Cloning and functional analysis of four O-Acetylserine (thiol) lyase family genes from foxtail millet

Abstract

Cysteine is the first organic molecule generated during the assimilation of sulfate. As such, cysteine and its derivatives are always essential signal molecules and thus have important roles in the regulation of many plant processes. O-acetylserine (thiol) lyase (OASTL) catalyzes the last step of the biosynthesis of cysteine. At present, detailed and comprehensive work about these enzymes has only been reported from the plant Arabidopsis thaliana, though sporadic studies on OASTL have been conducted on other dicots, such as spinach and soybean. However, few reports on the functions of OASTLs in monocots have been found in the literature. Here in this study, we obtained four SiOASTL genes (SiOASTL7, SiOASTL8, SiOASTL9 and SiOASTL10) from foxtail millet and analyzed their potential functions. Phylogenetically, the four SiOASTL genes did not belong to any published subfamily of the OASTL genes; instead they constituted a new subfamily specific to the OASTL genes from monocots. In sequencing, we found that with the exception of the pseudogene SiOASTL8, proteins encoded by the other three genes exhibited high similarity with OASTL proteins from Arabidopsis, though the critical PLP-binding sites of both SiOASTL7 and SiOASTL10 were missing. The enzymatic activity assays demonstrated that SiOASTL9 has the ability to catalyze the biosynthesis of both cysteine and S-sulfocysteine, while SiOASTL7 and SiOASTL10 did not possess any previously reported catalyzing abilities. In addition, the gene expression pattern analysis showed that all four genes were widely expressed in various tissues of foxtail millet, and all had a preference in the leaves. Under abiotic stresses, the expression of these genes could be induced by salt and drought stress. Our finding that cadmium could only up-regulate the transcription of SlOASTL8 and SlOASTL9, further indicates the diversified responses of SiOASTLs to abiotic stresses.