Arabidopsis thaliana CDKG1 belongs to the family of cyclin-dependent protein kinases (CDKs) which were originally characterized as cell cycle regulators in eukaryotes.Here we report that CDKG 1 regulates the pre-mRNA splicing for the callose synthase 5 (CalS5), and therefore pollen wall formation.The knockout mutant cdkg1 exhibits dramatically reduced male fertility with impaired callose synthesis and abnormal pollen wall formation.The sixth intron in CalS5 pre-mRNA, a rare type intron with the GC 5 splice site, is abnormally spliced in cdkg1.