Root to shoot defense: Indirect plant defenses are affected by rhizobia

Background/Question/Methods

Without an immune system, plants have been forced to be highly innovative throughout evolutionary time in terms of methods for resistance, such as in defense against herbivores. The emission of volatile organic compounds (VOCs) and secretion of extrafloral nectar (EFN) as a draw for ants and other predatory or parasitoid insects, and thereby as an indirect defense, appears throughout the plant kingdom. However, regarding resource allocation constraints, these defenses can be costly. Particularly, in legumes, which are generally associated with mutualistic nitrogen-fixing bacteria (rhizobia), constraints may arise between investment in defense and the microbial partner, as both represent strong carbon sinks. Despite the legume-rhizobia association critically affecting productivity and diversity in many terrestrial ecosystems, surprisingly, information on the costs and benefits of rhizobia and quantitative expression of carbon-based defenses such as VOC and EFN production is lacking so far. In our study, we used wild lima bean (Phaseolus lunatus) as an experimental plant to test the effects of rhizobia on these two indirect defenses. In comparative experiments using rhizobia-free plants and plants inoculated with a rhizobia strain isolated from wild lima bean plants, the induced production of VOCs and EFN were quantitatively and qualitatively analyzed using GC-MS and a refractometer.

Results/Conclusions

Our study shows that rhizobia have complex effects on plant traits. Fitness-relevant parameters (aboveground biomass, number of leaves, and seed number) were enhanced in rhizobia-colonized plants as compared to rhizobia-free controls. In addition, we found complex effects of rhizobia on indirect plant defenses with potential effects on higher trophic levels. Rhizobia-colonized plants showed a significantly reduced emission of distinct volatile organic compounds, while other volatile organic compounds were significantly increased. Compounds emitted at lower quantity were produced via the octadecanoid, mevalonate, and non-mevalonate pathways; compounds emitted at higher quantity were derived from shikimic acid. Particularly, indole, the only nitrogen-containing compound, was increased. Extrafloral nectar production by rhizobia-colonized plants was lower than production by rhizobia-free plants. Our results show that rhizobia have complex effects on various plant traits with potential cascading effects for plant herbivores and carnivorous arthropods. 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 in ecosystem function.