The ancestral TEV was poorly adapted to A. thaliana Ler and infection concurred with the development of very mild symptoms ( Figure 1 ). Furthermore, 21 days post-inoculation (dpi), the viral load in infected plants, measured as the number of lesion-forming units (LFU) produced per milligram of tissue, was low, 48.3362.95 LFU/mg (6SEM), and the infectivity of the newly produced viral particles (i.e., the efficiency of initiating a new infection using a normalized amount of viral particles) was as low as 17.95% [95% confidence interval (CI): 7.54-33.53%].