TRANSGENIC PLANTS EXPRESSING POTATO-VIRUS-X ORF2 PROTEIN (P24) ARE RESISTANT TO TOBACCO MOSAIC-VIRUS AND OB TOBAMOVIRUSES

The p24 protein, one of the three proteins implicated in local movemen t of potato virus X (PVX), was expressed in transgenic tobacco plants (Nicotiana tabacum Xanthi D8 NN). Plants with the highest level of p24 accumulation exhibited a stunted and slightly chlorotic phenotype. Th ese transgenic plants facilitate the cell-to-cell movement of a mutant of PVX that contained a frameshift mutation in p24. Upon inoculation with tobacco mosaic virus (TMV), the size of necrotic local lesions wa s significantly smaller in p24+ plants than in nontransgenic, control plants. Systemic resistance to tobamoviruses was also evidenced after inoculation of p24+ plants with Ob, a virus that evades the hypersensi tive response provided by the N gene. In the latter case, no systemic symptoms were observed, and virus accumulation remained low or undetec table by Western immunoblot analysis and back-inoculation assays. In c ontrast, no differences were observed in virus accumulation after inoc ulation with PVX, although more severe symptoms were evident on p24-ex pressing plants than on control plants. Similarly, infection assays co nducted with potato virus Y showed no differences between control and transgenic plants. On the other hand, a considerable delay in virus ac cumulation and symptom development was observed when transgenic tobacc o plants containing the movement protein (MP) of TMV were inoculated w ith PVX. Finally, a movement defective mutant of TMV was inoculated on p24+ plants or in mixed infections with PVX on nontransgenic plants. Both types of assays failed to produce TMV infections, implying that T MV MP is not interchangeable with the PVX MPs.