Interaction between the endophytic fungus Epichloe bromicola and the grassBromus erectus: effects of endophyte infection, fungal concentration and environment on grass growth and flowering

Epichloe bromicola is an endophytic fungal species that systemically and pe rennially colonizes intercellular spaces of leaf blades, leaf sheaths and c ulms of Bromus grass species. E. bromicola causes choke disease in B. erect us, suppressing maturation of most, if not all, host inflorescences. In an investigation of the interaction between fungus and host, we used a quantit ative polymerase chain reaction technique to estimate the amount of fungal DNA, and thereby fungal concentration, in host plants. Fungal concentration was directly correlated with vegetative vigour of the plant, as measured b y longest leaf length, number of tillers and vegetative above-ground biomas s, suggesting that, during vegetative growth, the endophytic fungus is most beneficial for the plant when present in high concentrations. In contrast, the reproduction of the plant, as measured by the number of functional inf lorescences, was inversely correlated with fungal concentration: the majori ty of infected plants, and all that were associated with high concentration s of fungi, were diseased. Thus, the benefit of endophyte infection for the plant is coupled with the disadvantages of infertility. Fungal concentrati on was shown to be at least in part genetically determined because fungal c oncentration differed significantly in different plant-endophyte genotype c ombinations (symbiotum). In a field experiment with normal and CO2-enriched environments, elevated CO2 levels favoured fungal reproductive vigour over host reproductive vigour suggesting that these plant endophytes would be a t a selective advantage in a corresponding environmental-change scenario. W e conclude that a dynamic and complex relationship between fungal endophyte infection, fungal concentration, genotype and environment affects growth a nd fecundity of B. erectus and should contribute to the evolution of these plant-fungal interactions.