Mechanisms and genetics of iron toxicity tolerance in African rice

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Rice is a major staple food across sub-Saharan Africa (SSA) and demand for rice is increasing rapidly with changes in consumer preferences and urbanization. However, domestic production currently accounts for only about 60% of consumption; imports to SSA amount to a third of the global rice trade. Accordingly there are various national and international initiatives with ambitious plans for increasing production across SSA. There are ample agro-climatically suitable wetlands in inland valleys and elsewhere to support a large intensification and expansion of rice area. During the rainy season, rice is the only possible crop on low-lying wetlands, so it does not compete with other crops for land or water. However, besides agricultural production, i.e. mainly rice-based systems including fish, vegetable, fruit and livestock production, inland valleys also provide local communities with forest, forage, hunting and fishing resources and they are important for water buffering and as biodiversity hot spots. Development for agricultural production must be done in such a way as to avoid compromising these locally- and regionally-important ecosystem services. This will require improved rice germplasm and nutrient and water management suited to these systems, so as to minimise the land area required. It will also require technologies for assessing which areas across SSA are best suited to intensification and expansion of rice-based farming, without compromising other ecosystem goods and services. It is estimated that with realistic improvements in germplasm and management, less than 10% of the total inland valley area in SSA would be sufficient to meet the total demand for rice in Africa. But various biophysical constraints need to be overcome; one of the most important is the soil health problem iron (Fe) toxicity.Iron toxicity is a set of severely yield-limiting disorders associated with high concentrations of reduced ferrous iron (Fe(II)) in flooded paddy soils. It is exclusively a problem of paddy rice, linked to the biogeochemistry of flooded, anaerobic soil. It is a particular problem in African rice systems because of the nature of the soils, which are highly weathered, nutrient-depleted and rich in Fe oxides, in contrast to the young alluvial rice soils of the Asian lowlands. It affects a large part of the existing and potential rice area in SSA (estimates vary from 20-60% of the area) and causes large yield losses (up to 90%). There are currently efforts to exploit tolerance of it in the indigenous African rice germplasm in breeding and management programmes at AfricaRice and elsewhere. But this is constrained by the complexity of the disorder and by poor understanding of the underlying mechanisms and genetics of tolerance, which reflects its relative unimportance in Asian rice systems where most rice research has been focused. We propose to provide the bioscience and tools required to (a) elucidate the mechanisms and genetics of tolerance to Fe toxicity in indigenous African germplasm, in support of rice breeding and management programmes, and (b) assess the potential of improved germplasm and management to raise the productivity of existing and new rice-based farming systems across SSA. We will especially focus on Oryza glaberrima ('African' rice) species, indigenous to W Africa, and sub-species of Oryza sativa ('Asian' rice) indigenous to Madagascar. We will use a combination of soil chemistry, plant physiology and molecular genetics approaches with field work in W Africa and Madagascar, supported by controlled-environment and laboratory work in the UK. We will also map the spatial extent of different types of Fe toxicity in existing and potential rice areas across SSA, and we will develop GIS tools for assessing the potential for improved germplasm and nutrient and water management to raise the productivity of rice-based farming systems in these areas.