Expression of MbR4, a TIR-NBS type of apple R gene, confers resistance to bacterial spot disease in Arabidopsis

Abstract

A single disease resistance gene candidate, MbR4, was isolated from the wild-type apple species Malus baccate. This gene was predicted to encode motifs characteristic of the Toll Interleukin 1 Receptor (TIR) - Nucleotide Binding Site (NBS) of the R gene. Starting with an isolated cDNA clone, genomic clones were obtained via inverse polymerase chain reaction (IPCR). The MbR4 gene has a single open reading frame (ORF) of 2178 nucleotides, a 41-b untranslated 5′ region, a 21-b untranslated 3′ region, and a predicted protein of 726 amino acids (82 kDa). Its deduced amino acid sequence resembles the N protein of tobacco and the NL25 protein of potato. Ectopic expression of MbR4 induced enhanced resistance in transgenic Arabidopsis plants against the virulent pathogen, Pseudomonas syringae pv. tomato DC3000. Microarray analysis confirmed the induction of defense-related gene expression in pathogen-free 35S::MbR4 heterologous Arabidopsis plants, thereby indicating that the MbR4 gene likely activates a pathogen-independent resistance pathway, rather than a gene-for-gene pathway. Our results suggest that MbR4 plays a role in the R gene, and may be a source of resistance for cultivated apple species.