Structural and Functional Properties of the Cotton Aquaporin Gene Family.

Water uptake and transport is a fundamental process of growth and development in living organisms. Aquaporins are transmembrane water channel proteins and are present as diverse forms in plants and animals where they facilitate transport of water and other small molecules. Plant aquaporins have been shown to be differentially regulated under abiotic stresses such as water deficit, high salt and low temperature. The plant aquaporin proteins belong to the large family of major intrinsic protein (MIP) and can be categorized in 5 subfamilies including plasma membrane intrinsic proteins (PIP), tonoplast intrinsic proteins (TIP), NOD26-like intrinsic proteins (NIP), small basic intrinsic proteins (SIP), and the recently discovered X intrinsic protein (XIP). The  purpose of this study was to identify the aquaporin genes in Upland cotton (Gossypium hirsutum L.) using a set of degenerate primers designed based on sequence alignments and conserved domain search from available aquaporin sequences in cotton and other plants. Together with a bioinformatic homology search, about 80 aquaporin genes were isolated from diverse cloning approaches using mRNAs and genomic DNAs as templates, and full-length cDNAs were obtained by 3’ and 5’ RACE (rapid amplification of cDNA ends) PCR. The large number of aquaporin gene sequences suggests that cotton aquaporins are highly divergent compared to other plants. They consist of more than 17 members each in the PIP, TIP and NIP subfamilies while also containing less divergent subfamilies of SIP and XIP with 5 and 2 members, respectively. Structural and functional characteristics were analyzed by sequence comparisons in each subfamily in addition to expression study using semi-quantitative RT-PCR among representative members from each subfamilies. Our results suggest the cotton aquaporin family is structurally diverse while conserving aquaporin specific properties. Many cotton aquaporin transcripts are regulated tissue specifically while others are expressed across tissues.