SUCCESS STORIES FROM THE FIELD
For all of the above scenarios there is promising evidence from lab
studies. Unfortunately, the latter do often not translate to successes
in the field for a plethora of reasons. To assess the efficacy of
various approaches we searched the literature to identify work that
combines efforts to improve metal(loid) tolerance with crop field
trials, for As, Cd, Hg and Zn. Table 1 shows that in general there are
very few studies that have evaluated the perceived benefits of lab-based
work in the field. Nevertheless, genetic approaches have resulted in
some highly successful outcomes: For example, the elegant work by Tang
et al. (Tang et al., 2017) used sophisticated CRISPR/Cas genome editing
to generate a dysfunctional rice NRAMP5, a plasma membrane transporter
that plays an important role in the acquisition of the micronutrient Mn
but also contributes to Cd uptake (Ishimaru et al., 2012). Loss of
function in NRAMP5 lowered root and shoot levels of Cd (Figure 4A) and
led to a 5 to 30 times reduction in grain Cd levels without a
significant yield penalty (an important parameter that is routinely
ignored in lab-based studies). The use of CRISPR/Cas allows generation
of genetically modified crops that do not contain foreign DNA, thus
circumventing problems of public acceptance. Root levels of other
essential metals such as Fe, Zn and Cu were not affected by the altered
NRAMP activity but the critical role of OsNRAMP5 in Mn nutrition
(Ishimaru et al., 2012) means this approach is predicated on sufficient
Mn supply. Trials on transgenic rice that overexpressed the sameNRAMP5 showed, as expected, increased root Cd and Mn levels
(Figure 4B). But the use of strong promoters in this study (OsActin1 and
Ubiquitin), altered the expression pattern of NRAMP5, thereby causing a
greatly reduced radial transport of Cd toward the xylem, and
surprisingly, this approach also caused a drastic decrease in grain Cd
albeit without compromising Mn supply (Chang et al., 2020).
Results from rhizosphere-based approaches are more of a mixed bag; a
long term field trial with mycorrhizas revealed no beneficial effects
(Gao et al., 2010). Soil amendment with a bacterial mixture (no details
are given) lowered bioavailability of Cd and Pb in soil but
surprisingly, the amendment increased bioavailability of As (Nong et
al., 2020). The treatment did not affect plant growth or yield as such
but did significantly reduce the level of all three metal(loid)s in the
rice grain. The endophytic fungus Piriformospora indica, a
regular tobacco symbiont that resides in the roots, caused more
efficient storage of Cd in the roots of colonised plants and, possibly
by upregulating genes involved in oxidative stress response, improved
tobacco Cd tolerance (Hui et al., 2015).
We could only detect one field study where root architecture was
assessed in the context of metal(loid) tolerance (Wu et al., 2011): This
report showed that rice genotypes with a relatively high proportion of
aerenchyma in their roots (i.e. a high ‘porosity’) show larger radial
oxygen loss and lower As contents in the grain. Maybe the extra oxygen
affects the AsIII: AsV ratio but the underlying mechanism remains to be
explained.