LUMINOMETRIC ANALYSES OF PLANT-ROOT COLONIZATION BY BIOLUMINESCENT PSEUDOMONADS

Bioluminescence was used to study root colonization in nonsterile soil by Tn5-luxAB mutants of four Pseudomonas sp. strains. These biolumine scent strains colonized cucumber, cotton, maize, and soybean roots fol lowing seed treatment. In general, there were significant interactions between host and strain. Soybean and cucumber supported the best bact erial colonization, followed by maize and cotton. Across all crops, th e best colonizing strains were GR7.4L and GR20.5L, with mean populatio ns of about log 6.0 (colony-forming units + 1)/root system, followed b y strains 61.9A.3L and 86.139LR with mean populations of about log 5.0 and 4.5 (colony-forming units + 1)/root system, respectively. The Lux + strains colonized abundantly the upper root system, while their popu lations decreased progressively down the root system, and their root-t ip populations were independent of mean populations at the crown and m iddle-root segments. Bioluminescence was observed visually from coloni es growing on selective media and photometrically on roots with a lumi nometer and a charge-coupled device camera. Embedding roots, in situ r oot luminometer readings, and root-imaging techniques were less sensit ive for detecting bacteria on roots than dilution-plating and broth en richment techniques using selective media. The number of bacteria and competition with other rhizosphere microorganisms affected the sensiti vity of the technique used. Selective broth enrichment was less time c onsuming than the dilution-plating technique. Bioluminescence is a sim ple and fast method to evaluate colonization of roots by bacteria.