Phosphorus: Back to The Roots

Phosphorus (P) is a pivotal nutrient for all life on Earth. It is poorly mobile in soil and inorganic P concentrations in the soil solution are <0.6 to 11 μM. Organic P concentrations in the soil solution tend to be higher, but organic P needs to be hydrolysed before it can be taken up by plant roots. Such hydrolysis involves phosphatases that are either released from the roots or derived from microorganisms in the rhizosphere. A large fraction of soil P is sorbed onto soil particles, and hence is unavailable to most plants. Roots that release large amounts of Psolubilising carboxylates can access some of this sorbed P. Rates of P uptake from the soil solution are determined predominantly by the movement of P in soil. Root traits that enhance P movement in soil increase P acquisition; however, the kinetic properties of P transporters that take up this P have little effect on net P uptake. The downregulation of genes encoding these transporters is important to avoid P toxicity at a high P supply. Species or genotypes that lack the capacity to downregulate their P-uptake capacity typically show P-toxicity symptoms at a high P supply. Mycorrhizal symbionts increase the soil volume that is available for P acquisition. Attempts to select or engineer genotypes with greater P-uptake capacities should consider both root and soil characteristics, including soil microorganisms such as mycorrhizal fungi.