Katz Yael S, Galili Gad, Amir Rachel
Plant Science Laboratory, Migal Galilee Technological Center, Kiryat Shmona, Israel.
Plant Mol Biol. 2006 May;61(1-2):255-68. doi: 10.1007/s11103-006-0009-8.
The essential amino acid methionine is a substrate for the synthesis of S-adenosyl-methionine (SAM), that donates its methyl group to numerous methylation reactions, and from which polyamines and ethylene are generated. To study the regulatory role of methionine synthesis in tomato fruit ripening, which requires a sharp increase in ethylene production, we cloned a cDNA encoding cystathionine gamma-synthase (CGS) from tomato and analysed its mRNA and protein levels during tomato fruit ripening. CGS mRNA and protein levels peaked at the "turning" stage and declined as the fruit ripened. Notably, the tomato CGS mRNA level in both leaves and fruit was negatively affected by methionine feeding, a regulation that Arabidopsis, but not potato CGS mRNA is subject to. A positive correlation was found between elevated ethylene production and increased CGS mRNA levels during the ethylene burst of the climacteric ripening of tomato fruit. In addition, wounding of pericarp from tomato fruit at the mature green stage stimulated both ethylene production and CGS mRNA level. Application of exogenous methionine to pericarp of mature green fruit increased ethylene evolution, suggesting that soluble methionine may be a rate limiting metabolite for ethylene synthesis. Moreover, treatment of mature green tomato fruit with the ethylene-releasing reagent Ethephon caused an induction of CGS mRNA level, indicating that CGS gene expression is regulated by ethylene. Taken together, these results imply that in addition to recycling of the methionine moieties via the Yang pathway, operating during synthesis of ethylene, de novo synthesis of methionine may be required when high rates of ethylene production are induced.
必需氨基酸甲硫氨酸是合成S-腺苷甲硫氨酸（SAM）的底物，SAM将其甲基提供给众多甲基化反应，并由此生成多胺和乙烯。为了研究甲硫氨酸合成在番茄果实成熟过程中的调控作用（番茄果实成熟需要乙烯产量急剧增加），我们从番茄中克隆了一个编码胱硫醚γ-合酶（CGS）的cDNA，并分析了其在番茄果实成熟过程中的mRNA和蛋白质水平。CGS的mRNA和蛋白质水平在“转色”阶段达到峰值，并随着果实成熟而下降。值得注意的是，叶片和果实中的番茄CGS mRNA水平均受到甲硫氨酸饲喂的负面影响，拟南芥CGS mRNA受此调控，但马铃薯CGS mRNA不受此调控。在番茄果实跃变型成熟的乙烯爆发期间，乙烯产量升高与CGS mRNA水平增加之间存在正相关。此外，对成熟绿熟期番茄果实的果皮进行创伤处理会刺激乙烯产量和CGS mRNA水平升高。向成熟绿熟果实的果皮施用外源甲硫氨酸会增加乙烯释放量，这表明可溶性甲硫氨酸可能是乙烯合成的限速代谢物。此外，用乙烯释放试剂乙烯利处理成熟绿熟番茄果实会诱导CGS mRNA水平升高，这表明CGS基因表达受乙烯调控。综上所述，这些结果表明，除了在乙烯合成过程中通过杨循环回收甲硫氨酸部分外，当诱导乙烯高产率时，可能还需要甲硫氨酸的从头合成。
