How the size and number of cells are determined during organ development remains a fundamental question in cell biology. Recently, Kanazawa University in Japan published online in the internationally renowned journal Nature Communications entitled "A hierarchical transcriptional network activates specific CDK inhibitors that regulate G2 to control cell size and number in Arabidopsis" Research papers. This paper identifies a GRAS family transcription factor called CARECROW-LIKE28 (SCL28) that plays a key role in determining Arabidopsis cell size.
During organ growth and development, cell proliferation is controlled in a complex way in space and time. Controlled by the developmental plan and influenced by the response of the active plant to environmental conditions, the number of cells produced during organ growth is determined by the rate of cell cycle during proliferation and the time point at which cells exit cell differentiation. Controlled cell proliferation requires coordinated regulation of gene expression during the cell cycle and after exiting the cell cycle. Typically, there are two broad classes of genes that show transcriptional fluctuations during the cell cycle: G1/S and G2/M-specific genes. G1/S-specific genes promote the initiation and progression of DNA replication and are often modulated by the activity of E2F transcription factors. Typically, E2F is dimerized with a dimerized chaperone (DP) protein to activate or inhibit its target gene, depending on their association with Theretinoblastoma-Related (RBR) repressor protein. On the other hand, most G2/M-specific genes are positively or negatively regulated by transcription factors of the MYB3R family in plants. Some members of MYB3R express specifically during G2/M and act as transcriptional activators, while others act as transcriptional inhibitors of G2/M-specific genes. These two sets of major transcription factors, E2Fs and MYB3Rs, were studied as part of the same polyprotein complex in Arabidopsis thaliana, respectively. The E2F-MYB3R complex is evolutionarily associated with the DREOM (DP, Retinoblastoma-like, E2F, and MuvB) complexes reported in Drosophila and human cells. Multicellular animal DREAM complexes play a major role in inhibiting G1/S and G2/M-specific genes, thereby promoting cell cycle exit and maintaining cell rest. THE DREAM COMPLEX IN ARABIDOPSIS SHOWS SIGNIFICANT DIFFERENCES FROM MULTICELLULAR ANIMAL COMPLEXES, WHICH INCLUDE THE PARTICIPATION OF PLANT-SPECIFIC SUBUNITS AND THE PRESENCE OF DIVERSE COMPLEXES WITH DIFFERENT SUBUNITARECTIONS COMPOSITION.
Transcriptional regulation in the cell cycle typically constitutes a multi-level hierarchical network in which the master regulator regulates other transcription factors that further regulate each other or downstream genes. Notably, studies of yeast have shown that cell cycle transcriptional activators that function at one stage of the cell cycle regulate transcriptional activators that play a role in the next phase, forming an interconnected regulatory network that is itself a cycle. In plants, however, such a hierarchical network of cell cycle transcription factors E2F and MYB3R has yet to be discovered.
In this study, the authors identified a mitosis-specific GRAS family transcription factor called CARECROW-LIKE28 (SCL28). A recent study found that the gene is directly regulated by MYB3Rs. The study demonstrated that SCL28 works with the AP2 transcription factor AtsmoS1 to directly activate transcription of a specific set of SMR family genes, encoding plant-specific cyclin-dependent kinase (CDK) inhibitors. This regulatory network inhibits the transition from the G2 to M phase in the cell cycle and facilitates the onset of internal replication, an atypical cell cycle consisting of repeated DNA replications without mitosis. The study identified a G2/M regulatory pathway that controls cell cycle length and may optimize cell function by setting cell size and number during organ growth.
Figure 1: Expression of SCL28 is regulated by the cell cycle regulated by the MYB3R transcription factor
Figure 2: SCL28 significantly affects cell size
Figure 3: SCL28 inhibits the G2 process and induces internal replication
Original link:
https://www.nature.com/articles/s41467-022-29316-2
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