Sulfur plays an important role in the regulation of plant growth and development. Inorganic sulfate, taken by the roots and translocated to different plant tissues, is assimilated into L-cysteine, the starting metabolite for futher synthesis of L-methionine and other S-compounds. Sulfate assimilation is a light-driven pathway, that has been considered to take place into the chloroplast. However, root cells have the capacity to assimilate sulfate since all the required enzyme activities have been detected in root plastids. Besides, molecular biology techniques like functional complementation of bacteria and yeast mutants have allowed the cloning of genes involved in sulfur metabolism. Furthermore, environmental stresses, like cold, dehydration, pathogenic attack and heavy metal exposition, generate plant adaptation mechanisms in which sulfur compounds are involved, thus producing activation of L-cysteine biosynthesis.