Preventive and curative effects of Apple latent spherical virus vectors harboring part of the target virus genome against potyvirus and cucumovirus infections.

Apple latent spherical virus (ALSV)-based vectors experimentally infect a broad range of plant species without causing symptoms and can effectively induce stable virus-induced gene silencing in plants. Here, we show that pre-infection of ALSV vectors harboring part of a target viral genome (we called ALSV vector vaccines here) inhibits the multiplication and spread of the corresponding challenge viruses [Bean yellow mosaic virus, Zucchini yellow mosaic virus (ZYMV), and Cucumber mosaic virus (CMV)] by a homology-dependent resistance. Further, the plants pre-infected with an ALSV vector having genome sequences of both ZYMV and CMV were protected against double inoculation of ZYMV and CMV. More interestingly, a curative effect of an ALSV vector vaccine could also be expected in ZYMV-infected cucumber plants, because the symptoms subsided on subsequent inoculation with an ALSV vector vaccine. This may be due to the invasion of ALSV, but not ZYMV, in the shoot apical meristem of cucumber.

Development of apple latent spherical virus-based vaccines against three tospoviruses.

Apple latent spherical virus (ALSV) is characterized by its relatively broad host range, latency in most host plants, and ability to induce gene silencing in host plants. Herein, we focus on the above characteristic of ALSV and describe our development of ALSV vector vaccines against three tospoviruses, namely, Impatiens necrotic spot virus (INSV), Iris yellow spot virus (IYSV), and Tomato spotted wilt virus (TSWV). DNA fragments of 201 nt of three tospovirus S-RNAs (silencing suppressor (NSS) and nucleocapsid protein (N) coding regions for each tospovirus) were inserted into an ALSV-RNA2 vector to obtain six types of ALSV vector vaccines. Nicotiana benthamiana plants at the five-leaf stage were inoculated with each ALSV vector vaccine and challenged with the corresponding tospovirus species. Tospovirus-induced symptoms and tospovirus replication after challenge were significantly suppressed in plants preinoculated with all ALSV vector vaccines having the N region fragment, indicating that strong resistance was acquired after infection with ALSV vector vaccines. On the other hand, cross protection was not significant in plants preinoculated with ALSV vectors having the NSs region fragment. Similarly, inoculation with an ALSV-RNA1 vector having the N region fragment in the 3'-noncoding region, but not the NSs region fragment, induced cross protection, indicating that cross protection is via RNA silencing, not via the function of the protein derived from the N region fragment. Our approach, wherein ALSV vectors and selected target inserts are used, enables rapid establishment of ALSV vector vaccines against many pathogenic RNA viruses with known sequences.