J. Indian bot. Soc. e-ISSN:2455-7218, ISSN:0019 - 4468 www.indianbotsoc.org AQUAPORINS IN PLANTS - EXPRESSION AND REGULATION UNDER STRESS * ARCHANA KUMARI, ADITI TAILOR AND SATISH C BHATLA Laboratory of Plant Physiology and Biochemistry, Department of Botany, University of Delhi, Delhi-11007, India * Email: bhatlasc@gmail.com DOI:10.5958/2455-7218.2020.00049.2 Plant cells are capable of adjusting their cellular water status in response to various stimuli by altering osmotic and water potential which may be achieved by regulation of transport of ions and accumulation of osmolytes inside the cell, and consequential influx or efflux of water. Transmembrane transport of water is facilitated by highly specialized water channels, categorized as 'aquaporins' (AQP) which allow rapid fluxes of water across the membranes to facilitate events, such as cell elongation and stomatal movement, and also maintain overall plant water relations. The discovery of AQPs dates back to early 1990s when first ever water permeable protein, CHIP28, was characterized as an integral protein spanning the membranes of human erythrocytes (Preston et al. 1993). CHIP28 was later renamed as AQP1 and was placed in the family 'Major Intrinsic Proteins' as it formed a major constituent of total membrane proteins, along with other aquaporins that share substantial homology with each other. The first MIP reported in plants was a 26.5 kDa transmembrane protein, Nodulin-26 (NOD26), isolated from the Plants continuously adjust their water status in response to external cues, which requires rapid and efficient transmembrane water transport, a feat which is made possible by highly specialized channels, collectively referred to as 'aquaporins'. Aquaporins (AQPs), belonging to the Major Intrinsic Protein superfamily of membrane-associated proteins, are located on cellular as well as various subcellular membranes, where they conduct not only water but also other substrates, such as urea, glycerol, boric acid, arsenite, CO , 2 NO and H O . AQP isoforms assume a conserved 'hourglass' structure, with two highly specialized constriction regions, the ar/A 2 2 selectivity filter, and the two NPA (Asparagine-Proline-Alanine) motifs, serving as primary determinants of their substrate selectivity and functional identity. Regulation of AQPs, both at expression and activity levels, is crucial for physiological and developmental processes, like cellular expansion and turgor-dependent movements of stomata and other organs, and comprises an important tolerance mechanism employed by plants to combat abiotic stresses, particularly those that impose osmotic stress, as well as biotic stresses. Overexpression of certain AQP genes (PIPs and TIPs) in different plant systems is beneficial for improving their tolerance to drought, salinity and low temperature stress. AQP isoforms, particularly belonging to TIP and NIP subfamilies, have been implicated in heavy metal toxicity due to their ability to conduct heavy metals like arsenic and antimony. AQPs are also involved in defense against biotic stress factors, including pathogen attack and herbivory, through deployment of H O as defense response, which may 2 2 also activate further immunity responses. TIPs and NIPs play significant roles in establishing symbiotic relationships via facilitation of bidirectional exchange of nutrients and water between host and symbiont. An interplay between ABA levels and changes in expression and activity of AQP isoforms has been established to be crucial for stress tolerance, which may involve H O , an important 2 2 player in ABA-mediated responses, most important being regulation of stomatal movements. Therefore, a complex mechanism of AQP regulation exists in plants which may be elicited in response to various stress conditions. This review attempts to summarize the function, regulation and modulation of different aquaporin isoforms under abiotic stress. Keywords: Abiotic stress, aquaporin, MIPs, water channel AQPs- Aquaporins Sp, Issue Vol. 100 (A) 2020:123-144 13 ER - Endoplasmic Reticulum 14 Cys - cysteine 15 ABA - Abscisic Acid 16 PEG - PolyEthylene Glycol 17 GFP - Green Fluorescent Protein 18 Si - Silicone 19 BL - Brassinolide 20 AM - Arbuscular Mycorrhiza 21 PTI - Pattern Triggered Immunity 22 SAR - Systemic Acquired Resistance 23 SOS - Salt Overly Sensitive 24 CMV - Cucumber Mosaic Virus 25 PM - Plasma Membrane 1 Abbreviations AQPs - Aquaporins 2 CHIP - Channel-forming Integral Protein of 28 kDa 3 MIPs - Major Intrinsic Proteins 4 TIP - Tonoplast membrane Intrinsic Protein 5 NPA - Asparagine-Proine-Alanine 6 NIP - Nodulin 26-like Intrinsic Proteins 7 PIP - Plasma membrane Intrinsic Protein 8 SIP - Small basic Intrinsic Protein 9 XIP - X Intrinsic Protein 10 GIP - GLP-like Intrinsic Protein 11 HIP - Hybrid Intrinsic Protein 12 LIP - Large Intrinsic Proteins