Transcription factor CsERF1B regulates postharvest citrus fruit resistance to Penicillium digitatum

Green mold, caused by Penicillium digitatum (P. digitatum), is among the most important postharvest disease of citrus fruit in terms of economic losses. Previous studies reported that AP2/ERFs in citrus pericarp positively respond to P. digitatum infection; therefore, it is presumed that AP2/ERFs could participate in the regulation of the fruit resistance response. However, limited research is available on AP2/ERF transcription factors in regulating the disease resistance of citrus fruit. In the present study, the regulatory role of CsERF1B in citrus green mold was clarified and the regulatory mechanism was initially explored. In our work, CsERF1B is a transcriptional activator with a nuclear localization signal. Transient overexpression of CsERF1B in citrus pericarp significantly enhanced fruit resistance to green mold and induced upregulated expression of pathogenesis-related genes in the pericarp. CsERF1B directly activated lignin synthesis genes, increased the activities of related enzymes, and promoted the accumulation of lignin. CsERF1B directly activated the genes related to jasmonic acid signaling and immunity, indirectly activated genes related to chitinase, and it also directly repressed salicylic acid signaling pathway genes, thus participating in the regulation of fruit disease resistance. This research provides novel insights into the mechanisms of disease resistance regulation by AP2/ERF transcription factors in citrus fruit.