1,2-DIPALMITOYL PHOSPHATIDYLCHOLINE, 1,2-DIPALMITOYL PHOSPHATIDIC-ACID OR 1,2-DIPALMITOYL-SN-GLYCEROL INHIBIT SPORANGIA FORMATION AND PROMOTE EXPONENTIAL-GROWTH OF VARIOUS FRANKIA ISOLATES FROM THE CASUARINACEAE FAMILY

Frankia strains from Casuarina (BR, S21, Thr), Allocasuarina (Allo2) a nd Gymnostoma (G80) genera were found to grow exponentially in stirred , propionate-containing BAP medium supplemented with 1.2 or 2.4 mum sy nthetic 1,2-dipalmitoyl phosphatidylcholine, 1,2-dipalmitoyl phosphati dic acid or 1,2-dipalmitoyl-sn-glycerol. Phosphatidylcholines containi ng C15:0 or C17:0 acyl residues instead of C16:0 (palmitoyl) residues were equally beneficial but stearoyl (C18:0) residues favored the form ation of small sporangia. Oleoyl residues (C18:1) appeared to be toxic as they produced a progressive post-exponential decrease in biomass. Our results suggest that phospholipids containing palmitoyl residues a re part of the beneficial compounds present in the complex egg yolk ph osphatidyl choline mixture which promote exponential growth of Frankia strains, inhibit sporangia formation and delay post-exponential bioma ss degradation. Phosphatidic acids do not appear to mediate the enhanc ing effect of phosphatidyl-choline derivatives. The fact that 1,2-dipa lmitoyl-sn-glycerol alone is beneficial, suggests that the phosphate a nd choline residues of the phosphatidylcholine molecule are not essent ial for balanced growth of Frankia under our conditions. Taken togethe r our results suggest that a primary target of the beneficial compound s may be the plasma membrane of Frankia cells.