Phomentrioloxin: A Phytotoxic Pentasubstituted Geranylcyclohexentriol Produced by Phomopsis sp., a Potential Mycoherbicide for Carthamus lanatus Biocontrol

A new phytotoxic geranylcyclohexenetriol, named phomentrioloxin, wasisolated from the liquid culture of Phomopsis sp., a fungal pathogen proposed for thebiological control of Carthamus lanatus, a widespread and troublesome thistle weedbelonging to the Asteraceae family causing severe crop and pastures losses in Australia.The structure of phomentrioloxin was established by spectroscopic, X-ray, and chemicalmethods as (1S,2S,3S,4S)-3-methoxy-6-(7-methyl-3-methylene-oct-6-en-1-ynyl)-cyclohex-5-ene-1,2,4-triol. At a concentration of 6.85 mM, the toxin causes theappearance of necrotic spots when applied to leaves of both host and nonhost plants. It also causes growth and chlorophyllcontent reduction of fronds of Lemna minor and inhibition of tomato rootlet elongation. Finally, in preliminary bioassays,phomentrioloxin did not show any antibacterial, fungicidal, or zootoxic activities.