Ecological importance of motility for the plant growth-promoting rhizopseudomonas strain ANP15

The motility of Pseudomonas fluorescens strain ANP15 was studied with respect to its plant growth-promoting capacities. By means of a non-motile Tn5 mutant, comparative studies were carried out. The maximum growth rate of the Tn5 non-motile mutant under nutrient-rich as well as under nutrient-limiting growth conditions, was 7.5–10.6% higher in comparison with the wild-type strain under the same conditions. The lack of motility of the mutant strain did not affect the ability to survive, nor to spread in the soil compared to the motile wild-type strain. Furthermore, the non-motile strain still exhibited plant growth-promoting effects and its root colonizing capacities appeared not to be altered. Capillary assays indicated that motility might, however, be vital in sites where a distinctive nutrient gradient is present. In agricultural soils and particularly in the densely colonized rhizosphere, motility appears not to be an important property with respect to the plant growth-promoting capacities of rhizopseudomonads. On the other hand, when plant growth-promoting bacteria are applied, it is very important to obtain a good homogenous distribution of the bacteria throughout the plough layer. The results reported here suggest that irrigation as well as the way of inoculation, are the main determining factors for a good distribution: inoculation throughout the plough layer rather than seed inoculation should be applied.