MUTATIONS AFFECTING HYPHAL COLONIZATION AND PYOVERDINE PRODUCTION IN PSEUDOMONADS ANTAGONISTIC TOWARD PHYTOPHTHORA-PARASITICA

In previous studies, Pseadomonas putida 06909 and Pseudomonas fluoresc ens 09906 suppressed populations of Phytophthora parasitica in the cit rus rhizosphere, suggesting that these bacteria may be useful in biolo gical control of citrus root rot. In this study we investigated the me chanisms of antagonism between the bacteria and the fungus. Both bacte ria colonized Phytophthora hyphae and inhibited the fungus on agar med ia. A hyphal column assay was developed to measure the colonization of bacteria on fungal hyphae and to enrich for colonization-deficient mu tants. In this way we identified Tn5 mutants of each pseudomonad that were not able to colonize the hyphae and inhibit fungal growth in vitr o. Colonization-deficient mutants were nonmotile and lacked flagella. Survival of nonmotile mutants in a citrus soil was similar to survival of a random Tn5 mutant over a 52-day period. Additional screening of random Tn5 mutants of both pseudomonads for loss of fungal inhibition in vitro yielded two distinct types of mutants. Mutants of the first t ype were deficient in production of pyoverdines and in inhibition of t he fungus in vitro, although they still colonized fungal hyphae. Mutan ts of the second type lacked flagella and were not able to colonize th e hyphae or inhibit fungal growth. No role was found for antibiotic pr oduction by the two bacteria in the inhibition of the fungus. Our resu lts suggest that both hyphal colonization and pyoverdine production ar e important in the inhibition of Phytophthora parasitica by P. fluores cens and P. putida in vitro.