Loss of the phosphate sensor <i>CsSPX2</i> impairs lateral root initiation and development in cucumber

Lateral root (LR) branching is a vital adaptive strategy for plants to optimize nutrient acquisition, particularly under phosphorus deficiency. While auxin triggers LR formation, the molecular link between phosphate (Pi) homeostasis and the developmental competence of xylem-pole-pericycle (XPP) cells in roots remains elusive. In this study, we identified CsSPX2 from the SPX (SYG1/Pho81/XPR1) family as a critical phosphate sensor in cucumber (Cucumis sativus L.) that is strongly induced by Pi starvation. CRISPR/Cas9-generated CsSPX2 knockout lines displayed reduced lateral root density, shorter lateral roots and abnormal root meristems. The loss of CsSPX2 resulted in phosphate overaccumulation and excessive lignin deposition under Pi-sufficient conditions. Concomitantly, we observed abnormal thickening of the XPP cell walls and subsequently impaired auxin responsiveness at LR initiation sites, as evidenced by diminished DR5::GUS activity. Notably, under Pi-deficient conditions, spx2 mutants exhibited obviously decreased lignin content and significantly reduced thickness of XPP cell wall compared to Pi-sufficient conditions. In addition, the LR developmental defects in spx2 mutants were partially rescued by exogenous application of either a lignin biosynthesis inhibitor or synthetic auxin. Furthermore, CsSPX2 appeared to be important for maintaining the stem cell niche organization and meristematic activity in root tips, as its loss altered quiescent center and columella stem cell behavior and impaired root cap differentiation. Collectively, our findings support a model where CsSPX2 might link phosphate homeostasis to lateral root development through its involvement in XPP cell wall properties and auxin response. This study deepens our understanding of plant nutrition stress responses and provides a key genetic target for developing phosphate-efficient crops.