Abstract
The tonsoku (tsk) was originally isolated as an Arabidopsis mutant showing short roots, disorganized phyllotaxy and fasciated stems. Further analysis found hypersensitivity to DNA-damaging agents and expression of mRNA from heterochromatic region in tsk. The tsk also showed increased ploidy and expression of CYCB1;1-GUS, suggesting that tsk has defects in cell cycle progression, especially in G2/M, which caused the disorganized cell division pattern and morphological phenotypes. It is known that cell cycle progression from G2 to M is regulated by checkpoint kinases, mainly by ATR. We hypothesized that the cell cycle in tsk mutant was temporally stopped by ATR, resulting in the disorganized cell division and structures of meristems. To probe this hypothesis, we generated the tsk/atr double mutant and found that developmental defects associated with tsk were almost fully recovered by atr. The increased expression of CYCB1;1-GUS in tsk was also repressed by atr. However, some phenotypes including hypersensitivity to DNA-damaging agents were not restored by atr. These results suggest that the developmental phenotypes observed in tsk is dependent on the activity of ATR.
