Evidence for posttranscriptional activation of γ-glutamylcysteine synthetase during plant stress responses

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Abstract
Glutathione (GSH) is a key component of plant antioxidant defenses. We have sought to determine how the rate-limiting step in GSH biosynthesis, catalyzed by γ-glutamylcysteine synthase (γECS) is regulated in Arabidopsis. Functional complementation of a yeast mutant deficient in this enzyme with an Arabidopsis expression library yielded two cDNAs with sequence identical to the previously described AtγECS. Nevertheless, the cellular concentration of GSH in these transformants was only 10% of wild-type concentrations and this was not a result of Cys availability. To explore the possibility that Arabidopsis γECS requires additional factors for full catalytic activity, we analyzed the GSH levels and the enzyme activities and transcript levels of both enzymes of the GSH biosynthetic pathway in Arabidopsis suspension cultures subjected to a variety of stresses that raise GSH levels. Our results demonstrate rapid posttranscriptional activation of Arabidopsis γECS. The implications of these findings for the mechanisms by which GSH concentrations are regulated during plant-stress responses are discussed.