Ultrastructural characterization of eubacteria residing within leaf cavities of symbiotic and cyanobiont-freeAzolla mexicana

Diversity analysis of endophytic bacteria withinAzolla microphyllausing PCR-DGGE and electron microscopy

Using 16S rDNA-polymerase chain reaction–denaturing gradient gel electrophoresis (PCR-DGGE), electron microscopy and a conventional plating method, the genetic diversity and phenotype polymorphism of the endophytic bacteria withinAzolla microphyllawere explored. The 16S rDNA-PCR-DGGE profile showed a complex and divergent bacterial community, withBacillus cereusas the dominant species, within theAzolla–cyanobacteria association. This result was supported by the fact that endobacterial cells exhibited distinct ultrastructural characteristicsin vivoand,in vitro, bacteria displayed various colonies with different sizes, shapes and colours. This study demonstrates that the genetic diversity of endophytic bacteria inAzollacan be investigated using the16S rDNA-PCR-DGGE technique.

Cellular responses in the cyanobacterial symbiont during its vertical transfer between plant generations in the Azolla microphylla symbiosis

The nitrogen-fixing symbiosis between cyanobacteria and the water fern Azolla microphylla is, in contrast to other cyanobacteria-plant symbioses, the only one of a perpetual nature. The cyanobacterium is vertically transmitted between the plant generations, via vegetative fragmentation of the host or sexually within megasporocarps. In the latter process, subsets of the cyanobacterial population living endophytically in the Azolla leaves function as inocula for the new plant generations. Using electron microscopy and immunogold-labeling, the fate of the cyanobacterium during colonization and development of the megasporocarp was revealed. On entering the indusium chamber of the megasporocarps as small-celled motile cyanobacterial filaments (hormogonia), these differentiated into large thick-walled akinetes (spores) in a synchronized manner. This process was accompanied by cytoplasmic reorganizations and the release of numerous membrane vesicles, most of which contained DNA, and the formation of a highly structured biofilm. Taken together the data revealed complex adaptations in the cyanobacterium during its transition between plant generations.

Occurrence and ultrastructural characterization of bacteria in association with and isolated from Azolla caroliniana

The occurrence and ultrastructure of bacteria in leaf cavities of symbiotic Azolla caroliniana were examined by transmission electron microscopy. Bacteria were observed in all leaf cavities of Azolla cultures. Five ultrastructurally distinct types of bacteria were observed in each individual leaf cavity. Features used to characterize the bacteria included morphology, cell wall structure, and cytoplasmic organization. At least one gram-positive and as many as four gram-negative types of bacteria reside in leaf cavities of A. caroliniana. The morphological and ultrastructural characteristics of the gram-positive bacterium suggest that it is an Arthrobacter sp. The gram-negative bacteria could not be cultured; therefore, they have not been classified further. Bacterial cell shape and cell wall structure were similar in leaf cavities of different ages, but cell size and cytoplasmic composition varied. The relative contributions of each bacterial type to the total community within individual leaves was determined. Ultrastructural characteristics of bacterial isolates cultured from A. caroliniana in a free-living state were also examined.

Immunocytochemical localization of nitrogenase in bacteria symbiotically associated with Azolla spp

In situ immunogold labeling and transmission electron microscopy were used to detect nitrogenase in bacteria (bactobionts) symbiotically associated with leaf cavities of Azolla caroliniana and Azolla filiculoides. In A. caroliniana, the Fe protein of the nitrogenase complex was detected in a subset of the distinct bactobiont types present in leaf cavities of all ages. Similar results were obtained for the bactobionts of A. filiculoides with antisera against both the Fe and MoFe subunits of nitrogenase.