plant growth stimulators. In plant-microbe interactions, IAA has recently

Some soil bacteria are capable of reducing the level of stress ethylene in plants as a result of cleaving its immediate precursor, 1-aminocyclopropane1-carboxylate (ACC), which is exuded from plant roots or seeds (Glick et al. 2001, Penrose and Glick 2001, Penrose el al. 2001), to a-ketobutyrate and ammonium, using the enzyme ACC deaminase. An interesting example of a PGPR, Kluyvera ascorbata SUD165, resistant to a range of heavy metals was reported (Burd et al. 1998) to protect plants from nickel toxicity without affecting nickel uptake by seedlings or its accumulation in roots and shoots. Rather, the plant growth-promoting effect in the presence of Ni may be attributed to the ACC deaminase activity (Shah et al. 1998) of this bacterium lowering the level of stress ethylene in plants induced by the uptaken Ni (Burd et al. 1998). At the same time, ACC deaminasedeficient mutant bacterial strains do not stimulate plant root elongation (Li et al. 2000, Belimov et al. 2001, and references therein).