZIP1 and bZIP53a by transcriptional and
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