Enhanced production of indole-3-acetic acid by a genetically modified strain of Pseudomonas fluorescens CHA0 affects root growth of cucumber, but does not improve protection of the plant against Pythium root rot

The biocontrol strain CHA0 of Pseudomonas fluorescens produces small amount s of indole-3-acetic acid via the tryptophan side chain oxidase and the try ptophan transaminase pathways. A recombinant plasmid (pME3468) expressing t he tryptophan monooxygenase pathway was introduced into strain CHA0; this r esulted in elevated synthesis of indole-3-acetic acid in vitro, especially after addition of L-tryptophan. In natural soil, strain CHA0/pME3468 increa sed fresh root weight of cucumber by 17-36%, compared to the effect of stra in CHA0; root colonization was about 10(6) cells per g of root. However, bo th strains gave similar protection of cucumber against Pythium ultimum. In autoclaved soil, at 6 x 10(7) cells per g of root, strain CHA0 stimulated g rowth of roots and shoots, whereas strain CHA0/pME3468 caused root stunting and strong reduction of plant weight. These results are in agreement with the known effects of exogenous indole-3-acetic acid on plant roots and sugg est that in the system examined, indole-3-acetic acid does not contribute t o the biocontrol properties of strain CHA0. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights r eserved.