Potential of utilizing pathogen-derived mycotoxins as alternatives to synthetic herbicides in controlling the noxious invasive plant Xanthium italicum.

Discovery of environmentally friendly agents for controlling alien invasive species (AIS) is challenging and in urgent need as their expansion continues to increase. Xanthium italicum is a notorious invasive weed that has caused serious ecological and economic impacts worldwide. For the purpose of exploring the possibility of utilizing herbicidal mycotoxins to control this species, three compounds, a new compound, curvularioxide (1), a new naturally occurring compound, dehydroradicinin (2), and a known compound, radicinin (3), were isolated via activity-guided fractionation from the secondary metabolites of the pathogenic Curvularia inaequalis, which was found to infect X. italicum in natural habitats. All isolated compounds exhibited potent herbicidal activity on receiver species. It is noteworthy to mention that their effects on X. italicum in our bioassays were equivalent to the commercial herbicide glyphosate. Subsequent morphological analysis revealed that application of radicinin (3) severely hindered X. italicum seedlings' hypocotyl and root development. Malondialdehyde content and the activity of catalase and peroxidase of the seedlings were also significantly different from the control, implying the occurrence of induced oxidative stress. Our results suggest that pathogens infecting invasive plants might be valuable resources for developing safer herbicides for controlling weeds. © 2023 Society of Chemical Industry.

Phytotoxic Compounds Isolated from Leaves of the Invasive Weed Xanthium spinosum.

The aim of this study was to identify bioactive compounds from leaves of the invasive plant Xanthium spinosum and assess their phytotoxic activity. Activity-guided fractionation led to the isolation of 6 bioactive compounds: xanthatin (1), 1α,5α-epoxyxanthatin (2), 4-epiisoxanthanol (3), 4-epixanthanol (4), loliolide (5) and dehydrovomifoliol (6). Of them, compounds 2⁻6 were isolated from the X. spinosum for the first time. The structures of 1⁻6 were elucidated on the basis of extensive NMR studies and ESI-MS measurements as well as comparison with literature data. All of compounds were evaluated for their phytotoxic activity. Among them, compounds 1⁻4 exhibited stronger activity on 2 receiver plants compared with the other 2 compounds, with xanthatin (1) being the most potent compound, which suppressed root growth of the dicot plant Amaranthus retroflexus by 32.5%, 39.4%, 84.7% when treated xanthatin (1) at 5, 20, and 100 µg/mL, while for the monocot plant, root growth was inhibited by 14.7%, 28.0%, and 40.0%, respectively. Seedling growth was nearly completely inhibited when the concentration of xanthanolides increased to 500 µg/mL, whereas there was still some seedling growth when loliolide (5) and dehydrovomifoliol (6) were applied at the same concentration. Dehydrovomifoliol (6) did not negatively affect seedling growth of P. annua at all tested concentrations, and root length was still 42.0% of the control when the highest concentration 500 µg/mL was used. This is the first report of the phytotoxicity of 1α,5α-epoxyxanthatin (2), 4-epiisxanthanol (3) and 4-epixanthanol (4). These compounds have the potential to be utilized as natural herbicides, especially 4-epiisoxanthanol (3), which exhibited significant selective activity between the dicot and monocot plants. On the other hand, whether these bioactive substances serve as allelochemicals to facilitate the invasion success of X. spinosum needs to be further studied.