1 ǀ Introduction
Iron toxicity is a set of severely yield-limiting disorders associated with high concentrations of reduced ferrous iron (Fe(II)) in submerged lowland rice soils (Becker & Asch; 2005; Sahrawat, 2005). It is exclusively a problem of submerged soils, because the low redox potential associated with exclusion of O2 from the soil leads to prevalence of reduced, soluble Fe(II), whereas in well-aerated soils, the dominant form is insoluble ferric Fe(III). It is particularly a problem on highly-weathered, nutrient-depleted soils rich in Fe oxides. These typify much of the current and potential rice area in sub-Saharan Africa, in contrast to the young, fertile, alluvial soils of the Asian lowlands, where most rice research has been done. Hence iron toxicity has not been a priority in much of the international rice breeding effort. However, it is a major constraint to rice in sub-Saharan Africa. Estimates of the rice area in sub-Saharan Africa affected by Fe toxicity vary from 20 to 60%, and estimated yield losses vary from 10 to 90% (Rodenburg et al., 2014; Sikirou et al., 2015). Ironically, most of the global hotspots of Fe toxicity in rice overlap with areas where Fe deficiency in human diets is acute, suggesting that currently-grown rice varieties are not effective in translocating excessively-available Fe from the soil into the grain (Frei et al., 2016).
There is large variation in tolerance of Fe toxicity in the rice germplasm, especially in the O. glaberrima species indigenous to West Africa (Sikirou et al., 2015), and in sub-species of O. sativa indigenous to Madagascar (Rakotoson et al., 2019). Modern high-yielding varieties are far more susceptible than locally-adapted but low-yielding traditional varieties. If tolerance traits in the indigenous African germplasm could be incorporated into improved varieties, this could have a huge impact on African rice productivity and the sustainable expansion of rice-based farming into new areas, and hence on overall African food security. However progress with breeding has been slow. Constraints include the complexity of the phenomenon, poor understanding of tolerance mechanisms, and a lack of reliable genetic markers for marker-assisted selection. The importance of particular mechanisms varies with the type of Fe toxicity, and there are multiple types and interactions with nutrient deficiencies.
Three distinct types of Fe toxic soil are recognised (Becker & Asch; 2005; Sahrawat, 2005):
  1. acid sulphate soils in coastal plains and river deltas, in which there is also extreme acidity and Al toxicity;
  2. clayey organic soils in swampy highland areas, in which the toxicity becomes acute later in the season as strongly reducing conditions develop, and so it tends to be less destructive; and
  3. poor sandy to coarse-loamy soils in inland valleys, where there is upwelling of interflow water from adjacent highlands with highly-weathered soils, and the toxicity lasts throughout the growing season.
As a result of sensitivity to the local hydrology, there is often large field-scale heterogeneity in toxicity and dependence on inter-annual variability in rainfall. Further, there are often also deficiencies of mineral nutrients, particularly P, K, Ca and Mg (Figure 1). These both compound the Fe toxicity and are exacerbated by it. Hence symptoms occur at widely differing Fe concentrations in the plant, associated with different soil types and landscape positions, and interactions with hydrology and nutrient levels.
Germplasm screening is complicated by large genotype by environment effects linked to these multiple interactions. Plant adaptations to the stress depend on complex below-ground plant-soil interactions. Hence yield losses are only weakly correlated with above-ground symptoms, though these are widely used for screening (Sikirou et al 2015). Most work on tolerance mechanisms has been done in hydroponics, far removed from field reality, and there has been limited progress with the genetics of tolerance and gene mapping (Dufey et al., 2015; Matthus et al., 2015; Melandri et al., 2021; Pawar et al., 2021; Sikirou et al., 2018). There is a need for an integrated approach to understand the mechanisms and genetics of Fe toxicity tolerance, taking account of the complex below-ground plant-soil interactions. In this review, we focus on these below-ground processes, and interactions between genotype adaptations and mineral nutrient deficiencies. Above-ground tolerance mechanisms have recently been reviewed by Aung & Masuda (2020) and Wu, Ueda, Lai & Frei (2017).