Identification and map location of TTR1, a single locus in Arabidopsis thaliana that confers tolerance to tobacco ringspot nepovirus

The interaction between Arabidopsis and the nepovirus tobacco ringspot virus (TRSV) was characterized. Of 97 Arabidopsis lines tested, all were susceptible when inoculated with TRSV grape strain. Even though there was systemic spread of the virus, there was a large degree of variation in symptoms as the most sensitive lines died 10 days after inoculation, while the most tolerant lines either were symptomless or developed only mild symptoms. Four lines were selected for further study based on their differential reactions to TRSV. Infected plants of line Col-0 and Col-0 gl1 flowered and produced seeds like noninfected plants, while those of lines Estland and 1155 died before producing seeds, Symptoms appeared on sensitive plants approximately 5 to 6 days after inoculation. Serological studies indicated that in mechanically inoculated seedlings, the virus, as measured by coat protein accumulation, developed at essentially the same rates and to the same levels in each of the four lines, demonstrating that differences in symptom development were not due to a suppression of virus accumulation, Two additional TRSV strains gave similar results when inoculated on the four lines. Genetic studies with these four Arabidopsis lines revealed segregation of a single incompletely dominant locus controlling tolerance to TRSV grape strain, We have designated this locus TTR1. By using SSLP and CAPS markers, TTR1 was mapped to chromosome V near the nga129 marker, Seed transmission frequency of TRSV for Col-0 and Col-0 gl1 was over 95% and their progeny from crosses all had seed transmission frequencies of over 83%, which made it possible to evaluate the segregation of TTRI in F-2 progeny from infected F-1 plants without inoculating F-2 plants, Seed transmission of TRSV will be further exploited to streamline selection of individuals for fine mapping the TTRI gene. The identification of tolerant and sensitive interactions between TRSV and A. thaliana lines provides a model system for genetic and molecular analysis of plant tolerance to virus infection.