Amplification, Cloning and Characterization of Resistance Gene Candidate Sequences In Gerbera and Caladium

Improving disease resistance has become an important objective in gerbera and caladium breeding in recent years.  In many cases, plant resistance traits are controlled by multiple genes and their expressions are affected by changes in the environment.  These factors make it difficult to select breeding lines with high levels of true resistance.  The use of molecular markers has been proposed to help overcome these difficulties.  One type of the molecular markers often used to tag disease resistance (R) genes is developed from resistance gene candidate (RGC) sequences, DNA sequences amplified with degenerate primers designed from cloned R genes.  The objectives of this study were to amplify, clone and characterize RGC sequences from powdery mildew-resistant gerbera and fusarium tuber rot- and pythium root rot-resistant caladium, with the goal to develop molecular markers linked to powdery mildew resistance in gerbera and fusarium tuber rot and pythium root rot resistance in caladium. Twenty pairs of degenerate PCR primers corresponding to the P-loop and GLPL motifs within the NBS domain of plant R genes were screened, and eight pairs produced PCR products in gerbera and caladium. The PCR products amplified by two pairs of primers from gerbera and by one pair of primer from caladium were cloned and sequenced.  Sixty-five gerbera sequences and 18 caladium sequences were homologous to R genes. In gerbera, 58 RGC sequences contained continuous open reading frames (ORF); they showed 57%-99% of protein sequence identity and were classified into three groups.  In caladium, the translated protein sequences of 15 ORF-containing RGC sequences showed 37%-98% sequence identity and were classified into three groups as well.  Phylogenetic analysis indicates gerbera and caladium RGCs belong to the TIR NBS-LRR and the Non-TIR NBS-LRR R gene subfamily, respectively.