Preliminary study on horizontal transfer and loss of the AM toxin gene of Alternaria

The genus Alternaria has a global distribution and consists of a diverse group of pathogens. Plant-pathogenic Alternaria spp. can reduce the crop yield and pose serious threats to agricultural production. The pathogen A. mali is recognised as the key the pathogenic mechanism in the early defoliation of apples, which produces the host specific toxin (HST) that was named as an apple specific toxin (a specialised toxin of A. alternata pv. mali, AM toxin). The phenomenon of horizontal transfer of the AM toxin gene from different strains of A. alternata was found, and the relationship between the AM toxin and pathogenicity was confirmed. The representative strain A. tuberculata with the AM toxin gene was co-cultured with sixteen Alternaria strains without the AM toxin gene. As a result, four strains from different Alternaria species obtained the AM toxin gene, which indicated that the AM toxin gene can transfer among different Alternaria species. The AM toxin gene is easy to be lost after subculture, and high temperature and low nutrition can promote this loss. The symptoms of the Alternaria pathogen with or without the AM toxin gene are obviously different on the host. When infected by a pathogen with the AM toxin gene, green spots formed on the apple leaves, and rotten disease spots appeared in the fruit carpels. Contrary to this, when infected by a pathogen without this gene, only small epidermal spots without chlorosis formed on the apple leaves, and mildew-heart spots appeared in the fruit carpels.