DIFFERENCES IN PROTECTION PHENOTYPES IN TOBACCO PLANTS EXPRESSING COAT PROTEIN GENES FROM PEANUT STRIPE POTYVIRUS WITH OR WITHOUT AN ENGINEERED ATG

Transgenic Nicotiana benthamiana plants expressing the full-length coat protein (CP) gene from the potyvirus, peanut stripe virus (PStV), with or without an engineered ATG initiation codon or containing an engineered ATG initiation codon with amino terminal deletions of 16 or 106 amino acids, were tested for resistance to virus infection. Two plant lines, both with engineered ATGs (ATG plus), did not accumulate virus in systemic leaves or show symptoms when mechanically inoculated. All other plant lines that were ATG plus (full length or deletions) exhibited a delay in the appearance of systemic symptoms and the symptoms were milder. Following the initial systemic spread of the virus, the younger systemic tissue appeared to escape infection, leading to a recovery of the plant. Western blot analysis revealed that virus-encoded CP was not detectable in the systemic leaf tissues from the plants which had recovered. The steady-state levels of both protein and mRNA from the genomic copy of the PStV CP gene were down-regulated in these upper systemic symptomless leaves. Upper systemic, symptomless leaves on recovered plants were resistant to infection by PStV upon reinoculation but susceptible to infection by tobacco etch virus. Protection or recovery phenotypes were not observed when transgenic plants, immune to PStV infection, were inoculated with other potyviruses or a tobamovirus. After challenge with PStV, plant lines expressing the CP messenger RNA without an engineered ATG (ATG minus) displayed an altered symptom phenotype compared with those on non-transgenic or ATG plus transgenic plants. Symptoms were delayed in appearance or did not appear on ATG minus plants and no recovery from the viral infection was seen even after an extended period of time on those plants exhibiting the delay phenotype. The possibility that an engineered ATG is necessary to obtain the recovery phenotype is discussed.