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).