Co-occurring species can have strong ecological impacts on their neighbors beyond competition due to neighbors altering resistance or susceptibility to natural enemies. Associational resistance occurs in plant communities when one species reduces natural enemy damage to neighboring species, typically via physical or chemical defenses. By contrast, associational susceptibility results in increased damage to neighboring species when enemies switch to less defended neighbors. Plant-microbe associations can also influence neighboring communities by increasing resistance or susceptibility of surrounding species to enemy attack. The hereditary fungus, Periglandula spp., and their bioactive ergot alkaloids, occur in many of the morning glory species (Convolvulaceae), especially within the genus Ipomoea. In a previous study, endosymbiotic I. tricolor ‘Pearly Gates’, where ergot alkaloids accumulate in roots, developed fewer nematode galls and produced less biomass compared to non-endosymbiotic plants. To test whether endosymbiont-mediated resistance extends to neighboring plant species, we performed two experiments where we grew endosymbiotic or non-endosymbiotic I. tricolor with corn (Zea mays) or the congeneric sweetpotato (I. batatas) in pots containing soil infested with high densities of Meloidogyne incognita nematodes or no nematodes. All treatments were grown for nine weeks before harvest and then separated into total pot and species-specific above- and below-ground biomass.
Results/Conclusions
In the corn experiment both the nematode and endosymbiont treatments reduced total biomass per pot, consistent with the earlier study with I. tricolor alone, but corn produced more biomass in the nematode addition treatment when morning glory was endosymbiotic, supporting the hypothesis of associational resistance. In the sweetpotato experiment, where gall numbers on morning glory roots were quantified, there was a significant effect of endosymbiont treatment on total pot biomass but no effect of nematode or endosymbiont treatments, or their interaction, on biomass of sweetpotato or morning glory individually. These results suggest that the Periglandula endosymbiont of I. tricolor can potentially protect co-occurring plant species from enemies but this effect may be variable among species, and that microbe-mediated interactions can play a significant role in structuring agricultural and natural plant communities.
