Implication of the hydrogen recycling capacity of rhizobia inoculants on the drought stress resistance of legume crops

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Besides the input of nitrogen in soil supported by legume crops, leakage of molecular hydrogen (H2) from nitrogen-fixing nodules is envisioned as a significant contributor to the crop rotation effect. Being an obligatory by-product of the nitrogen fixation reaction, large amounts of H2 are produced in agroecosystems, representing 240 000 liters per hectare per growing season according to previous estimates. However, there are nitrogen-fixing rhizobacteria endowed with the ability to capture the H2 produced, but these latter are rarely found in natural symbioses, whereas they are dominant in commercial inoculants. Two schools of thought attempt to explain this paradox. The first stipulates that rhizobium-legume symbioses recycling H2 (Hup+) are more effective in improving the energy balance of the symbiosis, justifying the interest in integrating them into commercial inoculants. The second sees H2 leakage from rhizobium-legume symbioses that do not recycle H2 (Hup-) as a currency for the enrichment of H2-oxidizing bacteria (BOH), using the gas as a source of energy in soil. BOH are ubiquitous in soil and are represented by several species that promote plant growth and stress tolerance. Could a transition to Hup- nitrogen-fixing rhizobium inoculants help improve crop resilience to climate change? This issue will be addressed through a workshop guiding the preparation of a review article to be held in Spain and through research activities that will be carried out to examine the impact of H2 leakage on plant tolerance to drought. Theoretical and experimental data will provide the first mechanistic information on the impact of H2 emitted by nitrogen-fixing nodules on "rhizobium-plant-microbe-soil" interactions. The results will be shared with Canadian pulse growers facing an increase in the frequency of drought events, consolidating new partnerships and guiding their selection of inoculants.