Relationship of in vitro Antibiosis of Plant Growth-Promoting Rhizobacteria to Plant Growth and the Displacement of Root Microflora

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Abstract
In preliminary experiments, plant growth-promoting rhizobacteria (PGPR) genetically marked for resistance to rifampicin and nalidixic acid caused significant (P = 0.01) increases ranging 300-500% in total weight of potato plants grown in field soils in the greenhouse. The 5 PGPR strains used were fluorescent Pseudomonas spp. that exhibited antibiosis on King''s B medium against 10 or more of 40 rhizosphere bacterial isolates. Mutants that caused no antibiotic effects did not induce plant growth increases. Growth rates of mutants were similar to that of wild-type strains and the mutants colonized plant roots with population densities similar to the wild-types. In field tests, wild-type PGPR caused increases in stolon development, but no increases were detected in plants grown from potato seed pieces treated with the mutants. Treatment of seed pieces with wild-type PGPR resulted in reductions in root zone fungal and Gram-positive bacterial population densities ranging 23 to 64% and 25 to 93%, respectively. No differences were detected in microbial populations on roots of control plants and plants treated with mutants having no antibiosis activity. The ability of PGPR to induce increased plant growth is related in part to antibiosis that occurs in root zones and the subsequent displacement of certain root colonizing microorganisms.

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