TOBACCO PLANTS PERTURBED IN THE UBIQUITIN-DEPENDENT PROTEIN-DEGRADATION SYSTEM ACCUMULATE CALLOSE, SALICYLIC-ACID, AND PATHOGENESIS-RELATEDPROTEIN-1

Transgenic tobacco plants expressing an inhibitor of ubiquitin-depende nt protein degradation, ubiquitin variant ubR48, spontaneously formed necrotic lesions and displayed altered responsiveness to tobacco mosai c virus attack. These plants were analyzed for the accumulation of def ense-related compounds and the expression of pathogenesis-related prot eins, which serve as convenient markers for systemic acquired resistan ce. Callose was detected in the cells of vascular bundles and in the l eaf blade. In addition, ubR48 transgenic plants constitutively accumul ated enhanced levels of salicylic acid (SA) and/or its glucoside. Accu mulation of SA glucoside coincided with high levels of pathogenesis-re lated protein 1, underscoring the similarity of certain changes in ubR 48-expressing plants to an authentic defense reaction.