Auxin production by bacteria associated with orchid roots

Bacteria associated with the roots of greenhouse tropical orchids were shown to produce indole-3-acetic acid (IAA) and to excrete it into the culture liquid. The presence and activity of IAA were demonstrated colorimetrically, by thin-layer chromatography, and by biotests. The associated bacteria varied in their ability to excrete indole compounds (1-28 mu g/ml nutrient broth). Addition of tryptophan to the growth medium enhanced phytohormone production. Upon addition of 200 mu g/ml tryptophan, the bacteria isolated from Dendrobium moschatum roots (Sphingomonas sp. 18, Microbacterium sp. 23, Mycobacterium sp. 1, Bacillus sp. 3, and Rhizobium sp. 5) produced 50.2, 53.1, 92.9, 37.6, and 60.4 mu g IAA/ml, respectively, while the bacteria isolated from Acampe papillosa roots (Sphingomonas sp. 42, Rhodococcus sp. 37, Cellulomonas sp. 23, Pseudomonas sp. 24, and Micrococcus luteus) produced 69.4,49.6,53.9, 31.0, and 39.2 mu g IAA/ml. Auxin production depended on cultivation conditions and on the growth phase of the bacterial cultures. Treatment of kidney bean cuttings with bacterial culture liquid promoted formation of a "root brush" with a location height 7.4-to 13.4-fold greater than the one in the control samples. The ability of IAA-producing associated bacteria to act as stimulants of the host plant root development is discussed.