CHARACTERIZATION OF TRANSCRIPTIONAL AND DEVELOPMENTAL REGULATORS OF ARBUSCULAR MYCORRHIZAL SYMBIOSIS IN THE LEGUME MEDICAGO TRUNCATULA

Abstract

Mineral nutrient acquisition in land plants is enhanced by participating in an endosymbiosis with arbuscular mycorrhizal fungi that has persisted for the past 400 million years. Highly branched fungal structures, called arbuscules, form inside root cortical cells. The arbuscule is continuously housed in a specialized plant membrane which contains nutrient transporters necessary for nutrient exchange. Together, the arbuscule-plant membrane interface defines the site of nutrient exchange between the symbiotic partners. Arbuscules can occupy more than a third of the plant cell volume at maturity, so there must be a sophisticated interplay of signaling and cellular remodeling to ensure that colonization levels by the fungus are sufficient, but do not exceed, the plant’s mineral nutrient requirements to minimize plant resource allocation toward the symbiosis. In this dissertation, I focus on transcriptional regulation of arbuscule development through the transcription factors IPD3 and IPD3L, as well as the regulatory function of VAPYRIN, an uncharacterized plant protein required for arbuscule development. Through the analysis of an ipd3 ipd3l double mutant, I show that under low phosphate conditions, symbiotic gene expression and arbuscule development can still occur, but under high phosphate conditions, fungal entry into the epidermis is restricted, implying that there is a phosphate-sensitive factor that can induce symbiotic gene expression in the absence of these two genes. IPD3 and IPD3L induce the expression of VAPYRIN, a gene encoding a putative scaffold protein. A structure-function analysis indicates a functional modularity of VAPYRIN’s protein domains. Three previously undescribed interactions of VAPYRIN are described. These proteins have different functions and locations within the cell, supporting the hypothesis that VAPYRIN is a scaffold protein. Because VAPYRIN can interact with disparate protein types, this suggests that this protein has the potential to serve as a nexus for multiple processes necessary for arbuscule development. This provides insight about how the plant cell can integrate signaling with cellular remodeling processes during AM symbiosis.