SHORT-ROOT Regulates Phosphate Allocation Between Root And Shoot In Arabidopsis

Phosphate(Pi)is one of the most important macronutrients for plants.Coordinated distribution of Pi between roots and shoots is an important process that plants use to maintain Pi homeostasis.Under Pi deficiency condition,the shoot growth is inhibited and more Pi is distributed to the root to promote its growth.PHO1 is specifically expressed in the root pericycle and is critical for Pi loading into the xylem.However,it was reported that PHO1 is positively regulated by Pi starvation at both transcriptional and post-transcriptional levels.The mechanism that retards Pi allocation from root to shoot was hitherto unknown.In this research,a forward genetic screen was performed in Arabidopsis which harbored a p PHT1;4::LUC reporter system,and a mutant named phod1 was recovered and displayed severe phosphate deficiency phenotypes in the shoot even under phosphate sufficient conditions.This phenotype can be rescued by grafting a wild type root.PHO1 was decreased in phod1 mutant and Pi translocation from the root the shoot was impaired.Increased PHO1 protein level or knock-out of PHO2 can partial rescued Pi content and phenotypes of phod1.We found phod1 is a weak allel of SHR(SHORT-ROOT),a key transcription factor that regulate radial patterning in the root apical meristem.SHR regulate the m RNA stability of HD-ZIPIII tracription factors,such as PHB PHV etc.Further,we show that Pi starvation suppresses SHR protein accumulation and releases its inhibition on PHB.PHB accumulates and directly binds the promoter of PHO2 to upregulate its transcription,resulting in PHO1 degradation in the xylem-pole pericycle cells.Therefore,more Pi is retained in the root to support its vitality and expansion to forage more Pi from the soil.Our findings reveal a previously unrecognized mechanism of how plants regulate Pi translocation from roots to shoots.