Accumulation and translocation of tomato yellow leaf curl virus (TYLCV) in a Lycopersicon esculentum breeding line containing the L. chilense TYLCV tolerance gene Ty-1

The major tomato yellow leaf curl virus (TYLCV) tolerance gene, Ty-1. of the wild tomato species Lyeopersicon chilense was mapped to chromosome 6 by using restriction fragment length polymorphism (RFLP) markers and introgressed into the domesticated tomato L. esculentum. Two nearly isogenic breeding lines were obtained. In infected fields, plants from line 52, which contained the Ty-1 allele, remained symptomless (tolerant line); plants from line 50, which did not contain the Ty-1 allele, were all symptomatic (susceptible line). The effect of the Ty-1 gene on TYLCV DNA accumulation and translocation was investigated by hybridizing plant DNA extracts with a viral DNA probe. After inoculation with a small number of viruliferous whiteflies (three insects per plant) plants of the tolerant line 52 had barely detectable levels of TYLCV DNA compared with plants of the susceptible line 50. Agroinoculation of whole plants showed that similar amounts of TYLCV DNA accumulated in both lines. The long-distance movement of TYLCV was impaired in the tolerant plants. After inoculation of the youngest leaf on each plant with 50-70 whiteflies, TYLCV DNA accumulated in the inoculated leaves of plants from both lines, although at a slower rate in plants of line 52. In plants from susceptible line 50, the viral DNA moved from the site of inoculation to the neighboring leaves and to the roots, in contrast to plants from the tolerant line, where it remained confined to the inoculated leaf. Western blot analysis of proteins from the inoculated plants with an antisera against TYLCV confirmed that the movement of the capsid protein was associated with that of the viral DNA. Removal of all leaves except the inoculated youngest leaf hastened the transport of virus towards the roots of the susceptible plants but not of the tolerant plants. Plants of line 50 developed symptoms in all experiments, while plants of line 52 always remained symptomless. Therefore, the Ty-1 gene is associated with inhibition of disease symptoms; and at low titer inoculum, viral accumulation is significantly reduced in inoculated tissue. At high titer inoculum, viral long-distance translocation is limited.