Effect of variation in nitrogen environment and legume and rhizobia genetics on the outcome of the legume-rhizobium mutualism.

Abstract

The legume-rhizobium mutualism has been studied for its agricultural importance from the nitrogen that the rhizobia fix in exchange for carbon from the plant, and additionally used as a model to understand the evolution of mutualisms. The objective of this research was to further understand the variation present in natural populations of legumes and rhizobia, and to use a population perspective to build upon the work done with inbred plant lines and single strains of rhizobia. I applied a gradient of nitrogen (N) to a single cultivar of Lotus corniculatus inoculated with a population of rhizobia to develop expectations of how L. corniculatus responds to N addition. I then used a full-factorial greenhouse experiment with natural populations of L. corniculatus and their associated rhizobia to assess the amount of variation present in natural populations, and how they respond to N addition. From this, I found that plant populations did not show variation in nodule traits that could affect rhizobial fitness, whereas rhizobial populations showed variation in all traits measured. The effect of N addition on L. corniculatus in general causes a decrease in nodule size, although when tested in the context of natural populations, there was a plant population-dependent effect, as some populations increased, decreased, or did not alter the size of their nodules. This work underscores the importance to incorporate population scale information in how this mutualism responds to varying environmental conditions. Furthermore, considering the amount of variation found in rhizobial populations, future work should focus on sampling legumes and their associated rhizobia in order to have a more accurate measure of the amount of variation present in the mutualism.