Yvonne Stahl

RPK2 is an essential receptor-like kinase that transmits the CLV3 signal in Arabidopsis.

The shoot apical meristem (SAM) is the fundamental structure that is located at the growing tip and gives rise to all aerial parts of plant tissues and organs, such as leaves, stems and flowers. In Arabidopsis thaliana, the CLAVATA3 (CLV3) pathway regulates the stem cell pool in the SAM, in which a small peptide ligand derived from CLV3 is perceived by two major receptor complexes, CLV1 and CLV2-CORYNE (CRN)/SUPPRESSOR OF LLP1 2 (SOL2), to restrict WUSCHEL (WUS) expression. In this study, we used the functional, synthetic CLV3 peptide (MCLV3) to isolate CLV3-insensitive mutants and revealed that a receptor-like kinase, RECEPTOR-LIKE PROTEIN KINASE 2 (RPK2), also known as TOADSTOOL 2 (TOAD2), is another key regulator of meristem maintenance. Mutations in the RPK2 gene result in stem cell expansion and increased number of floral organs, as seen in the other clv mutants. These phenotypes are additive with both clv1 and clv2 mutations. Moreover, our biochemical analyses using Nicotiana benthamiana revealed that RPK2 forms homo-oligomers but does not associate with CLV1 or CLV2. These genetic and biochemical findings suggest that three major receptor complexes, RPK2 homomers, CLV1 homomers and CLV2-CRN/SOL2 heteromers, are likely to mediate three signalling pathways, mainly in parallel but with potential crosstalk, to regulate the SAM homeostasis.

A Signaling Module Controlling the Stem Cell Niche in Arabidopsis Root Meristems

The niches of the Arabidopsis shoot and root meristems, the organizing center (OC) and the quiescent center (QC), orchestrate the fine balance of stem cell maintenance and the provision of differentiating descendants. They express the functionally related homeobox genes WUSCHEL (WUS) and WOX5, respectively, that promote stem cell fate in adjacent cells. Shoot stem cells signal back to the OC by secreting the CLAVATA3 (CLV3) dodecapeptide, which represses WUS expression. However, the signals controlling homeostasis of the root stem cell system are not identified to date. Here we show that the differentiating descendants of distal root stem cells express CLE40, a peptide closely related to CLV3. Reducing CLE40 levels delays differentiation and allows stem cell proliferation. Conversely, increased CLE40 levels drastically alter the expression domain of WOX5 and promote stem cell differentiation. We report that the receptor kinase ACR4, previously shown to control cell proliferation, is an essential component, and also a target, of CLE40 signaling. Our results reveal how, in contrast to the shoot system, signals originating from differentiated cells, but not the stem cells, determine the size and position of the root niche.

Moderation of Arabidopsis root stemness by CLAVATA1 and ARABIDOPSIS CRINKLY4 receptor kinase complexes.

BACKGROUND The root system of higher plants originates from the activity of a root meristem, which comprises a group of highly specialized and long-lasting stem cells. Their maintenance and number is controlled by the quiescent center (QC) cells and by feedback signaling from differentiated cells. Root meristems may have evolved from structurally distinct shoot meristems; however, no common player acting in stemness control has been found so far. RESULTS We show that CLAVATA1 (CLV1), a key receptor kinase in shoot stemness maintenance, performs a similar but distinct role in root meristems. We report that CLV1 is signaling, activated by the peptide ligand CLAVATA3/EMBRYO SURROUNDING REGION40 (CLE40), together with the receptor kinase ARABIDOPSIS CRINKLY4 (ACR4) to restrict root stemness. Both CLV1 and ACR4 overlap in their expression domains in the distal root meristem and localize to the plasma membrane (PM) and plasmodesmata (PDs), where ACR4 preferentially accumulates. Using multiparameter fluorescence image spectroscopy (MFIS), we show that CLV1 and ACR4 can form homo- and heteromeric complexes that differ in their composition depending on their subcellular localization. CONCLUSIONS We hypothesize that these homo- and heteromeric complexes may differentially regulate distal root meristem maintenance. We conclude that essential components of the ancestral shoot stemness regulatory system also act in the root and that the specific interaction of CLV1 with ACR4 serves to moderate and control stemness homeostasis in the root meristem. The structural differences between these two meristem types may have necessitated this recruitment of ACR4 for signaling by CLV1.

Plant primary meristems: shared functions and regulatory mechanisms.

Primary plant meristems are the shoot and root meristems that are initiated at opposite poles of the plant embryo. They contain stem cells, which remain undifferentiated, and supply new cells for growth and the formation of tissues. The maintenance of a long-lasting stem cell population in meristems is achieved by signal exchange between organizing regions and the stem cells, and also by feedback signals emanating from differentiating cells. Related peptide signals that make use of different receptor classes were found to control the stem cell populations in both meristem types by regulating evolutionarily conserved homeodomain transcription factors. The precise interplay of auxin and cytokinin signaling pathways is central to keep cells in the meristem, or direct them toward differentiation.

Tackling Drought Stress: RECEPTOR-LIKE KINASES Present New Approaches

Global climate change and a growing population require tackling the reduction in arable land and improving biomass production and seed yield per area under varying conditions. One of these conditions is suboptimal water availability. Here, we review some of the classical approaches to dealing with plant response to drought stress and we evaluate how research on RECEPTOR-LIKE KINASES (RLKs) can contribute to improving plant performance under drought stress. RLKs are considered as key regulators of plant architecture and growth behavior, but they also function in defense and stress responses. The available literature and analyses of available transcript profiling data indeed suggest that RLKs can play an important role in optimizing plant responses to drought stress. In addition, RLK pathways are ideal targets for nontransgenic approaches, such as synthetic molecules, providing a novel strategy to manipulate their activity and supporting translational studies from model species, such as Arabidopsis thaliana, to economically useful crops.

Real-time dynamics of peptide ligand–dependent receptor complex formation in planta

In plants, the flagellin and CLAVATA3 signaling pathways act through induced and preassembled receptor complexes, respectively. Monitoring receptor dynamics Plants use structurally related receptor complexes to respond to pathogens and growth signals, for example, using the flagellin (flg) and CLAVATA (CLV) receptors, respectively. Somssich et al. used multiparameter fluorescence imaging spectroscopy (MFIS) to assess the distribution of the receptor proteins and complexes at the membrane and the effect of their respective ligands. MFIS revealed that, before the presence of the bacterial peptide flg22, the receptors were kept apart and that the addition of flg22 triggered first receptor dimerization and then oligomerization of the dimeric complexes. In contrast, the receptors for the meristem-regulating peptide CLV3 existed as complexes before the presence of the ligand, and CLV3 induced their aggregation into membrane subdomains. This study demonstrates the usefulness of MFIS for analyzing receptor dynamics in living plant cells and reveals distinct characteristics of pathogen-sensing and growth-regulating pathways mediated by related receptor complexes. The CLAVATA (CLV) and flagellin (flg) signaling pathways act through peptide ligands and closely related plasma membrane–localized receptor-like kinases (RLKs). The plant peptide CLV3 regulates stem cell homeostasis, whereas the bacterial flg22 peptide elicits defense responses. We applied multiparameter fluorescence imaging spectroscopy (MFIS) to characterize the dynamics of RLK complexes in the presence of ligand in living plant cells expressing receptor proteins fused to fluorescent proteins. We found that the CLV and flg pathways represent two different principles of signal transduction: flg22 first triggered RLK heterodimerization and later assembly into larger complexes through homomerization. In contrast, CLV receptor complexes were preformed, and ligand binding stimulated their clustering. This different behavior likely reflects the nature of these signaling pathways. Pathogen-triggered flg signaling impedes plant growth and development; therefore, receptor complexes are formed only in the presence of ligand. In contrast, CLV3-dependent stem cell homeostasis continuously requires active signaling, and preformation of receptor complexes may facilitate this task.

Gated communities: apoplastic and symplastic signals converge at plasmodesmata to control cell fates

Due to their rigid cell walls, plant cells can only communicate with each other either by symplastic transport of diverse non-cell autonomous signalling molecules via plasmodesmata (PDs) or by endo- and exocytosis of signalling molecules via the extracellular apoplastic space. PDs are plasma membrane-lined channels spanning the cell wall between neighbouring cells, allowing the exchange of molecules by symplastic movement through them. This review focuses on developmental decisions that are coordinated by short- and long-distance communication of cells via PDs. We propose a model combining both apoplastic and symplastic signalling events via secreted ligands and their PD-localized receptor kinases which gate the symplastic transport of information molecules through PDs. Cell communities can thus coordinate cell-fate decisions non-cell autonomously by connecting or disconnecting symplastic subdomains. Here we concentrate on the establishment of such subdomains in the plants primary meristems that serve to maintain long-lasting stem cell populations in the shoot and root apical meristems, and discuss how apoplastic signalling via transport of information molecules through PDs is integrated with symplastic feedback signalling events.

Is the Arabidopsis root niche protected by sequestration of the CLE40 signal by its putative receptor ACR4

A tight but also dynamic regulation is necessary to control the size of stem cell populations in response to internal and external cues. The stem cells of the Arabidopsis shoot and root meristems are governed by the niche cells of the organizing centre (OC) and the quiescent centre (QC), respectively. The well characterized CLV3/WUS negative feedback loop adjusts homeostasis of the stem cell population in the shoot. Here, the CLAVATA3 (CLV3) dodecapeptide, expressed by the stem cells, signals to repress WUSCHEL (WUS), which is expressed in the subjacent OC cells, and in turn activates CLV3 expression non-cell autonomously. However, a similar signaling module controlling the root stem cell population was as yet unknown. In the June issue of Current Biology we report on such a signaling module comprising CLE40 (a CLV3 homologue) that acts via the receptor kinase Arabidopsis Crinkly4 (ACR4) to repress the WUS homologue WOX5 which maintains distal root stem cells. Furthermore, we showed that CLE40 peptide (CLE40p) treatment upregulates ACR4 expression. In this Addendum, we are further elaborating our hypothesis in which the upregulation of ACR4 as a consequence of ectopic CLE40p builds a protective barrier for the QC niche cells.

Function and regulation of transcription factors involved in root apical meristem and stem cell maintenance.

Plant roots are essential for overall plant development, growth, and performance by providing anchorage in the soil and uptake of nutrients and water. The primary root of higher plants derives from a group of pluripotent, mitotically active stem cells residing in the root apical meristem (RAM) which provides the basis for growth, development, and regeneration of the root. The stem cells in the Arabidopsis thaliana RAM are surrounding the quiescent center (QC), which consists of a group of rarely dividing cells. The QC maintains the stem cells in a non-cell-autonomous manner and prevents them from differentiation. The necessary dynamic but also tight regulation of the transition from stem cell fate to differentiation most likely requires complex regulatory mechanisms to integrate external and internal cues. Transcription factors play a central role in root development and are regulated by phytohormones, small signaling molecules, and miRNAs. In this review we give a comprehensive overview about the function and regulation of specific transcription factors controlling stem cell fate and root apical meristem maintenance and discuss the possibility of TF complex formation, subcellular translocations and cell-to-cell movement functioning as another level of regulation.

The CURLY LEAF Interacting Protein BLISTER Controls Expression of Polycomb-Group Target Genes and Cellular Differentiation of Arabidopsis thaliana

Pc-G proteins are key regulators of plant cell fate and development. This study examines the Pc-G interacting protein BLISTER, which has Pc-G related and unrelated functions and might link Pc-G proteins to specific developmental processes. Polycomb-group (Pc-G) proteins are important regulators of many developmental processes in plants and animals and repress gene expression by imparting histone H3 lysine 27 trimethylation (H3K27me3). Here, we present the identification of the novel, plant-specific Arabidopsis thaliana protein BLISTER (BLI), which interacts with the Pc-G histone methyltransferase CURLY LEAF (CLF). We map the interaction of BLI with CLF to a predicted coiled-coil domain in BLI that shares similarity with STRUCTURAL MAINTENANCE OF CHROMOSOMES proteins. BLI colocalizes with CLF in the nucleus, shows an overlapping expression pattern with CLF throughout plant development that is strongest in dividing cells, and represses a subset of Pc-G target genes. Loss of BLI results in a pleiotropic developmental mutant phenotype, indicating that BLI prevents premature differentiation. Furthermore, bli mutants exhibit severe epidermal defects, including loss of cell adhesion, outgrowth of cells, and increased cotyledon cell size. As these phenotypes have not been observed in Pc-G mutants, we propose that BLI has functions related to Pc-G proteins but can also act independently in Arabidopsis development.