Dual Interaction of 1-Aminocyclopropane-1-Carboxylate Utilizing Rhizobacterium Pseudomonas brassicacearum with Tomatoes under Water-Limited Conditions

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 well-watered 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 water-limitation 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.