Plant responses to sulphur deficiency and the genetic manipulation of sulphate transporters to improve S-utilization efficiency.

Decreased inputs of S have increased the incidence of S-deficiency in crops, resulting in decreased yields and quality. Remediation by fertilizer application is not always successful because this often results in an uneven supply of S. The ability to respond to S-deficiency stress varies between crops and this is a target for the genetic improvement of S-utilization efficiency. Improved capture of resources, the accumulation of greater reserves of S and improved mechanisms for the remobilization of these reserves are required. It is an inability to over-accumulate S and subsequently, effectively remobilize S-reserves, which restricts optimum S-use efficiency. Genetic manipulation of the transporters and their expression will contribute to overcoming these limitations. Control of gene expression limits excess uptake and activity of the assimilatory pathway: the endogenous expression of sulphate transporters is regulated by S-supply, with negative regulation from reduced S-containing compounds and positive regulation by O-acetylserine, the C/N skeleton precursor of cysteine. Constitutive expression of the transporter will remove this control and may enable the accumulation of sulphate reserves. Sulphate in the vacuole and other pools of reduced sulphur, such as glutathione or protein may be remobilized under S-limiting conditions. Low efficiencies of these remobilization processes, particularly the remobilization of vacuolar sulphate, suggest that the transporters involved in the remobilization are a target for modification. Transporters are involved in facilitating the multiple trans-membrane transport steps between uptake of sulphate from the soil solution, and delivery to the site of reduction in the chloroplast or plastid. A gene family has been identified and phylogenetic relationships based on primary sequence information indicate multiple sub-groups. Groups which are expressed in roots, in shoots and in both tissue types are postulated, however, the functional roles for these groups and the identification of transporters involved in recycling remain to be confirmed.