Hopanoid lipids compose the Frankia vesicle envelope, presumptive barrier of oxygen diffusion to nitrogenase.

Abstract

Biological nitrogen fixation in aerobic organisms requires a mechanism for excluding oxygen from the site of nitrogenase activity. Oxygen exclusion in Frankia spp., members of an actinomycetal genus that forms nitrogen-fixing root-nodule symbioses in a wide range of woody Angiosperms, is accomplished within specialized structures termed vesicles, where nitrogen fixation is localized. The lipidic vesicle envelope is apparently a functional analogue of the cyanobacterial heterocyst envelope, forming an external gas-diffusion barrier around the nitrogen-fixing cells. We report here that purified vesicle envelopes consist primarily of two hopanoid lipids, rather than of glycolipids, as is the case in cyanobacteria. One envelope hopanoid, bacteriohopanetetrol phenylacetate monoester, is vesicle-specific. The Frankic vesicle envelope thus represents a layer specific to the locus of nitrogen fixation that is biosynthetically uniquely derived.

Transcriptomics of Actinorhizal Symbioses Reveals Homologs of the Whole Common Symbiotic Signaling Cascade

“…In addition to its ability to synthesize an as yet uncharacterized root hair-deforming factor and specialized cells covered with a unique hopanoid lipid (Berry et al, 1993), it contains the sugar 2-O-methyl-D-Man (Mort et al, 1983), a determinant that is present in all the strains tested but otherwise very rare in the microbial world. Frankia is in symbiosis with plants scattered throughout the Fabid but that nevertheless share a pioneer lifestyle and the ability to tolerate nitrogen-poor soils and repopulate biotopes (Benson and Silvester, 1993).…”

“…The major difference between the two plants is oxygen level regulation in nodule cortical cells, which is a critical factor in Frankia. In Alnus, the oxygen level is not regulated, and Frankia has to adapt by synthesizing specialized cells called vesicles with thick hopanoid walls that form a diffusion barrier (Berry et al, 1993). When vesicles are absent, as in Casuarina, there is a low-oxygen tension in the infected cells (Tjepkema, 1979), presumably due to the lignification of their cell walls (Berg and McDowell, 1988), whereas a symbiotic hemoglobin facilitates the supply of oxygen to the bacterial respiration chain (Gherbi et al, 1997).…”

Rhizobial and Actinorhizal Symbioses: What Are the Shared Features?

“…In symbiosis as well as under nitrogen-limiting conditions in the free-living state, frankia can form nitrogen-fixing vesicles at the ends of its hyphae (reviewed in Benson and Silvester, 1993). In thesevesicles, bacterial nitrogenase is protected from oxygen by a multilayered envelope consisting of hopanoids, which are bacteria1 steroid lipids (Parsons et ai., 1987;Berry et al, 1993). In contrast, rhizobia cannot provide oxygen protection for nitrogenase themselves and, with afew exceptions (see, e.g., de Bruijn, 1989), are capable of living on dinitrogen Only in the symbiotic state.…”

Alnus peptides modify membrane porosity and induce the release of nitrogen-rich metabolites from nitrogen-fixing Frankia

“…Ag5 ASUP was most abundant on the vesicle surfaces, suggesting a direct relationship with the symbiotic process, beyond the first steps of the interaction. This specific binding to vesicles could be related to the presence of the thick hopanoid lipidic external layers in vesicles that are not present on the hyphae (Berry et al, 1993).…”

“…In both conditions, Frankia develops special cell types called vesicles for nitrogen fixation. These structures are formed at the tip of hyphae, are spherical with a diameter from 2 to 5 mm and are covered by several layers of hopanoid lipids to protect the nitrogenase enzyme from oxygen (Benson and Silvester, 1993;Berry et al, 1993). In vitro, growth in nitrogen-free medium (BAP À ) has been found to induce the formation of these structures (Murry et al, 1984).…”