Os11g0249000

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Annotated Information

Introduction

One of the main defense mechanisms is resistance (R) gene-mediated disease resistance. The majority of R genes characterized encode proteins that have a central nucleotide-binding region (NB) plus a C- terminal leucine-rich repeat (LRR).The NB-LRR proteins recognize the corresponding avirulence (Avr) effectors produced by the pathogen and activate defense responses. In most cases, these defense responses are accompanied by a hypersensitivity response (HR) in the infected cells, triggering the rapid production of reactive oxygen species (ROS).These defense responses are also associated with accumulation of salicylic acid (SA), which functions in the induction of numerous pathogenesis-related (PR) genes, and the establishment of systemic acquired resistance (SAR).SA is required in R gene-mediated defense signaling pathways. It is assumed that R proteins are present in an auto-inhibited state and are unable to activate defense responses in the absence of pathogens. Here, we describe the semi-dominant rice mutant nls1-1D ( necrotic leaf sheath 1 ), which displayed tissue-specific and age-dependent spontaneous necrotic lesions and defense-related phenotypes.

Keywords: NLS1, CC-NB-LRR, disease resistance protein, lesion, salicylic acid, rice.

Function

NLS1 displays spontaneous lesions, specifically on leaf sheaths, with 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 pathogens.NLS1 encodes a typical CC-NB-LRR-type protein in rice and causes a S367N substitution in the non-conserved region close to the GLPL motif of the NB domain. An adjacent S366T substitution was found in another semi-dominant mutant, nls1-2D, which exhibited the same phenotypes as nls1-1D. Combined analyses of wild-type plants transformed with the mutant NLS1 gene (nls1-1D), NLS1 RNAi and over-expression transgenic lines showed that nls1-2D is allelic to nls1-1D, and both mutations may cause constitutive auto-activation of the NLS1 R protein. Further real-time PCR analysis revealed that NLS1 is 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 mediates both SA and NPR1-independent defense signaling pathways in rice.

Expression

The nls1-1D mutant was identified initially in the hetero-zygous state by screening a T-DNA insertion population of rice cultivar Nipponbare ( Oryza sativa ssp. japonica) (Ma et al., 2009). Among offspring of the original mutant plants, approximately 25.9% of the progeny exhibited a wild-type phenotype, and 47.9% of the progeny exhibited a mild mutant phenotype: despite their morphological appearance of very aggressive lesions, the vast majority of these mild mutants developed reproductive tissues and set seeds, and their offspring also segregated in a 3:1 mutant:wild-type Mendelian ratio.

Evolution

In recent years, some reports have shown that R protein mutations are sufficient to cause pathogen-independent defense responses; for example, single amino acid substitutions in the TIR-NB-LRR-type R proteins SSI4 and SNC1 and the CC-NB-LRR-type R protein UNI all cause constitutive expression of defense responses in Arabidopsis (Shirano et al., 2002; Zhang et al., 2003; Noutoshi et al., 2005; Igari et al., 2008). Furthermore, a single amino acid insertion in the WRKY domain of Arabidopsis SLH1, a TIR-NB-LRR-WRKY-type R protein, also activates pathogen-independent defense responses (Noutoshi et al., 2005). In addition, modifications to the coding sequences of some CC-NB-LRR-type R genes, such as the tomato (Lycopersicon esculentum) Mi gene,the potato (Solanum tuberosum) Rx gene and the maize(Zea mays) Rp1 gene, often result in mutants with constitutive defense activity (Hulbert, 1997; Hwang et al., 2000;Bendahmane et al., 2002). On the other hand, several other studies have demonstrated that over-expression of R genes,for example Arabidopsis At4g16890 (SNC1) or tomato Pto or Prf, results in pathogen-independent resistance responses.Recently, it has been shown that constitutive expression of the Arabidopsis CC-NB-LRR-type R gene ADR1, caused by the insertion of CaMV 35S enhancers 4 kb upstream of its promoter, also leads to a constitutive defense phenotype .

Summary

In this study, we characterized the semi-dominant mutant nls1-1D (necrotic leaf sheath 1) of rice, which displays spontaneous lesions, specifically on leaf sheaths, with 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 pathogens. Map-based cloning revealed that NLS1 encodes a typical CC-NB-LRR-type protein in rice. The nls1-1D mutation causes a S367N substitution in the non-conserved region close to the GLPL motif of the NB domain. An adjacent S366T substitution was found in another semi-dominant mutant, nls1-2D, which exhibited the same phenotypes as nls1-1D. Combined analyses of wild-type plants transformed with the mutant NLS1 gene (nls1-1D), NLS1 RNAi and over-expression transgenic lines showed that nls1-2D is allelic to nls1-1D, and both mutations may cause constitutive auto-activation of the NLS1 R protein. Further real-time PCR analysis revealed that NLS1 is 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 mediates both SA and NPR1-independent defense signaling pathways in rice.

Labs working on this gene

State Key Laboratory of Plant Genomics and National Center for Plant Gene Research (Beijing), Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Beijing 100101, China,

Graduate School of the Chinese Academy of Sciences, Beijing 100101, China, and

China National Rice Research Institute, Chinese Academy of Agricultural Sciences, Hangzhou 310006, China

References

Jiuyou Tang, Xudong Zhu, Yiqin Wang, Linchuan Liu,BoXu, Feng Li, Jun Fang and Chengcai Chu.(2011)Semi-dominant mutations in the CC-NB-LRR-type R gene,NLS1, lead to constitutive activation of defense responses in rice.the Plant Journal,66,996-1007.

Tang, X., Xie, M., Kim, Y.J., Zhou, J., Klessig, D.F. and Martin, G.B. (1999) Overexpression of Pto activates defense responses and confers broad resistance. Plant Cell, 11, 15–29.

Takken, F.L., Albrecht, M. and Tameling, W.I. (2006) Resistance proteins: molecular switches of plant defence. Curr. Opin. Plant Biol. 9, 383–390.