Certain legumes form a symbiotic relationship with bacteria! Tucked into their roots, these bacteria increase the crop yield of these plants. These bacteria, capable of producing lipid molecules known as hopanoids, therefore seem to provide a distinct advantage.
They are among the “most wanted” of the 25,000 bacterial species identified throughout the world1 . What are they? Bradyrhizobium bacteria, of the order Rhizobium. Found in more than 99% of soil, they form symbiotic relationships with legumes (soybean, groundnut, cowpea, etc.), thereby increasing their crop yields. How? By creating nodules on their roots, they fix atmospheric nitrogen and transfer it to the plants in a form they can absorb. Their special value lies in their ability to withstand multiple stressors. They are in fact capable of disrupting symbiosis processes. For example, soil salinisation and acidification are threats to the bacterium, while the host plant can release toxic substances (acids, antimicrobial peptides, etc.). However, these bacteria have a secret weapon: they produce lipid molecules known as hopanoids. “By strengthening the wall around the bacterium, hopanoids help contain the bacterium in the host cell, explains Eric Giraud who has recently published a summary on this topic2. Without these lipids, the bacterium dies and the nodule disappears”. The preponderance of Bradyrhizobium bacteria in soil across the world is therefore explained in part by the existence of the hopanoids they produce.
The properties of these lipid molecules for withstanding stressors inside the host plant or in the outside environment make them particularly attractive in terms of agricultural research. Faced with increasingly drastic environmental conditions and depleted soil, farmers must implement new cultivation methods and refer to scientific research in the matter. “In Africa, farmers are currently using chemical nitrogen fertilisers to grow groundnuts and cowpeas, continues the researcher. This is a harmful practice because of its financial cost as well as negative impact on the environment and health. Instead, we could envisage the introduction of bacteria suited to these plants and these harsh environmental conditions”. The Rhizobium bacteria which will be used to inoculate the roots must be selected on the basis of their ability to produce vast amounts of hopanoids so as to effectively replace current chemical fertilisers.
From legumes to cereals
Improving the symbiosis between plants and bacteria is essential for world agriculture, in a context of global warming. However, while Bradyrhizobium bacteria can develop relationships with legumes, these legumes are mainly grown and eaten in Africa or Asia. In other continents, cereals are the dominant crops. One of tomorrow’s scientific challenges will be to transfer this ability to create symbiosis with rice, wheat or maize.
1. M. Delgado-Baquerizo, A. M. Oliverio, T. E. Brewer, A. Benavent-González, D. J. Eldridge, R. D. Bardgett, F. T. Maestre, B. K. Singh, N. Fierer. A global atlas of the dominant bacteria found in soil, Science, 2018.
2. B. J. Belin, N. Busset, E. Giraud, A. Molinaro, A. Silipo and D. K. Newman. Hopanoid lipids: from membranes to plant–bacteria interactions, Nature Reviews Microbiology, 2018
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