Aquaporins: Channels for the Molecule of Life

Abstract

Liquid water has unique properties that make it a universal solvent. Water is an essential component of almost all physiological and biochemical reactions; therefore, its presence is required everywhere within an organism. The circulatory systems of animals and the vascular system of plants move fluids over long distances. In the tallest trees, water moves across a height gradient of 100 m or more, thereby connecting roots and distant leaves. On a cellular and tissue level, water movement is facilitated by intrinsic membrane proteins called aquaporins. These water channels are found in all life forms. Aquaporins have been mostly studied in mammals and plants, where water channels play important physiological roles. This article gives an overview of the discovery, structure and regulation of aquaporins. Their roles in different life forms are discussed. Key Concepts Water can diffuse through the lipid bilayer, but without aquaporins flow across membranes is slow. Aquaporins are integral membrane transport proteins, and they facilitate water movement in cells, tissues and entire organisms. A single water channel allows the passive passage of more than one billion water molecules per second. The aquaporin protein family is ancient, and aquaporins can be found in a wide range of organisms. Aquaporins appear to play important physiological roles in mammals, plants and other organisms.

Plant xylem hydraulics: What we understand, current research, and future challenges

“…Here we focus on the physiological roles of PIPs located in or near vascular tissue. PIP water channels are prominent features of both xylem and phloem (Hacke and Laur ). This suggests that PIPs play important roles associated with long‐distance transport (Figure ).…”

DGE-seq analysis of MUR3-related Arabidopsis mutants provides insight into how dysfunctional xyloglucan affects cell elongation

“…This process requires an internal turgor pressure, which is generated by water uptake into the cell [62][63][64]. To maintain turgor during cell wall expansion, water movement through cell membranes is enabled by water channels referred to as aquaporins [65]. Aquaporins are integral membrane proteins that form pores which facilitate water transport at a rate higher than diffusion.…”

An ignored anatomical variable: pore shape shows a nonrandom variation pattern in xylem cross sections

“…This phenomenon was related to the variation in vessel diameter in trees. With decreasing vessel diameter, the number of vessels per group increased (Zhao ), which means that the circularity of grouped vessels decreased. Even so, the average circularity of all vessels (not only grouped vessels, but also solitary vessels) significantly increased (p < 0.01).…”

“…It should be noted that the hydraulic diameter of all vessels in B. platyphylla was published in our previous paper (Zhao 2015), as well as the diameter of solitary pores. The mean number of vessels per vessel group was counted and published by Zhao ().…”

“…Vessels constitute the xylem pipelines for long‐distance water transport in land plants, and their anatomical characteristics have a profound impact on the hydraulic conductivity of the xylem (Bolton and Robson , Longui et al 2017). Major restrictions to the hydraulic conductance of vessels in trees have traditionally been attributed to vessel size, vessel frequency, vessel grouping, vessel wall surface properties, perforation types of vessel and bordered pits structure (Nardini et al , Klepsch et al ). Unfortunately, the vessel pore shape, an important variable, has been ignored in previous studies (Pickard ).…”