In vitro sporulation of Frankia strain HFPCcI3 from Casuarina cunninghamiana

Optimization of the in vitro sporulation of Frankia is a prerequisite for the development of spore preparations as a practical inoculum for actinorhizal plants. The in vitro sporulation of Frankia strain HFPCcI3 previously isolated from Casuarina cunninghamiana was increased from 0 to 12 million spores per millilitre of culture medium when nitrogen and phosphorus were excluded from a defined nutrient medium. The same medium did not increase sporulation of Frankia isolates HFPArI3 from Alnus rubra and HFPCpI1 from Comptonia peregrina. The addition of 10 mM aliphatic L amino acids increased the in vitro sporulation of HFPCcI3 by as much as 39% when grown in a defined nutrient medium with 1 mM potassium phosphate lacking ammonium chloride, whereas the addition of 10 mM acidic, basic, aromatic, and sulfur amino acids decreased sporulation. The in vitro sporulation of HFPCcI3 was inhibited by culture at 33°C relative to culture at 23 and 28°C. Rotary shaking at 100 rpm increased sporulation at the low culture temperatures.