The thaxtomin phytotoxins: sources, synthesis, biosynthesis, biotransformation and biological activity

Thaxtomin phytotoxins, first reported in 1989, are cyclic dipeptides (2,5-diketopiperazines) formed from the condensation of 4-nitrotrytophan and phenylalanine groups. Inclusion of a 4-nitroindole moiety that is an essential requirement for their phytotoxicity makes them unique amongst microbial generated metabolites. Individual thaxtomins differ only in the presence or absence of N-methyl and hydroxyl groups and their respective substitution sites. The name "thaxtomin" was assigned to these compounds in honor of the eminent American phytopathologist Roland Thaxter who first described the nature of the microorganisms responsible for their production. The great interest in the thaxtomins derives mainly from their established roles as virulence factors in the common scab of potato disease and their apparent ability to inhibit cellulose synthesis in developing plant cells. Surveys of comparable scab-inducing organisms from potato producing regions throughout the world have confirmed the generality of the phytotoxin associated process. Descriptions of the isolation, chemical structure determination, total synthesis, biosynthesis, transformation by microorganisms and biological activity of the thaxtomins are comprehensively reviewed.