|Tabi Tambong, J.|
Submitted to: Molecular Plant Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/1998
Publication Date: N/A
Citation: Interpretive Summary: A naturally-occuring bacterium, found on roots of chickpea plants grown in India, could protect roots from infection by two fungi: Fusarium oxysporum f. sp. ciceris, which causes fusarium wilt; and Pythium splendens, which causes death of seedlings (damping-off). This bacterium, called PNA1, was classified as Pseudomonas aureofaciens, a commonly occuring species in soil. PNA1 inhibited F. oxysporum f. sp. ciceris and P. splendens when grown in culture, and we found that the inhibition was caused by the antibiotic phenazine-1- carboxylate, which we detected in cultures of PNA1. Two mutants of PNA1, which could not produce phenazine-1-carboxylate, were derived. Neither mutant could suppress Fusarium wilt, indicating that phenazine-1-carboxylate is required for the protection of plant roots from infection by F. oxysporum. One of the mutants had a genetic defect that prevented the production of both phenazine-1-carboxylate and tryptophan. This mutant excreted anthranilate, an proposed intermediate in both the phenazine-1-carboxylate and tryptophan biosynthesis pathways. Both anthranilate and the mutant that excreted anthranilate inhibited P. splendens in culture, and protected seedlings from infection by P. splendens. The second mutant was defective in a gene required for phenazine biosynthesis, and this mutant did not inhibit P. splendens and did not suppress damping-off. This research directly addresses the need to learn how biological control organisms function to suppress plant diseases. In this case, the antibiotic phenazine-1-carboxylate was involved in biological control of two plant diseases.
Technical Abstract: Pseudomonas aeruginosa PNA1, isolated from the rhizosphere of chickpea in India, suppressed Fusarium wilt of chickpea, caused by Fusarium oxysporum f. sp. ciceris, and Pythium damping-off of bean, caused by Pythium splendens. When grown in culture, PNA1 produced the phenazine antibiotics phenazine-1-carboxylic acid and oxychloraphine, and inhibited mycelial growth of F. oxysporum f. sp. ciceris, P.splendens, and certain other phytopathogenic fungi. Two mutants (FM29 and FM13) deficient in phenazine production were obtained following transposon mutagenesis of PNA1. The transposon in the genome of FM29 was localized to phnA, which is thought to encode a subunit of anthranilate synthase II involved in phenazine biosynthesis. The FM13 mutation was complemented by trpC, which encodes indole glycerol phosphate synthase in the tryptophan biosynthesis pathway; consequently, FM13 could not grow on a minimal medium in the absence of tryptophan. Neither FM29 nor FM13 suppressed Fusarium wilt of chickpea to the level achieved by the wildtype strain, indicating that phenazine production contributed to the biocontrol of this disease by P. aeruginosa PNA1. FM29 was also less effective than the phenazine-producing parental strain in biological control of Pythium damping-off of bean, but FM13 was as effective as the parental strain in suppressing this disease. Anthranilate, an intermediate in the tryptophan biosynthesis pathway, suppressed mycelial growth of Pythium spp. in culture and Pythium damping off of bean and lettuce. Anthranilate, excreted by FM13 as a consequence of the trpC mutation, may have contributed to the suppression of Pythium damping-off by the mutant.