Glucose-6-Phosphate 1-Epimerase Responds to Phosphate Starvation by Regulating Carbohydrate Homeostasis in Rice and Arabidopsis

Plants adapt to phosphate starvation by remodeling root architecture and reallocating carbohydrates. Glucose-6-phosphate 1-epimerase (G6PE), a key enzyme in carbon and energy metabolism, is hypothesized to contribute to phosphate starvation responses. Here, we investigated the role of G6PE in rice and <i>Arabidopsis</i> through phenotypic, physiological, and molecular analyses of <i>osg6pe</i> and <i>atg6pe</i> mutants. Under normal-phosphate (NP) conditions, both mutants exhibited significantly reduced biomass and fresh weight compared with the wild-type (WT) plants, indicating growth inhibition caused by the mutations. Under low-phosphate (LP) conditions, the mutants displayed enhanced root growth, suggesting that G6PE functions as a negative regulator of radial root growth under phosphate deficiency. The <i>osg6pe</i> mutant showed elevated phosphate content and increased leaf starch accumulation under LP, whereas it accumulated more phosphate but less starch under NP. Expression analysis revealed that G6PE transcripts were suppressed under NP but remained relatively stable under LP. Notably, among phosphate starvation-induced (PSI) genes, only <i>PHT1;4</i> showed notable transcriptional changes in both species. These findings indicate that G6PE contributes to phosphate homeostasis by modulating carbohydrate metabolism, restraining radial root growth, and selectively regulating <i>PHT1</i> expression under phosphate-deficient conditions.