A Reduced Starch Level in Plants at Early Stages of Infection by Viruses Can Be Considered a Broad-Range Indicator of Virus Presence

SIMPLE SUMMARY: In this report, we describe a potential broad-range indicator of virus presence in plants. This method is based on the observation that starch content in leaves is generally reduced upon virus infection at the early stages of infection. This method was also used to effectively detect the infection of three latent viruses in propagative apple materials. Hence, the plant with the low virus accumulation, latent infection or at early stage of infection can be indicated by assessing starch accumulation levels. The genes encoding key enzymes involved in starch biosynthesis are transcriptionally downregulated by virus infection, suggesting that plant virus infection generally affects cellular metabolic pathways. ABSTRACT: The diagnosis of virus infection can facilitate the effective control of plant viral diseases. To date, serological and molecular methods for the detection of virus infection have been widely used, but these methods have disadvantages if applied for broad-range and large-scale detection. Here, we investigated the effect of infection of several different plant RNA and DNA viruses such as cucumber mosaic virus (CMV), tobacco mosaic virus (TMV), potato virus X (PVX), potato virus Y (PVY) and apple geminivirus on starch content in leaves of Nicotiana benthamiana. Analysis showed that virus infection at an early stage was generally associated with a reduction in starch accumulation. Notably, a reduction in starch accumulation was readily apparent even with a very low virus accumulation detected by RT-PCR. Furthermore, we also observed that the infection of three latent viruses in propagative apple materials was associated with a reduction in starch accumulation levels. Analysis of transcriptional expression showed that some genes encoding enzymes involved in starch biosynthesis were downregulated at the early stage of CMV, TMV, PVX and PVY infections, suggesting that virus infection interferes with starch biosynthesis in plants. Our findings suggest that assessing starch accumulation levels potentially serve as a broad-range indicator for the presence of virus infection.