Investigation of the gene family encoding aquaporins, the protein channels regulating water movement, in wheat

Aquaporins are controllers of the selective transport of water molecules through cellular membranes and possess important roles in all aspects of plant-water relations, including response to abiotic stress. However, more information is required to gain further insight into their possible physiological functions. The increasing demand for wheat as a staple food source requires improvements to wheat quality and yield for growth in broader lands, including hostile environments. Consequently this study aimed to identify and characterise genes from the PIP and TIP aquaporin subfamilies of wheat and investigate their specific functions, in order to help address these problems in the future. PIP and TIP genes isolated from common wheat (T. aestivum L. cv. Cranbrook) varied in size due to primer annealing sites and the presence of non-coding DNA. Genes contained 0 to 3 introns, the positions of which are conserved within each subfamily. Analysis of these sequences (and those identified from an expressed sequence database, as well as from isolated cDNA) resulted in the identification of 24 PIPs and 11 TIPs (of which most are expressed), more than has been identified in any plant so far. The 35 aquaporins were predicted to be located over all 7 wheat chromosomes using a predictive mapping strategy. Selected PIP1 genes were physically mapped to chromosomes 2B, 2D, 6B and 7B, confirming some of these predictions, and led to the identification of homoeologues (TaPIP1;2 and TaPIP1;5). Putative proteins possessed a high degree of sequence conservation, including the expected six transmembrane helical domains and two conserved NPA motifs that make up the typical „hourglass‟ structure. The aquaporins are thus predicted to be involved in the selective transport of water (PIPs), as well as alternative molecules (TIPs), based on the six different ar/R selectivity filters identified. Wheat PIPs were predicted to be broadly expressed across a range of tissues, while TIPs were more tissue-specific (e.g. root, leaf, seed or flower), based on EST abundance. Thus wheat aquaporins are expected to be involved in a number of vital plant functions, of which some appear to have specific roles (e.g. seed germination) while others have more of a housekeeping nature (e.g. photosynthesis). Using a real time PCR assay, three wheat PIPs were found to be up-regulated during salt stress, and PIP2s were more strongly induced than PIP1s. These are expected to be regulated both at the transcriptional and post-translational levels due to the identification of cis-acting regulatory elements in related species, and conserved residues in putative proteins of wheat (e.g. gating via phosphorylation of Ser). This study is the first report on the characterisation of aquaporin genes in wheat, perhaps the most important global cereal crop. These results provide a framework for all kinds of further analyses into wheat aquaporins to tackle problems associated with wheat cultivation. For example, tolerant wheat genotypes could be identified to select and improve the best germplasm for cultivation in adverse environments through molecular breeding or genetic manipulation, in order to meet increasing food demands.