Leaf endophytes and host genotype in Populus affect pathogen severity

Background/Question/Methods

Fungal leaf endophytes—microfungi that infect the asymptomatic leaf tissues of plants—are ubiquitous in land plants.  A number of leaf endophytes are known to protect their hosts from disease, yet whether this group of plant symbionts alters disease severity by interacting with pathogens, host plants, or both is not well known.  

Results/Conclusions

Here, we show that different leaf endophytes and host plant genotype both affect disease outcomes in Populus.  However, we found no evidence that endophyte effects resulted from induction or suppression of host plant genetic resistance.  Plants inoculated with the endophyte Penicillium sp. nov. prior to inoculation with the necrotrophic leaf pathogen Drepanopeziza populi were characterized by significantly greater necrotic leaf area and premature leaf abscission than plants inoculated with the pathogen only.  Our qPCR analysis of expression levels of selected defense genes in the plant host yielded no evidence that greater disease severity was the result of the endophyte altering plant genetic resistance (i.e., enhanced susceptibility was not induced susceptibility).  A second endophyte, Truncatella angustata, did not affect either disease severity or defense gene expression.  Overall, our results are more consistent with a direct interaction hypothesis for fungal leaf endophytes and pathogens than with an indirect interaction hypothesis (i.e., induced susceptibility).  If confirmed to be true, this finding should have important implications for the use of fungal leaf endophytes in plant disease management.