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.