Arabidopsis F-BOX STRESS INDUCED 4 is required to repress excessive divisions in stomatal development

During the terminal stage of stomatal development,the R2R3-MYB transcription factors FOUR LIPS(FLP/MYB124) and MYB88 limit guard mother cell division by repressing the transcript levels of multiple cell-cycle genes.In Arabidopsis thaliana possessing the weak allele flp-1,an extra guard mother cell di-vision results in two stomata having direct contact.Here,we identified an ethylmethane sulfonate-mutagenized mutant,flp-1 xs01c,which exhibited more severe defects than fip-1 alone,producing giant tumor-like cell clusters.XS01C,encoding F-BOX STRESS-INDUCED 4 (FBS4),is preferentially ex-pressed in epidermal stomatal precursor cells.Overexpressing FBS4 rescued the defective stomatal phenotypes of flp-1 xs01c and flp-1 mutants.The deletion or substitution of a conserved residue (Pro-line166) within the F-box domain of FBS4 abolished or reduced,respectively,its interaction with Arabi-dopsis Skp1-Like1 (ASK1),the core subunit of the Skp1/Cullin/F-box E3 ubiquitin ligase complex.Fur-thermore,the FBS4 protein physically interacted with CYCA2;3 and induced its degradation through the ubiquitin-26S proteasome pathway.Thus,in addition to the known transcriptional pathway,the terminal symmetric division in stomatal development is en sured at the post-translational level,such as through the ubiquitination of target proteins recognized by the stomatal lineage F-box protein FBS4.