Morphogenesis, Growth, and Development of the Grass Symbiont Epichlöe festucae

Epichloe festucae and its asexual derivative Neotyphodium lolii are mutualistic symbionts that confer on their Festuca and Lolium grass hosts, protection from various biotic and abiotic stresses. The genetic tractability of E. festucae has led to its adoption as a model experimental system to study fungal–grass symbiotic interactions. Growth of E. festucae in Lolium perenne is both epiphytic and endophytic. Endophytic growth is characterized by hyphal tip growth and branching in the meristematic tissues but in the leaves hyphae divide and extend by intercalary growth in synchrony with the same pattern of growth of the leaves. Forward and reverse genetics approaches have shown that the NADPH oxidase (Nox) complex and MAP kinase signaling pathways are crucial for maintaining this restrictive pattern of hyphal growth in the leaves. Disruption of genes that encode components of these signaling complexes leads to proliferative (pathogenic) growth in the host and a breakdown in the symbiosis. This chapter provides an overview of morphogenesis, growth, and development of E. festucae in culture and in planta and an oversight of what is currently known about the fungal signaling mechanisms required for maintaining a balanced symbiosis.