6 .1 ǀ Tolerance within the Oryza gene pool
Numerous studies have screened sets of varieties or larger germplasm collections in attempts to identify donors for Fe toxicity, typically employing one of two main screening strategies. They may have been conducted in the field in Fe toxic hotspots, evaluating grain yield in addition to biomass and leaf symptoms (Melandri et al., 2021; Pawar et al. 2021; Sikirou et al. 2018), or they were designed as rapid screens in nutrient solution to which excess Fe had been added at high concentrations for short periods of time (Dufey et al. 2015; Matthus et al. 2015). From these and other studies it can be concluded that ample genetic variation for tolerance to Fe toxicity exists within theO. sativa gene pool (Matthus et al. 2015; Pawar et al. 2021), and that wild relatives may serve as sources of novel traits enhancing adaptation to Fe toxicity (Bierschenk et al. 2020; Wairich et al., 2021).
The indigenous African rice O. glaberrima , domesticated from its wild ancestor O. barthii about 3500 years ago (Heuer et al., 2003), includes accessions tolerant of a wide range of abiotic stresses, including Fe toxicity (Linares et al., 2002; Melandri et al., 2021; Sié et al., 2012; Sikirou et al., 2018). Sikirou et al. (2018) evaluated more than 2000 O. glaberrima accessions in the Africa Rice Center gene bank at multiple Fe toxic hotspots across West Africa, and identified highly tolerant accessions such as TOG 7250-A, TOG 14367, TOG 7206 and TOG 6218-B. Melandri et al. (2021) confirmed the tolerance of some of these accessions. This suggests transferring genes/alleles conferring Fe toxicity tolerance from O. glaberrima to high yielding O. sativa varieties as a promising breeding strategy. However, spikelet sterility in progenies of O. glaberrima andO. sativa interspecific crosses has been a major challenge for breeders (Ndjiondjop et al., 2018; Sié et al., 2012).