Effects of bacterial single inoculation and co-inoculation on growth and phytohormone production of sunflower seedlings under water stress

The purpose of the study was to measure shoot and root dry matter (DM) and production of auxins, salicylic acid, abscisic acid, and jasmonic acid in sunflower (Helianthus annuus L.) seedlings cultivated under water stress and singly inoculated or co-inoculated with Achromobacter xylosoxidans (SF2) and Bacillus pumilus (SF3 and SF4) bacterial strains. Shoot DM was higher in non-stressed seedlings than in stressed seedlings for all inoculation treatments. Water stress resulted in decreased relative water content and reduction of shoot DM. Root DM was higher in stressed seedlings than in non-stressed seedlings. Salicylic acid was the most abundant phytohormone in shoots of stressed, singly inoculated and co-inoculated seedlings. High salicylic acid content in stressed seedlings suggests that this hormone plays a key role in abiotic stress. Abscisic acid was higher in stressed and co-inoculated seedlings than in non-stressed seedlings but was lower than that of salicylic acid. Auxin profile was similar to that of abscisic acid in co-inoculated seedlings. Shoot jasmonic acid content was increased in stressed seedlings co-inoculated with SF2/SF3 or SF2/SF4. Shoot hormonal profiles were different from those of root, suggesting a differential effect of bacterial inoculation on these plant organs. Our findings will be useful in future strategies to mitigate drought effects on crop plants through bacterial inoculation treatments.