Abstract
The H2 is an obligate by-product of N-fixation. Recycling of H2 through uptake hydrogenase (Hup) inside the root nodules of leguminous plants is often considered an advantage for plants. However, many of the rhizobium-legume symbioses found in nature, especially those used in agriculture are shown to be Hup−, with the plants releasing H2 produced by nitrogenase activity from root nodules into the surrounding rhizosphere. Recent studies have suggested that, H2 induces plant-growth-promoting rhizobacteria, which may explain the widespread of Hup− symbioses in spite of the low energy efficiency of such associations. Wild legumes grown in Nova Scotia, Canada, were surveyed to determine if any plant-growth characteristics could give an indication of Hup choice in leguminous plants. Out of the plants sampled, two legumes, Securigera varia and Vicia cracca, showed Hup+ associations. Securigera varia exhibited robust root structure as compared with the other plants surveyed. Data from the literature and the results from this study suggested that plants with established root systems are more likely to form the energy-efficient Hup+ symbiotic relationships with rhizobia. Conversely, Hup− associations could be beneficial to leguminous plants due to H2-oxidizing plant-growth-promoting rhizobacteria that allow plants to compete successfully, early in the growing season. However, some nodules from V. cracca tested Hup+, while others were Hup−. This was similar to that observed in Glycine max and Pisum sativum, giving reason to believe that Hup choice might be affected by various internal and environmental factors.
Highlights
Rhizobia species have been credited with the fixation of atmospheric nitrogen by forming symbiotic relationships with leguminous plants
Root nodules from soybean plants inoculated with the H2 uptake (Hup)+ (JH) and HupÀ (JH47) strains of Bradyrhizobium japonicum were used as positive and negative controls, respectively
The nodule Hup statuses detected in this survey are listed in Table 2, similar in size to the majority of the surveyed plants
Summary
Rhizobia species have been credited with the fixation of atmospheric nitrogen by forming symbiotic relationships with leguminous plants. This process occurs within nodules, which are formed by the invasion of these bacteria into plant tissues, and allows the plant to utilize compounded nitrogen for amino and nucleic acid formation. The H2 produced diffuses into the surrounding soil, signifying a loss of energy for the plant. For this reason, nitrogen fixation is often seen as energy inefficient
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