Plant Aquaporins and Metalloids

The metalloids represent a group of physiologically important elements, some of which are essential or at least beneficial (boron and silicon) for plant growth and some of which are toxic (arsenic, antimony and germanium). Exposure to and availability of metalloids can have major effects on plant fitness and yield and can seriously downgrade the end-use quality of certain crop products. Plants have evolved various membrane transport systems to regulate metalloid transport both at the cellular and whole plant level. To date, the channel proteins referred to as aquaporins (AQPs) represent the most favored candidates ensuring metalloid homeostasis. AQPs are found in all living organisms. From bacteria to mammals and also in plants, several distinct AQP subfamilies facilitate the transmembrane diffusion of the set of physiologically and environmentally important metalloids. A subgroup of the Nodulin26-like intrinsic protein AQP subfamily (NIPs) has been designated as functional metalloidoporins. NIPs are the only known transport protein family in the plant kingdom which are essential for the uptake, translocation, or extrusion of various uncharged metalloid species. This chapter describes the various features, and particularly the metalloid transport properties of plant AQPs, and illustrates their physiologically important contributions to metalloid homeostasis. Their intimate involvement in metalloid transport underlines their relevance to plant nutrition, detoxification of toxic mineral elements phytoremediation, phytomining, and biofortification.