Nobuyuki Yoshikawa - Plant Pathology - yoshikawa@ Professor United Graduate School of Agricultural Sciences Strategic mechanism of biological systems for thermal energy - Molecular mechanism of propagation of plant virus and other pathogens under cold environment -

I. Research achievements
1. Genome structure, functions and molecular variability of viruses in fruit trees in the cold regions
My research group carried out analysis of the whole structure of the genome, gene expression strategy, and molecular variability of the viruses (Apple chlorotic leaf spot virus, ACLSV; Apple stem grooving virus, ASGV; Apple stem pitting virus, ASPV; Apple latent spherical virus, ALSV; and Grapevine berry inner necrosis virus, GINV) that infect apple and other fruit trees (i.e., deciduous fruit trees) in the cold regions. We also found that ACLSV, ASGV and ALSV and GINV are a novel type of viruses that belong to a new genus. These findings contributed greatly to the basic knowledge for identification, classification, and diagnostic techniques of viruses which infect deciduous fruit trees in the world. In addition, we succeeded in the construction of infectious cDNA clones of ACLSV, ASGV and ALSV and established the genetic manipulation system of viral genomes.

2. Virus resistance and its mechanism of transgenic plants expressing movement protein (MP) of plant viruses
Plant viruses that replicated in initially infected cells move from cell to cell through plasmodesmata which connects the cytoplasm between neighboring cells, and then spread systemically. The cell-to-cell movement of the viruses is the most critical step for the establishment of infection, and the movement proteins (MP) that are involved in this process are coded by the viruses themselves. My research group found that transgenic plants expressing ACLSV-MP showed a strong resistance against GINV infection. Further, we found that this resistance is a result of the interaction between MPs of ACLSV and GINV, which in turn inhibit the cell-to-cell and long-distance movement of GINV. Introduction of viral resistance to the plants using MP has high potential for improvement of the resistance because it can focus on the intercellular movement at the early stage of infection, and therefore can become a new strategy to develop crops with virus-tolerance in the future.

3. Development of gene transfer techniques using Apple latent spherical virus (ALSV)
We constructed infectious cDNA clones of latent virus (ALSV) isolated from apple and altered the genome into a virus vector for stable expression of foreign genes (patented). Gene transfer with conventional methods is very difficult to apply to fruit trees and also requires several years before the transgenic individuals are evaluated. In contrast, fruit tree virus vectors will allow the foreign genes to be introduced directly into the plant body, not requiring the processes of tissue culture or shoot regeneration. Then the viruses carry the genes throughout the plant for expression. For these advantages, the gene transfer with virus vectors has high potential for effective use in the future.

II. Goals in the COE Program
1. Understanding of interaction between the fruit trees in the cold regions and plant viruses that are adopted to those trees
Except for some cases where they cause serious disease, the viruses infected to apple and other fruit tree species grown in the cold regions are most frequently infected latently to the plant without showing any symptoms. It is considered that this phenomenon is a result of the balance between the defense mechanism against the viruses on the plant side and the pathogenic potential on the virus side. In the research for the COE program, we will place the focus of research on the fundamental plant defense mechanisms against viruses (virus induced gene silencing: VIGS). We aim to prove the VIGS in the fruit trees in the cold regions (e.g., apple), which has not been reported yet worldwide. In addition, influence of the heat (temperature) on the induction and the factors on the virus side (e.g., suppressor) associated with VIGS induction and suppression are also studied.

2. Development of a gene silencing induction system in the cold regions and its application
As an application, we carry out research and development of VIGS induction system in apple by using the fruit tree virus vectors we have developed. This system should be an effective measure for the post-genome research of the fruit trees, which is considered to be technically difficult at the current stage. When that happens, analysis of genes associated with flower bud development, tree shape development and other characteristics will become available. In addition, the gene silencing induction system can be applied as a new breeding technology to develop new varieties, such as the apple variety whose seedlings will bloom in 1 or 2 years, instead of 6 or 7 years as currently required.

III. Related website
http://news7a1.atm.iwate-u.ac.jp/~plvirus/

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