Signalling in the Epichloe festucae Perennial Ryegrass Mutualistic Symbiotic Interaction

Carla Eaton, Milena Mitic, and Barry Scott

Abstract Epichloe festucae is a biotrophic fungus that forms symbiotic associations with temperate grasses of the Festuca and Lolium spp. The association between E. festucae and L. perenne (perennial ryegrass) is a good experimental system to study these endophyte-grass associations. Integral to the establishment and maintenance of these mutualistic associations is mutual communication between the endophyte and host partner. This communication will likely involve many well-known signalling pathways. Mitogen-activated protein kinase (MAPK) cascades and second messenger signalling pathways involving cAMP and calcium are the main pathways for transduction of signals perceived by cell surface receptors such as G-protein coupled receptors and histidine kinases. Here we review what is currently known about signalling mechanisms in the E. festucae-L. perenne symbiosis including a bioinformatics analysis of the E. festucae genome to identify which components of these key signalling pathways have been conserved in this organism. Where pathways are yet to be functionally analysed in E. festucae we present data from fungal plant pathogens and try to predict possible functions for these pathways in endophyte associations.

1 Introduction

Epichloe' endophytes are a group of sexual and asexual clavicipitaceous fungi (Clavicipitaceae, Ascomycota) that form symbiotic associations (symbiota) with temperate grasses of the sub-family Pooideae. There are at least ten different sexual species (Schardl and Wilkinson 2000) including E. festucae, a natural symbiont of

Institute of Molecular Biosciences, Centre for Functional Genomics, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand e-mail: [email protected]; [email protected]; [email protected]

S. Perotto and F. Baluska (eds.), Signaling and Communication in Plant Symbiosis, 143

Signaling and Communication in Plants 11, DOI 10.1007/978-3-642-20966-6_7, © Springer-Verlag Berlin Heidelberg 2012

Festuca spp. (Leuchtmann et al. 1994). E.festucae also forms a stable mutualistic association with perennial ryegrass, Lolium perenne, and is proving to be a good experimental system to study these endophyte-grass associations (Christensen et al. 1997; Scott et al. 2007). The asexual Neotyphodium endophytes are predominantly interspecific hybrids that form mutualistic associations with their host (Moon et al. 2004).

These biotrophic fungi systemically colonise the intercellular spaces of leaf primordia, sheaths and blades of vegetative tillers and the inflorescence tissues of reproductive tillers. During the host reproductive phase, sexual species are capable of forming external reproductive structures (stromata) that prevent emergence of the host inflorescence, a disease known as "choke". Initiation of stroma formation appears to be triggered by a switch from restricted symbiotic hyphal growth in the apoplast of the leaf to proliferative pathogenic growth in the outer cell layers and surface of the leaf.

In these mutualistic associations, the major benefits to the fungal symbiont are access to nutrients from the host apoplast and a means of dissemination through the seed. Benefits to the host include increased tolerance to both biotic (e.g. insect and mammalian herbivory) and abiotic stresses (e.g. drought).

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