Alternatively spliced N resistance gene transcripts:: Their possible role in tobacco mosaic virus resistance

The N gene, a member of the Toll-IL-1 homology region-nucleotide binding site-leucine-rich repeat region (LRR) class of plant resistance genes, encodes two transcripts, N-S and N-L, via alternative splicing of the alternative exon present in the intron III. The N-S transcript, predicted to encode the full-length N protein containing the Toll-IL-1 homology region, nucleotide binding site, and LRR, is more prevalent before and for 3 hr after tobacco mosaic virus (TMV) infection. The N-L transcript, predicted to encode a truncated N protein (N-tr) lacking 13 of the 14 repeats of the LRR, is more prevalent 4-8 hr after TMV infection. Plants harboring a cDNA-N-S transgene, capable of encoding an N protein but not an N-tr protein, fail to exhibit complete resistance to TMV. Transgenic plants containing a cDNA-N-S-bearing intron III and containing 3' N-genomic sequences, encoding both N-S and N-L transcripts, exhibit complete resistance to TMV, These results suggest that both N transcripts and presumably their encoded protein products are necessary to confer complete resistance to TMV.