Zinc Transporter: Mechanism for Improving Zn Availability

Abstract

Zinc (Zn) is essentially required by plants for their growth and development. It plays very important role in various physiological procedures of plants such as photosynthesis, membrane integrity, protein synthesis, pollen formation, and immunity system. Although Zn is required by the plant in microconcentration, its bioavailable fraction in the soil is very low due to various soil factors. From soil solution it is absorbed by plants by root membrane transport mechanisms. After entering into plant system, it is neither oxidized nor reduced; but remains as divalent cation which has a great tendency to form tetrahedral complexes. From soil solution Zn reaches the plant root surface by three mechanisms, i.e., mass flow, diffusion, and root interception. Once it is absorbed, its transportation from roots to shoots occurs through the xylem and then easily retranslocated by phloem. This transport of ions and molecules from epidermal and cortical cell to xylem occurs through the symplastic or apoplastic route. The uptake of zinc into cells and its permeability into and out of intracellular organelles require some of the specific chemicals, generally known as transporter proteins. These proteins possess a quality to span the cell membranes which facilitate the movement of zinc. In recent years, a number of metal transporters have been identified in plants, including the P1B-ATPase family, zinc-regulated transporter (ZRT), iron-regulated transporter (IRT)-like protein (ZIP), natural resistance-associated macrophage protein (NRAMP) family, and cation diffusion facilitator (CDF) family. The bioavailable content of Zn in the soil can be increased using both chemical and biological approaches. Mineral fertilizers are considered a good source of Zn, but it gets fixed quickly on soil matrix, resulting in poor availability to plants. It is crucial to increase bioavailability of Zn to plants by solubilizing fixed Zn and/or by reducing fixation of the applied Zn fertilizers. This can be achieved either by using organic amendments or potential Zn solubilizing bioinoculants. Organic amendments improve bioavailability of Zn by increasing microbial biomass, which not only enhance the rate of decomposition of organic matter (source of Zn) but also enhance the bioavailability of indigenous Zn by lowering the soil pH and by releasing chelating agents. Similarly, exogenous application of some potential Zn solubilizing microflora has shown huge capability to improve bioavailable Zn content in the soil and its uptake by plant roots. This manuscript critically reviews about the Zn transporters and the role of rhizosphere microflora as a potential tool in enhancing its bioavailability to higher plants.