Alternaria alternata effector AaAlta1 targets CmWD40 and participates in regulating disease resistance in Chrysanthemum morifolium.

Black spot diseases caused by the necrotrophic fungal pathogen Alternaria alternata adversely affect the growth and yield of many plants worldwide. However, the molecular mechanisms underlying the virulence and pathogenicity of A. alternata remain largely unknown. In this study, we report the identification of a novel effector Alta1, secreted by A. alternata, which not only contributes to its virulence but also triggers the cell death and defense of the host plant. The expression of Alta1 in Chrysanthemum morifolium activated jasmonic acid (JA) signaling, which, in turn, enhanced plant resistance to A. alternata. Moreover, we found that Alta1 targeted the WD40-repeat protein of chrysanthemum (CmWD40) after entering host cells. Notably, the CmWD40 gene showed rhythmic basal expression, and the overexpression of CmWD40 increased the resistance of chrysanthemum leaves against A. alternata, whereas its loss of function led to a decrease in this resistance. The results of the comparative transcriptomics and JA content analyses indicated that CmWD40 is possibly involved in the accumulation and signaling of JA. The transcript levels of the MYC2 gene were significantly upregulated in lines overexpressing the CmWD40 gene compared with that in the wild type. Further, the results of the infection assay revealed that CmWD40 positively modulated Alta1-induced defense response by activating MYC2 transcription. Overall, the results obtained in this study demonstrate that identified effector Alta1, recognized by the circadian rhythm gene CmWD40, triggers JA-induced immune response and enhances disease resistance in chrysanthemum plants.