Signaling pathways driving the development of ectomycorrhizal symbiosis

Abstract

This chapter presents the current knowledge on signal molecules and putative receptors that promote and mediate the very early steps of ectomycorrhizal (ECM) establishment. It emphasizes both the hormone-based and symbiosis effector-based dialogues, and explains how ECM hyphae can proliferate in host roots without eliciting plant defenses. Host genes that are required for ECM mutualistic development are still unknown, as the model plants are perennial trees and more difficult to handle. Construction of symbiotic interface requires secretion of fungal proteins, such as hydrophobins and apoplastic Mycorrhiza-induced Small Secreted Proteins (MiSSPs), and cell wall remodeling. It is probable that hypaphorine acts as an auxin-mimic that is secreted from fungal hyphae, absorbed by the plant root where it enters root cells, and competes with plant-produced auxins for auxin-binding proteins and receptors. The chapter proposes that fungal colonization first triggers auxin-signaling pathway and auxin responses, such as root growth arrest and cell radial elongation.