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Review
. 2014 Jun 30;5:326.
doi: 10.3389/fmicb.2014.00326. eCollection 2014.

Nitrogen-fixing Rhizobium-legume Symbiosis: Are Polyploidy and Host Peptide-Governed Symbiont Differentiation General Principles of Endosymbiosis?

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Free PMC article
Review

Nitrogen-fixing Rhizobium-legume Symbiosis: Are Polyploidy and Host Peptide-Governed Symbiont Differentiation General Principles of Endosymbiosis?

Gergely Maróti et al. Front Microbiol. .
Free PMC article

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.

Keywords: Rhizobium-legume symbiosis; bacteroid differentiation; endosymbiont; host effector molecules; plant peptides; polyploidy.

Figures

FIGURE 1
FIGURE 1
Structure of nitrogen-fixing root nodules formed in S. meliloti M. truncatula symbiosis. The different nodules zones are indicated on the longitudinal nodule section: (I) meristem, (II) infection zone, (III) nitrogen fixation zone, (IV) senescence zone. Symbiotic cells in zone II contain the differentiating endosymbionts while in zone III the host cytoplasm is fully packed with long nitrogen-fixing bacteroids. Endosymbionts stained with Syto9 have green fluorescence.
FIGURE 2
FIGURE 2
Differential expression of symPEP genes in M. truncatula nodules. Black signal: in situ hybridization, blue signal: GUS activity of symPEP promoter-GUS fusions in transgenic nodules.

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