Asymmetric leaves2 and Elongator, a histone acetyltransferase complex, mediate the establishment of polarity in leaves of Arabidopsis thaliana

ASYMMETRIC LEAVES 2 and ASYMMETRIC LEAVES 2-LIKE are partially redundant genes and essential for fruit development in tomato

Fruit size and shape are controlled by genes expressed during the early developmental stages of fruit. Although the function of ASYMMETRIC LEAVES 2 (AS2) in promoting leaf adaxial cell fates has been well characterized in Arabidopsis thaliana, the molecular mechanisms conferring freshy fruit development as a spatial-temporal expression gene in tomato pericarp remain unclear. In the present study, we verified the transcription of SlAS2 and SlAS2L, two homologs of AS2, in the pericarp during early fruit development. Disruption of SlAS2 or SlAS2L caused a significant decrease in pericarp thickness as a result of a reduction in the number of pericarp cell layers and cell area, leading to smaller tomato fruit size, which revealed their critical roles in tomato fruit development. In addition, leaves and stamens exhibited severe morphological defects in slas2 and slas2l single mutants, as well as in the double mutants. These results demonstrated the redundant and pleiotropic functions of SlAS2 and SlAS2L in tomato fruit development. Yeast two-hybrid and split-luciferase complementation assays showed that both SlAS2 and SlAS2L physically interact with SlAS1. Molecular analyses further indicated that SlAS2 and SlAS2L regulate various downstream genes in leaf and fruit development, and that some genes participating in the regulation of cell division and cell differentiation in the tomato pericarp are affected by these genes. Our findings demonstrate that SlAS2 and SlAS2L are vital transcription factors required for tomato fruit development.

Molecular mechanism of leaf adaxial upward curling caused by BpPIN3 suppression in Betula pendula

Leaves are one of the vegetative organs of plants that are essential for plant growth and development. PIN-FORMED (PINs) gene is an indoleacetic acid (IAA) transporter that plays a critical role in leaf development. To determine the function of BpPIN3 in leaf polarity formation in Betula pendula, the transgenic lines with BpPIN3 overexpression (OE) and BpPIN3-reduced expression (RE) were analyzed using the Agrobacterium-mediated method. The RE lines displayed the characteristics of leaf margin adaxial upward curling, with lower expression of BpPIN3 resulting in greater rolling. Tissue localization of IAA in the auxin GUS reporter system proved that auxin in the RE was mainly distributed in the secondary veins, palisade tissues, and epidermal cells in the leaf margin area. The auxin content in the leaf margin area was significantly greater than that in the main vein tissue. The cell density of the palisade tissue and the ratio of palisade tissue to spongy tissue in the curled leaf margin of the RE lines were found to be significantly decreased. RNA-seq analysis revealed that the RE hormone-signaling pathway genes were significantly enriched compared with those of the OE and WT lines; in particular, the auxin response-related genes SAURs (i.e., SAUR23, SAUR24, SAUR28, and SAUR50) and GH3.10 were found to be significantly upregulated. qRT-PCR analysis indicated that BpPIN3 expression at the leaf margin was significantly lower than that near the main vein in the RE lines. In contrast, the expression levels of SAURs and GH3.10 were significantly higher than those near the midrib. In conclusion, BpPIN3 regulates the expression of auxin response-related genes and the polar transport of auxin to change the polar form of the proximal and distal axes of birch leaves.

AtELP4 a subunit of the Elongator complex in Arabidopsis, mediates cell proliferation and dorsoventral polarity during leaf morphogenesis

The Elongator complex in eukaryotes has conserved tRNA modification functions and contributes to various physiological processes such as transcriptional control, DNA replication and repair, and chromatin accessibility. ARABIDOPSIS ELONGATOR PROTEIN 4 (AtELP4) is one of the six subunits (AtELP1-AtELP6) in Arabidopsis Elongator. In addition, there is an Elongator-associated protein, DEFORMED ROOTS AND LEAVES 1 (DRL1), whose homolog in yeast (Kti12) binds tRNAs. In this study, we explored the functions of AtELP4 in plant-specific aspects such as leaf morphogenesis and evolutionarily conserved ones between yeast and Arabidopsis. ELP4 comparison between yeast and Arabidopsis revealed that plant ELP4 possesses not only a highly conserved P-loop ATPase domain but also unknown plant-specific motifs. ELP4 function is partially conserved between Arabidopsis and yeast in the growth sensitivity toward caffeine and elevated cultivation temperature. Either single Atelp4 or drl1-102 mutants and double Atelp4 drl1-102 mutants exhibited a reduction in cell proliferation and changed the adaxial-abaxial polarity of leaves. In addition, the single Atelp4 and double Atelp4 drl1-102 mutants showed remarkable downward curling at the whole part of leaf blades in contrast to wild-type leaf blades. Furthermore, our genetic study revealed that AtELP4 might epistatically act on DRL1 in the regulation of cell proliferation and dorsoventral polarity in leaves. Taken together, we suggest that AtELP4 as part of the plant Elongator complex may act upstream of a regulatory pathway for adaxial-abaxial polarity and cell proliferation during leaf development.

Bioenergy sorghum stem growth regulation: intercalary meristem localization, development, and gene regulatory network analysis

Bioenergy sorghum is a highly productive drought tolerant C₄ grass that accumulates 80% of its harvestable biomass in approximately 4 m length stems. Stem internode growth is regulated by development, shading, and hormones that modulate cell proliferation in intercalary meristems (IMs). In this study, sorghum stem IMs were localized above the pulvinus at the base of elongating internodes using magnetic resonance imaging, microscopy, and transcriptome analysis. A change in cell morphology/organization occurred at the junction between the pulvinus and internode where LATERAL ORGAN BOUNDARIES (SbLOB), a boundary layer gene, was expressed. Inactivation of an AGCVIII kinase in DDYM (dw2) resulted in decreased SbLOB expression, disrupted IM localization, and reduced internode cell proliferation. Transcriptome analysis identified approximately 1000 genes involved in cell proliferation, hormone signaling, and other functions selectively upregulated in the IM compared with a non-meristematic stem tissue. This cohort of genes is expressed in apical dome stem tissues before localization of the IM at the base of elongating internodes. Gene regulatory network analysis identified connections between genes involved in hormone signaling and cell proliferation. The results indicate that gibberellic acid induces accumulation of growth regulatory factors (GRFs) known to interact with ANGUSTIFOLIA (SbAN3), a master regulator of cell proliferation. GRF:AN3 was predicted to induce SbARF3/ETT expression and regulate SbAN3 expression in an auxin-dependent manner. GRFs and ARFs regulate genes involved in cytokinin and brassinosteroid signaling and cell proliferation. The results provide a molecular framework for understanding how hormone signaling regulates the expression of genes involved in cell proliferation in the stem IM.

The genomic basis of the plant island syndrome in Darwin's giant daisies

The repeated, rapid and often pronounced patterns of evolutionary divergence observed in insular plants, or the ‘plant island syndrome’, include changes in leaf phenotypes, growth, as well as the acquisition of a perennial lifestyle. Here, we sequence and describe the genome of the critically endangered, Galápagos-endemic species Scalesia atractyloides Arnot., obtaining a chromosome-resolved, 3.2-Gbp assembly containing 43,093 candidate gene models. Using a combination of fossil transposable elements, k-mer spectra analyses and orthologue assignment, we identify the two ancestral genomes, and date their divergence and the polyploidization event, concluding that the ancestor of all extant Scalesia species was an allotetraploid. There are a comparable number of genes and transposable elements across the two subgenomes, and while their synteny has been mostly conserved, we find multiple inversions that may have facilitated adaptation. We identify clear signatures of selection across genes associated with vascular development, growth, adaptation to salinity and flowering time, thus finding compelling evidence for a genomic basis of the island syndrome in one of Darwin’s giant daisies.

Many island plant species share a syndrome of characteristic phenotype and life history. Cerca et al. find the genomic basis of the plant island syndrome in one of Darwin’s giant daisies, while separating ancestral genomes in a chromosome-resolved polyploid assembly.

Arabidopsis ASYMMETRIC LEAVES2 (AS2): roles in plant morphogenesis, cell division, and pathogenesis

The ASYMMETRIC LEAVES2 (AS2) gene in Arabidopsis thaliana is responsible for the development of flat, symmetric, and extended leaf laminae and their vein systems. AS2 protein is a member of the plant-specific AS2/LOB protein family, which includes 42 members comprising the conserved amino-terminal domain referred to as the AS2/LOB domain, and the variable carboxyl-terminal region. Among the members, AS2 has been most intensively investigated on both genetic and molecular levels. AS2 forms a complex with the myb protein AS1, and is involved in epigenetic repression of the abaxial genes ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3), ARF4, and class 1 KNOX homeobox genes. The repressed expression of these genes by AS2 is markedly enhanced by the cooperative action of various modifier genes, some of which encode nucleolar proteins. Further downstream, progression of the cell division cycle in the developing organs is stimulated; meristematic states are suppressed in determinate leaf primordia; and the extension of leaf primordia is induced. AS2 binds the specific sequence in exon 1 of ETT/ARF3 and maintains methylated CpGs in several exons of ETT/ARF3. AS2 forms bodies (designated as AS2 bodies) at nucleolar peripheries. AS2 bodies partially overlap chromocenters, including inactive 45S ribosomal DNA repeats, suggesting the presence of molecular and functional links among AS2, the 45S rDNAs, and the nucleolus to exert the repressive regulation of ETT/ARF3. The AS2/LOB domain is characterized by three subdomains, the zinc finger (ZF) motif, the internally conserved-glycine containing (ICG) region, and the leucine-zipper-like (LZL) region. Each of these subdomains is essential for the formation of AS2 bodies. ICG to LZL are required for nuclear localization, but ZF is not. LZL intrinsically has the potential to be exported to the cytoplasm. In addition to its nuclear function, it has been reported that AS2 plays a positive role in geminivirus infection: its protein BV1 stimulates the expression of AS2 and recruits AS2 to the cytoplasm, which enhances virus infectivity by suppression of cytoplasmic post transcriptional gene silencing.

PtKTI12 genes influence wobble uridine modifications and drought stress tolerance in hybrid poplar

The multisubunit Elongator complex plays key roles in transcription by interacting with RNA polymerase II and chromatin modeling. Kti proteins have been identified as the auxiliary protein for the Elongator complex. However, our knowledge of Kti proteins in woody plants remains limited. In this study, in total 16 KTI gene homologs were identified in Populus trichocarpa. Among them, the two KTI12 candidates were named PtKTI12A and PtKTI12B. Although PtKTI12A and PtKTI12B were largely different in gene expression level and tissue specificity, both genes were induced by heat and drought stresses. PtKTI12A and PtKTI12B RNAi transgenic poplar plants showed reduced levels of modified nucleosides, in particular 5-carbamoylmethyluridine and 5-methoxycarbonylmethyl-2-thiouridine. Meanwhile, their tolerance to drought was improved when subjected to withdrawal of watering. Also, the protein products of PtKTI12A and PtKTI12B had similar subcellular localization and predicted tertiary structure. The results suggest that Kti12 proteins are involved in tRNA wobble uridine modification, stress response and drought stress tolerance in hybrid poplar.

Patterning a Leaf by Establishing Polarities

Leaves are the major organ for photosynthesis in most land plants, and leaf structure is optimized for the maximum capture of sunlight and gas exchange. Three polarity axes, the adaxial-abaxial axis, the proximal-distal axis, and the medial-lateral axis are established during leaf development to give rise to a flattened lamina with a large area for photosynthesis and blades that are extended on petioles for maximum sunlight. Adaxial cells are elongated, tightly packed cells with many chloroplasts, and their fate is specified by HD-ZIP III and related factors. Abaxial cells are rounder and loosely packed cells and their fate is established and maintained by YABBY family and KANADI family proteins. The activities of adaxial and abaxial regulators are coordinated by ASYMMETRIC LEAVES2 and auxin. Establishment of the proximodistal axis involves the BTB/POZ domain proteins BLADE-ON-PETIOLE1 and 2, whereas homeobox genes PRESSED FLOWER and WUSCHEL-RELATED HOMEOBOX1 mediate leaf development along the mediolateral axis. This review summarizes recent advances in leaf polarity establishment with a focus on the regulatory networks involved.

CURLED LATER1 encoding the largest subunit of the Elongator complex has a unique role in leaf development and meristem function in rice

The Elongator complex, which is conserved in eukaryotes, has multiple roles in diverse organisms. In Arabidopsis thaliana, Elongator is shown to be involved in development, hormone action and environmental responses. However, except for Arabidopsis, our knowledge of its function is poor in plants. In this study, we initially carried out a genetic analysis to characterize a rice mutant with narrow and curled leaves, termed curled later1 (cur1). The cur1 mutant displayed a heteroblastic change, whereby the mutant leaf phenotype appeared specifically at a later adult phase of vegetative development. The shoot apical meristem (SAM) was small and the leaf initiation rate was low, suggesting that the activity of the SAM seemed to be partially reduced in cur1. We then revealed that CUR1 encodes a yeast ELP1-like protein, the largest subunit of Elongator. Furthermore, disruption of OsELP3 encoding the catalytic subunit of Elongator resulted in phenotypes similar to those of cur1, including the timing of the appearance of mutant phenotypes. Thus, Elongator activity seems to be specifically required for leaf development at the late vegetative phase. Transcriptome analysis showed that genes involved in protein quality control were highly upregulated in the cur1 shoot apex at the later vegetative phase, suggesting the restoration of impaired proteins probably produced by partial defects in translational control due to the loss of function of Elongator. The differences in the mutant phenotype and gene expression profile between CUR1 and its Arabidopsis ortholog suggest that Elongator has evolved to play a unique role in rice development.

Plant Elongator-Protein Complex of Diverse Activities Regulates Growth, Development, and Immune Responses

Contrary to the conserved Elongator composition in yeast, animals, and plants, molecular functions and catalytic activities of the complex remain controversial. Elongator was identified as a component of elongating RNA polymerase II holoenzyme in yeast, animals, and plants. Furthermore, it was suggested that Elonagtor facilitates elongation of transcription via histone acetyl transferase activity. Accordingly, phenotypes of Arabidopsis elo mutants, which show development, growth, or immune response defects, correlate with transcriptional downregulation and the decreased histone acetylation in the coding regions of crucial genes. Plant Elongator was also implicated in other processes: transcription and processing of miRNA, regulation of DNA replication by histone acetylation, and acetylation of alpha-tubulin. Moreover, tRNA modification, discovered first in yeast and confirmed in plants, was claimed as the main activity of Elongator, leading to specificity in translation that might also result indirectly in a deficiency in transcription. Heterologous overexpression of individual Arabidopsis Elongator subunits and their respective phenotypes suggest that single Elongator subunits might also have another function next to being a part of the complex. In this review, we shall present the experimental evidence of all molecular mechanisms and catalytic activities performed by Elongator in nucleus and cytoplasm of plant cells, which might explain how Elongator regulates growth, development, and immune responses.

A G(enomic)P(ositioning)S(ystem) for Plant RNAPII Transcription

Post-translational modifications (PTMs) of histone residues shape the landscape of gene expression by modulating the dynamic process of RNA polymerase II (RNAPII) transcription. The contribution of particular histone modifications to the definition of distinct RNAPII transcription stages remains poorly characterized in plants. Chromatin immunoprecipitation combined with next-generation sequencing (ChIP-seq) resolves the genomic distribution of histone modifications. Here, we review histone PTM ChIP-seq data in Arabidopsis thaliana and find support for a Genomic Positioning System (GPS) that guides RNAPII transcription. We review the roles of histone PTM 'readers', 'writers', and 'erasers', with a focus on the regulation of gene expression and biological functions in plants. The distinct functions of RNAPII transcription during the plant transcription cycle may rely, in part, on the characteristic histone PTM profiles that distinguish transcription stages.

Genotyping by RAD Sequencing Analysis Assessed the Genetic Distinctiveness of Experimental Lines and Narrowed Down the Genomic Region Responsible for Leaf Shape in Endive (Cichorium endivia L.)

The characterization of genetic diversity in elite breeding stocks is crucial for the registration and protection of new varieties. Moreover, experimental population structure analysis and information about the genetic distinctiveness of commercial materials are essential for crop breeding programs. The purpose of our research was to assess the genetic relationships of 32 endive (Cichorium endivia L.) breeding lines, 18 from var. latifolium (escarole) and 14 from var. crispum (curly), using heterologous Cichorium intybus-derived simple sequence repeats (SSR) markers and single-nucleotide polymorphisms (SNP) markers. We found that 14 out of 29 SSR markers were successfully amplified, but only 8 of them were related to polymorphic loci. To overcome the limitation of the low number of informative SSR marker loci, an alternative SNP-based approach was employed. The 4621 SNPs produced by a restriction site-associated DNA marker sequencing approach were able to fully discriminate the 32 endive accessions; most importantly, as many as 50 marker loci were found to distinguish the curly group from the escarole group. Interestingly, 24 of the marker loci mapped within a peripheral segment of chromosome 8 of lettuce (Lactuca sativa L.), spanning a chromosomal region of 49.6 Mb. Following Sanger sequencing-based validation, three genes were determined to carry nonsynonymous SNPs, and one of them matched a putative ortholog of AtELP1, subunit 1 of the Elongator complex. Considering that several previously characterized Elongator complex subunit mutants exhibited elongated and/or curly leaf phenotypes, this gene should be taken into consideration for a better understanding of the underlying mechanism controlling leaf shape in endive.

The formation of perinucleolar bodies is important for normal leaf development and requires the zinc-finger DNA-binding motif in Arabidopsis ASYMMETRIC LEAVES2

In Arabidopsis, the ASYMMETRIC LEAVES2 (AS2) protein plays a key role in the formation of flat symmetric leaves via direct repression of the abaxial gene ETT/ARF3. AS2 encodes a plant-specific nuclear protein that contains the AS2/LOB domain, which includes a zinc-finger (ZF) motif that is conserved in the AS2/LOB family. We have shown that AS2 binds to the coding DNA of ETT/ARF3, which requires the ZF motif. AS2 is co-localized with AS1 in perinucleolar bodies (AS2 bodies). To identify the amino acid signals in AS2 required for formation of AS2 bodies and function(s) in leaf formation, we constructed recombinant DNAs that encoded mutant AS2 proteins fused to yellow fluorescent protein. We examined the subcellular localization of these proteins in cells of cotyledons and leaf primordia of transgenic plants and cultured cells. The amino acid signals essential for formation of AS2 bodies were located within and adjacent to the ZF motif. Mutant AS2 that failed to form AS2 bodies also failed to rescue the as2-1 mutation. Our results suggest the importance of the formation of AS2 bodies and the nature of interactions of AS2 with its target DNA and nucleolar factors including NUCLEOLIN1. The partial overlap of AS2 bodies with perinucleolar chromocenters with condensed ribosomal RNA genes implies a correlation between AS2 bodies and the chromatin state. Patterns of AS2 bodies in cells during interphase and mitosis in leaf primordia were distinct from those in cultured cells, suggesting that the formation and distribution of AS2 bodies are developmentally modulated in plants.

SIMON: Simple methods for analyzing DNA methylation by targeted bisulfite next-generation sequencing

DNA methylation in higher organisms has become an expanding field of study as it often involves the regulation of gene expression. Although Whole Genome Bisulfite Sequencing (WG-BS) based on next-generation sequencing (NGS) is the most versatile method, this is a costly technique that lacks in-depth analytic power. There are no conventional methods based on NGS that enable researchers to easily compare the level of DNA methylation from the practical number of samples handled in the laboratory. Although the targeted BS method based on Sanger sequencing is generally used in this case, it lacks in-depth analytic power. Therefore, we propose a new method that combines the high throughput analytic power of NGS and bioinformatics with the specificity and focus offered by PCR-amplification-based bisulfite sequencing methods. We use in silico size sieving of DNA-fragments and primer matchings instead of whole-fragment alignment in our bioinformatics analyses, and named our method SIMON (Simple Inference for Methylome based On NGS). The results of our targeted BS method based on NGS (SIMON method) show that small variations in DNA methylation patterns can be precisely and efficiently measured at a single nucleotide resolution. SIMON method combines pre-existing techniques to provide a cost-effective technique for in-depth studies that focus on pre-identified loci. It offers significant improvements with regard to workflow and the quality of the acquired DNA methylation information. Because of the high accuracy of the analysis, small variations of DNA methylation levels can be precisely determined even with large numbers of samples and loci.

tRNA Wobble Modification Affects Leaf Cell Development in Arabidopsis thaliana

The tRNA modification at the wobble position of Lys, Glu and Gln (wobbleU* modification) is responsible for the fine-tuning of protein translation efficiency and translation rate. This modification influences organism function in accordance with growth and environmental changes. However, the effects of wobbleU* modification at the cellular, tissue, or individual level have not yet been elucidated. In this study, we show that sulfur modification of wobbleU* of the tRNAs affects leaf development in Arabidopsis thaliana. The sulfur modification was impaired in the two wobbleU*-modification mutants: the URM1-like protein-defective mutant and the Elongator complex-defective mutants. Analyses of the mutant phenotypes revealed that the deficiency in the wobbleU* modification increased the airspaces in the leaves and the leaf size without affecting the number and the area of palisade mesophyll cells. On the other hand, both mutants exhibited increased number of leaf epidermal pavement cells but with reduced cell size. The deficiency in the wobbleU* modification also delayed the initiation of the endoreduplication processes of mesophyll cells. The phenotype of ASYMMETRIC LEAVES2-defective mutant was enhanced in the Elongator-defective mutants, while it was unchanged in the URM1-like protein-defective mutant. Collectively, the findings of this study suggest that the tRNA wobbleU* modification plays an important role in leaf morphogenesis by balancing the development between epidermal and mesophyll tissues.

Arabidopsis Zinc-Finger-Like Protein ASYMMETRIC LEAVES2 (AS2) and Two Nucleolar Proteins Maintain Gene Body DNA Methylation in the Leaf Polarity Gene ETTIN (ARF3)

Arabidopsis ASYMMETRIC LEAVES2 (AS2) plays a critical role in leaf adaxial-abaxial partitioning by repressing expression of the abaxial-determining gene ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3). We previously reported that six CpG dinucleotides in its exon 6 are thoroughly methylated by METHYLTRASFERASE1, that CpG methylation levels are inversely correlated with ETT/ARF3 transcript levels and that methylation levels at three out of the six CpG dinucleotides are decreased in as2-1. All these imply that AS2 is involved in epigenetic repression of ETT/ARF3 by gene body DNA methylation. The mechanism of the epigenetic repression by AS2, however, is unknown. Here, we tested mutations of NUCLEOLIN1 (NUC1) and RNA HELICASE10 (RH10) encoding nucleolus-localized proteins for the methylation in exon 6 as these mutations enhance the level of ETT/ARF3 transcripts in as2-1. Methylation levels at three specific CpGs were decreased in rh10-1, and two of those three overlapped with those in as2-1. Methylation levels at two specific CpGs were decreased in nuc1-1, and one of those three overlapped with that in as2-1. No site was affected by both rh10-1 and nuc1-1. One specific CpG was unaffected by these mutations. These results imply that the way in which RH10, NUC1 and AS2 are involved in maintaining methylation at five CpGs in exon 6 might be through at least several independent pathways, which might interact with each other. Furthermore, we found that AS2 binds specifically the sequence containing CpGs in exon 1 of ETT/ARF3, and that the binding requires the zinc-finger-like motif in AS2 that is structurally similar to the zinc finger-CxxC domain in vertebrate DNA methyltransferase1.

A genetic link between epigenetic repressor AS1-AS2 and DNA replication factors in establishment of adaxial-abaxial leaf polarity of Arabidopsis

Balanced development of adaxial and abaxial domains in leaf primordia is critical for the formation of flat symmetric leaf lamina. Arabidopsis ASYMMETRIC LEAVES1 (AS1) and AS2 proteins form a complex (AS1-AS2), which acts as key regulators for the adaxial development by the direct repression of expression of the abaxial gene ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3). Many modifier mutations have been identified, which enhance the defect of as1 and as2 mutations to generate abaxialized filamentous leaves without adaxial traits, suggesting that the development of the adaxial domain is achieved by cooperative repression by AS1-AS2 and the wild-type proteins corresponding to the modifiers. Mutations of several genes for DNA replication-related chromatin remodeling factors such as Chromatin Assembly Factor-1 (CAF-1) have been also identified as modifiers. It is still unknown, however, whether mutations in genes involved in DNA replication themselves might act as modifiers. Here we report that as1 and as2 mutants grown in the presence of hydroxyurea, a known inhibitor of DNA replication, form abaxialized filamentous leaves in a concentration-dependent manner. We further show that a mutation of the INCURVATA2 (ICU2) gene, which encodes the putative catalytic subunit of DNA polymerase α, and a mutation of the Replication Factor C Subunit3 (RFC3) gene, which encodes a protein used in replication as a clamp loader, act as modifiers. In addition, as2-1 icu2-1 double mutants showed increased mRNA levels of the genes for leaf abaxialization. These results suggest a tight link between DNA replication and the function of AS1-AS2 in the development of flat leaves.

Model for the role of auxin polar transport in patterning of the leaf adaxial-abaxial axis

Leaf adaxial-abaxial polarity refers to the two leaf faces, which have different types of cells performing distinct biological functions. In 1951, Ian Sussex reported that when an incipient leaf primordium was surgically isolated by an incision across the vegetative shoot apical meristem (SAM), a radialized structure without an adaxial domain would form. This led to the proposal that a signal, now called the Sussex signal, is transported from the SAM to emerging primordia to direct leaf adaxial-abaxial patterning. It was recently proposed that instead of the Sussex signal, polar transport of the plant hormone auxin is critical in leaf polarity formation. However, how auxin polar transport functions in the process is unknown. Through live imaging, we established a profile of auxin polar transport in and around young leaf primordia. Here we show that auxin polar transport in lateral regions of an incipient primordium forms auxin convergence points. We demonstrated that blocking auxin polar transport in the lateral regions of the incipient primordium by incisions abolished the auxin convergence points and caused abaxialized leaves to form. The lateral incisions also blocked the formation of leaf middle domain and margins and disrupted expression of the middle domain/margin-associated marker gene WUSCHEL-RELATED HOMEOBOX 1 (SlWOX1). Based on these results we propose that the auxin convergence points are required for the formation of leaf middle domain and margins, and the functional middle domain and margins ensure leaf adaxial-abaxial polarity. How middle domain and margins function in the process is discussed.

Genome-Wide Identification of Histone Modifiers and Their Expression Patterns during Fruit Abscission in Litchi

Modifications to histones, including acetylation and methylation processes, play crucial roles in the regulation of gene expression in plant development as well as in stress responses. However, limited information on the enzymes catalyzing histone acetylation and methylation in non-model plants is currently available. In this study, several histone modifier (HM) types, including six histone acetyltransferases (HATs), 11 histone deacetylases (HDACs), 48 histone methyltransferases (HMTs), and 22 histone demethylases (HDMs), are identified in litchi (Litchi chinensis Sonn. cv. Feizixiao) based on similarities in their sequences to homologs in Arabidopsis (A. thaliana), tomato (Solanum lycopersicum), and rice (Oryza sativa). Phylogenetic analyses reveal that HM enzymes can be grouped into four HAT, two HDAC, two HMT, and two HDM subfamilies, respectively, while further expression profile analyses demonstrate that 17 HMs were significantly altered during fruit abscission in two field treatments. Analyses reveal that these genes exhibit four distinct patterns of expression in response to fruit abscission, while an in vitro assay was used to confirm the HDAC activity of LcHDA2, LcHDA6, and LcSRT2. Our findings are the first in-depth analysis of HMs in the litchi genome, and imply that some are likely to play important roles in fruit abscission in this commercially important plant.

AUXIN RESPONSE FACTOR3 Regulates Compound Leaf Patterning by Directly Repressing PALMATE-LIKE PENTAFOLIATA1 Expression in Medicago truncatula

Diverse leaf forms can be seen in nature. In Medicago truncatula, PALM1 encoding a Cys(2)His(2) transcription factor is a key regulator of compound leaf patterning. PALM1 negatively regulates expression of SGL1, a key regulator of lateral leaflet initiation. However, how PALM1 itself is regulated is not yet known. To answer this question, we used promoter sequence analysis, yeast one-hybrid tests, quantitative transcription activity assays, ChIP-PCR analysis, and phenotypic analyses of overexpression lines and mutant plants. The results show that M. truncatula AUXIN RESPONSE FACTOR3 (MtARF3) functions as a direct transcriptional repressor of PALM1. MtARF3 physically binds to the PALM1 promoter sequence in yeast cells. MtARF3 selectively interacts with specific auxin response elements (AuxREs) in the PALM1 promoter to repress reporter gene expression in tobacco leaves and binds to specific sequences in the PALM1 promoter in vivo. Upregulation of MtARF3 or removal of both PHANTASTICA (PHAN) and ARGONAUTE7 (AGO7) pathways resulted in compound leaves with five narrow leaflets arranged in a palmate-like configuration. These results support that MtARF3, in addition as an adaxial-abaxial polarity regulator, functions to restrict spatiotemporal expression of PALM1, linking auxin signaling to compound leaf patterning in the legume plant M. truncatula.

Identification of Phosphoinositide-Binding Protein PATELLIN2 as a Substrate of Arabidopsis MPK4 MAP Kinase during Septum Formation in Cytokinesis

The phosphorylation of proteins by protein kinases controls many cellular and physiological processes, which include intracellular signal transduction. However, the underlying molecular mechanisms of such controls and numerous substrates of protein kinases remain to be characterized. The mitogen-activated protein kinase (MAPK) cascade is of particular importance in a variety of extracellular and intracellular signaling processes. In plant cells, the progression of cytokinesis is an excellent example of an intracellular phenomenon that requires the MAPK cascade. However, the way in which MAPKs control downstream processes during cytokinesis in plant cells remains to be fully determined. We show here that comparisons, by two-dimensional difference gel electrophoresis, of phosphorylated proteins from wild-type Arabidopsis thaliana and mutant plants defective in a MAPK cascade allow identification of substrates of a specific MAPK. Using this method, we identified the PATELLIN2 (PATL2) protein, which has a SEC14 domain, as a substrate of MPK4 MAP kinase. PATL2 was concentrated at the cell division plane, as is MPK4, and had binding affinity for phosphoinositides. This binding affinity was altered after phosphorylation of PATL2 by MPK4, suggesting a role for the MAPK cascade in the formation of cell plates via regeneration of membranes during cytokinesis.

A genetic link between epigenetic repressor AS1-AS2 and a putative small subunit processome in leaf polarity establishment of Arabidopsis

Although the DEAD-box RNA helicase family is ubiquitous in eukaryotes, its developmental role remains unelucidated. Here, we report that cooperative action between the Arabidopsis nucleolar protein RH10, an ortholog of human DEAD-box RNA helicase DDX47, and the epigenetic repressor complex of ASYMMETRIC-LEAVES1 (AS1) and AS2 (AS1-AS2) is critical to repress abaxial (ventral) genes ETT/ARF3 and ARF4, which leads to adaxial (dorsal) development in leaf primordia at shoot apices. Double mutations of rh10-1 and as2 (or as1) synergistically up-regulated the abaxial genes, which generated abaxialized filamentous leaves with loss of the adaxial domain. DDX47 is part of the small subunit processome (SSUP) that mediates rRNA biogenesis. In rh10-1 we found various defects in SSUP-related events, such as: accumulation of 35S/33S rRNA precursors; reduction in the 18S/25S ratio; and nucleolar hypertrophy. Double mutants of as2 with mutations of genes that encode other candidate SSUP-related components such as nucleolin and putative rRNA methyltransferase exhibited similar synergistic defects caused by up-regulation of ETT/ARF3 and ARF4. These results suggest a tight link between putative SSUP and AS1-AS2 in repression of the abaxial-determining genes for cell fate decisions for adaxial development.

Summary: This paper reports the importance of cooperative action between the nucleus-localized epigenetic repressor and the nucleolus-localized proteins involved in ribosomal RNA processing for polarity establishment of Arabidopsis leaves.

Plant Elongator-mediated transcriptional control in a chromatin and epigenetic context

Elongator (Elp) genes were identified in plants by the leaf growth-altering elo mutations in the yeast (Saccharomyces cerevisiae) gene homologs. Protein purification of the Elongator complex from Arabidopsis thaliana cell cultures confirmed its conserved structure and composition. The Elongator function in plant growth, development, and immune response is well-documented in the elp/elo mutants and correlated with the histone acetyl transferase activity of the ELP3/ELO3 subunit at the coding part of key regulatory genes of developmental and immune response pathways. Here we will focus on additional roles in transcription, such as the cytosine demethylation activity of ELP3/ELO3 at gene promoter regions and primary microRNA transcription and processing through the ELP2 subunit interaction with components of the small interference RNA machinery. Furthermore, specific interactions and upstream regulators support a role for Elongator in transcription and might reveal mechanistic insights into the specificity of the histone acetyl transferase and cytosine demethylation activities for target genes.

Behavior of Leaf Meristems and Their Modification

A major source of diversity in flowering plant form is the extensive variability of leaf shape and size. Leaf formation is initiated by recruitment of a handful of cells flanking the shoot apical meristem (SAM) to develop into a complex three-dimensional structure. Leaf organogenesis depends on activities of several distinct meristems that are established and spatiotemporally differentiated after the initiation of leaf primordia. Here, we review recent findings in the gene regulatory networks that orchestrate leaf meristem activities in a model plant Arabidopsis thaliana. We then discuss recent key studies investigating the natural variation in leaf morphology to understand how the gene regulatory networks modulate leaf meristems to yield a substantial diversity of leaf forms during the course of evolution.

The complex of ASYMMETRIC LEAVES (AS) proteins plays a central role in antagonistic interactions of genes for leaf polarity specification in Arabidopsis

Leaf primordia are born around meristem‐containing stem cells at shoot apices, grow along three axes (proximal–distal, adaxial–abaxial, medial–lateral), and develop into flat symmetric leaves with adaxial–abaxial polarity. Axis development and polarity specification of Arabidopsis leaves require a network of genes for transcription factor‐like proteins and small RNAs. Here, we summarize present understandings of adaxial‐specific genes, ASYMMETRIC LEAVES1 (AS1) and AS2. Their complex (AS1–AS2) functions in the regulation of the proximal–distal leaf length by directly repressing class 1 KNOX homeobox genes (BP, KNAT2) that are expressed in the meristem periphery below leaf primordia. Adaxial–abaxial polarity specification involves antagonistic interaction of adaxial and abaxial genes including AS1 and AS2 for the development of two respective domains. AS1–AS2 directly represses the abaxial gene ETTIN/AUXIN RESPONSE FACTOR3 (ETT/ARF3) and indirectly represses ETT/ARF3 and ARF4 through tasiR‐ARF. Modifier mutations have been identified that abolish adaxialization and enhance the defect in the proximal–distal patterning in as1 and as2. AS1–AS2 and its modifiers synergistically repress both ARFs and class 1 KNOXs. Repression of ARFs is critical for establishing adaxial–abaxial polarity. On the other hand, abaxial factors KANADI1 (KAN1) and KAN2 directly repress AS2 expression. These data delineate a molecular framework for antagonistic gene interactions among adaxial factors, AS1, AS2, and their modifiers, and the abaxial factors ARFs as key regulators in the establishment of adaxial–abaxial polarity. Possible AS1–AS2 epigenetic repression and activities downstream of ARFs are discussed. WIREs Dev Biol 2015, 4:655–671. doi: 10.1002/wdev.196

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Comparative analysis of the conserved functions of Arabidopsis DRL1 and yeast KTI12

Patterning of the polar axis during the early leaf developmental stage is established by cell-to-cell communication between the shoot apical meristem (SAM) and the leaf primordia. In a previous study, we showed that the DRL1 gene, which encodes a homolog of the Elongator-associated protein KTI12 of yeast, acts as a positive regulator of adaxial leaf patterning and shoot meristem activity. To determine the evolutionally conserved functions of DRL1, we performed a comparison of the deduced amino acid sequence of DRL1 and its yeast homolog, KTI12, and found that while overall homology was low, well-conserved domains were presented. DRL1 contained two conserved plant-specific domains. Expression of the DRL1 gene in a yeast KTI12-deficient yeast mutant suppressed the growth retardation phenotype, but did not rescue the caffeine sensitivity, indicating that the role of Arabidopsis Elongator-associated protein is partially conserved with yeast KTI12, but may have changed between yeast and plants in response to caffeine during the course of evolution. In addition, elevated expression of DRL1 gene triggered zymocin sensitivity, while overexpression of KTI12 maintained zymocin resistance, indicating that the function of Arabidopsis DRL1 may not overlap with yeast KTI12 with regards to toxin sensitivity. In this study, expression analysis showed that class-I KNOX genes were downregulated in the shoot apex, and that YAB and KAN were upregulated in leaves of the Arabidopsis drl1-101 mutant. Our results provide insight into the communication network between the SAM and leaf primordia required for the establishment of leaf polarity by mediating histone acetylation or through other mechanisms.

Elongator and its epigenetic role in plant development and responses to abiotic and biotic stresses

Elongator, a six-subunit protein complex, was initially isolated as an interactor of hyperphosphorylated RNA polymerase II in yeast, and was subsequently identified in animals and plants. Elongator has been implicated in multiple cellular activities or biological processes including tRNA modification, histone modification, DNA demethylation or methylation, tubulin acetylation, and exocytosis. Studies in the model plant Arabidopsis thaliana suggest that the structure of Elongator and its functions are highly conserved between plants and yeast. Disruption of the Elongator complex in plants leads to aberrant growth and development, resistance to abiotic stresses, and susceptibility to plant pathogens. The morphological and physiological phenotypes of Arabidopsis Elongator mutants are associated with decreased histone acetylation and/or altered DNA methylation. This review summarizes recent findings related to the epigenetic function of Elongator in plant development and responses to abiotic and biotic stresses.

Protein encoded by oncogene 6b from Agrobacterium tumefaciens has a reprogramming potential and histone chaperone-like activity

Crown gall tumors are formed mainly by actions of a group of genes in the T-DNA that is transferred from Agrobacterium tumefaciens and integrated into the nuclear DNA of host plants. These genes encode enzymes for biosynthesis of auxin and cytokinin in plant cells. Gene 6b in the T-DNA affects tumor morphology and this gene alone is able to induce small tumors on certain plant species. In addition, unorganized calli are induced from leaf disks of tobacco that are incubated on phytohormone-free media; shooty teratomas, and morphologically abnormal plants, which might be due to enhanced competence of cell division and meristematic states, are regenerated from the calli. Thus, the 6b gene appears to stimulate a reprogramming process in plants. To uncover mechanisms behind this process, various approaches including the yeast-two-hybrid system have been exploited and histone H3 was identified as one of the proteins that interact with 6b. It has been also demonstrated that 6b acts as a histone H3 chaperon in vitro and affects the expression of various genes related to cell division competence and the maintenance of meristematic states. We discuss current views on a role of 6b protein in tumorigenesis and reprogramming in plants.

Analysis of gene expression profiles of soft tissue sarcoma using a combination of knowledge-based filtering with integration of multiple statistics

The diagnosis and treatment of soft tissue sarcomas (STS) have been difficult. Of the diverse histological subtypes, undifferentiated pleomorphic sarcoma (UPS) is particularly difficult to diagnose accurately, and its classification per se is still controversial. Recent advances in genomic technologies provide an excellent way to address such problems. However, it is often difficult, if not impossible, to identify definitive disease-associated genes using genome-wide analysis alone, primarily because of multiple testing problems. In the present study, we analyzed microarray data from 88 STS patients using a combination method that used knowledge-based filtering and a simulation based on the integration of multiple statistics to reduce multiple testing problems. We identified 25 genes, including hypoxia-related genes (e.g., MIF, SCD1, P4HA1, ENO1, and STAT1) and cell cycle- and DNA repair-related genes (e.g., TACC3, PRDX1, PRKDC, and H2AFY). These genes showed significant differential expression among histological subtypes, including UPS, and showed associations with overall survival. STAT1 showed a strong association with overall survival in UPS patients (logrank p = 1.84 × 10(-6) and adjusted p value 2.99 × 10(-3) after the permutation test). According to the literature, the 25 genes selected are useful not only as markers of differential diagnosis but also as prognostic/predictive markers and/or therapeutic targets for STS. Our combination method can identify genes that are potential prognostic/predictive factors and/or therapeutic targets in STS and possibly in other cancers. These disease-associated genes deserve further preclinical and clinical validation.

Application of a combination of a knowledge-based algorithm and 2-stage screening to hypothesis-free genomic data on irinotecan-treated patients for identification of a candidate single nucleotide polymorphism related to an adverse effect

Interindividual variation in a drug response among patients is known to cause serious problems in medicine. Genomic information has been proposed as the basis for “personalized” health care. The genome-wide association study (GWAS) is a powerful technique for examining single nucleotide polymorphisms (SNPs) and their relationship with drug response variation; however, when using only GWAS, it often happens that no useful SNPs are identified due to multiple testing problems. Therefore, in a previous study, we proposed a combined method consisting of a knowledge-based algorithm, 2 stages of screening, and a permutation test for identifying SNPs. In the present study, we applied this method to a pharmacogenomics study where 109,365 SNPs were genotyped using Illumina Human-1 BeadChip in 168 cancer patients treated with irinotecan chemotherapy. We identified the SNP rs9351963 in potassium voltage-gated channel subfamily KQT member 5 (KCNQ5) as a candidate factor related to incidence of irinotecan-induced diarrhea. The p value for rs9351963 was 3.31×10⁻⁵ in Fisher's exact test and 0.0289 in the permutation test (when multiple testing problems were corrected). Additionally, rs9351963 was clearly superior to the clinical parameters and the model involving rs9351963 showed sensitivity of 77.8% and specificity of 57.6% in the evaluation by means of logistic regression. Recent studies showed that KCNQ4 and KCNQ5 genes encode members of the M channel expressed in gastrointestinal smooth muscle and suggested that these genes are associated with irritable bowel syndrome and similar peristalsis diseases. These results suggest that rs9351963 in KCNQ5 is a possible predictive factor of incidence of diarrhea in cancer patients treated with irinotecan chemotherapy and for selecting chemotherapy regimens, such as irinotecan alone or a combination of irinotecan with a KCNQ5 opener. Nonetheless, clinical importance of rs9351963 should be further elucidated.

The cell-cycle interactome: a source of growth regulators?

When plants develop, cell proliferation and cell expansion are tightly controlled in order to generate organs with a determinate final size such as leaves. Several studies have demonstrated the importance of the cell proliferation phase for leaf growth, illustrating that cell-cycle regulation is crucial for correct leaf development. A large and complex set of interacting proteins that constitute the cell-cycle interactome controls the transition from one cell-cycle phase to another. Here, we review the current knowledge on cell-cycle regulators from this interactome affecting final leaf size when their expression is altered, mainly in Arabidopsis. In addition to the description of mutants of CYCLIN-DEPENDENT KINASES (CDKs), CYCLINS (CYCs), and their transcriptional and post-translational regulators, a phenotypic analysis of gain- and loss-of-function mutants for 27 genes encoding proteins that interact with cell-cycle proteins is presented. This compilation of information shows that when cell-cycle-related genes are mis-expressed, leaf growth is often altered and that, seemingly, three main trends appear to be crucial in the regulation of final organ size by cell-cycle-related genes: (i) cellular compensation; (ii) gene dosage; and (iii) correct transition through the G2/M phase by ANAPHASE PROMOTING COMPLEX/CYCLOSOME (APC/C) activation. In conclusion, this meta-analysis shows that the cell-cycle interactome is enriched in leaf growth regulators, and illustrates the potential to identify new leaf growth regulators among putative new cell-cycle regulators.

Adaxial-abaxial polarity: the developmental basis of leaf shape diversity

Leaves of flowering plants are diverse in shape. Part of this morphological diversity can be attributed to differences in spatiotemporal regulation of polarity in the upper (adaxial) and lower (abaxial) sides of developing leaves. In a leaf primordium, antagonistic interactions between polarity determinants specify the adaxial and abaxial domains in a mutually exclusive manner. The patterning of those domains is critical for leaf morphogenesis. In this review, we first summarize the gene networks regulating adaxial-abaxial polarity in conventional bifacial leaves and then discuss how patterning is modified in different leaf type categories.

Gene expression profile analysis of Ligon lintless-1 (Li1) mutant reveals important genes and pathways in cotton leaf and fiber development

Ligon lintless-1 (Li1) is a monogenic dominant mutant of Gossypium hirsutum (upland cotton) with a phenotype of impaired vegetative growth and short lint fibers. Despite years of research involving genetic mapping and gene expression profile analysis of Li1 mutant ovule tissues, the gene remains uncloned and the underlying pathway of cotton fiber elongation is still unclear. In this study, we report the whole genome-level deep-sequencing analysis of leaf tissues of the Li1 mutant. Differentially expressed genes in leaf tissues of mutant versus wild-type (WT) plants are identified, and the underlying pathways and potential genes that control leaf and fiber development are inferred. The results show that transcription factors AS2, YABBY5, and KANDI-like are significantly differentially expressed in mutant tissues compared with WT ones. Interestingly, several fiber development-related genes are found in the downregulated gene list of the mutant leaf transcriptome. These genes include heat shock protein family, cytoskeleton arrangement, cell wall synthesis, energy, H2O2 metabolism-related genes, and WRKY transcription factors. This finding suggests that the genes are involved in leaf morphology determination and fiber elongation. The expression data are also compared with the previously published microarray data of Li1 ovule tissues. Comparative analysis of the ovule transcriptomes of Li1 and WT reveals that a number of pathways important for fiber elongation are enriched in the downregulated gene list at different fiber development stages (0, 6, 9, 12, 15, 18dpa). Differentially expressed genes identified in both leaf and fiber samples are aligned with cotton whole genome sequences and combined with the genetic fine mapping results to identify a list of candidate genes for Li1.

Macrophage migration inhibitory factor and stearoyl-CoA desaturase 1: potential prognostic markers for soft tissue sarcomas based on bioinformatics analyses

The diagnosis and treatment of soft tissue sarcomas (STSs) has been particularly difficult, because STSs are a group of highly heterogeneous tumors in terms of histopathology, histological grade, and primary site. Recent advances in genome technologies have provided an excellent opportunity to determine the complete biological characteristics of neoplastic tissues, resulting in improved diagnosis, treatment selection, and investigation of therapeutic targets. We had previously developed a novel bioinformatics method for marker gene selection and applied this method to gene expression data from STS patients. This previous analysis revealed that the extracted gene combination of macrophage migration inhibitory factor (MIF) and stearoyl-CoA desaturase 1 (SCD1) is an effective diagnostic marker to discriminate between subtypes of STSs with highly different outcomes. In the present study, we hypothesize that the combination of MIF and SCD1 is also a prognostic marker for the overall outcome of STSs. To prove this hypothesis, we first analyzed microarray data from 88 STS patients and their outcomes. Our results show that the survival rates for MIF- and SCD1-positive groups were lower than those for negative groups, and the p values of the log-rank test are 0.0146 and 0.00606, respectively. In addition, survival rates are more significantly different (p = 0.000116) between groups that are double-positive and double-negative for MIF and SCD1. Furthermore, in vitro cell growth inhibition experiments by MIF and SCD1 inhibitors support the hypothesis. These results suggest that the gene set is useful as a prognostic marker associated with tumor progression.

Identification of a candidate single-nucleotide polymorphism related to chemotherapeutic response through a combination of knowledge-based algorithm and hypothesis-free genomic data

Inter-individual variations in drug responses among patients are known to cause serious problems in medicine. Genome-wide association study (GWAS) is powerful for examining single-nucleotide polymorphisms (SNPs) and their relationships with drug response variations. However, no significant SNP has been identified using GWAS due to multiple testing problems. Therefore, we propose a combination method consisting of knowledge-based algorithm, two stages of screening, and permutation test for identifying SNPs in the present study. We applied this method to a genome-wide pharmacogenomics study for which 109,365 SNPs had been genotyped using Illumina Human-1 BeadChip for 119 gastric cancer patients treated with fluoropyrimidine. We identified rs2293347 in epidermal growth factor receptor (EGFR) is as a candidate SNP related to chemotherapeutic response. The p value for the rs2293347 was 2.19 × 10(-5) for Fisher's exact test, and the p value was 0.00360 for the permutation test (multiple testing problems are corrected). Additionally, rs2293347 was clearly superior to clinical parameters and showed a sensitivity value of 55.0% and specificity value of 94.4% in the evaluation by using multiple regression models. Recent studies have shown that combination chemotherapy of fluoropyrimidine and EGFR-targeting agents is effective for gastric cancer patients highly expressing EGFR. These results suggest that rs2293347 is a potential predictive factor for selecting chemotherapies, such as fluoropyrimidine alone or combination chemotherapies.

Dual regulation of ETTIN (ARF3) gene expression by AS1-AS2, which maintains the DNA methylation level, is involved in stabilization of leaf adaxial-abaxial partitioning in Arabidopsis

Leaf primordia are generated at the periphery of the shoot apex, developing into flat symmetric organs with adaxial-abaxial polarity, in which the indeterminate state is repressed. Despite the crucial role of the ASYMMETRIC LEAVES1 (AS1)-AS2 nuclear-protein complex in leaf adaxial-abaxial polarity specification, information on mechanisms controlling their downstream genes has remained elusive. We systematically analyzed transcripts by microarray and chromatin immunoprecipitation assays and performed genetic rescue of as1 and as2 phenotypic abnormalities, which identified a new target gene, ETTIN (ETT)/AUXIN RESPONSE FACTOR3 (ARF3), which encodes an abaxial factor acting downstream of the AS1-AS2 complex. While the AS1-AS2 complex represses ETT by direct binding of AS1 to the ETT promoter, it also indirectly activates miR390- and RDR6-dependent post-transcriptional gene silencing to negatively regulate both ETT and ARF4 activities. Furthermore, AS1-AS2 maintains the status of DNA methylation in the ETT coding region. In agreement, filamentous leaves formed in as1 and as2 plants treated with a DNA methylation inhibitor were rescued by loss of ETT and ARF4 activities. We suggest that negative transcriptional, post-transcriptional and epigenetic regulation of the ARFs by AS1-AS2 is important for stabilizing early leaf partitioning into abaxial and adaxial domains.

Abaxial Greening Phenotype in Hybrid Aspen

The typical angiosperm leaf, as in Arabidopsis, is bifacial consisting of top (adaxial) and bottom (abaxial) surfaces readily distinguishable by the underlying cell type (palisade and spongy mesophyll, respectively). Species of the genus Populus have leaves that are either conventionally bifacial or isobilateral. Isobilateral leaves have palisade mesophyll on the top and bottom of the leaf, making the two sides virtually indistinguishable at the macroscopic level. In poplars this has been termed the “abaxial greening” phenotype. Previous work has implicated ASYMMETRIC LEAVES1 (AS1) as an essential determinant of palisade mesophyll development. This gene, as well as other genes (84 in all) putatively involved in setting the dorsiventral axis of leaves, were investigated in two Populus species: black cottonwood (Populus trichocarpa) and hybrid aspen (P. tremula x tremuloides), representative of each leaf type (bifacial and isobilateral, respectively). Poplar orthologs of AS1 have significantly higher expression in aspen leaf blade and lower in the petiole, suggestive of a potential role in the isobilateral leaf phenotype consistent with the previously observed phenotypes. Furthermore, an ABERRANT TESTA SHAPE (ATS) ortholog has significantly lower expression in aspen leaf tissue, also suggesting a possible contribution of this gene to abaxial greening.

The Leaf Adaxial-Abaxial Boundary and Lamina Growth

In multicellular organisms, boundaries have a role in preventing the intermingling of two different cell populations and in organizing the morphogenesis of organs and the entire organism. Plant leaves have two different cell populations, the adaxial (or upper) and abaxial (or lower) cell populations, and the boundary is considered to be important for lamina growth. At the boundary between the adaxial and abaxial epidermis, corresponding to the margin, margin-specific structures are developed and structurally separate the adaxial and abaxial epidermis from each other. The adaxial and abaxial cells are determined by the adaxial and abaxial regulatory genes (including transcription factors and small RNAs), respectively. Among many lamina-growth regulators identified by recent genetic analyses, it has been revealed that the phytohormone, auxin, and the WOX family transcription factors act at the adaxial-abaxial boundary downstream of the adaxial-abaxial pattern. Furthermore, mutant analyses of the WOX genes shed light on the role of the adaxial-abaxial boundary in preventing the mixing of the adaxial and abaxial features during lamina growth. In this review, we highlight the recent studies on the dual role of the adaxial-abaxial boundary.

The potential of text mining in data integration and network biology for plant research: a case study on Arabidopsis

Despite the availability of various data repositories for plant research, a wealth of information currently remains hidden within the biomolecular literature. Text mining provides the necessary means to retrieve these data through automated processing of texts. However, only recently has advanced text mining methodology been implemented with sufficient computational power to process texts at a large scale. In this study, we assess the potential of large-scale text mining for plant biology research in general and for network biology in particular using a state-of-the-art text mining system applied to all PubMed abstracts and PubMed Central full texts. We present extensive evaluation of the textual data for Arabidopsis thaliana, assessing the overall accuracy of this new resource for usage in plant network analyses. Furthermore, we combine text mining information with both protein-protein and regulatory interactions from experimental databases. Clusters of tightly connected genes are delineated from the resulting network, illustrating how such an integrative approach is essential to grasp the current knowledge available for Arabidopsis and to uncover gene information through guilt by association. All large-scale data sets, as well as the manually curated textual data, are made publicly available, hereby stimulating the application of text mining data in future plant biology studies.

Meta-analyses of microarrays of Arabidopsis asymmetric leaves1 (as1), as2 and their modifying mutants reveal a critical role for the ETT pathway in stabilization of adaxial-abaxial patterning and cell division during leaf development

It is necessary to use algorithms to analyze gene expression data from DNA microarrays, such as in clustering and machine learning. Previously, we developed the knowledge-based fuzzy adaptive resonance theory (KB-FuzzyART), a clustering algorithm suitable for analyzing gene expression data, to find clues for identifying gene networks. Leaf primordia form around the shoot apical meristem (SAM), which consists of indeterminate stem cells. Upon initiation of leaf development, adaxial-abaxial patterning is crucial for lateral expansion, via cellular proliferation, and the formation of flat symmetric leaves. Many regulatory genes that specify such patterning have been identified. Analysis by the KB-FuzzyART and subsequent molecular and genetic analyses previously showed that ASYMMETRIC LEAVES1 (AS1) and AS2 repress the expression of some abaxial-determinant genes, such as AUXIN RESPONSE FACTOR3 (ARF3)/ETTIN (ETT) and ARF4, which are responsible for defects in leaf adaxial-abaxial polarity in as1 and as2. In the present study, genetic analysis revealed that ARF3/ETT and ARF4 were regulated by modifier genes, BOBBER1 (BOB1) and ELONGATA3 (ELO3), together with AS1-AS2. We analyzed expression arrays with as2 elo3 and as2 bob1, and extracted genes downstream of ARF3/ETT by using KB-FuzzyART and molecular analyses. The results showed that expression of Kip-related protein (KRP) (for inhibitors of cyclin-dependent protein kinases) and Isopentenyltransferase (IPT) (for biosynthesis of cytokinin) genes were controlled by AS1-AS2 through ARF3/ETT and ARF4 functions, which suggests that the AS1-AS2-ETT pathway plays a critical role in controlling the cell division cycle and the biosynthesis of cytokinin around SAM to stabilize leaf development in Arabidopsis thaliana.

Genome-wide analysis of histone modifiers in tomato: gaining an insight into their developmental roles

BACKGROUND

Histone post-translational modifications (HPTMs) including acetylation and methylation have been recognized as playing a crucial role in epigenetic regulation of plant growth and development. Although Solanum lycopersicum is a dicot model plant as well as an important crop, systematic analysis and expression profiling of histone modifier genes (HMs) in tomato are sketchy.

RESULTS

Based on recently released tomato whole-genome sequences, we identified in silico 32 histone acetyltransferases (HATs), 15 histone deacetylases (HDACs), 52 histone methytransferases (HMTs) and 26 histone demethylases (HDMs), and compared them with those detected in Arabidopsis (Arabidopsis thaliana), maize (Zea mays) and rice (Oryza sativa) orthologs. Comprehensive analysis of the protein domain architecture and phylogeny revealed the presence of non-canonical motifs and new domain combinations, thereby suggesting for HATs the existence of a new family in plants. Due to species-specific diversification during evolutionary history tomato has fewer HMs than Arabidopsis. The transcription profiles of HMs within tomato organs revealed a broad functional role for some HMs and a more specific activity for others, suggesting key HM regulators in tomato development. Finally, we explored S. pennellii introgression lines (ILs) and integrated the map position of HMs, their expression profiles and the phenotype of ILs. We thereby proved that the strategy was useful to identify HM candidates involved in carotenoid biosynthesis in tomato fruits.

CONCLUSIONS

In this study, we reveal the structure, phylogeny and spatial expression of members belonging to the classical families of HMs in tomato. We provide a framework for gene discovery and functional investigation of HMs in other Solanaceae species.

How do 'housekeeping' genes control organogenesis?--Unexpected new findings on the role of housekeeping genes in cell and organ differentiation

In recent years, an increasing number of mutations in what would appear to be 'housekeeping genes' have been identified as having unexpectedly specific defects in multicellular organogenesis. This is also the case for organogenesis in seed plants. Although it is not surprising that loss-of-function mutations in 'housekeeping' genes result in lethality or growth retardation, it is surprising when (1) the mutant phenotype results from the loss of function of a 'housekeeping' gene and (2) the mutant phenotype is specific. In this review, by defining housekeeping genes as those encoding proteins that work in basic metabolic and cellular functions, we discuss unexpected links between housekeeping genes and specific developmental processes. In a surprising number of cases housekeeping genes coding for enzymes or proteins with functions in basic cellular processes such as transcription, post-transcriptional modification, and translation affect plant development.

The Arabidopsis organelle-localized glycyl-tRNA synthetase encoded by EMBRYO DEFECTIVE DEVELOPMENT1 is required for organ patterning

Leaves develop as planar organs, with a morphology that is specialized for photosynthesis. Development of a planar leaf requires genetic networks that set up opposing adaxial and abaxial sides of the leaf, which leads to establishment of dorsoventral polarity. While many genes have been identified that regulate adaxial and abaxial fate there is little information on how this is integrated with cellular function. EMBRYO DEFECTIVE DEVELOPMENT1 (EDD1) is a nuclear gene that encodes a plastid and mitochondrial localized glycyl-tRNA synthetase. Plants with partial loss of EDD1 function have changes in patterning of margin and distal regions of the leaf. In combination with mutations in the MYB domain transcription factor gene ASYMMETRIC LEAVES1 (AS1), partial loss of EDD1 function results in leaves with reduced adaxial fate. EDD1 may influence leaf dorsoventral polarity through regulating the abaxial fate genes KANADI1 (KAN1) and ETTIN (ETT)/AUXIN RESPONSE FACTOR3 (ARF3) since these genes are upregulated in the edd1 as1 double mutant. SCABRA3 (SCA3), a nuclear gene that encodes the plastid RNA polymerase is also required for leaf adaxial fate in the absence of AS1. These results add a novel component to networks of genetic regulation of leaf development and suggest that organelles, particularly plastids, are required in leaf patterning. Potentially, signalling from organelles is essential for coordination of different cell fates within the developing leaf.

MicroRNA miR396 and RDR6 synergistically regulate leaf development

The microRNA (miRNA) miR396 regulates GROWTH-REGULATING FACTORs (GRFs), a plant specific family of transcription factors. Overexpression of miR396 causes a decrease in the GRFs that has been shown to affect cell proliferation in the meristem and developing leaves. To bring further insights into the function of the miR396 regulatory network we performed a mutant enhancer screen of a stable Arabidopsis transgenic line expressing 35S:miR396b, which has a reduction in leaf size. From this screen we recovered several mutants enhancing this phenotype and displaying organs with lotus- or needle-like shape. Analysis of these plants revealed mutations in as2 and rdr6. While 35S:miR396b in an as2 context generated organs with lotus-like shape, the overexpression of the miRNA in an rdr6 mutant background caused more important developmental defects, including pin-like organs and lobed leaves. Combination of miR396 overexpressors, and rdr6 and as2 mutants show additional organ defects, suggesting that the three pathways act in concert. Genetic interactions during leaf development were observed in a similar way between miR396 overexpression and mutants in RDR6, SGS3 or AGO7, which are known to participate in trans-acting siRNA (ta-siRNA) biogenesis. Furthermore, we found that miR396 can cause lotus- and pin-like organs per se, once a certain expression threshold is overcome. In good agreement, mutants accumulating high levels of TCP4, which induces miR396, interacted with the AS1/AS2 pathway to generate lotus-like organs. The results indicate that the miR396 regulatory network and the ta-siRNA biogenesis pathway synergistically interact during leaf development and morphogenesis.

Berberine enhances defects in the establishment of leaf polarity in asymmetric leaves1 and asymmetric leaves2 of Arabidopsis thaliana

Leaves develop as flat lateral organs from the indeterminate shoot apical meristem. The establishment of polarity along three-dimensional axes, proximal-distal, medial-lateral, and adaxial-abaxial axes, is crucial for the growth of normal leaves. The mutations of ASYMMETRIC LEAVES1 (AS1) and AS2 of Arabidopsis thaliana cause defects in repression of the indeterminate state and the establishment of axis formation in leaves. Although many mutations have been identified that enhance the adaxial-abaxial polarity defects of as1 and as2 mutants, the roles of the causative genes in leaf development are still unknown. In this study, we found that wild-type plants treated with berberine produced pointed leaves, which are often observed in the single mutants that enhance phenotypes of as1 and as2 mutants. The berberine-treated as1 and as2 mutants formed abaxialized filamentous leaves. Berberine, an isoquinoline alkaloid compound naturally produced in various plant sources, has a growth inhibitory effect on plants that do not produce berberine. We further showed that transcript levels of meristem-specific class 1 KNOX homeobox genes and abaxial determinant genes were increased in berberine-treated as1 and as2. Berberine treated plants carrying double mutations of AS2 and the large subunit ribosomal protein gene RPL5B showed more severe defects in polarity than did the as2 single mutant plants. We suggest that berberine inhibits (a) factor(s) that might be required for leaf adaxial cell differentiation through a pathway independent of AS1 and AS2. Multiple pathways might play important roles in the formation of flat symmetric leaves.

Leaf adaxial-abaxial polarity specification and lamina outgrowth: evolution and development

A key innovation in leaf evolution is the acquisition of a flat lamina with adaxial-abaxial polarity, which optimizes the primary function of photosynthesis. The developmental mechanism behind leaf adaxial-abaxial polarity specification and flat lamina formation has long been of interest to biologists. Surgical and genetic studies proposed a conceptual model wherein a signal derived from the shoot apical meristem is necessary for adaxial-abaxial polarity specification, and subsequent lamina outgrowth is promoted at the juxtaposition of adaxial and abaxial identities. Several distinct regulators involved in leaf adaxial-abaxial polarity specification and lamina outgrowth have been identified. Analyses of these genes demonstrated that the mutual antagonistic interactions between adaxial and abaxial determinants establish polarity and define the boundary between two domains, along which lamina outgrowth regulators function. Evolutionary developmental studies on diverse leaf forms of angiosperms proposed that alteration to the adaxial-abaxial patterning system can be a major driving force in the generation of diverse leaf forms, as represented by 'unifacial leaves', in which leaf blades have only the abaxial identity. Interestingly, unifacial leaf blades become flattened, in spite of the lack of adaxial-abaxial juxtaposition. Modification of the adaxial-abaxial patterning system is also utilized to generate complex organ morphologies, such as stamens. In this review, we summarize recent advances in the genetic mechanisms underlying leaf adaxial-abaxial polarity specification and lamina outgrowth, with emphasis on the genetic basis of the evolution and diversification of leaves.

ASYMMETRIC-LEAVES2 and an ortholog of eukaryotic NudC domain proteins repress expression of AUXIN-RESPONSE-FACTOR and class 1 KNOX homeobox genes for development of flat symmetric leaves in Arabidopsis

Leaf primordia form around the shoot apical meristem, which consists of indeterminate stem cells. Upon initiation of leaf development, adaxial-abaxial patterning is crucial for appropriate lateral expansion, via cellular proliferation, and the formation of flat symmetric leaves. Many genes that specify such patterning have been identified, but regulation by upstream factors of the expression of relevant effector genes remains poorly understood. In Arabidopsis thaliana, ASYMMETRIC LEAVES2 (AS2) and AS1 play important roles in repressing transcription of class 1 KNOTTED1-like homeobox (KNOX) genes and leaf abaxial-determinant effector genes. We report here a mutation, designated enhancer of asymmetric leaves2 and asymmetric leaves1 (eal), that is associated with efficient generation of abaxialized filamentous leaves on the as2 or as1 background. Levels of transcripts of many abaxial-determinant genes, including ETTIN (ETT)/AUXIN RESPONSE FACTOR3 (ARF3), and all four class 1 KNOX genes were markedly elevated in as2 eal shoot apices. Rudimentary patterning in as2 eal leaves was suppressed by the ett mutation. EAL encodes BOBBER1 (BOB1), an Arabidopsis ortholog of eukaryotic NudC domain proteins. BOB1 was expressed in plant tissues with division potential and bob1 mutations resulted in lowered levels of transcripts of some cell-cycle genes and decreased rates of cell division in shoot and root apices. Coordinated cellular proliferation, supported by BOB1, and repression of all class 1 KNOX genes, ETT/ARF3 by AS2 (AS1) and BOB1 might be critical for repression of the indeterminate state and of aberrant abaxialization in the presumptive adaxial domain of leaf primordia, which might ensure the formation of flat symmetric leaves.