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, by introducing it into the plant of a polynucleotide that shows RNA interference activity against threonine synthase mRNA, wherein threonine synthase comprises (i) a polynucleotide comprising or consisting of a sequence encoding a threonine synthase and having at least 90% identity in sequence with sec. with no. of ident.:1, sec. with no. of ident.:2, sec. with no. of ident.:3, sec. with no. of ident.:4, or sec. with no. from ident.:5； (ii) a polypeptide encoded by any of said polynucleotides set forth in (i)； or (iii) a polypeptide that has at least 95% sequence identity with sec. with no. of ident.:6, sec. with no. of ident.:7 or sec. with no. of ident.:8； (b) measure the concentration of free methionine in at least a part of the transgenic tobacco plant obtained in step (i)； and (c) identify a transgenic tobacco plant in which the concentration of free methionine has increased by at least 50% compared to a control tobacco plant in which the expression or activity of threonine synthase has not been reduced and where The visual appearance of said transgenic tobacco plant is essentially the same as the control tobacco plant three months after transplanting to the field or 36 days after topping.Un método para aumentar la concentración de metionina libre en al menos una parte de una planta de tabaco, que comprende las etapas de: (a) reducir la expresión o actividad de treonina sintasa en la planta de tabaco, mediante la introducción en la planta de un polinucleótido que muestra actividad de interferencia de ARN contra el ARNm de la treonina sintasa, en donde la treonina sintasa comprende (i) un polinucleótido que comprende o que consiste en una secuencia que codifica una treonina sintasa y que tiene al menos 90 % de identidad de secuencia con la sec. con núm. de
