A self-regulated transcription factor CsWRKY33 enhances resistance of citrus fruit to Penicillium digitatum

Green mold caused by Penicillium digitatum is a major postharvest disease in citrus fruit, but the molecular mechanism related to the induced disease resistance in citrus is still unclear. This study investigated the regulatory function of CsWRKY33 on citrus disease resistance. The results showed that CsWRKY33 was located in the nucleus, and the transient overexpression of CsWRKY33 significantly enhanced the host resistance against green mold. The DNA affinity purification sequencing and transcriptome sequencing analysis identified 485 direct target genes of CsWRKY33. KEGG pathway analysis revealed that the 250 up-regulated genes were mainly involved in the disease resistance response. And the 235 down regulated genes were mostly associated with plant growth and development. 9 up-regulated genes were selected for EMSA and DLR analysis, including CsRbohD, CsPODN1, CsPOD24, CsPOD, CsPAL, CsWRKY22, CsGSTU7, and CsHSP26A, and found CsWRKY33 activated the expression of these genes. Besides, PAL, POD, GST activities, H2O2 content and lignin content were increased when CsWRKY33 was transiently overexpressed. The results also found that CsWRKY33 could bind to its own promoter's W-box directly for positive regulation. In conclusion, CsWRKY33 plays an important regulatory role in citrus disease resistance, and the key mechanisms is the activation of disease resistance pathways such as phenylalanine metabolism and secondary metabolism.