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

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.

Determinants of Host Range Specificity in Legume-Rhizobia Symbiosis

“…Nodules belonging to the inverted-repeat lacking clade (IRLC) of legumes are marked by more extreme bacteroid differentiation, and this is mediated, at least in part, by antimicrobial peptides belonging to the nodule-specific cysteine-rich (NCR) family (Van de Velde et al, 2010;Roy et al, 2020). The role of NCR peptides is best understood in M. truncatula, which has over 700 inferred NCRs to date (Maróti et al, 2015), although it is unclear if all NCR family members are involved in regulating symbiosis since their expression levels and pattern can vary [reviewed in Roy et al, 2020]. Patterns of NCR peptide expression vary greatly between nodules of M. truncatula accessions but show little variation within accessions in response to different strains of rhizobia (Nallu et al, 2014).…”

“…There is strong evidence to suggest that rhizobial tolerance of NCR peptides depends on the activity of BacA and BacAlike proteins. These are membrane transport proteins that have been found to be essential for rhizobia to survive within the symbiosome of legume species belonging to the IRLC (reviewed in Roy et al, 2020), although their presence in rhizobia that do not interact with IRLC legumes and also many other bacteria besides, indicates they are likely to have functions outside of symbiosis. Deletion of the bacA gene of S. meliloti Rm2011 alone is sufficient to result in a fix − phenotype in previously compatible nodules of M. sativa and Melilotus alba.…”

The plant circadian clock gene LHY influences Medicago truncatula nodulation

“…The evolution of nodulation in legumes has been greatly shaped by a whole genome duplication event approximately 58 million years ago (MYA), resulting in amplified, rearranged gene families and retention of paralogous genes (24). Prominent amongst these is the Nodule Cysteine-Rich (NCR) gene family of small secreted peptides that are highly specific to nodules (25). Except for some Aeschynomene species from the relatively ancient dalbergoid lineage, NCRs are exclusively found in the Inverted Repeat-Lacking Clade (IRLC) of legumes which includes the model plant M. truncatula and many agriculturally important crops such as alfalfa, clovers, lentils, chickpea, garden pea and fava beans (26).…”

Unexplored Arsenals of Legume Peptides With Potential for Their Applications in Medicine and Agriculture

“…This terminal differentiation is associated with definitive loss of cell division potential, changes in the membrane composition and permeability, cell growth from moderate to extreme sizes coupled to genome amplification, altered cell morphology (Mergaert et al, 2006;Montiel et al, 2017), and more efficient nitrogen fixation (Oono and Denison, 2010). To accomplish this, legumes have evolved a spectacular arsenal of antimicrobial peptides (AMPs) which are targeted to the bacteroids and provoke their differentiation (Mergaert, 2018;Roy et al, 2020). In the IRLC legumes, the NCR peptides, while in Dalbegioids, the convergently evolved NCR-like peptides represent the vast majority of these host effectors (Van de Velde et al, 2010;Czernic et al, 2015;Montiel et al, 2017;Trujillo et al, 2019).…”