WOX5 Suppresses CYCLIN D Activity to Establish Quiescence at the Center of the Root Stem Cell Niche
Forzani, C. ; Aichinger, E. ; Willemsen, V. ; Murray, J.A. - \ 2014
Current Biology 24 (2014)16. - ISSN 0960-9822 - p. 1939 - 1944.
arabidopsis-thaliana root - division - meristem - differentiation - organization - transition - expression - complex - protein
In Arabidopsis, stem cells maintain the provision of new cells for root growth. They surround a group of slowly dividing cells named the quiescent center (QC), and, together, they form the stem cell niche (SCN). The QC acts as the signaling center of the SCN, repressing differentiation of the surrounding stem cells  and providing a pool of cells able to replace damaged stem cells [2, 3]. Maintenance of the stem cells depends on the transcription factor WUSCHEL-RELATED HOMEOBOX 5 (WOX5), which is specifically expressed in the QC . However, the molecular mechanisms by which WOX5 promotes stem cell fate and whether WOX5 regulates proliferation of the QC are unknown. Here, we reveal a new role for WOX5 in restraining cell division in the cells of the QC, thereby establishing quiescence. In contrast, WOX5 and CYCD3;3/CYCD1;1 both promote cell proliferation in the nascent columella. The additional QC divisions occurring in wox5 mutants are suppressed in mutant combinations with the D type cyclins CYCD3;3 and CYCD1;1. Moreover, ectopic expression of CYCD3;3 in the QC is sufficient to induce cell division in the QC. WOX5 thus suppresses QC divisions that are otherwise promoted by CYCD3;3 and CYCD1;1, in part by interacting with the CYCD3;3 promoter to repress CYCD3;3 expression in the QC. Therefore, we propose a specific role for WOX5 in initiating and maintaining quiescence of the QC by excluding CYCD activity from the QC.
Precise control of plant stem cell activity through parallel regulatory inputs
Bennett, T. ; Toorn, A. van; Willemsen, V. ; Scheres, B. - \ 2014
Development 141 (2014). - ISSN 0950-1991 - p. 4055 - 4064.
arabidopsis-thaliana root - transcription factor - shoot apex - meristem - gene - differentiation - organization - maintenance - homeostasis - sombrero
The regulation of columella stem cell activity in the Arabidopsis root cap by a nearby organizing centre, the quiescent centre, has been a key example of the stem cell niche paradigm in plants. Here, we investigate interactions between transcription factors that have been shown to regulate columella stem cells using a simple quantification method for stem cell activity in the root cap. Genetic and expression analyses reveal that the RETINOBLASTOMA-RELATED protein, the FEZ and SOMBRERO NAC-domain transcription factors, the ARF10 and ARF16 auxin response factors and the quiescent centre-expressed WOX5 homeodomain protein each provide independent inputs to regulate the number of columella stem cells. Given the tight control of columella development, we found that these inputs act in a surprisingly parallel manner. Nevertheless, important points of interaction exist; for example, we demonstrate the repression of SMB activity by non-autonomous action of WOX5. Our results suggest that the developmental progression of columella stem cells may be quantitatively regulated by several more broadly acting transcription factors rather than by a single intrinsic stem cell factor, which raises questions about the special nature of the stem cell state in plants.
Unraveling Root Developmental Programs Initiated by Beneficial Pseudomonas spp. Bacteria
Zamioudis, C. ; Mastranesti, P. ; Dhonukshe, P. ; Blilou, I. ; Pieterse, C.M.J. - \ 2013
Plant Physiology 162 (2013)1. - ISSN 0032-0889 - p. 304 - 318.
induced systemic resistance - stem-cell niche - arabidopsis-thaliana root - transcription factor myc2 - auxin biosynthesis - rhizosphere microbiome - biocontrol bacteria - promote growth - ethylene - rhizobacteria
Plant roots are colonized by an immense number of microbes, referred to as the root microbiome. Selected strains of beneficial soil-borne bacteria can protect against abiotic stress and prime the plant immune system against a broad range of pathogens. Pseudomonas spp. rhizobacteria represent one of the most abundant genera of the root microbiome. Here, by employing a germfree experimental system, we demonstrate the ability of selected Pseudomonas spp. strains to promote plant growth and drive developmental plasticity in the roots of Arabidopsis (Arabidopsis thaliana) by inhibiting primary root elongation and promoting lateral root and root hair formation. By studying cell type-specific developmental markers and employing genetic and pharmacological approaches, we demonstrate the crucial role of auxin signaling and transport in rhizobacteria-stimulated changes in the root system architecture of Arabidopsis. We further show that Pseudomonas spp.-elicited alterations in root morphology and rhizobacteria-mediated systemic immunity are mediated by distinct signaling pathways. This study sheds new light on the ability of soil-borne beneficial bacteria to interfere with postembryonic root developmental programs.