Endophytic bacteria of Mammillaria fraileana, an endemic rock-colonizing cactus of the southern Sonoran Desert

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The small cactus Mammillaria fraileana is a pioneer rock-colonizing plant harboring endophytic bacteria with the potential for nitrogen fixation and rock weathering (phosphate solubilization and rock degradation). In seeds, only a combination of culture-independent methods, such as fluorescence in situ hybridization, scanning electron microscopy, and fluorescence vital staining, detected significant amounts of non-culturable, but living, endophytic bacteria distributed underneath the membrane covering the embryo, in the undifferentiated tissue of the embryo, and in the vascular tissue. Large populations of culturable endophytic bacteria were detected in stems and roots of wild plants colonizing rocks in the southern Sonoran Desert, but not in seeds. Among 14 endophytic bacterial isolates found in roots, four isolates were identified by full sequencing of their 16S rRNA gene. In vitro tests indicated that Azotobacter vinelandii M2Per is a potent nitrogen fixer. Solubilization of inorganic phosphate was exhibited by Pseudomonas putida M5TSA, Enterobacter sakazakii M2PFe, and Bacillus megaterium M1PCa, while A. vinelandii M2Per, P. putida M5TSA, and B. megaterium M1PCa weathered rock by reducing the size of rock particles, probably by changing the pH of the liquid media. Cultivated seedlings of M. fraileana, derived from disinfected seeds and inoculated with endophytic bacteria, showed re-colonization 105 days after inoculation. Their densities decreased from the root toward the stem and apical zones. Functional traits in planta of culturable and non-culturable endophytic bacteria in seeds remain unknown.