Plasmodiophora brassicae is a biotrophic pathogen causing clubroots of cruciferous crops. The Brassica rapa accession T1-145 has an ability to produce small spheroid galls (SSGs), which represent neither a fully compatible interaction nor a complete resistance. To explore the defense response in SSGs induced by P. brassicae infection, global transcriptome profiling SSGs was performed at different time points. By comparing gene expression patterns, we identified many defense related genes. The first group included genes encoding receptor-like protein/kinases, such as cysteine-rich receptor-like protein kinases, receptor-like proteins, phloem intercalated with xylem/tracheary element differentiation inhibitory factor receptor (PXY/TDR), PXY-correlated 1, wall-associated kinases, nuclear shuttle protein-interacting kinase, lectin receptorlike kinases, and flagelin-sensitive 2, which might activate a basal defense. The second group involved robust effectortriggered immunity response genes such as resistance to leptosphaeria maculans 1B, constitutive shade-avoidance 1, target of avirulence B operation 1, ribosomal protein of the small subunit 6, resistance to Pseudomonas maculicola 1-interacting protein 4, enhanced disease resistance 2L, and recognition of Peronospora parasitica 13-like protein 4. The third group included genes encoding secondary cell wall formation related protein/s, a nodulin-like protein, a germin-like protein, a jacalin-related lectin, a defensin-like protein, tumor inhibitors, and sugars will eventually be exported transporter, which might contribute to quantitative resistance against P. brassicae. The gene expressions were the highest at the late stage of infection. To our knowledge, it is the first report on exploring defense response genes during SSG occurrence by a transcriptome analysis. Our data would provide useful information to further explore molecular mechanisms of the incomplete resistance.
