The cytosolic O-acetylserine(thiol)lyase gene is regulated by heavy metals and can function in cadmium tolerance

Regulation of the expression of the cytosolic O-acetyl-serine(thiol)lyase gene (Atcys-3A) from Arabidopsis thaliana under heavy metal stress conditions has been investigated. Northern blot analysis of Atcys-3A expression shows a 7-fold induction after 18 h of cadmium treatment. Addition of 50 muM CdCl2 to the irrigation medium of mature Arabidopsis plants induces a rapid accumulation of the mRNA throughout the leaf lamina, the root and stem cortex, and stem vascular tissues when compared with untreated plants, as observed by in situ hybridization. High pressure liquid chromatography analysis of GSH content shows a transient increase after 18 h of metal treatment. Our results are compatible with a high cysteine biosynthesis rate under heavy metal stress required for the synthesis of GSH and phytochelatins, which are involved in the plant detoxification mechanism. Arabidopsis-transformed plants overexpressing the Atcys-3A gene by up to g-fold show increased tolerance to cadmium when grown in medium containing 250 muM CdCl2, suggesting that increased cysteine availability is responsible for cadmium tolerance. In agreement with these results, exogenous addition of cystine can, to some extent, also favor the growth of wild type plants in cadmium-containing medium. Cadmium accumulates to higher levels in leaves of tolerant transformed lines than in wild-type plants.