Saprophytic growth of inoculated Frankia sp. in soil microcosms

Abstract

The potential of two Frankia strains to grow saprophytically was studied in nonsterile soil microcosms with ground leaf litter of Alnus glutinosa as the sole carbon and nitrogen sources. Strains Ag45/Mut15 and ArI3, which represent two taxonomic subgroups within the Alnus host infection group were inoculated alone, or together to investigate potential competition. Their growth was analyzed by in situ and dot-blot hybridization. A significant increase in cell numbers and filament length was observed during the first 6 weeks after inoculation for strain Ag45/Mut15, both alone and in mixed culture with strain ArI3, followed by a decrease until the end of the study after 12 weeks. The number of filaments remained unchanged. In contrast, the cell numbers and filament length of strain ArI3 were reduced significantly during the first 2 weeks and were undetectable for the remainder of the study. These results were comparable with those obtained in sterile mineral medium amended with leaf litter of A. glutinosa, although reductions in cell numbers and filament length were less pronounced than in soil microcosms. In concomitant control studies without leaf litter amendments for both experimental setups, filaments of both strains could only be detected immediately after inoculation. These results were matched in all experimental setups by concomitant shifts in the rRNA content of both strains, i.e., an immediate decline in the rRNA content for strain ArI3 after inoculation, and an increase in the rRNA content, followed by a late decline during incubation for strain Ag45/Mut15. These results demonstrated that Frankia strain Ag45/Mut15 could grow saprophytically in soil with complex carbon and nitrogen sources such as leaf litter, while the growth of strain ArI3 was not supported.

Abundance and Relative Distribution of Frankia Host Infection Groups Under Actinorhizal Alnus glutinosa and Non-actinorhizal Betula nigra Trees

“…In our previous analyses, only one group of frankiae, i.e., subgroup II of the Alnus host infection group represented by strain Ag45/Mut15, was detected by qPCR in both nodules on A. glutinosa and in soils, though with numbers in soils that corresponded largely to those retrieved for nitrogen-fixing frankiae [19]. This subgroup was dominant apparently independent of environmental characteristics such as organic matter content and matric potential that had been shown to affect the development of specific Frankia populations [9][10][11]31]. Our current analyses detected three of our four target subgroups within the genus demonstrating the potential of our qPCRbased quantification procedure for population analyses in soils.…”

“…In the absence of plants, amendment of soils with slowly available C resources such as leaf litter affected the nodulation capacity of different Frankia strains [11] and allowed growth of a small number of strains only, all of which belonged to a distinct phylogenetic cluster within the Alnus host infection group [9,10]. Casuarina-infective strains, another distinct phylogenetic cluster within the Alnus host infection group [12,13], also grew saprotrophically, however, only in the rhizosphere of their host plant [10] or with leaf litter of Casuarina sp.…”

“…Frankia strains of the Alnus and Elaeagnus host infection groups have been shown to grow saprotrophically in the rhizosphere of both host and non-host plants, likely using root exudates that are easily available carbon (C) resources [7][8][9][10]. In the absence of plants, amendment of soils with slowly available C resources such as leaf litter affected the nodulation capacity of different Frankia strains [11] and allowed growth of a small number of strains only, all of which belonged to a distinct phylogenetic cluster within the Alnus host infection group [9,10].…”

A previously uncharacterized gene, yjfO (bsmA), influences Escherichia coli biofilm formation and stress response

“…Biofilms were viewed using a Nikon Eclipse 80 I microscope. Images were obtained using a Nikon DXM 1200F digital camera and images were analysed using Image-Pro Plus (version 5.1) (Mirza et al, 2007). Adobe Photoshop CS3 (version 10.0.1) was used to convert the images to greyscale, invert the image (to make bacteria appear dark against a light background), and optimize contrast.…”

Diversity of Frankia Populations in Root Nodules of Geographically Isolated Arizona Alder Trees in Central Arizona (United States)

“…Some evidence suggests that active Frankia populations in the soil may be preferentially selected by the host for nodulation (33,36). Previous research in our laboratory has confirmed the importance of the plant host in selecting Frankia strains for symbiosis when the same soil was inoculated into six different actinorhizal plant species and resulted in six different diversity profiles (34).…”