Host plant peptides elicit a transcriptional response to control the Sinorhizobium meliloti cell cycle during symbiosis

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178

Symbiosis between rhizobia soil bacteria and legume plants results in the formation of root nodules where plant cells are fully packed with nitrogen fixing bacteria. In the host cells, the bacteria adapt to the intracellular environment and gain the ability for nitrogen fixation. Depending on the host plants, the symbiotic fate of bacteria can be either reversible or irreversible. In Medicago and related legume species, the bacteria undergo a host-directed multistep differentiation process culminating in the formation of elongated and branched polyploid bacteria with definitive loss of cell division ability. The plant factors are nodule-specific symbiotic peptides. Approximately 600 of them are nodule-specific cysteine-rich (NCR) peptides produced in the rhizobium-infected plant cells. NCRs are targeted to the endosymbionts, and concerted action of different sets of peptides governs different stages of endosymbiont maturation, whereas the symbiotic function of individual NCRs is unknown. This study focused on NCR247, a cationic peptide exhibiting in vitro antimicrobial activities. We show that NCR247 acts in those nodule cells where bacterial cell division is arrested and cell elongation begins. NCR247 penetrates the bacteria and forms complexes with many bacterial proteins. Interaction with FtsZ required for septum formation is one of the host interventions for inhibiting bacterial cell division. Complex formation with the ribosomal proteins affects translation and contributes to altered proteome and physiology of the endosymbiont. Binding to the chaperone GroEL amplifies the NCR247-modulated biological processes. We show that GroEL1 of Sinorhizobium meliloti is required for efficient infection, terminal differentiation, and nitrogen fixation.bacterial targets | antimicrobial peptides | protein interactions | translation inhibition | host peptides S ymbiosis between microbes and hosts, like legumes and various insects, often results in the formation of symbiotic organs where host cells harbor thousands of endosymbiotic bacteria. In rhizobium-legume symbiosis, rhizobia in the soil interact with their host legume and induce the formation of root nodules where bacteria reside in the host cytosol in membrane-enclosed vesicles called symbiosomes. The bacteria adapt to the endosymbiotic lifestyle in the plant cells and gain the capacity for nitrogen fixation, thereby ensuring the nitrogen need of the host plant.In addition to the physiological changes required for nitrogen fixation, in certain rhizobium-legume interactions, like in the Medicago truncatula-Sinorhizobium meliloti symbiosis, the bacteria undergo an additional remarkable differentiation process (1). The endosymbiotic bacteria called bacteroids become polyploid, noncultivable elongated, and frequently branched cells (2). This terminal bacterial differentiation is provoked by hundreds of nodule-specific cysteine-rich (NCR) host peptides that are exclusively expressed in the rhizobium-infected symbiotic cells (3).Nodule development and bacterial infection a...

Section: Involvement Of Ncr247 In Cell Division Arrest and Filamentousmentioning

confidence: 72%

Medicago truncatula symbiotic peptide NCR247 contributes to bacteroid differentiation through multiple mechanisms

Farkas

Maróti

Dürgő

et al. 2014

153

178

“…In this issue of PNAS, Penterman et al (9) found that a specific NCR peptide (NCR247) can alter S. meliloti cell cycle progression to cause a cell division block by at least in part manipulating the CtrA regulatory circuit. It will be important to continue to probe the role of the S. meliloti cell cycle regulation during symbiosis, including carrying out in vivo experiments to verify the predicted CtrA and DnaA regulons, as well as further elucidating the molecular mechanisms by which NCR247 and other NCR peptides modulate the S. meliloti cell cycle and prepare the organism for its important symbiotic role.…”

Section: Discussionmentioning

confidence: 99%

Global analysis of cell cycle gene expression of the legume symbiontSinorhizobium meliloti

Nisco

Abo

et al. 2014

Self Cite

118

In α-proteobacteria, strict regulation of cell cycle progression is necessary for the specific cellular differentiation required for adaptation to diverse environmental niches. The symbiotic lifestyle of Sinorhizobium meliloti requires a drastic cellular differentiation that includes genome amplification. To achieve polyploidy, the S. meliloti cell cycle program must be altered to uncouple DNA replication from cell division. In the α-proteobacterium Caulobacter crescentus, cell cycle-regulated transcription plays an important role in the control of cell cycle progression but this has not been demonstrated in other α-proteobacteria. Here we describe a robust method for synchronizing cell growth that enabled global analysis of S. meliloti cell cycle-regulated gene expression. This analysis identified 462 genes with cell cycle-regulated transcripts, including several key cell cycle regulators, and genes involved in motility, attachment, and cell division. Only 28% of the 462 S. meliloti cell cycle-regulated genes were also transcriptionally cell cycle-regulated in C. crescentus. Furthermore, CtrA-and DnaA-binding motif analysis revealed little overlap between the cell cycle-dependent regulons of CtrA and DnaA in S. meliloti and C. crescentus. The predicted S. meliloti cell cycle regulon of CtrA, but not that of DnaA, was strongly conserved in more closely related α-proteobacteria with similar ecological niches as S. meliloti, suggesting that the CtrA cell cycle regulatory network may control functions of central importance to the specific lifestyles of α-proteobacteria.cell cycle regulation | symbiosis | alpha-proteobacteria

“…NCR247 has been studied in particular detail, and has been reported to enter bacteroids and interact with a plethora of proteins (10). In addition, NCR247 treatment causes massive transcriptome changes in the bacteria (∼15% of the Sinorhizobium meliloti genome), affecting critical cell cycle regulators and cell division genes (11). Genetically, the only in planta evidence supporting the role of NCR peptides in bacterial differentiation comes from ectopic expression of NCR035 in Lotus japonicus (5).…”

mentioning

confidence: 99%

An antimicrobial peptide essential for bacterial survival in the nitrogen-fixing symbiosis

Kim

Chen

et al. 2015

135

124

In the nitrogen-fixing symbiosis between legume hosts and rhizobia, the bacteria are engulfed by a plant cell membrane to become intracellular organelles. In the model legume Medicago truncatula, internalization and differentiation of Sinorhizobium (also known as Ensifer) meliloti is a prerequisite for nitrogen fixation. The host mechanisms that ensure the long-term survival of differentiating intracellular bacteria (bacteroids) in this unusual association are unclear. The M. truncatula defective nitrogen fixation4 (dnf4) mutant is unable to form a productive symbiosis, even though late symbiotic marker genes are expressed in mutant nodules. We discovered that in the dnf4 mutant, bacteroids can apparently differentiate, but they fail to persist within host cells in the process. We found that the DNF4 gene encodes NCR211, a member of the family of nodule-specific cysteine-rich (NCR) peptides. The phenotype of dnf4 suggests that NCR211 acts to promote the intracellular survival of differentiating bacteroids. The greatest expression of DNF4 was observed in the nodule interzone II-III, where bacteroids undergo differentiation. A translational fusion of DNF4 with GFP localizes to the peribacteroid space, and synthetic NCR211 prevents free-living S. meliloti from forming colonies, in contrast to mock controls, suggesting that DNF4 may interact with bacteroids directly or indirectly for its function. Our findings indicate that a successful symbiosis requires host effectors that not only induce bacterial differentiation, but also that maintain intracellular bacteroids during the host-symbiont interaction. The discovery of NCR211 peptides that maintain bacterial survival inside host cells has important implications for improving legume crops.nitrogen-fixing symbiosis | Sinorhizobium meliloti | Medicago truncatula | legume | NCR antimicrobial peptides