Citrus tristeza virus p23 may suppress systemic silencing but is not related to the kind of viral syndrome

Citrus tristeza virus (CTV) the causal agent of the most devastating viral disease of citrus, has evolved three silencing suppressor proteins namely p20, p23 and p25 protein, previously reported for the monophyletic isolate T36. In this study the p23 suppressing activity from isolates of each of seven phylogenetic groups recognized for CTV was characterized in Nicotiana benthamiana 16C plants expressing GFP. Data revealed that the p23 protein from each phylogenetic group was able to transiently suppress the local but not the short-range silencing. p23 from Group 5 was the most efficient local suppressor followed by Groups 1, 3a, 3b and 4; the Groups 2 and M were less efficient. Different development of systemic silencing was observed among p23 proteins. This was most conspicuous for Gp 5, which completely blocked the GFP systemic silencing, suggesting that a quantitative relationship might exist between local and systemic silencing. A 3D model of the p23 protein was constructed showing differences within the Zn-finger region, which, however, do not totally explain the differences found. It was not possible to trace a relationship between the syndromes tribute to the various phylogenetic groups and p23 activity.