The Rhizobium-Legume Symbiosis

Dear Colleagues,

Despite its vast abundance in the atmosphere, the anthropological utilization of gaseous nitrogen is accomplished primarily via industrial or biological reduction. Chemical nitrogen fertilizers produced through industrial fixation have revolutionized agriculture. However, chemical nitrogen fertilizers are energy-intensive to produce and apply, and cause environmental pollution in the form of eutrophication and nitrogenous gas emissions. Biological nitrogen fixation by a group of diazotropic prokaryotic organisms with a specialized nitrogenase enzyme complex is a sustainable alternative to the use of chemical nitrogen fertilizers. The most significant form of biological nitrogen fixation in agriculture results from an intricate symbiosis between plants belonging to the Fabids clade, and a group of diazotropic bacteria collectively called Rhizobia. Among the Fabids, the family Fabaceae, also referred to as legumes, consists of the most agriculturally significant plant species, with the ability to form symbiosis with rhizobia.

Evolution of the rhizobium–legume symbiosis appears to be due to a single predisposition event. This fascinating symbiosis allows for the plants to thrive in nitrogen-deficient soils, and the rhizobia receive carbon in return for the fixed nitrogen. The process begins with a chemical signal exchange between the plant and the bacterium, to ensure compatibility. The plants produce specific flavonoid compounds that are perceived by compatible rhizobia that, in turn, produce distinct lipo-chitooligosaccharides, known as “Nod factors”. The perception of Nod factors by compatible plant hosts activates intrinsic plant signaling pathways in the root epidermis, resulting in the formation of specialized infection threads that enable the colonization of plant roots by compatible rhizobia. In parallel, plant hormone signaling initiates the division and differentiation of root cortex cells to give rise to root nodules that accommodate the rhizobia. The coordinated activation of the biochemical reactions and processes by the plant and the rhizobia enables biological nitrogen fixation, utilization, and nutrient exchange. Nodule formation and nitrogen fixation are tightly regulated by a number of factors, in particular, plant nitrogen status.

Genetic, genomic, and biochemical studies have provided significant, but fragmented insights into the nature of chemical signals, how they are perceived, components of the signal transduction pathway, hormone action associated with organogenesis, biochemical components and reactions associated with nitrogen fixation, and nitrogen transport. Significant knowledge gaps exist on the components of these processes and their precise regulation.

This Special Issue welcomes articles (original research papers, perspectives, reviews, and modeling approaches and methods) on “The Rhizobium–Legume Symbiosis” in four major sub-topics, namely: (i) Nod factor signaling, (ii) nodule organogenesis, (iii) nitrogen fixation, and (iv) agronomical and environmental impacts.

Dr. Senthil Subramanian
Dr. Georgina Hernández
Guest Editors

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• Nod factor signaling
• nodule organogenesis
• nitrogen fixation
• agronomical and environmental impacts