Involvement of gacS and rpoS in enhancement of the plant growth-promoting capabilities of Enterobacter cloacae CAL2 and UW4

The plant growth-promoting bacteria Enterobacter cloacae CAL2 and UW4 were genetically transformed with a multicopy plasmid containing an rpoS or gacS gene from Pseudomonas fluorescens. The transformed strains were compared w ith the nontransformed strains for growth, indoleacetic acid (IAA) producti on, antibiotic production, 1-aminocyclopropane-1-carboxylic acid (ACC) deam inase activity, siderophore production, cell morphology, and the ability to promote canola root elongation. All transformed strains had a longer lag p hase, were slower in reaching stationary phase, and attained a higher cell density than the nontransformed strains. Transformation resulted in cells t hat were significantly shorter than the nontransformed cells. The transform ed strains also produced significantly more IAA than the nontransformed str ains. Introduction of rpoS or gacS from Pseudomonas fluorescens was associa ted with a reduction in the production of both antibiotics, 2,4-diacetylphl oroglucinol and mono-acetylphloroglucinol, produced by Enterobacter cloacae CAL2. With Enterobacter cloacae CAL2, plasmid-borne rpoS, but not gacS, in creased the level of ACC deaminase activity, while introduction of rpoS in Enterobacter cloacae UW4 caused a decrease in ACC deaminase activity. Neith er gacS nor rpoS significantly affected the level of siderophores synthesiz ed by either bacterial strain. Overproduction of either GacA or RpoS in Ent erobacter cloacae CAL2 resulted in a significant increase in the root lengt hs of canola seedlings when seeds were treated with the bacteria, and overp roduction of RpoS caused an increase in canola shoot as well as root length s.