Sulphates taken up by the roots are the major sulfur source for growth, though it has to be reduced to sulphide before it is further metabolized. Root plastids contain all sulphates reduction enzymes, but the reduction of sulfate to sulphide and its subsequent incorporation into cysteine predominantly takes place in the shoot, in the chloroplast.
Sulphates is taken up by the roots that have high affinity. The formation of cysteine is the direct coupling step between sulfur and nitrogen assimilation in plants. This differs from the process in yeast, where sulfide must be incorporated first in homocysteine then converted in two steps to cysteine
All photosynthetic organisms, sulfate reduction occurs in the plastids, with the notable exception of Euglena gracilis, which locates the sulfate-reducing enzymes in mitochondria . Whereas the reductive steps are localized exclusively in the organelles, ATP sulfurylase is also present in the cytosol. Accordingly, plants and algae possess multiple isoforms of ATP sulfurylase . Very surprisingly, plant ATP sulfurylases have a different evolutionary origin than those from green algae, and they are more similar to their animal counterparts .
Sulfate uptake and assimilation are tightly regulated according to the plant demands for reduced sulfur. Among other compounds affecting sulfate assimilation, the phytohormones play the most prominent roles.
