RELATIVE IMPORTANCE OF MOTILITY AND ANTIBIOSIS IN THE RHIZOPLANE COMPETENCE OF A BIOCONTROL AGENT PSEUDOMONAS-FLUORESCENS MELRC2RIF

Mutants defective in motility or antibiotics production were obtained by Tn5 mutagenesis of a biocontrol agent Pseudomonas fluorescens MelRC 2Rif (wt). Tomato or melon seeds were co-inoculated with a Tn5 mutant and wt in a 1:1 ratio and then grown in soil for 10 days. There was no change in ratios of Tn5 mutants defective in antibiosis to wt in the process of rhizoplane colonization, suggesting little contribution of in vitro antibiosis to the rhizoplane competence of P. fluorescens Mel RC2Rif. Similar results were also obtained when seeds treated with bac teria were planted in soil artificially infested with fungal pathogens . In contrast, ratios of Tn5 mutants defective in motility to wt signi ficantly decreased, suggesting the contribution of motility to the rhi zoplane competence of this bacterium. When a non-motile Tn5 mutant and wt were co-inoculated into soil at a matric potential of pF 2.3 (-20 kPa) and plants were then grown, there was no change in the ratio in r hizoplane colonization, suggesting that motility might have a role in the movement along roots but an insignificant role in the movement fro m bulk soil towards roots. When they were co-inoculated into 0.2% wate r agar (WA) instead of soil, a remarkable decline in ratios was detect ed. Thus it was soil structure that hindered the efficiency of motilit y. Time course enumeration of rhizoplane colonization of tomatoes grow n in WA revealed that motility was an important means of movement towa rds and/or along roots rather than the multiplication on roots.