Characterization of the Tomato Mosaic Virus resistance gene Tm2² and its product.

 

Many vegetable and ornamental crops, amongst which tomato, sweet peppers and cucumbers, suffer from tobamoviruses. Infections with tobamoviruses, such as Tomato Mosaic Virus (ToMV), usually give characteristic mosaic symptoms and lead to considerable cosmetical damage and yield losses. Tobamoviruses are rod-shaped particles containing positive stranded linear RNA genomes. The viruses are mechanically transmitted, highly infectious and extremely stable.

When a plant is resistant against a ToMV infection the plant contains a resistance gene, which product recognizes the avirulence (avr) gene product of the pathogen. Recognition of the avr-product results in a hypersensitive response of the plant, which prevents spread of the virus through the plant. Disruption of the resistance gene will result in susceptibility.

Using the Activator/dissociation (Ac/Ds) transposon system from maize we isolated a tomato resistance gene by transposon tagging. The Ds element is mobile only in the presence of Ac. The Ac/Ds transposon system was introduced into the tomato plant (transgenic plants) and mutants were obtained in which the Ds-element had jumped into the resistance gene and disrupted the gene's function. Using plasmid rescue the gene was isolated and sequenced. The gene obtained codes for an approximately 800 aa-protein.

Several new lines of research can now be pursued.

1) Complementation studies should confirm if the gene isolated is indeed the resistance gene responsible for the ToMV-resistance.
2) Expression levels and patterns will be studied.
3) The recessive alleles will be analyzed.
4) The gene product will be produced in high levels and studied. The interaction between the avr-protein and the resistance protein will be analyzed.
5) Some mutant plants obtained from the transposon tagging experiments must be further analyzed because their relation with the ToMV-resistance is still obscure.