A hierarchical transcriptional network activates specific CDK inhibitors that regulate G2 to control cell size and number in Arabidopsis.

Yuji Nomoto, Hirotomo Takatsuka, Kesuke Yamada, Toshiya Suzuki, Takamasa Suzuki, Ying Huang, David Latrasse, Jing An, Magdolna Gombos, Christian Breuer, Takashi Ishida, Kenichiro Maeo, Miyu Imamura, Takafumi Yamashino, Keiko Sugimoto, Zoltán Magyar, László Bögre, Cécile Raynaud, Moussa Benhamed, Masaki Ito

How cell size and number are determined during organ development remains a fundamental question in cell biology. Here, we identified a GRAS family transcription factor, called SCARECROW-LIKE28 (SCL28), with a critical role in determining cell size in Arabidopsis. SCL28 is part of a transcriptional regulatory network downstream of the central MYB3Rs that regulate G2 to M phase cell cycle transition. We show that SCL28 forms a dimer with the AP2-type transcription factor, AtSMOS1, which defines the specificity for promoter binding and directly activates transcription of a specific set of SIAMESE-RELATED (SMR) family genes, encoding plant-specific inhibitors of cyclin-dependent kinases and thus inhibiting cell cycle progression at G2 and promoting the onset of endoreplication. Through this dose-dependent regulation of SMR transcription, SCL28 quantitatively sets the balance between cell size and number without dramatically changing final organ size. We propose that this hierarchical transcriptional network constitutes a cell cycle regulatory mechanism that allows to adjust cell size and number to attain robust organ growth.