Spore germination and the life cycle of Frankia in vitro

Bacterial spores of Frankia produced in defined culture media were collected by filtration after washing in amounts approximating lo6 spores/mL. Frankia strains UFGCeI5 from Casuarina equisetifolia and UFGCgIl from C. glauca showed spontaneous release of spores in culture; strains HFPCcI3 from C. cunningharniana and HFPAllIl from Allocasuarina lehmanniana showed low spore release in culture unless homogenized. Spore germination was tested on plates of agar nutrient media under different physical and chemical environments. Strain CeI5 showed about 15% germination within 2 days in a defined (BAP) medium with an optimum pH of 6.0-6.8 at 28-35°C. Under these conditions, strain CcI3 germinated less than 0.5%. In a series of trials with increasingly complex media, strain CeI5 showed 75 % spore germination in 3 days at 28°C and pH 6.7 in the most complex medium tested. Additions of specific single organic compounds to BAP medium caused either strong inhibition or slight stimulation of spore germination. Frankia strains that showed spontaneous spore release germinated better than strains that did not release. Spore germination in Frankia strains is markedly influenced by their strain origin and by the physical and chemical environment in which they are placed.