Stomata are key cellular structures for gas and water exchange in plants and their development is influenced by 
several genes. We found the A. thaliana bagel23-D(bgl23-D) mutant showing abnormal bagel-shaped single guard cells. The bgl23-D was a novel dominant mutation in the A. thaliana cellulose synthase-like D5(ATCSLD5) gene that was reported to function in the division of guard mother cells. The dominant character of bgl23-D was used to inhibit ATCSLD5 function in specific cells and tissues. Transgenic A. thaliana expressing bgl23-DcDNA with the promoter of stomata lineage genes, SDD1, MUTE, and FAMA, showed bagel-shaped stomata as observed in the bgl23-Dmutant. Especially, the FAMA promoter exhibited a higher frequency of bagel-shaped stomata with severe cytokinesis defects. Expression of bgl23-DcDNA in the tapetum with SP11promoter or in the anther with ATSP146promoter induced defects in exine pattern and pollen shape, novel phenotypes that were not shown in the bgl23-Dmutant. These results indicated that bgl23-D inhibited unknown ATCSLD(s) that exert the function of exine formation in the tapetum. Furthermore, transgenic A. thaliana expressing bgl23-DcDNA  with SDD1, MUTE, and FAMApromoters showed enhanced rosette diameter and increased leaf growth. Taken together, these findings suggest that the bgl23-Dmutation could be a helpful genetic tool for functional analysis of ATCSLDs and manipulating plant growth.