Azospirillum brasilense Sp 245 produces ABA in chemically-defined culture medium and increases ABA content in arabidopsis plants

Azospirillum sp. are plant growth promoting bacteria (PGPB) that increase grain yield in cereals and other species via growth promotion and/or stress alleviation. The PGPB beneficial effects have been partially attributed to bacterial production of plant hormones, especially growth promoters like auxins, gibberellins and cytokinins. This paper reports the characterization of the stress-like plant hormone abscisic acid (ABA) by GC-EIMS in cultures of A. brasilense Sp 245 after 120 h of incubation in chemically-defined media, and chemically-defined media with moderate stress (100 mM NaCl). Chemical characterization of ABA was done by gas chromatography-electron impact mass spectrometry (GC-EIMS) and quantification by selected ion monitoring (SIM) with a stable isotope of the hormone as internal standard in the media. A. brasilense cultures produced higher amounts of ABA per ml of culture when NaCl was incorporated in the culture medium. Inoculation of Arabidopsis thaliana with A. brasilense Sp 245 enhanced two-fold the plant's ABA content. These results contribute to explain, at least to some extent, the beneficial effects of Azospirillum sp. previously found in inoculated plants placed under adverse environmental conditions.