An isolated polynucleotide encoding a plant cysteine synthase is disclosed. Also disclosed is the construction of a recombinant construct comprising all or part of a coding or non-coding region of the isolated polynucleotide for use in co-suppression or antisense suppression of endogenous nculeic acid sequences encoding polypeptides having cysteine synthase activity. In another aspect, use of this construct to alter levels of cysteine synthase in a plant is disclosed.