There can be only one: two Flavescence dorée phytoplasma strains compete during plant infection

Flavescence dorée phytoplasmas (FDp, 16SrV) are plant pathogenic non-cultivable bacteria associated with a severe and epidemic disease of grapevine. Despite the high genome sequence identity of the two reference strains FD-C and FD-D, their frequencies in cultivated grapevine are unbalanced. In particular, FD-C overcomes FD-D during plant infection. To investigate this phenomenon, Catharanthus roseus plants were graft-infected with the two strains. A first group of plants was infected with the two strains simultaneously, whereas a second group was infected with either of the two strains sequentially. Since the apical grafting seemed to foster FDp infection, different combination of lateral and apical grafting were applied to avoid artificial benefit due to the graft position. The infection was monitored over time and strain-specific diagnosis of symptomatic leaves was periodically performed up to 120 days. At the end of the experiments, roots were also collected and FD-C and -D loads were measured. Since the first sampling date, FD-C was more effective in plant colonization, and this was confirmed until the end of the experiment, when more than 70% of plants were positive only for FD-C. Moreover, more than the 90% of the analyzed roots were infected with FD-C only. These results strongly support a competition between FD-C and FD-D strains during plant infection/colonization and suggest the hypothesis that FD-C is more effective in plant infection. Moreover, these data are coherent with the observed different frequencies of FD-C/D strains under field conditions, and may contribute to explain the biology behind this phenomenon.