Quantification of maize brace root formation after vertical stalk displacement

Maize brace roots develop from aboveground stem nodes in both upright and vertically displaced stalks. The cues that trigger brace root development after displacement are unknown. Possibilities include disturbance of the belowground roots, gravity, moisture, physical interaction, or node anatomical changes. We show that brace root formation occurs at all growth stages, with more nodes producing brace roots when plants are displaced at later growth stages. This occurs with the underground roots intact, without moisture accumulation and without physical interaction. We propose that the formation of brace roots after vertical stalk displacement is most likely due to gravity or anatomical changes at the node.

Graft incompatibility between pepper and tomato can be attributed to genetic incompatibility between diverged immune systems

Graft compatibility is the capacity of two plants to form cohesive vascular connections. Tomato and pepper are incompatible graft partners; however, the underlying cause of graft rejection between these two species remains unknown.We diagnosed graft incompatibility between tomato and diverse pepper varieties based on weakened biophysical stability, decreased growth, and persistent cell death using trypan blue and TUNEL assays. Transcriptomic analysis of cell death in the junction was performed using RNA-sequencing, and molecular signatures for incompatible graft response were characterized based on meta-transcriptomic comparisons with other biotic processes.We show that tomato is broadly incompatible with diverse pepper cultivars. These incompatible graft partners activate prolonged transcriptional changes that are highly enriched for defense processes. Amongst these processes was broad NLR upregulation and hypersensitive response. Using transcriptomic datasets for a variety of biotic stress treatments, we identified a significant overlap in the genetic profile of incompatible grafting and plant parasitism. In addition, we found over 1000 genes that are uniquely upregulated in incompatible grafts.Based on NLR overactivity, DNA damage, and prolonged cell death we have determined that tomato and pepper graft incompatibility is likely caused by a form of genetic incompatibility, which triggers a hyperimmune-response.

Root responses to abiotic stress: a comparative look at root system architecture in maize and sorghum

Under all environments, roots are important for plant anchorage and acquiring water and nutrients. However, there is a knowledge gap regarding how root architecture contributes to stress tolerance in a changing climate. Two closely related plant species, maize and sorghum, have distinct root system architectures and different levels of stress tolerance, making comparative analysis between these two species an ideal approach to resolve this knowledge gap. However, current research has focused on shared aspects of the root system that are advantageous under abiotic stress conditions rather than on differences. Here we summarize the current state of knowledge comparing the root system architecture relative to plant performance under water deficit, salt stress, and low phosphorus in maize and sorghum. Under water deficit, steeper root angles and deeper root systems are proposed to be advantageous for both species. In saline soils, a reduction in root length and root number has been described as advantageous, but this work is limited. Under low phosphorus, root systems that are shallow and wider are beneficial for topsoil foraging. Future work investigating the differences between these species will be critical for understanding the role of root system architecture in optimizing plant production for a changing global climate.

Brace roots in C 3 Poaceae: where have they gone?

Brace roots are common in large C 4 Poaceae species, such as maize and sorghum. However, in other species, these roots were either never reported, or the existence of the trait was neglected. Here we report the presence of brace roots in a high-performing Avena sativa L. (oat) line.

2020-2021 field seasons of Maize GxE project within the Genomes to Fields Initiative

OBJECTIVES

This release note describes the Maize GxE project datasets within the Genomes to Fields (G2F) Initiative. The Maize GxE project aims to understand genotype by environment (GxE) interactions and use the information collected to improve resource allocation efficiency and increase genotype predictability and stability, particularly in scenarios of variable environmental patterns. Hybrids and inbreds are evaluated across multiple environments and phenotypic, genotypic, environmental, and metadata information are made publicly available.

DATA DESCRIPTION

The datasets include phenotypic data of the hybrids and inbreds evaluated in 30 locations across the US and one location in Germany in 2020 and 2021, soil and climatic measurements and metadata information for all environments (combination of year and location), ReadMe, and description files for each data type. A set of common hybrids is present in each environment to connect with previous evaluations. Each environment had a collaborator responsible for collecting and submitting the data, the GxE coordination team combined all the collected information and removed obvious erroneous data. Collaborators received the combined data to use, verify and declare that the data generated in their own environments was accurate. Combined data is released to the public with minimal filtering to maintain fidelity to the original data.

Genomes to Fields 2022 Maize genotype by Environment Prediction Competition

OBJECTIVES

The Genomes to Fields (G2F) 2022 Maize Genotype by Environment (GxE) Prediction Competition aimed to develop models for predicting grain yield for the 2022 Maize GxE project field trials, leveraging the datasets previously generated by this project and other publicly available data.

DATA DESCRIPTION

This resource used data from the Maize GxE project within the G2F Initiative [1]. The dataset included phenotypic and genotypic data of the hybrids evaluated in 45 locations from 2014 to 2022. Also, soil, weather, environmental covariates data and metadata information for all environments (combination of year and location). Competitors also had access to ReadMe files which described all the files provided. The Maize GxE is a collaborative project and all the data generated becomes publicly available [2]. The dataset used in the 2022 Prediction Competition was curated and lightly filtered for quality and to ensure naming uniformity across years.

Characterization of mechanosensitive MSL gene family expression in Zea mays aerial and subterranean brace roots

Plants must be able to sense and respond to mechanical stresses encountered throughout their lifespan. The MscS-Like (MSL) family of mechanosensitive ion channels is one mechanism to perceive mechanical stresses. In maize, brace roots emerge from stem nodes above the soil and some remain aerial while some grow into the soil. We tested the hypothesis that MSL gene expression is higher in subterranean brace roots compared to those that remain aerial. However, there was no difference in MSL expression between the two environments. This work sets the foundation for a deeper understanding of MSL gene expression and function in maize.

Image-based assessment of plant disease progression identifies new genetic loci for resistance to Ralstonia solanacearum in tomato

A major challenge in global crop production is mitigating yield loss due to plant diseases. One of the best strategies to control these losses is through breeding for disease resistance. One barrier to the identification of resistance genes is the quantification of disease severity, which is typically based on the determination of a subjective score by a human observer. We hypothesized that image-based, non-destructive measurements of plant morphology over an extended period after pathogen infection would capture subtle quantitative differences between genotypes, and thus enable identification of new disease resistance loci. To test this, we inoculated a genetically diverse biparental mapping population of tomato (Solanum lycopersicum) with Ralstonia solanacearum, a soilborne pathogen that causes bacterial wilt disease. We acquired over 40 000 time-series images of disease progression in this population, and developed an image analysis pipeline providing a suite of 10 traits to quantify bacterial wilt disease based on plant shape and size. Quantitative trait locus (QTL) analyses using image-based phenotyping for single and multi-traits identified QTLs that were both unique and shared compared with those identified by human assessment of wilting, and could detect QTLs earlier than human assessment. Expanding the phenotypic space of disease with image-based, non-destructive phenotyping both allowed earlier detection and identified new genetic components of resistance.

Maize plants and the brace roots that support them

Brace roots are a unique but poorly understood set of organs found in some large cereal crops such as maize. These roots develop from aerial stem nodes and can remain aerial or grow into the ground. Despite their name, the function of these roots to brace the plant was only recently shown. In this article, I discuss the current understanding of brace root function and development, as well as the multitude of open questions that remain about these fascinating organs.

Maize brace root mechanics vary by whorl, genotype and reproductive stage

BACKGROUND AND AIMS

Root lodging is responsible for significant crop losses worldwide. During root lodging, roots fail by breaking, buckling or pulling out of the ground. In maize, above-ground roots, called brace roots, have been shown to reduce susceptibility to root lodging. However, the underlying structural-functional properties of brace roots that prevent root lodging are poorly defined. In this study, we quantified structural mechanical properties, geometry and bending moduli for brace roots from different whorls, genotypes and reproductive stages.

METHODS

Using 3-point bend tests, we show that brace root mechanics are variable by whorl, genotype and reproductive stage.

KEY RESULTS

Generally, we find that within each genotype and reproductive stage, the brace roots from the first whorl (closest to the ground) had higher structural mechanical properties and a lower bending modulus than brace roots from the second whorl. There was additional variation between genotypes and reproductive stages. Specifically, genotypes with higher structural mechanical properties also had a higher bending modulus, and senesced brace roots had lower structural mechanical properties than hydrated brace roots.

CONCLUSIONS

Collectively these results highlight the importance of considering whorl-of-origin, genotype and reproductive stage for the quantification of brace root mechanics, which is important for mitigating crop loss due to root mechanical failure.

Multiple brace root phenotypes promote anchorage and limit root lodging in maize

Plant mechanical failure (lodging) causes global yield losses of 7%-66% in cereal crops. We have previously shown that the above-ground nodal roots (brace roots) in maize are critical for anchorage. However, it is unknown how brace root phenotypes vary across genotypes and the functional consequence of this variation. This study quantifies the contribution of brace roots to anchorage, brace root traits, plant height, and root lodging susceptibility in 52 maize inbred lines. We show that the contribution of brace roots to anchorage and root lodging susceptibility varies among genotypes and this contribution can be explained by plant architectural variation. Additionally, supervised machine learning models were developed and show that multiple plant architectural phenotypes can predict the contribution of brace roots to anchorage and root lodging susceptibility. Together these data define the plant architectures that are important in lodging resistance and show that the contribution of brace roots to anchorage is a good proxy for root lodging susceptibility.

Bracing for sustainable agriculture: the development and function of brace roots in members of Poaceae

Optimization of crop production requires root systems to function in water uptake, nutrient use, and anchorage. In maize, two types of nodal roots-subterranean crown and aerial brace roots function in anchorage and water uptake and preferentially express multiple water and nutrient transporters. Brace root development shares genetic control with juvenile-to-adult phase change and flowering time. We present a comprehensive list of the genes known to alter brace roots and explore these as candidates for QTL studies in maize and sorghum. Brace root development and function may be conserved in other members of Poaceae, however research is limited. This work highlights the critical knowledge gap of aerial nodal root development and function and suggests new focus areas for breeding resilient crops.

Maize brace roots provide stalk anchorage

Mechanical failure, known as lodging, negatively impacts yield and grain quality in crops. Limiting crop loss from lodging requires an understanding of the plant traits that contribute to lodging-resistance. In maize, specialized aerial brace roots are reported to reduce root lodging. However, their direct contribution to plant biomechanics has not been measured. In this manuscript, we use a non-destructive field-based mechanical test on plants before and after the removal of brace roots. This precisely determines the contribution of brace roots to establish a rigid base (i.e. stalk anchorage) that limits plant deflection in maize. These measurements demonstrate that the more brace root whorls that contact the soil, the greater their overall contribution to anchorage, but that the contributions of each whorl to anchorage were not equal. Previous studies demonstrated that the number of nodes that produce brace roots is correlated with flowering time in maize. To determine if flowering time selection alters the brace root contribution to anchorage, a subset of the Hallauer's Tusón tropical population was analyzed. Despite significant variation in flowering time and anchorage, selection neither altered the number of brace root whorls in the soil nor the overall contribution of brace roots to anchorage. These results demonstrate that brace roots provide a rigid base in maize and that the contribution of brace roots to anchorage was not linearly related to flowering time.

Field-based mechanical phenotyping of cereal crops to assess lodging resistance

Plant mechanical failure, also known as lodging, is the cause of significant and unpredictable yield losses in cereal crops. Lodging occurs in two distinct failure modes-stalk lodging and root lodging. Despite the prevalence and detrimental impact of lodging on crop yields, there is little consensus on how to phenotype plants in the field for lodging resistance and thus breed for mechanically resilient plants. This review provides an overview of field-based mechanical testing approaches to assess stalk and root lodging resistance. These approaches are placed in the context of future perspectives. Best practices and recommendations for acquiring field-based mechanical phenotypes of plants are also presented.

Novel Imaging Modalities Shedding Light on Plant Biology: Start Small and Grow Big

The acquisition of quantitative information on plant development across a range of temporal and spatial scales is essential to understand the mechanisms of plant growth. Recent years have shown the emergence of imaging methodologies that enable the capture and analysis of plant growth, from the dynamics of molecules within cells to the measurement of morphometricand physiological traits in field-grown plants. In some instances, these imaging methods can be parallelized across multiple samples to increase throughput. When high throughput is combined with high temporal and spatial resolution, the resulting image-derived data sets could be combined with molecular large-scale data sets to enable unprecedented systems-level computational modeling. Such image-driven functional genomics studies may be expected to appear at an accelerating rate in the near future given the early success of the foundational efforts reviewed here. We present new imaging modalities and review how they have enabled a better understanding of plant growth from the microscopic to the macroscopic scale.

Minimum requirements for changing and maintaining endodermis cell identity in the Arabidopsis root

Changes in gene regulation during differentiation are governed by networks of transcription factors. The Arabidopsis root endodermis is a tractable model to address how transcription factors contribute to differentiation. We used a bottom-up approach to understand the extent to which transcription factors that are required for endodermis differentiation can confer endodermis identity to a non-native cell type. Our results show that the transcription factors SHORTROOT and MYB36 alone have limited ability to induce ectopic endodermal features in the absence of additional cues. The stele-derived signalling peptide CIF2 stabilizes SHORTROOT-induced endodermis identity acquisition. The outcome is a partially impermeable barrier deposited in the subepidermal cell layer, which has a transcriptional signature similar to the endodermis. These results demonstrate that other root cell types can be forced to differentiate into the endodermis and highlight a previously unappreciated role for receptor kinase signalling in maintaining endodermis identity.

Uncovering Gene Regulatory Networks Controlling Plant Cell Differentiation

The development of multicellular organisms relies on the precise regulation of cellular differentiation. As such, there has been significant effort invested to understand the process through which an immature cell undergoes differentiation. In this review, we highlight key discoveries and advances that have contributed to our understanding of the transcriptional networks underlying Arabidopsis root endodermal differentiation. To conclude, we propose perspectives on how advances in molecular biology, microscopy, and nucleotide sequencing will provide the tools to test the biological significance of these gene regulatory networks (GRN).

Morphological Plant Modeling: Unleashing Geometric and Topological Potential within the Plant Sciences

The geometries and topologies of leaves, flowers, roots, shoots, and their arrangements have fascinated plant biologists and mathematicians alike. As such, plant morphology is inherently mathematical in that it describes plant form and architecture with geometrical and topological techniques. Gaining an understanding of how to modify plant morphology, through molecular biology and breeding, aided by a mathematical perspective, is critical to improving agriculture, and the monitoring of ecosystems is vital to modeling a future with fewer natural resources. In this white paper, we begin with an overview in quantifying the form of plants and mathematical models of patterning in plants. We then explore the fundamental challenges that remain unanswered concerning plant morphology, from the barriers preventing the prediction of phenotype from genotype to modeling the movement of leaves in air streams. We end with a discussion concerning the education of plant morphology synthesizing biological and mathematical approaches and ways to facilitate research advances through outreach, cross-disciplinary training, and open science. Unleashing the potential of geometric and topological approaches in the plant sciences promises to transform our understanding of both plants and mathematics.

Reshaping Plant Biology: Qualitative and Quantitative Descriptors for Plant Morphology

An emerging challenge in plant biology is to develop qualitative and quantitative measures to describe the appearance of plants through the integration of mathematics and biology. A major hurdle in developing these metrics is finding common terminology across fields. In this review, we define approaches for analyzing plant geometry, topology, and shape, and provide examples for how these terms have been and can be applied to plants. In leaf morphological quantifications both geometry and shape have been used to gain insight into leaf function and evolution. For the analysis of cell growth and expansion, we highlight the utility of geometric descriptors for understanding sepal and hypocotyl development. For branched structures, we describe how topology has been applied to quantify root system architecture to lend insight into root function. Lastly, we discuss the importance of using morphological descriptors in ecology to assess how communities interact, function, and respond within different environments. This review aims to provide a basic description of the mathematical principles underlying morphological quantifications.

Morphological Plant Modeling: Unleashing Geometric and Topological Potential within the Plant Sciences

The geometries and topologies of leaves, flowers, roots, shoots, and their arrangements have fascinated plant biologists and mathematicians alike. As such, plant morphology is inherently mathematical in that it describes plant form and architecture with geometrical and topological techniques. Gaining an understanding of how to modify plant morphology, through molecular biology and breeding, aided by a mathematical perspective, is critical to improving agriculture, and the monitoring of ecosystems is vital to modeling a future with fewer natural resources. In this white paper, we begin with an overview in quantifying the form of plants and mathematical models of patterning in plants. We then explore the fundamental challenges that remain unanswered concerning plant morphology, from the barriers preventing the prediction of phenotype from genotype to modeling the movement of leaves in air streams. We end with a discussion concerning the education of plant morphology synthesizing biological and mathematical approaches and ways to facilitate research advances through outreach, cross-disciplinary training, and open science. Unleashing the potential of geometric and topological approaches in the plant sciences promises to transform our understanding of both plants and mathematics.

Tissue-Specific Transcriptome Profiling in Arabidopsis Roots

Spatiotemporal transcriptome profiles from specific tissues are critical for understanding plant development and responses to the environment. One approach to isolate specific tissues is fluorescence-activated cell sorting (FACS). In this chapter, we outline methods for the FACS isolation of root protoplasts followed by transcriptome profiling using RNA sequencing.

Establishment of Expression in the SHORTROOT-SCARECROW Transcriptional Cascade through Opposing Activities of Both Activators and Repressors

Tissue-specific gene expression is often thought to arise from spatially restricted transcriptional cascades. However, it is unclear how expression is established at the top of these cascades in the absence of pre-existing specificity. We generated a transcriptional network to explore how transcription factor expression is established in the Arabidopsis thaliana root ground tissue. Regulators of the SHORTROOT-SCARECROW transcriptional cascade were validated in planta. At the top of this cascade, we identified both activators and repressors of SHORTROOT. The aggregate spatial expression of these regulators is not sufficient to predict transcriptional specificity. Instead, modeling, transcriptional reporters, and synthetic promoters support a mechanism whereby expression at the top of the SHORTROOT-SCARECROW cascade is established through opposing activities of activators and repressors.

Genes and networks regulating root anatomy and architecture

The root is an excellent model for studying developmental processes that underlie plant anatomy and architecture. Its modular structure, the lack of cell movement and relative accessibility to microscopic visualization facilitate research in a number of areas of plant biology. In this review, we describe several examples that demonstrate how cell type-specific developmental mechanisms determine cell fate and the formation of defined tissues with unique characteristics. In the last 10 yr, advances in genome-wide technologies have led to the sequencing of thousands of plant genomes, transcriptomes and proteomes. In parallel with the development of these high-throughput technologies, biologists have had to establish computational, statistical and bioinformatic tools that can deal with the wealth of data generated by them. These resources provide a foundation for posing more complex questions about molecular interactions, and have led to the discovery of new mechanisms that control phenotypic differences. Here we review several recent studies that shed new light on developmental processes, which are involved in establishing root anatomy and architecture. We highlight the power of combining large-scale experiments with classical techniques to uncover new pathways in root development.

HEC of a job regulating stem cells

In this issue of Developmental Cell, Schuster et al. (2014) describe the signals regulated by the bHLH transcription factor HEC1 during Arabidopsis stem cell maintenance. HEC1 acts antagonistically with other factors, integrating multiple cues to provide a balance between cellular differentiation and proliferation.

3-dimensional resin casting and imaging of mouse portal vein or intrahepatic bile duct system

In organs, the correct architecture of vascular and ductal structures is indispensable for proper physiological function, and the formation and maintenance of these structures is a highly regulated process. The analysis of these complex, 3-dimensional structures has greatly depended on either 2-dimensional examination in section or on dye injection studies. These techniques, however, are not able to provide a complete and quantifiable representation of the ductal or vascular structures they are intended to elucidate. Alternatively, the nature of 3-dimensional plastic resin casts generates a permanent snapshot of the system and is a novel and widely useful technique for visualizing and quantifying 3-dimensional structures and networks. A crucial advantage of the resin casting system is the ability to determine the intact and connected, or communicating, structure of a blood vessel or duct. The structure of vascular and ductal networks are crucial for organ function, and this technique has the potential to aid study of vascular and ductal networks in several ways. Resin casting may be used to analyze normal morphology and functional architecture of a luminal structure, identify developmental morphogenetic changes, and uncover morphological differences in tissue architecture between normal and disease states. Previous work has utilized resin casting to study, for example, architectural and functional defects within the mouse intrahepatic bile duct system that were not reflected in 2-dimensional analysis of the structure(1,2), alterations in brain vasculature of a Alzheimer's disease mouse model(3), portal vein abnormalities in portal hypertensive and cirrhotic mice(4), developmental steps in rat lymphatic maturation between immature and adult lungs(5), immediate microvascular changes in the rat liver, pancreas, and kidney in response in to chemical injury(6). Here we present a method of generating a 3-dimensional resin cast of a mouse vascular or ductal network, focusing specifically on the portal vein and intrahepatic bile duct. These casts can be visualized by clearing or macerating the tissue and can then be analyzed. This technique can be applied to virtually any vascular or ductal system and would be directly applicable to any study inquiring into the development, function, maintenance, or injury of a 3-dimensional ductal or vascular structure.

Genetic interactions between hepatocyte nuclear factor-6 and Notch signaling regulate mouse intrahepatic bile duct development in vivo

Notch signaling and hepatocyte nuclear factor-6 (HNF-6) are two genetic factors known to affect lineage commitment in the bipotential hepatoblast progenitor cell (BHPC) population. A genetic interaction involving Notch signaling and HNF-6 in mice has been inferred through separate experiments showing that both affect BHPC specification and bile duct morphogenesis. To define the genetic interaction between HNF-6 and Notch signaling in an in vivo mouse model, we examined the effects of BHPC-specific loss of HNF-6 alone and within the background of BHPC-specific loss of recombination signal binding protein immunoglobulin kappa J (RBP-J), the common DNA-binding partner of all Notch receptors. Isolated loss of HNF-6 in this mouse model fails to demonstrate a phenotypic variance in bile duct development compared to control. However, when HNF-6 loss is combined with RBP-J loss, a phenotype consisting of cholestasis, hepatic necrosis, and fibrosis is observed that is more severe than the phenotype seen with Notch signaling loss alone. This phenotype is associated with significant intrahepatic biliary system abnormalities, including an early decrease in biliary epithelial cells, evolving to ductular proliferation and a decrease in the density of communicating peripheral bile duct branches. In this in vivo model, simultaneous loss of both HNF-6 and RBP-J results in down-regulation of both HNF-1β and Sox9 (sex determining region Y-related HMG box transcription factor 9).

CONCLUSION

HNF-6 and Notch signaling interact in vivo to control expression of downstream mediators essential to the normal development of the intrahepatic biliary system. This study provides a model to investigate genetic interactions of factors important to intrahepatic bile duct development and their effect on cholestatic liver disease phenotypes.

Defects in hepatic Notch signaling result in disruption of the communicating intrahepatic bile duct network in mice

Abnormal Notch signaling in humans results in Alagille syndrome, a pleiotropic disease characterized by a paucity of intrahepatic bile ducts (IHBDs). It is not clear how IHBD paucity develops as a consequence of atypical Notch signaling, whether by a developmental lack of bile duct formation, a post-natal lack of branching and elongation or an inability to maintain formed ducts. Previous studies have focused on the role of Notch in IHBD development, and demonstrated a dosage requirement of Notch signaling for proper IHBD formation. In this study, we use resin casting and X-ray microtomography (microCT) analysis to address the role of Notch signaling in the maintenance of formed IHBDs upon chronic loss or gain of Notch function. Our data show that constitutive expression of the Notch1 intracellular domain in bi-potential hepatoblast progenitor cells (BHPCs) results in increased IHBD branches at post-natal day 60 (P60), which are maintained at P90 and P120. By contrast, loss of Notch signaling via BHPC-specific deletion of RBP-J (RBP KO), the DNA-binding partner for all Notch receptors, results in progressive loss of intact IHBD branches with age. Interestingly, in RBP KO mice, we observed a reduction in bile ducts per portal vein at P60; no further reduction had occurred at P120. Thus, bile duct structures are not lost with age; instead, we propose a model in which BHPC-specific loss of Notch signaling results in an initial developmental defect resulting in fewer bile ducts being formed, and in an acquired post-natal defect in the maintenance of intact IHBD architecture as a result of irresolvable cholestasis. Our studies reveal a previously unappreciated role for Notch signaling in the post-natal maintenance of an intact communicating IHBD structure, and suggest that liver defects observed in Alagille syndrome patients might be more complex than bile duct paucity.

Notch signaling regulates formation of the three-dimensional architecture of intrahepatic bile ducts in mice

Alagille syndrome, a chronic hepatobiliary disease, is characterized by paucity of intrahepatic bile ducts (IHBDs). To determine the impact of Notch signaling specifically on IHBD arborization, we studied the influence of both chronic gain and loss of Notch function on the intact three-dimensional IHBD structure using a series of mutant mouse models and a resin casting method. Impaired Notch signaling in bipotential hepatoblast progenitor cells (BHPCs) dose-dependently decreased the density of peripheral IHBDs, whereas activation of Notch1 results in an increased density of peripheral IHBDs. Although Notch2 has a dominant role in IHBD formation, there is also a redundant role for other Notch receptors in determining the density of peripheral IHBDs. Because changes in IHBD density do not appear to be due to changes in cellular proliferation of bile duct progenitors, we suggest that Notch plays a permissive role in cooperation with other factors to influence lineage decisions of BHPCs and sustain peripheral IHBDs.

CONCLUSION

There is a threshold requirement for Notch signaling at multiple steps, including IHBD tubulogenesis and maintenance, during hepatic development that determines the density of three-dimensional peripheral IHBD architecture.

Rac1 promotes TGF-beta-stimulated mesangial cell type I collagen expression through a PI3K/Akt-dependent mechanism

Transforming growth factor (TGF)-beta is a central mediator in the progression of glomerulosclerosis, leading to accumulation of aberrant extracellular matrix proteins and inappropriate expression of smooth muscle alpha-actin in the kidney. Previously, we reported that disrupting the cytoskeleton diminished TGF-beta-stimulated type I collagen accumulation in human mesangial cells. As cytoskeletal signaling molecules, including the Rho-family GTPases, have been implicated in fibrogenesis, we sought to determine the respective roles of RhoA and Rac1 in HMC collagen I expression. TGF-beta1 activated both RhoA and Rac1 within 5 min of treatment, and this activation was dependent on the kinase activity of the type I TGF-beta receptor. TGF-beta1-stimulated induction of type I collagen mRNA expression and promoter activity was diminished by inhibiting Rac1 activity and was increased by a constitutively active Rac1 mutant, whereas inhibiting RhoA activity had no such effect. Rac1 activation required phosphatidylinositol-3-kinase (PI3K) activity. Furthermore, the PI3K antagonist, LY294002, reduced TGF-beta1-stimulated COL1A2 promoter activity and Rac1 activation. It also partially blocked active Rac1-stimulated collagen promoter activity, suggesting that PI3K activity contributes to both TGF-beta activation of Rac1 and signal propagation downstream of Rac1. Thus, while both Rac1 and RhoA are rapidly activated in response to TGF-beta1 in human mesangial cells, only Rac1 activation enhances events that contribute to mesangial cell collagen expression, through a positive feedback loop involving PI3K.