Nitrogen-fixing Rhizobium-legume symbiosis: are polyploidy and host peptide-governed symbiont differentiation general principles of endosymbiosis?

Abstract

The symbiosis between rhizobia soil bacteria and legumes is facultative and initiated by nitrogen starvation of the host plant. Exchange of signal molecules between the partners leads to the formation of root nodules where bacteria are converted to nitrogen-fixing bacteroids. In this mutualistic symbiosis, the bacteria provide nitrogen sources for plant growth in return for photosynthates from the host. Depending on the host plant the symbiotic fate of bacteria can either be reversible or irreversible. In Medicago plants 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. About 500 of them are cysteine-rich NCR peptides produced in the infected plant cells. NCRs are targeted to the endosymbionts and the concerted action of different sets of peptides governs different stages of endosymbiont maturation. This review focuses on symbiotic plant cell development and terminal bacteroid differentiation and demonstrates the crucial roles of symbiotic peptides by showing an example of multi-target mechanism exerted by one of these symbiotic peptides.

Burkholderia bacteria infectiously induce the proto-farming symbiosis of Dictyostelium amoebae and food bacteria

“…Taken together, this horizontal transfer suggests that little coevolution may be needed for the formation of novel symbiosis. However, in some plant-rhizobia or -mycorrhizal associations, the symbiont phenotype can vary substantially among hosts, and the outcome of the association is determined by the interaction of host and symbiont-derived factors (34)(35)(36)(37)(38). Similarly, a synergistic interplay between host and symbiont components mediates the initiation and persistence of the bobtail squid-Vibrio fischeri symbiosis (39)(40)(41).…”

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

“…The large size of the NCR family and the limited sequence homology among its members hinder efforts to generalize their role in the symbiosis. Their sequence diversity and distinct temporal and spatial expression patterns have led to the suggestion that they may perform diverse functions, with different bacterial targets and modes of action (9), but direct proof of functional diversity is still lacking.…”

The Plant Peptidome: An Expanding Repertoire of Structural Features and Biological Functions

“…Interestingly, in contrast to most AMP family members, a majority of these peptides are anionic or neutral (Durgo et al, 2015), and they were suggested to neutralize the toxic activity of cationic NCRs. However, although cationic NCRs display in vitro antimicrobial activity, it is doubtful that they exert this activity in vivo, as peptide concentrations in nodules are lower than those used in in vitro assays Tiricz et al, 2013;Maróti and Kondorosi, 2014). Instead, they are essential for bacteroid differentiation and for maintenance of their bacterial endosymbionts Farkas et al, 2014;Penterman et al, 2014).…”