Sulfur is taken up be plants as sulfate which must be reduced to sulfide in order for plants to make use of it. Sulfate must first be activated to adenosine 5-phosphosulfate (APS) which is catalyzed by ATP sulfurylase. The APS is then reduced to sulfite with the help of APS reductase with glutathione as a reductant. The sulfite is then reduced to sulfide by sulfite reductase with ferredoxin as a reductant.