HANABA TARANU Is a GATA Transcription Factor That Regulates Shoot Apical Meristem and Flower Development in ArabidopsisW⃞

A Dynamic Co-expression Map of Early Inflorescence Development in Setaria viridis Provides a Resource for Gene Discovery and Comparative Genomics

The morphological and functional diversity of plant form is governed by dynamic gene regulatory networks. In cereal crops, grain and/or pollen-bearing inflorescences exhibit vast architectural diversity and developmental complexity, yet the underlying genetic framework is only partly known. Setaria viridis is a small, rapidly growing grass species in the subfamily Panicoideae, a group that includes economically important cereal crops such as maize and sorghum. The S. viridis inflorescence displays complex branching patterns, but its early development is similar to that of other panicoid grasses, and thus is an ideal model for studying inflorescence architecture. Here we report a detailed transcriptional resource that captures dynamic transitions across six sequential stages of S. viridis inflorescence development, from reproductive onset to floral organ differentiation. Co-expression analyses identified stage-specific signatures of development, which include homologs of previously known developmental genes from maize and rice, suites of transcription factors and gene family members, and genes of unknown function. This spatiotemporal co-expression map and associated analyses provide a foundation for gene discovery in S. viridis inflorescence development, and a comparative model for exploring related architectural features in agronomically important cereals.

Characterization of the GATA gene family in Vitis vinifera: genome-wide analysis, expression profiles, and involvement in light and phytohormone response

The plant GATA family is one of the most important transcription factors involved in light-responsive development, nitrogen metabolism, phytohormone signaling, and source/sink balance. However, the function of the GATA gene is less known in grape (Vitis vinifera L.). In this study, we comprehensively analyzed the GATA family in grape, particularly the phylogenetic evolution, duplication patterns, conserved motifs, gene structures, cis-elements, tissue expression patterns, and predicted function of VvGATA genes in response to abiotic stress. The potential roles of VvGATA genes in berry development were also investigated. The GATA transcription factors displayed expression diversity among different grape organs and tissues, and some of them showed preferential expression in a specific tissue. Heterotrophic cultured cells were used as model systems for the functional characterization of the VvGATA gene and study of its response to light and phytohormone. Results indicated that some VvGATA genes displayed differential responses to light and phytohormones, suggesting their role in light and hormone signaling pathways. A thorough analysis of GATA transcription factors in grape (V. vinifera L.) presented the characterization and functional prediction of VvGATA genes. The data presented here lay the foundation for further functional studies of grape GATA transcription factors.

Transcriptomic analysis identifies the key genes involved in stamen petaloid in lotus (Nelumbo nucifera)

Background

Flower morphology, a phenomenon regulated by a complex network, is one of the vital ornamental features in Nelumbo nucifera. Stamen petaloid is very prevalent in lotus flowers. However, the mechanism underlying this phenomenon is still obscure.

Results

Here, the comparative transcriptomic analysis was performed among petal, stamen petaloid and stamen through RNA-seq. Using pairwise comparison analysis, a large number of genes involved in hormonal signal transduction pathways and transcription factors, especially the MADS-box genes, were identified as candidate genes for stamen petaloid in lotus.

Conclusions

Taken together, these results provide an insight into the molecular networks underlying lotus floral organ development and stamen petaloid.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4950-0) contains supplementary material, which is available to authorized users.

The early inflorescence of Arabidopsis thaliana demonstrates positional effects in floral organ growth and meristem patterning

Linear modelling approaches detected significant gradients in organ growth and patterning across early flowers of the Arabidopsis inflorescence and uncovered evidence of new roles for gibberellin in floral development. Most flowering plants, including the genetic model Arabidopsis thaliana, produce multiple flowers in sequence from a reproductive shoot apex to form a flower spike (inflorescence). The development of individual flowers on an Arabidopsis inflorescence has typically been considered as highly stereotypical and uniform, but this assumption is contradicted by the existence of mutants with phenotypes visible in early flowers only. This phenomenon is demonstrated by mutants partially impaired in the biosynthesis of the phytohormone gibberellin (GA), in which floral organ growth is retarded in the first flowers to be produced but has recovered spontaneously by the 10th flower. We presently lack systematic data from multiple flowers across the Arabidopsis inflorescence to explain such changes. Using mutants of the GA 20-OXIDASE (GA20ox) GA biosynthesis gene family to manipulate endogenous GA levels, we investigated the dynamics of changing floral organ growth across the early Arabidopsis inflorescence (flowers 1-10). Modelling of floral organ lengths identified a significant, GA-independent gradient of increasing stamen length relative to the pistil in the wild-type inflorescence that was separable from other, GA-dependent effects. It was also found that the first flowers exhibited unstable organ patterning in contrast to later flowers and that this instability was prolonged by exogenous GA treatment. These findings indicate that the development of individual flowers is influenced by hitherto unknown factors acting across the inflorescence and also suggest novel functions for GA in floral patterning.

Developmental transcriptome analysis of floral transition in Rosa odorata var. gigantea

Expression analyses revealed that floral transition of Rosa odorata var. gigantea is mainly regulated by VRN1, COLs, DELLA and KSN, with contributions by the effects of phytohormone and starch metabolism. Seasonal plants utilize changing environmental and developmental cues to control the transition from vegetative growth to flowering at the correct time of year. This study investigated global gene expression profiles at different developmental stages of Rosa odorata var. gigantea by RNA-sequencing, combined with phenotypic characterization and physiological changes. Gene ontology enrichment analysis of the differentially expressed genes (DEGs) between four different developmental stages (vegetative meristem, pre-floral meristem, floral meristem and secondary axillary buds) indicated that DNA methylation and the light reaction played a large role in inducing the rose floral transition. The expression of SUF and FLC, which are known to play a role in delaying flowering until vernalization, was down-regulated from the vegetative to the pre-floral meristem stage. In contrast, the expression of VRN1, which promotes flowering by repressing FLC expression, increased. The expression of DELLA proteins, which function as central nodes in hormone signaling pathways, and probably involve interactions between GA, auxin, and ABA to promote the floral transition, was well correlated with the expression of floral integrators, such as AGL24, COL4. We also identified DEGs associated with starch metabolism correlated with SOC1, AGL15, SPL3, AGL24, respectively. Taken together, our results suggest that vernalization and photoperiod are prominent cues to induce the rose floral transition, and that DELLA proteins also act as key regulators. The results summarized in the study on the floral transition of the seasonal rose lay a foundation for further functional demonstration, and have profound economic and ornamental values.

Longitudinal patterning in roots: a GATA2-auxin interaction underlies and maintains the root transition domain

In Arabidopsis thaliana root meristems the GATA2 transcription factor is a marker for the root transition domain, is auxin regulated, and functions to restrict cell division activity. The growing part of roots is comprised of three discrete regions; the proliferative domain (PD), an elongation zone, and interposed between these two, the transition domain (TD), which is the focus of this investigation. Within the TD, it is hypothesized that cells are reprogrammed, losing the capacity to divide and begin to differentiate. In recently germinated Arabidopsis thaliana seedlings, a TD is not anatomically evident, but subsequently forms in a region of the root in which there has occurred prior expression of both AUX1/PIN2 proteins and of transcripts of the GATA transcription factor family (pGATA2:H2B-YFP or pGATA2:GUS). pGATA2:GUS expression is regulated by auxin and is reduced in seedlings in which either auxin transport or auxin sensitivity is perturbed. Application of cytokinin results in a reduction in both pGATA2:GUS expression and in TD cell number, via a pathway involving ARR1 and ARR12. Overexpression of GATA2 is accompanied by a reduction in cell number in the PD, but has no effect on cell number in the TD, whereas in knockdowns of GATA transcription factors, cell number is reduced in both the PD and TD. We conclude: (1) that GATA2 expression is localized to (a marker for) the TD; (2) that development and maintenance of the TD are associated with an auxin-regulation of GATA2 expression; (3) that GATA transcription factors function to restrict cell division activity.

Grass inflorescence architecture and meristem determinacy

The grass inflorescence is striking not only for its beauty and diversity, but also for its developmental complexity. While models of inflorescence architecture have been proposed in both eudicots and grasses, these are inadequate to fully explain the complex branching events that occur during the development of the grass inflorescence. Key to understanding grass inflorescence architecture is the meristem determinacy/indeterminacy decision, which regulates the number of branching events that occur. Here we review what has been learned about meristem determinacy from grass mutants with defects in inflorescence development. A picture is emerging of a complex network of signaling molecules and meristem identity factors that interact to regulate inflorescence meristem activity, many of which have been modified during crop domestication directly affecting yield traits.

Grass inflorescence architecture and evolution: the origin of novel signaling centers

Contents 367 I. 367 II. 368 III. 370 IV. 371 371 References 371 SUMMARY: A central goal of evo-devo is to understand how morphological diversity arises from existing developmental mechanisms, requiring a clear, predictive explanatory framework of the underlying developmental mechanisms. Despite an ever-increasing literature on genes regulating grass inflorescence development, an effective model of inflorescence patterning is lacking. I argue that the existing framework for grass inflorescence development, which invokes homeotic shifts in multiple distinct meristem identities, obscures a recurring theme emerging from developmental genetic studies in grass models, that is that inflorescence branching is regulated by novel localized signaling centers. Understanding the origin and function of these novel signaling centers will be key to future evo-devo work on the grass inflorescence.

Comprehensive identification and clustering of CLV3/ESR-related (CLE) genes in plants finds groups with potentially shared function

CLV3/ESR (CLE) proteins are important signaling peptides in plants. The short CLE peptide (12-13 amino acids) is cleaved from a larger pre-propeptide and functions as an extracellular ligand. The CLE family is large and has resisted attempts at classification because the CLE domain is too short for reliable phylogenetic analysis and the pre-propeptide is too variable. We used a model-based search for CLE domains from 57 plant genomes and used the entire pre-propeptide for comprehensive clustering analysis. In total, 1628 CLE genes were identified in land plants, with none recognizable from green algae. These CLEs form 12 groups within which CLE domains are largely conserved and pre-propeptides can be aligned. Most clusters contain sequences from monocots, eudicots and Amborella trichopoda, with sequences from Picea abies, Selaginella moellendorffii and Physcomitrella patens scattered in some clusters. We easily identified previously known clusters involved in vascular differentiation and nodulation. In addition, we found a number of discrete groups whose function remains poorly characterized. Available data indicate that CLE proteins within a cluster are likely to share function, whereas those from different clusters play at least partially different roles. Our analysis provides a foundation for future evolutionary and functional studies.

Genome-wide binding analysis of AtGNC and AtCGA1 demonstrates their cross-regulation and common and specific functions

GATA transcription factors are involved in multiple processes in plant growth and development. Two GATA factors, NITRATE-INDUCIBLE,CARBON METABOLISM-INVOLVED (GNC) and CYTOKININ-RESPONSIVE GATA FACTOR 1 (CGA1, also named GNL), are important regulators in greening, flowering, senescence, and hormone signaling. However, their direct target genes related to these biological processes are poorly characterized. Here, GNC and CGA1 are shown to be transcription activators and by using chromatin immunoprecipitation sequencing (ChIP-seq), 1475 and 638 genes are identified to be associated with GNC and CGA1 binding, respectively. Enrichment of diverse motifs in the peak binding regions for GNC and CGA1 suggests the possibility that these two transcription factors also interact with other transcription factors and in addition genes coding for DNA-binding proteins are highly enriched among GNC- and CGA1-associated genes. Despite the fact that these two GATA factors are known to share a large portion of co-expressed genes, our analysis revealed a low percentage of overlapping binding-associated genes for these two homologues. This suggests a possible cross-regulation between these, which is verified using ChIP-qPCR. The common and specific biological processes regulated by GNC and CGA1 also support this notion. Functional analysis of the binding-associated genes revealed that those encoding transcription factors, E3 ligase, as well as genes with roles in plant development are highly enriched, indicating that GNC and CGA1 mediate complex genetic networks in regulating different aspects of plant growth and development.

Genome-wide identification, evolution, and expression analysis of GATA transcription factors in apple (Malus×domestica Borkh.)

Plant GATA transcription factors are type-IV zinc-finger proteins that play important regulatory roles in plant growth and development. In this study, we identified 35 GATA genes classified into four groups in the whole genome sequence of Malus domestica. A physiochemical property analysis indicated that GATA proteins are largely unstable hydrophilic proteins. An analysis of conserved protein motifs uncovered three highly conserved motifs, in addition to the GATA motif, in all MdGATA proteins. These three motifs, CCT, TIFY, and ASXH, were found to occur in specific GATA groups and may be related to GATA gene function. We identified 10 pairs of putative paralogs, indicating that MdGATA genes have mainly undergone whole genome duplication. Eighteen orthologous gene pairs were also identified between Arabidopsis thaliana and M. domestica. Furthermore, many light-responsive cis-elements were found in MdGATA gene promoters. Tissue-specific expression analysis performed by quantitative real-time reverse transcription PCR showed that MdGATA genes were preferentially expressed in flowers, leaves, and buds. Apple seedlings maintained in darkness for 7days exhibited a moderate decline in chlorophyll content along with significant down-regulation of most MdGATA genes, suggesting that MdGATA genes may be involved in light-responsive development and chlorophyll-level regulation. The distinctly higher expression levels observed for many MdGATA genes during three stages of floral induction also indicate that MdGATA genes may play a role in the apple flowering transition. The results presented here lay the foundation for further investigation of MdGATA gene family putative functions and improvement of apple yields.

Hormone Distribution and Transcriptome Profiles in Bamboo Shoots Provide Insights on Bamboo Stem Emergence and Growth

Growth and development are tightly co-ordinated events in the lifetime of living organisms. In temperate bamboo plants, spring is the season when environmental conditions are suitable for the emergence of new shoots. Previous studies demonstrated that bamboo plants undergo an energy-consuming 'fast stem growth' phase. However, the events during the initiation of stem elongation in bamboo are poorly understood. To understand the onset of bamboo stem growth, we performed hormone and transcriptome profiling of tissue regions in newly elongating shoots of the Moso bamboo Phyllostachys edulis. The growth hormones auxins, cytokinins and gibberellins accumulated in the shoot apex, while the stress hormones ABA, salicylic acid (SA) and jasmonic acid (JA) are predominantly found in the lower part of the stem. The mature basal part of the stem showed enrichment of transcripts associated with cell wall metabolism and biosynthesis of phenylpropanoid metabolites, such as lignin. In the young upper stem region, expression of cell formation- and DNA synthesis-related genes was enriched. Moreover, the apical region showed enhanced expression of genes involved in meristem maintenance, leaf differentiation and development, abaxial/adaxial polarity and flowering. Our findings integrate the spatial regulation of hormones and transcriptome programs during the initiation of bamboo stem growth.

Abiotic Stresses Cause Differential Regulation of Alternative Splice Forms of GATA Transcription Factor in Rice

The GATA gene family is one of the most conserved families of transcription factors, playing a significant role in different aspects of cellular processes, in organisms ranging from fungi to angiosperms. GATA transcription factors are DNA-binding proteins, having a class IV zinc-finger motif CX₂CX_17-20CX₂C followed by a highly basic region and are known to bind a consensus sequence WGATAR. In plants, GATAs are known to be involved in light-dependent gene regulation and nitrate assimilation. However, a comprehensive analysis of these GATA gene members has not yet been highlighted in rice when subjected to environmental stresses. In this study, we present an overview of the GATA gene family in rice (OsGATA) in terms of, their chromosomal distribution, domain architecture, and phylogeny. Our study has revealed the presence of 28 genes, encoding 35 putative GATA transcription factors belonging to seven subfamilies in the rice genome. Transcript abundance analysis in contrasting genotypes of rice-IR64 (salt sensitive) and Pokkali (salt tolerant), for individual GATA members indicated their differential expression in response to various abiotic stresses such as salinity, drought, and exogenous ABA. One of the members of subfamily VII-OsGATA23a, emerged as a multi-stress responsive transcription factor giving elevated expression levels in response to salinity and drought. ABA also induces expression of OsGATA23a by 35 and 55-folds in IR64 and Pokkali respectively. However, OsGATA23b, an alternative splice variant of OsGATA23 did not respond to above-mentioned stresses. Developmental regulation of the OsGATA genes based on a publicly available microarray database showed distinct expression patterns for most of the GATA members throughout different stages of rice development. Altogether, our results suggest inherent roles of diverse OsGATA factors in abiotic stress signaling and also throw some light on the tight regulation of the spliced variants of OsGATA genes in response to different environmental conditions.

Transcription dynamics of Saltol QTL localized genes encoding transcription factors, reveals their differential regulation in contrasting genotypes of rice

Salinity is one of the major environmental factors affecting the growth and yield of rice crop. Salinity stress response is a multigenic trait and numerous approaches have been used to dissect out the key determinants of complex salt tolerance trait and their regulation in plant. In the current study, we have investigated expression dynamics of the genes encoding transcription factors (SalTFs) localized within a major salinity tolerance related QTL-'Saltol' in the contrasting cultivars of rice. SalTFs were found to be differentially regulated between the contrasting genotypes of rice, with higher constitutive expression in the salt tolerant landrace, Pokkali than the cultivar IR64. Moreover, SalTFs were found to exhibit inducibility in the salt sensitive cultivar at late duration (after 24 h) of salinity stress. Further, the transcript abundance analysis of these SalTFs at various developmental stages of rice revealed that low expressing genes may be involved in developmental responses, while high expressing genes can be linked with the salt stress response. Grouping of these genes was well supported by in silico protein-protein interaction studies and distribution of single-nucleotide polymorphisms (SNPs) and insertions/deletions (InDels) in the promoter and genic regions of these genes. Taken together, we propose that out of 14 SalTFs, eight members are strongly correlated with the salinity stress tolerance in rice and six are involved in plant growth and development.

Transcriptomic analysis of differentially expressed genes in the floral transition of the summer flowering chrysanthemum

Background

Chrysanthemum is a leading cut flower species. Most conventional cultivars flower during the fall, but the Chrysanthemum morifolium ‘Yuuka’ flowers during the summer, thereby filling a gap in the market. To date, investigations of flowering time determination have largely focused on fall-flowering types. Little is known about molecular basis of flowering time in the summer-flowering chrysanthemum. Here, the genome-wide transcriptome of ‘Yuuka’ was acquired using RNA-Seq technology, with a view to shedding light on the molecular basis of the shift to reproductive growth as induced by variation in the photoperiod.

Results

Two sequencing libraries were prepared from the apical meristem and leaves of plants exposed to short days, three from plants exposed to long days and one from plants sampled before any photoperiod treatment was imposed. From the ~316 million clean reads obtained, 115,300 Unigenes were assembled. In total 70,860 annotated sequences were identified by reference to various databases. A number of transcription factors and genes involved in flowering pathways were found to be differentially transcribed. Under short days, genes acting in the photoperiod and gibberellin pathways might accelerate flowering, while under long days, the trehalose-6-phosphate and sugar signaling pathways might be promoted, while the phytochrome B pathway might block flowering. The differential transcription of eight of the differentially transcribed genes was successfully validated using quantitative real time PCR.

Conclusions

A transcriptome analysis of the summer-flowering cultivar ‘Yuuka’ has been described, along with a global analysis of floral transition under various daylengths. The large number of differentially transcribed genes identified confirmed the complexity of the regulatory machinery underlying floral transition.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3024-4) contains supplementary material, which is available to authorized users.

LLM-Domain Containing B-GATA Factors Control Different Aspects of Cytokinin-Regulated Development in Arabidopsis thaliana

Leu-Leu-Met (LLM)-domain B-GATAs are a subfamily of the 30-membered GATA transcription factor family from Arabidopsis. Only two of the six Arabidopsis LLM-domain B-GATAs, i.e. GATA, NITRATE-INDUCIBLE, CARBON METABOLISM-INVOLVED (GNC) and its paralog GNC-LIKE/CYTOKININ-RESPONSIVE GATA FACTOR1 (GNL), have already been analyzed with regard to their biological function. Together, GNC and GNL control germination, greening, flowering time, and senescence downstream from auxin, cytokinin (CK), gibberellin (GA), and light signaling. Whereas overexpression and complementation analyses suggest a redundant biochemical function between GNC and GNL, nothing is known about the biological role of the four other LLM-domain B-GATAs, GATA15, GATA16, GATA17, and GATA17L (GATA17-LIKE), based on loss-of-function mutant phenotypes. Here, we examine insertion mutants of the six Arabidopsis B-GATA genes and reveal the role of these genes in the control of greening, hypocotyl elongation, phyllotaxy, floral organ initiation, accessory meristem formation, flowering time, and senescence. Several of these phenotypes had previously not been described for the gnc and gnl mutants or were enhanced in the more complex mutants when compared to gnc gnl mutants. Some of the respective responses may be mediated by CK signaling, which activates the expression of all six GATA genes. CK-induced gene expression is partially compromised in LLM-domain B-GATA mutants, suggesting that B-GATA genes play a role in CK responses. We furthermore provide evidence for a transcriptional cross regulation between these GATAs that may, in at least some cases, be at the basis of their apparent functional redundancy.

Molecular Mechanisms of Floral Boundary Formation in Arabidopsis

Boundary formation is a crucial developmental process in plant organogenesis. Boundaries separate cells with distinct identities and act as organizing centers to control the development of adjacent organs. In flower development, initiation of floral primordia requires the formation of the meristem-to-organ (M-O) boundaries and floral organ development depends on the establishment of organ-to-organ (O-O) boundaries. Studies in this field have revealed a suite of genes and regulatory pathways controlling floral boundary formation. Many of these genes are transcription factors that interact with phytohormone pathways. This review will focus on the functions and interactions of the genes that play important roles in the floral boundaries and discuss the molecular mechanisms that integrate these regulatory pathways to control the floral boundary formation.

LLM-Domain B-GATA Transcription Factors Promote Stomatal Development Downstream of Light Signaling Pathways in Arabidopsis thaliana Hypocotyls

Stomata are pores that regulate the gas and water exchange between the environment and aboveground plant tissues, including hypocotyls, leaves, and stems. Here, we show that mutants of Arabidopsis thaliana LLM-domain B-GATA genes are defective in stomata formation in hypocotyls. Conversely, stomata formation is strongly promoted by overexpression of various LLM-domain B-class GATA genes, most strikingly in hypocotyls but also in cotyledons. Genetic analyses indicate that these B-GATAs act upstream of the stomata formation regulators SPEECHLESS(SPCH), MUTE, and SCREAM/SCREAM2 and downstream or independent of the patterning regulators TOO MANY MOUTHS and STOMATAL DENSITY AND DISTRIBUTION1 The effects of the GATAs on stomata formation are light dependent but can be induced in dark-grown seedlings by red, far-red, or blue light treatments. PHYTOCHROME INTERACTING FACTOR(PIF) mutants form stomata in the dark, and in this genetic background, GATA expression is sufficient to induce stomata formation in the dark. Since the expression of the LLM-domain B-GATAs GNC(GATA, NITRATE-INDUCIBLE, CARBON METABOLISM-INVOLVED) and GNC-LIKE/CYTOKININ-RESPONSIVE GATA FACTOR1 as well as that of SPCH is red light induced but the induction of SPCH is compromised in a GATA gene mutant background, we hypothesize that PIF- and light-regulated stomata formation in hypocotyls is critically dependent on LLM-domain B-GATA genes.

HANABA TARANU regulates the shoot apical meristem and leaf development in cucumber (Cucumis sativus L.)

The shoot apical meristem (SAM) is essential for continuous organogenesis in higher plants, while the leaf is the primary source organ and the leaf shape directly affects the efficiency of photosynthesis. HANABA TARANU (HAN) encodes a GATA3-type transcription factor that functions in floral organ development, SAM organization, and embryo development in Arabidopsis, but is involved in suppressing bract outgrowth and promoting branching in grass species. Here the function of the HAN homologue CsHAN1 was characterized in cucumber, an important vegetable with great agricultural and economic value. CsHAN1 is predominantly expressed at the junction of the SAM and the stem, and can partially rescue the han-2 floral organ phenotype in Arabidopsis. Overexpression and RNAi of CsHAN1 transgenic cucumber resulted in retarded growth early after embryogenesis and produced highly lobed leaves. Further, it was found that CsHAN1 may regulate SAM development through regulating the WUSCHEL (WUS) and SHOOT MERISTEMLESS (STM) pathways, and mediate leaf development through a complicated gene regulatory network in cucumber.

Regulation of plant growth and development by the GROWTH-REGULATING FACTOR and GRF-INTERACTING FACTOR duo

Transcription factors are key regulators of gene expression and play pivotal roles in all aspects of living organisms. Therefore, identification and functional characterization of transcription factors is a prerequisite step toward understanding life. This article reviews molecular and biological functions of the two transcription regulator families, GROWTH-REGULATING FACTOR (GRF) and GRF-INTERACTING FACTOR (GIF), which have only recently been recognized. A myriad of experimental evidence clearly illustrates that GRF and GIF are bona fide partner proteins and form a plant-specific transcriptional complex. One of the most conspicuous outcomes from this research field is that the GRF-GIF duo endows the primordial cells of vegetative and reproductive organs with a meristematic specification state, guaranteeing the supply of cells for organogenesis and successful reproduction. It has recently been shown that GIF1 proteins, also known as ANGUSTIFOLIA3, recruit chromatin remodelling complexes to target genes, and that AtGRF expression is directly activated by the floral identity factors, APETALA1 and SEPALLATA3, providing an important insight into understanding of the action of GRF-GIF. Moreover, GRF genes are extensively subjected to post-transcriptional control by microRNA396, revealing the presence of a complex regulatory circuit in regulation of plant growth and development by the GRF-GIF duo.

Bridging the gaps: evolution and development of perianth fusion

One of the most striking innovations in flower development is the congenital or postgenital union of petals (sympetaly) which has enabled dramatic specialization in flower structure and possibly accelerated speciation rates. Sympetalous flowers exhibit extraordinary variation in development, including the degree and timing of fusion, and fusion with other floral organs. Different axes of corolla tube complexity can be disentangled at the developmental level, with most variation being explained by differences in coordinated growth between interconnected and lobed regions of neighboring petal primordia, and between lower and upper portions of the corolla tube, defined by the stamen insertion boundary. Genetically, inter- and intra-specific variation in the degree of petal fusion is controlled by various inputs from genes that affect organ boundary and lateral growth, signaling between different cell types, and production of the cuticle. It is thus hypothesized that the evolution and diversification of fused petals, at least within the megadiverse Asteridae clade of core eudicots, have occurred through the modification of a conserved genetic pathway previously involved in free petal development.

HANABA TARANU (HAN) Bridges Meristem and Organ Primordia Boundaries through PINHEAD, JAGGED, BLADE-ON-PETIOLE2 and CYTOKININ OXIDASE 3 during Flower Development in Arabidopsis

Shoot organ primordia are initiated from the shoot apical meristem and develop into leaves during the vegetative stage, and into flowers during the reproductive phase. Between the meristem and the newly formed organ primordia, a boundary with specialized cells is formed that separates meristematic activity from determinate organ growth. Despite interactions that have been found between boundary regulators with genes controlling meristem maintenance or primordial development, most boundary studies were performed during embryogenesis or vegetative growth, hence little is known about whether and how boundaries communicate with meristem and organ primordia during the reproductive stage. We combined genetic, molecular and biochemical tools to explore interactions between the boundary gene HANABA TARANU (HAN) and two meristem regulators BREVIPEDICELLUS (BP) and PINHEAD (PNH), and three primordia-specific genes PETAL LOSS (PTL), JAGGED (JAG) and BLADE-ON-PETIOLE (BOP) during flower development. We demonstrated the key role of HAN in determining petal number, as part of a set of complex genetic interactions. HAN and PNH transcriptionally promote each other, and biochemically interact to regulate meristem organization. HAN physically interacts with JAG, and directly stimulates the expression of JAG and BOP2 to regulate floral organ development. Further, HAN directly binds to the promoter and intron of CYTOKININ OXIDASE 3 (CKX3) to modulate cytokinin homeostasis in the boundary. Our data suggest that boundary-expressing HAN communicates with the meristem through the PNH, regulates floral organ development via JAG and BOP2, and maintains boundary morphology through CKX3 during flower development in Arabidopsis.

Genome-wide survey of the soybean GATA transcription factor gene family and expression analysis under low nitrogen stress

GATA transcription factors are transcriptional regulatory proteins that contain a characteristic type-IV zinc finger DNA-binding domain and recognize the conserved GATA motif in the promoter sequence of target genes. Previous studies demonstrated that plant GATA factors possess critical functions in developmental control and responses to the environment. To date, the GATA factors in soybean (Glycine max) have yet to be characterized. Thus, this study identified 64 putative GATA factors from the entire soybean genomic sequence. The chromosomal distributions, gene structures, duplication patterns, phylogenetic tree, tissue expression patterns, and response to low nitrogen stress of the 64 GATA factors in soybean were analyzed to further investigate the functions of these factors. Results indicated that segmental duplication predominantly contributed to the expansion of the GATA factor gene family in soybean. These GATA proteins were phylogenetically clustered into four distinct subfamilies, wherein their gene structure and motif compositions were considerably conserved. A comparative phylogenetic analysis of the GATA factor zinc finger domain sequences in soybean, Arabidopsis (Arabidopsis thaliana), and rice (Oryza sativa) revealed four major classes. The GATA factors in soybean exhibited expression diversity among different tissues; some of these factors showed tissue-specific expression patterns. Numerous GATA factors displayed upregulation or downregulation in soybean leaf in response to low nitrogen stress, and two GATA factors GATA44 and GATA58 were likely to be involved in the regulation of nitrogen metabolism in soybean. Overexpression of GmGATA44 complemented the reduced chlorophyll phenotype of the Arabidopsis ortholog AtGATA21 mutant, implying that GmGATA44 played an important role in modulating chlorophyll biosynthesis. Overall, our study provides useful information for the further analysis of the biological functions of GATA factors in soybean and other crops.

Transcriptomic analysis reveals the roles of microtubule-related genes and transcription factors in fruit length regulation in cucumber (Cucumis sativus L.)

Cucumber (Cucumis sativus L.) fruit is a type of fleshy fruit that is harvested immaturely. Early fruit development directly determines the final fruit length and diameter, and consequently the fruit yield and quality. Different cucumber varieties display huge variations of fruit length, but how fruit length is determined at the molecular level remains poorly understood. To understand the genes and gene networks that regulate fruit length in cucumber, high throughout RNA-Seq data were used to compare the transcriptomes of early fruit from two near isogenic lines with different fruit lengths. 3955 genes were found to be differentially expressed, among which 2368 genes were significantly up-regulated and 1587 down-regulated in the line with long fruit. Microtubule and cell cycle related genes were dramatically activated in the long fruit, and transcription factors were implicated in the fruit length regulation in cucumber. Thus, our results built a foundation for dissecting the molecular mechanism of fruit length control in cucumber, a key agricultural trait of significant economic importance.

B-GATA transcription factors - insights into their structure, regulation, and role in plant development

GATA transcription factors are evolutionarily conserved transcriptional regulators that recognize promoter elements with a G-A-T-A core sequence. In comparison to animal genomes, the GATA transcription factor family in plants is comparatively large with approximately 30 members. Here, we review the current knowledge on B-GATAs, one of four GATA factor subfamilies from Arabidopsis thaliana. We show that B-GATAs can be subdivided based on structural features and their biological function into family members with a C-terminal LLM- (leucine-leucine-methionine) domain or an N-terminal HAN- (HANABA TARANU) domain. The paralogous GNC (GATA, NITRATE-INDUCIBLE, CARBON-METABOLISM INVOLVED) and CGA1/GNL (CYTOKININ-INDUCED GATA1/GNC-LIKE) are introduced as LLM-domain containing B-GATAs from Arabidopsis that control germination, greening, senescence, and flowering time downstream from several growth regulatory signals. Arabidopsis HAN and its monocot-specific paralogs from rice (NECK LEAF1), maize (TASSEL SHEATH1), and barley (THIRD OUTER GLUME) are HAN-domain-containing B-GATAs with a predominant role in embryo development and floral development. We also review GATA23, a regulator of lateral root initiation from Arabidopsis that is closely related to GNC and GNL but has a degenerate LLM-domain that is seemingly specific for the Brassicaceae family. The Brassicaceae-specific GATA23 and the monocot-specific HAN-domain GATAs provide evidence that neofunctionalization of B-GATAs was used during plant evolution to expand the functional repertoire of these transcription factors.

Beyond the Divide: Boundaries for Patterning and Stem Cell Regulation in Plants

The initiation of plant lateral organs from the shoot apical meristem (SAM) is closely associated with the formation of specialized domains of restricted growth known as the boundaries. These zones are required in separating the meristem from the growing primordia or adjacent organs but play a much broader role in regulating stem cell activity and shoot patterning. Studies have revealed a network of genes and hormone pathways that establish and maintain boundaries between the SAM and leaves. Recruitment of these pathways is shown to underlie a variety of processes during the reproductive phase including axillary meristems production, flower patterning, fruit development, and organ abscission. This review summarizes the role of conserved gene modules in patterning boundaries throughout the life cycle.

Transcript profiling reveals auxin and cytokinin signaling pathways and transcription regulation during in vitro organogenesis of Ramie (Boehmeria nivea L. Gaud)

In vitro organogenesis, one of the most common pathways leading to in vitro plant regeneration, is widely used in biotechnology and the fundamental study of plant biology. Although previous studies have constructed a complex regulatory network model for Arabidopsis in vitro organogenesis, no related study has been reported in ramie. To generate more complete observations of transcriptome content and dynamics during ramie in vitro organogenesis, we constructed a reference transcriptome library and ten digital gene expression (DGE) libraries for illumina sequencing. Approximately 111.34 million clean reads were obtained for transcriptome and the DGE libraries generated between 13.5 and 18.8 million clean reads. De novo assembly produced 43,222 unigenes and a total of 5,760 differentially expressed genes (DEGs) were filtered. Searching against the Kyoto Encyclopedia of Genes and Genomes Pathway database, 26 auxin related and 11 cytokinin related DEGs were selected for qRT-PCR validation of two ramie cultivars, which had high (Huazhu No. 5) or extremely low (Dazhuhuangbaima) shoot regeneration abilities. The results revealed differing regulation patterns of auxin and cytokinin in different genotypes. Here we report the first genome-wide gene expression profiling of in vitro organogenesis in ramie and provide an overview of transcription and phytohormone regulation during the process. Furthermore, the auxin and cytokinin related genes have distinct expression patterns in two ramie cultivars with high or extremely low shoot regeneration ability, which has given us a better understanding of the in vitro organogenesis mechanism. This result will provide a foundation for future phytohormone research and lead to improvements of the ramie regeneration system.

Functional diversification within the family of B-GATA transcription factors through the leucine-leucine-methionine domain

The transcription of the Arabidopsis (Arabidopsis thaliana) GATA transcription factors GATA, NITRATE-INDUCIBLE, CARBON METABOLISM-INVOLVED (GNC) and GNC-LIKE (GNL)/CYTOKININ-RESPONSIVE GATA FACTOR1 is controlled by several growth regulatory signals including light and the phytohormones auxin, cytokinin, and gibberellin. To date, GNC and GNL have been attributed functions in the control of germination, greening, flowering time, floral development, senescence, and floral organ abscission. GNC and GNL belong to the 11-member family of B-class GATA transcription factors that are characterized to date solely by their high sequence conservation within the GATA DNA-binding domain. The degree of functional conservation among the various B-class GATA family members is not understood. Here, we identify and examine B-class GATAs from Arabidopsis, tomato (Solanum lycopersicon), Brachypodium (Brachypodium distachyon), and barley (Hordeum vulgare). We find that B-class GATAs from these four species can be subdivided based on their short or long N termini and the presence of the 13-amino acid C-terminal leucine-leucine-methionine (LLM) domain with the conserved motif LLM. Through overexpression analyses and by complementation of a gnc gnl double mutant, we provide evidence that the length of the N terminus may not allow distinguishing between the different B-class GATAs at the functional level. In turn, we find that the presence and absence of the LLM domain in the overexpressors has differential effects on hypocotyl elongation, leaf shape, and petiole length, as well as on gene expression. Thus, our analyses identify the LLM domain as an evolutionarily conserved domain that determines B-class GATA factor identity and provides a further subclassification criterion for this transcription factor family.

Characterization of putative cis-regulatory elements in genes preferentially expressed in Arabidopsis male meiocytes

Meiosis is essential for plant reproduction because it is the process during which homologous chromosome pairing, synapsis, and meiotic recombination occur. The meiotic transcriptome is difficult to investigate because of the size of meiocytes and the confines of anther lobes. The recent development of isolation techniques has enabled the characterization of transcriptional profiles in male meiocytes of Arabidopsis. Gene expression in male meiocytes shows unique features. The direct interaction of transcription factors (TFs) with DNA regulatory sequences forms the basis for the specificity of transcriptional regulation. Here, we identified putative cis-regulatory elements (CREs) associated with male meiocyte-expressed genes using in silico tools. The upstream regions (1 kb) of the top 50 genes preferentially expressed in Arabidopsis meiocytes possessed conserved motifs. These motifs are putative binding sites of TFs, some of which share common functions, such as roles in cell division. In combination with cell-type-specific analysis, our findings could be a substantial aid for the identification and experimental verification of the protein-DNA interactions for the specific TFs that drive gene expression in meiocytes.

AINTEGUMENTA-LIKE proteins: hubs in a plethora of networks

Members of the AINTEGUMENTA-LIKE (AIL) family of APETALA 2/ETHYLENE RESPONSE FACTOR (AP2/ERF) domain transcription factors are expressed in all dividing tissues in the plant, where they have central roles in developmental processes such as embryogenesis, stem cell niche specification, meristem maintenance, organ positioning, and growth. When overexpressed, AIL proteins induce adventitious growth, including somatic embryogenesis and ectopic organ formation. The Arabidopsis (Arabidopsis thaliana) genome contains eight AIL genes, including AINTEGUMENTA, BABY BOOM, and the PLETHORA genes. Studies on these transcription factors have revealed their intricate relationship with auxin as well as their involvement in an increasing number of gene regulatory networks, in which extensive crosstalk and feedback loops have a major role.

Arabidopsis JAGGED links floral organ patterning to tissue growth by repressing Kip-related cell cycle inhibitors

Plant morphogenesis requires coordinated cytoplasmic growth, oriented cell wall extension, and cell cycle progression, but it is debated which of these processes are primary drivers for tissue growth and directly targeted by developmental genes. Here, we used ChIP high-throughput sequencing combined with transcriptome analysis to identify global target genes of the Arabidopsis transcription factor JAGGED (JAG), which promotes growth of the distal region of floral organs. Consistent with the roles of JAG during organ initiation and subsequent distal organ growth, we found that JAG directly repressed genes involved in meristem development, such as CLAVATA1 and HANABA TARANU, and genes involved in the development of the basal region of shoot organs, such as BLADE ON PETIOLE 2 and the GROWTH REGULATORY FACTOR pathway. At the same time, JAG regulated genes involved in tissue polarity, cell wall modification, and cell cycle progression. In particular, JAG directly repressed KIP RELATED PROTEIN 4 (KRP4) and KRP2, which control the transition to the DNA synthesis phase (S-phase) of the cell cycle. The krp2 and krp4 mutations suppressed jag defects in organ growth and in the morphology of petal epidermal cells, showing that the interaction between JAG and KRP genes is functionally relevant. Our work reveals that JAG is a direct mediator between genetic pathways involved in organ patterning and cellular functions required for tissue growth, and it shows that a regulatory gene shapes plant organs by releasing a constraint on S-phase entry.

Genetic and phenotypic analysis of shoot apical and floral meristem development

The shoot apical and floral meristems (SAM and FM, respectively) of Arabidopsis thaliana contain reservoirs of self-renewing stem cells that function as sources of progenitor cells for organ formation during development. The primary SAM produces all of the aerial structures of the adult plant, whereas the FMs generate the four types of floral organs. Consequently, aberrant SAM and FM activity can profoundly affect vegetative and reproductive plant morphology. The embedded location and small size of Arabidopsis meristems make accessing these structures difficult, so specialized techniques have been developed to facilitate their analysis. Microscopic, histological, and molecular techniques provide both qualitative and quantitative data on meristem organization and function, which are crucial for the normal growth and development of the entire plant.

Flower development in Arabidopsis: there is more to it than learning your ABCs

The field of Arabidopsis flower development began in the early 1980s with the initial description of several mutants including apetala1, apetala2, and agamous that altered floral organ identity (Koornneef and van der Veen, Theor Appl Genet 58:257-263, 1980; Koornneef et al., J Hered 74:265-272, 1983). By the end of the 1980s, these mutants were receiving more focused attention to determine precisely how they affected flower development (Komaki et al., Development 104:195-203, 1988; Bowman et al., Plant Cell 1:37-52, 1989). In the last quarter century, impressive progress has been made in characterizing the gene products and molecular mechanisms that control the key events in flower development. In this review, we briefly summarize the highlights of work from the past 25 years but focus on advances in the field in the last several years.

Auxin regulation of embryonic root formation

The plant hormone auxin was initially identified as the bioactive substance that induces roots in plant tissue culture. In the past decades, mechanisms for auxin action, including its transport and response, have been described in detail. However, a molecular and cellular description of its role in root initiation is far from complete. In this review, we discuss recent advances in our understanding of auxin-dependent embryonic root formation. During this process, a root meristem is initiated in a precise and predictable position, and at a stage when the organism consists of relatively few cells. Recent studies have revealed mechanisms for local control of auxin transport, for cellular differences in auxin response components and cell type-specific chromatin regulation. The recent identification of biologically relevant target genes for auxin regulation during embryonic root initiation now also allows dissection of auxin-activated cellular processes. Finally, we discuss the potential for hormonal cross-regulation in embryonic root formation.

Transcription repressor HANABA TARANU controls flower development by integrating the actions of multiple hormones, floral organ specification genes, and GATA3 family genes in Arabidopsis

Plant inflorescence meristems and floral meristems possess specific boundary domains that result in proper floral organ separation and specification. HANABA TARANU (HAN) encodes a boundary-expressed GATA3-type transcription factor that regulates shoot meristem organization and flower development in Arabidopsis thaliana, but the underlying mechanism remains unclear. Through time-course microarray analyses following transient overexpression of HAN, we found that HAN represses hundreds of genes, especially genes involved in hormone responses and floral organ specification. Transient overexpression of HAN also represses the expression of HAN and three other GATA3 family genes, HANL2 (HAN-LIKE 2), GNC (GATA, NITRATE-INDUCIBLE, CARBON-METABOLISM-INVOLVED), and GNL (GNC-LIKE), forming a negative regulatory feedback loop. Genetic analysis indicates that HAN and the three GATA3 family genes coordinately regulate floral development, and their expression patterns are partially overlapping. HAN can homodimerize and heterodimerize with the three proteins encoded by these genes, and HAN directly binds to its own promoter and the GNC promoter in vivo. These findings, along with the fact that constitutive overexpression of HAN produces an even stronger phenotype than the loss-of-function mutation, support the hypothesis that HAN functions as a key repressor that regulates floral development via regulatory networks involving genes in the GATA3 family, along with genes involved in hormone action and floral organ specification.

PETAL LOSS is a boundary gene that inhibits growth between developing sepals in Arabidopsis thaliana

Flower primordia are partitioned by boundaries during their early development. Such boundaries occur between whorls of organs, and also between organs within whorls. PETAL LOSS (PTL) is a trihelix transcription factor gene that is expressed in boundaries between sepal primordia in the outer whorl. Over-expression of PTL results in growth suppression suggesting that PTL normally inhibits growth between newly arising sepals. We have tested this by examining the consequences of loss of PTL function using confocal imaging. The size of the inter-sepal zone in stage 4 buds expands radially by 35-40% in ptl-1 mutants as a consequence of additional cell proliferation. There is no change in the size of PTL-expressing cells. PTL expression does not overlap with the sites of petal initiation identified using the DR5 auxin response reporter. The latter are closer to the centre of the flower. Thus the consequence of loss of PTL function on petal initiation is indirect, perhaps through interference with a mobile petal-initiation signal or movement of the PTL protein. CUP-SHAPED COTYLEDON (CUC) genes are also involved in defining inter-sepal boundaries. However, genetic studies combining ptl with loss of cuc1 function, and gain of CUC function in extra early petals-1 (miR164c) mutants, have revealed that CUC and PTL act differently. CUC suppresses growth of sepal tissues from the boundary region whereas PTL acts to keep the size of the boundary in check.

Stable establishment of cotyledon identity during embryogenesis in Arabidopsis by ANGUSTIFOLIA3 and HANABA TARANU

In seed plants, the shoot apical and root apical meristems form at the apical and basal poles of the embryonic axis, and leaves form at the flanks of the shoot apical meristem. ANGUSTIFOLIA3/GRF INTERACTING FACTOR1 (AN3/GIF1) encodes a putative transcriptional co-activator involved in various aspects of shoot development, including the maintenance of shoot apical meristems, cell proliferation and expansion in leaf primordia, and adaxial/abaxial patterning of leaves. Here, we report a novel function of AN3 involved in developmental fate establishment. We characterised an an3-like mutant that was found to be an allele of hanaba taranu (han), named han-30, and examined its genetic interactions with an3. an3 han double mutants exhibited severe defects in cotyledon development such that ectopic roots were formed at the apical region of the embryo, as confirmed by pWOX5::GFP expression. Additionally, gif2 enhanced the ectopic root phenotype of an3 han. Although the auxin accumulation pattern of the embryo was correct in an3 han-30, based on DR5rev::GFP expression at the globular stage, expression of the PLETHORA1 (PLT1), a master regulator of root development, expanded from the basal embryonic region to the apical region during the same developmental stage. Furthermore, the plt1 mutation suppressed ectopic root formation in an3 han. These data suggest that establishing cotyledon identity requires both AN3 and HAN to repress ectopic root formation by repressing PLT1 expression.

Ontogeny of the maize shoot apical meristem

The maize (Zea mays) shoot apical meristem (SAM) arises early in embryogenesis and functions during stem cell maintenance and organogenesis to generate all the aboveground organs of the plant. Despite its integral role in maize shoot development, little is known about the molecular mechanisms of SAM initiation. Laser microdissection of apical domains from developing maize embryos and seedlings was combined with RNA sequencing for transcriptomic analyses of SAM ontogeny. Molecular markers of key events during maize embryogenesis are described, and comprehensive transcriptional data from six stages in maize shoot development are generated. Transcriptomic profiling before and after SAM initiation indicates that organogenesis precedes stem cell maintenance in maize; analyses of the first three lateral organs elaborated from maize embryos provides insight into their homology and to the identity of the single maize cotyledon. Compared with the newly initiated SAM, the mature SAM is enriched for transcripts that function in transcriptional regulation, hormonal signaling, and transport. Comparisons of shoot meristems initiating juvenile leaves, adult leaves, and husk leaves illustrate differences in phase-specific (juvenile versus adult) and meristem-specific (SAM versus lateral meristem) transcript accumulation during maize shoot development. This study provides insight into the molecular genetics of SAM initiation and function in maize.

Transcriptome sequencing of Hevea brasiliensis for development of microsatellite markers and construction of a genetic linkage map

To obtain more information on the Hevea brasiliensis genome, we sequenced the transcriptome from the vegetative shoot apex yielding 2 311 497 reads. Clustering and assembly of the reads produced a total of 113 313 unique sequences, comprising 28 387 isotigs and 84 926 singletons. Also, 17 819 expressed sequence tag (EST)-simple sequence repeats (SSRs) were identified from the data set. To demonstrate the use of this EST resource for marker development, primers were designed for 430 of the EST-SSRs. Three hundred and twenty-three primer pairs were amplifiable in H. brasiliensis clones. Polymorphic information content values of selected 47 SSRs among 20 H. brasiliensis clones ranged from 0.13 to 0.71, with an average of 0.51. A dendrogram of genetic similarities between the 20 H. brasiliensis clones using these 47 EST-SSRs suggested two distinct groups that correlated well with clone pedigree. These novel EST-SSRs together with the published SSRs were used for the construction of an integrated parental linkage map of H. brasiliensis based on 81 lines of an F1 mapping population. The map consisted of 97 loci, consisting of 37 novel EST-SSRs and 60 published SSRs, distributed on 23 linkage groups and covered 842.9 cM with a mean interval of 11.9 cM and ∼4 loci per linkage group. Although the numbers of linkage groups exceed the haploid number (18), but with several common markers between homologous linkage groups with the previous map indicated that the F1 map in this study is appropriate for further study in marker-assisted selection.

Organogenesis from stem cells in planta: multiple feedback loops integrating molecular and mechanical signals

In multicellular organisms, the coordination of cell behaviors largely relies on biochemical and biophysical signals. Understanding how such signals control development is often challenging, because their distribution relies on the activity of individual cells and, in a feedback loop, on tissue behavior and geometry. This review focuses on one of the best-studied structures in biology, the shoot apical meristem (SAM). This tissue is responsible for the production of all the aerial parts of a plant. In the SAM, a population of stem cells continuously produces new cells that are incorporated in lateral organs, such as leaves, branches, and flowers. Organogenesis from stem cells involves a tight regulation of cell identity and patterning as well as large-scale morphogenetic events. The gene regulatory network controlling these processes is highly coordinated in space by various signals, such as plant hormones, peptides, intracellular mobile factors, and mechanical stresses. Many crosstalks and feedback loops interconnecting these pathways have emerged in the past 10 years. The plant hormone auxin and mechanical forces have received more attention recently and their role is more particularly detailed here. An integrated view of these signaling networks is also presented in order to help understanding how robust shape and patterning can emerge from these networks.

Nitrogen metabolite repression of metabolism and virulence in the human fungal pathogen Cryptococcus neoformans

Proper regulation of metabolism is essential to maximizing fitness of organisms in their chosen environmental niche. Nitrogen metabolite repression is an example of a regulatory mechanism in fungi that enables preferential utilization of easily assimilated nitrogen sources, such as ammonium, to conserve resources. Here we provide genetic, transcriptional, and phenotypic evidence of nitrogen metabolite repression in the human pathogen Cryptococcus neoformans. In addition to loss of transcriptional activation of catabolic enzyme-encoding genes of the uric acid and proline assimilation pathways in the presence of ammonium, nitrogen metabolite repression also regulates the production of the virulence determinants capsule and melanin. Since GATA transcription factors are known to play a key role in nitrogen metabolite repression, bioinformatic analyses of the C. neoformans genome were undertaken and seven predicted GATA-type genes were identified. A screen of these deletion mutants revealed GAT1, encoding the only global transcription factor essential for utilization of a wide range of nitrogen sources, including uric acid, urea, and creatinine-three predominant nitrogen constituents found in the C. neoformans ecological niche. In addition to its evolutionarily conserved role in mediating nitrogen metabolite repression and controlling the expression of catabolic enzyme and permease-encoding genes, Gat1 also negatively regulates virulence traits, including infectious basidiospore production, melanin formation, and growth at high body temperature (39°-40°). Conversely, Gat1 positively regulates capsule production. A murine inhalation model of cryptococcosis revealed that the gat1Δ mutant is slightly more virulent than wild type, indicating that Gat1 plays a complex regulatory role during infection.

TF-Cluster: a pipeline for identifying functionally coordinated transcription factors via network decomposition of the shared coexpression connectivity matrix (SCCM)

BACKGROUND

Identifying the key transcription factors (TFs) controlling a biological process is the first step toward a better understanding of underpinning regulatory mechanisms. However, due to the involvement of a large number of genes and complex interactions in gene regulatory networks, identifying TFs involved in a biological process remains particularly difficult. The challenges include: (1) Most eukaryotic genomes encode thousands of TFs, which are organized in gene families of various sizes and in many cases with poor sequence conservation, making it difficult to recognize TFs for a biological process; (2) Transcription usually involves several hundred genes that generate a combination of intrinsic noise from upstream signaling networks and lead to fluctuations in transcription; (3) A TF can function in different cell types or developmental stages. Currently, the methods available for identifying TFs involved in biological processes are still very scarce, and the development of novel, more powerful methods is desperately needed.

RESULTS

We developed a computational pipeline called TF-Cluster for identifying functionally coordinated TFs in two steps: (1) Construction of a shared coexpression connectivity matrix (SCCM), in which each entry represents the number of shared coexpressed genes between two TFs. This sparse and symmetric matrix embodies a new concept of coexpression networks in which genes are associated in the context of other shared coexpressed genes; (2) Decomposition of the SCCM using a novel heuristic algorithm termed "Triple-Link", which searches the highest connectivity in the SCCM, and then uses two connected TF as a primer for growing a TF cluster with a number of linking criteria. We applied TF-Cluster to microarray data from human stem cells and Arabidopsis roots, and then demonstrated that many of the resulting TF clusters contain functionally coordinated TFs that, based on existing literature, accurately represent a biological process of interest.

CONCLUSIONS

TF-Cluster can be used to identify a set of TFs controlling a biological process of interest from gene expression data. Its high accuracy in recognizing true positive TFs involved in a biological process makes it extremely valuable in building core GRNs controlling a biological process. The pipeline implemented in Perl can be installed in various platforms.

Establishment of the embryonic shoot apical meristem in Arabidopsis thaliana

In higher plants, shoot organs such as leaves, branches, and flowers are generated from the shoot apical meristem (SAM), a small group of undifferentiated cells located at the tip of the shoot. The SAM maintains its pluripotency and simultaneously produces lateral organs at its periphery. The SAM arises during embryogenesis and its positioning requires axis-dependent embryo patterning and compartmentalization of the embryo apex. Here, we introduce major factors involved in these processes in Arabidopsis thaliana and discuss how the embryonic SAM is established.

Integration of light- and brassinosteroid-signaling pathways by a GATA transcription factor in Arabidopsis

Light and brassinosteroid (BR) antagonistically regulate the developmental switch from etiolation in the dark to photomorphogenesis in the light in plants. Here, we identify GATA2 as a key transcriptional regulator that mediates the crosstalk between BR- and light-signaling pathways. Overexpression of GATA2 causes constitutive photomorphogenesis in the dark, whereas suppression of GATA2 reduces photomorphogenesis caused by light, BR deficiency, or the constitutive photomorphogenesis mutant cop1. Genome profiling and chromatin immunoprecipitation experiments show that GATA2 directly regulates genes that respond to both light and BR. BR represses GATA2 transcription through the BR-activated transcription factor BZR1, whereas light causes accumulation of GATA2 protein and feedback inhibition of GATA2 transcription. Dark-induced proteasomal degradation of GATA2 is dependent on the COP1 E3 ubiquitin ligase, and COP1 can ubiquitinate GATA2 in vitro. This study illustrates a molecular framework for antagonistic regulation of gene expression and seedling photomorphogenesis by BR and light.

Cell-type specific analysis of translating RNAs in developing flowers reveals new levels of control

Combining translating ribosome affinity purification with RNA-seq for cell-specific profiling of translating RNAs in developing flowers.

Cell type comparisons of cell type-specific hormone responses, promoter motifs, coexpressed cognate binding factor candidates, and splicing isoforms.

Widespread post-transcriptional regulation at both the intron splicing and translational stages.

A new class of noncoding RNAs associated with polysomes.

What constitutes a differentiated cell type? How much do cell types differ in their transcription of genes? The development and functions of tissues rely on constant interactions among distinct and nonequivalent cell types. Answering these questions will require quantitative information on transcriptomes, proteomes, protein–protein interactions, protein–nucleic acid interactions, and metabolomes at cellular resolution. The systems approaches emerging in biology promise to explain properties of biological systems based on genome-wide measurements of expression, interaction, regulation, and metabolism. To facilitate a systems approach, it is essential first to capture such components in a global manner, ideally at cellular resolution.

Recently, microarray analysis of transcriptomes has been extended to a cellular level of resolution by using laser microdissection or fluorescence-activated sorting (for review, see Nelson et al, 2008). These methods have been limited by stresses associated with cellular separation and isolation procedures, and biases associated with mandatory RNA amplification steps. A newly developed method, translating ribosome affinity purification (TRAP; Zanetti et al, 2005; Heiman et al, 2008; Mustroph et al, 2009), circumvents these problems by epitopetagging a ribosomal protein in specific cellular domains to selectively purify polysomes. We combined TRAP with deep sequencing, which we term TRAP-seq, to provide cell-level spatiotemporal maps for Arabidopsis early floral development at single-base resolution.

Flower development in Arabidopsis has been studied extensively and is one of the best understood aspects of plant development (for review, see Krizek and Fletcher, 2005). Genetic analysis of homeotic mutants established the ABC model, in which three classes of regulatory genes, A, B and C, work in a combinatorial manner to confer organ identities of four whorls (Coen and Meyerowitz, 1991). Each class of regulatory gene is expressed in a specific and evolutionarily conserved domain, and the action of the class A, B and C genes is necessary for specification of organ identity (Figure 1A).

Using TRAP-seq, we purified cell-specific translating mRNA populations, which we and others call the translatome, from the A, B and C domains of early developing flowers, in which floral patterning and the specification of floral organs is established. To achieve temporal specificity, we used a floral induction system to facilitate collection of early stage flowers (Wellmer et al, 2006). The combination of TRAP-seq with domain-specific promoters and this floral induction system enabled fine spatiotemporal isolation of translating mRNA in specific cellular domains, and at specific developmental stages.

Multiple lines of evidence confirmed the specificity of this approach, including detecting the expression in expected domains but not in other domains for well-studied flower marker genes and known physiological functions (Figures 1B–D and 2A–C). Furthermore, we provide numerous examples from flower development in which a spatiotemporal map of rigorously comparable cell-specific translatomes makes possible new views of the properties of cell domains not evident in data obtained from whole organs or tissues, including patterns of transcription and cis-regulation, new physiological differences among cell domains and between flower stages, putative hormone-active centers, and splicing events specific for flower domains (Figure 2A–D). Such findings may provide new targets for reverse genetics studies and may aid in the formulation and validation of interaction and pathway networks.

Beside cellular heterogeneity, the transcriptome is regulated at several steps through the life of mRNA molecules, which are not directly available through traditional transcriptome profiling of total mRNA abundance. By comparing the translatome and transcriptome, we integratively profiled two key posttranscriptional control points, intron splicing and translation state. From our translatome-wide profiling, we (i) confirmed that both posttranscriptional regulation control points were used by a large portion of the transcriptome; (ii) identified a number of cis-acting features within the coding or noncoding sequences that correlate with splicing or translation state; and (iii) revealed correlation between each regulation mechanism and gene function. Our transcriptome-wide surveys have highlighted target genes transcripts of which are probably under extensive posttranscriptional regulation during flower development.

Finally, we reported the finding of a large number of polysome-associated ncRNAs. About one-third of all annotated ncRNA in the Arabidopsis genome were observed co-purified with polysomes. Coding capacity analysis confirmed that most of them are real ncRNA without conserved ORFs. The group of polysome-associated ncRNA reported in this study is a potential new addition to the expanding riboregulator catalog; they could have roles in translational regulation during early flower development.

Determining both the expression levels of mRNA and the regulation of its translation is important in understanding specialized cell functions. In this study, we describe both the expression profiles of cells within spatiotemporal domains of the Arabidopsis thaliana flower and the post-transcriptional regulation of these mRNAs, at nucleotide resolution. We express a tagged ribosomal protein under the promoters of three master regulators of flower development. By precipitating tagged polysomes, we isolated cell type-specific mRNAs that are probably translating, and quantified those mRNAs through deep sequencing. Cell type comparisons identified known cell-specific transcripts and uncovered many new ones, from which we inferred cell type-specific hormone responses, promoter motifs and coexpressed cognate binding factor candidates, and splicing isoforms. By comparing translating mRNAs with steady-state overall transcripts, we found evidence for widespread post-transcriptional regulation at both the intron splicing and translational stages. Sequence analyses identified structural features associated with each step. Finally, we identified a new class of noncoding RNAs associated with polysomes. Findings from our profiling lead to new hypotheses in the understanding of flower development.

LOST MERISTEMS genes regulate cell differentiation of central zone descendants in Arabidopsis shoot meristems

Meristems of seed plants continuously produce new cells for incorporation into maturing tissues. A tightly controlled balance between cell proliferation in the center and cell differentiation at the periphery of the shoot meristem maintains its integrity. Here, we describe the role of three GRAS genes, named LOST MERISTEMS genes, in shoot apical meristem maintenance and axillary meristem formation. Under short photoperiods, the lom1 lom2 and lom1 lom2 lom3 mutants have arrested meristems characterized by an over-proliferation of meristematic cells and loss of polar organization. They also show early arrest of axillary meristem development and formation of ectopic meristematic cell clusters within the stem. LOM1 and LOM2 transcripts accumulate in the peripheral and basal zones of the SAM and in vascular strands. We show that LOM1 and LOM2 promote cell differentiation at the periphery of shoot meristems and help to maintain their polar organization.

Flower development

Flowers are the most complex structures of plants. Studies of Arabidopsis thaliana, which has typical eudicot flowers, have been fundamental in advancing the structural and molecular understanding of flower development. The main processes and stages of Arabidopsis flower development are summarized to provide a framework in which to interpret the detailed molecular genetic studies of genes assigned functions during flower development and is extended to recent genomics studies uncovering the key regulatory modules involved. Computational models have been used to study the concerted action and dynamics of the gene regulatory module that underlies patterning of the Arabidopsis inflorescence meristem and specification of the primordial cell types during early stages of flower development. This includes the gene combinations that specify sepal, petal, stamen and carpel identity, and genes that interact with them. As a dynamic gene regulatory network this module has been shown to converge to stable multigenic profiles that depend upon the overall network topology and are thus robust, which can explain the canalization of flower organ determination and the overall conservation of the basic flower plan among eudicots. Comparative and evolutionary approaches derived from Arabidopsis studies pave the way to studying the molecular basis of diverse floral morphologies.

The maize SBP-box transcription factor encoded by tasselsheath4 regulates bract development and the establishment of meristem boundaries

Plant architecture consists of repeating units called phytomers, each containing an internode, leaf and axillary meristem. The formation of boundaries within the phytomer is necessary to differentiate and separate these three components, otherwise some will grow at the expense of others. The microRNA-targeted SBP-box transcription factor tasselsheath4 (tsh4) plays an essential role in establishing these boundaries within the inflorescence. tsh4 mutants display altered phyllotaxy, fewer lateral meristems and ectopic leaves that grow at the expense of the meristem. Double-mutant analyses of tsh4 and several highly branched mutants, such as ramosa1-3 and branched silkless1, demonstrated a requirement for tsh4 in branch meristem initiation and maintenance. TSH4 protein, however, was localized throughout the inflorescence stem and at the base of lateral meristems, but not within the meristem itself. Double labeling of TSH4 with the ramosa2, branched silkless1 and knotted1 meristem markers confirmed that TSH4 forms a boundary adjacent to all lateral meristems. Indeed, double labeling of miR156 showed a meristem-specific pattern complementary to that of TSH4, consistent with tsh4 being negatively regulated by this microRNA. Thus, downregulation of TSH4 by a combination of microRNAs and branching pathway genes allows the establishment of lateral meristems and the repression of leaf initiation, thereby playing a major role in defining meristem versus leaf boundaries.