Defensive mutualism between plants and endophytic fungi?

Endophyte-grass symbiosis is generally considered to be a classic example of microbe-plant symbiosis in which the fitness of the microbial symbiont and its host plant is closely linked, and thus, presumed to align the interests of partners toward mutually beneficial cooperation. Accumulating evidence seems to suggest that defensive mutualism provides the best framework for understanding plant-endophyte interactions in general. We conducted a meta-analysis of 99 published studies on 36 plant (inc. both grass and tree species), 62 herbivore and 17 predator or parasitoid taxons to test the importance of defensive mutualism in multitrophic interactions. In general, statistical perusal revealed that we still know little about these seemingly well-studied biological interactions. The conceptual framework for endophyte-grass interactions has largely been based on endophyte-plant-herbivore studies of two, economically important, artificially selected and introduced agricultural grass species, tall fescue and perennial ryegrass, and two generalist invertebrate pests. Only 10 original publications provided data of higher trophic levels. Consistent with the defensive mutualism hypothesis, the meta-analysis indicates that endophytes slightly increase grass resistance to herbivores, and the defensive mutualism appears to be most commonly detected in systemic and vertically transmitted grass endophytes compared to horizontally transmitted tree endophytes. However, variation appears to increase when higher trophic levels are considered. In addition to taxonomical bias, the literature is strongly biased toward short-term laboratory and greenhouse experiments rather than field conditions. Thus, current literature is insufficient to capture the breadth of variability inherent in the wild grass-endophyte populations and communities, and the general importance of defensive mutualism remains to be solved in future studies.