SiPHO2 orchestrates phosphorus homeostasis and regulates heading date through SiFTc-dependent pathway in foxtail millet (Setaria italica)

Phosphorus (P) is a crucial macronutrient and its deficiency severely limits plant growth and yield. Although multiple inorganic phosphate (Pi) signaling regulators have been identified, the function of them in plant development and flowering time regulatory remains inadequately characterized in C4 model species like Setaria italica . Here, CRISPR/Cas9-generated SiPHO2 knockout lines exhibited disrupted Pi homeostasis, and the lines showed shoot Pi accumulation, leaf tip necrosis, modified root architecture and reduced yield compared with wildtype (Ci846) under Pi deficient conditions. Transcriptome analysis suggested these phenotypic abnormalities might due to expression patterns alteration of Pi starvation-responsive genes. Notably, SiPHO2 knockout lines displayed earlier heading date under Pi deficiency but delayed heading date under normal conditions compared to Ci846 plants. Expression profiling and transgenic functional verification revealed that the heading date reversal correlated with the expression pattern of FLOWERING LOCUS T c ( SiFTc ), rather than SiFTa , which is the closest homolog of Heading date 3a ( OsHd3a ). This study identifies a novel flowering regulator as a potential target for coordinating phosphorus-mediated heading date regulation and yield production. Our findings elucidate genetic mechanisms underlying phosphorus-dependent developmental regulation and propose a strategic approach for improving crop yield under Pi starvation.