Soil Microorganisms

Pseudomonas spp. are known to be a well rhizosphere colonizer in plant roots and to have a high potential in biological control (BC) of soilborne plant diseases and in growth promotion of plants (PGP). While, these are also known to have high productivities of second metabolites concerning with microbial antagonism, as antibiotics, cyanide, siderophores and bacteriocins. The role of these antagonistic substances in BC and/or PGP has been evaluated by following evidences; 1) production of the substances in culture, 2) effect of application of the purified substances, 3) detection and determination of the substances in situ, and/or 4) alteration of the effect by the substance-deficient mutants. This paper reviews recent knowledge on the role of antagonistic substances produced by Pseudomonas spp. in BC and PGP.

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By using a spontaneous mutant strain FP-16 R, resistant to streptomycin, ampicillin and nalidixic acid, which was derived from Pseudomonas putida strain FP-16, and suppressed bacterial wilt of tomato caused by P. solanacearum, experiments on the colonizing ability of the bacterium at the root surface of tomato were carried out. When the tomato plants were inoculated with the strain FP-16 R by dipping and were then grown in root boxes the strain colonized the surface of primary roots which were inoculated and moved downward with the elongation of roots. The colonization of the tomato roots by the strain FP-16 R in the field was influenced by the kinds of organic matter applied to soil. Root colonization by the introduced strain was markedly restricted in the field applied with either dried pig manure or a mixture of cattle feces and saw dust. After 1 year, inoculated FP-16 R could not be detected either at the root surface or in the rhizosphere soil under field conditions. When the strain FP-16 R adsorbed onto charcoal was inoculated into field soil it moved from charcoal to the root surface of tomato 46 days after inoculation, although it remained in soil until 7 to 17 days.

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