Genome-Wide Analysis of Shaggy-like Kinase (SK) Family Genes in Brassica rapa and Functional Characterization of BrSKβ-2 in Pollen Development

The SHAGGY-like kinase (SK) gene family regulates diverse developmental and abiotic stress response processes in plants. Although genome-wide analyses of SKs have been conducted in model plants such as Arabidopsis thaliana and rice, their characterization in the economically important crop Brassica rapa remains limited. In this study, we conducted a systematic genome-wide analysis of SK genes in three Brassica species. A total of 18, 16, and 18 SK members were identified in B. rapa, B. nigra, and B. oleracea, respectively, and phylogenetic analysis classified them into four distinct clades. Expression profiling revealed that BrSKβ-1 and BrSKβ-2 were specifically expressed in fertile floral buds, suggesting their critical roles in pollen development. Furthermore, co-expression analysis indicated that both genes were co-expressed with key regulators involved in pollen development, pollen sperm cell differentiation and pollen tube growth. Loss of BrSKβ-2 via CRISPR/Cas9 resulted in 25–65% pollen abnormality and reduced the germination rate of normal-appearing pollen to only 10%, confirming its essential role in male fertility. Together, these findings provide a comprehensive characterization of the SK gene family in Brassica and position BrSKβ-2 as a promising candidate for gene editing-based male sterility systems in B. rapa and related crops.