Policing the legume-Rhizobium symbiosis: a critical test of partner choice

A new paper by Annet Westhoek, Elsa Field, Finn Rehling, Geraldine Mulley, Isabel Webb, Philip Poole, and Lindsday Turnbull has been published by Scientific Reports. 

Leguminous plants like beans, peas and lentils form symbiosis with nitrogen-fixing soil bacteria called rhizobia. Rhizobia infect the root and reside in root nodules, where they fix atmospheric nitrogen in return for carbon from the plant. The symbiosis makes leguminous crops rich in protein and is of major importance in global food security. Plants are usually infected by multiple rhizobial strains, which differ in how effective they are at providing the plant with nitrogen. Ineffective strains are common and affect yields.

This study looked at how plants react to strains providing different amounts of nitrogen. Using colourfully marked effective and ineffective mutant strains, it was found that plants cannot detect how much nitrogen a strain will provide before establishing the symbiosis. Effective and ineffective strains are equally likely to form a nodule. After nodule formation though, the plant does selectively provide resources to the more effective strain, resulting in larger nodules. This finding will help in understanding how to best reap the benefits of this special relationship between plants and bacteria. 

Find the paper here: https://www.nature.com/articles/s41598-017-01634-2