Plant and Cell Physiology Advance Access originally published online on August 18, 2008
Plant and Cell Physiology 2008 49(9):1378-1389; 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: E-mail, ezura{at}gene.tsukuba.ac.jp; Fax, +81-29-853-7734.
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Abstract |
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A large amount of 
-aminobutyric 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 hyperaccumulation 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 the analysis 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 glutamate decarboxylase (GAD) which is a key enzyme of GABA biosynthesis. During GABA catabolism, we found a strong correlation between GABA contents and enzyme activity of 
-ketoglutarate-dependent GABA transaminase (GABA-TK). The contents of glutamate and aspartate, 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, respectively, in tomato fruits.
Keywords: GABA - GABA-TK - Glutamate - Micro-Tom
Abbreviations:
AAT, aspartate transaminase; Asp, aspartic acid; CaM, calmodulin; DAF, days after flowering; DIG, digoxigenin; DTT, dithiothreitol; GABA, -aminobutyric acid; GABA-T, GABA transaminase; GABA-TK, -ketoglutarate-dependent GABA transaminase; GABA-TP, pyruvate-dependent GABA transaminase; GAD, glutamate decarboxylase; GC–MS, gas chromatography–mass spectrometry; Glu, glutamate; PVDF, polyvinylidene difluoride; RACE, rapid amplification of cDNA ends; RT–PCR, reverse transcription–PCR; SSADH, succinic semialdehyde dehydrogenase.
5These authors contributed equally to this work.
(Received May 17, 2008; Accepted August 6, 2008)
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