Citrus psorosis virus: insights into virus biology

EDUARDO PEÑA, GABRIEL ROBLES LUNA, M. CECILIA ZANEK AND M. LAURA GARCIA

Campinas

Congreso; Proceeding of the XVII International Conference on Citrus Virology; 2010

IOCV -EMBRAPA

Citrus psorosis virus (CPsV), the type member of the Ophiovirus genus, causes serious disease in citrus plants grown in Argentina and Uruguay. The viral genome consists of three negative-sense ssRNAs. RNA 1 codes for 24K, a 24 kDa putative protein as well as a protein predicted to be a RNA-dependent RNA polymerase. A 54 kDa protein (54K), encoded by RNA 2 and RNA 3 encodes the coat protein (CP or 48K). The molecular and cellular aspects of the infection cycle of ophioviruses have not been investigated yet. To gain insight into CPsV biology, we study the subcellular localization and functions of the proteins 24K, 48K and 54K. The subcellular localization of the CP and 54K proteins was analyzed by subcellular fractionation of infected Chenopodium quinoa and by confocal laser scanning microscopy (CLSM) using GFP fusion proteins expressed transiently in Nicotiana benthamiana. The 54K protein was found in the nuclear, cytoplasmic, cell wall and microsomal fractions. By CLSM, the 54-GFP fusion protein was found in the nucleus, cytoplasm and in the plasmodesmata. Applying the same strategies, the CP was only localized to the cytoplasm but not in other subcellular fractions. In plants, post transcriptional gene silencing (PTGS) is a conserved mechanism of defense against viruses. Viruses often encode proteins which can suppress this mechanism by the action of suppressor protein. To assess which protein/s of CPsV suppress PTGS, an Agrobacterium-mediated transient expression assays using the GFP expressing N. benthamiana line 16c, were employed to address local and/or systemic suppressor activity. None of CPsV proteins (24K, 48K or 54K) suppress local silencing when express individually. On the other hand, 54K and 24K proteins, but not CP, have shown systemic suppressor activity. To determinate which of the CPsV proteins allow cell-to-cell and systemic viral movement, a complementation experiment using a Potato virus X (PVX) mutant defective in the movement function was performed. Preliminary results suggest that 54K and 24K proteins may complement PVX, thus providing a first indication of a movement function of these proteins during CPsV infection. These results indicate that CP is limited to encapsidation, the 24K and 54K proteins could be involved in suppression of systemic RNA silencing and viral movement.