Isolation and Bioactivity of Secondary Metabolites from Solid Culture of the Fungus, Alternaria sonchi

The fungus, Alternaria sonchi is considered to be a potential agent for the biocontrol of perennial sowthistle (Sonchus arvensis). A new chlorinated xanthone, methyl 8-hydroxy-3-methyl-4-chloro-9-oxo-9H-xanthene-1-carboxylate (1) and a new benzophenone derivative, 5-chloromoniliphenone (2), were isolated together with eleven structurally related compounds (3-13) from the solid culture of the fungus, which is used for the production of bioherbicidal inoculum of A. sonchi. Their structures were determined by spectroscopic (mostly by NMR and MS) methods. Alternethanoxins A and B, which were reported in A. sonchi earlier, were re-identified as moniliphenone and pinselin, respectively. The isolated compounds were tested for phytotoxic, antimicrobial, insecticidal, cytotoxic and esterase-inhibition activities. They did not demonstrate high phytotoxicity (lesions up to 2.5 mm in diameter/length at a concentration of 2 mg/mL) when tested on leaf disks/segments of perennial sowthistle (Sonchus arvensis) and couch grass (Elytrigia repens). They did not possess acute toxicity to Paramecium caudatum, and showed moderate to low cytotoxicity (IC50 > 25 µg/mL) for U937 and K562 tumor cell lines. However, chloromonilicin and methyl 3,8-dihydroxy-6-methyl-4-chloro-9-oxo-9H-xanthene-1-carboxylate (4) were shown to have antimicrobial properties with MIC 0.5-5 µg/disc. Compound 4 and chloromonilinic acid B were found to have contact insecticidal activity to wheat aphid (Schizaphis graminum) at 1 mg/mL. Compounds 2 and methyl 3,8-dihydroxy-6-methyl-9-oxo-9H-xanthene-1-carboxylate displayed selective carboxylesterase inhibition activity at concentration of 100 µg/mL. Therefore, the waste solid substrate for production of A. sonchi spores can be re-utilized for the isolation of a number of valuable natural products.

Alternethanoxins C-E, Further Polycyclic Ethanones Produced by Alternaria sonchi, a Potential Mycoherbicide for Sonchus arvensis Biocontrol

Three new polycyclic ethanones, named alternethanoxins C-E, were isolated together with the well-known and closely related alternethanoxins A and B, from the solid culture of Alternaria sonchi, a fungal pathogen proposed for perennial sowthistle (Sonchus arvensis L.) biocontrol. Alternethanoxins C-E were characterized by spectroscopic methods (essentially NMR and HRESI MS) as 2'-dihydroxymethyl-2,5,6,6'-tetrahydroxy-3'-methoxy-biphenyl-3-carboxylic acid methyl ester, 1,4,6,9,10-pentahydroxy-7-methoxy-6H-benzo[c]chromene-2-carboxylic acid methyl ester, and 7,9-dihydroxy-2-methoxy-9H-4,8-dioxa-cyclopenta[def]phenanthrene-5-carboxylic acid methyl ester. When assayed on leaf segments of weeds (Sonchus arvensis and Elytrigia repens), alternethanoxins A and C showed phytotoxic activity inducing notable necrotic lesions. Alternethanoxins C and D possess notable antimicrobial activity when tested against Bacillus subtilis (MIC 10 μg/disc) and Candida tropicalis (MIC 25 μg/disc). Alternethanoxins A and B had low activity against these microbes, while alternethanoxin E was inactive.