Quantitative Analysis of Southern rice black-streaked dwarf virus in Sogatella furcifera and Virus Threshold for Transmission.

We investigated Southern rice black-streaked dwarf virus (SRBSDV) accumulation in a vector insect, the whitebacked planthopper (Sogatella furcifera), to elucidate the association of virus accumulation in the vector with virus transmission efficiency. Real-time quantitative reverse transcription polymerase chain reaction analysis confirmed that this virus is transmitted in a persistent-propagative manner. SRBSDV was successfully transmitted by S. furcifera males in which RNA accumulation of the capsid protein gene of SRBSDV was >10(3) in the whole body of S. furcifera, indicating that the threshold accumulation of the virus RNA for virus transmission is 10(3) in an S. furcifera male. The SRBSDV detection rate in the immigrant population of S. furcifera was high in 2011 (39.5%); however, most of the insects contained fewer than 10(3) RNAs of the capsid protein gene. This result indicates that the risk of SRBSDV epidemics could be estimated from the proportion of virus-transmissible S. furcifera (i.e., S. furcifera that contained more than 10(3) RNAs of the virus capsid protein gene) rather than the SRBSDV detection rate in S. furcifera.

Biological and genetic diversity of Wheat yellow mosaic virus (Genus Bymovirus).

The biological and genetic diversity of Wheat yellow mosaic virus (WYMV) isolates in Japan was characterized. On the basis of wheat cultivar reactions, 14 WYMV isolates from various places were classified into pathotypes I, II, or III. These were distributed in central, northern, and southern areas of Japan, respectively. WYMV isolates comprised three genotypes (A, A' and B) based on amino acid differences in RNA1 and two genotypes (a and b) based on amino acid differences in RNA2. A correlation was found between the WYMV RNA1-based genotype and pathotype, suggesting that factors associated with pathogenicity map to RNA1. Genotype Aa and A'a were distributed mainly in the central to southern areas of Japan, and genotype Bb was found in northern areas of Japan, as shown by reverse-transcription polymerase chain reaction restriction fragment length polymorphism analysis. Chinese isolates YA and YZ were closely related to genotypes Bb and Aa, respectively. Wheat was introduced from China to Japan in the 4th and 5th centuries, and the two genotypes of WYMV might also have been introduced with the crop from China and later adapted to local wheat cultivars in Japan.

Transgenic strategies to confer resistance against viruses in rice plants.

Rice (Oryza sativa L.) is cultivated in more than 100 countries and supports nearly half of the world's population. Developing efficient methods to control rice viruses is thus an urgent necessity because viruses cause serious losses in rice yield. Most rice viruses are transmitted by insect vectors, notably planthoppers and leafhoppers. Viruliferous insect vectors can disperse their viruses over relatively long distances, and eradication of the viruses is very difficult once they become widespread. Exploitation of natural genetic sources of resistance is one of the most effective approaches to protect crops from virus infection; however, only a few naturally occurring rice genes confer resistance against rice viruses. Many investigators are using genetic engineering of rice plants as a potential strategy to control viral diseases. Using viral genes to confer pathogen-derived resistance against crops is a well-established procedure, and the expression of various viral gene products has proved to be effective in preventing or reducing infection by various plant viruses since the 1990s. RNA interference (RNAi), also known as RNA silencing, is one of the most efficient methods to confer resistance against plant viruses on their respective crops. In this article, we review the recent progress, mainly conducted by our research group, in transgenic strategies to confer resistance against tenuiviruses and reoviruses in rice plants. Our findings also illustrate that not all RNAi constructs against viral RNAs are equally effective in preventing virus infection and that it is important to identify the viral "Achilles' heel" gene to target for RNAi attack when engineering plants.

Dynamics of Southern rice black-streaked dwarf virus in rice and implication for virus acquisition.

A novel viral disease of rice caused by Southern rice black-streaked dwarf virus (SRBSDV) has spread throughout East and Southeast Asia since the mid-2000s. Outbreaks of this viral disease occur yearly in southern parts of Japan concurrently with overseas migration of the planthopper vector Sogatella furcifera from southern China during the rainy season (from late June to early July). We examined the dynamics (changes in titer and localization) of SRBSDV on rice using reverse-transcription real-time polymerase chain reaction and determined the relationship between virus titer in plants and virus acquisition by S. furcifera. Under a constant temperature of 27°C, a substantial increase of SRBSDV titer in the leaf sheath together with typical symptoms (stunted growth and twisting of leaf tips) was observed at 20 days after the end of a 7-day exposure of viruliferous S. furcifera. Approximately 40% of S. furcifera acquired SRBSDV through feeding for 5 days on rice plants that were infected following exposure to viruliferous vectors for 10 to 15 days. These results suggest that rice infected by S. furcifera can be a source of SRBSDV before the next generation of S. furcifera emerges.

Pathogenicity and insect transmission of a begomovirus complex between tomato yellow leaf curl virus and Ageratum yellow vein betasatellite.

Tomato yellow leaf curl virus (TYLCV) and Ageratum yellow vein betasatellite (AYVB) are members of the genus Begomovirus (family Geminiviridae). TYLCV and AYVB have been found in Japan over the last 15 years, and are associated with tomato leaf curl and the tomato yellow leaf curl diseases (TYLCD). AYVB is also associated with some monopartite begomoviruses. We have cloned both TYLCV and AYVB and demonstrated that TYLCV can trans-replicate with AYVB in Nicotiana benthamiana and tomato plants. A mixed infection of TYLCV and AYVB induced more severe symptoms of upward leaf curl, stunting, vein thickening, and swelling compared with TYLCV infection alone. The symptoms induced by infection of AYVB included a rise in abnormal cell proliferation, and pigmentation around leaf vein tissues. This is the first study to show that a complex of TYLCV and AYVB can be transmitted by vector insects among tomato plants. These results indicate that TYLCV possesses the potential to induce severe TYLCD by associating with AYVB.