Terminal bacteroid differentiation in the legume−rhizobium symbiosis: nodule‐specific cysteine‐rich peptides and beyond

Abstract

Summary Terminal bacteroid differentiation (TBD) is a remarkable case of bacterial cell differentiation that occurs after rhizobia are released intracellularly within plant cells of symbiotic legume organs called nodules. The hallmarks of TBD are cell enlargement, genome amplification and membrane permeabilization. This plant‐driven process is governed by a large family of bacteroid‐targeted nodule‐specific cysteine‐rich (NCR) peptides that were until recently thought to be restricted to a specific lineage of the legume family, including the model plant Medicago truncatula. Recently, new plant and bacterial factors involved in TBD have been identified, challenging our view of this phenomenon at mechanistic and evolutionary levels. Here, we review the recent literature and discuss emerging questions about the mechanisms and the role(s) of TBD.

Autopolyploidy alters nodule‐level interactions in the legume–rhizobium mutualism

“…However, additional studies using a broad range of diploid and neotetraploid host genotypes as well as diverse rhizobial symbionts are needed to empirically evaluate this hypothesis. Symbiosome size was far less variable, which may be due to greater host control over these structures (Mergaert et al., ; Oldroyd et al., ; Alunni and Guorion, ; Coba de la Peña et al., ). Although ploidy differences in host control over symbiosome size was not explicitly tested here, if this trait is not shaped by interactions between host and symbiont genotypes to the same extent as nodule area and N‐fixation zone area, then greater host control may explain the drastic differences observed between neotetraploids and diploids.…”

Transcriptomic dissection of Bradyrhizobium sp. strain ORS285 in symbiosis with Aeschynomene spp. inducing different bacteroid morphotypes with contrasted symbiotic efficiency

“…During nodule formation, rhizobia are released inside the host plant cells by an endocytosis‐like process and transform into nitrogen‐fixing bacteroids. In several legume lineages, a terminal bacteroid differentiation (TBD) process occurs after their intracellular uptake, producing elongated polyploid bacterial cells that switched their cell cycle towards endoreduplication (Mergaert et al ., ; Kondorosi et al ., ; Alunni and Gourion, ). TBD is controlled by the host plant, which produces defensin‐like peptides called Nodule‐specific Cysteine Rich (NCR) peptides (Mergaert et al ., ; Van de Velde et al ., ; Czernic et al ., ).…”

Antifungal symbiotic peptide NCR044.1 exhibits unique structure and multi-faceted mechanisms of action that confer plant protection

“…The 1 H, 13 C, and 15 N chemical shift assignments and peak-picked NOESY data were used as initial experimental inputs in iterative structure calculations with the program CYANA (v 2.1) 48 .…”

“…A mode-of-action (MOA) study showed that NCR247 blocks cell division, induces cell elongation, and formed complexes with bacterial ribosomal proteins affecting protein synthesis and the bacterial proteome 12 . Since M. truncatula expresses several cationic NCR peptides, it is likely that they exhibit different modes of antibacterial action in different compartments of the nodule 13 .…”