Phosphate and salinity differentially affect root skewing and circumnutation

Phosphorus (P) is essential for plant growth, forming key macromolecules like ATP, phospholipids, and nucleic acids. Inorganic phosphate (Pi) availability in soils is limited due to low solubility and mobility. This thesis investigates how plant roots perceive and respond to Pi availability. We discovered that Arabidopsis roots exhibit a clear Pi-specific directional-growth response, however, always to the left direction independent of the direction of the Pi gradient. This increasing level of Arabidopsis root skewing induced by increasing concentration of Pi was termed "Phosphate Dependent Skewing" (PDS). This response is caused by a right-handed, clockwise, circumnutation of the root tip, which is driven by left-handed, counterclockwise, epidermal cell file rotation (CFR) in the root elongation zone, and involves a reorientation of the cortical microtubule cytoskeleton. Transcriptomic and genome-wide association studies identified critical roles for the microtubule cytoskeleton, cell wall, and Pi signalling pathways in regulating PDS responses. Interestingly, increasing NaCl concentrations reverse PDS to a rightward skewing response, termed “Salt Induced-Rightward Skewing” (SIRS), with associated reversals in helicities. This work provides a deeper understanding of root behaviour under Pi availability and salinity stress, linking cellular mechanics to genetic pathways. The findings emphasise the relationship between root skewing, circumnutation, and tropisms, and offer insights into how roots adapt their growth to optimise Pi acquisition. These discoveries lay the groundwork for improving nutrient use efficiency in crops, potentially addressing challenges in sustainable agriculture.