Root Phenes for Enhanced Soil Exploration and Phosphorus Acquisition: Tools for Future Crops

Plant growth in the vast majority of terrestrial ecosystems is limited by low phosphorus availability.Over 70% of all terrestrial biomass occurs in low-phosphorus soils, including over half of agricultural land (Fig. 1).Phosphorus availability is declining in many systems because of soil degradation, which has affected over half of global agricultural land and 75% of agricultural land in Africa.Intensive phosphorus fertilization is uncommon in the low-input agriculture common in poor nations and has limitations as a long-term strategy because of limited reserves of high-grade phosphate ore deposits, the energy costs of producing fertilizer, and the environmental cost associated with intensive fertilization.The development of crops with greater phosphorus efficiency, defined as the ability to grow and yield in soils with reduced phosphorus availability, would substantially improve food security in developing nations, while enhancing the sustainability of agriculture in rich nations (Lynch, 2007). PHOSPHORUS IS AN IMMOBILE SOIL NUTRIENTPhosphate is highly immobile in soil, because it reacts with many chemical and biological soil constituents.Plant strategies to acquire phosphorus are therefore oriented around two basic themes: (1) soil exploration and (2) mobilization of phosphate from poorly available phosphorus pools in the rhizosphere.This Update focuses on phenes controlling soil exploration by roots, since they are subject to selection in crop breeding programs, and since root deployment is senior to many other root phenes affecting phosphorus acquisition, by determining the placement of root exudates and symbionts in specific soil domains, and thereby their functional benefit.