The View From the Bottom: Root Traits for Breeding P Efficient Crops.

Annual crops show substantial genotypic variation for P acquisition, caused primarily by variation in root traits that determine soil exploration and exploitation. Phosphorus availability is typically greatest in surface soil and therefore traits that enhance topsoil foraging are particularly important for P acquisition, including adventitious rooting, shallow root growth angles, and lateral branching. A second set of traits enhance rhizosphere exploitation, including the exudation of protons, carboxylates, and phosphatases, and root hair length and density. A third set of traits reduce the metabolic costs of soil exploration, including root cortical aerenchyma and root etiolation. Rapid phenotyping methods have been developed for many of these traits suitable for screening large numbers of genotypes. Several of these traits are being deployed in crop breeding programs that have resulted in new cultivars of soybean and common bean with superior yield in low P soils of Africa, Asia, and Latin America. Phosphorus efficient crop genotypes are improving food availability in low input agroecosystems in developing nations, and in rich nations would reduce the environmental and economic costs associated with intensive fertilization. Additional research is needed to understand the costs and benefits associated with specific traits, how they interact to form integrated phenotypes, and the agroecological and socioeconomic effects such phenotypes may have in rural communities.