Current Research

Functional redundancy of Aquaporins during Cold acclimation and Freeze tolerance

Water plays an important role in regulating the growth and developments of plants, both at optimal and adverse environmental conditions. The uptake of water and its flow across the cell membranes is a fundamental requirement for plant growth, both at cellular and whole plant. Response to cold and adaptation to the freezing condition is a complex process that involves complex interactions of many gene products. The chilling-tolerant plants, e.g., Arabidopsis thaliana, have mechanisms to avoid and/or minimize cold stress. In the last two decades aquaporin proteins have been revealed to be the major facilitators of water transport across the membrane, which resulted in a paradigm shift in understanding the plant water relations. This also facilitates our understanding on their role in plant development.

Several recent studies elegantly demonstrated that the decrease in root hydraulic conductivity (Lpr) under cold acclimation is linked to the function of aquaporins. In many of these studies, it has been shown that aquaporins are transcriptionally down regulated during whole plant chilling treatment. For instance, a reduction in water uptake has been observed in rice under cold stress with a down regulation of aquaporin expression. In order to fight the cold stress, plants acclimate themselves in the adverse environment. Acclimation mechanism plays a vital role in enhancing the cold resistance of plant. Also, in freezing-resistant plants, the adaptation is achieved through cold acclimation, where plants are treated at slightly above freezing temperature. In these circumstances, plant water uptake mechanism and cell membrane water permeability are tightly regulated by aquaporins. In higher plants, based on subcellular localization and sequence similarity, aquaporins are classified into five clades: plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin26-like intrinsic proteins (NIPs), and small and basic intrinsic proteins (SIPs). Recently, a fifth uncharacterized subfamily (XIPs) has also been. These transmembrane aquaporins response to various abiotic stresses, and has been shown to enhance the water uptake during long term LRT (low root temperature) treatment. In plants, PIPs are divided into two sequence-related groups, PIP1 and PIP2, the members of which exhibit different water channel activities when expressed in Xenopus oocytes. The conserved structure of the aquaporins as the membrane channels facilitate the transport of water and/or small neutral solutes (urea, boric acid, and silicic acid) or gases (ammonia and carbon dioxide). The permeability of membranes of plants to water is regulated by both direct interactions and phosphorylation of aquaporin isoforms and this harmonized ambience is hindered during the abiotic stresses, including cold and salt stress. However, the high isoform multiplicity of plant aquaporins, with, for instance, 35 homologs in Arabidopsis, has rendered the significance of individual gene regulation events difficult to assess in terms of water transport regulation.

November 2020