In planta regulation of extension of an endophytic fungus and maintenance of high metabolic rates in its mycelium in the absence of apical extension

The fun-us Neotyphodium lolii is an endophytic symbiont. It grows in the in tercellular spaces of the perennial ryegrass Lolium perenne, producing seco ndary metabolites which enhance the fitness of the association over that of uninfected L. perenne. We report that the average number of hyphal strands in a given section of a leaf remains constant during the life of a leaf, i ndicating synchrony of leaf and hyphal extension, including cessation of hy phal extension when leaf extension ceases. We used a constitutively express ed reporter gene as an indicator of the mycelium's metabolic activity durin g and after hyphal extension. Reporter gene activity decreased when the myc elium stopped extending in liquid culture but not in planta. This indicates that in planta endophyte hyphae remain metabolically highly active when ex tension has ceased and throughout the life of the leaf they are colonizing. The behavior of the fungus in planta indicates the existence of signaling pathways which (i) synchronize the extension of leaf and hypha by regulatin g hyphal extension, (ii) suppress hyphal branching, and (iii) stop apical e xtension of fungal hyphae, without reducing the mycelium's metabolic activi ty. These signals may be crucial for the symbiosis, by allowing the endophy te to switch the focus of its metabolic activity from extension to the prod uction of secondary metabolites.