EVIDENCE FOR POSTTRANSCRIPTIONAL ACTIVATION OF GAMMA-GLUTAMYLCYSTEINESYNTHETASE DURING PLANT STRESS RESPONSES

Glutathione (GSH) is a key component of plant antioxidant defenses. We have sought to determine how the rate-limiting step in GSH biosynthes is, catalyzed by gamma-glutamylcysteine synthase (gamma ECS) is regula ted in Arabidopsis. Functional complementation of a yeast mutant defic ient in this enzyme with an Arabidopsis expression library yielded two cDNAs with sequence identical to the previously described At gamma EC S, Nevertheless, the cellular concentration of GSH in these transforma nts was only 10% of wild-type concentrations and this was not a result of Cys availability. To explore the possibility that Arabidopsis gamm a ECS requires additional factors for full catalytic activity, we anal yzed the GSH levels and the enzyme activities and transcript levels of both enzymes of the GSH biosynthetic pathway in Arabidopsis suspensio n cultures subjected to a variety of stresses that raise GSH levels. O ur results demonstrate rapid posttranscriptional activation of Arabido psis gamma ECS. The implications of these findings for the mechanisms by which GSH concentrations are regulated during plant-stress response s are discussed.