Autopolyploidy alters nodule-level interactions in the legume-rhizobium mutualism

PREMISE

Polyploidy is a major genetic driver of ecological and evolutionary processes in plants, yet its effects on plant interactions with mutualistic microbes remain unresolved. The legume-rhizobium symbiosis regulates global nutrient cycles and plays a role in the diversification of legume species. In this mutualism, rhizobia bacteria fix nitrogen in exchange for carbon provided by legume hosts. This exchange occurs inside root nodules, which house bacterial cells and represent the interface of legume-rhizobium interactions. Although polyploidy may directly impact the legume-rhizobium mutualism, no studies have explored how it alters the internal structure of nodules.

METHODS

We created synthetic autotetraploids using Medicago sativa subsp. caerulea. Neotetraploid plants and their diploid progenitors were singly inoculated with two strains of rhizobia, Sinorhizobium meliloti and S. medicae. Confocal microscopy was used to quantify internal traits of nodules produced by diploid and neotetraploid plants.

RESULTS

Autotetraploid plants produced larger nodules with larger nitrogen fixation zones than diploids for both strains of rhizobia, although the significance of these differences was limited by power. Neotetraploid M. sativa subsp. caerulea plants also produced symbiosomes that were significantly larger, nearly twice the size, than those present in diploids.

CONCLUSIONS

This study sheds light on how polyploidy directly affects a plant-bacterium mutualism and uncovers novel mechanisms. Changes in plant-microbe interactions that directly result from polyploidy likely contribute to the increased ability of polyploid legumes to establish in diverse environments.

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

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.