Abstract
Phosphate (P(i)) plays a key role in plant growth and development. Hence, plants display a range of adaptations to acquire it, including changes in root system architecture (RSA). Whether P(i) triggers directional root growth is unknown. We investigated whether Arabidopsis roots sense P(i) and grow towards it, that is whether they exhibit phosphotropism. While roots did exhibit a clear P(i)-specific directional growth response, it was, however, always to the left, independent of the direction of the P(i) gradient. We discovered that increasing concentrations of KH(2)PO(4), trigger a dose-dependent skewing response, in both primary and lateral roots. This phenomenon is P(i)-specific - other nutrients do not trigger this - and involves the reorganisation of the microtubule cytoskeleton in epidermal cells of the root elongation zone. Higher P(i) levels promote left-handed cell file rotation that results in right-handed, clockwise, root growth and leftward skewing as a result of the helical movement of roots (circumnutation). Our results shed new light on the role of P(i) in root growth, and may provide novel insights for crop breeding to optimise RSA and P-use efficiency.