Eukaryotic control on bacterial cell cycle and differentiation in the Rhizobium –legume symbiosis

Abstract

Symbiosis between legumes and Rhizobium bacteria leads to the formation of root nodules where bacteria in the infected plant cells are converted into nitrogen-fixing bacteroids. Nodules with a persistent meristem are indeterminate, whereas nodules without meristem are determinate. The symbiotic plant cells in both nodule types are polyploid because of several cycles of endoreduplication (genome replication without mitosis and cytokinesis) and grow consequently to extreme sizes. Here we demonstrate that differentiation of bacteroids in indeterminate nodules of Medicago and related legumes from the galegoid clade shows remarkable similarity to host cell differentiation. During bacteroid maturation, repeated DNA replication without cytokinesis results in extensive amplification of the entire bacterial genome and elongation of bacteria. This finding reveals a positive correlation in prokaryotes between DNA content and cell size, similar to that in eukaryotes. These polyploid bacteroids are metabolically functional but display increased membrane permeability and are nonviable, because they lose their ability to resume growth. In contrast, bacteroids in determinate nodules of the nongalegoid legumes lotus and bean are comparable to free-living bacteria in their genomic DNA content, cell size, and viability. Using recombinant Rhizobium strains nodulating both legume types, we show that bacteroid differentiation is controlled by the host plant. Plant factors present in nodules of galegoid legumes but absent from nodules of nongalegoid legumes block bacterial cell division and trigger endoreduplication cycles, thereby forcing the endosymbionts toward a terminally differentiated state. Hence, Medicago and related legumes have evolved a mechanism to dominate the symbiosis.antimicrobial activity ͉ bacteroid ͉ endoreduplication ͉ Medicago ͉ nitrogen fixation

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…NIH-PA Author Manuscript NIH-PA Author Manuscript which might allow them to reach a higher metabolic rate to support nitrogen fixation 65,68 . The intensive DNA synthesis that is required for endoreduplication within bacteroids requires a large quantity of dNTPs that must be supplied by ribonucleotide reductase (RNR) 89 .…”

“…However, what makes comparative study of both these systems most interesting is that they form different types of nodules, in which the nodule structure and the differentiation programme imposed on the symbiotic bacteria are different. M. truncatula and Medicago sativa are examples of indeterminate legumes, which form nodules that possess a persistent meristem, whereas L. japonicus forms determinate nodules, which lack a persistent meristem and in which all cells in the interior of the nodule proliferate, differentiate and senesce synchronously 65 .…”

“…Engineering of bacterial symbionts that nodulate the opposite type of host has determined that the plant controls the degree of differentiation and the ultimate fate of the bacteria 65 . Differentiated bacterial symbionts isolated from L. japonicus nodules have not undergone genomic endoreduplication and can resume growth 65 .…”

“…Differentiated bacterial symbionts isolated from L. japonicus nodules have not undergone genomic endoreduplication and can resume growth 65 . However, fully differentiated bacteroids isolated from M. truncatula are unable to resume growth, perhaps because of effects of extensive genomic endoreduplication on the bacterial cell cycle 65 .…”

“…Surprisingly, plants that form indeterminate nodules impose a programme of genomic endoreduplication on the invading bacterial cells 65 . Bacteroids within indeterminate nodules increase their DNA content and cell size, …”