Plant Growth-Promoting Rhizobacteria and Plant Growth Under Gnotobiotic Conditions

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Abstract
Increases in radish and potato plant growth caused by inoculating seeds or seed pieces with plant growth-promoting rhizobacteria (PGPR) were apparently not related to the production of bacterial products that directly stimulated growth. Radish seeds inoculated with PGPR and grown under gnotobiotic conditions did not produce larger plants than water-treated controls, even though the PGPR colonized the plant roots. When radishes were grown under the same but unsterile conditions, plants grown from seeds treated with PGPR exhibited significantly greater growth (.ltoreq. 150%) than did untreated controls. Radish seeds treated with rhizobacteria in sterile cellophane growth packets produced hormonal-type increases in branching or total length of roots; however, there was no relationship between increased root development in growth packets and subsequent growth responses by radish inoculated with the same PGPR. These results suggest that PGPR increase plant growth indirectly by interacting with the native root microflora rather than directly by producing growth-promoting substances.