Species Pseudomonas brassicacearum was described both as plant growth-promoting (PGP) rhizobacterium and necrotrophic opportunistic phytopathogen. An important PGP property of this rhizobacterium
is the activity of 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which reduces abiotic stress in plants. Tomatoes are an important but drought-sensitive agricultural crop. Growth and physiological and biochemical responses of well-watered and water-limited tomatoes to such kind of rhizobacteria were addressed using Ps. brassicacearum Am3 or its ACC deaminase deficient mutant T8-1. The counts of the introduced P. brassicacearum Am3 and T8-1 in the tomato rhizosphere at the end of experiment were similar under both water regimes. The applied drought and inoculations did not yield differences in the content of water, chlorophyll, indole-3-acetic and indole-3-lactic acids in leaves. Drought reduced stomatal conductance and inhibited shoot growth, whereas only wild type Am3 maintained shoot biomass on the level of wellwatered plants. Strain Am3 lowered difference in the contents of thiobarbithuric acid reactive substances (expressed as malondialdehyde equivalents) between well-watered and droughted plants. This strain also decreased abscisic acid content in roots, as well as hydrogen peroxide, salicylic and indole-carboxylic acid contents in leaves of water-limited plants. The mutant T8-1 decreased content of abscisic and indole-3-lactic acids in roots and abscisic and indole-3-carboxylic acids in leaves of water-limited plants. Thus, under waterlimitation Ps. brassicacearum Am3 having high ACC deaminase activity exhibited the properties of PGPR and stimulated plant growth, but the deficiency of ACC deaminase in T8-1 redirected the action of bacteria towards saprophytic or even deleterious effects.