BRAVO self-confined expression through WOX5 in the Arabidopsis root stem-cell niche.

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.

Brassinosteroid signaling in plant development and adaptation 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.

Precise transcriptional control of cellular quiescence by BRAVO/WOX5 complex in Arabidopsis roots.

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.

TOPLESS mediates brassinosteroid control of shoot boundaries and root meristem development in Arabidopsis thaliana.

The transcription factor BRI1-EMS-SUPRESSOR 1 (BES1) is a master regulator of brassinosteroid (BR)-regulated gene expression. BES1 together with BRASSINAZOLE-RESISTANT 1 (BZR1) drive activated or repressed expression of several genes, and have a prominent role in negative regulation of BR synthesis. Here, we report that BES1 interaction with TOPLESS (TPL), via its ERF-associated amphiphilic repression (EAR) motif, is essential for BES1-mediated control of organ boundary formation in the shoot apical meristem and the regulation of quiescent center (QC) cell division in roots. We show that TPL binds via BES1 to the promoters of the CUC3 and BRAVO targets and suppresses their expression. Ectopic expression of TPL leads to similar organ boundary defects and alterations in QC cell division rate to the bes1-d mutation, while bes1-d defects are suppressed by the dominant interfering protein encoded by tpl-1, with these effects respectively correlating with changes in CUC3 and BRAVO expression. Together, our data unveil a pivotal role of the co-repressor TPL in the shoot and root meristems, which relies on its interaction with BES1 and regulation of BES1 target gene expression.

Phosphoenolpyruvate carboxylase (PEPC) and PEPC-kinase (PEPC-k) isoenzymes in Arabidopsis thaliana: role in control and abiotic stress conditions.

MAIN CONCLUSION: 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.

Effect of Facetectomy on the Three-Dimensional Biomechanical Properties of the Fourth Canine Cervical Functional Spinal Unit: A Cadaveric Study.

Objective To study the biomechanical effect of facetectomy in 10 large breed dogs (>24 kg body weight) on the fourth canine cervical functional spinal unit. Methods Canine cervical spines were freed from all muscles. Spines were mounted on a six-degrees-of-freedom spine testing machine for three-dimensional motion analysis. Data were recorded with an optoelectronic motion analysis system. The range of motion was determined in all three primary motions as well as range of motion of coupled motions on the intact specimen, after unilateral and after bilateral facetectomy. Repeated-measures analysis of variance models were used to assess the changes of the biomechanical properties in the three treatment groups considered. Results Facetectomy increased range of motion of primary motions in all directions. Axial rotation was significantly influenced by facetectomy. Coupled motion was not influenced by facetectomy except for lateral bending with coupled motion axial rotation. The coupling factor (coupled motion/primary motion) decreased after facetectomy. Symmetry of motion was influenced by facetectomy in flexion-extension and axial rotation, but not in lateral bending. Clinical Significance Facet joints play a significant role in the stability of the cervical spine and act to maintain spatial integrity. Therefore, cervical spinal treatments requiring a facetectomy should be carefully planned and if an excessive increase in range of motion is expected, complications should be anticipated and reduced via spinal stabilization.

Enzymatic activity, gene expression and posttranslational modifications of photosynthetic and non-photosynthetic phosphoenolpyruvate carboxylase in ammonium-stressed sorghum plants.

Sorghum plants grown with 5mM (NH4)2SO4 showed symptoms of stress, such as reduced growth and photosynthesis, leaf chlorosis, and reddish roots. Phosphoenolpyruvate carboxylase (PEPC) activity, by supplying carbon skeletons for ammonium assimilation, plays a pivotal role in tolerance to ammonium stress. This work investigated the effect of ammonium nutrition on PPC and PPCK gene expression, on PEPC activity, and on post-translational modifications (PTMs) of PEPC in leaves and roots of sorghum plants. Ammonium increased PEPC kinase (PEPCk) activity and the phosphorylation state of PEPC in leaves, both in light and in the dark, due to increased PPCK1 expression in leaves. This result resembled the effect of salinity on sorghum leaf PEPC and PEPCk, which is thought to allow a better functioning of PEPC in conditions that limit the income of reduced C. In roots, ammonium increased PEPC activity and the amount of monoubiquitinated PEPC. The first effect was related to increased PPC3 expression in roots. These results highlight the relevance of this specific isoenzyme (PPC3) in sorghum responses to ammonium stress. Although the role of monoubiquitination is not fully understood, it also increased in germinating seeds along with massive mobilization of reserves, a process in which the anaplerotic function of PEPC is of major importance.

Proline synthesis in barley under iron deficiency and salinity.

This work investigates proline synthesis in six barley varieties subjected to iron deficiency, salinity or both stresses. The highest growth under Fe sufficiency corresponded to Belgrano and Shakira. A moderate augment of leaf phosphoenolpyruvate carboxylase (PEPC) activity was observed in all six varieties in response to Fe deficiency, consistently in leaves and sporadically in roots. All six varieties accumulated proline under Fe deficiency, to a higher extent in leaves than in roots. The decrease of Fe supply from 100 µM NaFe(III)-EDTA to 0.5 µM NaFe(III)-EDTA reduced growth and photosynthetic pigments similarly in the six barley varieties. On the contrary, differences between varieties could be observed with respect to increased or, conversely, decreased proline content as a function of the amount of NaFe(III)-EDTA supplied. These two opposite types were represented by Belgrano (higher proline under Fe deficiency) and Shakira (higher proline under Fe sufficiency). Time-course experiments suggested that leaf PEPC activity was not directly responsible for supplying C for proline synthesis under Fe deficiency. High proline levels in the leaves of Fe-deficient Belgrano plants in salinity were associated to a better performance of this variety under these combined stresses.