Regulation switching of Epichloë typhina within elongating perennial ryegrass leaves.

Epichloë and Neotyphodium spp. are clavicipitaceous fungi that form symbiotic endophytic associations with temperate grasses. The growth of these endophytes is strictly intercellular and tightly regulated, being synchronized with that of the host grass. The exception to this synchronized form of growth is the profuse growth of hyphae during development of stromata on reproductive tillers, thereby preventing emergence of the inflorescence. Here we report the occurrence of stromata on leaves of vegetative tillers of perennial ryegrass infected with E. typhina. Stroma formation was confined to leaf blades, being present as a distinct zone within an otherwise symptomless leaf. In some leaf blades two or more stromata zones were present, separated by symptomless tissue. Growth of hyphae in the symptomless zones was seldom branched, orientated parallel with the longitudinal leaf axis, and synchronised with plant growth. In contrast, hyphal growth in the stroma zones was unrestricted and highly branched. Thus in stable zones along a single leaf blade the interaction between E. typhina and its host grass alternated between synchronised, symptomless growth and a pathogenic state with unrestricted hyphal growth. These results demonstrate that the development of an inflorescence is not a prerequisite for hyphal growth leading to stroma formation and provide an ideal experimental system for investigating further how these fungi are able to switch from synchronized, symptomless growth to unrestricted stromal expression.