Genome-wide analysis of rubber tree (Hevea brasiliensis Muell. Arg.) aquaporin genes reveals a crucial role of the PIP subfamily in the water balance of laticifers

Abstract

The movement of water is a matter of great importance to all plant species. This is particularly true for the rubber tree ( Hevea brasiliensis Muell. Arg.) which is tapped for the laticifer cytoplasm in the form of aqueous latex. Due to the lack of plasmodesmata, the laticifer water balance is mediated largely by aquaporins (AQPs), a class of channel-forming integral membrane proteins that transport water and other small solutes. Our studies reported the characterization of 51 full-length AQP genes from the rubber tree genome, and further assigned them to five subfamilies, i.e. plasma membrane intrinsic proteins (PIPs: 15), tonoplast intrinsic proteins (TIPs: 17), NOD26-like intrinsic proteins (NIPs: 9), small basic intrinsic proteins (SIPs: 4) and X intrinsic proteins (XIPs: 6). Although the classification of subfamily/subgroup is the same as in Ricinus communis , genome-wide comparative analysis revealed the PIP and TIP subfamilies in rubber tree are highly expanded, corresponding to their high water permeability and the particular importance of water balance in a big tree and a highly differentiated laticifer tissue. Functional prediction based on the analysis of the aromatic/arginine (ar/R) selectivity filter, Froger’s positions and specificity-determining positions (SDPs) further suggested the potentially key role of HbPIPs and HbTIPs in the laticifer water balance. Moreover, deep sequencing of the latex transcriptome and qRT-PCR analysis supported a crucial role of several PIP subfamily members in the laticifer water balance under both normal and ethephon-stimulated conditions.