Spore-formation of inoculated Frankia strains in nodules formed on Alnus glutinosa

Date

2020-05

Authors

Chahine, Christophe

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Abstract

Members of the genus Frankia are soil-dwelling actinobacteria that are generally characterized as nitrogen-fixing symbionts forming root nodules on specific woody plants. In nodules, the presence or absence of spores formed by Frankia populations has led to the recognition of two distinct phenotypes, i.e. spore-positive and spore-negative frankiae. Isolates obtained so far have not been shown to form spores in nodules on their host plants after re-inoculation, however, are able to produce spores in pure culture in stationary phase. We therefore speculated that isolates might form spores as a function of environmental triggers related to plant dormancy and assessed spore-formation in nodules formed by nitrogen-fixing Frankia strains ArI3 and Ag45/Mut15, both isolated from spore-negative nodules, on dormant and on growing plants of Alnus glutinosa. The presence of sporangia and spores in nodules of dormant plants, but not of growing plants was documented by scanning electron microscopy and in situ hybridizations with Cy3-labeled, 23S rRNA-targeted oligonucleotide probes of nodule sections and crushed nodules, while the identity of both inoculated strains in nodules of both dormant and growing plants was indicated by qPCR analyses and confirmed by comparative sequence analyses of a 23S rRNA gene insertion, nifH and nifD-K IGS gene fragments. These results support our hypothesis that limited availability of carbon sources during plant dormancy could potentially result in dormant stages, i.e. spores, of the bacterial symbiont in the nodules as well.

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Keywords

Frankia, Nodules, Alnus glutinosa, Nitrogen fixation

Citation

Chahine, C. (2020). <i>Spore-formation of inoculated Frankia strains in nodules formed on Alnus glutinosa</i> (Unpublished thesis). Texas State University, San Marcos, Texas.

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