| Summary: | Understanding how phytoplasmas move and multiply within the host plant is fundamental for plant–pathogen interaction studies. In recent years, the tomato has been used as a model plant to study this type of interaction. In the present work, we investigated the distribution and multiplication dynamics of one strain of ‘<i>Candidatus</i> Phytoplasma (<i>Ca</i>. P.) solani’ (16SrXII-A) in tomato (<i>Solanum lycopersicum</i> L., cv. Micro-Tom) plants. We obtained infected plants by grafting, a fast and effective method to maintain phytoplasma infection. <i>In planta</i> spread and multiplication of ‘<i>Ca</i>. P. solani’ was monitored over time using qualitative and quantitative qPCR. Root, apical shoot, lower leaves, and upper leaves were sampled at each sampling time. We hypothesized that ‘<i>Ca</i>. P. solani’ from the grafting site reached firstly the highest leaf, the apex and the roots; subsequently, the phytoplasmas spread to the rest of the upper leaves and then progressively to the lower leaves. Significant differences were found in ‘<i>Ca</i>. P. solani’ titer among different plant tissues. In particular, the concentration of phytoplasma in the roots was significantly higher than that in the other plant compartments in almost all the sampling dates. Since the roots show rapid colonization and the highest concentration of phytoplasmas, they represent the ideal tissue to sample for an early, sensitive and robust diagnosis.
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