Effect of wild-type and mutant plant growth-promoting rhizobacteria on therooting of mung bean cuttings

Mung bean cuttings were dipped in solutions of wild type and mutant forms o f the plant growth-promoting rhizobacterium Pseudomonas putida GR12-2 and t hen incubated for several days until roots formed. The bacteria P. putida G R12-2 and P. putida GR12-2/aux1 mutant do not produce detectable levels of the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, whereas P. pu tida GR12-2/acd36 is an ACC deaminase minus mutant. All bacteria produce th e phytohormone indole-3-acetic acid (IAA), and P. putida GR12-2/aux1 overpr oduces it. Treatment of cuttings with the above-mentioned bacteria affected the rates of ethylene production in the cuttings in a way that can be expl ained by the combined effects of the activity of ACC deaminase localized in the bacteria and bacterial produced IAA. P. putida GR12-2 and P. putida GR 12-2/acd36-treated cuttings had a significantly higher number of roots comp ared with cuttings rooted in water. In addition, the wild type influenced t he development of longer roots. P. putida GR12-2/aux1 stimulated the highes t rates of ethylene production but did not influence the number of roots. T hese results are consistent with the notion that ethylene is involved ire t he initiation and elongation of adventitious roots in mung bean cuttings.