Targeting Aquaporins for Conferring Salinity Tolerance in Crops

Abstract

Salinity is one of the well-known abiotic stresses which affects crop productivity through imposing ion imbalance and disrupting the metabolic pathways. Soil salinity is dramatically increasing throughout the world because of climate change, rise in sea levels, excessive irrigation, and natural leaching process. To overcome this problem, many approaches were reported including selection of natural salt-tolerant variety, breeding program, and genetic-engineered plants. Membrane intrinsic proteins (MIPs; also called aquaporins) are membrane channel proteins initially discovered as water channels, but their roles in the transport of small neutral solutes, metal ions, and gasses are now well established. Based on homology and subcellular localization, plant MIPs are divided into four major subfamilies: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), NOD26-like intrinsic proteins (NIPs), and small basic intrinsic proteins (SIPs). Besides these four subfamilies, some unique subfamilies were reported such as GlpF-like intrinsic proteins (GIPs), hybrid intrinsic proteins (HIPs), and the uncategorized X intrinsic proteins (XIPs). In plants, MIPs are involved in diverse physiological roles such as seed germination; fruit ripening; leaf, petal, and stomata movement; phloem loading and unloading; reproduction; and stress response. However, a large number of studies have suggested the involvement of MIPs in various abiotic stresses, including drought, salt, and cold stress. PIPs and TIPs have shown differential regulation pattern in roots and shoots of Arabidopsis, barley, and maize in salinity stress. Moreover, overexpression studies of various PIPs and TIPs in plant suggest their possible role in salt tolerance. Transcriptome analyses of citrus under salt stress showed that in addition to PIPs and TIPs, most of the NIPs, XIPs, and SIPs were differentially regulated in root tissues. In the present chapter, we discussed roles of plant aquaporins in salinity stress and exploitating the same for genetic engineering approach.