Genome-Wide Characterization and Expression Profiling of the Chitinase Gene Family in Radish (Raphanus sativus L.) Under Clubroot Stress

Clubroot, caused by the obligate biotrophic protist Plasmodiophora brassicae（P. brassicae）, is a destructive soil-borne disease that severely threatens the production of radish (Raphanus sativus L.). Although chitinases are known to execute critical defense functions by degrading pathogen chitin, a comprehensive genome-wide characterization of the radish chitinase (RsChi) gene family and its specific role in clubroot resistance remains lacking. Here, we systematically identified 24 RsChi genes in the radish genome, characterizing their chromosomal distribution, structural organization, and promoter regulatory networks. These genes are unevenly distributed across seven chromosomes and cluster into four subfamilies, with tandem duplication driving family expansion, particularly on Chromosome 3. Promoter analysis revealed a significant enrichment of jasmonic acid- and abscisic acid-responsive cis-elements, implicating RsChi genes in hormone-mediated defense signaling. Using qRT-PCR to profile expression dynamics during P. brassicae infection across contrasting radish lines, we identified strong genotype- and stage-specific transcriptional responses. Notably, TRs0x1c000780 remained transcriptionally silent prior to infection but was specifically induced over 10-fold in the resistant line at 28 days post-inoculation. This infection-triggered induction positions TRs0x1c000780 as a promising candidate defense gene. Together, these findings provide structural and functional insights into the RsChi family and highlight candidate targets for breeding clubroot-resistant radish cultivars.