A novel impact of a novel weapon: allelochemicals in Alliaria petiolata disrupt the legume-rhizobia mutualism

Abstract

Some introduced species become invasive by releasing novel allelochemicals into the soil, directly harming nearby plants and soil microbes. Alliaria petiolata (garlic mustard) is an invasive plant, well known to excrete a suite of phytotoxic and anti-microbial allelochemicals, including allyl isothiocyanate (AITC) and benzyl isothiocyanate (BITC). While the effects of these chemicals on plant-mycorrhizae mutualisms are well documented, the effects on other plant-soil microbe interactions, such as the legume-rhizobia mutualism, have not yet been tested. Here, we performed laboratory and greenhouse experiments with both synthetic chemicals and leaf extracts to investigate the effects of allelochemicals in A. petiolata on a native leguminous plant, Amphicarpaea bracteata, and its rhizobia mutualists. We found that BITC reduced rhizobia growth rate in the lab, but had no effect on nodulation in the greenhouse when rhizobia were grown in the presence of plants. AITC did not directly harm either plants or rhizobia, though plants and rhizobia grown in the presence of AITC showed reduced nodulation, indicating that it disrupted the formation of the mutualism itself. We found no effects of A. petiolata allelochemical leaf extracts on plant performance or nodulation. Our data suggest that AITC causes mutualism disruption in this system by preventing the formation of nodules, which reduces plant growth and could threaten the long-term performance of rhizobia. Our study shows that the allelochemicals in A. petiolata disrupt the legume-rhizobia resource mutualism, adding another impact of these novel weapons in addition to their well-documented role in disrupting plant-mycorrhizae symbioses.