Transport of inorganic phosphate (Pi) through plant membranes is mediated by a number of families of transporter proteins. Studies on the topology, function, regulation and sites of expression of the genes that encode the members of these transporter families are enabling roles to be ascribed to each of them. The Pht1 family, of which there are nine members in the Arabidopsis genome, includes proteins involved in the uptake of Pi from the soil solution and the redistribution of Pi within the plant. Members of this family are H2PO4−/H+ symporters. Most of the genes of the Pht1 family that are expressed in roots are up-regulated in P-stressed plants. Two members of the Pht1 family have been isolated from the cluster roots of white lupin. These same genes are expressed in non-cluster roots. The evidence available to date suggests that there are no major differences between the types of transport systems that cluster roots and non-cluster roots use to acquire Pi. Differences in uptake rates between cluster and non-cluster roots can be ascribed to more high-affinity Pi transporters in the plasma membranes of cluster roots, rather than any difference in the characteristics of the transporters. The efficient acquisition of Pi by cluster roots arises primarily from their capacity to increase the availability of soil Pi immediately adjacent to the rootlets by excretion of carboxylates, protons and phosphatases within the cluster. This paper reviews Pi transport processes, concentrating on those mediated by the Pht1 family of transporters, and attempts to relate those processes involved in Pi acquisition to likely Pi transport processes in cluster roots.
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