Trichomes: different regulatory networks lead to convergent structures

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.

Polyploidization altered gene functions in cotton (Gossypium spp.)

Cotton (Gossypium spp.) is an important crop plant that is widely grown to produce both natural textile fibers and cottonseed oil. Cotton fibers, the economically more important product of the cotton plant, are seed trichomes derived from individual cells of the epidermal layer of the seed coat. It has been known for a long time that large numbers of genes determine the development of cotton fiber, and more recently it has been determined that these genes are distributed across At and Dt subgenomes of tetraploid AD cottons. In the present study, the organization and evolution of the fiber development genes were investigated through the construction of an integrated genetic and physical map of fiber development genes whose functions have been verified and confirmed. A total of 535 cotton fiber development genes, including 103 fiber transcription factors, 259 fiber development genes, and 173 SSR-contained fiber ESTs, were analyzed at the subgenome level. A total of 499 fiber related contigs were selected and assembled. Together these contigs covered about 151 Mb in physical length, or about 6.7% of the tetraploid cotton genome. Among the 499 contigs, 397 were anchored onto individual chromosomes. Results from our studies on the distribution patterns of the fiber development genes and transcription factors between the At and Dt subgenomes showed that more transcription factors were from Dt subgenome than At, whereas more fiber development genes were from At subgenome than Dt. Combining our mapping results with previous reports that more fiber QTLs were mapped in Dt subgenome than At subgenome, the results suggested a new functional hypothesis for tetraploid cotton. After the merging of the two diploid Gossypium genomes, the At subgenome has provided most of the genes for fiber development, because it continues to function similar to its fiber producing diploid A genome ancestor. On the other hand, the Dt subgenome, with its non-fiber producing D genome ancestor, provides more transcription factors that regulate the expression of the fiber genes in the At subgenome. This hypothesis would explain previously published mapping results. At the same time, this integrated map of fiber development genes would provide a framework to clone individual full-length fiber genes, to elucidate the physiological mechanisms of the fiber differentiation, elongation, and maturation, and to systematically study the functional network of these genes that interact during the process of fiber development in the tetraploid cottons.

Mapping of an anthocyanin-regulating MYB transcription factor and its expression in red and green pear, Pyrus communis

'Max Red Bartlett' is a red bud mutation of the yellow pear (Pyrus communis L.) cultivar 'Williams' (known as 'Bartlett' in North America). Anthocyanins are the most important pigments for red colour in fruits. Synthesis of anthocyanins is mediated by a number of well-characterized enzymes that include chalcone synthase (CHS), flavanone-3-hydroxylase (F3H), dihydroflavonol-4-reductase (DFR), anthocyanidin synthase (ANS), and UDP-glucose:flavonoid-3-O-glucosyltransferase (UFGT). Expression of the genes encoding these five enzymes was examined in pear fruit skin in order to elucidate the molecular mechanism for red coloration. In addition, the gene PcMYB10, encoding an R2R3 MYB transcription factor involved in anthocyanin biosynthetic pathway regulation, was isolated from both 'Williams' and 'Max Red Bartlett'. Analysis of the deduced amino acid sequence suggests that this gene is an ortholog of anthocyanin regulators known in other plant species. Its expression level was significantly higher in 'Max Red Bartlett' (red pear) compared with the original yellow variety 'Williams'. Although the map position of PcMYB10 corresponds to that of MdMYBa and MdMYB10, which control pigmentation of apple fruit skin, PcMYB10 is not directly responsible for red versus yellow colour in the two pear varieties, as the mutation underlying this difference maps to a different region of the pear genome.

Analysis of purified glabra3-shapeshifter trichomes reveals a role for NOECK in regulating early trichome morphogenic events

Transcriptome analysis using the Affymetrix ATH1 platform has been completed on purified trichomes from the gl3-sst mutant. These trichomes display immature features, such as glassy cell walls and blunted branches. The gl3-sst trichome transcriptome was greatly enriched for genes involved in lipid biosynthesis, including those mediating the synthesis of fatty acids and wax. In addition, gl3-sst trichomes displayed reduced expression of the R3 MYBs TRY and CPC, which normally function to limit trichome development. The expression of the MIXTA-like MYB gene NOK was elevated. Members of the MIXTA-like family promote conical cell outgrowth, and in some cases, trichome initiation in diverse plant species. In contrast, NOK limits trichome outgrowth in wild-type Arabidopsis plants. Similar to other MIXTA-like genes, NOK was required for the expansion of gl3-sst trichomes, as the gl3-sst nok double mutant trichomes were greatly reduced in size. Expression of NOK in nok mutants reduced branch formation, whereas in gl3-sst nok, NOK expression promoted trichome cell outgrowth, illustrating duel roles for NOK in both promoting and limiting trichome development. MIXTA-like genes from phylogenetically diverse plant species could substitute for NOK in both nok and gl3-sst nok backgrounds. These findings suggest that certain aspects of NOK and MIXTA-like gene function have been conserved.

Characterization and fine mapping of the glabrous leaf and hull mutants (gl1) in rice (Oryza sativa L.)

The glabrous leaf and hull (gl1) mutants were isolated from M(2) generation of indica cultivar 93-11. These mutants produced smooth leaves and hairless glumes under normal growth conditions. By analyzing through scanning electron microscope, it was revealed that the leaf trichomes, including macro and micro hairs, were deficient in these mutants. Genetic analysis indicated that the mutation was controlled by a single recessive gene. Using nine SSR markers and one InDel marker, the gl1 gene was mapped between RM1200 and RM2010 at the short arm of chromosome 5, which was consistent with the mapping of gl1 in previous studies. To facilitate the map-based cloning of the gl1 gene, 12 new InDel markers were developed. A high-resolution genetic and physical map was constructed by using 1,396 mutant individuals of F(2) mapping population. Finally, the gl1 was fine mapped in 54-kb region containing 10 annotated genes. Cloning and sequencing of the target region from four gl1 mutants (gl1-1, gl1-2, gl1-3 and gl1-4) and four glabrous rice varieties (Jackson, Jefferson, Katy and Lemont) all showed that the same single point mutation (A-->T) occurred in the 5'-untranslated region (UTR) of the locus Os05g0118900 (corresponding to the 3'-UTR of STAR2). RT-PCR analysis of the locus Os05g0118900 revealed that its mRNA expression level was normal in gl1 mutant. RNA secondary structure prediction showed that the single point mutation resulted in a striking RNA conformational change. These results suggest that the single point mutation is most likely responsible for the glabrous leaf and hull phenotypes in rice.

Fine mapping of pepper trichome locus 1 controlling trichome formation in Capsicum annuum L. CM334

Trichomes are present on nearly all land plants and protect plants against insect herbivores, drought and UV radiation. The trichome-bearing phenotype is conferred by the dominant allele of the pepper trichome locus 1 (Ptl1) in Capsicum annuum, Mexican 'Criollo de Morelos-334' (CM334). A genetic analysis using simple sequence repeats from pepper cDNA identified the HpmsE031 marker as tightly linked to Ptl1 in 653 individuals of an F(2) population derived from a cross between CM334 and Chilsungcho varieties. A bacterial artificial chromosome (BAC) library from CM334 covering 12x of the genome was screened using the HpmsE031 SSR marker as a probe and three BAC clones were identified. The Ptl1 region was covered by one 80 kb BAC clone, TT1B7. Fluorescence in situ hybridization (FISH) confirmed that TT1B7 localized to pepper chromosome 10. One co-dominant marker, Tco, and one dominant marker, Tsca, were successfully developed from the TT1B7 BAC sequence. Tco mapped 0.33 cM up from Ptl1 and Tsca mapped 0.75 cM down from Ptl1. Analysis of the BAC sequence predicts the presence of 14 open reading frames including 60S ribosomal protein L21-like protein (Solanum demissum), protein kinase 2 (Nicotiana tabacum), hypothetical proteins, and unnamed protein products. These results will provide not only useful information for map-based cloning of Ptl1 in Capsicum but also the starting points for analysis of R-gene cluster inked with Ptl1.

Phylogenetically distinct cellulose synthase genes support secondary wall thickening in arabidopsis shoot trichomes and cotton fiber

Abstract Through exploring potential analogies between cotton seed trichomes (or cotton fiber) and arabidopsis shoot trichomes we discovered that CesAs from either the primary or secondary wall phylogenetic clades can support secondary wall thickening. CesA genes that typically support primary wall synthesis, AtCesA1,2,3,5, and 6, underpin expansion and secondary wall thickening of arabidopsis shoot trichomes. In contrast, apparent orthologs of CesA genes that support secondary wall synthesis in arabidopsis xylem, AtCesA4,7, and 8, are up-regulated for cotton fiber secondary wall deposition. These conclusions arose from: (a) analyzing the expression of CesA genes in arabidopsis shoot trichomes; (b) observing birefringent secondary walls in arabidopsis shoot trichomes with mutations in AtCesA4, 7, or 8; (c) assaying up-regulated genes during different stages of cotton fiber development; and (d) comparing genes that were co-expressed with primary or secondary wall CesAs in arabidopsis with genes up-regulated in arabidopsis trichomes, arabidopsis secondary xylem, or cotton fiber during primary or secondary wall deposition. Cumulatively, the data show that: (a) the xylem of arabidopsis provides the best model for secondary wall cellulose synthesis in cotton fiber; and (b) CesA genes within a "cell wall toolbox" are used in diverse ways for the construction of particular specialized cell walls.

Transgenic apple plants overexpressing the Lc gene of maize show an altered growth habit and increased resistance to apple scab and fire blight

Transgenic apple plants (Malus x domestica cv. 'Holsteiner Cox') overexpressing the Leaf Colour (Lc) gene from maize (Zea mays) exhibit strongly increased production of anthocyanins and flavan-3-ols (catechins, proanthocyanidins). Greenhouse plants investigated in this study exhibit altered phenotypes with regard to growth habit and resistance traits. Lc-transgenic plants show reduced size, transversal gravitropism of lateral shoots, reduced trichome development, and frequently reduced shoot diameter and abnormal leaf development with fused leaves. Such phenotypes seem to be in accordance with a direct or an indirect effect on polar-auxin-transport in the transgenic plants. Furthermore, leaves often develop necrotic lesions resembling hypersensitive response lesions. In tests, higher resistance against fire blight (caused by the bacterium Erwinia amylovora) and against scab (caused by the fungus Venturia inaequalis) is observed. These phenotypes are discussed with respect to the underlying altered physiology of the Lc-transgenic plants. The results are expected to be considered in apple breeding strategies.

Hormone-mediated promotion of trichome initiation in plants is conserved but utilizes species- and trichome-specific regulatory mechanisms

Plant trichome initiation is steered by diverse developmental and environmental cues, through molecular mechanisms that remain elusive in most plant species. Using a robust experimental method to investigate the molecular mechanisms by which phytohormones modulate leaf trichome formation, we verified the effect of jasmonates, cytokinins and gibberellins in Arabidopsis (Arabidopsis thaliana). All three phytohormones promoted Arabidopsis trichome initiation, but caused divergent effects on trichome maturation and other leaf parameters. Molecular analysis indicated that the phytohormones mediated trichome initiation by the transcriptional regulation of the components of the TRANSPARENT TESTA GLABRA1 (TTG1) activator/inhibitor complex. In this addendum, we additionally studied the effects of jasmonates, cytokinins and gibberellins on leaf trichome formation in a representative set of plant species, spanning the angiosperm lineage and covering different trichome types. We found that the general ability of the three phytohormones to impinge on trichome initiation is conserved across angiosperms, but that within a particular plant species distinct regulatory networks might be activated to steer the formation of the various trichome types.

A WD40 repeat protein from Medicago truncatula is necessary for tissue-specific anthocyanin and proanthocyanidin biosynthesis but not for trichome development

WD40 repeat proteins regulate biosynthesis of anthocyanins, proanthocyanidins (PAs), and mucilage in the seed and the development of trichomes and root hairs. We have cloned and characterized a WD40 repeat protein gene from Medicago truncatula (MtWD40-1) via a retrotransposon-tagging approach. Deficiency of MtWD40-1 expression blocks accumulation of mucilage and a range of phenolic compounds, including PAs, epicatechin, other flavonoids, and benzoic acids, in the seed, reduces epicatechin levels without corresponding effects on other flavonoids in flowers, reduces isoflavone levels in roots, but does not impair trichome or root hair development. MtWD40-1 is expressed constitutively, with highest expression in the seed coat, where its transcript profile temporally parallels those of PA biosynthetic genes. Transcript profile analysis revealed that many genes of flavonoid biosynthesis were down-regulated in a tissue-specific manner in M. truncatula lines harboring retrotransposon insertions in the MtWD40-1 gene. MtWD40-1 complemented the anthocyanin, PA, and trichome phenotypes of the Arabidopsis (Arabidopsis thaliana) transparent testa glabrous1 mutant. We discuss the function of MtWD40-1 in natural product formation in M. truncatula and the potential use of the gene for engineering PAs in the forage legume alfalfa (Medicago sativa).

An ortholog of MIXTA-like2 controls epidermal cell shape in flowers of Thalictrum

Here, we investigated the genetic underpinnings of pollination-related floral phenotypes in Thalictrum, a ranunculid with apetalous flowers. The variable presence of petaloid features in other floral organs correlates with distinct adaptations to insect vs. wind pollination. Conical cells are present in sepals or stamens of insect-pollinated species, and in stigmas. We characterized a Thalictrum ortholog of the Antirrhinum majus transcription factor MIXTA-like2, responsible for conical cells, from three species with distinct floral morphologies, representing two pollination syndromes. Genes were cloned by PCR and analysed phylogenetically. Expression analyses were conducted by quantitative PCR and in situ hybridization, followed by functional studies in transgenic tobacco. The cloned genes encode R2R3 MYB proteins closely related to Antirrhinum AmMYBML2 and Petunia hybrida PhMYB1. Spatial expression by in situ hybridization overlaps areas of conical cells. Overexpression in tobacco induces cell outgrowths in carpel epidermis and significantly increases the height of petal conical cells. We have described the first orthologs of AmMIXTA-like2 outside the core eudicots, likely ancestral to the MIXTA/MIXTA-like1 duplication. The conserved role in epidermal cell elongation results in conical cells, micromorphological markers for petaloidy. This adaptation to attract insect pollinators was apparently lost after the evolution of wind pollination in Thalictrum.

CPC, a single-repeat R3 MYB, is a negative regulator of anthocyanin biosynthesis in Arabidopsis

Single-repeat R3 MYB transcription factors like CPC (CAPRICE) are known to play roles in developmental processes such as root hair differentiation and trichome initiation. However, none of the six Arabidopsis single-repeat R3 MYB members has been reported to regulate flavonoid biosynthesis. We show here that CPC is a negative regulator of anthocyanin biosynthesis. In the process of using CPC to test GAL4-dependent driver lines, we observed a repression of anthocyanin synthesis upon GAL4-mediated CPC overexpression. We demonstrated that this is not due to an increase in nutrient uptake because of more root hairs. Rather, CPC expression level tightly controls anthocyanin accumulation. Microarray analysis on the whole genome showed that, of 37 000 features tested, 85 genes are repressed greater than three-fold by CPC overexpression. Of these 85, seven are late anthocyanin biosynthesis genes. Also, anthocyanin synthesis genes were shown to be down-regulated in 35S::CPC overexpression plants. Transient expression results suggest that CPC competes with the R2R3-MYB transcription factor PAP1/2, which is an activator of anthocyanin biosynthesis genes. This report adds anthocyanin biosynthesis to the set of programs that are under CPC control, indicating that this regulator is not only for developmental programs (e.g. root hairs, trichomes), but can influence anthocyanin pigment synthesis.

The MYB transcription factor GhMYB25 regulates early fibre and trichome development

Little is still known about the developmental control of the long seed coat trichomes of cotton (Gossypium hirsutum L.). In Arabidopsis, leaf trichome initiation is regulated by a group of well-defined transcription factors that includes MYB and homeodomain types. Many MYBs are expressed in fibres, but their roles in fibre development remain unclear. We analysed the function of one MYB transcription factor, GhMYB25, identified from transcriptome comparisons between wild-type and fibreless cotton mutants. A GhMYB25 promoter-GUS construct in transgenic cotton was expressed in the epidermis of ovules, developing fibre initials and fibres, in the trichomes of a number of tissues including leaves, stems and petals, as well as in the anthers, pollen and the epidermal layers of roots and root initials, but not in root hairs. Cotton is an allotetraploid with two very similar GhMYB25 genes that were silenced by a single RNAi construct. GhMYB25-silenced cotton showed alterations in the timing of rapid fibre elongation, resulting in short fibres, dramatic reductions in trichomes on other parts of the plant, and reductions in seed production. Reciprocal crosses between transgenic and non-transgenic plants indicated that pollen and ovule viability per se were not disrupted. Ectopic over-expression of GhMYB25 had more subtle impacts, with increases in cotton fibre initiation and leaf trichome number. High expression appeared to adversely affect fertility. Our results provide convincing evidence for a role of GhMYB25, like other MIXTA-like MYBS, in regulating specialized outgrowths of epidermal cells, including, in this case, cotton fibres.

Jasmonic acid control of GLABRA3 links inducible defense and trichome patterning in Arabidopsis

Once attacked by herbivores, plants regenerate new leaves with increased trichome density as an inducible defense. Trichomes are specified from neighboring epidermal cells through local cell-cell interactions in the leaf primordia. However, the molecular mechanism of how herbivore-induced damage at older leaves remodels the pattern of trichome fate specification at newly forming leaves is largely unknown. In this study, we show that mutations in either the biosynthetic or signaling pathway of jasmonates (JAs),long-distance wound signals, abolish the wound-induced formation of trichomes. To identify the factors linking JA signaling to trichome fate specification,we isolated a novel class of mutants, unarmed (urm), which lack trichome induction but show otherwise normal responses to JAs. URM9 encodes an Importin β family protein, and URM23 is identical to TRANSPARENT TESTA GLABRA1 (TTG1), the product of which interacts with the bHLH transcription factor GLABRA3 (GL3). Loss of either URM9 or URM23 disrupts the subnuclear localization of GL3, thus implicating GL3 in trichome induction. The expression of GL3 was enhanced by JA treatment prior to trichome initiation. Genetic analysis of multiple trichome mutants shows that GL3, in concert with the R2R3-Myb transcription factor GLABRA1 (GL1), promotes trichome fate in response to JA in a dosage-dependent manner. These results indicate that GL3 is a key transcription factor of wound-induced trichome formation acting downstream of JA signaling in Arabidopsis.

A systems approach reveals regulatory circuitry for Arabidopsis trichome initiation by the GL3 and GL1 selectors

Position-dependent cell fate determination and pattern formation are unique aspects of the development of plant structures. The establishment of single-celled leaf hairs (trichomes) from pluripotent epidermal (protodermal) cells in Arabidopsis provides a powerful system to determine the gene regulatory networks involved in cell fate determination. To obtain a holistic view of the regulatory events associated with the differentiation of Arabidopsis epidermal cells into trichomes, we combined expression and genome-wide location analyses (ChIP-chip) on the trichome developmental selectors GLABRA3 (GL3) and GLABRA1 (GL1), encoding basic helix-loop-helix (bHLH) and MYB transcription factors, respectively. Meta-analysis was used to integrate genome-wide expression results contrasting wild type and gl3 or gl1 mutants with changes in gene expression over time using inducible versions of GL3 and GL1. This resulted in the identification of a minimal set of genes associated with the differentiation of epidermal cells into trichomes. ChIP-chip experiments, complemented by the targeted examination of factors known to participate in trichome initiation or patterning, identified about 20 novel GL3/GL1 direct targets. In addition to genes involved in the control of gene expression, such as the transcription factors SCL8 and MYC1, we identified SIM (SIAMESE), encoding a cyclin-dependent kinase inhibitor, and RBR1 (RETINOBLASTOMA RELATED1), corresponding to a negative regulator of the cell cycle transcription factor E2F, as GL3/GL1 immediate targets, directly implicating these trichome regulators in the control of the endocycle. The expression of many of the identified GL3/GL1 direct targets was specific to very early stages of trichome initiation, suggesting that they participate in some of the earliest known processes associated with protodermal cell differentiation. By combining this knowledge with the analysis of genes associated with trichome formation, our results reveal the architecture of the top tiers of the hierarchical structure of the regulatory network involved in epidermal cell differentiation and trichome formation.

The establishment of single-celled leaf hairs (trichomes) from pluripotent epidermal (protodermal) cells provides a powerful system to determine the gene regulatory networks involved in plant cell fate determination. Two transcription factors—GL1 and GL3—have been associated with the initiation of trichome formation; yet only a handful of GL1-GL3–regulated genes have previously been characterized. In this study, we combined expression analyses performed in a number of different genotypes to identify a minimal set of about 500 genes associated with trichome formation. We also used ChIP-chip to identify a set of about 20 genes that are immediate targets of GL3 and GL1. Many more genes are targeted by GL1 or by GL3, likely in cooperation with other bHLH of MYB partners, but not by both GL1 and GL3. As predicted for genes involved in the initiation of epidermal cell fate determination, several of the GL3/GL1 direct targets are expressed early during trichome formation, including the transcription factors MYC1 (bHLH), SCL8 (GRAS), and genes involved in the regulation of the endocycle (SIM and RBR1). Co-expression analyses permitted us to identify sets of target genes likely downstream of the GL3/GL1 regulated transcription factors, providing the first steps towards building the regulatory network associated with the differentiation of protodermal cells into trichomes.

Floral isolation, specialized pollination, and pollinator behavior in orchids

Floral isolation is a form of prepollination reproductive isolation mediated by floral morphology (morphological isolation) and pollinator behavior (ethological isolation). Here we review mechanisms and evolutionary consequences of floral isolation in various pollination systems. Furthermore, we compare key features of floral isolation, i.e., pollinator sharing and specialization in pollination, in different orchid pollination systems. In orchid pollination, pollinator sharing is generally low, indicating strong floral isolation. The pollinators' motivation to visit flowers (specifically) can be due to both foraging or reproductive behavior. In both types of behavior, innate preferences for floral signals can be quickly overruled by learning. In pollination systems in which reproductive behavior of pollinators triggers flower visits, lower pollinator sharing was evident compared with systems with foraging behavior, probably because pollinators displaying reproductive behavior show higher fidelity in their visitation patterns. Orchids pollinated through reproductive behavior also use fewer pollinators than orchids pollinated through foraging behavior. No association between specialization and pollinator sharing was found. Thus, generalized pollination does not impede floral isolation, as orchids with many pollinators may nonetheless have low pollinator sharing. Specialization in pollination was, however, linked to orchid species richness in our analysis. Flower size, spur, and column morphology are most important for morphological isolation, and floral scent is most important for ethological isolation. These traits may be based on few genes, implying that floral isolation can be brought about by few genes of large effect.

The Arabidopsis MYB5 transcription factor regulates mucilage synthesis, seed coat development, and trichome morphogenesis

The Arabidopsis thaliana MYB5 gene is expressed in trichomes and seeds, including the seed coat. Constitutive expression of MYB5 resulted in the formation of more small trichomes and ectopic trichomes and a reduction in total leaf trichome numbers and branching. A myb5 mutant displayed minimal changes in trichome morphology, while a myb23 mutant produced increased numbers of small trichomes and two-branched trichomes. A myb5 myb23 double mutant developed more small rosette trichomes and two-branched trichomes than the single mutants. These results indicate that MYB5 and MYB23 regulate trichome extension and branching. The seed coat epidermal cells of myb5 and myb5 myb23 were irregular in shape, developed flattened columellae, and produced less mucilage than those of the wild type. Among the downregulated genes identified in the myb5 seeds using microarray analysis were ABE1 and ABE4 (alpha/beta fold hydrolase/esterase genes), MYBL2, and GLABRA2. The same genes were also downregulated in transparent testa glabra1 (ttg1) seeds, suggesting that MYB5 collaborates with TTG1 in seed coat development. These genes were upregulated in leaves and roots by ectopically expressed MYB5. The MYBL2, ABE1, and ABE4 promoters were active in seeds, including seed coats, and the latter two also in trichomes. Models of the MYB5 regulatory networks involved in seed coat and trichome development are presented.

Arabidopsis transient expression analysis reveals that activation of GLABRA2 may require concurrent binding of GLABRA1 and GLABRA3 to the promoter of GLABRA2

Transcription factors regulate gene expression by directly binding the cis-acting regulatory elements of target genes via their DNA-binding domains or by interacting with other transcription factors. Trichome cell fate determination in Arabidopsis utilizes a lateral inhibition mechanism that relies on the interplay of transcription factors. GLABRA1 (GL1), an R2R3 MYB transcription factor, GLABRA3 (GL3), a basic helix-loop-helix (bHLH) transcription factor, and TRANSPARENT TESTA GLABRA1 (TTG1), a WD40 protein, are believed to form a transcriptional activator complex to control the transcription of GLABRA2 (GL2), which in turn induces trichome formation in shoots. However, the molecular mechanism of the regulation of GL2 expression by this activator complex is still poorly understood. Here we report that GL1 and GL3 control GL2 expression by a previously unrecognized mechanism in which in addition to the protein-protein interaction between GL1 and GL3, concurrent binding of GL1 and GL3 to the promoter of GL2 via their own DNA-binding domains is probably required to activate GL2. We demonstrate that disruption or deletion of the DNA-binding domains in either GL1 or GL3 completely abolishes the transcriptional activity of the GL1-GL3 complex in activating GL2. These results provide new insight into the interplay of GL1 and GL3 transcription factors in the activation of GL2.

Transcriptional profiling of mature Arabidopsis trichomes reveals that NOECK encodes the MIXTA-like transcriptional regulator MYB106

Leaf hairs (trichomes) of Arabidopsis (Arabidopsis thaliana) have been extensively used as a model to address general questions in cell and developmental biology. Here, we lay the foundation for a systems-level understanding of the biology of this model cell type by performing genome-wide gene expression analyses. We have identified 3,231 genes that are up-regulated in mature trichomes relative to leaves without trichomes, and we compared wild-type trichomes with two mutants, glabra3 and triptychon, that affect trichome morphology and physiology in contrasting ways. We found that cell wall-related transcripts were particularly overrepresented in trichomes, consistent with their highly elaborated structure. In addition, trichome expression maps revealed high activities of anthocyanin, flavonoid, and glucosinolate pathways, indicative of the roles of trichomes in the biosynthesis of secondary compounds and defense. Interspecies comparisons revealed that Arabidopsis trichomes share many expressed genes with cotton (Gossypium hirsutum) fibers, making them an attractive model to study industrially important fibers. In addition to identifying physiological processes involved in the development of a specific cell type, we also demonstrated the utility of transcript profiling for identifying and analyzing regulatory gene function. One of the genes that are differentially expressed in fibers is the MYB transcription factor GhMYB25. A combination of transcript profiling and map-based cloning revealed that the NOECK gene of Arabidopsis encodes AtMYB106, a MIXTA-like transcription factor and homolog of cotton GhMYB25. However, in contrast to Antirrhinum, in which MIXTA promotes epidermal cell outgrowth, AtMYB106 appears to function as a repressor of cell outgrowth in Arabidopsis.

Functional specialization of the TRANSPARENT TESTA GLABRA1 network allows differential hormonal control of laminal and marginal trichome initiation in Arabidopsis rosette leaves

Trichome initiation in Arabidopsis (Arabidopsis thaliana) is controlled by the TRANSPARENT TESTA GLABRA1 (TTG1) network that consists of R2R3- and R1-type MYB-related transcription factors, basic helix-loop-helix (bHLH) proteins, and the WD40 protein TTG1. An experimental method was designed to investigate the molecular mechanisms by which jasmonates, cytokinins, and gibberellins modulate Arabidopsis leaf trichome formation. All three phytohormones provoked a seemingly common effect on cell patterning by promoting trichome initiation but caused strikingly distinct effects on cell and trichome maturation. The phytohormonal control was mediated by transcriptional regulation of the established TTG1 complex and depended on the R2R3-MYB factor GLABRA1. However, unsuspected degrees of functional specialization of the bHLH factors and a resultant differential molecular regulation of trichome initiation on leaf lamina and leaf margins were revealed. Trichome formation on leaf lamina relied entirely on GLABRA3 and ENHANCER OF GLABRA3. Conversely, TRANSPARENT TESTA8 (TT8) was particularly important for marginal trichome development. This hitherto unknown role for TT8 in trichome formation further underscored the functional redundancy between the three TTG1-dependent bHLH proteins.

The genus antirrhinum (snapdragon): a flowering plant model for evolution and development

INTRODUCTIONThe Antirrhinum species group comprises approximately 20 morphologically diverse members that are able to form fertile hybrids. It includes the cultivated snapdragon Antirrhinum majus, which has been used as a model for biochemical and developmental genetics for more than 75 yr. The research infrastructure for A. majus, together with the interfertility of the species group, allows Antirrhinum to be used to examine the genetic basis for plant diversity.

The R2R3 MYB transcription factor GhMYB109 is required for cotton fiber development

Cotton (Gossypium hirsutum L.) fibers are single highly elongated cells derived from the outer epidermis of ovules. A large number of genes are required for fiber differentiation and development, but so far, little is known about how these genes control and regulate the process of fiber development. Here we examine the role of the cotton-fiber-specific R2R3 MYB gene GhMYB109 in cotton fiber development. Transgenic reporter gene analysis revealed that a 2-kb GhMYB109 promoter was sufficient to confirm its fiber-specific expression. Antisense-mediated suppression of GhMYB109 led to a substantial reduction in fiber length. Consistently, several genes related to cotton fiber growth were found to be significantly reduced in the transgenic cotton. Our results showed that GhMYB109 is required for cotton fiber development and reveal a largely conserved mechanism of the R2R3 MYB transcription factor in cell fate determination in plants.

MYBL2 is a new regulator of flavonoid biosynthesis in Arabidopsis thaliana

In Arabidopsis thaliana, several MYB and basic helix-loop-helix (BHLH) proteins form ternary complexes with TTG1 (WD-Repeats) and regulate the transcription of genes involved in anthocyanin and proanthocyanidin (PA) biosynthesis. Similar MYB-BHLH-WDR (MBW) complexes control epidermal patterning and cell fates. A family of small MYB proteins (R3-MYB) has been shown to play an important role in the regulation of epidermal cell fates, acting as inhibitors of the MBW complexes. However, so far none of these small MYB proteins have been demonstrated to regulate flavonoid biosynthesis. The genetic and molecular analyses presented here demonstrated that Arabidopsis MYBL2, which encodes a R3-MYB-related protein, is involved in the regulation of flavonoid biosynthesis. The loss of MYBL2 activity in the seedlings of two independent T-DNA insertion mutants led to a dramatic increase in the accumulation of anthocyanin. In addition, overexpression of MYBL2 in seeds inhibited the biosynthesis of PAs. These changes in flavonoid content correlate well with the increased level of mRNA of several structural and regulatory anthocyanin biosynthesis genes. Interestingly, transient expression analyses in A. thaliana cells suggested that MYBL2 interacts with MBW complexes in planta and directly modulates the expression of flavonoid target genes. These results are fully consistent with the molecular interaction of MYBL2 with BHLH proteins observed in yeast. Finally, MYBL2 expression studies, including its inhibition by light-induced stress, allowed us to hypothesise a physiological role for MYBL2. Taken together, these results bring new insights into the transcriptional regulation of flavonoid biosynthesis and provide new clues and tools for further investigation of its developmental and environmental regulation.

The HD-Zip IV gene GaHOX1 from cotton is a functional homologue of the Arabidopsis GLABRA2

Most of the plant homeodomain-containing proteins play important roles in organ patterning and development, and Arabidopsis GLABRA2 (GL2), a member of the class IV homeodomain-leucine zipper (HD-ZIP) proteins, is a trichome and non-root hair cell regulator. Here we report the analysis of two cotton homeodomain-containing proteins, GaHOX1 and GaHOX2, isolated from the diploid cotton Gossypium arboreum. Both GaHOX1 and GaHOX2 belong to the class IV HD-ZIP family. When expressed under the control of the GL2 promoter, GaHOX1 rescued trichome development of an Arabidopsis glabrous mutant of gl2-2 (SALK_130213), whereas GaHOX2 did not. On the other hand, expression of GaHOX1 with a Cauliflower mosaic virus (CaMV) 35S promoter in the wild-type Arabidopsis plants suppressed the trichome development just as the GL2 ectopic expression. Expression analysis by Northern, RT-PCR and in situ hybridization indicated that GaHOX1 is predominantly expressed in cotton fiber cells at early developmental stages, consistent with its putative role in regulating cotton fiber development, while GaHOX2 is expressed in both fiber and other ovular tissues, including outer and inner integuments. Our results suggest that GaHOX1 is a functional homolog of GL2 in plant trichome development.

Promoter of a cotton fibre MYB gene functional in trichomes of Arabidopsis and glandular trichomes of tobacco

Cotton fibres are unicellular seed trichomes. Our previous study suggested that the cotton R2R3 MYB transcript factor GaMYB2 is a functional homologue of the Arabidopsis trichome regulator GLABRA1 (GL1). Here, the GaMYB2 promoter activity is reported in cotton (Gossypium hirsutum), tobacco (Nicotiana tabacum), and Arabidopsis plants. A 2062 bp promoter of GaMYB2 was isolated from G. arboreum, and fused to a beta-glucuronidase (GUS) reporter gene. In cotton, the GaMYB2 promoter exhibited activities in developing fibre cells and trichomes of other aerial organs, including leaves, stems and bracts. In Arabidopsis the promoter was specific to trichomes. Different from Arabidopsis and cotton that have unicellular non-glandular simple trichomes, tobacco plants contain more than one type of trichome, including multicellular simple and glandular secreting trichomes (GSTs). Interestingly, in tobacco plants the GaMYB2 promoter directed GUS expression exclusively in glandular cells of GSTs. A series of 5'-deletions revealed that a 360 bp fragment upstream to the translation initiation codon was sufficient to drive gene expression. A putative cis-element of the T/G-box was located at -233 to -214; a yeast one-hybrid assay showed that Arabidopsis bHLH protein GLABRA3 (GL3), also a trichome regulator, and GhDEL65, a GL3-like cotton protein, had high binding activities to the T/G-box motif. Overexpression of GL3 or GhDEL65 enhanced the GaMYB2 promoter activity in transgenic Arabidopsis plants. A comparison of GaMYB2 promoter specificities in trichomes of different plant species with different types of trichomes provides a tool for further dissection of plant trichome structure and development.

Gene-rich islands for fiber development in the cotton genome

Cotton fiber is an economically important seed trichome and the world's leading natural fiber used in the manufacture of textiles. As a step toward elucidating the genomic organization and distribution of gene networks responsible for cotton fiber development, we investigated the distribution of fiber genes in the cotton genome. Results revealed the presence of gene-rich islands for fiber genes with a biased distribution in the tetraploid cotton (Gossypium hirsutum L.) genome that was also linked to discrete fiber developmental stages based on expression profiles. There were 3 fiber gene-rich islands associated with fiber initiation on chromosome 5, 3 islands for the early to middle elongation stage on chromosome 10, 3 islands for the middle to late elongation stage on chromosome 14, and 1 island on chromosome 15 for secondary cell wall deposition, for a total of 10 fiber gene-rich islands. Clustering of functionally related gene clusters in the cotton genome displaying similar transcriptional regulation indicates an organizational hierarchy with significant implications for the genetic enhancement of particular fiber quality traits. The relationship between gene-island distribution and functional expression profiling suggests for the first time the existence of functional coupling gene clusters in the cotton genome.

SAD2 in Arabidopsis functions in trichome initiation through mediating GL3 function and regulating GL1, TTG1 and GL2 expression

Most genes identified that control Arabidopsis trichome initiation and formation are transcription factors or regulatory components in transcriptional networks and include GLABROUS1 (GL1), GLABRA2 (GL2), GLABRA3 (GL3) and TRANSPARENT TESTA GLABRA1 (TTG1). Herein, we report that an importin beta-like protein, SENSITIVE TO ABA AND DROUGHT2 (SAD2), is required for trichome initiation. Mutations in SAD2 disrupted trichome initiation resulting in reduced trichome number, but had no effect on trichome development or root hair number and development. Expression levels of GL1, MYB23, GL2 and TTG1 were reduced in shoots of sad2 mutants while expression levels of GL3 and ENHANCER OF GLABRA3 (EGL3) were enhanced. Overexpression of GL3 increased trichome numbers in wild type but not in sad2 mutants, indicating that the function of the GL3 protein is altered in the sad2 mutants. In contrast, overexpression of GFP-GL1 decreased trichome number in both wild type and sad2. Double mutant analysis of gl1 sad2 and gl3 sad2 indicated that SAD2 functions genetically, at least in part, in the same pathway with these two genes. Co-immunoprecipitation indicated that the sad2 mutation does not disrupt formation of the TTG1-GL3-GL1 complex. Analysis of GFP fusions of GL1, GL2, GL3 and TTG1 suggested that these proteins are most likely not direct cargo of SAD2. Our data suggest that SAD2 is involved in trichome initiation by regulating these nuclear genes.

Mutagenesis in Petunia x hybrida Vilm. and isolation of a novel morphological mutant

Traditionally, mutagenesis has been used to introduce novel genetic variability in ornamental crops. More recently, it has become a powerful tool in gene discovery and functional analyses in reverse genetics approaches. The present work aimed to compare the efficiency of physical and chemical agents in generating mutant populations of petunia. We have indirectly evaluated the genomic damage by analyzing developmental characteristics of the plantlets derived from treated seeds employing gamma radiation at 0, 20, 40, 60, 80 and 100 Gy and the alkylating agent ethyl-methanesulfonate (EMS) at 0, 0.05, 0.1, 0.15, 0.2 and 0.25% (v/v). Gamma rays and EMS caused developmental defects and decreased seedling viability in plants obtained from the mutagenized seeds. High mutagen doses reduced in approximately 44% the number of plants with primary leaves at 15 days after sowing (DAS) and decreased seedling survival rates to 55% (gamma) and 28% (EMS), in comparison to untreated controls. Seedling height decrease was proportional to increasing EMS dosage, whereas 40 and 60 Gy of gamma irradiation caused the most significant reduction in height. Moderate DNA damage allowing a high saturation of mutant alleles in the genome and the generation of viable plants for reverse genetics studies was correlated to the biological parameter LD50, the dose required to kill half of the tested population. It corresponded to 100 Gy for gamma radiation and 0.1% for EMS treatment. The optimized mutagen treatments were used to develop petunia mutant populations (M1 and M2) and novel morphological mutants were identified.

Molecular mechanisms of floral mimicry in orchids

Deceptive plants do not produce floral rewards, but attract pollinators by mimicking signals of other organisms, such as food plants or female insects. Such floral mimicry is particularly common in orchids, in which flower morphology, coloration and odour play key roles in deceiving pollinators. A better understanding of the molecular bases for these traits should provide new insights into the occurrence, mechanisms and evolutionary consequences of floral mimicry. It should also reveal the molecular bases of pollinator-attracting signals, in addition to providing strategies for manipulating insect behaviour in general. Here, we review data on the molecular bases for traits involved in floral mimicry, and we describe methodological advances helpful for the functional evaluation of key genes.

Harnessing plant trichome biochemistry for the production of useful compounds

Plant trichomes come in a variety of shapes, sizes and cellular composition. Some types, commonly called glandular trichomes, produce large amounts of specialized (secondary) metabolites of diverse classes. Trichomes are implicated in a variety of adaptive processes, including defense against herbivores and micro-organisms as well as in ion homeostasis. Because trichomes protrude from the epidermis and can often be easily separated from it and harvested, the mRNAs, proteins and small molecules that they contain are unusually accessible to analysis. This property makes them excellent experimental systems for identification of the enzymes and pathways responsible for the synthesis of the specialized metabolites found in these structures and sometimes elsewhere in the plant. We review the literature on the biochemistry of trichomes and consider the attributes that might make them highly useful targets for plant metabolic engineering.

Plant systems biology comes of age

'Omics' research approaches have produced copious data for living systems, which have necessitated the development of systems biology to integrate multidimensional biological information into networks and models. Applications of systems biology to plant science have been rapid, and have increased our knowledge about circadian rhythms, multigenic traits, stress responses and plant defenses, and have advanced the virtual plant project.

Cell division and endoreduplication: doubtful engines of vegetative growth

Currently, there is little information to indicate whether plant cell division and development is the collective effect of individual cell programming (cell-based) or is determined by organ-wide growth (organismal). Modulation of cell division does not confirm cell autonomous programming of cell expansion; instead, final cell size seems to be determined by the balance between cells formed and subsequent tissue growth. Control of growth in regions of the plant therefore has great importance in determining cell, organ and plant development. Here, we question the view that formation of new cells and their programmed expansion is the driving force of growth. We believe there is evidence that division does not drive, but requires, cell growth and a similar requirement for growth is detected in the modified cycle termed endoreduplication.

Regulation of the anthocyanin biosynthetic pathway by the TTG1/bHLH/Myb transcriptional complex in Arabidopsis seedlings

In all higher plants studied to date, the anthocyanin pigment pathway is regulated by a suite of transcription factors that include Myb, bHLH and WD-repeat proteins. However, in Arabidopsis thaliana, the Myb regulators remain to be conclusively identified, and little is known about anthocyanin pathway regulation by TTG1-dependent transcriptional complexes. Previous overexpression of the PAP1 Myb suggested that genes from the entire phenylpropanoid pathway are targets of regulation by Myb/bHLH/WD-repeat complexes in Arabidopsis, in contrast to other plants. Here we demonstrate that overexpression of Myb113 or Myb114 results in substantial increases in pigment production similar to those previously seen as a result of over-expression of PAP1, and pigment production in these overexpressors remains TTG1- and bHLH-dependent. Also, plants harboring an RNAi construct targeting PAP1 and three Myb candidates (PAP2, Myb113 and Myb114) showed downregulated Myb gene expression and obvious anthocyanin deficiencies. Correlated with these anthocyanin deficiencies is downregulation of the same late anthocyanin structural genes that are downregulated in ttg1 and bHLH anthocyanin mutants. Expression studies using GL3:GR and TTG1:GR fusions revealed direct regulation of the late biosynthetic genes only. Functional diversification between GL3 and EGL3 with regard to activation of gene targets was revealed by GL3:GR studies in single and double bHLH mutant seedlings. Expression profiles for Myb and bHLH regulators are also presented in the context of pigment production in young seedlings.

Identification of target genes for a MYB-type anthocyanin regulator in Gerbera hybrida

Genetic modification of the flavonoid pathway has been used to produce novel colours and colour patterns in ornamental plants as well as to modify the nutritional and pharmaceutical properties of food crops. It has been suggested that co-ordinate control of multiple steps of the pathway with the help of regulatory genes would lead to a more predictable control of metabolic flux. Regulation of anthocyanin biosynthesis has been studied in a common ornamental plant, Gerbera hybrida (Asteraceae). An R2R3-type MYB factor, GMYB10, shares high sequence similarity and is phylogenetically grouped together with previously characterized regulators of anthocyanin pigmentation. Ectopic expression of GMYB10 leads to strongly enhanced accumulation of anthocyanin pigments as well as to an altered pigmentation pattern in transgenic gerbera plants. Anthocyanin analysis indicates that GMYB10 specifically induces cyanidin biosynthesis in undifferentiated callus and in vegetative tissues. Furthermore, in floral tissues enhanced pelargonidin production is detected. Microarray analysis using the gerbera 9K cDNA array revealed a highly predicted set of putative target genes for GMYB10 including new gene family members of both early and late biosynthetic genes of the flavonoid pathway. However, completely new candidate targets, such as a serine carboxypeptidase-like gene as well, as two new MYB domain factors, GMYB11 and GMYB12, whose exact function in phenylpropanoid biosynthesis is not clear yet, were also identified.

Participation of the Arabidopsis bHLH factor GL3 in trichome initiation regulatory events

The development of trichomes (leaf hairs) from pluripotent epidermal cells in Arabidopsis (Arabidopsis thaliana) provides a powerful system to investigate the regulatory motifs involved in plant cell differentiation. We show here that trichome initiation is triggered within 4 h of the induction of the GLABRA3 (GL3) basic helix-loop-helix transcription factor. Within this developmental window, GL3 binds to the promoters of at least three genes previously implicated in the development and patterning of trichomes (GL2, CAPRICE, and ENHANCER OF TRIPTYCHON AND CAPRICE1) and activates their transcription. The in vivo binding of GL3 to the promoters of these genes requires the presence of the R2R3-MYB factor GL1, supporting a model in which a GL3-GL1 complex is part of the trichome initiation enhanceosome. In contrast, GL3 is recruited to its own promoter in a GL1-independent manner, and this results in decreased GL3 expression, suggesting the presence of a GL3 negative autoregulatory loop. In support of genetic analyses indicating that ENHANCER OF GL3 (EGL3) is partially redundant with GL3, we show that EGL3 shares some direct targets with GL3. However, our results suggest that GL3 and EGL3 work independently of each other. Taken together, our results provide a regulatory framework to understand early events of epidermal cell differentiation.

bHLH proteins know when to make a stoma

In Arabidopsis thaliana, stomata develop through a stereotypical pattern of cell divisions. Three recent publications demonstrate that three bHLH proteins act successively in such lineages to drive the formation of stomata. SPEECHLES drives the division that initiates the stomatal-cell lineage. Then MUTE induces the formation of the immediate stomatal precursor cell. Finally, FAMA causes the stomatal precursor cell to divide into the two guard cells that surround each stomatal pore.

TRICHOMELESS1 regulates trichome patterning by suppressingGLABRA1inArabidopsis

The patterning of epidermal cell types in Arabidopsis is a simple and useful model for studying the molecular basis of cell specification in plants. The distribution of different cell types in the Arabidopsisepidermis is regulated by a lateral inhibition mechanism that relies on interactions between transcription factors. However, it is unclear how temporal- or organ-specific differences in epidermal patterning are achieved. Here we identify TRICHOMELESS1 (TCL1) as a new and major single-repeat MYB-type transcription factor that negatively regulates trichome formation in the inflorescence epidermis. A dominant mutant with elevated expression of TCL1 has a glabrous (trichomeless) phenotype, whereas a loss-of-function mutation in TCL1 uniquely confers ectopic trichome formation on inflorescence stem and pedicels. Genetic analyses demonstrate that TCL1 and CAPRICE work synergistically to regulate trichome patterning on these organs. Interestingly, overexpression of TCL1 specifically suppresses the expression of GLABRA1 (GL1), a crucial component in the trichome initiation complex, whereas loss-of-function of TCL1enhances GL1 expression. Chromatin immunoprecipitation results show that TCL1 can be recruited to the cis-acting regulatory elements of GL1. These results provide the first molecular and genetic evidence that an R3 MYB may negatively regulate trichome cell specification in a novel manner by directly suppressing the transcription of GL1.

Identification of genes expressed in the Arabidopsis female gametophyte

The angiosperm female gametophyte typically consists of one egg cell, two synergid cells, one central cell, and three antipodal cells. Each of these four cell types has unique structural features and performs unique functions that are essential for the reproductive process. The gene regulatory networks conferring these four phenotypic states are largely uncharacterized. As a first step towards dissecting the gene regulatory networks of the female gametophyte, we have identified a large collection of genes expressed in specific cells of the Arabidopsis thaliana female gametophyte. We identified these genes using a differential expression screen based on reduced expression in determinant infertile1 (dif1) ovules, which lack female gametophytes. We hybridized ovule RNA probes with Affymetrix ATH1 genome arrays and validated the identified genes using real-time RT-PCR. These assays identified 71 genes exhibiting reduced expression in dif1 ovules. We further validated 45 of these genes using promoter::GFP fusions and 43 were expressed in the female gametophyte. In the context of the ovule, 11 genes were expressed exclusively in the antipodal cells, 11 genes were expressed exclusively or predominantly in the central cell, 17 genes were expressed exclusively or predominantly in the synergid cells, one gene was expressed exclusively in the egg cell, and three genes were expressed strongly in multiple cells of the female gametophyte. These genes provide insights into the molecular processes functioning in the female gametophyte and can be used as starting points to dissect the gene regulatory networks functioning during differentiation of the four female gametophyte cell types.

Control of cell and petal morphogenesis by R2R3 MYB transcription factors

Petals of animal-pollinated angiosperms have adapted to attract pollinators. Factors influencing pollinator attention include colour and overall size of flowers. Colour is determined by the nature of the pigments, their environment and by the morphology of the petal epidermal cells. Most angiosperms have conical epidermal cells, which enhance the colour intensity and brightness of petal surfaces. The MYB-related transcription factor MIXTA controls the development of conical epidermal cells in petals of Antirrhinum majus. Another gene encoding an R2R3 MYB factor very closely related to MIXTA, AmMYBML2, is also expressed in flowers of A. majus. We have analysed the roles of AmMYBML2 and two MIXTA-related genes, PhMYB1 from Petunia hybrida and AtMYB16 from Arabidopsis thaliana, in petal development. The structural similarity between these genes, their comparable expression patterns and the similarity of the phenotypes they induce when ectopically expressed in tobacco, suggest they share homologous functions closely related to, but distinct from, that of MIXTA. Detailed phenotypic analysis of a phmyb1 mutant confirmed the role of PhMYB1 in the control of cell morphogenesis in the petal epidermis. The phmyb1 mutant showed that epidermal cell shape affects petal presentation, a phenotypic trait also observed following re-examination of mixta mutants. This suggests that the activity of MIXTA-like genes also contributes to petal form, another important factor influencing pollinator attraction.