Trichomes: different regulatory networks lead to convergent structures

Autofluorescence as a Signal to Sort Developing Glandular Trichomes by Flow Cytometry

The industrial relevance of a number of metabolites produced in plant glandular trichomes (GTs) has spurred research on these specialized organs for a number of years. Most of the research, however, has focused on the elucidation of secondary metabolite pathways and comparatively little has been undertaken on the development and differentiation of GTs. One way to gain insight into these developmental processes is to generate stage-specific transcriptome and metabolome data. The difficulty for this resides in the isolation of early stages of development of the GTs. Here we describe a method for the separation and isolation of intact young and mature type VI trichomes from the wild tomato species Solanum habrochaites. The final and key step of the method uses cell sorting based on distinct autofluorescence signals of the young and mature trichomes. We demonstrate that sorting by flow cytometry allows recovering pure fractions of young and mature trichomes. Furthermore, we show that the sorted trichomes can be used for transcript and metabolite analyses. Because many plant tissues or cells have distinct autofluorescence components, the principles of this method can be generally applicable for the isolation of specific cell types without prior labeling.

The development of type VI glandular trichomes in the cultivated tomato Solanum lycopersicum and a related wild species S. habrochaites

BACKGROUND

Type VI glandular trichomes represent the most abundant trichome type on leaves and stems of tomato plants and significantly contribute to herbivore resistance, particularly in the wild species. Despite this, their development has been poorly studied so far. The goal of this study is to fill this gap. Using a variety of cell imaging techniques, a detailed record of the anatomy and developmental stages of type VI trichomes in the cultivated tomato (Solanum lycopersicum) and in a related wild species (S. habrochaites) is provided.

RESULTS

In both species, the development of these structures follows a highly reproducible cell division pattern. The two species differ in the shape of the trichome head which is round in S. habrochaites and like a four-leaf clover in S. lycopersicum, correlating with the presence of a large intercellular cavity in S. habrochaites where the produced metabolites accumulate. In both species, the junction between the intermediate cell and the four glandular cells constitute a breaking point facilitating the decapitation of the trichome and thereby the quick release of the metabolites. A strongly auto-fluorescent compound transiently accumulates in the early stages of development suggesting a potential role in the differentiation process. Finally, immuno-labelling with antibodies recognizing specific cell wall components indicate a key role of pectin and arabinogalactan components in the differentiation of type VI trichomes.

CONCLUSIONS

Our observations explain the adaptive morphologies of type VI trichomes for metabolite storage and release and provide a framework for further studies of these important metabolic cellular factories. This is required to better exploit their potential, in particular for the breeding of pest resistance in tomato.

Auxin Response Gene SlARF3 Plays Multiple Roles in Tomato Development and is Involved in the Formation of Epidermal Cells and Trichomes

The auxin response factor (ARF) genes encode a large family of proteins involved in auxin signaling transduction. SlARF3, a member of the ARF gene family, encodes a protein containing two conserved domains, B3 and ARF, and lacking an Aux/IAA domain. Expression analysis showed that SlARF3 has a particularly high expression level in trichomes. In situ hybridization also detected the SlARF3 transcripts in epidermal pavement cells of leaves. The physiological function of SlARF3 was studied by using the RNA interference (RNAi) strategy. SlARF3-down-regulated plants exhibited decreased density of epidermal pavement cells and obviously reduced density of type I, V and VI trichomes of leaves, which indicates the important role of SlARF3 in the formation of trichomes and epidermal cells in tomato. The number of shoot xylem cells was also decreased in SlARF3-down-regulated lines. Furthermore, RNA-sequencing (RNA-Seq) analysis identified 51 differentially expressed genes (DEGs) belonging to 14 transcription factor (TF) families, such as MYB, bHLH, WD40 and C2H2 zinc finger. Twenty-seven DEGs were involved in the metabolism and signaling transduction of phytohormones, such as auxin, ethylene and gibberellin. These results indicated the important roles of the TFs and hormones in auxin-dependent transcriptional regulation of trichome formation in tomato. Taken together, our results demonstrate that SlARF3 plays an important role in the formation of epidermal cells and trichomes and reveal novel and specific functions for ARFs in tomato developmental processes.

Transcriptome profile analysis of cell proliferation molecular processes during multicellular trichome formation induced by tomato Wov gene in tobacco

BACKGROUND

Trichomes, developing from the epidermis of nearly all terrestrial plants, provide good structural resistance against insect herbivores and an excellent model for studying the molecular mechanisms underlying cell fate determination. Regulation of trichomes in Rosids has been well characterized. However, little is known about the cell proliferation molecular processes during multicellular trichome formation in Asterids.

RESULTS

In this study, we identified two point mutations in a novel allele (Wov) at Wo locus. Ectopic expression of Wov in tobacco and potato induces much more trichome formation than wild type. To gain new insights into the underlying mechanisms during the processes of these trichomes formation, we compared the gene expression profiles between Wov transgenic and wild-type tobacco by RNA-seq analysis. A total of 544 co-DEGs were detected between transgenic and wild-type tobacco. Functional assignments of the co-DEGs indicated that 33 reliable pathways are altered in transgenic tobacco plants. The most noticeable pathways are fatty acid metabolism, amino acid biosynthesis and metabolism, and plant hormone signal transduction. Results suggest that these enhanced processes are critical for the cell proliferation during multicellular trichome formation in transgenic plants. In addition, the transcriptional levels of homologues of trichome regulators in Rosids were not significantly changed, whereas homologues of genes (Wo and SlCycB2) in Asterids were significantly upregulated in Wov transgenic tobacco plants.

CONCLUSIONS

This study presents a global picture of the gene expression changes induced by Wov-gene in tobacco. And the results provided us new insight into the molecular processes controlling multicellular formation in tobacco. Furthermore, we inferred that trichomes in solanaceous species might share a common network.

Molecular Framework of a Regulatory Circuit Initiating Two-Dimensional Spatial Patterning of Stomatal Lineage

Stomata, valves on the plant epidermis, are critical for plant growth and survival, and the presence of stomata impacts the global water and carbon cycle. Although transcription factors and cell-cell signaling components regulating stomatal development have been identified, it remains unclear as to how their regulatory interactions are translated into two-dimensional patterns of stomatal initial cells. Using molecular genetics, imaging, and mathematical simulation, we report a regulatory circuit that initiates the stomatal cell-lineage. The circuit includes a positive feedback loop constituting self-activation of SCREAMs that requires SPEECHLESS. This transcription factor module directly binds to the promoters and activates a secreted signal, EPIDERMAL PATTERNING FACTOR2, and the receptor modifier TOO MANY MOUTHS, while the receptor ERECTA lies outside of this module. This in turn inhibits SPCH, and hence SCRMs, thus constituting a negative feedback loop. Our mathematical model accurately predicts all known stomatal phenotypes with the inclusion of two additional components to the circuit: an EPF2-independent negative-feedback loop and a signal that lies outside of the SPCH•SCRM module. Our work reveals the intricate molecular framework governing self-organizing two-dimensional patterning in the plant epidermis.

The identification of Cucumis sativus Glabrous 1 (CsGL1) required for the formation of trichomes uncovers a novel function for the homeodomain-leucine zipper I gene

The spines and bloom of cucumber (Cucumis sativus L.) fruit are two important quality traits related to fruit market value. However, until now, none of the genes involved in the formation of cucumber fruit spines and bloom trichomes has been identified. Here, the characterization of trichome development in wild-type (WT) cucumber and a spontaneous mutant, glabrous 1 (csgl1) controlled by a single recessive nuclear gene, with glabrous aerial organs, is reported. Via map-based cloning, CsGL1 was isolated and it was found that it encoded a member of the homeodomain-leucine zipper I (HD-Zip I) proteins previously identified to function mainly in the abiotic stress responses of plants. Tissue-specific expression analysis indicated that CsGL1 was strongly expressed in trichomes and fruit spines. In addition, CsGL1 was a nuclear protein with weak transcriptional activation activity in yeast. A comparative analysis of the digital gene expression (DGE) profile between csgl1 and WT leaves revealed that CsGL1 had a significant influence on the gene expression profile in cucumber, especially on genes related to cellular process, which is consistent with the phenotypic difference between csgl1 and the WT. Moreover, two genes, CsMYB6 and CsGA20ox1, possibly involved in the formation of cucumber trichomes and fruit spines, were characterized. Overall, the findings reveal a new function for the HD-Zip I gene subfamily, and provide some candidate genes for genetic engineering approaches to improve cucumber fruit external quality.

Protein expression changes during cotton fiber elongation in response to drought stress and recovery

An investigation to better understand the molecular mechanism of cotton (Gossypium hirsutum L.) fiber elongation in response to drought stress and recovery was conducted using a comparative proteomics analysis. Cotton plants (cv. NuCOTN 33B) were subjected to water deprivation for 10 days followed by a recovery period (with watering) of 5 days. The temporal changes in total proteins in cotton fibers were examined using 2DE. The results revealed that 163 proteins are significantly drought responsive. MS analysis led to the identification of 132 differentially expressed proteins that include some known as well as some novel drought-responsive proteins. These drought responsive fiber proteins in NuCOTN 33B are associated with a variety of cellular functions, i.e. signal transduction, protein processing, redox homeostasis, cell wall modification, metabolisms of carbon, energy, lipid, lignin, and flavonoid. The results suggest that the enhancement of the perception of drought stress, a new balance of the metabolism of the biosynthesis of cell wall components and cytoskeleton homeostasis plays an important role in the response of cotton fibers to drought stress. Overall, the current study provides an overview of the molecular mechanism of drought response in cotton fiber cells.

Transcriptional control of flavonoid biosynthesis by MYB-bHLH-WDR complexes

Flavonoids are widely known for the colors they confer to plant tissues, their contribution to plant fitness and health benefits, and impact on food quality. As convenient biological markers, flavonoids have been instrumental in major genetic and epigenetic discoveries. We review recent advances in the characterization of the underlying regulatory mechanisms of flavonoid biosynthesis, with a special focus on the MBW (MYB-bHLH-WDR) protein complexes. These proteins are well conserved in higher plants. They participate in different types of controls ranging from fine-tuned transcriptional regulation by environmental factors to the initiation of the flavonoid biosynthesis pathway by positive regulatory feedback. The MBW protein complexes provide interesting models for investigating developmentally or environmentally controlled transcriptional regulatory networks.

A landscape of hairy and twisted: hunting for new trichome mutants in the Saskatoon Arabidopsis T-DNA population

A total of 88 new Arabidopsis lines with trichome variation were recovered by screening 49,200 single-seed descent T3 lines from the SK activation-tagged population and from a new 20,000-line T-DNA insertion population (called pAG). Trichome variant lines were classified into 12 distinct phenotype categories. Single or multiple T-DNA insertion sites were identified for 89% of these mutant lines. Alleles of the well-known trichome genes TRY, GL2 and TTG1 were recovered with atypical phenotype variation not reported previously. Moreover, atypical gene expression profiles were documented for two additional mutants specifying TRY and GL2 disruptions. In remaining mutants, ten lines were disrupted in genes coding for proteins not implicated in trichome development, five were disrupted in hypothetical proteins and 11 were disrupted in proteins with unknown function. The collection represents new opportunities for the plant biology community to define trichome development more precisely and to refine the function of individual trichome genes.

The R3-MYB gene GhCPC negatively regulates cotton fiber elongation

Cotton (Gossypium spp.) fibers are single-cell trichomes that arise from the outer epidermal layer of seed coat. Here, we isolated a R3-MYB gene GhCPC, identified by cDNA microarray analysis. The only conserved R3 motif and different expression between TM-1 and fuzzless-lintless mutants suggested that it might be a negative regulator in fiber development. Transgenic evidence showed that GhCPC overexpression not only delayed fiber initiation but also led to significant decreases in fiber length. Interestingly, Yeast two-hybrid analysis revealed an interaction complex, in which GhCPC and GhTTG1/4 separately interacted with GhMYC1. In transgenic plants, Q-PCR analysis showed that GhHOX3 (GL2) and GhRDL1 were significantly down regulated in -1-5 DPA ovules and fibers. In addition, Yeast one-hybrid analysis demonstrated that GhMYC1 could bind to the E-box cis-elements and the promoter of GhHOX3. These results suggested that GhHOX3 (GL2) might be downstream gene of the regulatory complex. Also, overexpression of GhCPC in tobacco led to differential loss of pigmentation. Taken together, the results suggested that GhCPC might negatively regulate cotton fiber initiation and early elongation by a potential CPC-MYC1-TTG1/4 complex. Although the fibers were shorter in transgenic cotton lines than in the wild type, no significant difference was detected in stem or leaf trichomes, even in cotton mutants (five naked seed or fuzzless), suggesting that fiber and trichome development might be regulated by two sets of genes sharing a similar model.

Mirid (Hemiptera: Heteroptera) specialists of sticky plants: adaptations, interactions, and ecological implications

Sticky plants-those having glandular trichomes (hairs) that produce adhesive, viscous exudates-can impede the movement of, and entrap, generalist insects. Disparate arthropod groups have adapted to these widespread and taxonomically diverse plants, yet their interactions with glandular hosts rarely are incorporated into broad ecological theory. Ecologists and entomologists might be unaware of even well-documented examples of insects that are sticky-plant specialists. The hemipteran family Miridae (more specifically, the omnivorous Dicyphini: Dicyphina) is the best-known group of arthropods that specializes on sticky plants. In the first synthesis of relationships with glandular plants for any insect family, we review mirid interactions with sticky hosts, including their adaptations (behavioral, morphological, and physiological) and mutualisms with carnivorous plants, and the ecological and agricultural implications of mirid-sticky plant systems. We propose that mirid research applies generally to tritrophic interactions on trichome-defended plants, enhances an understanding of insect-plant interactions, and provides information useful in managing crop pests.

Promoters of AaGL2 and AaMIXTA-Like1 genes of Artemisia annua direct reporter gene expression in glandular and non-glandular trichomes

Herein, we report cloning and analysis of promoters of GLABRA2 (AaGL2) homolog and a MIXTA-Like (AaMIXTA-Like1) gene from Artemisia annua. The upstream regulatory regions of AaGL2 and AaMIXTA-Like1 showed the presence of several crucial cis-acting elements. Arabidopsis and A. annua seedlings were transiently transfected with the promoter-GUS constructs using a robust agro-infiltration method. Both AaGL2 and AaMIXTA-Like1 promoters showed GUS expression preferentially in Arabidopsis single-celled trichomes and glandular as well as T-shaped trichomes of A. annua. Transgenic Arabidopsis harboring constructs in which AaGL2 or AaMIXTA-Like1 promoters would control GFP expression, showed fluorescence emanating specifically from trichome cells. Our study provides a fast and efficient method to study trichome-specific expression, and 2 promoters that have potential for targeted metabolic engineering in plants.

Finding our way through phenotypes

Despite a large and multifaceted effort to understand the vast landscape of phenotypic data, their current form inhibits productive data analysis. The lack of a community-wide, consensus-based, human- and machine-interpretable language for describing phenotypes and their genomic and environmental contexts is perhaps the most pressing scientific bottleneck to integration across many key fields in biology, including genomics, systems biology, development, medicine, evolution, ecology, and systematics. Here we survey the current phenomics landscape, including data resources and handling, and the progress that has been made to accurately capture relevant data descriptions for phenotypes. We present an example of the kind of integration across domains that computable phenotypes would enable, and we call upon the broader biology community, publishers, and relevant funding agencies to support efforts to surmount today's data barriers and facilitate analytical reproducibility.

RNA-seq analysis of Quercus pubescens Leaves: de novo transcriptome assembly, annotation and functional markers development

Quercus pubescens Willd., a species distributed from Spain to southwest Asia, ranks high for drought tolerance among European oaks. Q. pubescens performs a role of outstanding significance in most Mediterranean forest ecosystems, but few mechanistic studies have been conducted to explore its response to environmental constrains, due to the lack of genomic resources. In our study, we performed a deep transcriptomic sequencing in Q. pubescens leaves, including de novo assembly, functional annotation and the identification of new molecular markers. Our results are a pre-requisite for undertaking molecular functional studies, and may give support in population and association genetic studies.

254,265,700 clean reads were generated by the Illumina HiSeq 2000 platform, with an average length of 98 bp. De novo assembly, using CLC Genomics, produced 96,006 contigs, having a mean length of 618 bp. Sequence similarity analyses against seven public databases (Uniprot, NR, RefSeq and KOGs at NCBI, Pfam, InterPro and KEGG) resulted in 83,065 transcripts annotated with gene descriptions, conserved protein domains, or gene ontology terms. These annotations and local BLAST allowed identify genes specifically associated with mechanisms of drought avoidance. Finally, 14,202 microsatellite markers and 18,425 single nucleotide polymorphisms (SNPs) were, in silico, discovered in assembled and annotated sequences. We completed a successful global analysis of the Q. pubescens leaf transcriptome using RNA-seq. The assembled and annotated sequences together with newly discovered molecular markers provide genomic information for functional genomic studies in Q. pubescens, with special emphasis to response mechanisms to severe constrain of the Mediterranean climate. Our tools enable comparative genomics studies on other Quercus species taking advantage of large intra-specific ecophysiological differences.

Genome-wide identification and characterisation of R2R3-MYB genes in sugar beet (Beta vulgaris)

BACKGROUND

The R2R3-MYB genes comprise one of the largest transcription factor gene families in plants, playing regulatory roles in plant-specific developmental processes, metabolite accumulation and defense responses. Although genome-wide analysis of this gene family has been carried out in some species, the R2R3-MYB genes in Beta vulgaris ssp. vulgaris (sugar beet) as the first sequenced member of the order Caryophyllales, have not been analysed heretofore.

RESULTS

We present a comprehensive, genome-wide analysis of the MYB genes from Beta vulgaris ssp. vulgaris (sugar beet) which is the first species of the order Caryophyllales with a sequenced genome. A total of 70 R2R3-MYB genes as well as genes encoding three other classes of MYB proteins containing multiple MYB repeats were identified and characterised with respect to structure and chromosomal organisation. Also, organ specific expression patterns were determined from RNA-seq data. The R2R3-MYB genes were functionally categorised which led to the identification of a sugar beet-specific clade with an atypical amino acid composition in the R3 domain, putatively encoding betalain regulators. The functional classification was verified by experimental confirmation of the prediction that the R2R3-MYB gene Bv_iogq encodes a flavonol regulator.

CONCLUSIONS

This study provides the first step towards cloning and functional dissection of the role of MYB transcription factor genes in the nutritionally and evolutionarily interesting species B. vulgaris. In addition, it describes the flavonol regulator BvMYB12, being the first sugar beet R2R3-MYB with an experimentally proven function.

Transcriptome profiling reveals roles of meristem regulators and polarity genes during fruit trichome development in cucumber (Cucumis sativus L.)

Trichomes are epidermal hair-like structures that function in plant defence against biotic and abiotic stresses. Extensive studies have been performed on foliar trichomes development in Arabidopsis and tomato, but the molecular mechanism of fruit trichome formation remains elusive. Cucumber fruit is covered with trichomes (spines) that directly affect the appearance and quality of cucumber products. Here, we characterized the fruit spine development in wild-type (WT) cucumber and a spontaneous mutant, tiny branched hair (tbh). Our data showed that the cucumber trichome was multicellular and non-glandular, with malformed organelles and no endoreduplication. Fruit spine development was generally homogenous and marked by a rapid base expansion stage. Trichomes in the tbh mutant were tiny and branched, with increased density and aberrant cell shape. Transcriptome profiling indicated that meristem-related genes were highly enriched in the upregulated genes in the tbh versus the WT, as well as in WT spines after versus before base expansion, and that polarity regulators were greatly induced during spine base expansion. Quantitative reverse transcription PCR and in situ hybridization confirmed the differential expression of CUP-SHAPED COTYLEDON3 (CUC3) and SHOOT MERISTEMLESS (STM) during spine development. Therefore, cucumber trichomes are morphologically different from those of Arabidopsis and tomato, and their development may be regulated by a distinct pathway involving meristem genes and polarity regulators.

Genome-wide identification and characterization of R2R3MYB family in Solanum lycopersicum

The R2R3MYB proteins comprise one of the largest families of transcription factors and play regulatory roles in developmental processes and defense responses in plants. However, there has been relatively little effort to systematically carry out comprehensive genomic and functional analyses of these genes in tomato (Solanum lycopersicum L.), a reference species for Solanaceae plants, and the model plant for fruit development. In this study, a total of 121 R2R3MYB genes were identified in the tomato genome released recently and further classified into 29 subgroups based on the phylogenetic analysis of the complete protein sequences. Phylogenetic comparison of the members of this superfamily among tomato, Arabidopsis, grape, rice, poplar, soybean, cucumber and apple revealed that the putative functions of some tomato R2R3MYB proteins were clustered into the Arabidopsis functional clades. The chromosome distribution pattern revealed that tomato R2R3MYB genes were enriched on several chromosomes and 52 % of the family members were tandemly duplicated genes. Tissue specificity or different expression levels of SlR2R3MYBs in different tissues suggested differential regulation of tissue development as well as metabolic regulation. The transcript abundance level analysis during abiotic conditions identified a group of R2R3MYB genes that responded to one or more treatments suggesting that the SlR2R3MYBs played major roles in the plant response to abiotic conditions and involved in signal transduction pathways. This study not only provides a solid foundation for further functional dissection of tomato R2R3MYB family genes, but may also be profitable for, in the future, the improvement of tomato stress tolerance and fruit quality.

MYB82 functions in regulation of trichome development in Arabidopsis

Trichome initiation and patterning are controlled by the TTG1-bHLH-MYB regulatory complex. Several MYB transcription factors have been determined to function in trichome development via incorporation into this complex. This study examined the role of MYB82, an R2R3-MYB transcription factor, in Arabidopsis trichome development. MYB82 was revealed to be a nuclear-localized transcription activator. Suppression of MYB82 function by fusion with a dominant repression domain (SRDX) resulted in glabrous leaves, as did overexpression of N-terminal-truncated MYB82. Overexpression of MYB82 genomic sequence, but not its cDNA sequence, led to reduced trichome numbers. Further investigation indicated that at least one of the two introns in MYB82 is essential to the protein's trichome developmental function. An MYB-binding box was identified in the third exon of MYB82, which was inferred to be crucial for MYB82 function because the mutation of this box interfered with the ability of MYB82 to rescue the gl1 mutant. Protein interaction analysis revealed that MYB82 physically interacts with GLABRA3 (GL3). In addition, MYB82 and GL1 can form homodimers and heterodimers at R2R3-MYB domains, which may explain why their overexpression reduces trichome numbers. These results demonstrate the functional diversification of MYB82 and GL1 in trichome development.

Silencing the vacuolar invertase gene GhVIN1 blocks cotton fiber initiation from the ovule epidermis, probably by suppressing a cohort of regulatory genes via sugar signaling

Cotton fibers, the most important source of cellulose for the global textile industry, are single-celled trichomes derived from the ovule epidermis at or just prior to anthesis. Despite progress in understanding cotton fiber elongation and cell-wall biosynthesis, knowledge regarding the molecular basis of fiber cell initiation, the first step of fiber development determining the fiber yield potential, remains elusive. Here, we provide evidence that expression of a vacuolar invertase (VIN) is an early event that is essential for cotton fiber initiation. RNAi-mediated suppression of GhVIN1, a major VIN gene that is highly expressed in wild-type fiber initials, resulted in significant reduction of VIN activity and consequently a fiberless seed phenotype in a dosage dependent manner. The absence of a negative effect on seed development in these fiberless seeds indicates that the phenotype is unlikely to be due to lack of carbon nutrient. Gene expression analyses coupled with in vitro ovule culture experiments revealed that GhVIN1-derived hexose signaling may play an indispensable role in cotton fiber initiation, probably by regulating the transcription of several MYB transcription factors and auxin signaling components that were previously identified as required for fiber initiation. Together, the data represent a significant advance in understanding the mechanisms of cotton fiber initiation, and provide the first indication that VIN-mediated hexose signaling may act as an early event modulating the expression of regulatory genes and hence cell differentiation from the ovule epidermis.

Analysis of TTG1 function in Arabis alpina

BACKGROUND

In Arabidopsis thaliana (A. thaliana) the WD40 protein TRANSPARENT TESTA GLABRA1 (TTG1) controls five traits relevant for the adaptation of plants to environmental changes including the production of proanthocyanidin, anthocyanidin, seed coat mucilage, trichomes and root hairs. The analysis of different Brassicaceae species suggests that the function of TTG1 is conserved within the family.

RESULTS

In this work, we studied the function of TTG1 in Arabis alpina (A. alpina). A comparison of wild type and two Aattg1 alleles revealed that AaTTG1 is involved in the regulation of all five traits. A detailed analysis of the five traits showed striking phenotypic differences between A. alpina and A. thaliana such that trichome formation occurs also at later stages of leaf development and that root hairs form at non-root hair positions.

CONCLUSIONS

The evolutionary conservation of the regulation of the five traits by TTG1 on the one hand and the striking phenotypic differences make A. alpina a very interesting genetic model system to study the evolution of TTG1-dependent gene regulatory networks at a functional level.

An overview of the gene regulatory network controlling trichome development in the model plant, Arabidopsis

Trichomes are specialized epidermal cells located on aerial parts of plants and are associated with a wide array of biological processes. Trichomes protect plants from adverse conditions including UV light and herbivore attack and are also an important source of a number of phytochemicals. The simple unicellular trichomes of Arabidopsis serve as an excellent model to study molecular mechanism of cell differentiation and pattern formation in plants. The emerging picture suggests that the developmental process is controlled by a transcriptional network involving three major groups of transcription factors (TFs): the R2R3 MYB, basic helix-loop-helix (bHLH), and WD40 repeat (WDR) protein. These regulatory proteins form a trimeric activator complex that positively regulates trichome development. The single repeat R3 MYBs act as negative regulators of trichome development. They compete with the R2R3 MYBs to bind the bHLH factor and form a repressor complex. In addition to activator-repressor mechanism, a depletion mechanism may operate in parallel during trichome development. In this mechanism, the bHLH factor traps the WDR protein which results in depletion of WDR protein in neighboring cells. Consequently, the cells with high levels of bHLH and WDR proteins are developed into trichomes. A group of C2H2 zinc finger TFs has also been implicated in trichome development. Phytohormones, including gibberellins and jasmonic acid, play significant roles in this developmental process. Recently, microRNAs have been shown to be involved in trichome development. Furthermore, it has been demonstrated that the activities of the key regulatory proteins involved in trichome development are controlled by the 26S/ubiquitin proteasome system (UPS), highlighting the complexity of the regulatory network controlling this developmental process. To complement several excellent recent relevant reviews, this review focuses on the transcriptional network and hormonal interplay controlling trichome development in Arabidopsis.

Functional analysis of the seed coat-specific gene GbMYB2 from cotton

MYB transcription factors are essential for cotton fiber development. We isolated the R2R3-MYB gene GbMYB2 from Gossypium barbadense. RNA in situ hybridization analysis showed that GbMYB2 is mainly expressed in the outer integuments of cotton ovules and in elongating fibers. GbMYB2 expression increased throughout fiber initiation and during the elongation stage. The expression level of GbMYB2 was higher in the Gossypium hirsutum cultivar Xu142 than in the Xu142 (fl) mutant. Overexpression of GbMYB2 in Arabidopsis caused thicker leaf trichomes and longer roots to develop due to the activation of trichome development-related genes such as GL2. These results indicate that GbMYB2 is an R2R3-MYB gene that is involved in fiber development.

Induction of cotton ovule culture fibre branching by co-expression of cotton BTL, cotton SIM, and Arabidopsis STI genes

The highly elongated single-celled cotton fibre consists of lint and fuzz, similar to the Arabidopsis trichome. Endoreduplication is an important determinant in Arabidopsis trichome initiation and morphogenesis. Fibre development is also controlled by functional homologues of Arabidopsis trichome patterning genes, although fibre cells do not have a branched shape like trichomes. The identification and characterization of the homologues of 10 key Arabidopsis trichome branching genes in Gossypium arboreum are reported here. Nuclear ploidy of fibres was determined, and gene function in cotton callus and fibre cells was investigated. The results revealed that the nuclear DNA content was constant in fuzz, whereas a limited and reversible change occurred in lint after initiation. Gossypeum arboreum branchless trichomes (GaBLT) was not transcribed in fibres. The homologue of STICHEL (STI), which is essential for trichome branching, was a pseudogene in Gossypium. Targeted expression of GaBLT, Arabidopsis STI, and the cytokinesis-repressing GaSIAMESE in G. hirsutum fibre cells cultured in vitro resulted in branching. The findings suggest that the distinctive developmental mechanism of cotton fibres does not depend on endoreduplication. This important component may be a relic function that can be activated in fibre cells.

Functional roles of flavonoids in photoprotection: new evidence, lessons from the past

We discuss on the relative significance of different functional roles potentially served by flavonoids in photoprotection, with special emphasis to their ability to scavenge reactive oxygen species (ROS) and control the development of individual organs and whole plant. We propose a model in which chloroplast-located flavonoids scavenge H2O2 and singlet oxygen generated under excess light-stress, thus avoiding programmed cell death. We also draw a picture in which vacuolar flavonoids in conjunction with peroxidases and ascorbic acid constitute a secondary antioxidant system aimed at detoxifying H2O2, which may diffuse out of the chloroplast at considerable rates and enter the vacuole following excess light stress-induced depletion of ascorbate peroxidase. We hypothesize for flavonols key roles as developmental regulators in early and current-day land-plants, based on their ability to modulate auxin movement and auxin catabolism. We show that antioxidant flavonoids display the greatest capacity to regulate key steps of cell growth and differentiation in eukaryotes. These regulatory functions of flavonoids, which are shared by plants and animals, are fully accomplished in the nM concentration range, as likely occurred in early land plants. We therefore conclude that functions of flavonoids as antioxidants and/or developmental regulators flavonoids are of great value in photoprotection. We also suggest that UV-B screening was just one of the multiple functions served by flavonoids when early land-plants faced an abrupt increase in sunlight irradiance.

Mutual control of intracellular localisation of the patterning proteins AtMYC1, GL1 and TRY/CPC in Arabidopsis

Trichome and root hair patterning is governed by a gene regulatory network involving TTG1 and several homologous MYB and bHLH proteins. The bHLH proteins GL3 and EGL3 are core components that serve as a regulatory platform for the activation of downstream genes. In this study we show that a homologue of GL3 and EGL3, AtMYC1, can regulate the intracellular localisation of GL1 and TRY. AtMYC1 protein is predominantly localised in the cytoplasm and can relocate GL1 from the nucleus into the cytoplasm. Conversely, AtMYC1 can be recruited into the nucleus by TRY and CPC, concomitant with a strong accumulation of TRY and CPC in the nucleus. When AtMYC1 is targeted to the nucleus or cytoplasm by nuclear localisation or export signals (NLS or NES), respectively, the intracellular localisation of GL1 and TRY also changes accordingly. The biological significance of this intracellular localisation is suggested by the finding that the efficiency of rescue of trichome number is significantly altered in NES and NLS fusions as compared with wild-type AtMYC1. Genetic analysis of mutants and overexpression lines supports the hypothesis that AtMYC1 represses the activity of TRY and CPC.

Genome-wide analysis of small RNA and novel microRNA discovery during fiber and seed initial development in Gossypium hirsutum. L

Cotton is the source of the most important, renewable natural textile fiber and oil in the world. MicroRNAs (miRNAs) are endogenous, non-coding, approximately 18-24 nucleotides long RNAs and function in the negative regulation of their target genes. Two mostly overlapping libraries of small RNA molecules were constructed and sequenced, and served as repetition sets of data to identify miRNAs involved in fiber initiation and seed development. The D genome sequence of Gossypium raimondii was used in conjunction with EST sequences to predict miRNA precursors. Overall, 93 new miRNA precursors were identified, of which 28 belonged to 10 known families and the other 65 were considered to be novel miRNAs. Seven hundred EST sequences were proposed to be candidate target genes which involved in the regulation of a diverse group of genes with diverse functions and transcription factors. Some of the novel miRNAs and candidate target genes were validated by the Northern blot and rapid amplification of 5' cDNA ends (5' RACE).

Identification and characterization of cotton genes involved in fuzz-fiber development

Cotton fuzz fibers, like Arabidopsis trichomes, are elongated unicells. It is postulated that a transcriptional complex of GLABRA1 (GL1), GLABRA3 (GL3), and TRANSPARENT TESTAGLABRA1 (TTG1) might be in existence in Arabidopsis as evidenced by their physical interaction in yeast, and the complex regulates expression of GLABRA2 (GL2) controlling trichome cell differentiation; it is also assumed that TRIPTYCHON (TRY) and CAPRICE (CPC) counteract the complex formation in neighboring cells. Here, the homologs GaMYB23 (a homolog of GL1), GaDEL65 (a homolog of GL3), GaTTG1, GaCPC and GaTRY were identified in Gossypium arboreum. We show that GaMYB23 can bind to and activate the promoters of GaCPC, GaGL2 and GaTRY, and that GaMYB23, GaTRY and GaTTG1 could interact with GaDEL65 in yeast and in planta. In situ analysis showed that GaMYB23, GaGL2, GaDEL65, and GaTRY were predominantly expressed in fuzz fiber, but GaTRY proteins were primarily found in undeveloped epidermal cells. A G. arboreum fuzzless mutant with consistently high level GaMYB23 transcript has lost the detectable GaMYB23-promoter of GaGL2 complex, corresponding to sharply reduced transcription of GaGL2. Our results support that cotton homologs to the genetic molecules regulating Arabidopsis trichome differentiation interacted in the epidermis of ovules and the redundant GaMYB23 serves as a negative regulator in fuzz-fiber patterning.

Trichomes as models for studying plant cell differentiation

Trichomes, originating from epidermal cells, are present on nearly all terrestrial plants. They exist in diverse forms, are readily accessible, and serve as an excellent model system for analyzing the molecular mechanisms in plant cell differentiation, including cell fate choices, cell cycle control, and cell morphogenesis. In Arabidopsis, two regulatory models have been identified that function in parallel in trichome formation; the activator-inhibitor model and the activator-depletion model. Cotton fiber, a similar unicellular structure, is controlled by some functional homologues of Arabidopsis trichome-patterning genes. Multicellular trichomes, as in tobacco and tomato, may form through a distinct pathway from unicellular trichomes. Recent research has shown that cell cycle control participates in trichome formation. In this review, we summarize the molecular mechanisms involved in the formation of unicellular and multicellular trichomes, and discuss the integration of the cell cycle in its initiation and morphogenesis.

The interacting MYB75 and KNAT7 transcription factors modulate secondary cell wall deposition both in stems and seed coat in Arabidopsis

The Arabidopsis thaliana KNAT7 (KNOX family) and MYB75 (MYB family) transcription factors were each shown earlier to interact in yeast two-hybrid assays, and to modulate secondary cell wall formation in inflorescence stems. We demonstrate here that their interaction also occurs in vivo, and that specific domains of each protein mediate this process. The participation of these interacting transcription factors in secondary cell wall formation was then extended to the developing seed coat through the use of targeted transcript analysis and SEM in single loss-of-function mutants. Novel genetic and protein-protein interactions of MYB75 and KNAT7 with other transcription factors known to be involved in seed coat regulation were also identified. We propose that a MYB75-associated protein complex is likely to be involved in modulating secondary cell wall biosynthesis in both the Arabidopsis inflorescence stem and seed coat, and that at least some parts of the transcriptional regulatory network in the two tissues are functionally conserved.

Arabidopsis homeodomain-leucine zipper IV proteins promote stomatal development and ectopically induce stomata beyond the epidermis

The shoot epidermis of land plants serves as a crucial interface between plants and the atmosphere: pavement cells protect plants from desiccation and other environmental stresses, while stomata facilitate gas exchange and transpiration. Advances have been made in our understanding of stomatal patterning and differentiation, and a set of 'master regulatory' transcription factors of stomatal development have been identified. However, they are limited to specifying stomatal differentiation within the epidermis. Here, we report the identification of an Arabidopsis homeodomain-leucine zipper IV (HD-ZIP IV) protein, HOMEODOMAIN GLABROUS2 (HDG2), as a key epidermal component promoting stomatal differentiation. HDG2 is highly enriched in meristemoids, which are transient-amplifying populations of stomatal-cell lineages. Ectopic expression of HDG2 confers differentiation of stomata in internal mesophyll tissues and occasional multiple epidermal layers. Conversely, a loss-of-function hdg2 mutation delays stomatal differentiation and, rarely but consistently, results in aberrant stomata. A closely related HD-ZIP IV gene, Arabidopsis thaliana MERISTEM LAYER1 (AtML1), shares overlapping function with HDG2: AtML1 overexpression also triggers ectopic stomatal differentiation in the mesophyll layer and atml1 mutation enhances the stomatal differentiation defects of hdg2. Consistently, HDG2 and AtML1 bind the same DNA elements, and activate transcription in yeast. Furthermore, HDG2 transactivates expression of genes that regulate stomatal development in planta. Our study highlights the similarities and uniqueness of these two HD-ZIP IV genes in the specification of protodermal identity and stomatal differentiation beyond predetermined tissue layers.

A WUSCHEL-related homeobox 3B gene, depilous (dep), confers glabrousness of rice leaves and glumes

BACKGROUND

Glabrousness is an important agricultural trait for the practical breeding of rice. In this study, depilous (dep), the gene responsible for glabrous leaves and glumes of rice was identified by map-based cloning.

RESULTS

The dep gene encodes a WUSCHEL-related homeobox 3B that was fine-mapped to a 22-kb region on the short arm of chromosome 5 using progenies derived from crosses between Koshihikari (pubescent) and GLSL15, an Oryza glaberrima chromosome segment substitution line (glabrous). Complementation tests confirmed the conditioning of the glabrous phenotype by the dep gene. Phylogenetic analysis showed that dep groups with the WOX3 family of plant-specific homeobox transcription factors that are involved in regulating lateral organ development. Localization of dep in the nucleus indicates the function of the gene as a transcription factor. Spatial expression of the gene was observed in the base of young shoots, the leaf sheath, midrib, young roots and nodal structures.

CONCLUSION

The identification and cloning of dep will not only provide basis for future research on the elucidation of the molecular mechanisms underlying trichome formation in rice but will also aid in breeding programs for the development of glabrous varieties.

A WUSCHEL-like homeobox gene, OsWOX3B responses to NUDA/GL-1 locus in rice

BACKGROUND

Most of the rice varieties are pubescent. However, the presence of trichomes is an undesirable characteristic in rice production because trichomes can cause atmospheric pollution. The use of glabrous rice varieties represents a solution to this problem. Yunnan Nuda Rice, a glabrous cultivar that constitutes approximately 20% of rice germplasms in Yunnan can provide important recourse for breeding of glabrous rice varieties.

RESULTS

The "Nuda" phenotype in Yunnan Nuda Rice was found to be controlled by a single recessive allelic gene within the well-characterized GL-1 locus. A high-resolution genetic and physical map was constructed using 1,192 Nuda individuals from the F2 population that was delivered from the cross between the Yunnan Nuda variety HMK and the pubescent TN1 variety. The NUDA/GL-1 gene was mapped to a 28.5 kb region containing six annotated genes based on the Nipponbare genomic sequence. By comparing the sequences and expression patterns of different pubescent and glabrous varieties, LOC_Os05g02730, a WUSCHEL-like homeobox gene (OsWOX3B) was identified as the candidate gene. This hypothesis was confirmed by RNA interference (RNAi) and transgenic complementation. Trichome deficiency in RNAi lines was associated with increased efficiency of grain packaging but did not affect the main agronomic traits.

CONCLUSION

NUDA/GL-1 locus encodes OsWOX3B gene.

Evolutionary analysis of the MIXTA gene family highlights potential targets for the study of cellular differentiation

Differentiated epidermal cells such as trichomes and conical cells perform numerous essential functions in plant biology and are important for our understanding of developmental patterning and cell shape regulation. Many are also commercially significant, such as cotton fibers and trichomes that secrete pharmaceutically useful or herbivore-deterring compounds. Here, we focus on the phylogeny and evolution of the subgroup 9 R2R3 MYB gene transcription factors, which include the MIXTA gene, and that are important for the specification and regulation of plant cellular differentiation. We have sequenced 49 subgroup 9 R2R3 MYB genes from key experimental taxa and combined these sequences with those identified by an exhaustive bioinformatic search, to compile a data set of 223 subgroup 9 R2R3 MYB genes. Our phylogenetic analyses demonstrate, for the first time, the complex evolutionary history of the subgroup 9 R2R3 MYB genes. A duplication event is inferred before the origin of seed plants giving rise to two major gene lineages, here termed SBG9-A and SBG9-B. The evolutionary conservation of the SBG9-B gene lineage has not been previously recognized and its role in cellular differentiation is unknown, thus an entire clade of potential candidate genes for epidermal cell regulation remains to be explored. Using a heterologous transformation bioassay, we provide functional data that implicate members of the SBG9-B lineage in the specification of epidermal projections. Furthermore, we reveal numerous putative duplication events in both SBG9-A and SBG9-B lineages, resolving uncertainty about orthology and paralogy among the subgroup 9 R2R3 MYB genes. Finally, we provide a robust framework over which to interpret existing functional data and to direct ongoing comparative genetic research into the evolution of plant cellular diversity.

Flavonoids as antioxidants in plants: location and functional significance

Stress-responsive dihydroxy B-ring-substituted flavonoids have great potential to inhibit the generation of reactive oxygen species (ROS) and reduce the levels of ROS once they are formed, i.e., to perform antioxidant functions. These flavonoids are located within or in the proximity of centers of ROS generation in severely stressed plants. Efficient mechanisms have been recently identified for the transport of flavonoids from the endoplasmic reticulum, the site of their biosynthesis, to different cellular compartments. The mechanism underlying flavonoid-mediated ROS reduction in plants is still unclear. 'Antioxidant' flavonoids are found in the chloroplast, which suggests a role as scavengers of singlet oxygen and stabilizers of the chloroplast outer envelope membrane. Dihydroxy B-ring substituted flavonoids are present in the nucleus of mesophyll cells and may inhibit ROS-generation making complexes with Fe and Cu ions. The genes that govern the biosynthesis of antioxidant flavonoids are present in liverworts and mosses and are mostly up-regulated as a consequence of severe stress. This suggests that the antioxidant flavonoid metabolism is a robust trait of terrestrial plants. Vacuolar dihydroxy B-ring flavonoids have been reported to serve as co-substrates for vacuolar peroxidases to reduce H(2)O(2) escape from the chloroplast, following the depletion of ascorbate peroxidase activity. Antioxidant flavonoids may effectively control key steps of cell growth and differentiation, thus acting regulating the development of the whole plant and individual organs.

Genome-wide identification and characterization of R2R3MYB family in Cucumis sativus

BACKGROUND

The R2R3MYB proteins comprise one of the largest families of transcription factors in plants. Although genome-wide analysis of this family has been carried out in some species, little is known about R2R3MYB genes in cucumber (Cucumis sativus L.).

PRINCIPAL FINDINGS

This study has identified 55 R2R3MYB genes in the latest cucumber genome and the CsR2R3MYB family contained the smallest number of identified genes compared to other species that have been studied due to the absence of recent gene duplication events. These results were also supported by genome distribution and gene duplication analysis. Phylogenetic analysis showed that they could be classified into 11 subgroups. The evolutionary relationships and the intron-exon organizations that showed similarities with Arabidopsis, Vitis and Glycine R2R3MYB proteins were also analyzed and suggested strong gene conservation but also the expansions of particular functional genes during the evolution of the plant species. In addition, we found that 8 out of 55 (∼14.54%) cucumber R2R3MYB genes underwent alternative splicing events, producing a variety of transcripts from a single gene, which illustrated the extremely high complexity of transcriptome regulation. Tissue-specific expression profiles showed that 50 cucumber R2R3MYB genes were expressed in at least one of the tissues and the other 5 genes showed very low expression in all tissues tested, which suggested that cucumber R2R3MYB genes took part in many cellular processes. The transcript abundance level analysis during abiotic conditions (NaCl, ABA and low temperature treatments) identified a group of R2R3MYB genes that responded to one or more treatments.

CONCLUSIONS

This study has produced a comparative genomics analysis of the cucumber R2R3MYB gene family and has provided the first steps towards the selection of CsR2R3MYB genes for cloning and functional dissection that can be used in further studies to uncover their roles in cucumber growth and development.

Epidermal cell differentiation in cotton mediated by the homeodomain leucine zipper gene, GhHD-1

Gossypium hirsutum L. (cotton) fibres are specialized trichomes a few centimetres in length that grow from the seed coat. Few genes directly involved in the differentiation of these epidermal cells have been identified. These include GhMYB25-like and GhMYB25, two related MYB transcription factors that regulate fibre cell initiation and expansion. We have also identified a putative homeodomain leucine zipper (HD-ZIP) transcription factor, GhHD-1, expressed in trichomes and early fibres that might play a role in cotton fibre initiation. Here, we characterize GhHD-1 homoeologues from tetraploid G. hirsutum and show, using reporter constructs and quantitative real-time PCR (qRT-PCR), that they are expressed predominantly in epidermal tissues during early fibre development, and in other tissues bearing epidermal trichomes. Silencing of GhHD-1 reduced trichome formation and delayed the timing of fibre initiation. Constitutive overexpression of GhHD-1 increased the number of fibres initiating on the seed, but did not affect leaf trichomes. Expression of GhHD-1 in cotton silenced for different fibre MYBs suggest that in ovules it acts downstream of GhMYB25-like, but is unaffected in GhMYB25- or GhMYB109-silenced plants. Microarray analysis of silencing and overexpression lines of GhHD-1 indicated that it potentially regulates the levels of ethylene and reactive oxidation species (ROS) through a WRKY transcription factor and calcium-signalling pathway genes to activate downstream genes necessary for cell expansion and elongation.

The apple WD40 protein MdTTG1 interacts with bHLH but not MYB proteins to regulate anthocyanin accumulation

The abundance of anthocyanins and proanthocyanins in apples is tightly regulated by three classes of regulatory factors, MYB, bHLH and WD40 proteins, only some of which have been previously identified. In this study, we identified an apple WD40 protein (MdTTG1) that promotes the accumulation of anthocyanins. The biosynthetic genes required downstream in the flavonoid pathway were up-regulated when MdTTG1 was over-expressed in Arabidopsis. Consistent with its role as a transcriptional regulator, an MdTTG1-GFP fusion protein was observed only in the nucleus. We assayed the expression patterns of this gene in different organs and found that they were positively correlated with anthocyanin accumulation in the apple. Yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated that MdTTG1 interacted with bHLH transcription factors (TFs) but not MYB protein, whereas bHLH was known to interact with MYB in apples. However, based on a ChIP assay, MdTTG1 does not appear to bind to the promoter of the anthocyanin biosynthetic genes MdDFR and MdUFGT. Taken together, these results suggest that the apple WD40 protein MdTTG1 interacts with bHLH but not MYB proteins to regulate anthocyanin accumulation.

The tomato SlIAA15 is involved in trichome formation and axillary shoot development

The Aux/IAA genes encode a large family of short-lived proteins known to regulate auxin signalling in plants. Functional characterization of SlIAA15, a member of the tomato (Solanum lycopersicum) Aux/IAA family, shows that the encoded protein acts as a strong repressor of auxin-dependent transcription. The physiological significance of SlIAA15 was addressed by a reverse genetics approach, revealing that SlIAA15 plays multiple roles in plant developmental processes. The SlIAA15 down-regulated lines display lower trichome number, reduced apical dominance with associated modified pattern of axillary shoot development, increased lateral root formation and decreased fruit set. Moreover, the leaves of SlIAA15-inhibited plants are dark green and thick, with larger pavement cells, longer palisade cells and larger intercellular space of spongy mesophyll cells. The SlIAA15-suppressed plants exhibit a strong reduction in type I, V and VI trichome formation, suggesting that auxin-dependent transcriptional regulation is required for trichome initiation. Concomitant with reduced trichome formation, the expression of some R2R3 MYB genes, putatively involved in the control of trichome differentiation, is altered. These phenotypes uncover novel and specialized roles for Aux/IAAs in plant developmental processes, clearly indicating that members of the Aux/IAA gene family in tomato perform both overlapping and specific functions.

Glandular trichomes: what comes after expressed sequence tags?

Glandular trichomes cover the surface of many plant species. They exhibit tremendous diversity, be it in their shape or the compounds they secrete. This diversity is expressed between species but also within species or even individual plants. The industrial uses of some trichome secretions and their potential as a defense barrier, for example against arthropod pests, has spurred research into the biosynthesis pathways that lead to these specialized metabolites. Because complete biosynthesis pathways take place in the secretory cells, the establishment of trichome-specific expressed sequence tag libraries has greatly accelerated their elucidation. Glandular trichomes also have an important metabolic capacity and may be considered as true cell factories. To fully exploit the potential of glandular trichomes as breeding or engineering objects, several research areas will have to be further investigated, such as development, patterning, metabolic fluxes and transcription regulation. The purpose of this review is to provide an update on the methods and technologies which have been used to investigate glandular trichomes and to propose new avenues of research to deepen our understanding of these specialized structures.

Fine-mapping of the woolly gene controlling multicellular trichome formation and embryonic development in tomato

Trichomes are small hairs that originate from the epidermal cells of nearly all land plants, and they exist in unicellular and multicellular forms. The regulatory pathway of unicellular trichomes in Arabidopsis is well characterized. However, little is known about the multicellular trichome formation in tomato (Solanum lycopersicum). The woolly (Wo) gene controls multicellular trichome initiation and leads to embryonic lethality when homozygous in tomato. To clone and characterize Wo, the gene was fine-mapped to a DNA fragment of ~200 kb using the map-based cloning strategy. A series of sequence-based molecular markers, including simple sequence repeat, sequence characterized amplified region, and cleaved amplified polymorphic sequence were utilized in this study. Analysis of the sequence indicated that this region carries 19 putative open reading frames. These results will provide not only the important information for the isolation and characterization of Wo but also the starting point for studying the regulatory pathway responsible for trichome formation and embryonic lethality in tomato.

A regulatory gene induces trichome formation and embryo lethality in tomato

Trichomes are universal biological structures originating from the aerial epidermis, which serve as an excellent model to study plant differentiation at the cell level. Although the pathway regulating trichome formation in the Rosids has been well characterized, only very recently a few genes were identified for trichome initiation in the Asterids. In this study, we cloned Woolly (Wo), essential for trichome formation in tomato. Transgenic experiments revealed that the woolly phenotype is caused by the mutation in Wo which encodes a homeodomain protein containing a bZIP motif and a START domain. We identified three alleles of Wo and found that each allele contains a missense mutation, which respectively results in an amino acid substitution at the C terminus. Microarray and expression analysis showed that the expression of a B-type cyclin gene, SlCycB2, is possibly regulated by Wo, which also participates in trichome formation. Suppression of Wo or SlCycB2 expression by RNAi decreased the number of type I trichomes, and direct protein-protein interaction was detected between them, implying that both proteins may work together in the regulation of this type of trichome formation. Cytological observation and Wo transcript analysis in the developing seeds showed that embryo development was also correlated with Wo.

Differential regulation of a MYB transcription factor is correlated with transgenerational epigenetic inheritance of trichome density in Mimulus guttatus

• Epigenetic inheritance, transgenerational transmission of traits not proximally determined by DNA sequence, has been linked to transmission of chromatin modifications and gene regulation, which are known to be sensitive to environmental factors. Mimulus guttatus increases trichome (plant hair) density in response to simulated herbivore damage. Increased density is expressed in progeny even if progeny do not experience damage. To better understand epigenetic inheritance of trichome production, we tested the hypothesis that candidate gene expression states are inherited in response to parental damage. • Using M. guttatus recombinant inbred lines, offspring of leaf-damaged and control plants were raised without damage. Relative expression of candidate trichome development genes was measured in offspring. Line and parental damage effects on trichome density were measured. Associations between gene expression, trichome density, and response to parental damage were determined. • We identified M. guttatus MYB MIXTA-like 8 as a possible negative regulator of trichome development. We found that parental leaf damage induces down-regulation of MYB MIXTA-like 8 in progeny, which is associated with epigenetically inherited increased trichome density. • Our results link epigenetic transmission of an ecologically important trait with differential gene expression states - providing insight into a mechanism underlying environmentally induced 'soft inheritance'.

The Jasmonate-ZIM-domain proteins interact with the WD-Repeat/bHLH/MYB complexes to regulate Jasmonate-mediated anthocyanin accumulation and trichome initiation in Arabidopsis thaliana

Jasmonates (JAs) mediate plant responses to insect attack, wounding, pathogen infection, stress, and UV damage and regulate plant fertility, anthocyanin accumulation, trichome formation, and many other plant developmental processes. Arabidopsis thaliana Jasmonate ZIM-domain (JAZ) proteins, substrates of the CORONATINE INSENSITIVE1 (COI1)-based SCF(COI1) complex, negatively regulate these plant responses. Little is known about the molecular mechanism for JA regulation of anthocyanin accumulation and trichome initiation. In this study, we revealed that JAZ proteins interact with bHLH (Transparent Testa8, Glabra3 [GL3], and Enhancer of Glabra3 [EGL3]) and R2R3 MYB transcription factors (MYB75 and Glabra1), essential components of WD-repeat/bHLH/MYB transcriptional complexes, to repress JA-regulated anthocyanin accumulation and trichome initiation. Genetic and physiological evidence showed that JA regulates WD-repeat/bHLH/MYB complex-mediated anthocyanin accumulation and trichome initiation in a COI1-dependent manner. Overexpression of the MYB transcription factor MYB75 and bHLH factors (GL3 and EGL3) restored anthocyanin accumulation and trichome initiation in the coi1 mutant, respectively. We speculate that the JA-induced degradation of JAZ proteins abolishes the interactions of JAZ proteins with bHLH and MYB factors, allowing the transcriptional function of WD-repeat/bHLH/MYB complexes, which subsequently activate respective downstream signal cascades to modulate anthocyanin accumulation and trichome initiation.

Evolutionary and comparative analysis of MYB and bHLH plant transcription factors

The expansion of gene families encoding regulatory proteins is typically associated with the increase in complexity characteristic of multi-cellular organisms. The MYB and basic helix-loop-helix (bHLH) families provide excellent examples of how gene duplication and divergence within particular groups of transcription factors are associated with, if not driven by, the morphological and metabolic diversity that characterize the higher plants. These gene families expanded dramatically in higher plants; for example, there are approximately 339 and 162 MYB and bHLH genes, respectively, in Arabidopsis, and approximately 230 and 111, respectively, in rice. In contrast, the Chlamydomonas genome has only 38 MYB genes and eight bHLH genes. In this review, we compare the MYB and bHLH gene families from structural, evolutionary and functional perspectives. The knowledge acquired on the role of many of these factors in Arabidopsis provides an excellent reference to explore sequence-function relationships in crops and other plants. The physical interaction and regulatory synergy between particular sub-classes of MYB and bHLH factors is perhaps one of the best examples of combinatorial plant gene regulation. However, members of the MYB and bHLH families also interact with a number of other regulatory proteins, forming complexes that either activate or repress the expression of sets of target genes that are increasingly being identified through a diversity of high-throughput genomic approaches. The next few years are likely to witness an increasing understanding of the extent to which conserved transcription factors participate at similar positions in gene regulatory networks across plant species.

GhMYB25-like: a key factor in early cotton fibre development

MYB transcription factors have been implicated in regulation of the development of ovule epidermal cells into the elongated seed fibres of cotton. An R2R3 MYB, GhMYB25-like, identified from its reduced expression in a fibreless mutant of cotton (Xu142 fl), is here shown to play a key role in the very early stages of fibre cell differentiation. A GhMYB25-like promoter-GUS construct was expressed predominantly in the epidermal layers of cotton ovules before anthesis (-3days post-anthesis, dpa), increasing in expression in 0-dpa ovules, primarily in those epidermal cells expanding into fibres, and then in elongating fibres at +3dpa, declining thereafter. This was consistent with GhMYB25-like transcript abundance during fibre development. RNA interference suppression of GhMYB25-like resulted in cotton plants with fibreless seeds, but normal trichomes elsewhere, phenocopying the Xu142 fl mutant. Like Xu142 fl these plants had reduced expression of the fibre-expressed MYBs, GhMYB25 and GhMYB109, indicating that GhMYB25-like is upstream from those MYBs. This hierarchy was supported by the absence of any change in transcript level of GhMYB25-like in GhMYB25- and GhMYB109-silenced transgenic lines. Transgenic cotton with an additional copy of the native gene had elevated expression of GhMYB25-like in ovules, but no obvious increase in fibre initials, suggesting that there may be other factors that interact with GhMYB25-like to differentiate epidermal cells into fibre cells.

Dissection of the phytohormonal regulation of trichome formation and biosynthesis of the antimalarial compound artemisinin in Artemisia annua plants

• Biosynthesis of the sesquiterpene lactone and potent antimalarial drug artemisinin occurs in glandular trichomes of Artemisia annua plants and is subjected to a strict network of developmental and other regulatory cues. • The effects of three hormones, jasmonate, gibberellin and cytokinin, were studied at the structural and molecular levels in two different A. annua chemotypes by microscopic analysis of gland development, and by targeted metabolite and transcript profiling. Furthermore, a genome-wide cDNA-amplified fragment length polymorphism (AFLP)-based transcriptome profiling was carried out of jasmonate-elicited leaves at different developmental stages. • Although cytokinin and gibberellin positively affected at least one aspect of gland formation, these two hormones did not stimulate artemisinin biosynthesis. Only jasmonate simultaneously promoted gland formation and coordinated transcriptional activation of biosynthetic gene expression, which ultimately led to increased sesquiterpenoid accumulation with chemotype-dependent effects on the distinct pathway branches. Transcriptome profiling revealed a trichome-specific fatty acyl- coenzyme A reductase, trichome-specific fatty acyl-CoA reductase 1 (TFAR1), the expression of which correlates with trichome development and sesquiterpenoid biosynthesis. • TFAR1 is potentially involved in cuticular wax formation during glandular trichome expansion in leaves and flowers of A. annua plants. Analysis of phytohormone-modulated transcriptional regulons provides clues to dissect the concerted regulation of metabolism and development of plant trichomes.

New insights into the mechanism of development of Arabidopsis root hairs and trichomes

Epidermis cell differentiation in Arabidopsis thaliana is a model system for understanding the mechanisms leading to the developmental end state of plant cells. Both root hairs and trichomes differentiate from epidermal cells and molecular genetic analyses using Arabidopsis mutants have demonstrated that the differentiation of root hairs and trichomes is regulated by similar molecular mechanisms. Molecular-genetic approaches have led to the identification of many genes that are involved in epidermal cell differentiation, most of which encode transcription factors that induce the expression of genes active in both root hair and trichome development. Control of cell growth after fate determination has also been studied using Arabidopsis mutants.