One of the important traits of many plant growth-promoting rhizobacteria (PGPR) is the biocontrol of phytopathogens. Some PGPR metabolize phytohormone abscisic acid (ABA); however, the role of this trait in plant–microbe interactions is scarcely understood. Phytopathogenic fungi produce ABA and use this property as a negative regulator of plant resistance. Therefore, interactions between ABA-producing necrotrophic phytopathogen Botrytis sp. BA3 with ABA-metabolizing rhizobacterium Rhodococcus sp. P1Y were studied in a batch culture and in gnotobiotic hydroponics with sunflower seedlings. Rhizobacterium P1Y possessed no antifungal activity against BA3 and metabolized ABA, which was synthesized by BA3 in vitro and in associations with sunflower plants infected with this fungus. Inoculation with BA3 and the application of exogenous ABA increased the root ABA concentration and inhibited root and shoot growth, suggesting the involvement of this phytohormone in the pathogenesis process. Strain P1Y eliminated negative effects of BA3 and exogenous ABA on root ABA concentration and plant growth. Both microorganisms significantly modulated the hormonal status of plants, affecting indole-3-acetic, salicylic, jasmonic and gibberellic acids, as well as cytokinins concentrations in sunflower roots and/or shoots. The hormonal effects were complex and could be due to the production of phytohormones by microorganisms, changes in ABA concentrations and multiple levels of crosstalk in hormone networks regulating plant defense. The results suggest the counteraction of rhizobacteria to ABA-producing phytopathogenic fungi through the metabolism of fungal ABA. This expands our understanding of the mechanisms related to the biocontrol of phytopathogens by PGPR.

One of the important traits of plant growth promoting rhizobacteria (PGPR) is biocontrol of phytopathogens. Some PGPR metabolize phytohormone abscisic acid (ABA), however the role of this trait in plant-microbe interactions is scarcely understood. Phytopathogenic fungi produce ABA and use this property as a negative regulator of plant resistance. Therefore, interactions between ABA-producing necrotrophic phytopathogen Botrytis sp. BA3 with ABA-metabolizing rhizobacterium Rhodococcus sp. P1Y was studied in batch culture and in gnotobiotic hydroponics with sunflower seedlings. Rhizobacterium P1Y possessed no antifungal activity against BA3 and metabolized ABA, which was synthesized by BA3 in vitro and in associations with sunflower plants infected with this fungus. Inoculation with BA3 and application of exogenous ABA increased root ABA concentration and inhibited root and shoot growth, suggesting involvement of this phytohormone in pathogenesis process. Strain P1Y eliminated negative effects of BA3 and exogenous ABA on root ABA concentration and plant growth. Both microorganisms significantly modulated hormonal status of plants affecting indole-3-acetic, salicylic, jasmonic and gibberellic acids, as well as cytokinins concentrations in sunflower roots and/or shoots. The hormonal effects were complex and could be due to production of phytohormones by microorganisms, changes in ABA concentrations and multiple levels of crosstalk in hormone networks regulating plant defense. The results suggest the counteraction of rhizobacteria to ABA-producing phytopathogenic fungi through the metabolism of fungal ABA. This expands our understanding of the mechanisms related to biocontrol of phytopathogens by PGPR.

Phytopathogenic fungi of the genus Alternaria cause black spot (helminthosporiasis) disease and therefore apply to develop bioherbicides for controlling weeds and illegal vegetation of narcotic plants such as opium poppy (Papaver somniferum subsp. somniferum). In this report, Alternaria papavericola strain 1.39 was identified using sequencing of the ITS region and characterized. Strain 1.39 produced auxins and abscisic acid, did not produce salicylic, jasmonic, or gibberellic (GA3) acids, and did not metabolize any of these phytohormones. Strain 1.39 produced siderophores but showed no 1-aminocyclopropane-1-carboxylate deaminase activity and did not solubilize phosphates Ca₃(PO₄)₂, Ca-phytate, Al₃PO₄, or Fe₃PO₄. Extracts from the culture liquid of strain 1.39 caused necrosis in leaves of poppy and, to a lesser extent, on creeping thistle, tomato, and wheat leaves. Strain 1.39 inhibited poppy seed germination energy by 80%, reduced shoot biomass by 77%, and resulted in the death of 75% of poppy plants in a pot experiment. The phytotoxin was isolated from the culture liquid of this fungus using chromatographic methods. Detailed study of structure of this phytotoxin by a complex of physicochemical methods revealed that it is (+)-cis-sclerosporide(1S,2S,3S,4R,5S,6R)-2,3,5-trihydroxy-4,6-dimethoxycyclohexyl-(1R,5S,6S)-5-isopropyl-8-methylbicyclo[4.4.0]deca-2,7-diene-2-carboxylate). The results showed possibility for application of A. papavericola 1.39 as bioherbicide against opium poppy.