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Rhizobia and Legume Roots: Why Do They Bond?

Pea Field with closeup of nodulated roots

Do bacteria have tastes and preferences? In this week’s edition of Growing Possibilities, we answer the question:

Why do different legumes need different inoculants?

While bacteria can’t make conscious decisions, like humans, they do experience biochemical attraction. Plants release compounds called flavonoids to attract rhizobia (bacteria) and provoke their root colonization. Flavonoids are like a natural “perfume” found in the largest concentrations near the plant’s root tips, where rhizobial colonization takes place (1). After colonization, the flavonoids take on a new role, regulating rhizobial growth, nodulation, and nitrogen (N) fixation (1). While bacteria can, in a primitive way, smell flavonoids, these compounds are not named after the word flavour. Flavonoids are named because of the Latin word flavus, meaning yellow, their color in nature (2).

Bacterial motility (movement) is imperative for nodulation, which is part of the reason why rhizobia excel as an inoculant – they are flagellated bacteria. Their flagella are like rotating ‘tails’, propelling them through the soil in search of optimal environments, which they seek out using chemotaxis (3). Chemotaxis allows the rhizobia to sense the plant-released flavonoids in the soil, and in response they “swim” closer to the specific plant roots (4).

Once the rhizobia reach the rhizosphere (the area right around the roots & root hairs), they find and adhere to the roots, and release their own plant-specific compounds. These compounds are signaling molecules called LCO (lipochitooligosaccharides) (1). LCOs signal to the plant to accept the colonization of rhizobia on its roots and stimulate the plant to wrap its root tips around them (5). This symbiosis forms the N-supplying nodules that farmers love to see in their crops.

The process of nodulation only works when the plant and rhizobia have the correct receptors for the compounds released by the other. If the rhizobium doesn’t have a receptor for the plant’s expressed flavonoid, it will not seek out the roots. Likewise, if a plant doesn’t have a receptor for the rhizobium’s LCO, it will not accept colonization and/or it will not form nodules with its roots (6).

It’s for this reason that certain rhizobia are more efficient at N fixation than others for each type of crop (6). Bradyrhizobium japonicum is well-known as the best N-fixing rhizobium for soybean crops, but it is not native to North American soils. Using an inoculant that contains B. japonicum, such as XiteBio® SoyRhizo®, is crucial when planting soybeans for the first time in a previously uninoculated field (7). Rhizobium leguminosarum is used to fix N in pulse crops, like peas, lentils & faba beans. Using an inoculant that contains R. leguminosarum like XiteBio® PulseRhizo® is the best way to ensure high yielding pulse crops. To learn more about these inoculant products, contact us at xitebio.ca or call directly to one of our knowledgeable sales team members.

Resources

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3442858/pdf/psb-7-636.pdf
2. https://en.wikipedia.org/wiki/Flavonoid
3. https://www.frontiersin.org/articles/10.3389/fpls.2021.725338/full
4. https://pubmed.ncbi.nlm.nih.gov/26797793/
5. https://www.grainews.ca/features/how-pulse-crops-work-with-bacteria-to-fix-their-own-nitrogen/
6. https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI.1999.12.10.839
7. https://www.topcropmanager.com/making-the-best-inoculant-choices-12882/

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