The present invention provides a method for altering or modifying the content and/or composition of one or more metabolites in the storage organs of a plant, at least comprising the expression of a sulfur-rich protein therein. In particular, the invention provides a method of modifying the fiber content and/or fiber composition and/or starch content and/or nitrogen content and/or non-sulfur-containing amino acid composition and/or anti-nutritional protein content and/or fatty acid content and/or fatty acid composition of the storage organs of a plant. The invention clearly extends to the plants produced by the performance of the inventive method and to genetic constructs which in use produce such plant material.