1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID DEAMINASE MUTANTS OF THE PLANT-GROWTH PROMOTING RHIZOBACTERIUM PSEUDOMONAS-PUTIDA GR12-2 DO NOT STIMULATE CANOLA ROOT ELONGATION

The plant growth promoting rhizobacterium Pseudomonas putida GR12-2 wa s mutagenized with nitrosoguanidine and three separate mutants that we re unable to utilize 1-aminocyclopropane-1-carboxylic acid (ACC) as a sole nitrogen source were selected. These mutants are devoid of the AC C deaminase activity that is present in wild-type P. putida GR12-2 cel ls. Only wild-type cells, but not any of the ACC deaminase mutants, pr omoted root elongation of developing canola seedlings under gnotobioti c conditions. These results are interpreted in terms of a model in whi ch P. putida GR12-2 promotes root elongation by binding to germinating seeds and sequesters and hydrolyzes some of the unbound ACC, thereby lowering the level of ACC and hence the endogenous ethylene concentrat ion, allowing the roots to grow longer.