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).