Effect of transferring 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase genes intoPseudomonas fluorescensstrain CHA0 and itsgacA derivative CHA96 on their growth-promoting and disease-suppressive capacities

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Abstract
Pseudomonas fluorescens strain CHA0, a root colonizing bacterium, has a broad 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 suggests that CHA0 does not contain the enzyme ACC deaminase, which cleaves ACC to ammonia and α-ketobutyrate, and was previously shown to promote root elongation of plant seedlings treated with bacteria containing this enzyme. An ACC deaminase gene, together with its regulatory region, was transferred into P. fluorescens strains CHA0 and CHA96, a global regulatory gacA mutant of CHA0. ACC deaminase activity was expressed in both CHA0 and CHA96. Transformed strains with ACC deaminase activity increased root length of canola plants under gnotobiotic conditions, whereas strains without this activity 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 containers. In contrast, ACC deaminase activity had no significant effect on the ability of CHA0 to protect tomato against Fusarium crown and root rot, and potato tubers against soft rot in large hermetically sealed containers. These results suggest that (i) ACC deaminase activity may have lowered the level of plant ethylene thereby increasing root length; (ii) the role of stress-generated plant ethylene in susceptibility or resistance depends on the host-pathogen system, and on the experimental conditions used; and (iii) the constructed strains could be developed as biosensors for the role of ethylene in plant diseases.Key words: biocontrol, plant growth-promoting rhizobacteria, ACC deaminase, ethylene biosensors.

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