Light availability and rhizobium variation interactively mediate the outcomes of legume–rhizobium symbiosis

Abstract

Premise Nutrients, light, water, and temperature are key factors limiting the growth of individual plants in nature. Mutualistic interactions between plants and microbes often mediate resource limitation for both partners. In the mutualism between legumes and rhizobia, plants provide rhizobia with carbon in exchange for fixed nitrogen. Because partner quality in mutualisms is genotype‐dependent, within‐species genetic variation is expected to alter the responses of mutualists to changes in the resource environment. Here we ask whether partner quality variation in rhizobia mediates the response of host plants to changing light availability, and conversely, whether light alters the expression of partner quality variation. Methods We inoculated clover hosts with 11 strains of Rhizobium leguminosarum that differed in partner quality, grew plants under either ambient or low light conditions in the greenhouse, and measured plant growth, nodule traits, and foliar nutrient composition. Results Light availability and rhizobium inoculum interactively determined plant growth, and variation in rhizobium partner quality was more apparent in ambient light. Conclusions Our results suggest that variation in the costs and benefits of rhizobium symbionts mediate host responses to light availability and that rhizobium strain variation might more important in higher‐light environments. Our work adds to a growing appreciation for the role of microbial intraspecific and interspecific diversity in mediating extended phenotypes in their hosts and suggests an important role for light availability in the ecology and evolution of legume–rhizobium symbiosis.

Host-Associated Rhizobial Fitness: Dependence on Nitrogen, Density, Community Complexity, and Legume Genotype

“…In this mutualistic relationship, rhizobia convert atmospheric nitrogen (N 2 ) into a plant-useable form to support host growth and reproduction while rhizobia gain carbon resources from the plant to support the growth and reproduction prior to release back into the soil ( 19 , 20 ). While this relationship is commonly beneficial to the plant host ( 21 ), the magnitude of these benefits to the plant depends on the identity of the rhizobial strain as well as additional environmental parameters including nitrogen (N), phosphorous, water and light availability, and temperature ( 17 , 22 – 24 ). Experiments in which plants are inoculated with a single rhizobium strain, often at a very high density, have shown that the benefits rhizobia obtain from symbiosis also can be context dependent (e.g., Batstone et al 2020 [ 25 ] and Friel and Friesen [ 26 ]).…”

A private channel of nitrogen alleviates interspecific competition for an annual legume

“…In our system, and for other legumes, spatiotemporal variation in soil N, as well as variation in the competitive environment, should result in context-dependent ecological outcomes and favor the evolution of plasticity. Plasticity in associations and allocation appears to be dependent not only on competition, but also on abiotic factors such as light availability and the genotypic match with particular rhizobia strains (Heath et al 2020). In our field surveys, we found spatial variation in the outcome of legume-rhizobia interactions that we speculate were dependent on the build-up of soil N in population centers, where rhizobial N-fixation was reduced (based on d 15 N) and where nodule mass was not associated with plant productivity (Fig.…”

Plant–environment interactions from the lens of plant stress, reproduction, and mutualisms

“…Additionally, the environment experienced by the host can mitigate host-mutalist interactions. Heath et al (2020) ask if variation in symbiont partner quality for their legume host plants is influenced by changing light availability. They show that light availability and symbiont inocula interact to influence plant responses to light, and moreover that variation in partner quality is more apparent in ambient light.…”