About this Research Topic
Rhizobia comprise a wide group of soil-dwelling bacteria that have the ability of establishing nitrogen-fixing symbiosis with leguminous plants through a specific interaction. Under appropriate conditions, these bacteria invade plant roots and elicit the formation of nodules: new organs in which endosymbiotic differentiated forms of bacteria reduce nitrogen to ammonia that will be used by the plant.
This Research Topic addresses the molecular bases underlying the mutual recognition that takes place at early stages of the Rhizobium-legume interaction before bacterial release within the plant symbiotic cells and the onset of nitrogen fixation. The formation of nitrogen-fixing nodules is a very complex process, in which rhizobial infection initiated at the epidermis needs to be co-ordinated with the nodule developmental program in the cortical cells. Because of this, the interaction is highly specific, requiring a continuous and adequate signaling in such a way that each rhizobial strain is able to nodulate a definite set of host legumes.
Investigations carried out in the last century allowed the identification of key players in the early recognition between Rhizobia and their plant hosts. The interaction begins when bacteria are attracted to the legume rhizosphere by different cues present in root exudates. Specific (iso)flavonoids, host root phenolic compounds, trigger the production of bacterial Nod-factors (NF), lipochitooligosaccharides that elicit plant responses crucial in the infection and nodule formation processes. Additional microbial signals such as different surface polysaccharides, as well as proteins secreted through specialized translocation systems also play essential roles in the early stages of the symbiosis. The knowledge on the regulatory mechanisms that control the production of these molecules and how they are perceived by the plant is an intense research topic. Although in the last twenty years remarkable progress has been made on the identification of plant receptors for rhizobial signals, many aspects of the downstream signaling components and early plant responses elicited upon contact with the microbial partner still remain unknown. Especially important for the establishment of a compatible Rhizobium-legume symbiosis is the control of plant innate immunity by mechanisms that are not fully understood.
This article collection welcomes manuscripts that contribute to the understanding of the early molecular communication between plants and rhizobia, including but not limited to:
• Chemoattraction of rhizobia to plant roots
• Plant symbiotic early signaling components
• Plant biochemical and physiological responses to microbial signals
• Rhizobial surface polysaccharides
• Rhizobial effectors and effector-triggered responses in the plant
• Control of plant defense responses
• Cross-talk between the plant immune system and symbiosis
• Plant hormones with a role in the early stages of the interaction
• Cell reorganization during infection thread formation
• Inter-kingdom communication mediated by signals that fine-tune the Rhizobium-legume interaction
Original Research articles but also Mini-Reviews, Perspectives, Hypothesis and Opinion articles are welcome.
Keywords: Legumes, Rhizobial infection, Symbiosis, Root nodule, Signaling, Rhizobial effectors, Surface polysaccharides, Plant immunity, Phytohormones
Important Note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.