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In animals and plants, stem-cell niches are local microenvironments that are tightly regulated to preserve their unique identity while communicating with adjacent cells that will give rise to specialized cell types. In the primary root of Arabidopsis thaliana, two transcription factors, BRAVO and WOX5, among others, are expressed in the stem-cell niche. Intriguingly, BRAVO, a repressor of quiescent center divisions, confines its own gene expression to the stem-cell niche, as evidenced in a bravo mutant background. Here, we propose through mathematical modeling that BRAVO confines its own expression domain to the stem-cell niche by attenuating a WOX5-dependent diffusible activator of BRAVO. This negative feedback drives WOX5 activity to be spatially restricted as well. The results show that WOX5 diffusion and sequestration by binding to BRAVO are sufficient to drive the experimentally observed confined BRAVO expression at the stem-cell niche. We propose that the attenuation of a diffusible activator can be a general mechanism acting at other stem-cell niches to spatially confine genetic activity to a small region while maintaining signaling within them and with the surrounding cells., This work was supported by the Ministerio de Ciencia e Innovación (MCIN), Agencia Estatal de Investigación (AEI) and European Regional Development Fund (Fondo Europeo de Desarrollo Regional, FEDER) program ‘Una manera de hacer Europa’ MCIN/AEI/10.13039/501100011033/FEDER (PGC2018-101896-B-I00 to M.I.); Ministerio de Ciencia e Innovación, Agencia Estatal de Investigación and European Social Fund (Fondo Social Europeo, FSE) program ‘El FSE invierte en tu futuro’ (BES-2016-078218 to J.M.); Ministerio de Ciencia e Innovación (MCIN), Agencia Estatal de Investigación (AEI) and European Regional Development Fund (ERDF) (BIO2016-78955 to A.I.C.-D.); the European Research Council (ERC) Consolidator Grant (ERC-2015-CoG–683163 to A.I.C.-D.); Generalitat de Catalunya (M.I. and J.M. in Grup de Recerca Consolidat 2017 SGR 1061; FI-DGR 2016FI-B 00472, also funded by FSE, to N.B.); and the Ministerio de Educación, Cultura y Deporte (FPU15/02822 to I.B.-P.).

Understanding stem cell regulatory circuits is the next challenge in plant biology, as these cells are essential for tissue growth and organ regeneration in response to stress. In the Arabidopsis primary root apex, stem cell-specific transcription factors BRAVO and WOX5 co-localize in the quiescent centre (QC) cells, where they commonly repress cell division so that these cells can act as a reservoir to replenish surrounding stem cells, yet their molecular connection remains unknown. Genetic and biochemical analysis indicates that BRAVO and WOX5 form a transcription factor complex that modulates gene expression in the QC cells to preserve overall root growth and architecture. Furthermore, by using mathematical modelling we establish that BRAVO uses the WOX5/BRAVO complex to promote WOX5 activity in the stem cells. Our results unveil the importance of transcriptional regulatory circuits in plant stem cell development., A.I.C-D. is a recipient of a BIO2016-78955 grant from the Spanish Ministry of Economy and Competitiveness and a European Research Council, ERC Consolidator Grant (ERC-2015-CoG – 683163). I.B-P. is funded by the FPU15/02822 grant from the Spanish Ministry of Education, Culture and Sport; N.B. by the FI-DGR 2016FI_B 00472 grant from the AGAUR, Generalitat de Catalunya; and A.P-R. by the SEV-2015-0533 from the Severo Ochoa Programme for Centers of Excellence in R&D. M.I. and J.M. acknowledge support from the Spanish Ministry of Economy and Competitiveness and FEDER (EU) through grant FIS2015-66503-C3-3-P, from Ministerio de Ciencia, Innovación y Universidades / Agencia Estatal de Investigación / Fondo Europeo de Desarrollo Regional, Unión Europea through grant PGC2018-101896-B-I00 and from the Generalitat de Catalunya through Grup de Recerca Consolidat 2014 SGR 878 and 2017 SGR 1061. J.M. is funded by the Spanish Ministry of Education through BES-2016-078218. Y.S. and R.C.D. were funded by the Deutsche Forschungsgesellschaft (DFG) (grant STA12/12 1-1). A.I.C.-D.–A.C. collaboration was funded by the European Regional Development Funds and Marie Curie IRSES Project DEANN (PIRSES-GA-2013-612583). CRAG is funded by “Severo Ochoa Programme” from Centers of Excellence in R&D 2016-2019 (SEV-2015-485 0533).

Arabidopsis ppc3 mutant has a growth-arrest phenotype and is affected in phosphate- and salt-stress responses, showing that this protein is crucial under control or stress conditions. Phosphoenolpyruvate carboxylase (PEPC) and its dedicated kinase (PEPC-k) are ubiquitous plant proteins implicated in many physiological processes. This work investigates specific roles for the three plant-type PEPC (PTPC) and the two PEPC-k isoenzymes in Arabidopsis thaliana. The lack of any of the PEPC isoenzymes reduced growth parameters under optimal growth conditions. PEPC activity was decreased in shoots and roots of ppc2 and ppc3 mutants, respectively. Phosphate starvation increased the expression of all PTPC and PPCK genes in shoots, but only PPC3 and PPCK2 in roots. The absence of any of these two proteins was not compensated by other isoforms in roots. The effect of salt stress on PTPC and PPCK expression was modest in shoots, but PPC3 was markedly increased in roots. Interestingly, both stresses decreased root growth in each of the mutants except for ppc3. This mutant had a stressed phenotype in control conditions (reduced root growth and high level of stress molecular markers), but was unaffected in their response to high salinity. Salt stress increased PEPC activity, its phosphorylation state, and L-malate content in roots, all these responses were abolished in the ppc3 mutant. Our results highlight the importance of the PPC3 isoenzyme for the normal development of plants and for root responses to stress.

Brassinosteroids (BRs) are steroid hormones that are essential for plant growth and development. These hormones control the division, elongation and differentiation of various cell types throughout the entire plant life cycle. Our current understanding of the BR signaling pathway has mostly been obtained from studies using Arabidopsis thaliana as a model. In this context, the membrane steroid receptor BRI1 (BRASSINOSTEROID INSENSITIVE 1) binds directly to the BR ligand, triggering a signal cascade in the cytoplasm that leads to the transcription of BR-responsive genes that drive cellular growth. However, recent studies of the primary root have revealed distinct BR signaling pathways in different cell types and have highlighted cell-specific roles for BR signaling in controlling adaptation to stress. In this Review, we summarize our current knowledge of the spatiotemporal control of BR action in plant growth and development, focusing on BR functions in primary root development and growth, in stem cell self-renewal and death, and in plant adaption to environmental stress., A.P.-R. is recipient of a PhD fellowship from the “Severo Ochoa Programme for Centers of Excellence in R&D” 2016-2019 from the Ministerio de Ciencia e Innovación (SEV-2015-0533). A.G. is a recipient of a post-doctoral fellowship from the “Severo Ochoa Programme for Centers of Excellence in R&D” 2016-2019 from the Ministerio de Ciencia e Innovación (SEV-2015-0533). I.B.-P. is funded by a grant from the Ministerio de Educación, Cultura y Deporte (FPU15/02822). N.B. is funded by a grant from the Agència de Gestió d'Ajuts Universitaris i de Recerca, Generalitat de Catalunya (FI-DGR 2016FI_B 00472). M.I. acknowledges support from the Ministerio de Economía y Competitividad (Spain) and FEDER (European Regional Development Fund of the European Union) (FIS2015-66503-C3-3-P) and from the Generalitat de Catalunya through Grup de Recerca Consolidat (2017 SGR 1061). A.I.C.-D. is recipient of a grant from the Ministerio de Economía y Competitividad (BIO2013-43873) and of an ERC Consolidator Grant from the European Research Council (ERC-2015-CoG – 683163).