SPX1

SPX1 is a gene found in the model flowering plant Arabidopsis thaliana. The gene encodes a protein that contains an SPX domain, a conserved motif that is implicated in phosphate (Pi) signaling in eukaryotic organisms. In Arabidopsis, SPX1 is one of two paralogues in the SPX gene family that is expressed primarily in roots and shoots, with higher transcript accumulation in the root apex and in tissues that are actively absorbing nutrients.

Functional studies indicate that the SPX1 protein acts as a negative regulator of Pi homeostasis. It interacts physically with the transcription factor PHR1, sequestering PHR1 in the cytoplasm during Pi replete conditions and thereby repressing the expression of Pi-starvation responsive genes. When Pi levels decline, SPX1 releases PHR1, which then enters the nucleus and activates genes that enhance Pi uptake and recycling. Mutants lacking SPX1 exhibit increased expression of Pi‑uptake genes and accumulate higher intracellular Pi levels, whereas overexpression of SPX1 suppresses these responses, confirming its regulatory role.

The Arabidopsis SPX1 protein is ~380 amino acids long and contains a single SPX domain at the N‑terminus, followed by a long disordered tail that enables interaction with PHR1. Sequence homology analysis shows that SPX1 shares conserved residues with SPX proteins from soybean, rice, and maize, suggesting that the Pi‑sensing function is conserved across angiosperms. In crop species, orthologues of SPX1 have been studied for potential roles in improving phosphate use efficiency, a key trait for sustainable agriculture. The Arabidopsis SPX1 locus is located on chromosome 1 and is annotated in the TAIR database with the locus identifier AT1G33730.