Deciphering the regulatory roles of phosphate transporters at the symbiotic interface in arbuscular mycorrhizal symbiosis

Boyce Thompson Institute Plant Research, Ithaca NY

Investigators

Abstract

Over 72% of flowering plant species are capable of mutualistic symbioses with arbuscular mycorrhizal (AM) fungi. In this nutritionally-based mutualism, the fungi live within and around the root and transfer phosphate and nitrogen from the soil to the root. In return, the plant cell provides the fungi with carbon, in the form of lipid and sugars. The symbiosis has a huge impact on plant mineral nutrition and on the level of carbon directed underground, both of which influence many aspects of ecosystem functioning. Phosphate transport proteins move phosphate across the fungal and plant membranes. Previous work has identified phosphate transport proteins active during symbiosis and has revealed that phosphate transport is a key regulator of the association. This project seeks a mechanistic understanding of the regulatory process and aims to identify how the plant root cells sense phosphate and regulate their cell biology to maintain the symbiosis. The function of a fungal phosphate transporter will also be assessed. This project will provide a mechanistic understanding of phosphate transport and its role in regulating the symbiosis, which could provide key targets for breeding crops that are optimized for phosphate acquisition through AM symbiosis. The project will provide training opportunities for scientists at all levels of their careers, from high school students to postdocs. Public engagement activities will include “Discover Friendly Fungi”, a Foldscope activity designed to increase awareness of AM symbiosis. During AM symbioses, phosphate and carbon transfer between the plant and fungus occurs over the interface between the fungal arbuscules and the plant periarbuscular membrane (PAM). Phosphate transporters of the PHT1- sub-family I, are conserved in AM symbiosis host plants and as revealed by Medicago truncatula PT4 (and orthologs in other plant species), they are essential for maintenance of the symbiosis. In pt4 loss-of-function mutants, each arbuscule dies prematurely and the symbiosis is terminated, thus, PT4/ phosphate transport is a key regulator of the symbiosis. The work proposed seeks a mechanistic understanding of this regulatory process. pt4 missense alleles uncouple phosphate transport from premature arbuscule death and several candidate PT4-interacting proteins have been identified. Candidate interactors and their interactions with PT4 wild type and mutant proteins will be evaluated in planta. The significance of these interactions for maintenance of symbiosis will be assessed and the hypothesis that PT4 function is mechanistically linked to fungal lipid provisioning will be tested. Two PAM-resident kinases are required for arbuscule survival; these kinases will be characterized and their roles in phosphorylation of PT4 assessed. The work proposed includes studies of an AM fungal phosphate transporter that shows arbuscule-enhanced expression. Using host-induced gene silencing, the hypothesis that this phosphate transporter mediates phosphate export from the arbuscule will be tested. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.