The cell cycle plays an important role in the development and adaptation of multicellular organisms; specifically, it
allows them to optimally adjust their architecture in response to environmental changes. Kip-related proteins
(KRPs) are important negative regulators of cyclin-dependent kinases (CDKs), which positively control the cell cycle
during plant development. The Arabidopsis genome possesses seven KRP genes with low sequence similarity
and distinct expression patterns; however, why Arabidopsis needs seven KRP genes and how these genes function
in cell cycle regulation are unknown. Here, we focused on the characterization of KRP3, which was found to have
unique functions in the shoot apical meristem (SAM) and leaves. KRP3 protein was localized to the SAM, including
the ground meristem and vascular tissues in the ground part of the SAM and cotyledons. In addition, KRP3 protein
was stabilized when treated with MG132, an inhibitor of the 26S proteasome, indicating that the protein may be regulated
by 26S proteasome-mediated protein degradation. KRP3-overexpressing (KRP3 OE) transgenic plants showed
reduced organ size, serrated leaves, and reduced fertility. Interestingly, the KRP3 OE transgenic plants showed a
significant reduction in the size of the SAM with alterations in cell arrangement. In addition, compared to the wild type,
the KRP3 OE transgenic plants had a higher DNA ploidy level in the SAM and leaves. Taken together, our data suggest
that KRP3 plays important regulatory roles in the cell cycle and endoreduplication in the SAM and leaves.

Keywords: Arabidopsis, endoreduplication, KRP3, leaf development, shoot apical meristem