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The stoichiometric state of the SHR-SCR complex as a modulator of divisions of the root stem cell niche

Continuous growth and development of plant roots is maintained by a group of cells located at the apical area of the root called the stem cell niche. This niche contains an organizing center which rarely divides known as a quiescent center (QC). The QC maintains the initial cells in an undifferentiated state, while the initial cells give rise to the different tissues of the root by asymmetric divisions.
The mobile TF SHORTROOT (SHR) has a key role in controlling the asymmetric divisions of the cortex endodermis initial (CEI) cells. After SHR is transcribed and translated in the stele, the protein moves to the CEI cells, QC, and endodermis. In these cells SHR forms a complex with the TF SCARECROW (SCR) which restricts its movement and regulates the division that gives rise to the cortex and endodermis.
In this collaboration between researchers of the CBGP and North Carolina State University (NCSU), a predictive model has been generated thanks to the combination of mathematical equations and experimental designs which shows that the levels of SHR and SCR transcripts as well as differences in the stoichiometric state of the SHR-SCR protein complex are associated with differences in the division of the QC and the CEI (Panel A). High levels of the SHR-SCR complex promotes CEI divisions, while suppressing QC divisions.
These data are supported by confocal analyses and the “Number and Brightness” statistical technique in a mutant which does not express SCR (scr4). These analyses show, lack of SHR-SCR complex formation which associates with a greater number of QC divisions. (Panel B).
Finally, using this model, two new transcription factors have been identified as regulators of SHR levels in the stem cell niche: SEU as a positive regulator and WOX5 as a repressor.

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Original Paper:

Clark, N.M., Fisher, A.P., Berckmans, B., Broeck, L.V. den, Nelson, E.C., Nguyen, T.T., Bustillo-Avendaño, E., Zebell, S.G., Moreno-Risueno, M.A., Simon, R., Gallagher, K.L., Sozzani, R. 2020. Protein complex stoichiometry and expression dynamics of transcription factors modulate stem cell division. Proceedings of the National Academy of Sciences 117, 15332–15342. DOI: 10.1073/pnas.2002166117

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