​Researchers from CEA, CNRS and the Leibniz Institute of Plant Biochemistry in Halle, Germany, have discovered a mechanism activated by phosphate starvation, that inhibits the plant primary root growth. Plants use their roots to extract from soil the nutrients required for their growth. Phosphorus is an essential element for life, but phosphate, its soluble form that plants can absorb, is often present in small quantities in the soil. The researchers identified a mechanism that enables the plant to slow down the growth of its primary root when it encounters a low-phosphate zone, while stimulating the growth of its lateral roots. This preferential exploration of the soil upper layers, where phosphate is concentrated, is particularly developed in plants adapted to poor soils.

According to this study, when the primary root tip encounters a low-phosphate environment, it stimulates the production of a protein allowing cells to release malate in the extracellular environment. Malate is a small organic acid that binds to metal ions such as iron. The association of malate with iron quickly triggers a reaction, via a ferroxidase[1], that inhibits both the elongation and division of root cells (Figure 1).

This discovery could help to select plants adapted to low-phosphate soils, useful for a more sustainable agriculture. Furthermore, organic acids as malate, promote solubilization of metals such as cesium and uranyl - a uranium oxide – in soils as well as their absorption by plants. Stimulating this root mechanism could thus be part of a new strategy for improving the extraction of these metallic pollutants by plants.

Model of root growth arrest in low-phosphate conditions

Phosphate deficiency stimulates the entry of the transcriptional regulator (in green) into the cell nucleus, where it activates the expression of the malate exporter (in blue). In conjunction with malate, iron inhibits the longitudinal expansion of the cell walls, thus preventing root cell elongation.

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