Phosphate promotes Arabidopsis root skewing and circumnutation through reorganisation of the microtubule cytoskeleton.

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 KH₂PO₄, 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.