Transgenic expression in citrus of single-chain antibody fragments specific to Citrus tristeza virus confers virus resistance.

Citrus tristeza virus (CTV) causes one of the most destructive viral diseases of citrus worldwide. Generation of resistant citrus genotypes through genetic engineering could be a good alternative to control CTV. To study whether production of single-chain variable fragment (scFv) antibodies in citrus could interfere and immunomodulate CTV infection, transgenic Mexican lime plants expressing two different scFv constructs, separately and simultaneously, were generated. These constructs derived from the well-referenced monoclonal antibodies 3DF1 and 3CA5, specific against CTV p25 major coat protein, whose mixture is able to detect all CTV isolates characterized so far. ScFv accumulation levels were low and could be readily detected just in four transgenic lines. Twelve homogeneous and vigorous lines were propagated and CTV-challenged by graft inoculation with an aggressive CTV strain. A clear protective effect was observed in most transgenic lines, which showed resistance in up to 40-60% of propagations. Besides, both a delay in symptom appearance and attenuation of symptom intensity were observed in infected transgenic plants compared with control plants. This effect was more evident in lines carrying the 3DF1scFv transgene, being probably related to the biological functions of the epitope recognized by this antibody. This is the first report describing successful protection against a pathogen in woody transgenic plants by ectopic expression of scFv recombinant antibodies.

Interference Between D and M Types of Plum pox virus in Japanese Plum Assessed by Specific Monoclonal Antibodies and Quantitative Real-Time Reverse Transcription-Polymerase Chain Reaction.

ABSTRACT The dynamics of virus interference between two isolates of Plum pox virus (PPV) belonging to the main PPV types, D and M, were analyzed in Japanese plum (Prunus salicina) by challenge inoculations. To assess the consequences of a PPV-M infection on plum already infected with PPV-D, and vice versa (predominance of one of the strains, recombination, synergism, symptoms aggravation, and so on), 30 Japanese plum trees were graft inoculated with PPV-D or PPV-M isolates in quarantine conditions. One year postinoculation, in the event that the inoculated isolates were detected in the whole plant, a second challenge inoculation (PPV-M or PPV-D, respectively) was performed by grafting. The presence of PPV-D, PPV-M, or both was monitored for 7 years by double-antibody sandwich indirect enzyme-linked immunosorbent assay using specific monoclonal antibodies. Reverse transcription-polymerase chain reaction (RT-PCR) with D- and M-specific primers confirmed the serological typing. Real-time RT-PCR assays were performed using D- and M-specific fluorescent 3' minor groove binder-DNA probes, which were able to detect and quantify PPV populations in the inoculated plants with greater precision. The presence of PPV-D in Japanese plum did not cross-protect the trees against PPV-M infection. In PPV-D-infected plants, the PPV-M strain used as challenge inoculum behaved differently depending on the plum cultivar assayed. In cv. Black Diamond, PPV-M invaded the plant progressively, displacing the previous PPV-D population; whereas, in cv. Sun Gold, both PPV isolates coexisted in the plant. In contrast, the PPV-D isolate used was unable to infect plants of both cultivars in which a PPV-M population already was established. After 7 years, no synergism was observed and no recombination event between PPV-D and PPV-M genomes was detected.