Characterization of resistance to Papaya ringspot virus (PRSV) in non transgenic papaya (Carica papaya L.) varieties and resistance evaluation of selected transgenic lines carrying the PRSV coat protein gene

The resistance of three papaya varieties such as Sinta, Cariflora and Red Lady was characterized based on disease onset, rate of disease development, disease incidence, symptom severity and virus level in inoculated plant. Two-month old papaya seedlings were mechanically inoculated with each of three PRSV isolates; PRSV Laguna 2010 field isolate, purified PRSV Laguna 2005 and purified PRSV Laguna 2010. The development of infection in inoculated plants of the tolerant varieties was compared with that of the susceptible variety, Davao Solo and Cavite Special and infection was assessed weekly after inoculation for a period of 8 weeks. Cariflora had delayed onset, reduced rate of disease development, reduced disease incidence and reduced symptom severity while Sinta had delayed onset and reduced symptom severity only when inoculated with PRSV Laguna 2010 field isolate. Disease incidence in Cariflora was significantly lower than Sinta, Cavite Special and Davao Solo. Sinta had delayed onset and reduced symptom severity when inoculated with purified PRSV Laguna 2005 but not with purified PRSV Laguna 2010 isolate. Red Lady had disease onset and symptom severity similar to Davao Solo when inoculated with either purified PRSV Laguna 2005 or 2010 isolates. Cavite Special had delayed disease development when inoculated with purified PRSV Laguna 2005 isolate. At later stages of infection, no differences in incidence among the four varieties was observed when plants were inoculated with either purified PRSV Laguna 2005 or 2010 isolates. Sinta had lower virus level in plant than Red Lady, Davao Solo and Cavite Special. Transgenic papaya T4 lines of Events 124 and 132 carrying the PRSV coat protein gene were evaluated to confirm their resistance to PRSV. One inoculated plant of line 132-2.9-13.27-F.262-1 and 1 out of 3 inoculated plants of line 132-2.9-13.27-F.262-3b from Event 132 were negative to PRSV infection. No PRSV symptoms were observed until 8 upi and these plants were negative to PRSV infection in Enzyme-linked Immunosorbent Assay (ELISA). The reaction to PRSV of these selected lines needs to be further evaluated using more number of plants to confirm of the negative reaction observed in this study was indeed due to PRSV resistance and not to disease escape.