Date Published: August 29, 2018
Publisher: Public Library of Science
Author(s): Asigul Ismayil, Yakupjan Haxim, Yunjing Wang, Huangai Li, Lichao Qian, Ting Han, Tianyuan Chen, Qi Jia, Alexander Yihao Liu, Songbiao Zhu, Haiteng Deng, Rena Gorovits, Yiguo Hong, Linda Hanley-Bowdoin, Yule Liu, Savithramma P. Dinesh-Kumar.
Gene silencing is a natural antiviral defense mechanism in plants. For effective infection, plant viruses encode viral silencing suppressors to counter this plant antiviral response. The geminivirus-encoded C4 protein has been identified as a gene silencing suppressor, but the underlying mechanism of action has not been characterized. Here, we report that Cotton Leaf Curl Multan virus (CLCuMuV) C4 protein interacts with S-adenosyl methionine synthetase (SAMS), a core enzyme in the methyl cycle, and inhibits SAMS enzymatic activity. By contrast, an R13A mutation in C4 abolished its capacity to interact with SAMS and to suppress SAMS enzymatic activity. Overexpression of wild-type C4, but not mutant C4R13A, suppresses both transcriptional gene silencing (TGS) and post-transcriptional gene silencing (PTGS). Plants infected with CLCuMuV carrying C4R13A show decreased levels of symptoms and viral DNA accumulation associated with enhanced viral DNA methylation. Furthermore, silencing of NbSAMS2 reduces both TGS and PTGS, but enhanced plant susceptibility to two geminiviruses CLCuMuV and Tomato yellow leaf curl China virus. These data suggest that CLCuMuV C4 suppresses both TGS and PTGS by inhibiting SAMS activity to enhance CLCuMuV infection in plants.
In the course of plant-virus interactions, plants have evolved ingenious counter-attack mechanisms to diminish or eliminate invading viral pathogens. Of various plant antiviral defenses, gene silencing can target either viral RNAs for degradation through post-transcriptional gene silencing (PTGS) or DNA sequences of DNA viruses for epigenetic modification through transcriptional gene silencing (TGS) [1,2].
In this study, we showed that CLCuMuV C4 suppresses both TGS and PTGS by inhibiting SAMS activity through its interaction with SAMS in plants. Furthermore, a point mutation from arginine to alanine at position 13 in C4 abolishes the C4-NbSAMS2 interaction and C4-mediated TGS/PTGS suppression, resulting in impaired viral infection and enhanced viral DNA methylation. In addition, silencing of NbSAMS2 reversed methylation-mediated TGS and PTGS and enhanced geminiviral infection. Our work provides direct evidence that a geminivirus is able to interfere with the methyl cycle to promote the effective infection, and reinforces the importance of methylation as an epigenetic defense against geminiviruses.