Physiological importance and possible applications of β-substituted alanine synthase in plants

Cysteine, as well as its precursors and derivatives, plays important roles in plant development and stress responses. In plants, a diverse range of reactions affecting cysteine content are catalyzed by the β-substituted alanine synthase (BSAS) enzyme family. Individual BSAS family members use similar reaction mechanisms involving pyridoxal phosphate cofactors and show catalytic preferences for biosynthesis, degradation, or modification of the cysteine amino acid. In Arabidopsis thaliana (Arabidopsis) of the Brassicaceae family, four distinct biochemical activities are characterized at the gene level, namely, O-acetylserine sulfhydrylase, β-cyanoalanine synthase, l-cysteine desulfhydrase, and S-sulfocysteine synthase activities. Reverse genetic approaches in Arabidopsis were used to elucidate the physiological roles of metabolites of cysteine metabolism (O-acetylserine, sulfide, cysteine, cyanide, and S-sulfocysteine) during the processes of root hair development, pollen tube germination, heavy metal tolerance, defense responses, stomatal closure, and autophagy. Key catalytic residues determining reaction specificities in different BSAS enzymes are being identified, along with the roles of macromolecular complexes involving BSAS. The biochemical properties of BSAS active sites are being investigated in various organisms, including plants, for possible application to the development of new biological materials and drugs. Systematic and comparative genomic studies of BSAS enzymes in Brassica plants, close relatives of Arabidopsis, requiring high cysteine production for optimum growth and disease resistance, will be useful for the future study of the diversification of BSAS and the biotechnological improvement of these important crop plants.