The Effect of Sulfur Nutrition on Plant Glucosinolate Content: Physiology and Molecular Mechanisms

Glucosinolates are sulfur-rich plant metabolites of the order Brassicales that function in the defense of plants against pests and pathogens. They are also important in human society as flavor components, cancer-prevention agents, and crop biofumigants. Since glucosinolates may represent up to 30 % of the total sulfur content of plant organs, their accumulation should depend intimately on the sulfur status of the entire plant. Here we review the literature on how sulfur supply affects glucosinolate content. In field and greenhouse experiments involving soil, hydroponic and tissue culture media, sulfur fertilisation usually led to an increase in glucosinolate content ranging from 25 % to more than 50-fold, depending on the plant species, amount of sulfur applied, and type of treatment. The effect was greater on glucosinolates derived from the sulfur amino acid, methionine, than on glucosinolates derived from tryptophan. These changes are regulated not by simple mass action effects, but by extensive changes in gene transcription. In sulfur-deficient plants, there is a general down-regulation of glucosinolate biosynthetic genes which accompanies an up-regulation of genes controlling sulfur uptake and assimilation. Glucosinolates may be considered a potential source of sulfur for other metabolic processes under low-sulfur conditions, since increased breakdown of glucosinolates has been reported under sulfur deficiency. However, the pathway for sulfur mobilisation from glucosinolates has not been determined. The breakdown of indolic glucosinolates to form auxin in roots under sulfur-deficient conditions may help stimulate root formation for sulfur uptake.