Distribution and characterization of AKT homologs in the tangerine pathotype of Alternaria alternata

The tangerine pathotype of Alternaria alternata produces a host-selective t oxin (HST), known as ACT-toxin, and causes Alternaria brown spot disease of citrus. The structure of ACT-toxin is closely related to AK- and AF-toxins , which are HSTs produced by the Japanese pear and strawberry pathotypes of A. alternata, respectively. AC-, AK-, and AF-toxins are chemically similar and share a 9,10-epoxy-8-hydroxy-9-methyldecatrienoic acid moiety. Two gen es controlling AK-toxin biosynthesis (AKT1 and AKT2) were recently cloned f rom the Japanese pear pathotype of A. alternata. Portions of these genes we re used as heterologous probes in Southern blots, that detected homologs in 13 isolates of A. alternata tangerine pathotype from Minneola tangelo in F lorida. Partial sequencing of the homologs in one of these isolates demonst rated high sequence similarity to AKT1 (89.8%) and to AKT2 (90.7%). AKT hom ologs were not detected in nine isolates of A. alternata from rough lemon, six isolates of nonpathogenic A. alternata, and one isolate of A. citri tha t causes citrus black rot. The presence of homologs in the Minneola isolate s and not in the rough lemon isolates, nonpathogens or black rot isolates, correlates perfectly to pathogenicity on Iyo tangerine and ACT-toxin produc tion. Functionality of the homologs was demonstrated by detection of transc ripts using reverse transcription-polymerase chain reaction (RT-PCR) in tot al RNA of the tangerine pathotype of A. alternata. The high sequence simila rity of AKT and AKT homologs in the tangerine pathotype, combined with the structural similarity of AK-toxin and ACT-toxin, may indicate that these ho mologs are involved in the biosynthesis of the decatrienoic acid moiety of ACT-toxin.