title: Biochemical mechanism on GABA accumulation during fruit development in tomato.
abstract: A large amount of gamma-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 alpha-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.
authors: Graduate School of Life and Environmental Sciences, University of Tsukuba, Tsukuba, Ibaraki, 305-8572 Japan.
authors: Takashi
authors: T
authors: Akihiro
authors: Satoshi
authors: S
authors: Koike
authors: Ryoji
authors: R
authors: Tani
authors: Takehiro
authors: T
authors: Tominaga
authors: Shin
authors: S
authors: Watanabe
authors: Yoko
authors: Y
authors: Iijima
authors: Koh
authors: K
authors: Aoki
authors: Daisuke
authors: D
authors: Shibata
authors: Hiroshi
authors: H
authors: Ashihara
authors: Chiaki
authors: C
authors: Matsukura
authors: Kazuhito
authors: K
authors: Akama
authors: Tatsuhito
authors: T
authors: Fujimura
authors: Hiroshi
authors: H
authors: Ezura
mesh_terms: D000612:4-Aminobutyrate Transaminase / Q000378:metabolism*; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D005638:Fruit / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D008401:Gas Chromatography-Mass Spectrometry; D015870:Gene Expression; D018745:Genome, Plant; D005968:Glutamate Decarboxylase / Q000378:metabolism*; D018551:Solanum lycopersicum / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D012333:RNA, Messenger / Q000032:analysis; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D018076:DNA, Complementary; D018744:DNA, Plant; D012333:RNA, Messenger; D005680:gamma-Aminobutyric Acid; D000612:4-Aminobutyrate Transaminase; D005968:Glutamate Decarboxylase