Xanthomonas oryzae pv. oryzae (Xoo) is the causal agent of bacterial blight of rice, which is one of the most important diseases of rice, along with rice blast. The disease occurs almost throughout the rice-growing areas of the world, especially in temperate to tropical zones. In severe cases, the yield loss might be as much as 60%. When the "Green Revolution" in rice appeared successful, the new rice variety IR8 was severely attacked by the pathogen, and as a result, it broke the myth of the revolution. Recently, the outbreak of bacterial blight became a serious problem again, as hybrid rice spread to many countries. Thus, the reason why our paper was highly cited is the importance of Xoo as a plant pathogen.

“Plants are attacked by many plant pathogens and get diseased as we have many diseases caused by various kinds of microbes.”

The bacterium is not only important as a pathogen of destructive disease, but it has significance as a model organism in the research field of plant-microbe interaction. Actually, Xoo is the third Xanthomonas species for which genome sequencing was completed. In the US, Korea, and China, genome sequencing of several strains was finished or already under way. In France, the genome of an African strain of Xoo is to be sequenced. Thus, the genome sequencing of Xoo is rapidly progressing.

The genome structure is very unique; because the genome harbors numerous effector genes (avirulence genes) and IS (mobile elements) compared with the genome structure of plant pathogenic bacteria whose sequencing was completed before our Xoo. This was a very important finding. The structure suggests their contribution to race diversity, since more than 40 races differing in pathogenicity have been reported worldwide. It is also enormously important that pathogenicity-related genes were scattered throughout the genome and did not form an island of pathogenicity.

Plants are attacked by many plant pathogens and we have many diseases caused by various kinds of microbes. Our target of genome sequencing was representative of such plant pathogens which are classified as a bacterium. Our paper provides ideas of what kind of genome plant pathogen Xoo has. In addition, its unique genome structure has caused diversity as a highly-evolved pathogen.

I have worked on the bacterial blight of rice for more than 30 years, since 1974. I have worked on genetic and pathogenic diversity of the pathogen Xoo, as well as varietal resistance and resistance mechanisms in host plants. In 1998, a round-table session was held among plant bacteriologists for the initiation of the genome sequencing of plant pathogenic bacteria at the International Congress on Plant Pathology held in Edinburgh, Scotland.

I presented a description of the sequencing of the Xoo genome at this meeting. However, it appeared to be only a dream, because we could not get any dedicated funding for sequencing. Fortunately, we were able to join in the one big project for plant-microbe interaction supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan. Genome sequencing was required as the basis for the project, and additional funds were provided by the National Institute of Agrobiological Sciences (NIAS).

I’m quite proud that our sequencing was achieved in a small, factory-like laboratory. Approximately a dozen young scientists and research assistants worked very hard, using a rather old-type of DNA sequencer.

I believe that our research will be able to contribute mainly to two research areas. One is for comparative genomics with other plant pathogenic bacteria, animal and human pathogenic bacteria, or plant symbiotic bacteria. The other is for the analysis in plant-pathogen interaction. As rice genome sequencing was also completed at NIAS, this became the first case where both genomes of host plant and parasite were elucidated. This rice and Xoo combination will be the model system for elucidation of plant-microbe interaction based on genomics.

If our work aids in the development of a novel control method against plant diseases, these are implications enough for us.

We express hearty thanks to Drs. Naoki Katsura, Kenichi Higo, Takuji Sasaki, and Jun-ichi Kurisaki for their encouragement and valuable advice.