Differential expression of TOR complex 1 components in Colletotrichum camelliae isolates confers natural resistance to rapamycin

Rapamycin is a lipophilic macrolide antibiotic which is famous for its immunosuppressive and anticancer activity. In recent years, rapamycin showed significant activity against various plant pathogenic fungi. However, the sensitivity of Colletotrichunm fungi to rapamycin is scarcely reported. In this study, we evaluated the sensitivity of 116 Colletotrichum isolates from tea-oil trees to rapamycin. Most isolates exhibited natural resistance with inhibition rates of 50 to 70% at 50 μg/mL. Three Colletotrichum camelliae isolates were found to be sensitive to rapamycin. No mutations were detected in the direct target FKBP12 and indirect target TOR-FRB domain of resistant and sensitive C. camelliae isolates. Notably, the expression of the TOR homolog (CcTOR) was higher in resistant C. camelliae isolates compared to the sensitive ones and overexpression of CcTOR in the sensitive isolate CcS1 resulted in decreased sensitivity to rapamycin. Moreover, ribosomal protein S6 phosphorylation was abolished in the sensitive isolate CcS1 but not in the resistant isolate CcR1 under rapamycin treatment. In addition, the expression levels of ribosome biogenesis genes and two other components of TORC1 were higher in CcR1 compared to CcS1 under the same treatment, which suggested that the abundance of TORC1 in CcR1 was greater than in CcS1, leading to more active TORC1 signaling in CcR1. These results provided a better understanding about natural resistance of C. camelliae isolates to rapamycin and could help for developing new TORC1 signaling-targeting fungicides.