SlMIPS2, a myo-inositol phosphate synthase, regulates phosphate homeostasis by influencing SlPHL1 and SlSPX2 levels...

Phosphorus (P) is an essential macronutrient utilized by plants to support various metabolic processes during growth and development. Recent studies have revealed the pivotal role of inositol hexakis/pyrophosphates (InsP6-8), the derivatives of myo-inositol (MI), in facilitating the interaction between SYG1/PHO81/XPR1 (SPX) and phosphate starvation response (PHR) proteins. myo-Inositol phosphate synthase (MIPS) catalyses the first committed step in MI biosynthesis. Although the role of MIPS genes in mediating stress responses in plants is well elucidated, their role in phosphate (Pi) deficiency remains largely unexplored. This study demonstrates that out of the five MIPS genes encoded by the tomato genome, only SlMIPS2 is sharply induced at an early stage of Pi starvation in tomato seedlings. Silencing of SlMIPS2 led to improved seedling growth with enhanced total soluble Pi and total P levels in silenced plants under high Pi availability. SlMIPS2 silencing also caused a significant reduction in MI and InsP6 content in tomato seedlings. These seedlings with depleted InsP6 levels accumulated lower levels of SlSPX2 protein. In contrast, stabilized SlPHL1 levels were noticed in these plants, directly implicating this transcription factor in activating phosphate-starvation-inducible genes in the SlMIPS2-silenced seedlings, even under high Pi conditions. The results assign a novel role to SlMIPS2 in regulating cellular InsP6 levels and SPX-PHR interactions to control Pi homeostasis in tomato seedlings.