Isolation, characterization and expression analysis of resistance gene candidates in pear (Pyrus spp.)

According to the conservative regions of the nucleotide-binding site and leucine-rich repeat domain (NBS-LRR) in resistance genes (R-gene), a homology-based cloning method was used to isolate disease resistance gene candidates (RGCs) from six species of pear, including Pyrus betulaefolia, P. bretschneideri, P. pyrifolia, P. ussriensis, P. sinkiangensis, P. communis and their interspecific hybrids. Approximately 100 disease resistance gene candidates from 39 cultivars in these pear species were identified. Among these RGC, 98 of genomic sequences which could be translated into polypeptides without stop codons, while the other two sequences presented multiple stop codons. The deduced amino acid sequences of the 98 RGC displayed high diversity, ranging from 20 to 100%, and could be divided into 17 distinct RGC families (RGC 1-RGC 17) based on the phylogenetic analysis with the threshold of 68% identity. The 98 RGC contained both the toll interleukin receptor (TIR) group and non-toll interleukin receptor (non-TIR) group. RGC families RGC 16 and RGC 17 belonged to non-TIR group, the other 15 RGC families were classified to TIR group. Sequences analysis indicated that there was a strong identity of these RGCs to the known R-gene and RGC from other plants. In each RGC family, one representative RGC was Selected to test the expression profile at transcriptional level, and all the 17 selected RGCs could be detected at mRNA level. In response to Venturia nashi cola inoculation or salicylic acid (SA) treatment, the expression level of RGC Pb-Zs1 (RGC 1) from pear scab-resistant cultivar 'Zaosu' pear changed while the expression level of RGC Pb-Dshs2 (RGC 8) from pear scab-susceptible pear cultivar 'Dangshansu' seemed less affected. These RGCs isolated in this study could help understand the R-gene in pear species and will serve as a potential resource for future improvement of disease resistance in pear. (C) 2010 Elsevier B.V. All rights reserved.