Search

Eukaryotic development and stem cell control depend on the integration of cell positional sensing with cell cycle control and cell wall positioning, yet few factors that directly link these events are known. The DEFECTIVE KERNEL1 (DEK1) gene encoding the unique plant calpain protein is fundamental for development and growth, being essential to confer and maintain epidermal cell identity that allows development beyond the globular embryo stage. We show that DEK1 expression is highest in the actively dividing cells of seeds, meristems and vasculature. We further show that eliminating Arabidopsis DEK1 function leads to changes in developmental cues from the first zygotic division onward, altered microtubule patterns and misshapen cells, resulting in early embryo abortion. Expression of the embryonic marker genes WOX2, ATML1, PIN4, WUS and STM, related to axis organization, cell identity and meristem functions, is also altered in dek1 embryos. By monitoring cell layerspecific DEK1 down-regulation, we show that L1- and 35Sinduced down-regulation mainly affects stem cell functions, causing severe shoot apical meristem phenotypes. These results are consistent with a requirement for DEK1 to direct layer-specific cellular activities and set downstream developmental cues. Our data suggest that DEK1 may anchor cell wall positions and control cell division and differentiation, thereby balancing the plant's requirement to maintain totipotent stem cell reservoirs while simultaneously directing growth and organ formation. A role for DEK1 in regulating microtubule-orchestrated cell wall orientation during cell division can explain its effects on embryonic development, and suggests a more general function for calpains in microtubule organization in eukaryotic cells. [ABSTRACT FROM AUTHOR]

Organ formation relies upon precise patterns of gene expression that are under tight spatial and temporal regulation. Transcription patterns are specified by several cellular processes during development, including chromatin remodeling, but little is known about how chromatin-remodeling factors contribute to plant organogenesis. We demonstrate that the trithorax group (trxG) gene ULTRAPETALA1 (ULT1) and the GARP transcription factor gene KANADI1 (KAN1) organize the Arabidopsis thaliana gynoecium along two distinct polarit y axes. We show that ULT1 activity is required for the kan1 adaxialized polarity defect, indicating that ULT1 and KAN1 act oppositely to regulate the adaxial-abaxial axis. Conversely, ULT1 and KAN1 together establish apical-basal polarity by promoting basal cell fate in the gynoecium, restricting the expression domain of the basic helix-loop-helix transcription factor gene SPATULA. Finally, we show that ult alleles display dose-dependent genetic interactions with kan alleles and that ULT and KAN proteins can associate physically. Our findings identify a dual role for plant trxG factors in organ patterning, with ULT1 and KAN1 acting antagonistically to pattern the adaxial-abaxial polarity axis but jointly to pattern the apical-basal axis. Our data indicate that the ULT proteins function to link chromatin-remodeling factors with DNA binding transcription factors to regulate target gene expression. [ABSTRACT FROM AUTHOR]

Background and Aims The morphological variability of the flower in angiosperms, combined with its relatively simple structure, makes it an excellent model to study cell specification and the establishment of morphogenetic patterns. Flowers are the products of floral meristems, which are determinate structures that generate four different types of floral organs before terminating. The precise organization of the flower in whorls, each defined by the identity and number of organs it contains, is controlled by a multi-layered network involving numerous transcriptional regulators. In particular, the AGAMOUS (AG) MADS domain-containing transcription factor plays a major role in controlling floral determinacy in Arabidopsis thaliana in addition to specifying reproductive organ identity. This study aims to characterize the genetic interactions between the ULTRAPETALA1 (ULT1) and LEAFY (LFY) transcriptional regulators during flower morphogenesis, with a focus on AG regulation. Methods Genetic and molecular approaches were used to address the question of redundancy and reciprocal interdependency for the establishment of flower meristem initiation, identity and termination. In particular, the effects of loss of both ULT1 and LFY function were determined by analysing flower developmental phenotypes of double-mutant plants. The dependency of each factor on the other for activating developmental genes was also investigated in gain-of-function experiments. Key Results The ULT1 and LFY pathways, while both activating AG expression in the centre of the flower meristem, functioned independently in floral meristem determinacy. Ectopic transcriptional activation by ULT1 of AG and AP3, another gene encoding a MADS domain-containing flower architect, did not depend on LFY function. Similarly, LFY did not require ULT1 function to ectopically determine floral fate. Conclusions The results indicate that the ULT1 and LFY pathways act separately in regulating identity and determinacy at the floral meristem. In particular, they independently induce AG expression in the centre of the flower to terminate meristem activity. A model is proposed whereby these independent contributions bring about a switch at the AG locus from an inactive to an active transcriptional state at the correct time and place during flower development. [ABSTRACT FROM AUTHOR]

We demonstrate that the Arabidopsis trxG genes ULTRAPETALA1 (ULT1) and ULT2 together restrict shoot and floral stem cell activity and establish apical-basal gynoecium polarity. The proteins physically associate and regulate meristem and flower transcription factor genes, indicating that plant trxG factors control key developmental targets.The epigenetic regulation of gene expression is critical for ensuring the proper deployment and stability of defined genome transcription programs at specific developmental stages. The cellular memory of stable gene expression states during animal and plant development is mediated by the opposing activities of Polycomb group (PcG) factors and trithorax group (trxG) factors. Yet, despite their importance, only a few trxG factors have been characterized in plants and their roles in regulating plant development are poorly defined. In this work, we report that the closely related Arabidopsis trxG genes ULTRAPETALA1 (ULT1) and ULT2 have overlapping functions in regulating shoot and floral stem cell accumulation, with ULT1 playing a major role but ULT2 also making a minor contribution. The two genes also have a novel, redundant activity in establishing the apical–basal polarity axis of the gynoecium, indicating that they function in differentiating tissues. Like ULT1 proteins, ULT2 proteins have a dual nuclear and cytoplasmic localization, and the two proteins physically associate in planta. Finally, we demonstrate that ULT1 and ULT2 have very similar overexpression phenotypes and regulate a common set of key development target genes, including floral MADS-box genes and class I KNOX genes. Our results reveal that chromatin remodeling mediated by the ULT1 and ULT2 proteins is necessary to control the development of meristems and reproductive organs. They also suggest that, like their animal counterparts, plant trxG proteins may function in multi-protein complexes to up-regulate the expression of key stage- and tissue-specific developmental regulatory genes. [ABSTRACT FROM AUTHOR]

Epigenetic regulation of gene expression is of fundamental importance for eukaryotic development. EMBRYONIC FLOWER1 (EMF1) is a plant-specific gene that participates in Polycomb group-mediated transcriptional repression of target genes such as the flower MADS box genes AGAMOUS, APETALA3, and PISTILLATA. Here, we investigated the molecular mechanism underlying the curly leaf and early flowering phenotypes caused by reducing EMF1 activity in the leaf primordia of LFYasEMF1 transgenic plants and propose a combined effect of multiple flower MADS box gene activities on these phenotypes. ULTRAPETALA1 (ULT1) functions as a trithorax group factor that counteracts Polycomb group action in Arabidopsis (Arabidopsis thaliana). Removing ULT1 activity rescues both the abnormal developmental phenotypes and most of the misregulated gene expression of LFYasEMFl plants. Reducing EMF1 activity increases salt tolerance, an effect that is diminished by introducing the ultl-3 mutation into the LFYasEMFl background. EMF1 is required for trimethylating lysine-27 on histone 3 (H3K27me3), and ULT1 associates with ARABIDOPSIS TRITHORAX1 (ATX1) for trimethylating lysine-3 on histone 4 (H3K4me3) at flower MADS box gene loci. Reducing EMF1 activity decreases H3K27me3 marks and increases H3K4me3 marks on target gene loci. Removing ULT1 activity has the opposite effect on the two histone marks. Removing both gene activities restores the active and repressive marks to near wild-type levels. Thus, ULT1 acts as an antirepressor that counteracts EMF1 action through modulation of histone marks on target genes. Our analysis indicates that, instead of acting as off and on switches, EMF1 and ULT1 mediate histone mark deposition and modulate transcriptional activities of the target genes. [ABSTRACT FROM AUTHOR]

The plant seed is a major nutritional source for humans as well as an essential embryo development and dispersal unit. To ensure proper seed formation, fine spatial and temporal coordination between the embryo, endosperm, and maternal seed components must be achieved. However, the intercellular signaling pathways that direct the synchronous development of these tissues are poorly understood. Here we show that the Arabidopsis thaliana peptide ligand CLAVATA3/EMBRYO SURROUNDING REGION-RELATED8 (CLE8) is exclusively expressed in young embryos and endosperm, and that it acts cell and noncell autonomously to regulate basal embryo cell division patterns, endosperm proliferation, and the timing of endosperm differentiation. CLE8 positively regulates expression of the transcription factor gene WUSCHEL - LIKE HOMEOBOX8 (WOX8), and together CLE8 and WOX8 form a signaling module that promotes seed growth and overall seed size. These results demonstrate that seed development is coordinated by a secreted peptide ligand that plays a key early role in orchestrating cell patterning and proliferation in the embryo and endosperm. [ABSTRACT FROM AUTHOR]

Intercellular signaling is essential for the coordination of growth and development in higher plants. Although hundreds of putative receptors have been identified in Arabidopsis (Arabidopsis thaliana), only a few families of extracellular signaling molecules have been discovered, and their biological roles are largely unknown. To expand our insight into the developmental processes potentially regulated by ligand-mediated signal transduction pathways, we undertook a systematic expression analysis of the members of the Arabidopsis CLAVATA3/ESR-RELATED (CLE) small signaling polypeptide family. Using reporter constructs, we show that the CLE genes have distinct and specific patterns of promoter activity. We find that each Arabidopsis tissue expresses at least one CLE gene, indicating that CLE-mediated signaling pathways are likely to play roles in many biological processes during the plant life cycle. Some CLE genes that are closely related in sequence have dissimilar expression profiles, yet in many tissues multiple CLE genes have overlapping patterns of promoter-driven reporter activity. This observation, plus the general absence of detectable morphological phenotypes in cle null mutants, suggest that a high degree of functional redundancy exists among CLE gene family members. Our work establishes a community resource of CLE-related biological materials and provides a platform for understanding and ultimately manipulating many different plant signaling systems. [ABSTRACT FROM AUTHOR]

The CLE (CLVATA3/ESR-related) family of plant polypeptide signaling molecules shares a conserved 14-amino-acid (aa) motif, designated the CLE motif, which recent studies suggest is sufficient for CLE function in vitro. In this study, we report that Arabidopsis CLE proteins can function in a tissue-specific manner and confirm some CLE factors can act through different receptors. Using domain swapping, we show for the first time that the CLE motif likely determines much of the functional tissue-specificity of the proteins in planta. However, we also provide evidence in support of the new view that sequences outside the CLE motif (14 aa) contribute to CLE function and functional specificity in vivo. Additionally, we report that deletion of the putative signal peptide from different CLE proteins completely inactivates CLE function in vivo, whereas exchanging the CLE signal peptides with a conventional signal peptide from a rice glycine-rich cell wall protein also influences CLE function. We thus propose that the CLE motif itself determines its functional tissue-specificity by dictating the direct recognition and interaction of each CLE peptide with its optimal receptor(s), whereas the receptor(s) may be available in a tissue-specific manner. On the other hand, the sequences outside the CLE motif may influence CLE function by affecting the processing of CLE peptides, resulting in a change in the availability and/or abundance of CLE peptides in specific tissues and/or cells. [ABSTRACT FROM PUBLISHER]

Continuous organ formation is a hallmark of plant development that requires organ-specific gene activity to establish determinacy and axial patterning, yet the molecular mechanisms that coordinate these events remain poorly understood. Here, we show that the organ-specific BTB-POZ domain proteins BLADE-ON-PETIOLE1 (BOP1) and BOP2 function as transcriptional activators during Arabidopsis thaliana leaf formation. We identify as a direct target of BOP1 induction the ASYMMETRIC LEAVES2 (AS2) gene, which promotes leaf cell fate specification and adaxial polarity. We find that BOP1 associates with the AS2 promoter and that BOP1 and BOP2 are required for AS2 activation specifically in the proximal, adaxial region of the leaf, demonstrating a role for the BOP proteins as proximal-distal as well as adaxial-abaxial patterning determinants. Furthermore, repression of BOP1 and BOP2 expression by the indeterminacy-promoting KNOX gene SHOOTMERISTEMLESS is critical to establish a functional embryonic shoot apical meristem. Our data indicate that direct activation of AS2 transcription by BOP1 and BOP2 is vital for generating the conditions for KNOX repression at the leaf base and may represent a conserved mechanism for coordinating leaf morphogenesis with patterning along the adaxial-abaxial and the proximal-distal axes. [ABSTRACT FROM AUTHOR]

We report a novel function for BLADE-ON-PETIOLE1 (BOP1) and BOP2 in regulating Arabidopsis thaliana lateral organ cell fate and polarity, through the analysis of loss-of-function mutants and transgenic plants that ectopically express BOP1 or BOP2. 35S:BOP1 and 35S:BOP2 plants exhibit a very short and compact stature, hyponastic leaves, and downwardorienting siliques. We show that the LATERAL ORGAN BOUNDARIES (LOB) domain genes ASYMMETRIC LEAVES2 (AS2) and LOB are upregulated in 35S:BOP and downregulated in bop mutant plants. Ectopic expression of BOP1 or BOP2 also results in repression of class I knox gene expression. We further demonstrate a role for BOP1 and BOP2 in establishing the adaxial-abaxial polarity axis in the leaf petiole, where they regulate PHB and FIL expression and overlap in function with AS1 and AS2. Interestingly, during this study, we found that KANADI1 (KAN1) and KAN2 act to promote adaxial organ identity in addition to their well-known role in promoting abaxial organ identity. Our data indicate that BOP1 and BOP2 act in cells adjacent to the lateral organ boundary to repress genes that confer meristem cell fate and induce genes that promote lateral organ fate and polarity, thereby restricting the developmental potential of the organ-forming cells and facilitating their differentiation. [ABSTRACT FROM AUTHOR]

The BLADE-ON-PETIOLE1 (BOP1) gene of Arabidopsis thaliana is required for proper leaf morphogenesis. BOP1 regulates leaf differentiation in a proximal-distal manner, and represses the expression of three class I knotted-like homeobox (knox) genes during leaf formation. Utilizing a map-based approach, we identified the molecular nature of the BOP1 gene, which encodes a BTB/POZ domain protein with ankyrin repeats. BOP1 is a member of a small gene family in Arabidopsis that includes the disease resistance regulatory protein NPR1. Insertions in and around BOP1 cause distinct lesions in leaf morphogenesis, revealing complex regulation of the locus. BOP1 transcripts are initially detectable in embryos, where they specifically localize to the base of the developing cotyledons near the SAM. During vegetative development, BOP1 is expressed in young leaf primordia and at the base of the rosette leaves on the adaxial side. During reproductive development, BOP1 transcripts are detected in young floral buds, and at the base of the sepals and petals. Our results indicate that BOP1 encodes a putative regulatory protein that modulates meristematic activity at discrete locations in developing lateral organs. This is the first report on a plant protein that plays a key role in morphogenesis with the distinctive combinatorial architecture of the BTB/POZ and ankyrin repeat domains. [ABSTRACT FROM AUTHOR]

We have isolated a new mutant, hanaba taranu (han), which affects both flower and shoot apical meristem (SAM) development in Arabidopsis thaliana. Mutants have fused sepals and reduced organ numbers in all four whorls, especially in the 2nd (petal) and 3rd (stamen) whorls. han meristems can become flatter or smaller than in the wild type. HAN encodes a GATA-3-like transcription factor with a single zinc finger domain. HAN is transcribed at the boundaries between the meristem and its newly initiated organ primordia and at the boundaries between different floral whorls. It is also expressed in vascular tissues, developing ovules and stamens, and in the embryo. han interacts strongly with clavata (clv) mutations (clv1, clv2, and clv3), resulting in highly fasciated SAMs, and we find that WUS expression is altered in han mutants from early embryogenesis. In addition, HAN is ectopically expressed both in clv1 and clv3 mutants. We propose that HAN is normally required for establishing organ boundaries in shoots and flowers and for controlling the number and position of WUS-expressing cells. Ectopic HAN expression causes growth retardation, aberrant cell division patterns, and loss of meristem activity, suggesting that HAN is involved in controlling cell proliferation and differentiation. [ABSTRACT FROM AUTHOR]

Focuses on the isolation of four independent mutations as a result of identifying novel genes required for early Arabidopsis flower development in a screen. Information on the three complementation groups which the mutants fell into; Revelation of elegant genetic and molecular studies; Consistency of the mature Arabidopsis flower.

Examines whether the Broad-Complex (BR-C) and E74 early genes function together during Drosophila development. Direct induction of BR-C and E74 by ecdysone; Encoding of families of transcription factors that regulate ecdysone primary and secondary-response genes; Analysis of the morphological and molecular phenotypes of the double-mutant animals.

In eukaryotes, Polycomb group (PcG) and trithorax group (trxG) factors oppositely regulate gene transcription during development through histone modifications, with PcG factors repressing and trxG factors activating the expression of their target genes. Although plant trxG factors regulate many developmental and physiological processes, their downstream targets are poorly characterized. Here we use transcriptomics to identify genome-wide targets of the Arabidopsis thaliana trxG factor ULTRAPETALA1 (ULT1) during vegetative and reproductive development and compare them with those of the PcG factor CURLY LEAF (CLF). We find that genes involved in development and transcription regulation are overrepresented among ULT1 target genes. In addition, stress response genes and defense response genes such as those in glucosinolate metabolic pathways are enriched, revealing a previously unknown role for ULT1 in controlling biotic and abiotic response pathways. Finally, we show that many ULT1 target genes can be oppositely regulated by CLF, suggesting that ULT1 and CLF may have antagonistic effects on plant growth and development in response to various endogenous and environmental cues. [ABSTRACT FROM AUTHOR]

Shoot and floral meristem activity in higher plants is controlled by complex signaling networks consisting of positive and negative regulators. The Arabidopsis ULTRAPETALA1 (ULT1) gene has been shown to act as a negative regulator of meristem cell accumulation in inflorescence and floral meristems, as loss-of-function ult1 mutations cause inflorescence meristem enlargement, the production of extra flowers and floral organs, and a decrease in floral meristem determinacy. To investigate whether ULT1 functions in known meristem regulatory pathways, we generated double mutants between ult1 alleles and null alleles of the meristem-promoting genes SHOOTMERISTEMLESS (STM) and WUSCHEL (WUS). We found that, although the ult1 alleles have no detectable embryonic or vegetative phenotypes, ult1 mutations restored extensive organ-forming capability to stm null mutants after germination and increased leaf and floral organ production in stm partial loss-of-function mutants. Mutations in ULT1 also partially suppressed the wus shoot and floral meristem phenotypes. However, wus was epistatic to ult1 in the center of the flower, and WUS transcriptional repression was delayed in ult1 floral meristems. Our results show that during the majority of the Arabidopsis life cycle, ULT1 acts oppositely to STM and WUS in maintaining meristem activity and functions in a separate genetic pathway. However, ULT1 negatively regulates WUS to establish floral meristem determinacy, acting through the WUS-AG temporal feedback loop. [ABSTRACT FROM AUTHOR]