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Satellite Panicum Mosaic Virus Capsid Protein Elicits Symptoms on a Nonhost Plant and Interferes with a Suppressor of Virus-Induced Gene Silencing

The capsid protein (CP) of satellite panicum mosaic virus (SPMV) has been implicated as a pathogenicity factor, inducing severe chlorosis on millet plants co-infected with SPMV and its helper virus, Panicum mosaic virus (PMV). In this study, we tested the effects of SPMV CP on Nicotiana benthamiana, a plant that does not support PMV+SPMV infections. SPMV CP expressed from a Potato virus X (PVX) gene vector elicited necrotic lesions on N. benthamiana. Pathogenicity factors often have the additional feature of acting as suppressors of gene silencing; therefore, several assays were developed to test if SPMV CP could act in such a capacity. The results showed that SPMV CP failed to act as a suppressor of posttranscriptional gene silencing when such tests were performed with transgenic N. benthamiana plants silenced for green fluorescent protein (GFP) expression by agroinfiltration or plant virus vectors. However SPMV CP expressed from the PVX gene vector did interfere with suppressor activity associated with PVX p25. This included a rebounded level of GFP silencing along the vascular tissues, including the veins on upper noninoculated leaves. Therefore, the roles of the SPMV CP now include encapsidation of the SPMV RNA, activity as a pathogenicity factor in both host and nonhost plants, and the enigmatic feature of interfering with suppression of gene silencing.