Antimicrobial Diterpenes from Rough Goldenrod (Solidago rugosa Mill.)

Abstract

Solidago rugosa is one of the goldenrod species native to North America but has sporadically naturalized as an alien plant in Europe. The investigation of the root and leaf ethanol extracts of the plant using a bioassay-guided process with an anti-Bacillus assay resulted in the isolation of two antimicrobial components. Structure elucidation was performed based on high-resolution tandem mass spectrometric and one- and two-dimensional NMR spectroscopic analyses that revealed (-)-hardwickiic acid (Compound 1) and (-)-abietic acid (Compound 2). The isolates were evaluated for their antimicrobial properties against several plant pathogenic bacterial and fungal strains. Both compounds demonstrated an antibacterial effect, especially against Gram-positive bacterial strains (Bacillus spizizenii, Clavibacter michiganensis subsp. michiganensis, and Curtobacterium flaccumfaciens pv. flaccumfaciens) with half maximal inhibitory concentration (IC₅₀) between 1 and 5.1 µg/mL (5-20 times higher than that of the positive control gentamicin). In the used concentrations, minimal bactericidal concentration (MBC) was reached only against the non-pathogen B. spizizenii. Besides their activity against Fusarium avenaceum, the highest antifungal activity was observed for Compound 1 against Bipolaris sorokiniana with an IC₅₀ of 3.8 µg/mL.

Keywords: (–)-abietic acid; (–)-hardwickiic acid; antibacterial effect; antifungal effect; bioassay-guided isolation; high-performance thin-layer chromatography–effect-directed analysis.

Figure 1

Young shoots with root (A), young (B), and flowering (C) plants and cascading flowerhead (D) of Solidago rugosa.

Figure 2

HPTLC chromatograms of S. rugosa root (1) and leaf (5) extracts, their main flash fractions (2 and 6, respectively), and Isolates 1 (3) and 2 (4), developed with n-hexane–isopropyl acetate 4:1 v/v, and detected at UV 254 nm (A), at white light illumination after derivatization with vanillin–sulfuric acid reagent (B), and bioautogram after Bacillus subtilis assay (C).

Figure 3

HPLC–ESI-qTOF-MS(/MS) spectra of Compounds 1 (left) and 2 (right) isolated from Solidago rugosa root and leaf, respectively. For fragmentation the collision energy was set to 20 eV.