Introns of the tobacco resistance gene N play important roles in elicitor-responsive upregulation and efficient induction of defense responses

The tobacco resistance gene N is known to be upregulated transcriptionally upon infection with Tobacco mosaic virus or the transient expression of the virus elicitor (termed p50), which is the helicase domain of the virus replicase, leading to cell death-associated defense responses. To investigate the roles of the encoded protein and introns of the N gene in its elicitor-triggered upregulation, we developed an Agrobacterium-mediated transient gene transfer system to express the N transgene controlled by the upstream region of the N gene itself in N-lacking tobacco. The transcript level of the N transgene with the introns was enhanced sharply when the p50 transgene was coexpressed. Introduction of a frameshift mutation in the intron-containing N transgene abolished its response to the elicitor, but an additional expression of the functional N protein supplied in trans complemented the upregulation of the mutant transgene. In contrast, an intron-less N transgene failed to be upregulated by the p50 coexpression, resulting in delayed and attenuated cell death and slow induction of defense-related gene expression. In addition, introduction of a frameshift mutation in the intron-less N transgene resulted in no response to the elicitor even with coexpression of the functional N protein. Our data provide the first evidence for important roles of introns of a resistance gene in elicitor-responsive gene regulation and efficient defense induction.