Undescribed antimicrobial labdane diterpenes from the flowers of Euthamia graminifolia

The demand for previously undescribed antimicrobial agents is increasing due to the emergence of resistant plant pathogens. One of the untapped sources of new biopesticides is the plant kingdom. A bioassay-guided process comprising TLC-Bacillus subtilis bioassay, TLC-MS, and preparative flash column chromatography enabled the isolation of five previously undescribed antimicrobial labdane diterpenes (graminifolins A-E, 1-5) from the flower extract of grass-leaved goldenrod (Euthamia graminifolia, formerly known as Solidago graminifolia). Their structures were elucidated by NMR spectroscopy, supported by HRMS/MS, polarimetry, and UV, and ATR-FTIR spectroscopy. Graminifolins A-C (1-3) displayed low to moderate antibacterial and bactericidal activity against the Gram-positive plant pathogens Curtobacterium flaccumfaciens pv. flaccumfaciens and Clavibacter michiganensis, with MIC values between 67 and 533 μg/mL and MBC values ranging from 133 to 533 μg/mL. In contrast, graminifolin D+E (4+5) was active only against C. michiganensis. Among the isolated compounds, graminifolin A exhibited the strongest antibacterial effect and demonstrated weak antifungal activity against the crop pathogen Bipolaris sorokiniana. These findings underscore the potential of bioassay-guided fractionation in discovering previously undescribed bioactive compounds.