The emerging importance of nodule-associated bacteria in sustainable agriculture

Abstract

Legume root nodules host a diverse community of microorganisms that play essential roles in biological nitrogen fixation and plant health. While classical rhizobia are well-known for their ability to convert atmospheric nitrogen into plant-available forms via nitrogenase, increasing attention is now directed toward nodule-associated bacteria (NAB), a group of non-rhizobial endophytes once dismissed as contaminants. NAB contribute to plant growth through multiple mechanisms, including enhanced nutrient acquisition, phytohormone production, modulation of plant defenses, and increased tolerance to drought, salinity, and heavy metal stress. Their interactions with rhizobia can further improve nodulation efficiency and nitrogen-fixing potential. Understanding the cross-inoculation capabilities, specificity, and metabolic functions of both rhizobia and NAB- supported by metagenomics, isotopic and non-isotopic nitrogen-fixation assays, and classical techniques such as ARA and ¹⁵N dilution- is crucial for optimizing legume productivity. Collectively, these insights highlight the potential of rhizobia- NAB consortia as powerful bioinoculants for sustainable agriculture.