Citation: Tsyganova, A.V.; Tsyganov, V.E. Rhizobial Symbiosis in Crop Legumes: Molecular and Cellular Aspects. Agronomy 2022, 12, 2857. https://doi.org/10.3390/ agronomy12112857 Received: 13 October 2022 Accepted: 14 November 2022 Published: 15 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). agronomy Editorial Rhizobial Symbiosis in Crop Legumes: Molecular and Cellular Aspects Anna V. Tsyganova * and Viktor E. Tsyganov * Laboratory of Molecular and Cellular Biology, All-Russia Research Institute for Agricultural Microbiology, Saint Petersburg 196608, Russia * Correspondence: avtsyganova@arriam.ru (A.V.T.); vetsyganov@arriam.ru (V.E.T.); Tel.: +7-812-470-5100 (A.V.T. & V.E.T.) The production of high-value, environmentally friendly and healthy food has been the major global focus of sustainable agriculture in recent years [1,2]. Further progress in this area requires the development, testing and introduction of new agricultural technologies that would minimise environmental risks, help maintain or even improve the fertility of soils and support the creation of new types of agricultural products. To fulfil these condi- tions, new approaches are needed, including better use of genetic resources such as plants and microorganisms. The use of microorganisms will dramatically increase the diversity of genetic resources and contribute to increasing the resilience of agricultural systems [3]. Numerous studies show that during evolution, plants have used certain features of microorganisms to enhance their own adaptive capacities. For example, the plant genome incorporated some genetic factors that help create new ecological niches for microorganisms, wherein the genes providing the expression of adaptation remained in the genomes of microorganisms. Recent studies have shown that the symbiotic signalling pathways in plant species that form intracellular symbioses (including arbuscular mycorrhiza, ericoid and orchid mycorrhizae in angiosperms; ericoid-like mycorrhiza in bryophytes; legume– rhizobial and actinorhizal symbioses) is conserved [4]. The fact that symbiotic signalling has been conserved over 450 million years of evolution indicates the great importance of these associations for the successful spreading of land plants. One of the best demonstrations of how the adaptive capacity of plants is expanded through co-evolution with microorganisms is the formation of symbiotic nitrogen-fixing nodules on the roots of legumes. Numerous genes in legumes are involved in the formation of the nodule. More than 40 regulatory symbiotic genes have been identified in the garden pea [5], while the process of nitrogen fixation is controlled by bacterial genes [6]. Therefore, the use of plant–microbe systems which are based on a nitrogen-fixing symbiosis between legumes and rhizobia is of significant interest for developing new approaches in sustainable agriculture [3,7–9]. The widespread use of legumes in sustainable agriculture will increase biological nitrogen fixation, reduce energy costs, improve the physical properties of the soil and increase soil microbial biodiversity [10,11]. In addition, legumes are important food and feed crops and are staples in some regions of the world [12]. The nitrogen-fixing nodule is a unique ecological niche for rhizobia in which micro- aerobic conditions enable the functioning of the main enzyme of nitrogen fixation, nitro- genase, which is highly sensitive to oxygen [13]. In the symbiotic nodule, specialised infected plant cells, which are increased in size due to endoreduplication, provide shelter to thousands of bacteria [14]. Bacteria are isolated from the cytoplasm of the plant cell through a membrane of plant origin, which has inclusions of bacterial proteins: the so- called symbiosome membrane. Within the symbiosome membrane, bacteria differentiate into a specialised form, bacteroids capable of nitrogen fixation, and together with the surrounding symbiosome membrane form a symbiosome [14]. The infected cells of the symbiotic nodule can be seen as a unique system in legumes that have appeared during Agronomy 2022, 12, 2857. https://doi.org/10.3390/agronomy12112857 https://www.mdpi.com/journal/agronomy