review · Frontiers in Microbiology
Legume root nodules are widely known for harbouring rhizobia that fix atmospheric nitrogen, but they also host diverse non-rhizobial endophytic bacteria. A systematic review reveals that Bacillus and Pseudomonas are the most prevalent non-rhizobial genera found inside nodules, followed by Paenibacillus, Enterobacter, Pantoea, Agrobacterium, and Microbacterium. The most frequently documented host species are soya bean, mung bean, common bean, and lentil. Although these non-rhizobial endophytes do not stimulate nodule creation, they display crucial plant growth-promoting traits, including biofertilisation, phytostimulation, biocontrol, and stress tolerance. Furthermore, co-inoculation of legumes with both rhizobia and non-rhizobial endophytes produces synergistic effects. This dual inoculation approach enhances overall plant growth, crop yield, and nodulation under both normal and stressful growing conditions, surpassing the performance of single inoculations with rhizobia alone.
Legumes are vital global crops, yet conventional inoculants typically focus solely on single-strain rhizobia. Recognising that beneficial non-rhizobial bacteria inhabit root nodules offers a pathway to create superior multi-strain biofertilisers. These microbial partnerships can enhance plant vigour, increase harvest yields, and help crops tolerate environmental stresses, providing sustainable options to improve food production without relying entirely on synthetic fertilisers.
The findings could enable agricultural biotechnology companies to design mixed microbial inoculants combining rhizobia with genera like Bacillus and Pseudomonas for legume farmers. As this work synthesises experimental data demonstrating synergistic yield improvements, the concept sits at an applied research stage, requiring scalable formulation and field-level validation before commercial production.
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Legumes are renowned for their distinctive biological characteristic of forming symbiotic associations with soil bacteria, mostly belonging to the <i>Rhizobiaceae</i> familiy, leading to the establishment of symbiotic root nodules. Within these nodules, rhizobia play a pivotal role in converting atmospheric nitrogen into a plant-assimilable form. However, it has been discerned that root nodules of legumes are not exclusively inhabited by rhizobia; non-rhizobial endophytic bacteria also reside within them, yet their functions remain incompletely elucidated. This comprehensive review synthesizes available data, revealing that <i>Bacillus</i> and <i>Pseudomonas</i> are the most prevalent genera of nodule endophytic bacteria, succeeded by <i>Paenibacillus</i>, <i>Enterobacter</i>, <i>Pantoea</i>, <i>Agrobacterium</i>, and <i>Microbacterium</i>. To date, the bibliographic data available show that <i>Glycine max</i> followed by <i>Vigna radiata, Phaseolus vulgaris</i> and <i>Lens culinaris</i> are the main hosts for nodule endophytic bacteria. Clustering analysis consistently supports the prevalence of <i>Bacillus</i> and <i>Pseudomonas</i> as the most abundant nodule endophytic bacteria, alongside <i>Paenibacillus</i>, <i>Agrobacterium</i>, and <i>Enterobacter</i>. Although non-rhizobial populations within nodules do not induce nodule formation, their presence is associated with various plant growth-promoting properties (PGPs). These properties are known to mediate important mechanisms such as phytostimulation, biofertilization, biocontrol, and stress tolerance, emphasizing the multifaceted roles of nodule endophytes. Importantly, interactions between non-rhizobia and rhizobia within nodules may exert influence on their leguminous host plants. This is particularly shown by co-inoculation of legumes with both types of bacteria, in which synergistic effects on plant growth, yield, and nodulation are often measured. Moreover these effects are pronounced under both stress and non-stress conditions, surpassing the impact of single inoculations with rhizobia alone.
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DOI: 10.3389/fmicb.2024.1386742
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