Acclimatization of plant growth promoting Pseudomonas strain 7NSK2 in soil : effect on population dynamics and plant growth

To acclimatize a plant growth promoting rhizobacterium (PGPR) to the ambient soil environment, the pyoverdin-producing Pseudomonas aeruginosa strain 7NSK2 and its pyoverdin-deficient mutant MPFM1 were grown in soil from 0 to 4 weeks prior to sowing with or without adding easily-degradable sugars. Pot experiments with corn salad, spinach and maize revealed that the acclimatized strain 7NSK2 showed a more pronounced beneficial effect (from 13 to 32%) on plant growth than the non-acclimatized strain (from 7 to 19%). On the contrary, the mutant strain MPFM1 did not influence plant growth. Acclimatization of strain 7NSK2 in soil for 2 or 4 weeks caused a significant decrease in its population density from 6.32 to 5.82 log cfu g-1 soil. The remaining bacteria colonized the roots actively, resulting in a significant decrease in the fungal populations on the roots (from 25 to 88%) and in the endorhizosphere (ca 70%). MPFM1 did not influence the fungal population. These results substantiate the hypothesis that active pyoverdin production in the root environment is a prerequisite to plant growth stimulation by rhizopseudomonad strain 7NSK2