Emerging parallels between stomatal and muscle cell lineages

RNA polymerase II associated proteins regulate stomatal development through direct interaction with stomatal transcription factors in Arabidopsis thaliana

RNA polymerase II (Pol II) associated proteins (RPAPs) have been ascribed diverse functions at the cellular level; however, their roles in developmental processes in yeasts, animals and plants are very poorly understood. Through screening for interactors of NRPB3, which encodes the third largest subunit of Pol II, we identified RIMA, the orthologue of mammalian RPAP2. A combination of genetic and biochemical assays revealed the role of RIMA and other RPAPs in stomatal development in Arabidopsis thaliana. We show that RIMA is involved in nuclear import of NRPB3 and other Pol II subunits, and is essential for restraining division and for establishing cell identity in the stomatal cell lineage. Moreover, plant RPAPs IYO/RPAP1 and QQT1/RPAP4, which interact with RIMA, are also crucial for stomatal development. Importantly, RIMA and QQT1 bind physically to stomatal transcription factors SPEECHLESS, MUTE, FAMA and SCREAMs. The RIMA-QQT1-IYO complex could work together with key stomatal transcription factors and Pol II to drive cell fate transitions in the stomatal cell lineage. Direct interactions with stomatal transcription factors provide a novel mechanism by which RPAP proteins may control differentiation of cell types and tissues in eukaryotes.

SPEECHLESS and MUTE Mediate Feedback Regulation of Signal Transduction during Stomatal Development

Stomatal density, spacing, and patterning greatly influence the efficiency of gas exchange, photosynthesis, and water economy. They are regulated by a complex of extracellular and intracellular factors through the signaling pathways. After binding the extracellular epidermal patterning factor 1 (EPF1) and 2 (EPF2) as ligands, the receptor-ligand complexes activate by phosphorylation through the MAP-kinase cascades, regulating basic helix-loop-helix (bHLH) transcription factors SPEECHLESS (SPCH), MUTE, and FAMA. In this review, we summarize the molecular mechanisms and signal transduction pathways running within the transition of the protodermal cell into a pair of guard cells with a space (aperture) between them, called a stoma, comprising asymmetric and symmetric cell divisions and draw several functional models. The feedback mechanisms involving the bHLH factors SPCH and MUTE are not fully recognized yet. We show the feedback mechanisms driven by SPCH and MUTE in the regulation of EPF2 and the ERECTA family. Intersections of the molecular mechanisms for fate determination of stomatal lineage cells with the role of core cell cycle-related genes and stabilization of SPCH and MUTE are also reported.

Decreased Levels of Thioredoxin o1 Influences Stomatal Development and Aperture but Not Photosynthesis under Non-Stress and Saline Conditions

Salinity has a negative impact on plant growth, with photosynthesis being downregulated partially due to osmotic effect and enhanced cellular oxidation. Redox signaling contributes to the plant response playing thioredoxins (TRXs) a central role. In this work we explore the potential contribution of Arabidopsis TRXo1 to the photosynthetic response under salinity analyzing Arabidopsis wild-type (WT) and two Attrxo1 mutant lines in their growth under short photoperiod and higher light intensity than previous reported works. Stomatal development and apertures and the antioxidant, hormonal and metabolic acclimation are also analyzed. In control conditions mutant plants displayed less and larger developed stomata and higher pore size which could underlie their higher stomatal conductance, without being affected in other photosynthetic parameters. Under salinity, all genotypes displayed a general decrease in photosynthesis and the oxidative status in the Attrxo1 mutant lines was altered, with higher levels of H₂O₂ and NO but also higher ascorbate/glutathione (ASC/GSH) redox states than WT plants. Finally, sugar changes and increases in abscisic acid (ABA) and NO may be involved in the observed higher stomatal response of the TRXo1-altered plants. Therefore, the lack of AtTRXo1 affected stomata development and opening and the mutants modulate their antioxidant, metabolic and hormonal responses to optimize their adaptation to salinity.

The Arabidopsis thaliana Mediator subunit MED8 regulates plant immunity to Botrytis Cinerea through interacting with the basic helix-loop-helix (bHLH) transcription factor FAMA

The Mediator complex is at the core of transcriptional regulation and plays a central role in plant immunity. The MEDIATOR25 (MED25) subunit of Arabidopsis thaliana regulates jasmonate-dependent resistance to Botrytis cinerea through interacting with the basic helix-loop-helix (bHLH) transcription factor of jasmonate signaling, MYC2. Another Mediator subunit, MED8, acts independently or together with MED25 in plant immunity. However, unlike MED25, the underlying action mechanisms of MED8 in regulating B. cinerea resistance are still unknown. Here, we demonstrated that MED8 regulated plant immunity to B. cinerea through interacting with another bHLH transcription factor, FAMA, which was previously shown to control the final proliferation/differentiation switch during stomatal development. Our research demonstrates that FAMA is also an essential component of B. cinerea resistance. The fama loss-of-function mutants (fama-1 and fama-2) increased susceptibility to B. cinerea infection and reduced defense-gene expression. On the contrary, transgenic lines constitutively overexpressing FAMA showed opposite B. cinerea responses compared with the fama loss-of-function mutants. FAMA-overexpressed plants displayed enhanced resistance to B. cinerea infection and increased expression levels of defensin genes following B. cinerea treatment. Genetic analysis of MED8 and FAMA suggested that FAMA-regulated pathogen resistance was dependent on MED8. In addition, MED8 and FAMA were both associated with the G-box region in the promoter of ORA59. Our findings indicate that the MED8 subunit of the A. thaliana Mediator regulates plant immunity to B. cinerea through interacting with the transcription factor FAMA, which was discovered to be a key component in B. cinerea resistance.

Molecular control of stomatal development

Plants have evolved developmental plasticity which allows the up- or down-regulation of photosynthetic and water loss capacities as new leaves emerge. This developmental plasticity enables plants to maximise fitness and to survive under differing environments. Stomata play a pivotal role in this adaptive process. These microscopic pores in the epidermis of leaves control gas exchange between the plant and its surrounding environment. Stomatal development involves regulated cell fate decisions that ensure optimal stomatal density and spacing, enabling efficient gas exchange. The cellular patterning process is regulated by a complex signalling pathway involving extracellular ligand–receptor interactions, which, in turn, modulate the activity of three master transcription factors essential for the formation of stomata. Here, we review the current understanding of the biochemical interactions between the epidermal patterning factor ligands and the ERECTA family of leucine-rich repeat receptor kinases. We discuss how this leads to activation of a kinase cascade, regulation of the bHLH transcription factor SPEECHLESS and its relatives, and ultimately alters stomatal production.

Feedback Regulation of DYT1 by Interactions with Downstream bHLH Factors Promotes DYT1 Nuclear Localization and Anther Development

Transcriptional regulation is one of the most important mechanisms controlling development and cellular functions in plants and animals. The Arabidopsis thaliana bHLH transcription factor (TF) DYSFUNCTIONL TAPETUM1 (DYT1) is required for normal male fertility and anther development and activates the expression of the bHLH010/bHLH089/bHLH091 genes. Here, we showed that DYT1 is localized to both the cytoplasm and nucleus at anther stage 5 but specifically to the nucleus at anther stage 6 and onward. The bHLH010/bHLH089/bHLH091 proteins have strong nuclear localization signals, interact with DYT1, and facilitate the nuclear localization of DYT1. We further found that the conserved C-terminal BIF domain of DYT1 is required for its dimerization, nuclear localization, transcriptional activation activity, and function in anther development. Interestingly, when the BIF domain of DYT1 was replaced with that of bHLH010, the DYT1(N)-bHLH010(BIF) chimeric protein shows nuclear-preferential localization at anther stage 5 but could not fully rescue the dyt1-3 phenotype, suggesting that the normal spatio-temporal subcellular localization of DYT1 is important for DYT1 function and/or that the BIF domains from different bHLH members might be functionally distinct. Our results support an important positive feedback regulatory mechanism whereby downstream TFs increase the function of an upstream TF by enhancing its nucleus localization through the BIF domain.

NRPB3, the third largest subunit of RNA polymerase II, is essential for stomatal patterning and differentiation in Arabidopsis

Stomata are highly specialized epidermal structures that control transpiration and gas exchange between plants and the environment. Signal networks underlying stomatal development have been previously uncovered but much less is known about how signals involved in stomatal development are transmitted to RNA polymerase II (Pol II or RPB), which plays a central role in the transcription of mRNA coding genes. Here, we identify a partial loss-of-function mutation of the third largest subunit of nuclear DNA-dependent Pol II (NRPB3) that exhibits an increased number of stomatal lineage cells and paired stomata. Phenotypic and genetic analyses indicated that NRPB3 is not only required for correct stomatal patterning, but is also essential for stomatal differentiation. Protein-protein interaction assays showed that NRPB3 directly interacts with two basic helix-loop-helix (bHLH) transcription factors, FAMA and INDUCER OF CBF EXPRESSION1 (ICE1), indicating that NRPB3 serves as an acceptor for signals from transcription factors involved in stomatal development. Our findings highlight the surprisingly conserved activating mechanisms mediated by the third largest subunit of Pol II in eukaryotes.

Summary: RNA polymerase II subunit NRPB3 interacts with stomatal bHLH transcription factors FAMA and ICE1, connecting the stomatal development pathway to the general transcription machinery.

Endosperm breakdown in Arabidopsis requires heterodimers of the basic helix-loop-helix proteins ZHOUPI and INDUCER OF CBP EXPRESSION 1

In Arabidopsis seeds, embryo growth is coordinated with endosperm breakdown. Mutants in the endosperm-specific gene ZHOUPI (ZOU), which encodes a unique basic helix-loop-helix (bHLH) transcription factor, have an abnormal endosperm that persists throughout seed development, significantly impeding embryo growth. Here we show that loss of function of the bHLH-encoding gene INDUCER OF CBP EXPRESSION 1 (ICE1) causes an identical endosperm persistence phenotype. We show that ZOU and ICE1 are co-expressed in the endosperm and interact in yeast via their bHLH domains. We show both genetically and in a heterologous plant system that, despite the fact that both ZOU and ICE1 can form homodimers in yeast, their role in endosperm breakdown requires their heterodimerization. Consistent with this conclusion, we confirm that ZOU and ICE1 regulate the expression of common target genes in the developing endosperm. Finally, we show that heterodimerization of ZOU and ICE1 is likely to be necessary for their binding to specific targets, rather than for their nuclear localization in the endosperm. By comparing our results with paradigms of bHLH function and evolution in animal systems we propose that the ZOU/ICE1 complex might have ancient origins, acquiring novel megagametophyte-specific functions in heterosporous land plants that were conserved in the angiosperm endosperm.

Positive and negative peptide signals control stomatal density

The stoma is a micro valve found on aerial plant organs that promotes gas exchange between the atmosphere and the plant body. Each stoma is formed by a strict cell lineage during the early stages of leaf development. Molecular genetics research using the model plant Arabidopsis has revealed the genes involved in stomatal differentiation. Cysteine-rich secretory peptides of the EPIDERMAL PATTERNING FACTOR-LIKE (EPFL) family play crucial roles as extracellular signaling factors. Stomatal development is orchestrated by the positive factor STOMAGEN/EPFL9 and the negative factors EPF1, EPF2, and CHALLAH/EPFL6 in combination with multiple receptors. EPF1 and EPF2 are produced in the stomatal lineage cells of the epidermis, whereas STOMAGEN and CHALLAH are derived from the inner tissues. These findings highlight the complex cell-to-cell and intertissue communications that regulate stomatal development. To optimize gas exchange, particularly the balance between the uptake of carbon dioxide (CO(2)) and loss of water, plants control stomatal activity in response to environmental conditions. The CO(2) level and light intensity influence stomatal density. Plants sense environmental cues in mature leaves and adjust the stomatal density of newly forming leaves, indicating the involvement of long-distance systemic signaling. This review summarizes recent research progress in the peptide signaling of stomatal development and discusses the evolutionary model of the signaling machinery.

Molecular characterization of vernalization loci VRN1 in wild and cultivated wheats

BACKGROUND

Variability of the VRN1 promoter region of the unique collection of spring polyploid and wild diploid wheat species together with diploid goatgrasses (donor of B and D genomes of polyploid wheats) were investigated. Accessions of wild diploid (T. boeoticum, T. urartu) and tetraploid (T. araraticum, T. timopheevii) species were studied for the first time.

RESULTS

Sequence analysis indicated great variability in the region from -62 to -221 nucleotide positions of the VRN1 promoter region. Different indels were found within this region in spring wheats. It was shown that VRN1 promoter region of B and G genome can also contain damages such as the insertion of the transposable element.Some transcription factor recognition sites including hybrid C/G-box for TaFDL2 protein known as the VRN1 gene upregulator were predicted inside the variable region. It was shown that deletions leading to promoter damage occurred in diploid and polyploid species independently. DNA transposon insertions first occurred in polyploid species. At the same time, the duplication of the promoter region was observed in A genomes of polyploid species.

CONCLUSIONS

We can conclude that supposed molecular mechanism of the VRN1 gene activating in cultivated diploid wheat species T. monococcum is common also for wild T. boeoticum and was inherited by T. monococcum. The spring polyploids are not related in their origin to spring diploids. The spring T. urartu and goatgrass accessions have another mechanism of flowering activation that is not connected with indels in VRN1 promoter region. All obtained data may be useful for detailed insight into origin of spring wheat forms in evolution and domestication process.

Arabidopsis RETINOBLASTOMA-RELATED is required for stem cell maintenance, cell differentiation, and lateral organ production

Several genes involved in the regulation of postembryonic organ initiation and growth have been identified. However, it remains largely unclear how developmental cues connect to the cell cycle. RETINOBLASTOMA RELATED (RBR) is a plant homolog of the tumor suppressor Retinoblastoma (pRb), which is a key regulator of the cell cycle. Using inducible RNA interference (RNAi) against Arabidopsis thaliana RBR (RBRi), we reduced RBR expression levels at different stages of plant development. Conditional reduction or loss of RBR function disrupted cell division patterns, promoted context-dependent cell proliferation, and negatively influenced establishment of cell differentiation. Several lineages of toti- and pluripotent cells, including shoot apical meristem stem cells, meristemoid mother cells, and procambial cells, failed to produce appropriately differentiated cells. Meristem activity was altered, leading to a disruption of the CLAVATA-WUSCHEL feedback loop and inhibition of lateral organ formation. Release of RBR from RNAi downregulation restored meristem activity. Gene profiling analyses soon after RBRi induction revealed that a change in RBR homeostasis is perceived as a stress, even before genes regulated by RBR-E2F become deregulated. The results establish RBR as a key cell cycle regulator required for coordination of cell division, differentiation, and cell homeostasis.

Origin and diversification of basic-helix-loop-helix proteins in plants

Basic helix-loop-helix (bHLH) proteins are a class of transcription factors found throughout eukaryotic organisms. Classification of the complete sets of bHLH proteins in the sequenced genomes of Arabidopsis thaliana and Oryza sativa (rice) has defined the diversity of these proteins among flowering plants. However, the evolutionary relationships of different plant bHLH groups and the diversity of bHLH proteins in more ancestral groups of plants are currently unknown. In this study, we use whole-genome sequences from nine species of land plants and algae to define the relationships between these proteins in plants. We show that few (less than 5) bHLH proteins are encoded in the genomes of chlorophytes and red algae. In contrast, many bHLH proteins (100-170) are encoded in the genomes of land plants (embryophytes). Phylogenetic analyses suggest that plant bHLH proteins are monophyletic and constitute 26 subfamilies. Twenty of these subfamilies existed in the common ancestors of extant mosses and vascular plants, whereas six further subfamilies evolved among the vascular plants. In addition to the conserved bHLH domains, most subfamilies are characterized by the presence of highly conserved short amino acid motifs. We conclude that much of the diversity of plant bHLH proteins was established in early land plants, over 440 million years ago.