Exploiting flavonoid diversity in legumes to improve nitrogen-fixing symbioses

A project undertaken at the Research School of Biology, Australian National University, and supervised by Ulrike Mathesius
Co-investigator: Giel van Noorden

Crop plants require large amounts of nitrogen fertilizer to optimize yields. This has created two major problems: Industrial production of nitrogen fertilizer is expensive and requires large amounts of fossil fuels, and application of excess fertilizer causes eutrophication of water ways. Most legume plants can form a symbiosis with nitrogen-fixing bacteria (rhizobia) that fix atmospheric nitrogen into ammonia. Biological nitrogen fixation significantly reduces the need for nitrogen fertilizers for the legume, and legume crops grown in rotation with other crops improve soil nitrogen content.

Nodulation and nitrogen fixation do not always work at optimal levels and can be limited by the types of rhizobia present in the soil, the ability of the legume to supply the symbiont with energy and the control of infection and nodule development by signals operating in the legume root. This project aims to test whether it would be feasible to improve legume nodulation and nitrogen fixation by improving the content or composition of a class of signaling molecules called flavonoids. Flavonoids control the attraction of rhizobia to legume roots, the activation of nodulation genes, the development of nodules and the successful infection of rhizobia into the root.

Flavonoids are a diverse class of phenolic compounds synthesized by all plants. In legumes, flavonoids that are exuded into the soil have long been known to control the activity of rhizobial nodulation genes. They also have roles in rhizobial infection and in nodule development. Flavonoids also play additional roles in influencing mycorrhizal symbioses, in nutrient uptake and in controlling quorum sensing-regulated behaviors in bacteria. While several studies have shown that flavonoid exudation activates rhizobial nodulation genes, it is not clear whether this would translate to better nodulation, nitrogen fixation or yield.

In this project we will test whether the abundance and diversity of flavonoid metabolites in legumes can be harnessed to optimize nodulation and nitrogen fixation. In the future, the content of key flavonoid metabolites with specific functions in nodulation could be used as a screenable marker to select legume crops with improved nodulation and nitrogen fixation.

Key questions investigated in this project:

  1. Which are the crucial flavonoid metabolites that control successful nodulation in Medicago?
  2. What is the diversity of flavonoid metabolites in roots and root exudates across a collection of diverse genotypes of the model legume, Medicago truncatula?
  3. Does the composition and abundance of key flavonoid metabolites from diverse Medicago genotypes correlate with the efficiency of nitrogen fixation in the host?
Figure 1. Medicago nodule.

Figure 2. Flavonoids in nodulated roots.

Figure 3. Flavonoids in root tip.