Methods for modulating levels of at least one organic sulfur compound in plants are provided. Also provided are plants, plant seeds, and plant cells produced by the methods. The methods comprise stably transforming a plant with a DNA construct encoding at least one APS reductase enzyme or PAPS reductase enzyme (herein, "(P)APS reductase") so that the transformed plant exhibits altered levels of at least one organic sulfur compound. APS reductase is an enzyme classified as EC 1.8.4.9 and PAPS reductase is an enzyme classified as EC 1.8.99.4; these enzymes are capable of reducing sulfur in the form of APS or PAPS to produce sulfite. Also provided are methods for reducing oxidative stress in plants and for increasing the nutritional quality of plants and seeds.