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. 2020 Feb 18;12(2):126.
doi: 10.3390/toxins12020126.

Phytotoxic Metabolites Isolated from Neufusicoccum batangarum, the Causal Agent of the Scabby Canker of Cactus Pear (Opuntia ficus-indica L.)

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Phytotoxic Metabolites Isolated from Neufusicoccum batangarum, the Causal Agent of the Scabby Canker of Cactus Pear (Opuntia ficus-indica L.)

Marco Masi et al. Toxins (Basel). .

Abstract

Six phytotoxins were obtained from the culture filtrates of the ascomycete Neofusicoccum batangarum, the causal agent of the scabby canker of cactus pear (Opuntia ficus-indica L.) in minor Sicily islands. The phytotoxins were identified as (-)-(R)-mellein (1); (±)-botryoisocoumarin A (2); (-)-(3R,4R)- and (-)-(3R,4S)-4-hydroxymellein (3 and 4); (-)-terpestacin (5); and (+)-3,4-dihydro-4,5,8-trihydroxy-3-methylisocoumarin, which we named (+)-neoisocoumarin (6). This identification was done by comparing their spectral and optical data with those already reported in literature. The absolute configuration (3R,4S) to (+)-neoisocoumarin (6) was determined using the advanced Mosher method. All six metabolites were shown to have phytotoxicity on the host (cactus pear) and non-host (tomato) plants, and the most active compounds were (±)-botryoisocoumarin A (2), (-)-terpestacin (5), and (+)-neoisocoumarin (6).

Keywords: Neofusicoccum batangarum; cactus pear; phytotoxins; scabby cankers.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Symptoms of scabby cankers on cladodes of pear cactus (O. ficus-indica L.), including radially expanding, crusty, concentric, silvery, perennial cankers, with a leathery, brown halo (left panel); and an abundant milky viscous exudate, caking on contact with air, which leaked from cankers and formed strips or cerebriform masses (right panel).
Figure 2
Figure 2
The structures of (−)-(R)-mellein (1), (±)-botryoisocoumarin A (2), (−)-(3R,4R)- and (−)-(3R,4S)-hydroxymellein (3 and 4), (−)-terpestacin (5), and (+)-neoisocoumarin (6).
Figure 3
Figure 3
Mechanism of the conversion of the dimethylether of (+)-neoisocoumarin (7) into the corresponding diastereomeric benzofuranones (8 and 9) by reaction with R-(−)-α-methoxy-α-trifluoromethylphenylacetyl (MTPA) and S-(+)-MTPA chlorides.
Figure 4
Figure 4
Structures of 4-O-S- and 4-O-R-MTPA esters of 5,8-O,O′-dimethyl ether of (+)-neoisocoumarin (8 and 9, respectively), reporting the Δδ value of each proton system.
Figure 5
Figure 5
Control cladode (MeOH 4%, v/v) (a), necrotic areas produced by (−)-(R)-mellein (1), (±)-botryoisocoumarin A (2), (−)-(3R,4R)-4-hydroxymellein (3), (−)-(3R,4S)-4-hydroxymellein (4), (−)-terpestacin (5), and (+)-neoisocoumarin (6) on cladodes of cactus pear.
Figure 6
Figure 6
Control leaves with sterile distilled water (A) and MeOH (4% v/v) (a); necrotic areas produced by (−)-(R)-mellein (1), (±)-botryoisocoumarin A (2), (−)-(3R,4R)-4-hydroxymellein (3), (−)-(3R,4S)-4-hydroxymellein (4), (−)-terpestacin (5), and (+)-neoisocoumarin (6) on tomato leaves.

References

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