Identification and Analysis of a Candidate WRKY Transcription Factor Gene Affecting Adventitious Root Formation Using Association Mapping in Catalpa Scop.

The capacity to develop adventitious roots (AR) from cuttings is a key factor for the mass deployment of superior genotypes in the woody plant, including Catalpa Scop. Catalpa Scop. was characterized as having excellent woody qualities in China. However, the knowledge of the molecular mechanisms of AR formation is limited in Catalpa Scop. In this study, for the first time, association mapping for AR formation was performed on a selected sample of 108 Catalpa accessions. Genetic diversity and population structure was estimated on the basis of 54 gene-derived simple sequence repeat markers. Genetic diversity analysis revealed that four accessions belonging to Catalpa duclouxii and eight belonging to Catalpa fargesii formed one clade, providing molecular evidence for C. duclouxii belonging to C. fargesii. Marker-trait association analysis revealed four genes associated with three rooting traits, namely AR rating, adventitious root numbers (ARN), and maximal AR length, with phenotypic variation explained for these traits of 10.77-18.49% in experiments in 2 years. Among the four genes, a WRKY transcription factor gene CbNN1 was the only gene that showed association with the ARN in both years, and expression of this gene (determined by analysis by real-time quantitative polymerase chain reaction) increased with increasing rooting ability. These results indicated that the gene CbNN1 might play a positive role in AR formation. The findings from this study will not only be beneficial to the research of AR formation, but also contribute to the phylogeny of interspecies in Catalpa Scop.