Aquaporins and their regulation for osmotic adjustment of plants

Cellular transport of water, i.e., essential for cell survival, takes place through a specialized water channel, referred to as an aquaporin (AQP). It falls under the category of the membrane intrinsic protein (MIP) superfamily and possesses several isoforms. AQP functions are directly related to an osmotic adjustment of plants facing different types of stresses. PIPs are the conserved ones among plant AQP subfamilies as found in most of the plant groups. About 30 kDa-sized monomers constitute the tetrameric structure of AQPs. Six transmembrane α-helices constitute the water transport channel where NPA, AEFL, and HW[V/I][F/Y]WXGP are the conserved sequences. Rapid water movement through this water channel occurs as a single-file configuration of water by means of channel selectivity with respective pore size. Specialized alignment of water molecules along with dipole charges plays a significant role in maintaining this water conduction. Basically, water stress is one of the substantial reasons for creating osmotic imbalances. Plasma membrane intrinsic proteins (PIPs) and tonoplast intrinsic proteins (TIPs) are expressed in most of the plants with deficiency of optimum water content for normal physiological activities. Each type of biotic and abiotic stress condition eventually causes cell-damaging osmotic stress. There are different factors like protons (H +), divalent cations, pH status, reactive oxygen species (ROS), hormones, phosphorylation, cellular trafficking, etc., which regulate the expression of AQPs in response to osmotic imbalance by different mechanisms. Presently, study of the role of AQPs in osmoregulation is very important in advanced research. Moreover, a multidirectional investigation is essential to answer questions regarding unrevealed issues in this area of plant physiology.