Plant and Cell Physiology Advance Access published online on August 18, 2008
Plant and Cell Physiology, doi:10.1093/pcp/pcn113
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Biochemical mechanism on GABA accumulation during fruit development in tomato.
1 Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan
2 Kazusa DNA Research Institute, Kazusa-Kamatari 2-6-7, Kisarazu, Chiba 292-0818, Japan
3 Faculty of Life and Environmental Science, Shimane University, Matsue city, Shimane 690-8504, Japan
4 Department of Advanced Bioscience, Graduate School of Humanities and Sciences, Ochanomizu University, Ohtsuka, Bunkyo-ku, Tokyo, 112-8610, Japan
Corresponding author: Hiroshi Ezura, Street addressTennodai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan, Fax number +81-29-853-7734, ezura{at}gene.tsukuba.ac.jp
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Abstract |
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A large amount of 
-amino butyric acid (GABA) was found to accumulate in tomato (Solanum lycopersicum) fruits before the breaker stage. Shortly thereafter, GABA was rapidly catabolized after the breaker stage. We screened the GABA-rich tomato cultivar ‘DG03-9’ which did not show rapid GABA catabolism after the breaker stage. Although GABA hyper-accumulation and rapid catabolism in fruits is well known, the mechanisms are not clearly understood. In order to clarify these mechanisms, we performed comparative studies of ‘Micro-Tom’ and ‘DG03-9’ fruits for analyses of gene expression levels, protein levels and enzymatic activity levels of GABA biosynthesis and catabolism-related enzymes. During GABA accumulation, we found positive correlations among GABA contents and expression levels of SlGAD2 and SlGAD3. Both of these genes encode for glutamate decarboxylase (GAD) which is key enzyme of GABA biosynthesis. During GABA catabolism, we found strong correlation between GABA contents and enzyme activity of 
-ketoglutarate-dependent GABA transaminase (GABA-TK). The contents of glutamate and aspatate, which are synthesized from GABA and glutamate, respectively, increased with elevation of GABA-TK enzymatic activity. GABA-TK is the major GABA-transaminase form in animals and appears to be a minor form in plants. In ‘DG03-9’ fruits, GAD enzymatic activity was prolonged until the ripening stage and GABA-TK activity was significantly low. Taken together, our results suggest that GAD and GABA-TK play crucial roles in GABA accumulation and catabolism in tomato fruits, respectively.
Keywords: GABA-TK - GABA - Glu - Micro-Tom
5 These authors contributed equally to this work.
(Received May 17, 2008; Accepted August 6, 2008)
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