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Journal of Virology, April 2008, p. 3250-3260, Vol. 82, No. 7
0022-538X/08/$08.00+0     doi:10.1128/JVI.02139-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Antiviral RNA Silencing Is Restricted to the Marginal Region of the Dark Green Tissue in the Mosaic Leaves of Tomato Mosaic Virus-Infected Tobacco Plants{triangledown}

Katsuyuki Hirai,1,2 Kenji Kubota,1,2,{dagger} Tomofumi Mochizuki,3 Shinya Tsuda,3 and Tetsuo Meshi1,4*

Division of Plant Sciences, National Institute of Agrobiological Sciences, Tsukuba 305-8602, Japan,1 Department of Botany, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan,2 National Agricultural Research Center, Tsukuba 305-8666, Japan,3 CREST, Japan Science and Technology Agency, Kawaguchi 322-0012, Japan4

Received 28 September 2007/ Accepted 15 January 2008

Mosaic is a common disease symptom caused by virus infection in plants. Mosaic leaves of Tomato mosaic virus (ToMV)-infected tobacco plants consist of yellow-green and dark green tissues that contain large and small numbers of virions, respectively. Although the involvement of RNA silencing in mosaic development has been suggested, its role in the process that results in an uneven distribution of the virus is unknown. Here, we investigated whether and where ToMV-directed RNA silencing was established in tobacco mosaic leaves. When transgenic tobaccos defective in RNA silencing were infected with ToMV, little or no dark green tissue appeared, implying the involvement of RNA silencing in mosaic development. ToMV-related small interfering RNAs were rarely detected in the dark green areas of the first mosaic leaves, and their interior portions were susceptible to infection. Thus, ToMV-directed RNA silencing was not effective there. By visualizing the cells where ToMV-directed RNA silencing was active, it was found that the effective silencing occurs only in the marginal regions of the dark green tissue (~0.5 mm in width) and along the major veins. Further, the cells in the margins were resistant against recombinant potato virus X carrying a ToMV-derived sequence. These findings demonstrate that RNA silencing against ToMV is established in the cells located at the margins of the dark green areas, restricting the expansion of yellow-green areas, and consequently defines the mosaic pattern. The mechanism of mosaic symptom development is discussed in relation to the systemic spread of the virus and RNA silencing.

* Corresponding author. Mailing address: Division of Plant Sciences, National Institute of Agrobiological Sciences, 2-1-2 Kannondai, Tsukuba 305-8602, Japan. Phone: 81-29-838-8361. Fax: 81-29-838-8347. E-mail: tmeshi{at}nias.affrc.go.jp

{triangledown} Published ahead of print on 23 January 2008.

{dagger} Present address: National Agricultural Research Center for Kyushu Okinawa Region, Koshi, Kumamoto 861-1192, Japan.

Journal of Virology, April 2008, p. 3250-3260, Vol. 82, No. 7
0022-538X/08/$08.00+0     doi:10.1128/JVI.02139-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.





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