Invasive legume leverages rhizobia to increase chemical herbivory defense

Abstract Differences in soil microbial communities between sites can contribute to variation in plant performance across populations; however, this relationship remains poorly understood in invasive plants. To address this gap, we conducted a greenhouse experiment using the herbaceous legume Lupinus polyphyllus , including plants originating from both native and invasive populations. Plants were grown with either intact or autoclaved soil inoculum obtained from three invasive sites. We measured plant growth, nodulation, leaf quinolizidine alkaloids, and resistance to the snail Arianta arbustorum and additionally analyzed soil bacterial communities. The soil inoculum source had detectable albeit small effects on plant height, leaf number, and total alkaloids. Nodulation positively influenced leaf alkaloid levels, which in turn increased resistance to snail herbivory. Differences between plants of invasive and native origins were clear, with invasive plants outperforming the native ones in most traits (plant growth, nodulation, total alkaloids, herbivore resistance). Leaf alkaloid composition also differed between invasive and native plants and among plant organs (leaves, roots, nodules). Our results demonstrate that rhizobia improve plant chemical herbivore defenses presumably through their effect on nitrogen availability, and suggest that the tripartite microbe-plant–herbivore interaction has evolved in the invasive range to the great benefit of the invader. These findings provide novel mechanistic insights into how mutualistic soil microbes and their benefits in defense against generalist herbivores contribute to plant invasions.