Cloning and Analysis of NBS Class Resistance Gene Analogues in Ipomoea trifida

Most plant disease-resistance genes(R-genes)isolated so far encode proteins with nucleotide binding site(NBS)domain which belong to a superfamily.NBS domains related to R-genes show a highly conserved backbone of amino acid motif,which makes it possible to isolate resistance gene analogues(RGAs)by degenerate primers.Degenerate primers based on conserved motif(P-loop and GLPL)of NBS domain from R-genes were used to isolate RGAs from genomic DNA of Ipomoea trifida,a close wild relative of sweet potato,six I.trifida RGA sequences were obtained.Sequence similarity among the 6 RGA nucleotide sequences ranged from 48% to 97%,while the deduced amino acid sequences similarity from the 6 RGAs ranged from 25.2% to 95.1%.The phylogenetic analyses for RGA nucleotide sequences and the deduced amino acids showed that RGAs from I.trifida could be classified into two distinct groups,TIR(toll and interleukin receptor-1)-NBS and nonTIR-NBS.The high similarity between I.trifida RGAs and sweet potato RGAs in GenBank database indicates an ancestral relationship.The I.trifida RGAs have been designated ItRGA1~ItRGA6 and deposited in the GenBank database under Accession Nos.DQ849027~DQ849032,respectively.