A method for increasing the concentration of free methionine in at least a part of a tobacco plant, comprising the steps of:(a) reducing the expression or activity of threonine synthase in the tobacco plant, wherein the threonine synthase comprises(i) a polynucleotide comprising or consisting of a sequence encoding a threonine synthase and having at least 90% sequence identity to SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, 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；(b) measuring the concentration of free methionine in at least a part of the mutant or non-naturally occurring plant obtained in step (i)； and(c) identifying a mutant or non-naturally occurring tobacco plant in which the concentration of free methionine therein has increased by at least 50% in comparison to a control tobacco plant in which the expression or activity of threonine synthase has not been reduced and wherein the visual appearance of said mutant or non-naturally occurring tobacco plant is substantially the same as the control tobacco plant three months after field transplant or 36 days after topping.
