~90% of the organic sulfur in plants is concentrated in the amino acids cysteine and methionine. Cysteine acts as the direct or indirect precursor to any other organic sulfur compounds in plants such as  coenzyme-A, methionine, biotin, lipoic acid and glutathione. The carbon skeleton necessary for sulfur assimilation are provided by glycolysis (acetyl-CoA), respiration (aspartic acid, Asp, which derives from oxaloacetate) and photorespiration (serine, Ser). Because cysteine is a direct precursor to methionine, methionine is naturally S-depleted in comparison to cysteine. The majority of sulfur is generally in the organic form but, when excess sulfur is available in the environment, inorganic sulfate becomes the major sulfur form. In most plants, S discrimination is minimal, and in a study of rice plants it was observed that discrimination takes place in the uptake stage, depleting imported sulfate by 1–2‰ from the source. This effect is through the expression of SO transporter genes (SULTR), 14 of which have been identified – which are expressed dependent on the availability of sulfate in the environment. When sulfate is plentiful low affinity transporters are expressed and when sulfate is scarce high affinity genes with greater S discrimination are expressed.