Abstract Resistance to an yellow strain of Cucumber mosaic virus [CMV(Y)] in Arabidopsis thaliana eco- type C24 is conferred by the CC-NBS-LRR type R gene, RCY1. RCY1-conferred resistance is accompa- nied by a hypersensitive response (HR), which is characterized by the development of necrotic local le- sion (NLL) at the site of infection that restricts viral spread. To further characterize the role of RCY1 in NLL formation we have identified six recessive CMV(Y)-susceptible rcy1 mutants, four of which con- tain single amino acid substitutions in RCY1: rcy1-2 contains a D to N substitution in the CC domain, rcy1- 3 and rcy1-4 contain R to K and E to K substitutions, respectively, in the LRR domain, and rcy1-6 contains a W to C substitution in the NBS domain. The rcy1-5 and rcy1-7 contain nonsense mutations in the LRR and NBS domains, respectively. Although the virus sys- temically spread in all six rcy1 mutants, HR-associated cell death was differentially induced in these mutants. In comparison to the wild type C24 plant, HR was not observed in the CMV(Y)-inoculated leaves of the rcy1- 3, rcy1-5, rcy1-6 and rcy1-7 mutants. In contrast, delayed NLL development was observed in the virus inoculated leaves of the rcy1-2 and rcy1-4 mutants. In addition, necrosis accompanied by elevated accumu- lation of PR gene transcript also appeared in the non- inoculated leaves of the rcy1-2 and rcy1-4 mutants. Trans-complementation was observed between the rcy1-2 and rcy1-4 alleles; in F1 plants derived from a cross between rcy1-2 and rcy1–4, HR associated cell death was accelerated and systemic spread of the virus was partially suppressed than in the homozygous rcy1-2 and rcy1-4 plants. Our results suggest that the CC, NBS and LRR domains of RCY1 are required for restriction of virus spread but differentially impact the induction of HR-like cell death. Furthermore, these results also predict inter-molecular interaction involving RCY1 in Arabidopsis resistance to CMV(Y). Keywords Resistance gene Æ Gene-for-gene interaction Æ Viral resistance Abbreviations CC Coiled-coil CMV(Y) An yellow strain of Cucumber mosaic virus EMS Ethyl methanesulfonate NBS Nucleotide-binding site NLL Necrotic local lesion LRR Leucine-rich repeats RCY1 RESISTANCE TO CMV(Y) 1 TIR Toll-interleukin-1 receptor Introduction In many cases of plant-pathogen interaction initial rec- ognition of the pathogen occurs via the products of a K.-T. Sekine Æ T. Ishihara Æ S. Hase Æ H. Takahashi (&) Department of Life Science, Graduate School of Agricultural Science, Tohoku University, 1-1 Tsutsumidori- Amamiyamachi, Aoba-ku, Miyagi, Sendai 981-8555, Japan e-mail: takahash@bios.tohoku.ac.jp T. Kusano Graduate School of Life Sciences, Tohoku University, 2-1-1 Katahira, Aoba-ku, Miyagi , Sendai 980-8577, Japan J. Shah Division of Biology and the Molecular, Cellular and Developmental Biology Program, , Kansas State University, 303 Ackert Hall, Manhattan, KS 66506-4901, U.S.A Plant Mol Biol (2006) 62:669–682 DOI 10.1007/s11103-006-9048-4 123 Single amino acid alterations in Arabidopsis thaliana RCY1 compromise resistance to Cucumber mosaic virus, but differentially suppress hypersensitive response-like cell death Ken-Taro Sekine Æ Takeaki Ishihara Æ Shu Hase Æ Tomonobu Kusano Æ Jyoti Shah Æ Hideki Takahashi Received: 28 April 2006 / Accepted: 6 July 2006 / Published online: 29 August 2006 Ó Springer Science+Business Media B.V. 2006