Is IAA the major root growth factor secreted from plant-growth-mediating bacteria?

The phytotoxic or promoting effect of bacterial secretions on root growth of young lettuce seedlings (Lactuca sativa) was measured under axenic conditions. It was assumed that the inhibitory or promoting effects of either deleterious rhizobacteria (DRB) or of plant growth promoting rhizobacteria (PGPR) were auxin mediated. To avoid measurements of either parasitism or competition, seedlings were placed adjacent to bacterial colonies, with no contact between the organisms. Auxin excretion rate, evaluated by thin-layer chromatography (TLC) combined with Salkowski's reagent, indicated that all bacteria examined produced and released indole-3-acetic acid (IAA). High levels of IAA (76.6 mu M) were excreted by four DRB (Micrococcus luteus, Streptoverticillium sp., Pseudomonas putida, and Gluconobacter sp.) during 84 hr of incubation. High concentrations of IAA released by DRB accounted for the suppression of root growth. Other unidentified fractions in the eluates of DRB also inhibited root elongation, but to a lesser extent. Like DRB, four isolates of PGPR (Agrobacterium sp., Alcaligenes piechaudii, and two different strains of Comamonas acidovorans) secreted IAA, but at lower levels (16.4 mu M during a similar period of incubation). PGPR secreted growth promoting substances other than IAA, and these are now being investigated.