The p23 Protein from Citrus tristeza virus is a Pathogenicity Determinant in Transgenic Citrus Hosts

The 23 kDa protein (p23) coded by the 3 -terminal gene of Citrus tristeza virus (CTV), is a RNA-binding protein that contains a motif rich in cysteine and histidine residues in the core of a putative zinc-finger domain. Transgenic Mexican lime plants were generated carrying a p23 transgene, or a truncated version thereof, under the control of the Cauliflower mosaic virus (CaMV) 35S promoter. Ectopic expression of the p23 gene from a severe (T36) strain of CTV induced phenotypic aberrations that resembled symptoms incited by CTV in non-transgenic lime plants, whereas transgenic plants expressing the p23 truncated version were normal. The intensity of the CTV-like symptoms in p23-transgenic plants was associated with the p23 accumulation level. Besides, expressing the same gene from a mild strain (T-317) induced similar symptoms irrespective of the source CTV strain. Transformation of CTV-susceptible sweet and sour orange and CTV-resistant trifoliate orange with p23-T36 also led to CTV-like symptoms that did not develop when plants were transformed with a truncated p23 version. In these transgenic citrus species, symptom intensity correlated with levels of p23 transcripts, as protein accumulation was barely detectable. Conversely, transgenic expression of p23 in CTV non-host Nicotiana spp. led to accumulation of p23 without phenotypic aberrations, indicating that p23 interferes with plant development only in Citrus species and relatives. This was the first case in which a protein encoded by a woody plant infecting RNA virus was identified as being directly involved in pathogenesis in its natural host. In this note, we summarize our results on the role of p23 as a pathogenicity determinant of CTV in citrus hosts. _____________________________________________________________________________________________________________