Involved in maintaining Cys homeostasis through the desulfuration of L-cysteine. Modulates the generation of the signaling molecule sulfide in plant cytosol. Probably unable to interact with SAT and to form the decameric Cys synthase complex (CSC) and is therefore not an enzymatically true OASTL protein. H2O + L,L-cystathionine = 2-oxobutanoate + L-cysteine + NH4(+) hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Inhibited by aminooxyacetate. Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Homodimer. No visible phenotype. Light early-flowering and premature leaf senescence phenotype. Increased total cysteine content and increased resistance to pathogens. Belongs to the cysteine synthase/cystathionine beta-synthase family.