Functional Differentiation in the Leucine-Rich Repeat Domains of Closely Related Plant Virus-Resistance Proteins That Recognize Common Avr Proteins

The N′ gene of Nicotiana sylvestris and L genes of Capsicum plants confer the resistance response accompanying the hypersensitive response (HR) elicited by tobamovirus coat proteins (CP) but with different viral specificities. Here, we report the identification of the N′ gene. We amplified and cloned an N′ candidate using polymerase chain reaction primers designed from L gene sequences. The N′ candidate gene was a single 4143 base pairs fragment encoding a coiled-coil nucleotide-binding leucine-rich repeat (LRR)-type resistance protein of 1,380 amino acids. The candidate gene induced the HR in response to the coexpression of tobamovirus CP with the identical specificity as reported for N′. Analysis of N′-containing and tobamovirus-susceptible N. tabacum accessions supported the hypothesis that the candidate is the N′ gene itself. Chimera analysis between N′ and L3 revealed that their LRR domains determine the spectrum of their tobamovirus CP recognition. Deletion and mutation analyses of N′ and L3 revealed that the conserved sequences in their C-terminal regions have important roles but contribute differentially to the recognition of common avirulence proteins. The results collectively suggest that Nicotiana N′ and Capsicum L genes, which most likely evolved from a common ancestor, differentiated in their recognition specificity through changes in the structural requirements for LRR function.