Mechanisms of vacuolar phosphate efflux supporting soybean root hair growth in response to phosphate deficiency

Phosphorus is an essential macronutrient for plant growth and development.In response to phosphate(Pi)deficiency,plants rapidly produce a substitutive amount of root hairs;however,the mechanisms underlying Pi supply for root hair growth remain unclear.Here,we observed that soybean(Glycine max)plants maintain a consistent level of Pi within root hairs even under external Pi deficiency.We therefore investigated the role of vacuole-stored Pi,a major Pi reservoir in plant cells,in supporting root hair growth under Pi-deficient conditions.Our findings indicated that two vacuolar Pi efflux(VPE)transporters,GmVPE1 and GmVPE2,remobilize vacuolar stored Pi to sustain cytosolic Pi content in root hair cells.Genetic analysis showed that double mutants of GmVPE1 and GmVPE2 exhibited reduced root hair growth under low Pi conditions.Moreover,GmVPE1 and GmVPE2 were highly expressed in root hairs,with their expression levels significantly upregulated by low Pi treatment.Further analysis revealed that GmRSL2(ROOT HAIR DEFECTIVE 6-like 2),a transcription factor involved in root hair morphogenesis,directly binds to the promoter regions of GmVPE1 and GmVPE2,and promotes their expressions under low Pi conditions.Additionally,mutants lacking both GmRSL2 and its homolog GmRSL3 exhibited impaired root hair growth under low Pi stress,which was rescued by overexpressing either GmVPE1 or GmVPE2.Taken together,our study has identified a module comprising vacuolar Pi exporters and transcription factors responsible for remobilizing vacuolar Pi to support root hair growth in response to Pi deficiency in soybean.

A SPX domain vacuolar transporter links phosphate sensing to homeostasis in Arabidopsis

Excess phosphate(Pi)is stored into the vacuole through Pi transporters so that cytoplasmic Pi levels remain stable in plant cells.We hypothesized that the vacuolar Pi transporters may harbor a Pi-sensing mechanism so that they are activated to deliver Pi into the vacuole only when cytosolic Pi reaches a threshold high level.We tested this hypothesis using Vacuolar Phosphate Transporter 1(VPT1),a SPX domain-containing vacuolar Pi transporter,as a model.Recent studies have defined SPX as a Pi-sensing module that binds inositol polyphosphate signaling molecules(InsPs)produced at high cellular Pi status.We showed here that Pi-deficient conditions or mutation of the SPX domain severely impaired the transport activity of VPT1.We further identified an auto-inhibitory domain in VPT1 that suppresses its transport activity.Taking together the results from detailed structure-function analyses,our study suggests that VPT1 is in the auto-inhibitory state when Pi status is low,whereas at high cellular Pi status InsPs are produced and bind SPX domain to switch on VPT1 activity to deliver Pi into the vacuole.This thus provides an auto-regulatory mechanism for VPT1-mediated Pi sensing and homeostasis in plant cells.