Zinc
Zinc is an essential micronutrient for plant growth and development. It acts as a cofactor of more than 300 proteins which primarily are zinc-finger proteins, DNA polymerases and RNA polymerases. In these enzymes Zn regulates catalytic activity, conformational stability, protein folding and protein interactions (particularly with nucleic acids) (Kambe et al., 2015). Remarkable is also the function of Zn in the stabilisation of biomembranes and its crucial role in lipid and nucleic acid synthesis. However, in higher concentrations Zn can become toxic for plants, harming growth, development and a host of essential functions. In A. thaliana, treatment with concentrations above 0.1 mM are toxic (Marschner, 2012, Jain et al., 2013).
Processes in the rhizosphere related to Zn toxicity At the root, Zn is taken up predominantly as a divalent cation (Zn2+). However, it can also bind to organic ligands and enter the root as a chelation complex. Depending on the ligand, this can occur through lowering the pH by extrusion of H+and/or organic acids to enhance the solubility of Zn-complexes such as Zn phosphates to release Zn2+. Subsequently Zn2+ can then be absorbed by the epidermal cells of the roots. The second strategy is based on exudation of phytosiderophores. Zn2+ uptake from the soil solution is driven by the negative membrane potential of root cells. Mycorrhizas also contribute to Zn uptake in plants. As much as 24% of the shoot Zn in wheat and tomato and 12% in barley are provided through the arbuscular mycorrhizal fungi (Coccina et al., 2019, Watts-Williams et al., 2015).
Zinc uptake and distribution Zn homeostasis in plantaconsists of a complex of cellular functions such as uptake, efflux, accumulation, sequestration, remobilization and detoxification (Figure 3). Membrane transporters that catalyse the movement of Zn2+ mostly belong to the ZIP family. ZIPs are expressed in multiple membranes and tissues including roots, leaves, nodules and flowers and typically transport Zn2+ from the extracellular space to the root symplast or from organellar lumens into the cytoplasm (López-Millán et al., 2004, Krishna et al., 2020). ZIP proteins have a histidine-rich domain, which might be involved in metal binding and transport regulation (Jeong and Eide, 2013, Zhang et al., 2019).
Expression of ZIPs and other transporters is strictly regulated in order to provide the necessary quantity of Zn into all cell types and at all stages of development. Several transcription factors involved in Zn homeostasis belong to the F group of the basic-region leucine zipper (bZIP) factors and have histidine-rich motifs at their N-terminal region capable of Zn binding. Thus, bZIPs act as Zn sensors that are crucial in maintaining adequate Zn acquisition (Krishna et al., 2020, Lilay et al., 2021).
In addition to ZIPs, Zn uptake can occur in the form of Zn phytosiderophores, mediated by members of the YSL transporter family (von Wiren et al., 1996, Erenoglu et al., 2000, Suzuki et al., 2006).
Translocation of Zn from roots to the above ground organs via the xylem is catalysed by members of P1b-type ATPases, localized to the plasma membrane cells in the root pericycle. HMAs such as AtHMA2 and AtHMA4 actively upload Zn and Cd into the xylem (e.g. (Verret et al., 2004, Hanikenne et al., 2008, Migeon et al., 2010, Wong et al., 2009).
The tonoplast HMAs are involved in the vacuolar accumulation of metals such as Cd and Zn (Morel et al., 2009) whilst remobilisation of Zn from the vacuoles is carried out by ZIP and NRAMP transporters. For instance, the tonoplast localised AtZIP1 releases Zn and Mn from the vacuolar lumen, presumably for subsequent long distance transport in the xylem (Milner et al., 2013).
Cellular detoxification of Zn To avoid the damaging effects of metal toxicity a fine balance of processes is necessary to maintain cytosolic metal concentration. A significant part of that is the use of vacuoles as a storage buffer. This is an important characteristic of the root cells for they are able to prevent translocation of toxic ions to the shoot. The loading of Zn2+ into vacuoles is facilitated by members of the Cation Diffusion Facilitator Family known in plants as MTPs that are located in the tonoplast (Arrivault et al., 2006, Gustin et al., 2009, Tanaka et al., 2013). Within the cytosol the exchangeable Zn2+ fraction is rapidly bound to ligands such as nicotianamine (NA), histidine, glutathione, phytochelatin or phosphate (Clemens, 2019). Conjugated and liganded Zn2+ is subsequently moved into the vacuole by ABC transporters and vacuolar sequestration in this manner is an important factor for retaining most Zn in the root. However, vacuolar sequestration of Zn2+ (but also of Cd2+, Co2+ and Pb2+) is further achieved through HMA ATPases such as HMA3 in Arabidopsis thaliana (Morel et al., 2009), while excess Zn in this species leads to upregulation of the Zinc-Induced Facilitator 1 (AtZIF1), a vacuolar Zn-NA importer into the vacuole (Haydon and Cobbett, 2007).