Identification of two CiGADs from Caragana intermedia and their transcriptional responses to abiotic stresses and exogenous abscisic acid.

BACKGROUND: Glutamate decarboxylase (GAD), as a key enzyme in the γ -aminobutyric acid (GABA) shunt, catalyzes the decarboxylation of L-glutamate to form GABA. This pathway has attracted much interest because of its roles in carbon and nitrogen metabolism, stress responses, and signaling in higher plants. The aim of this study was to isolate and characterize genes encoding GADs from Caragana intermedia, an important nitrogen-fixing leguminous shrub.

METHODS: Two full-length cDNAs encoding GADs (designated as CiGAD1 and CiGAD2) were isolated and characterized. Multiple alignment and phylogenetic analyses were conducted to evaluate their structures and identities to each other and to homologs in other plants. Tissue expression analyses were conducted to evaluate their transcriptional responses to stress (NaCl, ZnSO4, CdCl2, high/low temperature, and dehydration) and exogenous abscisic acid.

RESULTS: The CiGADs contained the conserved PLP domain and calmodulin (CaM)-binding domain in the C-terminal region. The phylogenetic analysis showed that they were more closely related to the GADs of soybean, another legume, than to GADs of other model plants. According to Southern blotting analysis, CiGAD1 had one copy and CiGAD2-related genes were present as two copies in C. intermedia. In the tissue expression analyses, there were much higher transcript levels of CiGAD2 than CiGAD1 in bark, suggesting that CiGAD2 might play a role in secondary growth of woody plants. Several stress treatments (NaCl, ZnSO4, CdCl2, high/low temperature, and dehydration) significantly increased the transcript levels of both CiGADs, except for CiGAD2 under Cd stress. The CiGAD1 transcript levels strongly increased in response to Zn stress (74.3-fold increase in roots) and heat stress (218.1-fold increase in leaves). The transcript levels of both CiGADs significantly increased as GABA accumulated during a 24-h salt treatment. Abscisic acid was involved in regulating the expression of these two CiGADs under salt stress.

DISCUSSION: This study showed that two CiGADs cloned from C. intermedia are closely related to homologs in another legume, soybean. CiGAD2 expression was much higher than that of CiGAD1 in bark, indicating that CiGAD2 might participate in the process of secondary growth in woody plants. Multiple stresses, interestingly, showed that Zn and heat stresses had the strongest effects on CiGAD1 expression, suggesting that CiGAD1 plays important roles in the responses to Zn and heat stresses. Additionally, these two genes might be involved in ABA dependent pathway during stress. This result provides important information about the role of GADs in woody plants' responses to environmental stresses.