A calmodulin-like protein suppresses RNA silencing and promotes geminivirus infection by degrading SGS3 via the autophagy pathway in Nicotiana benthamiana

Citation

Li, F., Zhao, N., Li, Z., Xu, X., Wang, Y., Yang, X., Liu, S.S., Wang, A., Zhou, X. (2017). A calmodulin-like protein suppresses RNA silencing and promotes geminivirus infection by degrading SGS3 via the autophagy pathway in Nicotiana benthamiana. PLoS Pathogens, [online] 13(2), http://dx.doi.org/10.1371/journal.ppat.1006213

Plain language summary

Post-transcriptional gene silencing (PTGS) is an elaborately regulated process for defense against virus infection in plants. To achieve effective infection, a betasatellite molecule associated with geminivirus induced high levels of an endogenous RNA silencing suppressor, calmodulin-like protein (CaM), to counter host defenses. However, although CaM is one of the first identified cellular suppressors of RNA silencing, the mechanism of PTGS suppression is still poorly understood. This study demonstrates that CaM interacts with and degrades Suppressor of Gene Silencing 3 (SGS3) in Nicotiana benthamiana. We found that domains essential for the interaction between NbSGS3 and NbCaM are also required for the subcellular localization of NbSGS3 and for NbCaM suppressor activity. Moreover, NbCaM mediated NbSGS3 protein degradation can be blocked using the autophagy inhibitors 3-methyladenine and E64d, and by knock-down of key autophagy-related genes within the phosphatidylinositol 3-kinase (PI3K) complex. Silencing of the PI3K complex also inhibited geminivirus infection, which is consistent with autophagy playing an important role in RNA silencing suppression pathway and geminivirus infection.

Abstract

A recently characterized calmodulin-like protein is an endogenous RNA silencing suppressor that suppresses sense-RNA induced post-transcriptional gene silencing (S-PTGS) and enhances virus infection, but the mechanism underlying calmodulin-like protein-mediated S-PTGS suppression is obscure. Here, we show that a calmodulin-like protein from Nicotiana benthamiana (NbCaM) interacts with Suppressor of Gene Silencing 3 (NbSGS3). Deletion analyses showed that domains essential for the interaction between NbSGS3 and NbCaM are also required for the subcellular localization of NbSGS3 and NbCaM suppressor activity. Overexpression of NbCaM reduced the number of NbSGS3-associated granules by degrading NbSGS3 protein accumulation in the cytoplasm. This NbCaM-mediated NbSGS3 degradation was sensitive to the autophagy inhibitors 3-methyladenine and E64d, and was compromised when key autophagy genes of the phosphatidylinositol 3-kinase (PI3K) complex were knocked down. Meanwhile, silencing of key autophagy genes within the PI3K complex inhibited geminivirus infection. Taken together these data suggest that NbCaM acts as a suppressor of RNA silencing by degrading NbSGS3 through the autophagy pathway.

Publication date

2017-02-01

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