Bioassay-Directed Isolation and Identification of Phytotoxic and Fungitoxic Acetylenes from Conyza canadensis

Abstract

Conyza canadensis (L.) Cronquist syn. (horseweed) is a problematic and invasive weed with reported allelopathic properties. To identify the phytotoxic constituents of the aerial parts, a systematic bioactivity-guided fractionation of the dichloromethane extract was performed. Three active enyne derivatives, (2Z,8Z)-matricaria acid methyl ester, (4Z,8Z)-matricaria lactone, and (4Z)-lachnophyllum lactone, were identified. The lactones inhibited growth of the monocot Agrostis stolonifera (bentgrass) and the dicot Lactuca sativa (lettuce) at 1 mg mL(-1), while the (2Z,8Z)-matricaria acid methyl ester was less active. In a dose-response screening of the lactones for growth inhibitory activity against Lemna paucicostata , (4Z)-lachnophyllum lactone was the most active with an IC50 of 104 μM, while the (4Z,8Z)-matricaria lactone was less active (IC50 of 220 μM). In a fungal direct bioautography assay, the two lactones at 10 and 100 μg/spot inhibited growth of the plant pathogenic fungi Colletotrichum acutatum , Colletotrichum fragariae , and Colletotrichum gloeosporioides . In a dose-response screening of the lactones against six different plant pathogenic fungi, (4Z,8Z)-matricaria lactone was more active than the commercial fungicide azoxystrobin on Col. acutatum , Col. fragariae , and Col. gloeosporioides at 30 μM and about as active as the commercial fungicide captan against Col. gloeosporioides , while (4Z)-lachnophyllum lactone was less active.

Conyza canadensis: Green Extraction Method of Bioactive Compounds and Evaluation of Their Antifungal Activity

“…Compound 3 was also detected in both fractions, but in very small quantities, rendering its isolation impossible. The low concentrations of compound 3 in the extracts differed from the earlier findings of Queiroz et al, 7 who isolated it in larger amounts from North American specimens. This was not unexpected, because the composition of C. canadensis essential oil can vary according to the collection site and the growth phase of the plant.…”

“…In a first trial, C. canadensis specimens collected in Brazil were extracted using dichloromethane, chloroform, and hexane, as described by Queiroz et al,7 in order to determine whether the compounds isolated from North American specimens were also present in specimens growing in Brazil. Using GC-MS analysis, it was possible to confirm that compounds 1 and 2 were present in both hexanic and chloroformic fractions.…”

“…1,6 Given the importance of developing new biopesticides and investigating their spectra of action, a recent survey showed promising results with the allelopathic plant Conyza canadensis (L.) Cronquist, which belongs to the Asteraceae family and is popularly known as horseweed. 7 This plant is a potential source of bioactive compounds 8 and several studies have reported the fungitoxic effects of its essential oils, [9][10][11] although the compounds responsible for such activity were not identified. Queiroz et al 7 found three fungitoxic compounds in dichloromethane extracts of C. canadensis specimens collected in the USA: (4Z)-lachnophyllum lactone (1), (4Z,8Z)-matricaria lactone (2), and (2Z,8Z)-matricaria acid methyl ester (3), whose chemical structures are shown in Figure 1.…”

“…7 This plant is a potential source of bioactive compounds 8 and several studies have reported the fungitoxic effects of its essential oils, [9][10][11] although the compounds responsible for such activity were not identified. Queiroz et al 7 found three fungitoxic compounds in dichloromethane extracts of C. canadensis specimens collected in the USA: (4Z)-lachnophyllum lactone (1), (4Z,8Z)-matricaria lactone (2), and (2Z,8Z)-matricaria acid methyl ester (3), whose chemical structures are shown in Figure 1. Compounds 1 and 2 showed antifungal activity against three phytopathogenic fungi: Colletotrichum acutatum, Colletotrichum fragariae and Colletotrichum gloeosporioides.…”

Plant-Derived Natural Products From the American Continent for the Control of Phytopathogenic Fungi: A Review

“…About antifungal mechanisms, it was reported that the compound 22 is a fungal endo-β-(1,3)-glucanase inhibitor (Vargas-Arispuru et al, 2009). Others active compounds (Figure 1) are acetylenic lactone (27) (Queiroz et al, 2012), 2-phenylethanol ester (28) (Labbé et , 2005), and the unusual 2-hydroxy-1-H-phenalen-one (29) and 2-methoxy-1-H-phenalen-one, which are fungicides against Colletotrichum, Cladosporium, and Mycosphaerella, respectively (Otálvaro et al, 2007). Other compound also toxic to Colletotrichum was the known glucosinolate glucoraphanin, isolated from Brassica oleracea (Lara-Viveros et al, 2014) Plants are able to produce proteinaceous compounds, between those described with fungicide activity are lectins (glycoproteins that agglutinate and immobilize microorganisms) and microbial proteinase inhibitors (inhibit enzymes that degrade plant cell walls).…”