Symbiotic Outcome Modified by the Diversification from 7 to over 700 Nodule-Specific Cysteine-Rich Peptides.

Proyash Roy,Miriam L Gifford,Mingkee Achom,Helen Wilkinson,Beatriz Lagunas

doi:10.3390/genes11040348

Proyash Roy, Miriam L Gifford + Show 3 more

Open Access

https://doi.org/10.3390/genes11040348

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Journal: Genes	Publication Date: Mar 25, 2020
Citations: 26	License type: CC BY 4.0

Affiliation: University of Warwick, University of Dhaka, Cornell University

Abstract

Legume-rhizobium symbiosis represents one of the most successfully co-evolved mutualisms. Within nodules, the bacterial cells undergo distinct metabolic and morphological changes and differentiate into nitrogen-fixing bacteroids. Legumes in the inverted repeat lacking clade (IRLC) employ an array of defensin-like small secreted peptides (SSPs), known as nodule-specific cysteine-rich (NCR) peptides, to regulate bacteroid differentiation and activity. While most NCRs exhibit bactericidal effects in vitro, studies confirm that inside nodules they target the bacterial cell cycle and other cellular pathways to control and extend rhizobial differentiation into an irreversible (or terminal) state where the host gains control over bacteroids. While NCRs are well established as positive regulators of effective symbiosis, more recent findings also suggest that NCRs affect partner compatibility. The extent of bacterial differentiation has been linked to species-specific size and complexity of the NCR gene family that varies even among closely related species, suggesting a more recent origin of NCRs followed by rapid expansion in certain species. NCRs have diversified functionally, as well as in their expression patterns and responsiveness, likely driving further functional specialisation. In this review, we evaluate the functions of NCR peptides and their role as a driving force underlying the outcome of rhizobial symbiosis, where the plant is able to determine the outcome of rhizobial interaction in a temporal and spatial manner.

Highlights

nodule-specific cysteine-rich (NCR) are likely to be more effective against bacterial defence (e.g., HrrP would not be efficient for NCRs with varying pI)
While the host benefit from bacterial terminal bacteroid differentiation (TBD) is quite obvious, how bacterial partners benefit from TBD is not yet clear
It could be that the subset of bacteria that are proliferating in the infection thread or those saprophytic bacteria at the proximal region of senescence zone, contribute significantly to the bacterial population and their natural persistence from an evolutionary perspective

Summary

The Role of NCRs in the Legume-Rhizobia Arms Race

Legume-rhizobia symbiosis has mostly been depicted as an altruistic mutualism between two partners. The outcome of the nitrogen fixation symbiosis depends on the host legume-endosymbiont balance to control each other’s functions. One observation suggests some rhizobial strains might have evolved mechanisms to avoid terminal differentiation entirely when interacting with IRLC legumes. Glycyrrhiza uralensis (liquorice), an IRLC legume able to trigger terminal differentiation with some rhizobial species HH103 escapes terminal differentiation, it still fixes nitrogen for liquorice, resembling non-IRLC legume bacteroids. When HH103 infects non-IRLC legumes, Glycine max or Cajanus cajan, such modification of LPS was not observed [49]. LPS-modifications could be a possible bacterial defence mechanism against NCR-mediated control.

NCRs Can Act as Determinants of Host-Symbiont Compatibility

Discussion and Perspectives