Development of plant virus symptoms and attenuated virus strains to control viral disease
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Development of plant virus symptoms and attenuated virus strains to control viral disease Shinya Tsuda1 Received: 11 August 2020 / Accepted: 12 August 2020 © The Phytopathological Society of Japan and Springer Japan KK, part of Springer Nature 2020
Introduction After plant viruses infect a host plant, symptoms usually develop and, in many cases, can lead to significant losses in crop yield. A mosaic of dark green islands (DGIs) and yellow tissues (YTs) on leaves is a common symptom induced by viruses on infected plants. The development of mosaic symptoms caused by tobamoviruses, cucumoviruses, and potyviruses has been cytologically and biochemically analyzed since the beginning of the last century. The DGIs caused by tobamoviruses usually contain little to no infectious virus, although the YTs areas include large numbers of viral particles. Histochemical observations have led to the suggestion that virus-induced host defense mechanisms are activated in the DGIs, resulting in resistance against the same virus or closely related viruses (Hull 2014). Attenuated viruses of the same or related viral species provide cross protection, or protection of plants from infection from the wild-type virus isolate, also known as the severe strain (McKinney 1929). The phenomenon of cross protection is similar to vaccination: once an attenuated virus has been used to inoculate and infect a whole plant, it cannot be infected by the same or a related viral species. The attenuated virus thus acts as a form of biological control. Many attenuated viruses have been developed for practical use worldwide (Fulton 1986; Kosaka et al. 2006; Lecoq 1998; Oshima 1981; Ziegreen and Carr 2010). For pepper mild mottle virus (PMMoV), belonging to the Tobamovirus genus, infection of Capsicum plants suppresses plant growth and induces leaf mosaic with DGIs and YTs on the upper leaves, and fruit malformation, which affects pepper salability (Wetter and Conti 1988). PMMoV infects * Shinya Tsuda [email protected] 1
Department of Clinical Plant Science, Faculty of Bioscience and Applied Chemistry, Hosei University, Koganei, Tokyo 184‑8584, Japan
cultivated pepper plants through seed, soil, and physical transmission in fields. Some wild species of Capsicum carry the hypersensitive response (HR)-associated tobamoviral resistance genes, inherited in a Mendelian fashion (Boukema 1984). Today, the cultivation of pepper varieties carrying virus resistance genes grown for market has become commonplace. Unfortunately, new wild-type isolates of PMMoV that are capable of systemic infection of the resistant varieties have immediately emerged in fields (Hamada et al. 2007; Tsuda et al. 1998). PMMoV is a positive-sense ssRNA virus about 6,400 nucleotides long. The PMMoV genome encodes at least four proteins: two replicase components (130 k and 180 k proteins), a movement protein (MP), and a coat protein (CP). The 130 k and 180 k proteins are translated from the genomic RNA, and MP and CP from the respective subgenomic mRNAs (Kirita et al. 1997). This rev
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