The Arabidopsis checkpoint protein Bub3.1 is essential for gametophyte development

Except for Mad2 homologues of maize and wheat, no spindle checkpoint proteins which regulate the onset of anaphase during nuclear divisions have been reported for plants so far. We found three homologues of the spindle checkpoint protein Bub3 in Arabidopsis thaliana. Bub3.1 and Bub3.2 are 88% identical at the amino acid level, while Bub3.3 is more distantly related. Bub3.1 and Bub3.2 mRNA appeared preferentially in mitotically active tissues. Bub3.1 but not Bub3.2 showed increased expression during mitosis. Nuclear but no centromeric localization was shown for EYFP-Bub3.1 and Bub3.1-ECFP fusion constructs. Three T-DNA insertion mutants each were identified for Bub3.1 and Bub3.2 encoding genes. One bub3.1 mutant survived only in heterozygous state and displayed defects in development of male and female gametophytes, while two other homozygous bub3.1 mutants revealed transcripts and are therefore no null mutants. In contrast, all three homozygous bub3.2 mutants appeared to be viable and fertile without generating full-length transcripts.