Characterization of BSK Homologs in Brassica rapa subsp. chinensis and Their Transcriptional and Physiological Alterations Under Thermal Stress

Plant steroid hormones, namely brassinosteroids (BRs), govern growth and resilience to environmental stress, yet little is known about how BR-signaling kinases (BSKs) operate in non-model horticultural species. Here, we carried out a whole-genome interrogation of the BSK family in Brassica rapa subsp. chinensis and examined its potential involvement in high-temperature stress responses. The search yielded 20 BcBSK members, each featuring a conserved kinase domain at the N-terminus and TPR repeats at the C-terminus. Phylogenetic reconstruction assigned them to separate subgroups, while collinearity assessment detected 16 duplicated gene pairs evolving under strong selection constraints. Upstream regulatory sequences harbored numerous cis-motifs linked to hormonal signals and stress perception. Interactome modeling pinpointed BcBSK2, BcBSK5, BcBSK14, and BcBSK18 as hub components. RNA-seq analysis under elevated temperature (38℃) uncovered distinct expression behaviors between cultivars: in the susceptible line “Aijiaohuang”, BcBSK1 and BcBSK2 transcripts increased sharply, whereas the resistant line “SHI” exhibited little fluctuation. Quantitative PCR results aligned with the RNA-seq findings. Exogenous application of 0.5 mg·L⁻¹ BR improved the activities of catalase, peroxidase, and superoxide dismutase, boosted proline levels, lowered malondialdehyde content, and preserved chlorophyll and carotenoid concentrations under heat exposure. Taken together, these data imply that BcBSK family members contribute to BR-facilitated heat adaptation by orchestrating changes at both transcript and metabolite levels, thus laying a groundwork for genetic enhancement of thermotolerance in this vegetable species.