This Research Topic is part of the article collection series - Plant-Rhizobia Symbiosis and Nitrogen Fixation in Legumes.

The reciprocal exchange of chemical signals between legume and rhizobium leads to the establishment of Root Nodule Symbiosis (RNS). The discoveries of the Nodulation Factor (NF) and nod genes, along with the discoveries of the relevant genetic plant factors, have expanded our understanding of how legume-rhizobial interactions constitute a successful mutualistic symbiosis. Symbiotic nodule formation can be divided into the following molecular events: (i) the reciprocal exchange of signals; (ii) epidermal recognition of symbiotic partner and infection thread formation; (iii) nodule organogenesis; (iv) the establishment of a microaerophilic environment to provide nitrogenase the ideal condition to work; and, (v) modulation of immunity to host rhizobium inside the nodule cells. The number of nodules in a plant is maintained via a process of systemic signaling, known as ‘autoregulation of nodulation’ (AON), that acts as a negative signal to control nodule number in low nitrogen conditions and inhibit nodule initiation in an optimally nitrogen-fed plant. Over 200 plant genes have been discovered up to now and these discoveries have given us a broad understanding of RNS.

More recently, the discoveries of NF-independent nodule development and the involvement of the type III secretion system that drives plant signalling pathways have highlighted the complexity present in nature. Several different modalities of symbiotic colonization and organogenesis programs in legumes exist. To understand the depths of the diversity of RNS, the molecular mechanisms that drive these processes need to be understood in greater detail. The domestication of crop legumes has forced them to adopt rhizobial partners in new environments. The molecular framework developed by the symbiotic nitrogen fixation community has allowed researchers to work with less explored crop species and answer questions such as:

1. How can nitrogen fixation efficiency be manipulated?

2. What are the genetic differences that impart variation in nitrogen fixation efficiency among legumes?

3. How many parallel signaling pathways activate/modulate RNS?4. How can plant-microbe and microbe-microbe interactions influence SNF in field conditions?

In the lab, under sterile and artificially created nutrient-limiting conditions, legume-rhizobial symbiosis is a binary interaction. Under field conditions, this interaction is also significantly affected by the available soil nutrients (especially nitrogen), the microbial community present in the rhizosphere, and the compatibility of the plant ecotype with the available rhizobial partners. These interactions and the genetic background of particular crop legumes result in differential nitrogen fixation efficiency by root nodules.

This Research Topic welcomes original research, methods, reviews, mini-reviews, and opinion articles that provide new insights into the molecular mechanisms that influence nitrogen fixation efficiency. It will also accept articles on other factors that influence nitrogen fixation efficiency in model and crop legumes.

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Plant-Rhizobia Symbiosis and Nitrogen Fixation in Legumes, Volume II