Phosphorus speciation and high-affinity transporters are influenced by humic substances

Phosphorus (P) is a limiting factor for plant growth, especially in highly weathered tropical soils. Plants have several mechanisms to overcome low P availability in soil, such as humic substances, that reduce phosphate (Pi) adsorption on oxide surfaces and enhance soil P availability. However, the direct influence of humic substances on Pi transporters in root cells or the distribution of P species in leaves remains unclear. Tomato seedlings were grown in a sand–vermiculite mixture with low or high P concentrations (10 or 100 mg kg–1 KH2PO4, respectively) and humic acids (0 or 48 mg C L–1) isolated from vermicompost. Plant responses were evaluated in the fifth week by measuring root and shoot weights and P concentration, and differential expression in the roots of the high-affinity Pi transporter genes LePT1 and LePT2. In addition, the distribution of P species in the leaves was assessed using 31P-NMR. Humic acids increased the root biomass and changed the distribution of P species in the leaves. Inorganic phosphate was the major compound in plants supplied with a high P concentration, whereas in plants supplied with a low P concentration, Pi was only identified in plants not treated with humic acids. Glycerophosphodiester and phosphorylcholine accumulated in plants treated with humic acid, indicating a modified metabolic pathway for economical P consumption at low P concentrations. High transcript accumulation of LePT2 was observed in roots treated with humic acids at both P concentrations. Our results show that humic substances are strategically involved in plant adaptation to P availability.