Figure 1: Generic, root and rhizosphere located processes that contribute to plant metal(loid) tolerance.
See text for details.
Figure 2: Overview of root and rhizosphere located response mechanisms to metal(loid) toxicity. 1) The uptake, efflux and long distance transport of metal(loid)s critically influences metal(loid) tolerance. The contributing transport steps, which may be controlled by transcriptional and post- transcriptional mechanisms, determine the partitioning of metal(loid)s within cells and in different organs. 2) Detoxification within the cells often depends on metal(loid) conjugation to thiol rich compounds and subsequent sequestration in the vacuole via specific transporters on the tonoplast. 3) Root architecture adapts to metal(loid) toxicity for example via reduced root growth (‘avoidance response’) or the formation of physical barriers in root tissues by deposition of callose, suberin or lignin. 4) The presence of bacteria and fungi in the rhizosphere impacts on metal(loid) bioavailability via the chelating activity of microbial exudates (PGP compounds). Microbes can also be involved in the bioconversion of metal(loid)s, for example via redox reactions or methylation. 5) Like microbial exudates, plant exudates such as organic acids and phytosiderophores can impact on metal(loid) solubility and bioavailability. Abbreviations: GSH: reduced glutathione; PC: phytochelatins; PGP: plant growth promoting ; TO: tonoplast; PM: plasma membrane; Me: Metal(loid).
Figure 3: Uptake and detoxification of As, Cd, Hg and Zn. The uptake of As, Cd, Hg and Zn is largely catalysed by transporters that reside in the plasma membrane (PM) and derive from various families. Very often, selectivity is not perfect which leads to substrate interactions such as As affecting uptake of Si and phosphate (Pi) and Cd interference with Fe, Zn and Mn nutrition. High affinity thiol groups of reducing compounds such as glutathione (GSH) and phytochelatin (PC) catalyse binding of metal(loid)s for delivery to the vacuolar lumen, neutralising much of the potential toxicity. Transport of the complexed metals into the vacuole (Vac) mainly relies on the activity of ABC transporters and heavy metal ATPases expressed on the tonoplast (TO). Abbreviations: GSH: reduced glutathione; PC: phytochelatins; PGP: plant growth promoting ; TO: tonoplast; PM: plasma membrane; Pi: inorganic phosphate. For abbreviations of specific transporters, see text.
Figure 4: Altering NRAMP expression lowers Cd in rice grain.The physiological function of OsNRAMP5 is in the acquisition of the micronutrient Mn, however, NRAMP5 can also mediate Cd uptake. In wild-type (WT) plants, NRAMP5 is expressed on the distal side of the exo- and endodermis while OsMTP9 is expressed on the opposite site of the cell as NRAMP5. This polar arrangement ensures radial transport of Mn toward the xylem. (A) Field trials show that loss of function in NRAMP5 greatly reduces Cd levels in roots, shoots and grains but it also compromises Mn uptake, making this approach viable only in the presence of sufficient Mn. Other micronutrients were not affected and no yield penalty was observed (based on data from Tang et al, 2017). (B) The use of strong promoters can disrupt the expression tissue patterns and polar arrangement of transporters and thereby disrupt radial transport toward the xylem. Hence,overexpression of NRAMP5, paradoxically, also reduced Cd levels of the grain. In this case, Mn provision to the shoot remained largely intact since MTP9 does not transport Cd (based on data from Chang et al., 2020).