Biochemical Mechanism on GABA Accumulation During Fruit Development in Tomato.

Takashi Akihiro; Satoshi Koike; Ryoji Tani; Takehiro Tominaga; Shin Watanabe; Yoko Iijima; Koh Aoki; Daisuke Shibata; Hiroshi Ashihara; Chiaki Matsukura; Kazuhito Akama; Tatsuhito Fujimura; Hiroshi Ezura

A large amount of γ-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 α-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.

AMINOBUTYRIC acid; TOMATOES; GENE expression; METABOLISM; BIOACCUMULATION; GENETIC code

Plant & Cell Physiology, 2008, Vol 49, Issue 9, p1378

0032-0781

Academic Journal

10.1093/pcp/pcn113