Abstract
Expression levels of mitotic cyclins are important for controlling entry into and progression through M phase during the cell cycle. Levels of cyclins are, in turn, regulated in two major steps, transcription and proteolysis. We have previously shown that cell cycle-regulated transcription of mitotic cyclin genes is activated by MYB3R1 and MYB3R4. Here, we report identification of a gene that may regulate proteolysis of mitotic cyclins in Arabidopsis thaliana. In a screen for enhancer of myb3r4, we isolated a mutant, designated as gigas cell1 (gig1), which has giant guard cell-like cells in leaf epidermis. The abnormal giant cells showed expression of multiple guard cell markers, and had double the number of chromosomes in nuclei, suggesting that they might be generated by aberrant mitosis in a course of cell division during stomatal development. Overexpression of GIG1 caused macroscopic and cellular abnormalities that were very similar to those in knockdown mutant of APC (anaphase-promoting complex), which acts as E3 ubiquitin ligase that targets mitotic cyclins. Our observations suggested that GIG1 might be an endogenous inhibitor of APC in Arabidopsis thaliana.
