Molecular phylogeny of Myricaceae: a reexamination of host-symbiont specificity

The phylogeny of 13 species of Myricaceae, the most ancient actinorhizal family involved in a nitrogen-fixing symbiosis with the actinomycete Frankia, was established by the analysis of their rbcL gene and 18S-26S ITS. The phylogenetic position of those species was then compared to their specificity of association with Frankia in their natural habitat and to their nodulation potential determined on greenhouse-grown seedlings. The results showed that Genus Myrica, including M. gale and M. hartwegii, and Genus Comptonia, including C. peregrina, belong to a phylogenetic cluster distinct from the other Myrica species transferred in a new genus, Morella. This grouping parallels the natural specificity of each cluster with Comptonia-Myrica and Morella being nodulated by two phylogenetically divergent clusters of Frankia strains, the Alnus and Elaeagnaceae-infective strains clusters, respectively. Under laboratory conditions, Comptonia and Morella had a nodulation potential larger than under natural conditions. From this study it appears that the Myricaceae are split into two different specificity groups. It can be hypothesized that the early divergence of the genera led to the selection of genetically diverse Frankia strains which is contradictory to the earlier proposal that evolution has proceeded toward narrower promiscuity within the family.

Ultrastructure of cortical root cells of Alnus incana infected by Penicillium nodositatum

Penicillium nodositatum Valla penetrates Alnus incana (L.) Moench roots through deformed root hairs. It then colonizes the cortical cells and induces the formation of myconodules which have structures similar to those of young actinorhizas. The ultrastructure of infected cortical cells has been compared with that of cells infected by pathogenic or symbiotic fungi as well as with that of cells infected by Frankia. Our observations show that an infected host cell shows few visible signs of resistance to the fungal invasion. As soon as P. nodositatum enters the cell, it is surrounded by a wall-like pecto-cellulosic matrix produced by the host cell. The host plasmalemma, invaginated around the endophyte, keeps its integrity as it does in symbiotic associations. At the beginning of the infection, the host cytoplasm and the fungus are intact but when the cell is completely invaded, the host cytoplasm becomes disorganized and subsequently the fungus also senesces, as in roots of Ericaceae infected by an endomycorrhizal fungus or as in older tissues of actinorhizas. P. nodositatum could survive the dead host cell by using its own reserves. Uninfected neighbouring host cells remain intact. P. nodositatum can be considered to behave as a neutral microsymbiont of A. incana, in a way similar to that of a compatible but ineffective Frankia strain.

Influence of host (Alnus and Myrica) genotype on infectivity, N2 fixation, spore formation and hydrogenase activity in Frankia

Nine species of Alnus, two of them represented by several provenances, and Myrica gale were inoculated with a 'local source' of Frankia. The inoculum is an effective (N₂ -fixing), spore-Forming type of Frankia that lacks H, uptake (hydrogenase) activity in symbiosis with A. incana. Despite their wide geographical origin (Europe, China, North America) and their various taxonomie grouping within the genus, all Alnus species/provenances formed Infixing nodules. In all studied nodules Frankia had the phenotype Spore⁺ . None of the studied symbioses had hydrogenase activity. The Frankia inoculum was infective also on M. gale, and gave effective nodules without hydrogenase activity. Unlike some other spore-forming types of symbiotic Frankia, all studied symbiotic phenotypes of the 'local source' of Frankia were thus stable characters not influenced by the various host genotypes included in the study.

Effects of Long-Term Preservation of Frankia Strains on Infectivity, Effectivity, and In Vitro Nitrogenase Activity

Frankia strain HFP ArI3 which had been preserved for 27 months by being lyophilized, frozen in glycerol, or stored in complex medium was successfully used as an inoculum after being subcultured for inducing nodulation and nitrogen fixation of Alnus rubra. Glycerol-preserved HFP ArI3 produced significantly lower rates of nitrogenase activity than did lyophilized or complex-medium-preserved inocula. Bacteria that had been preserved by all three methods were successfully induced to fix atmospheric nitrogen by being cultured in nitrogen-free medium. Subculturing of these cells in nitrogen-free medium a second and third time yielded increasing rates of nitrogenase activity. Initial nitrogenase activity was detected on days 5, 4, and 3 during the first, second, and third subcultures after preservation, respectively. Maximum activity was observed on days 11, 10, and 8 during the first, second, and third subcultures, respectively. A description is given of standard culture techniques used in our laboratory for Frankia isolates, and methods used to distribute Frankia cultures by mail are described.

Initiation and ontogeny of vesicles in cultured Frankia sp. strain HFPArI3

Removal of combined nitrogen from the medium of Frankia sp. strain HFPArI3 induced the formation of specialized structures, called vesicles, which are the proposed site of nitrogen fixation. After 5 to 6 h of culture on N-free medium, newly formed vesicles, termed provesicles, arose from the tips of some hyphae. These structures were spherical, phase dark, ca. 1.5 to 2.0 micron in diameter, and were not associated with acetylene reduction (nitrogenase) activity. Provesicles reached their greatest frequency after ca. 24 h of N-free culture. Provesicles increased in size to become mature vesicles which first appeared after 18 to 20 h of N-free culture. They were ca. 2.5 micron in diameter, phase bright, and reached their greatest frequency after 5 to 6 days, at which time nitrogenase activity peaked. Some vesicles eventually became damaged structurally and took on the appearance of ghosts. Transmission electron micrographs revealed an increase in size from provesicle to mature vesicle. Also evident with the micrographs were the presence of a septum between the young provesicle and parental hypha, the presence of glycogen in some young vesicles, the development of internal septations as vesicles matured, and the degradation of cytoplasm and internal septae in ghost vesicles. The extent to which the formation of vesicles is reversible by the addition of NH4+ was investigated. Commitment times of 3.2 and 6.5 h were obtained for provesicles and vesicles, respectively. A concentration-dependent inhibition of nitrogenase by NH4+ was demonstrated. The structure of preexisting vesicles was also affected by addition of NH4+ to the culture medium.