A SCARECROW-based regulatory circuit controls Arabidopsis thaliana meristem size from the root endodermis.
ABSTRACT: SCARECROW controls Arabidopsis root meristem size from the root endodermis tissue by regulating the DELLA protein RGA that in turn mediates the regulation of ARR1 levels at the transition zone. Coherent organ growth requires a fine balance between cell division and cell differentiation. Intriguingly, plants continuously develop organs post-embryonically thanks to the activity of meristems that allow growth and environmental plasticity. In Arabidopsis thaliana, continued root growth is assured when division of the distal stem cell and their daughters is balanced with cell differentiation at the meristematic transition zone (TZ). We have previously shown that at the TZ, the cytokinin-dependent transcription factor ARR1 controls the rate of differentiation commitment of meristematic cells and that its activities are coordinated with those of the distal stem cells by the gene SCARECROW (SCR). In the stem cell organizer (the quiescent center, QC), SCR directly suppresses ARR1 both sustaining stem cell activities and titrating non-autonomously the ARR1 transcript levels at the TZ via auxin. Here, we show that SCR also exerts a fine control on ARR1 levels at the TZ from the endodermis by sustaining gibberellin signals. From the endodermis, SCR controls the RGA REPRESSOR OF ga1-3 (RGA) DELLA protein stability throughout the root meristem, thus controlling ARR1 transcriptional activation at the TZ. This guarantees robustness and fineness to the control of ARR1 levels necessary to balance cell division to cell differentiation in sustaining coherent root growth. Therefore, this work advances the state of the art in the field of root meristem development by integrating the activity of three hormones, auxin, gibberellin, and cytokinin, under the control of different tissue-specific activities of a single root key regulator, SCR.
Project description:During plant development, because no cell movement takes place, control of the timing and extent of cell division and coordination of the direction and extent of cell expansion are particularly important for growth and development. The plant hormone gibberellins (GAs) play key roles in the control of these developmental processes. However, little is known about the molecular components that integrate the generic GA signaling into a specific cell/tissue to coordinate cell division and cell expansion. Here we report that scarecrow-like 3 (SCL3), a GRAS protein, acts as a positive regulator to integrate and maintain a functional GA pathway by attenuating the DELLA repressors in the root endodermis. The tissue-specific maintenance of GA signaling in the root endodermis plays distinct roles along the longitudinal root axis. While in the elongation/differentiation zone (EDZ), the endodermis-confined GA pathway by SCL3 controls primarily coordination of root cell elongation; in the meristem zone (MZ) SCL3 in conjunction with the short-root/scarecrow (SHR/SCR) pathway controls GA-modulated ground tissue maturation. Our findings highlight the regulatory network of the GRAS transcription regulators (SCL3, DELLAs, and SHR/SCR) in the root endodermis, shedding light on how GA homeostasis is achieved and how the maintenance of GA signaling controls developmental processes in roots.
Project description:In the Arabidopsis root meristem, ground tissue stem cell daughters perform an asymmetric division to form endodermis and cortex. The putative transcription factors SCARECROW (SCR) and SHORTROOT (SHR) regulate this radial patterning event, and the mixed cell fate in scr mutants suggests an atypical role of the SCR gene in asymmetric cell division. Here we use a newly developed site-specific gene activation/deletion system in which induced clones are positively marked with green fluorescent protein (GFP). Using this system, we show that SCR acts cell-autonomously to control asymmetric cell division within the ground tissue. We provide evidence that SCR gene expression is under autoregulatory control, that SCR limits SHR movement, and that transient SCR action is sufficient to separate endodermis and cortex fates by asymmetric cell division.
Project description:A critical issue in development is the coordination of the activity of stem cell niches with differentiation of their progeny to ensure coherent organ growth. In the plant root, these processes take place at opposite ends of the meristem and must be coordinated with each other at a distance. Here, we show that in Arabidopsis, the gene SCR presides over this spatial coordination. In the organizing center of the root stem cell niche, SCR directly represses the expression of the cytokinin-response transcription factor ARR1, which promotes cell differentiation, controlling auxin production via the ASB1 gene and sustaining stem cell activity. This allows SCR to regulate, via auxin, the level of ARR1 expression in the transition zone where the stem cell progeny leaves the meristem, thus controlling the rate of differentiation. In this way, SCR simultaneously controls stem cell division and differentiation, ensuring coherent root growth.
Project description:Maintenance of a functional root is fundamental to plant survival in response to some abiotic stresses, such as water deficit. In this study, we found that overexpression of Arabidopsis class 1 phytoglobin (AtPgb1) alleviated the growth retardation of polyethylene glycol (PEG)-induced water stress by reducing programmed cell death (PCD) associated with protein folding in the endoplasmic reticulum (ER). This was in contrast to PEG-stressed roots down-regulating AtPgb1 that exhibited extensive PCD and reduced expression of several attenuators of ER stress, including BAX Inhibitor-1 (BI-1). The death program experienced by the suppression of AtPgb1 in stressed roots was mediated by reactive oxygen species (ROS) and ethylene. Suppression of ROS synthesis or ethylene perception reduced PCD and partially restored root growth. The PEG-induced cessation of root growth was preceded by structural changes in the root apical meristem (RAM), including the loss of cell and tissue specification, possibly as a result of alterations in PIN1- and PIN4-mediated auxin accumulation at the root pole. These events were attenuated by the overexpression of AtPgb1 and aggravated when AtPgb1 was suppressed. Specifically, suppression of AtPgb1 compromised the functionality of the WOX5-expressing quiescent cells (QCs), leading to the early and premature differentiation of the adjacent columella stem cells and to a rapid reduction in meristem size. The expression and localization of other root domain markers, such as SCARECROW (SCR), which demarks the endodermis and QCs, and WEREWOLF (WER), which specifies the lateral root cap, were also most affected in PEG-treated roots with suppressed AtPgb1. Collectively, the results demonstrate that AtPgb1 exercises a protective role in roots exposed to lethal levels of PEG, and suggest a novel function of this gene in ensuring meristem functionality through the retention of cell fate specification.
Project description:Plant growth depends on stem cell niches in meristems. In the root apical meristem, the quiescent center (QC) cells form a niche together with the surrounding stem cells. Stem cells produce daughter cells that are displaced into a transit-amplifying (TA) domain of the root meristem. TA cells divide several times to provide cells for growth. SHORTROOT (SHR) and SCARECROW (SCR) are key regulators of the stem cell niche. Cytokinin controls TA cell activities in a dose-dependent manner. Although the regulatory programs in each compartment of the root meristem have been identified, it is still unclear how they coordinate one another. Here, we investigate how PHABULOSA (PHB), under the posttranscriptional control of SHR and SCR, regulates TA cell activities. The root meristem and growth defects in shr or scr mutants were significantly recovered in the shr phb or scr phb double mutant, respectively. This rescue in root growth occurs in the absence of a QC. Conversely, when the modified PHB, which is highly resistant to microRNA, was expressed throughout the stele of the wild-type root meristem, root growth became very similar to that observed in the shr; however, the identity of the QC was unaffected. Interestingly, a moderate increase in PHB resulted in a root meristem phenotype similar to that observed following the application of high levels of cytokinin. Our protoplast assay and transgenic approach using ARR10 suggest that the depletion of TA cells by high PHB in the stele occurs via the repression of B-ARR activities. This regulatory mechanism seems to help to maintain the cytokinin homeostasis in the meristem. Taken together, our study suggests that PHB can dynamically regulate TA cell activities in a QC-independent manner, and that the SHR-PHB pathway enables a robust root growth system by coordinating the stem cell niche and TA domain.
Project description:The Scarecrow (SCR) transcription factor plays a crucial role in root cell radial patterning and is required for maintenance of the quiescent centre and differentiation of the endodermis. In response to phosphorus (P) deficiency, white lupin (Lupinus albus L.) root surface area increases some 50-fold to 70-fold due to the development of cluster (proteoid) roots. Previously it was reported that SCR-like expressed sequence tags (ESTs) were expressed during early cluster root development. Here the cloning of two white lupin SCR genes, LaSCR1 and LaSCR2, is reported. The predicted amino acid sequences of both LaSCR gene products are highly similar to AtSCR and contain C-terminal conserved GRAS family domains. LaSCR1 and LaSCR2 transcript accumulation localized to the endodermis of both normal and cluster roots as shown by in situ hybridization and gene promoter::reporter staining. Transcript analysis as evaluated by quantitative real-time-PCR (qRT-PCR) and RNA gel hybridization indicated that the two LaSCR genes are expressed predominantly in roots. Expression of LaSCR genes was not directly responsive to the P status of the plant but was a function of cluster root development. Suppression of LaSCR1 in transformed roots of lupin and Medicago via RNAi (RNA interference) delivered through Agrobacterium rhizogenes resulted in decreased root numbers, reflecting the potential role of LaSCR1 in maintaining root growth in these species. The results suggest that the functional orthologues of AtSCR have been characterized.
Project description:The plant-specific GAI, RGA and SCR (GRAS) family proteins play critical roles in plant development and signalling. Two GRAS proteins, SHORT-ROOT (SHR) and SCARECROW (SCR), cooperatively direct asymmetric cell division and the patterning of root cell types by transcriptional control in conjunction with BIRD/INDETERMINATE DOMAIN (IDD) transcription factors, although precise details of these specific interactions and actions remain unknown. Here, we present the crystal structures of the SHR-SCR binary and JACKDAW (JKD)/IDD10-SHR-SCR ternary complexes. Each GRAS domain comprises one ?/? core subdomain with an ?-helical cap that mediates heterodimerization by forming an intermolecular helix bundle. The ?/? core subdomain of SHR forms the BIRD binding groove, which specifically recognizes the zinc fingers of JKD. We identified a conserved SHR-binding motif in 13 BIRD/IDD transcription factors. Our results establish a structural basis for GRAS-GRAS and GRAS-BIRD interactions and provide valuable clues towards our understanding of these regulators, which are involved in plant-specific signalling networks.
Project description:Stem cells are defined by their ability to self-renew and produce daughter cells that proliferate and mature. These maturing cells transition from a proliferative state to a terminal state through the process of differentiation. In the Arabidopsis thaliana root the transcription factors SCARECROW and SHORTROOT regulate specification of the bipotent stem cell that gives rise to cortical and endodermal progenitors. Subsequent progenitor proliferation and differentiation generate mature endodermis, marked by the Casparian strip, a cell-wall modification that prevents ion diffusion into and out of the vasculature. We identified a transcription factor, MYB DOMAIN PROTEIN 36 (MYB36), that regulates the transition from proliferation to differentiation in the endodermis. We show that SCARECROW directly activates MYB36 expression, and that MYB36 likely acts in a feed-forward loop to regulate essential Casparian strip formation genes. We show that myb36 mutants have delayed and defective barrier formation as well as extra divisions in the meristem. Our results demonstrate that MYB36 is a critical positive regulator of differentiation and negative regulator of cell proliferation.
Project description:Inadequate availability of inorganic phosphate (Pi) in the rhizosphere is a common challenge to plants, which activate metabolic and developmental responses to maximize Pi acquisition. The sensory mechanisms that monitor environmental Pi status and regulate root growth via altered meristem activity are unknown. Here, we show that PHOSPHATE DEFICIENCY RESPONSE 2 (PDR2) encodes the single P(5)-type ATPase of Arabidopsis thaliana. PDR2 functions in the endoplasmic reticulum (ER) and is required for proper expression of SCARECROW (SCR), a key regulator of root patterning, and for stem-cell maintenance in Pi-deprived roots. We further show that the multicopper oxidase encoded by LOW PHOSPHATE ROOT 1 (LPR1) is targeted to the ER and that LPR1 and PDR2 interact genetically. Because the expression domains of both genes overlap in the stem-cell niche and distal root meristem, we propose that PDR2 and LPR1 function together in an ER-resident pathway that adjusts root meristem activity to external Pi. Our data indicate that the Pi-conditional root phenotype of pdr2 is not caused by increased Fe availability in low Pi; however, Fe homeostasis modifies the developmental response of root meristems to Pi availability.
Project description:GRAS transcriptional factors have diverse functions in plant growth and development, and are named after the first three transcription factors, namely, GAI (GIBBERELLIC ACID INSENSITIVE), RGA (REPRESSOR OF GAI) and SCR (SCARECROW) identified in this family. Knowledge of the GRAS gene family in maize remains was largely unknown, and their characterization is necessary to understand their importance in the maize life cycle. This study identified 86 GRAS genes in maize, and further characterized with phylogenetics, gene structural analysis, genomic loci, and expression patterns. The 86 GRAS genes were divided into 8 groups (SCL3, HAM, LS, SCR, DELLA, SHR, PAT1 and LISCL) by phylogenetic analysis. Most of the maize GRAS genes contain one exon (80.23%) and closely related members in the phylogenetic tree had similar structure and motif composition. Different motifs especially in the N-terminus might be the sources of their functional divergence. Segmental- and tandem-duplication occurred in this family leading to expansion of maize GRAS genes and the expression patterns of the duplicated genes in the heat map according to the published microarray data were very similar. Quantitative RT-PCR (qRT-PCR) results demonstrated that the expression level of genes in different tissues were different, suggesting their differential roles in plant growth and development. The data set expands our knowledge to understanding the function of GRAS genes in maize, an important crop plant in the world.