1047 PSEUDOMONADS AND THE ROLE OF-THE ANTIBIOTICS IN THE CONTROL OF GREEN MOLD OF LEMONS.

Abstract

Pseudomonas cepacia LT412W reduced green mold on lemons caused by Penicillium digitatum. It produces phenylpyrrole antibiotics which cause inhibition zones in co-culture with the pathogen. Their role in control of the disease was investigated. Mutagenesis of P. cepacia (rifampicin resistant) was performed by mating it with E. coli S-17 pSUP1021 (kanamycin resistant), which carries the transposon Tn5. Transconjugate selection and screening for absence of inhibition zones identified a stable mutant. Growth of parent and mutant were comparable. When the mutant was co-cultured with the pathogen on lemon albedo agar, no inhibition zone appeared. Similar co-culture on potato dextrose agar with tryptophan (0.05 g/L), a precursor of phenylpyrroles, did not induce inhibition zones. This suggests the mutation is not in tryptophan biosynthesis. Parent and mutant were assayed for phenylpyrroles. They were cultured in nutrient broth, centrifuged, and the cells extracted with acetone. The extract was dried and dissolved in chloroform. It was spotted on nano-SIL Cl8 TLC plates, run one hour (methanol:acetonitrile:water, 1:1:1), dried, developed with sulfanilic acid, and observed under UV light. The relative mobility of spots from extracts of the parent matched phenylpyrroles, whereas the mutant produced none. Control of decay by the mutant and parent were equal, suggesting no role for phenylpyrroles in suppression of the disease.