Transporter gene families in plants: the sulphate transporter gene family - redundancy or specialization?

Analysis of the Arabidopsis genome has indicated that the gene families encoding for specific solute transporters are surprisingly large. An example is the sulphate transporter family, which in Arabidopsis , consists of 14 isoforms showing homology to one another. H+-sulphate co-transport has been demonstrated for some, but not all, of these Arabidopsis isoforms. Occurrence of the sulphate transporter is ubiquitous and many examples have been cloned from a variety of plant species, animals and yeast. This is a unique transporter family with no apparent homology to any other transporter type. Phylogenetic analysis of the plant gene or amino acid sequences indicates that there are 5 or more distinguishable clusters within the family of sulphate transporters. Analysis of functional characteristics and patterns of regulation together with localization data, suggests that these groups have specific roles, such as high affinity uptake in the root, translocation in vascular tissues and cell to cell transfer in leaves and seeds. Additionally, some members of this sulphate transporter family may have discrete subcellular locations in plastid or tonoplast membranes. Within individual groups, there are often multiple isoforms from the same species, indicating an even greater degree of specialization or alternatively, redundancy. This mini-review will summarize the available data to distinguish between specialization and redundancy.