Genome-wide identification and characterization of sweet orange (Citrus sinensis) aquaporin genes and their expression in two citrus cultivars differing in drought tolerance

Aquaporins (AQPs), which belong to a highly conserved superfamily of major intrinsic proteins (MIPs), play key roles in regulation of movement of water and other small molecules across membranes. However, information concerning the AQP gene family in citrus is limited. Here, we conducted a genome-wide search for the homologs of AQPs in sweet orange and identified 34 full-length AQP genes (CsAQPs) that were located on all nine chromosomes. Phylogenetic analysis revealed that the CsAQPs could be classified into five subfamilies, including 11 plasma membrane intrinsic proteins (PIPs), nine tonoplast intrinsic proteins (TIPs), eight NOD26-like intrinsic proteins (NIPs), three small basic intrinsic proteins (SIPs), and three X intrinsic proteins (XIPs). Gene structure was generally conserved within each subfamily, with intron numbers ranging from zero to four. Functional prediction based on the analysis of the NPA motifs, aromatic/arginine (ar/R) selectivity filter, Froger’s positions, and specificity-determining positions (SDPs) revealed remarkable differences in substrate specificity among subfamilies. Furthermore, analysis of the transcription profile of CsAQP genes in the roots and leaves of drought-tolerant (HJ) and drought-sensitive (HH) cultivars under drought treatment revealed that most CsPIPs and CsTIPs were down-regulated in roots of both treated cultivars. In addition, the down-regulation of CsPIP1;2, CsTIP3;2, and CsNIP2;1 in roots and up-regulation of CsNIP1;1, CsNIP1;3, CsNIP4;1, and CsNIP5;1 in leaves revealed obvious differences between tolerant and sensitive cultivars during drought. Collectively, these findings provide valuable knowledge that furthers our understanding of the potential biological functions of AQP genes in drought tolerance of citrus.