Associative Biological Nitrogen Fixation in Maize: From Diazotroph Recruitment to Sustainable Crop Nutrition

Abstract

Biological nitrogen fixation (BNF) represents a promising approach for decreasing maize's reliance on synthetic nitrogen fertilisers while at the same time improving the sustainability of low-input agricultural production systems. Unlike the highly efficient symbiotic nitrogen fixation seen in the association between legumes and rhizobium bacteria, maize mainly depends on associative and endophytic diazotrophic microorganisms which colonise the rhizosphere, the root surfaces and the internal tissues of the plant. Recent discoveries, especially the identification of mucilage-associated nitrogen-fixing microbiomes in Sierra Mixe maize, have once again stimulated interest in the possible role that BNF can play in the nitrogen nutrition of maize. This review brings together current knowledge of the biological, biochemical, genetic, and ecological mechanisms responsible for associative BNF in maize. The main areas covered are the diversity and functional roles of diazotrophic bacteria such as Azospirillum, Herbaspirillum, Gluconacetobacter, Azoarcus and Klebsiella; the regulation of nitrogenase activity and the nitrogen assimilation pathways; and the impact of carbon supply, oxygen availability and host-microbe interactions on the efficiency of fixation. The review also covers recent developments in maize genetics, including genome-wide association studies, quantitative trait locus mapping, and the identification of candidate genes involved in aerial root development and mucilage secretion. Moreover, it assesses the effects of soil fertility management, environmental conditions, and agronomic practices on microbial recruitment and nitrogen fixation. Lastly, the review examines opportunities and challenges in breeding, biotechnology, and microbiome engineering, highlighting their potential to improve associative BNF and support climate-smart, resource-efficient maize production. Combining knowledge of host genetics, rhizosphere ecology, and microbial function may reduce the need for fertilizers and improve nitrogen use efficiency in future maize production systems.