Acts as a major beta-cyanoalanine synthase. The cyanoalanine synthesis reaction is more efficient than the cysteine synthase activity. Probably unable to interact with SAT and to form the decameric Cys synthase complex (CSC) and is therefore not an enzymatically true OASTL protein. Probably involved in the detoxification of cyanide that arises from ethylene biosynthesis. Maintains a low level of cyanide for proper root hair development. hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine hydrogen cyanide + L-cysteine = 3-cyano-L-alanine + H(+) + hydrogen sulfide kcat is 120 min(-1) for O(3)-acetyl-L-serine for the cysteine synthase activity, kcat is 90 min(-1) for Na(2)S for the cysteine synthase activity, kcat is 160 min(-1) for cysteine for the L-3-cyanoalanine synthase activity, kcat is 130 min(-1) for KCN for the L-3-cyanoalanine synthase activity. Mainly expressed in mature rosette leaves. Also detected in roots, young rosette leaves, stems, cauline leaves, and flowers. Defective in root hair formation and accumulates cyanide in root tissues. No effects on growth. Belongs to the cysteine synthase/cystathionine beta-synthase family.