PS 26-99
Bacterial symbionts of endophytic fungi mediate functional shifts in plant-fungal interactions
Plant-associated fungi provide important ecosystem services as pathogens, mycorrhizae, endophytes, and saprotrophs. Endophytic fungi colonize healthy, living plant tissues in all biomes, often providing protection from pathogens, herbivores, and other environmental stressors. Saprotrophic fungi are the primary decomposers of senescent plant materials, cycling nutrients by breaking down cellulose and lignin. Many fungi play more than one of these roles throughout their life cycles, with functional switches in ecological modes driven in part by interactions with additional microbes. Recently diverse endophytic and saprotrophic fungi were found to harbor endohyphal bacteria (EHB), which live within apparently healthy, viable fungal cells. As part of a larger study examining the specificity and plasticity of EHB effects on fungal phenotypes and the outcome of plant-fungal interactions, we used seven fungal endophyte/EHB associations isolated from cupressaceous plants to examine endophytic and saprotrophic traits of EHB-infected, EHB-cured, EHB-cross-infected and resynthesized fungal strains.
Results/Conclusions
We found that the presence and identity of EHB significantly influenced the success of plant infection by fungal endophytes. Particular EHB aided or inhibited fungal infection by closely related endophytes in a partnership-specific manner. Relative to axenic controls, fungal cultures infected with EHB grew significantly more rapidly on senescent tissue of their native host, and more effectively degraded senescent foliage. However, EHB-infected and control strains did not differ in their capacity to grow on or degrade fresh plant material or material from related hosts. In vitro assays revealed that EHB can significantly alter cellulase activity (3 of 7 fungal-EHB partnerships) and ligninase production (1 of 7 partnerships). Under nutrient-rich conditions at 22°C, fungi grew significantly more slowly when EHB were present, but some EHB restored fungal growth under high thermal stress. Because EHB can have drastic effects on fungal phenotypes and plant-fungal symbioses, and can be horizontally acquired, we suggest that they can influence plant-fungal interactions, resultant ecosystem services, and the functional diversification of plant-associated fungi, especially with regard to the saprotroph-endophyte continuum.