Novel seco-dammarane triterpenes with antimicrobial activity against plant pathogens from Ailanthus altissima petioles
TL;DR
Results indicate that A. altissima petioles represent a promising source of antimicrobial phytochemicals, and could serve as lead compounds for the development of novel antimicrobial agents against certain phytopathogens.
Abstract
There is a growing demand for effective, environmentally friendly pesticides due to increasing environmental awareness and stricter regulations on chemical use. Natural products, including secondary metabolites of the invasive tree of heaven (Ailanthus altissima), could represent promising sources for biopesticide development. A bioassay-guided isolation workflow involving thin-layer chromatography (TLC)–Bacillus subtilis bioassay, TLC–high-resolution mass spectrometry (HRMS), and preparative flash column chromatography yielded three antimicrobial seco-dammarane triterpenes from the methanolic extract of A. altissima petioles, namely the known shoreic acid (1), and the previously undescribed foveolin C (2), and (20S,24R)-epoxy-25-hydroxy-A-homo-4-oxadammaran-3-one (3). Their structures were elucidated by nuclear magnetic resonance (NMR) spectroscopy, supported by high-resolution tandem mass spectrometry (HRMS/MS) and polarimetry. The presence of the hydroxylated, polyunsaturated fatty acids 13-hydroxy-9,11-octadecadienoic acid (13-HODE, 4) and 9-hydroxy-10,12-octadecadienoic acid (9-HODE, 5), previously isolated from A. altissima bark, was also confirmed in the petioles by high-performance liquid chromatography (HPLC)–HRMS/MS. To the best of our knowledge, compounds 2 and 3 are reported here as new natural products, while compounds 1, 4, and 5 were detected in A. altissima petioles for the first time. In microdilution assays, compounds 1–5 showed antimicrobial activity against B. subtilis, the plant pathogenic bacteria Curtobacterium flaccumfaciens pv. flaccumfaciens, Clavibacter michiganensis, and Rhodococcus fascians, as well as the plant pathogenic fungus Fusarium graminearum. Compound 1 exhibited the strongest antibacterial activity, especially against C. michiganensis and R. fascians, with minimal inhibitory concentration (MIC) values two-fold lower than those of the positive control neomycin. Compounds 2–5 were the most effective against R. fascians, with MIC values the same as or two-fold higher than those of neomycin. These results indicate that A. altissima petioles represent a promising source of antimicrobial phytochemicals. After proper structure optimization, they could serve as lead compounds for the development of novel antimicrobial agents against certain phytopathogens.