Over-expression of the triploid white poplar PtDrl01 gene in tobacco enhances resistance to tobacco mosaic virus.

A full-length cDNA, designated as the Populus tomentosa disease resistance-like 01 (PtDrl01) gene, was isolated from triploid white poplar [(Populus tomentosa × P. bolleana) × P. tomentosa]. The protein thought to be produced by the PtDrl01 gene contains a nuclear localisation sequence (NLS), a toll/interleukin-1 receptor (TIR) homologue region, a nucleotide binding site (NBS) and a leucine-rich repeat (LRR) domain. The protein also exhibits a considerable degree of homology to N-like resistance proteins. Real-time quantitative RT-PCR analysis revealed that expression of the PtDrl01 gene in triploid white poplar leaves could be induced by two defence signalling molecules: methyl jasmonate (MeJA) and salicylic acid (SA). Over-expression of the PtDrl01 gene in transgenic tobacco induced enhanced resistance to tobacco mosaic virus (TMV). Long-term resistance from the PtDrl01 gene to TMV infection was also observed in transgenic tobacco plants. Additionally, over-expression of the PtDrl01 gene resulted in transcriptional changes in genes expressing pathogenesis-related proteins in transgenic tobacco under non-stress conditions. These data strongly suggest that the PtDrl01 gene is involved in plant defence responses to pathogen infection.

Characterization of resistance gene analogs with a nucleotide binding site isolated from a triploid white poplar.

The majority of cloned plant disease resistance genes (R genes) encode a nucleotide binding site (NBS) and a leucine-rich repeat (LRR) domain. In this study, to better understand the R genes in white poplar, 59 resistance gene analogues (RGAs) were identified from a triploid white poplar [(Populus tomentosa x Populus bolleana) x P. tomentosa], based on conserved NBS regions. The 59 RGAs were phylogenetically classified into 10 subfamilies, and 54 RGAs with open-reading frames (ORFs) were further grouped into two classes, toll and interleukin-1 receptor (TIR) and non-TIR. BLAST searches with reference to the genomic sequence of Populus trichocarpa found 96 highly homologous regions distributed in 37 loci, suggesting the abundance and divergence of NBS-encoding genes in the triploid poplar genome. Within subfamilies 1-3, the average non-synonymous/synonymous substitution (omega) rates were < 1, indicating purifying selection on these RGAs, but some sites were clearly under diversifying selection with omega > 1. Many intergenic exchanges were also detected among these RGAs, indicating a probable role in homogenising NBS domains. Quantitative real-time PCR analysis revealed dramatic variations in the transcript level of 18 RGAs in the mature leaves, bark and roots of the triploid poplar, and identified two RGAs that had significantly higher level of transcripts in bark, four RGAs in mature leaves, and 14 in the above-ground portion of poplars, suggesting their probable roles in resistance against diseases attacking the organs. Our results shed light on genetic resources of poplar resistance and will be useful for further resistance gene isolation and exploitation.