Effect of transferring 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase genes into Pseudomonas fluorescens strain CHA0 and its gacA derivative CHA96 on their growth-promoting and disease-suppressive capacities

Pseudomonas fluorescens strain CHA0, a root colonizing bacterium, has a bro ad spectrum of biocontrol activity against plant diseases. However, strain CHA0 is unable to utilize 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of plant ethylene, as a sole source of nitrogen. This s uggests that CHA0 does not contain the enzyme ACC deaminase, which cleaves ACC to ammonia and alpha-ketobutyrate, and was previously shown to promote root elongation of plant seedlings treated with bacteria containing this en zyme. An ACC deaminase gene, together with its regulatory region, was trans ferred into P. fluorescens strains CHA0 and CHA96, a global regulatory gacA mutant of CHA0. ACC deaminase activity was expressed in both CHA0 and CHA9 6. Transformed strains with ACC deaminase activity increased root length of canola plants under gnotobiotic conditions, whereas strains without this a ctivity had no effect. Introduction of ACC deaminase genes into strain CHA0 improved its ability to protect cucumber against Pythium damping-off, and potato tubers against Erwinia soft rot in small hermetically sealed contain ers. In contrast, ACC deaminase activity had no significant effect on the a bility of CHA0 to protect tomato against Fusarium crown and root rot, and p otato tubers against soft rot in large hermetically sealed containers. Thes e results suggest that (i) ACC deaminase activity may have lowered the leve l of plant ethylene thereby increasing root length; (ii) the role of stress -generated plant ethylene in susceptibility or resistance depends on the ho st-pathogen system, and on the experimental conditions used; and (iii) the constructed strains could be developed as biosensors for the role of ethyle ne in plant diseases.