Sulphur and phosphorus transport systems in plants

Our understanding of the mechanisms of sulphate and phosphate transport in plants has advanced considerably in recent years as a result of the application of molecular techniques in nutritional physiology. Genes encoding the transporters involved in the uptake of sulphate and phosphate by plant roots and the distribution of these anions throughout the plant have been identified. Many of these genes have been characterised, their regulatory systems studied and, for some, the tissues in which they are expressed delineated. This information is providing important clues about the roles of these genes. Most work to date has been done on the sulphate and phosphate transporters that are found in roots. These transporters are SO42-/H+ or H2PO4-/H+ symporters. The expression of the genes that encode these transporters in roots is transcriptionally regulated by feedback signals that respond to the sulphate or phosphate status of the plant. However, there is evidence that genes encoding high-affinity sulphate transporters can also be regulated by nutrient demand and the availability of precursors needed for sulphur assimilation. Genes encoding high-affinity sulphate and phosphate transporters are expressed in cells in close contact with soil solution, especially epidermal cells with their associated root hairs. Genes encoding other lower-affinity transporters are expressed in cells associated with the vascular system where they play a role in the internal redistribution of sulphate or phosphate. Transgenic plants in which genes encoding sulphate or phosphate transporters are over-expressed are now emerging from transformation programs. Data from these studies, together with some thoughts on possible implications of this technology for genetic improvement of commercial crop plants will be presented.