Role of extracellular pectinases in the rhizoplane competence of a rhizobacterium Burkholderia pickettii MSP3Rif

The contribution of extracellular pectinase to the rhizoplane competence of a rhizobacterium Burkholderia pickettii MSP3Rif was investigated. A mutant (Exp1), defective in extracellular pectinase production, was obtained by Tn5-mutagenesis. The ratio of Exp1-to-the parental strain (wt) significantly decreased during rhizoplane colonisation, when melon seeds were co-inoculated with Exp1 in a 1-to-1 ratio and grown in soil. Then, melon plants were grown in highly compacted soil, where root exudation was most likely to be stimulated, resulting in more remarkable decrease in the ratio of Exp1-to-wt. Declines in the ratio of Exp1-to-wt was also observed on the roots of maize and tomato. There was no change in the ratio when Exp1 and wt were incubated together in 10−1 strength nutrient broth (1/10 NB) and galacturonic acid medium. In addition, the ratio did not change significantly in the process of survival in soil. Co-inoculation of the two strains into sterile root exudates collected from melon plants brought about a significant decrease in the ratio of Exp1 during growth in the insoluble root exudates (mucigels), but not in the soluble exudates. These results suggest that extracellular pectinase production may play a significant role in the rhizoplane competence of MSP3Rif. A role for extracellular pectinase in the rhizoplane competence of B. pickettii MSP3Rif is discussed.