Rhizobial symbiosis affects higher trophic levels by altering direct and indirect plant defenses

Background/Question/Methods

Legume-associated nitrogen-fixing bacteria play a key role for plant performance and productivity in natural and agricultural ecosystems. Although this plant-microbe mutualism has been intensely studied for decades, studies on effects of rhizobia colonization on higher trophic levels are scarce. Nitrogen-fixing rhizobia can substantially influence plant-herbivore interactions by altering food plant quality through additional nitrogen. However, whether or not nitrogen provided by the bacteria also affects plant defenses remains widely elusive so far. We investigated effects of rhizobial colonization on cyanogenesis (a direct defense), soluble proteins (as nutritive trait), and jasmonic acid (JA)-induced volatile organic compounds (VOCs). VOCs serve as indirect and direct defenses against arthropod herbivores as well as signals in defense-associated plant-plant and within-plant signaling. In addition to characterizing defensive and nutritive plant traits of rhizobia-colonized and rhizobia-free lima bean plants (Phaseolus lunatus), we tested effects of altered plant biochemistry on a specialist insect herbivore (Mexican bean beetle; Epilachna varivestis) in feeding and experiments. Additionally, effects of volatile bouquets released from rhizobia-colonized and rhizobia-free control plants on this specialist insect herbivore were investigated in olfactometer choice experiments.

Results/Conclusions

Our study suggests that nitrogen provided by rhizobia is allocated to nitrogen-containing defense-associated compounds, and thereby crucially determines the outcome of plant-herbivore interactions. Direct defense via cyanogenesis was enhanced in rhizobia-colonized plants, as were fitness relevant plant growth parameters, while soluble protein concentration was not affected. Following induction with JA, rhizobia-colonized plants released significantly higher amounts of shikimic acid derived compounds, whereas the emission of compounds produced via the octadecanoid, mevalonate, and non-mevalonate pathway was reduced. These changes affected beetles' choice behavior as the preference of non-induced plants was much more pronounced for rhizobia-colonized plants. Indole was the causing agent determining repellent effects of JA-induced VOCs of rhizobia-colonized plants. Our study supports the view that the fitness benefit of bacterial root symbioses includes plant defenses and thus extends beyond the mere promotion of plant growth. Due to the enhanced direct as well as indirect defense, we interpret rhizobia as integral part of the defense system in legumes. Since the associations between legumes and nitrogen-fixing rhizobia are ubiquitous in terrestrial ecosystems, improved knowledge on rhizobia-mediated effects on plant traits—and the resulting effects on higher trophic levels—is important to better understand the role of these microbes for ecosystem functioning.