Review. Root growth under phosphorus deficiency: Revisiting the role of TOR signaling

While phosphorus (P) ranks the second most essential mineral nutrient for plant growth, its availability in natural soils is often very low. To cope with this limitation, plant roots have evolved sophisticated molecular mechanisms and phenotypic plasticity to maximize root surface area and enhance P uptake—typically by promoting lateral root and root hair proliferation at the expense of primary root elongation. Several key regulatory nodes have been implicated in the adaptive response to P deficiency, including the phosphate starvation response (PSR) signaling network, the Target of Rapamycin (TOR) pathway, hormonal regulation, and, interestingly, iron (Fe) homeostasis. Focusing on TOR signaling and Fe, Choi and colleagues revisit root growth regulation under P deficiency and propose new perspectives. It has long been observed that P deficiency is accompanied by Fe accumulation in the root, a correlation that may contribute to toxicity symptoms and restricted root growth. More recent studies, however, have looked deeper into the molecular mechanisms, suggesting that Fe, among other regulators, modifies cell wall structure to constrain meristem size, thereby reinforcing growth limitation. Additionally, Fe seems to be required for the proliferation of lateral roots via a TOR-requiring, auxin-dependent mechanism. Although TOR has been implicated in diverse environmental responses, it remains unknown how these diverse pathways interact with TOR. Consequently, the precise roles of these known regulatory nodes—and their potential interactions—remain unresolved, presenting new opportunities for research and application. (Summary by Ching Chan @ntnuchanlab) J. Exp. Bot. 10.1093/jxb/eraf204