Decoding phosphate starvation in plants: local, internal and systemic signaling

Phosphorus (P) is essential for plant growth. However, low inorganic phosphate (Pi) alters plant development and activates a suite of adaptive responses known as phosphate-starvation responses (PSRs). Plants perceive Pi at three distinct levels: external Pi at the root surface, internal Pi status within the cell, and the Pi status of distant organs communicated through the vasculature. This review keeps these levels separate throughout. It first defines what phosphorus limitation means quantitatively, distinguishing deficiency from starvation. It then examines the phenotypic adjustments that low Pi induces in roots and shoots, before treating each of the three sensing levels in turn: local sensing at the root apex, intracellular sensing through inositol pyrophosphates and SPX-domain proteins acting on PHR-family transcription factors, and long-distance signalling through xylem Pi transport, phloem-mobile microRNAs and carbon signals. Transcriptional, post-transcriptional and epigenetic regulation are treated as a separate layer, and hormones as integrators acting across all these levels. Finally, this review assesses how far this knowledge transfers from Arabidopsis to crops, distinguishing phosphorus acquisition from utilization efficiency and identifying which mechanisms are conserved, which are model-specific and which remain untested in other crops.