There is disclosed a mutant, non-naturally occurring or transgenic plant cell comprising: (i) a polynucleotide comprising, consisting or consisting essentially of a sequence encoding a threonine synthase and having at least 90% sequence identity to SEQ ID NO:1, SEQ ID NO:2 or SEQ ID NO:3 or at least 87% sequence identity to SEQ ID NO:4, or SEQ ID NO:5； (ii) a polypeptide encoded by any one of said polynucleotides set forth in (i)； or (iii) a polypeptide having at least 95% sequence identity to SEQ ID NO:6, SEQ ID NO:7 or SEQ ID NO:8； or (iv) a construct, vector or expression vector comprising the polynucleotide as set forth in (i).
