Investigating the phytotoxic potential of Verbesina encelioides: effect on growth and performance of co-occurring weed species

Allelopathy has been proposed as an efficient mechanism of invasion by plant species via growth inhibition and suppression of the resident plant community. Verbesina encelioides (Cav.) Benth. & Hook. f. ex A. Gray (golden crownbeard; Asteraceae), a native of south-western USA and Mexican Plateau, is an emerging troublesome invasive weed species of north-western states of India. We investigated the allelopathic potential of the aqueous extracts prepared from the fresh foliage and leaf litter of V. encelioides on its co-occurring species, Amaranthus viridis and Senna occidentalis. Phytotoxicity bioassay showed concentration-dependent (control < 0.5% < 1% < 2% < 4% extract) inhibition of growth and photosynthetic parameters in the test plants. Both the extracts induced ~ 50% inhibition of germination compared to control at 4% concentration. The maximum synthesis effect (collective effect on seedling length and dry weight) was observed to be - 0.69 and - 0.62 in A. viridis and - 0.68 and - 0.57 in S. occidentalis for the fresh leaf and leaf litter extracts, respectively, at 4% concentration. Also, an antagonistic concentration-dependent impact was observed on the photosynthetic pigments (total chlorophyll and chlorophyll a content) and photosynthetic efficiency. The liquid chromatography-mass spectrometry assay of leaf extracts revealed the presence of 15 allelochemicals including phenolic acids, flavonoids, phytosterols, phytophenols, dicarboxylic acid, guanidine, and triterpenes. Of these, 14 compounds were present in both fresh and leaf litter materials. However, a guanidine derivative, galegine, was only found in the fresh leaf material of the plant. The findings support the novel weapon hypothesis and suggest that V. encelioides competitively excludes its neighboring plants by virtue of allelopathic interference.

Identification of Anticryptococcal Bornyl Compounds from Verbesina turbacensis and Their Structure-Activity Relationships

Cryptococcus neoformans is an opportunistic fungal pathogen that has limited treatment options. Natural product plant extracts offer a cost-effective option for the discovery of new anticryptococcal lead compounds. The acetone bark extract of Verbesina turbacensis was found to potently inhibit C. neoformans and was subjected to bioautography. Two compounds that inhibited the growth of C. neoformans were isolated and displayed minimum inhibitory concentration values of 10 and 310 µg/mL. The compounds were identified as the bornyl hydroxycinnamic esters bornyl caffeate and bornyl ferulate, respectively. To better understand initial structure-activity relationships, anticryptococcal activity was characterized for similar compounds. All compounds were further evaluated for mammalian cell toxicity using the MTT assay with MCF-7 and HEK-293 cell lines. Overall, bornyl caffeate demonstrated promising anticryptococcal potential given its potent inhibition of C. neoformans and low mammalian cell toxicity.