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 applica tion of molecular techniques in nutritional physiology. Genes encoding the transporters involved in the uptake of sulphate and phosphate by plant root s and the distribution of these anions throughout the plant have been ident ified. Many of these genes have been characterised, their regulatory system s studied and, for some, the tissues in which they are expressed delineated . This information is providing important clues about the roles of these ge nes. Most work to date has been done on the sulphate and phosphate transpor ters that are found in roots. These transporters are SO42-/H+ or H2PO4-/Hsymporters. The expression of the genes that encode these transporters in r oots 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 b y nutrient demand and the availability of precursors needed for sulphur ass imilation. Genes encoding high-affinity sulphate and phosphate transporters are expressed in cells in close contact with soil solution, especially epi dermal cells with their associated root hairs. Genes encoding other lower-a ffinity transporters are expressed in cells associated with the vascular sy stem where they play a role in the internal redistribution of sulphate or p hosphate. Transgenic plants in which genes encoding sulphate or phosphate t ransporters are over-expressed are now emerging from transformation program s. Data from these studies, together with some thoughts on possible implica tions of this technology for genetic improvement of commercial crop plants will be presented.