Ethylene production by Alternaria alternata and its association with virulence on inoculated grape berries

Ethylene has been shown to promote spore germination and hyphal growth in the phytopathogenic fungus Alternaria alternata. However, little is known about the ethylene biosynthetic pathway in this fungus. In the present study, the ethylene biosynthetic pathway in A. alternata was investigated to explore ethylene-associated virulence of this fungus. The strain A0 of A. alternata did not produce ethylene on basal medium with different possible precursors or intermediates for ethylene biosynthesis (glutamate, aspartate, 2-oxoglutarate and 1-aminocyclopropano −1-carboxylic acid). However, ethylene production was observed when methionine was added as a precursor to the medium and was further promoted by continuous light illumination. Furthermore, addition of 2-keto-4-methylthiobutyric acid (KMBA) promoted ethylene production in the absence of methionine, indicating that the KMBA pathway was mainly responsible for ethylene biosynthesis in this fungus. The strain A0 was inoculated into grape berries to examine the effect of ethylene production on its virulence (as assessed by lesion formation at the inoculation site). The results indicated that higher ethylene production caused larger lesion formation. Similar results were also obtained when isolates of A. alternata, obtained from infected grapes, were inoculated. Thus, the present study thus demonstrated that A. alternata produces ethylene via the KMBA pathway and utilizes it for enhanced virulence expression during infection.