NCR peptides - plant effectors governing terminal differentiation of nodule bacteria into the symbiotic form

Uptake of mineral nutrients from the soil is the challenge of plant survival. In particular, the availability of such macro-elements as nitrogen and phosphorus is the limiting factor for plant growth and development. Some plant genera overcome this limitation by establishing symbiotic relationships with microorganisms. A remarkable example of such symbiosis is one between legumes and rhizobia - a group of nitrogen fixing soil bacteria. Rhizobial penetration into roots of a specific host plant causes initiation of a specialized organ, symbiotic nodule. Within cells of symbiotic nodule free-living bacteria differentiate into a symbiotic form called «bacteroids». Such organelle-like structures provide plants with fixed nitrogen in exchange for nutrients (B.J. Ferguson et al., 2010). A number of legumes form nodules, in which bacteria terminally (irreversibly) differentiate into bacteroids, thus losing the opportunity to return to the free-living state. Terminal differentiation of bacteroids begins soon after release of the rhizobia into plant cells and leads to morphological, physiological and genetic changes in bacterial cells. It has been shown that a large family of antimicrobial peptides of plants called Nodule-specific Cysteine-Rich peptides (NCR peptides) plays a key regulatory role in this process (P. Mergaert et al., 2003). Its representatives are similar in structure and mode of action to defensins - plant innate immunity factors; however, NCR genes are expressed only in nodules, which fact is reflected in their name. At the moment, about 700 genes encoding NCR peptides that are highly variable in their amino acid sequence but possess a distinct conservative cysteine motif required for the adoption of correct conformation were identified in the genome of the model legume Medicago truncatula Gaertn. NCR peptides are delivered to their intracellular target symbiosome (сell compartments containing bacteroides) triggering the process of differentiation by interacting with the components of membranes and various intracellular targets of bacteria (D. Wang et al., 2010). The most studied member of this family in M. truncatula is MtNCR247 a cationic peptide with four cysteines forming two disulfide bonds in oxidized form. MtNCR247 affects transcription, translation and cell division processes in M. truncatula microsymbiont Sinorhizobium meliloti at low concentrations, and also exhibits antimicrobial activity at higher concentrations (A. Farkas et al., 2014). To date, NCR peptides are identified only in plants belonging to IRLC (Inverted Repeat-lacking Clade) legumes which are characterized by terminal differentiation of bacteria into bacteroids. Probably, evolutionary acquisition of the variable gene family encoding NCR peptides has been the selective advantage of this group of plants.