Grafting and its behavior on CPsV-resistant transgenic oranges

AGUSTINA DE FRANCESCO; CARINA REYES; NORMA COSTA; MARIA LAURA GARCIA

Conferencia; IXX Conference of the IOCV; 2013

International Orgnization of Citrus Virologists (IOCV)

Citrus psorosis virus(CPsV) is the causal agent of Psorosis, an important disease causing significant economic losses in Argentina and Uruguay. CPsV is the type member of the genus Ophiovirus, family Ophioviridae. Its genome has three single-stranded RNAs of negative polarity, encapsidated with a coat protein (CP). As there is no natural resistance to Psorosis, transgenic Pineapple sweet orange (SwO) plants were obtained expressing an intron-RNA hairpin from the cp gene (ihpCP). This RNA transcript induces the post-transcriptional gene silencing (PTGS) mechanism, degrading all RNA with cp gene sequences, thus preventing the progress of the infection. When ihpCP citrus trees were CPsV-challenged by graft-transmission, at least two SwO lines (6110 and 6115) did not show any symptoms and CPsV was not detected by molecular and serological analysis. The hallmark of the PTGS is the presence of small RNAs (siRNA), which were detected in the transgenic tissue indicating that resistance is PTGS-mediated. Our interest is to evaluate whether ihpCP is able to transfer this feature of resistance to susceptible grafted-tissue. For that, non-transgenic buds were grafted on the two resistant ihpCP lines used as rootstocks, which had previously been inoculated with CPsV. In another assay, a susceptible CP-line, expressing the complete CP-mRNA, was grafted on the same two ihpCP resistant rootstocks. Both, the non-transgenic and transgenic CP were susceptible. New flushes showed characteristic symptoms (flecking, spot and shock) and TAS-ELISA and qRT-PCR analysis were positive. These results suggest that the ihpCP lines, with no evidence of infection, allow virus movement to the susceptible tissues (non-transgenic and CP-line). It seems that the virus could reach susceptible tissues before PTGS has been established. Therefore, we are currently performing a new assay grafting the susceptible non-transgenic and CP-line scions before CPsV infection, giving kinetic advantage to PTGS establishment.