In this study we characterized the semidominant mutant nls1-1D (necrotic leaf sheath 1) of rice, which displayed spontaneous lesions specifically on leaf sheaths following a developmental pattern. nls1-1D plants also exhibited constitutively activated defense responses, including extensive cell death, excess hydrogen peroxide and salicylic acid (SA) accumulation, up-regulated expressions of pathogenesis-related genes and enhanced resistance to bacterial pathogen. Map-based cloning revealed that NLS1 encodes a typical CC-NB-LRR-type protein in rice. The nls1-1D mutation causes a S367N substitution located in the nonconserved region closed to GLPL motif of the NB domain. Furthermore, an adjacent S366T substitution was found in another semidominant mutant, nls1-2D, which exhibited the same phenotypes as nls1-1D. Combined analyses of wild-type plants transformed with mutant NLS1 gene (nls1-1D), NLS1 RNAi or overexpression transgenic lines showed that nls1-2D is allelic to nls1-1D and both mutations may cause the constitutive autoactivation of the NLS1 R protein, respectively. Further real-time PCR analysis revealed that NLS1 expressed constitutively in an age-dependent manner. In addition, because the morphology and constitutive defense responses of nls1-1D were not suppressed by blocking SA or NPR1 transcript accumulation, we suggest that NLS1 mediated both SA and NPR1-independent defense signaling pathways in rice.

Keywords:NLS1;CC-NB-LRR;disease resistance protein;lesion;rice