Tobacco mosaic virus (TMV) elicits a hypersensitive response (HR) in tobacco plants that carry the N gene. To identify the elicitor of this HR, Agrobacteriumn tumefaciens was used as a vector for the transient expression of TMV replicase proteins, movement protein, and coat protein in NN and nn tobacco. Transient expression of the 126K protein and fragments thereof containing the helicase motifs induced necrosis and systemic expression of the pathogenesis-related PR-1a gene in NN plants but not in nn plants. The results confirm previous evidence that the TMV helicase sequence is the elicitor of the HR (H. S. Padgett, Y. Watanabe, and R. N. Beachy, Mol. Plant-Microbe Interact. 10:709-715, 1997) and demonstrate that this helicase sequence acts as an elicitor of HR in the absence of other viral proteins or RNA replication.