FIDDLEHEAD, a gene required to suppress epidermal cell interactions in Arabidopsis, encodes a putative lipid biosynthetic enzyme

Fatty acid elongase is required for shoot development in rice

Organisms are covered extracellularly with cuticular waxes that consist of various fatty acids. In higher plants, extracellular waxes act as indispensable barriers to protect the plants from physical and biological stresses such as drought and pathogen attacks. However, the effect of fatty acid composition on plant development under normal growth conditions is not well understood. Here we show that the ONION1 (ONI1) gene, which encodes a fatty acid elongase (β-ketoacyl CoA synthase) involved in the synthesis of very-long-chain fatty acids, is required for correct fatty acid composition and normal shoot development in rice. oni1 mutants containing a reduced amount of very-long-chain fatty acids produced very small shoots, with an aberrant outermost epidermal cell layer, and ceased to grow soon after germination. These mutants also showed abnormal expression of a KNOX family homeobox gene. ONI1 was specifically expressed in the outermost cell layer of the shoot apical meristem and developing lateral organs. These results show that fatty acid elongase is required for formation of the outermost cell layer, and this layer is indispensable for entire shoot development in rice.

The Arabidopsis ABCG13 transporter is required for flower cuticle secretion and patterning of the petal epidermis

Previous studies showed that ABCG11 and ABCG12, two ATP-binding-cassette (ABC) transporters, are required for cuticular lipids extracellular secretion. Here, we characterized ABCG13, a third clade member, to widen our limited knowledge regarding assembly of the plant's cuticle. We isolated an abcg13 knockout mutant and used RNAi and artificial microRNA approaches to study the effect of ABCG13 loss-of-function. These plants were subsequently used to conduct a detailed analysis of cuticular lipids composition and cytological observations. ABCG13 loss-of-function resulted in cuticle-related phenotypes that were restricted to flowers, including inter-organ post-genital fusions. Apart from a significant reduction in flower cutin monomers, the macromorphology and micromorphology of abcg13 petal epidermis was strongly affected. We also found that ABCG13 is highly expressed in flowers, predominantly in petals and carpels. The results suggest that ABCG13 is required for the transport of flower cuticular lipids. This work introduces a new component to the recently emerging genetic network that makes the archetypal exterior of Arabidopsis flowers. While the question regarding the substrate specificity of the ABCG12-clade members remains open, these findings will facilitate future investigations regarding the interaction between the half-size ABCG-type transporters that likely take part in cuticle assembly.

Molecular events of apical bud formation in white spruce, Picea glauca

Bud formation is an adaptive trait that temperate forest trees have acquired to facilitate seasonal synchronization. We have characterized transcriptome-level changes that occur during bud formation of white spruce [Picea glauca (Moench) Voss], a primarily determinate species in which preformed stem units contained within the apical bud constitute most of next season's growth. Microarray analysis identified 4460 differentially expressed sequences in shoot tips during short day-induced bud formation. Cluster analysis revealed distinct temporal patterns of expression, and functional classification of genes in these clusters implied molecular processes that coincide with anatomical changes occurring in the developing bud. Comparing expression profiles in developing buds under long day and short day conditions identified possible photoperiod-responsive genes that may not be essential for bud development. Several genes putatively associated with hormone signalling were identified, and hormone quantification revealed distinct profiles for abscisic acid (ABA), cytokinins, auxin and their metabolites that can be related to morphological changes to the bud. Comparison of gene expression profiles during bud formation in different tissues revealed 108 genes that are differentially expressed only in developing buds and show greater transcript abundance in developing buds than other tissues. These findings provide a temporal roadmap of bud formation in white spruce.

Epidermis: the formation and functions of a fundamental plant tissue

Epidermis differentiation and maintenance are essential for plant survival. Constant cross-talk between epidermal cells and their immediate environment is at the heart of epidermal cell fate, and regulates epidermis-specific transcription factors. These factors in turn direct epidermal differentiation involving a whole array of epidermis-specific pathways including specialized lipid metabolism necessary to build the protective cuticle layer. An intact epidermis is crucial for certain key processes in plant development, shoot growth and plant defence. Here, we discuss the control of epidermal cell fate and the function of the epidermal cell layer in the light of recent advances in the field.

The glabra1 mutation affects cuticle formation and plant responses to microbes

Systemic acquired resistance (SAR) is a form of defense that provides resistance against a broad spectrum of pathogens in plants. Previous work indicates a role for plastidial glycerolipid biosynthesis in SAR. Specifically, mutations in FATTY ACID DESATURASE7 (FAD7), which lead to reduced trienoic fatty acid levels and compromised plastidial lipid biosynthesis, have been associated with defective SAR. We show that the defective SAR in Arabidopsis (Arabidopsis thaliana) fad7-1 plants is not associated with a mutation in FAD7 but rather with a second-site mutation in GLABRA1 (GL1), a gene well known for its role in trichome formation. The compromised SAR in gl1 plants is associated with impairment in their cuticles. Furthermore, mutations in two other components of trichome development, GL3 and TRANSPARENT TESTA GLABRA1, also impaired cuticle development and SAR. This suggests an overlap in the biochemical pathways leading to cuticle and trichome development. Interestingly, exogenous application of gibberellic acid (GA) not only enhanced SAR in wild-type plants but also restored SAR in gl1 plants. In contrast to GA, the defense phytohoromes salicylic acid and jasmonic acid were unable to restore SAR in gl1 plants. GA application increased levels of cuticular components but not trichome formation on gl1 plants, thus implicating cuticle, but not trichomes, as an important component of SAR. Our findings question the prudence of using mutant backgrounds for genetic screens and underscore a need to reevaluate phenotypes previously studied in the gl1 background.

Transcriptome profiling of early developing cotton fiber by deep-sequencing reveals significantly differential expression of genes in a fuzzless/lintless mutant

Cotton fiber as a single-celled trichome is a biological model system for studying cell differentiation and elongation. However, the complexity of its gene expression and regulatory mechanism allows only marginal progress. Here, we report the high-throughput tag-sequencing (Tag-seq) analysis using Solexa Genome Analyzer platform on transcriptome of -2 to 1 (fiber initiation, stage I) and 2-8 (fiber elongation, stage II) days post anthesis (DPA) cotton (Gossypium hirsutum) ovules (wild type: WT; Xuzhou 142 and its mutant: fuzzless/lintless or flM, in the same background). To this end, we sequenced 3.5-3.8 million tags representing 0.7-1.0 million unique transcripts for each library (WT1, WT2, M1, and M2). After removal of low quality tags, we obtained a total of 2,973,104, 3,139,306, 2,943,654, and 3,392,103 clean sequences that corresponded to 357,852, 280,787, 372,952, and 382,503 distinct tags for WT1, WT2, M1, and M2, respectively. All clean tags were aligned to the publicly available cotton transcript database (TIGR, http://www.tigr.org). About 15% of the distinct tags were uniquely mapped to the reference genes, and 31.4% of existing genes were matched by tags. The tag mapping to the database sequences generated 23,854, 24,442, 23,497, and 19,957 annotated genes for WT1, WT2, M1, and M2 libraries, respectively. Analyses of differentially expressed genes revealed the substantial changes in gene type and abundance between the wild type and mutant libraries. Among the 20 most differentially expressed genes in WT1/M1 and WT2/M2 libraries were cellulose synthase, phosphatase, and dehydrogenase, all of which are involved in the fiber cell development. Overall, the deep-sequencing analyses demonstrate the high degree of transcriptional complexity in early developing fibers and represent a major improvement over the microarrays for analyzing transcriptional changes on a large scale.

AtsPLA2-alpha nuclear relocalization by the Arabidopsis transcription factor AtMYB30 leads to repression of the plant defense response

The hypersensitive response (HR), characterized by a rapid and localized cell death at the inoculation site, is one of the most efficient resistance reactions to pathogen attack in plants. The transcription factor AtMYB30 was identified as a positive regulator of the HR and resistance responses during interactions between Arabidopsis and bacteria. Here, we show that AtMYB30 and the secreted phospholipase AtsPLA(2)-alpha physically interact in vivo, following the AtMYB30-mediated specific relocalization of AtsPLA(2)-alpha from cytoplasmic vesicles to the plant cell nucleus. This protein interaction leads to repression of AtMYB30 transcriptional activity and negative regulation of plant HR. Moreover, Atspla(2)-alpha mutant plants are more resistant to bacterial inoculation, whereas AtsPLA(2)-alpha overexpression leads to decreased resistance, confirming that AtsPLA(2)-alpha is a negative regulator of AtMYB30-mediated defense. These data underline the importance of cellular dynamics and, particularly, protein translocation to the nucleus, for defense-associated gene regulation in plants.

Acyl-lipid metabolism

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

The Arabidopsis DSO/ABCG11 transporter affects cutin metabolism in reproductive organs and suberin in roots

Apart from its significance in the protection against stress conditions, the cuticular cover is essential for proper development of the diverse surface structures formed on aerial plant organs. This layer mainly consists of a cutin matrix, embedded and overlaid with cuticular waxes. Following their biosynthesis in epidermal cells, cutin and waxes were suggested to be exported across the plasma membrane by ABCG-type transporters such as DSO/ABCG11 to the cell wall and further to extracellular matrix. Here, additional aspects of DSO/ABCG11 function were investigated, predominantly in reproductive organs, which were not revealed in the previous reports. This was facilitated by the generation of a transgenic DSO/ABCG11 silenced line (dso-4) that displayed relatively subtle morphological and chemical phenotypes. These included altered petal and silique morphology, fusion of seeds, and changes in levels of cutin monomers in flowers and siliques. The dso-4 phenotypes corresponded to the strong DSO/ABCG11 gene expression in the embryo epidermis as well as in the endosperm tissues of the developing seeds. Moreover, the DSO/ABCG11 protein displayed polar localization in the embryo protoderm. Transcriptome analysis of the dso-4 mutant leaves and stems showed that reduced DSO/ABCG11 activity suppressed the expression of a large number of cuticle-associated genes, implying that export of cuticular lipids from the plasma membrane is a rate-limiting step in cuticle metabolism. Surprisingly, root suberin composition of dso-4 was altered, as well as root expression of two suberin biosynthetic genes. Taken together, this study provides new insights into cutin and suberin metabolism and their role in reproductive organs and roots development.

Pollen-pistil interactions regulating successful fertilization in the Brassicaceae

In the Brassicaceae, the acceptance of compatible pollen and the rejection of self-incompatible pollen by the pistil involves complex molecular communication systems between the pollen grain and the female reproductive structures. Preference towards species related-pollen combined with self-recognition systems, function to select the most desirable pollen; and thus, increase the plant's chances for the maximum number of successful fertilizations and vigorous offspring. The Brassicaceae is an ideal group for studying pollen-pistil interactions as this family includes a diverse group of agriculturally relevant crops as well as several excellent model organisms for studying both compatible and self-incompatible pollinations. This review will describe the cellular systems in the pistil that guide the post-pollination events, from pollen capture on the stigmatic papillae to pollen tube guidance to the ovule, with the final release of the sperm cells to effect fertilization. The interplay of other recognition systems, such as the self-incompatibility response and interspecific interactions, on regulating post-pollination events and selecting for compatible pollen-pistil interactions will also be explored.

Suppression of peroxisome biogenesis factor 10 reduces cuticular wax accumulation by disrupting the ER network in Arabidopsis thaliana

Peroxisome biogenesis factor 10 (PEX10) is a component of the peroxisomal matrix protein import machinery. To analyze the physiological function of PEX10, we used transgenic AtPEX10i Arabidopsis plants that had suppressed expression of the PEX10 gene due to RNA interference. AtPEX10i plants had patches of paleness on leaves, and abnormal floral organs that were typical of cuticular wax-deficient mutants. Quantitative analysis of cuticular wax revealed that the amount of wax in AtPEX10i plants was indeed lower than that in control plants. This result was confirmed by toluidine blue staining and scanning electron microscopic analysis of AtPEX10i. The CER1, CER4, WAX2 and SHN1 genes are known to be responsible for wax biosynthesis in Arabidopsis. Of these, CER1, CER4 and WAX2 were found to be localized on the endoplasmic reticulum (ER). In AtPEX10i plants, the expression of these genes was down-regulated, and CER1, CER4 and WAX2 were mislocalized to the cytosol. We also found that AtPEX10i plants had defects in ER morphology. Based on these results, we propose that PEX10 is essential for the maintenance of ER morphology and for the expression of CER1, CER4, WAX2 and SHN1 genes, which contribute to the biosynthesis of cuticular wax.

Dissection of the complex phenotype in cuticular mutants of Arabidopsis reveals a role of SERRATE as a mediator

Mutations in LACERATA (LCR), FIDDLEHEAD (FDH), and BODYGUARD (BDG) cause a complex developmental syndrome that is consistent with an important role for these Arabidopsis genes in cuticle biogenesis. The genesis of their pleiotropic phenotypes is, however, poorly understood. We provide evidence that neither distorted depositions of cutin, nor deficiencies in the chemical composition of cuticular lipids, account for these features, instead suggesting that the mutants alleviate the functional disorder of the cuticle by reinforcing their defenses. To better understand how plants adapt to these mutations, we performed a genome-wide gene expression analysis. We found that apparent compensatory transcriptional responses in these mutants involve the induction of wax, cutin, cell wall, and defense genes. To gain greater insight into the mechanism by which cuticular mutations trigger this response in the plants, we performed an overlap meta-analysis, which is termed MASTA (MicroArray overlap Search Tool and Analysis), of differentially expressed genes. This suggested that different cell integrity pathways are recruited in cesA cellulose synthase and cuticular mutants. Using MASTA for an in silico suppressor/enhancer screen, we identified SERRATE (SE), which encodes a protein of RNA-processing multi-protein complexes, as a likely enhancer. In confirmation of this notion, the se lcr and se bdg double mutants eradicate severe leaf deformations as well as the organ fusions that are typical of lcr and bdg and other cuticular mutants. Also, lcr does not confer resistance to Botrytis cinerea in a se mutant background. We propose that there is a role for SERRATE-mediated RNA signaling in the cuticle integrity pathway.

The extracellular lipase EXL4 is required for efficient hydration of Arabidopsis pollen

Pollination in species with dry stigmas begins with the hydration of desiccated pollen grains on the stigma, a highly regulated process involving the proteins and lipids of the pollen coat and stigma cuticle. Self-incompatible species of the Brassicaceae block pollen hydration, and while the early signaling steps of the self-incompatibility response are well studied, the precise mechanisms controlling pollen hydration are poorly understood. Both lipids and proteins are important for hydration; loss of pollen coat lipids and proteins results in defective or delayed hydration on the stigma surface. Here, we examine the role of the pollen coat protein extracellular lipase 4 (EXL4), in the initial steps of pollination, namely hydration on the stigma. We identify a mutant allele, exl4-1, that shows a reduced rate of pollen hydration. exl4-1 pollen is normal with respect to pollen morphology and the downstream steps in pollination, including pollen tube germination, growth, and fertilization of ovules. However, owing to the delay in hydration, exl4-1 pollen is at a disadvantage when competed with wild-type pollen. EXL4 also functions in combination with GRP17 to promote the initiation of hydration. EXL4 is similar to GDSL lipases, and we show that it functions in hydrolyzing ester bonds. We report a previously unknown function for EXL4, an abundant pollen coat protein, in promoting pollen hydration on the stigma. Our results indicate that changes in lipid composition at the pollen-stigma interface, possibly mediated by EXLs, are required for efficient pollination in species with dry stigmas.

Characterization of Glossy1-homologous genes in rice involved in leaf wax accumulation and drought resistance

The outermost surfaces of plants are covered with an epicuticular wax layer that provides a primary waterproof barrier and protection against different environmental stresses. Glossy 1 (GL1) is one of the reported genes controlling wax synthesis. This study analyzed GL1-homologous genes in Oryza sativa and characterized the key members of this family involved in wax synthesis and stress resistance. Sequence analysis revealed 11 homologous genes of GL1 in rice, designated OsGL1-1 to OsGL1-11. OsGL1-1, -2 and -3 are closely related to GL1. OsGL1-4, -5, -6, and -7 are closely related to Arabidopsis CER1 that is involved in cuticular wax biosynthesis. OsGL1-8, -9, -10 and -11 are closely related to SUR2 encoding a putative sterol desaturase also involved in epicuticular wax biosynthesis. These genes showed variable expression levels in different tissues and organs of rice, and most of them were induced by abiotic stresses. Compared to the wild type, the OsGL1-2-over-expression rice exhibited more wax crystallization and a thicker epicuticular layer; while the mutant of this gene showed less wax crystallization and a thinner cuticular layer. Chlorophyll leaching experiment suggested that the cuticular permeability was decreased and increased in the over-expression lines and the mutant, respectively. Quantification analysis of wax composition by GC-MS revealed a significant reduction of total cuticular wax in the mutant and increase of total cuticular wax in the over-expression plants. Compared to the over-expression and wild type plants, the osgl1-2 mutant was more sensitive to drought stress at reproductive stage, suggesting an important role of this gene in drought resistance.

Early gene expression programs accompanying trans-differentiation of epidermal cells of Vicia faba cotyledons into transfer cells

Transfer cells (TCs) trans-differentiate from differentiated cells by developing extensive wall ingrowths that enhance plasma membrane transport of nutrients. Here, we investigated transcriptional changes accompanying induction of TC development in adaxial epidermal cells of cultured Vicia faba cotyledons. Global changes in gene expression revealed by cDNA-AFLP were compared between adaxial epidermal cells during induction (3 h) and subsequent building (24 h) of wall ingrowths, and in cells of adjoining storage parenchyma tissue, which do not form wall ingrowths. A total of 5795 transcript-derived fragments (TDFs) were detected; of these, 264 TDFs showed epidermal-specific changes in gene expression and a further 207 TDFs were differentially expressed in both epidermal and storage parenchyma cells. Genes involved in signalling (auxin/ethylene), metabolism (mitochondrial; storage product hydrolysis), cell division, vesicle trafficking and cell wall biosynthesis were specifically induced in epidermal TCs. Blockers of auxin action and vesicle trafficking inhibited ingrowth formation and marked increases in cell division accompanied TC development. Auxin and possibly ethylene signalling cascades induce epidermal cells of V. faba cotyledons to trans-differentiate into TCs. Trans-differentiation is initiated by rapid de-differentiation to a mitotic state accompanied by mitochondrial biogenesis driving storage product hydrolysis to fuel wall ingrowth formation orchestrated by a modified vesicle trafficking mechanism.

Isolation of high salinity stress tolerant genes from Pisum sativum by random overexpression in Escherichia coli and their functional validation

Salinity stress is one of the major factors which reduce crop plants growth and productivity resulting in significant economic losses worldwide. Therefore, it would be fruitful to isolate and functionally identify new salinity stress-induced genes for understanding the mechanism and developing salinity stress tolerant plants. Based on functional gene screening assay, we have isolated few salinity tolerant genes out of one million Escherichia coli (SOLR) transformants containing pea cDNAs. Sequence analysis of three of these genes revealed homology to Ribosomal-L30E (RPL30E), Chlorophyll-a/b-binding protein (Chla/bBP) and FIDDLEHEAD (FDH). The salinity tolerance of these genes in bacteria was further confirmed by using another strain of E. coli (DH5alpha) transformants. The homology based computational modeling of these proteins suggested the high degree of conservation with the conserved domains of their homologous partners. The reverse transcriptase polymerase chain reaction (RT-PCR) analysis showed that the expression of these cDNAs (except the FDH) was upregulated in pea plants in response to NaCl stress. We observed that there was no significant effect of Li(+) ion on the expression level of these genes, while an increase in response to K(+) ion was observed. Overall, this study provides an evidence for a novel function of these genes in high salinity stress tolerance. The PsFDH showed constitutive expression in planta suggesting that it can be used as constitutively expressed marker gene for salinity stress tolerance in plants. This study brings new direction in identifying novel function of unidentified genes in abiotic stress tolerance without previous knowledge of the genome sequence.

Misexpression of FATTY ACID ELONGATION1 in the Arabidopsis epidermis induces cell death and suggests a critical role for phospholipase A2 in this process

Very-long-chain fatty acids (VLCFAs) are important functional components of various lipid classes, including cuticular lipids in the higher plant epidermis and lipid-derived second messengers. Here, we report the characterization of transgenic Arabidopsis thaliana plants that epidermally express FATTY ACID ELONGATION1 (FAE1), the seed-specific beta-ketoacyl-CoA synthase (KCS) catalyzing the first rate-limiting step in VLCFA biosynthesis. Misexpression of FAE1 changes the VLCFAs in different classes of lipids but surprisingly does not complement the KCS fiddlehead mutant. FAE1 misexpression plants are similar to the wild type but display an essentially glabrous phenotype, owing to the selective death of trichome cells. This cell death is accompanied by membrane damage, generation of reactive oxygen species, and callose deposition. We found that nuclei of arrested trichome cells in FAE1 misexpression plants cell-autonomously accumulate high levels of DNA damage, including double-strand breaks characteristic of lipoapoptosis. A chemical genetic screen revealed that inhibitors of KCS and phospholipase A2 (PLA2), but not inhibitors of de novo ceramide biosynthesis, rescue trichome cells from death. These results support the functional role of acyl chain length of fatty acids and PLA2 as determinants for programmed cell death, likely involving the exchange of VLCFAs between phospholipids and the acyl-CoA pool.

[Progress in the study on genes encoding enzymes involved in biosynthesis of very long chain fatty acids and cuticular wax in plants]

Very long chain fatty acids (VLCFAs) play a comprehensive role in organisms. They are essential biological components in seed storage triacylglycerols (TAGs), membrane lipids, and sphingolipids. They also serve as precursors of wax layer compounds. The cuticle covers the aerial surface of land plants, which consists of cutin and wax. The wax, including amorphous intracuticular wax embedded in cutin polymer and epicuticular wax crystalloids that cover the outer plant surface, plays crucial roles in plant growth and development, and adaptation to environment. Biosynthesis of VLCFAs is catalyzed by the fatty acyl-CoA elongase, a membrane-bound enzymatic complex containing 3-ketoacyl-CoA synthase (KCS), 3-ketoacyl-CoA reductase (KCR), 3-hydroxacyl-CoA dehydratase (HCD), and trans-2, 3-enoyl-CoA reductase (ECR). Very long chain fatty acid wax precursors flux into cuticular wax biosynthetic pathways through acyl reduction and decarbonylation, and then are converted to all kinds of wax components. This article reviews the functions of VLCFAs and cuticular wax, and the recent progress in cloning and characterization of genes encoding enzymes involved in catalyzing VLCFAs and cuticular wax biosynthesis. The problems existing in researches of wax genes are also discussed.

Expression-based discovery of candidate ovule development regulators through transcriptional profiling of ovule mutants

BACKGROUND

Arabidopsis ovules comprise four morphologically distinct parts: the nucellus, which contains the embryo sac, two integuments that become the seed coat, and the funiculus that anchors the ovule within the carpel. Analysis of developmental mutants has shown that ovule morphogenesis relies on tightly regulated genetic interactions that can serve as a model for developmental regulation. Redundancy, pleiotropic effects and subtle phenotypes may preclude identification of mutants affecting some processes in screens for phenotypic changes. Expression-based gene discovery can be used access such obscured genes.

RESULTS

Affymetrix microarrays were used for expression-based gene discovery to identify sets of genes expressed in either or both integuments. The genes were identified by comparison of pistil mRNA from wild type with mRNA from two mutants; inner no outer (ino, which lacks the outer integument), and aintegumenta (ant, which lacks both integuments). Pools of pistils representing early and late stages of ovule development were evaluated and data from the three genotypes were used to designate genes that were predominantly expressed in the integuments using pair-wise and cluster analyses. Approximately two hundred genes were found to have a high probability of preferential expression in these structures, and the predictive nature of the expression classes was confirmed with reverse transcriptase polymerase chain reaction and in situ hybridization.

CONCLUSION

The results showed that it was possible to use a mutant, ant, with broad effects on plant phenotype to identify genes expressed specifically in ovules, when coupled with predictions from known gene expression patterns, or in combination with a more specific mutant, ino. Robust microarray averaging (RMA) analysis of array data provided the most reliable comparisons, especially for weakly expressed genes. The studies yielded an over-abundance of transcriptional regulators in the identified genes, and these form a set of candidate genes for evaluation of roles in ovule development using reverse genetics.

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.

Very long chain fatty acid and lipid signaling in the response of plants to pathogens

Recent findings indicate that lipid signaling is essential for plant resistance to pathogens. Besides oxylipins and unsaturated fatty acids known to play important signaling functions during plant-pathogen interactions, the very long chain fatty acid (VLCFA) biosynthesis pathway has been recently associated to plant defense through different aspects. VLCFAs are indeed required for the biosynthesis of the plant cuticle and the generation of sphingolipids. Elucidation of the roles of these lipids in biotic stress responses is the result of the use of genetic approaches together with the identification of the genes/proteins involved in their biosynthesis. This review focuses on recent observations which revealed the complex function of the cuticle and cuticle-derived signals, and the key role of sphingolipids as bioactive molecules involved in signal transduction and cell death regulation during plant-pathogen interactions.

The DAISY gene from Arabidopsis encodes a fatty acid elongase condensing enzyme involved in the biosynthesis of aliphatic suberin in roots and the chalaza-micropyle region of seeds

Suberin is a hydrophobic polyester found in the cell walls of various plant-environment interfaces, including shoot and root peridermal tissue, and the root hypodermis and endodermis. Suberin deposits form apoplastic barriers that control water and nutrient transport, protect against pathogens and seal wounded tissue. Despite this physiological importance, and the detailed information on the suberin composition of many plants, there is a great gap in our knowledge of the molecular mechanism of suberin biosynthesis, caused in part by a lack of mutants in suberin formation. Here, we report the characterization of daisy, an Arabidopsis mutant that is defective in a fatty acid elongase condensing enzyme. The daisy mutant roots exhibit disturbed growth, and the suberin level is reduced in C(22) and C(24) very long chain fatty acid derivatives, whereas C(16), C(18) and C(20) derivatives accumulate, compared with wild-type suberin, indicating that DAISY functions as a docosanoic acid synthase. Consistent with a significantly increased level of suberin in the roots of NaCl-stressed plants, DAISY is transcriptionally activated by NaCl application, and also by polyethylene glycol-induced drought stress and wounding. Expression analysis using RT-PCR and promoter-GUS fusions demonstrated a distinct DAISY expression pattern in the root stele, senescing sepals, siliques abscission zones and the chalaza-micropyle region of seeds. Together, these results indicate that DAISY is involved in suberin biosynthesis and in the formation of protective layers in these tissues, and in the response to unfavourable environmental conditions.

The oxygen status of the developing seed

Recent applications of oxygen-sensitive microsensors have demonstrated steep oxygen gradients in developing seeds of various crops. Here, we present an overview on oxygen distribution, major determinants of the oxygen status in the developing seed and implications for seed physiology. The steady-state oxygen concentration in different seed tissues depends on developmental parameters, and is determined to a large extent by environmental factors. Photosynthetic activity of the seed significantly diminishes hypoxic constraints, and can even cause transient, local hyperoxia. Changes in oxygen availability cause rapid adjustments in mitochondrial respiration and global metabolism. We argue that nitric oxide (NO) is a key player in the oxygen balancing process in seeds, avoiding fermentation and anoxia in vivo. Molecular approaches aiming to increase oxygen availability within the seed are discussed.

Transmitting tissue architecture in basal-relictual angiosperms: Implications for transmitting tissue origins

Carpel transmitting tissue is a major floral innovation that is essential for angiosperm success. It facilitates the rapid adhesion, hydration, and growth of the male gametophyte to the female gametophyte. As well, it functions as a molecular screen to promote male gametophytic competition and species-specific recognition and compatibility. Here, we characterize the transmitting tissue extracellular matrix (ECM) and pollen tube growth in basal-relictual angiosperms and test the hypothesis that a freely flowing ECM (wet stigma) was ancestral to a cuticle-bound ECM (dry stigma). We demonstrate that the most recent common ancestor of extant angiosperms produced an ECM that was structurally and functionally equivalent to a dry stigma. Dry stigmas are composed of a cuticle and primary wall that contains compounds that facilitate the adhesion and growth of the male gametophyte. These compounds include methyl-esterified homogalacturonans, arabinogalactan-proteins, and lipids. We propose that transmitting tissue evolved in concert with an increase in cuticle permeability that resulted from modifications in the biosynthesis and secretion of fatty acids needed for cuticle construction. Increased cuticle permeability exposed the male gametophyte to pre-existing molecules that enabled rapid male gametophyte adhesion, hydration, and growth as well as species-specific recognition and compatibility.

HOS3, an ELO-like gene, inhibits effects of ABA and implicates a S-1-P/ceramide control system for abiotic stress responses in Arabidopsis thaliana

A hyper-osmotically sensitive mutant of Arabidopsis thaliana, designated hos3-1 (high expression of osmotically responsive genes), was identified based on its hyper-luminescence of RD29A:LUC promoter fusion plants upon treatment with NaCl and ABA. These responses implicate the disrupted gene as a direct or indirect negative regulator of the RD29A stress-responsive pathway. By sequencing the flanking regions of the T-DNA borders, it was determined that the disrupted gene is at locus At4g36830, annotated as encoding a putative protein with high homology to CIG30 (ELO2/FEN1). CIG30 has been implicated in synthesis of very long chain fatty acids (VLCFA), which are essential precursors for sphingolipids and ceramides. Altered stress responses characteristic of ABA-hypersensitivity, including reduced root growth inhibition and reduced germination with ABA treatment and reduced water loss from leaves, were exhibited by allelic hos3-1 and hos3-2 mutants. The hos3-2 mutant is partially suppressed in its transcript abundance and is inherited as a recessive trait. Further, the HOS3 ORF under the control of the 35SCaMV promoter restored wild-type NaCl- and ABA-root growth sensitivity as well as RD29A:LUC luminescence in mutant plants. We also show here that the HOS3 wild-type gene functionally complements the sensitivity of elo2 and elo3 yeast mutants to monensin. Furthermore, both hos3-1 and hos3-2 alleles shared increased sensitivity to the herbicide Metolachlor, which inhibits acyl chain elongation in synthesis of VLCFA, and HOS3 functionally complemented both elo2 and elo3 and restored levels of VLCFA. Together, these data establish that HOS3 inhibits ABA-mediated stress responses and implicate the VLCFA pathway and products as control points for several aspects of abiotic stress signaling and responses. The results also provide support for a role of ceramide in the control of stomatal behavior.

Changes to gene expression associated with hybrid speciation in plants: further insights from transcriptomic studies in Senecio

Interspecific hybridization is an important mechanism of speciation in higher plants. In flowering plants, hybrid speciation is usually associated with polyploidy (allopolyploidy), but hybrid speciation without genome duplication (homoploid hybrid speciation) is also possible, although it is more difficult to detect. The combination of divergent genomes within a hybrid can result in profound changes to both genome and transcriptome. Recent transcriptomic studies of wild and resynthesized homoploid and allopolyploid hybrids have revealed widespread changes to gene expression in hybrids relative to expression levels in their parents. Many of these changes to gene expression are 'non-additive', i.e. not simply the sum of the combined expression levels of parental genes. Some gene expression changes are far outside the range of gene expression in either parent, and can therefore be viewed as 'transgressive'. Such profound changes to gene expression may enable new hybrids to survive in novel habitats not accessible to their parent species. Here, we give a brief overview of hybrid speciation in plants, with an emphasis on genomic change, before focusing discussion on findings from recent transcriptomic studies. We then discuss our current work on gene expression change associated with hybrid speciation in the genus Senecio (ragworts and groundsels) focusing on the findings from a reanalysis of gene expression data obtained from recent microarray studies of wild and resynthesized allopolyploid Senecio cambrensis. These data, showing extensive non-additive and transgressive gene expression changes in Senecio hybrids, are discussed in the light of findings from other model systems, and in the context of the potential importance of gene expression change to hybrid speciation in plants.

Various spatiotemporal expression profiles of anther-expressed genes in rice

The male gametophyte and tapetum play different roles during anther development although they are differentiated from the same cell lineage, the L2 layer. Until now, it has not been possible to delineate their transcriptomes due to technical difficulties in separating the two cell types. In the present study, we characterized the separated transcriptomes of the rice microspore/pollen and tapetum using laser microdissection (LM)-mediated microarray. Spatiotemporal expression patterns of 28,141 anther-expressed genes were classified into 20 clusters, which contained 3,468 (12.3%) anther-enriched genes. In some clusters, synchronous gene expression in the microspore and tapetum at the same developmental stage was observed as a novel characteristic of the anther transcriptome. Noteworthy expression patterns are discussed in connection with gene ontology (GO) categories and gene annotations, which are related to important biological events in anther development, such as pollen maturation, pollen germination, pollen tube elongation and pollen wall formation.

Wax Crystal-Sparse Leaf1 encodes a beta-ketoacyl CoA synthase involved in biosynthesis of cuticular waxes on rice leaf

Cuticular waxes, forming the plant/atmosphere interface of plants colonizing the terrestrial environment, are complex mixtures of very-long chain fatty acids (VLCFAs) and their derivatives. In VLCFAs biosynthesis, beta-ketoacyl CoA synthase (E.C.2.3.1.119, KCS) is the key enzyme. Using T-DNA insertional mutagenesis, we identified a cuticle-deficient rice mutant, which displayed a pleiotropic phenotype including reduced growth, leaf fusion, sparse wax crystals, enhanced sensitivity to drought and low fertility. Further analysis indicated that T-DNA was inserted in the 5'-UTR intron of the affected gene, Wax Crystal-Sparse Leaf1 (WSL1), and abnormal transcript caused the loss-of-function of WSL1 gene. Genetic complementation experiment confirmed the function of the candidate gene. WSL1 was predicted to encode a polypeptide containing a conserved FAE1_CUT1_RppA domain typical of the KCS family proteins. Qualitative and quantitative wax composition analyses by gas chromatography-mass spectrometry (GC-MS) demonstrated a marked reduction of total cuticular wax load on wsl1 leaf blades and sheaths, and VLCFA precursors of C20-C24 decreased in both. Moreover, ubiquitous expression of the WSL1 gene gave a hint that WSL1-catalyzed elongation of VLCFAs might participate in a wide range of rice growth and development processes beyond biosynthesis of cuticular waxes.

Wax Crystal-Sparse Leaf1 encodes a beta-ketoacyl CoA synthase involved in biosynthesis of cuticular waxes on rice leaf

Cuticular waxes, forming the plant/atmosphere interface of plants colonizing the terrestrial environment, are complex mixtures of very-long chain fatty acids (VLCFAs) and their derivatives. In VLCFAs biosynthesis, beta-ketoacyl CoA synthase (E.C.2.3.1.119, KCS) is the key enzyme. Using T-DNA insertional mutagenesis, we identified a cuticle-deficient rice mutant, which displayed a pleiotropic phenotype including reduced growth, leaf fusion, sparse wax crystals, enhanced sensitivity to drought and low fertility. Further analysis indicated that T-DNA was inserted in the 5'-UTR intron of the affected gene, Wax Crystal-Sparse Leaf1 (WSL1), and abnormal transcript caused the loss-of-function of WSL1 gene. Genetic complementation experiment confirmed the function of the candidate gene. WSL1 was predicted to encode a polypeptide containing a conserved FAE1_CUT1_RppA domain typical of the KCS family proteins. Qualitative and quantitative wax composition analyses by gas chromatography-mass spectrometry (GC-MS) demonstrated a marked reduction of total cuticular wax load on wsl1 leaf blades and sheaths, and VLCFA precursors of C20-C24 decreased in both. Moreover, ubiquitous expression of the WSL1 gene gave a hint that WSL1-catalyzed elongation of VLCFAs might participate in a wide range of rice growth and development processes beyond biosynthesis of cuticular waxes.

Diverse cell signalling pathways regulate pollen-stigma interactions: the search for consensus

Siphonogamy, the delivery of nonmotile sperm to the egg via a pollen tube, was a key innovation that allowed flowering plants (angiosperms) to carry out sexual reproduction on land without the need for water. This process begins with a pollen grain (male gametophyte) alighting on and adhering to the stigma of a flower. If conditions are right, the pollen grain germinates to produce a pollen tube. The pollen tube invades the stigma and grows through the style towards the ovary, where it enters an ovule, penetrates the embryo sac (female gametophyte) and releases two sperm cells, one of which fertilizes the egg, while the other fuses with the two polar nuclei of the central cell to form the triploid endosperm. The events before fertilization (pollen-pistil interactions) comprise a series of complex cellular interactions involving a continuous exchange of signals between the haploid pollen and the diploid maternal tissue of the pistil (sporophyte). In recent years, significant progress has been made in elucidating the molecular identity of these signals and the cellular interactions that they regulate. Here we review our current understanding of the cellular and molecular interactions that mediate the earliest of these interactions between the pollen and the pistil that occur on or within the stigma - the 'pollen-stigma interaction'.

The VLCFA elongase gene family in Arabidopsis thaliana: phylogenetic analysis, 3D modelling and expression profiling

As precursors of wax compounds, very long chain fatty acids participate in the limitation of non-stomatal water loss and the prevention of pathogen attacks. They also serve as energy storage in seeds and as membrane building blocks. Their biosynthesis is catalyzed by the acyl-CoA elongase, a membrane-bound enzymatic complex containing four distinct enzymes (KCS, KCR, HCD and ECR). Twenty-one 3-ketoacyl-CoA synthase (KCS) genes have been identified in Arabidopsis thaliana genome. In this paper we present an overview of the acyl-CoA elongase genes in Arabidopsis focusing on the entire KCS family. We show that the KCS family is made up of 8 distinct subclasses, according to their phylogeny, duplication history, genomic organization, protein topology and 3D modelling. The analysis of the subcellular localization in tobacco cells of the different subunits of the acyl-CoA elongase shows that all these proteins are localized in the endoplasmic reticulum demonstrating that VLCFA production occurs in this compartment. The expression patterns in Arabidopsis of the acyl-CoA elongase genes suggest several levels of regulations at the tissular or organ level but also under stress conditions suggesting a complex organization of this multigenic family.

GASSHO1 and GASSHO2 encoding a putative leucine-rich repeat transmembrane-type receptor kinase are essential for the normal development of the epidermal surface in Arabidopsis embryos

Receptor-like kinases (RLKs) containing leucine-rich repeats (LRRs) act as both signal receptor and signal transducer in ligand-mediated communication between cells. It is believed that many LRR-RLKs are present in the Arabidopsis genome, but the functions of most are unknown. We recently identified Bnms4D-82, an expressed sequence tag (EST) in Brassica napus that encodes an LRR-RLK and is expressed at an early stage of its microspore embryogenesis. To elucidate the function of this gene we used GASSHO1 (GSO1) and GSO2, two Arabidopsis genes with a high degree of homology with Bnms4D-82. The products of transcripts of GSO1 and GSO2 accumulate in parts of the embryo and in seedlings, but not in true leaves. Plants that lacked both GSO1 and GSO2 exhibited pleiotropy, including abnormal bending of embryos, ectopic adhesion between cotyledons, a highly permeable epidermal structure, and an abnormal pattern of distribution of stomata on cotyledons in embryos and seedlings. However, plants homozygous for either gso1-1 or gso2-1 had no visible abnormality. These results suggest that GASSHO genes are essential for the formation of a normal epidermal surface during embryogenesis.

A MYB transcription factor regulates very-long-chain fatty acid biosynthesis for activation of the hypersensitive cell death response in Arabidopsis

Plant immune responses to pathogen attack include the hypersensitive response (HR), a form of programmed cell death occurring at invasion sites. We previously reported on Arabidopsis thaliana MYB30, a transcription factor that acts as a positive regulator of a cell death pathway conditioning the HR. Here, we show by microarray analyses of Arabidopsis plants misexpressing MYB30 that the genes encoding the four enzymes forming the acyl-coA elongase complex are putative MYB30 targets. The acyl-coA elongase complex synthesizes very-long-chain fatty acids (VLCFAs), and the accumulation of extracellular VLCFA-derived metabolites (leaf epidermal wax components) was affected in MYB30 knockout mutant and overexpressing lines. In the same lines, a lipid extraction procedure allowing high recovery of sphingolipids revealed changes in VLCFA contents that were amplified in response to inoculation. Finally, the exacerbated HR phenotype of MYB30-overexpressing lines was altered by the loss of function of the acyl-ACP thioesterase FATB, which causes severe defects in the supply of fatty acids for VLCFA biosynthesis. Based on these findings, we propose a model in which MYB30 modulates HR via VLCFAs by themselves, or VLCFA derivatives, as cell death messengers in plants.

The Arabidopsis DESPERADO/AtWBC11 transporter is required for cutin and wax secretion

The cuticle fulfills multiple roles in the plant life cycle, including protection from environmental stresses and the regulation of organ fusion. It is largely composed of cutin, which consists of C(16-18) fatty acids. While cutin composition and biosynthesis have been studied, the export of cutin monomers out of the epidermis has remained elusive. Here, we show that DESPERADO (AtWBC11) (abbreviated DSO), encoding a plasma membrane-localized ATP-binding cassette transporter, is required for cutin transport to the extracellular matrix. The dso mutant exhibits an array of surface defects suggesting an abnormally functioning cuticle. This was accompanied by dramatic alterations in the levels of cutin monomers. Moreover, electron microscopy revealed unusual lipidic cytoplasmatic inclusions in epidermal cells, disappearance of the cuticle in postgenital fusion areas, and altered morphology of trichomes and pavement cells. We also found that DSO is induced by salt, abscisic acid, and wounding stresses and its loss of function results in plants that are highly susceptible to salt and display reduced root branching. Thus, DSO is not only essential for developmental plasticity but also plays a vital role in stress responses.

An ABC transporter gene of Arabidopsis thaliana, AtWBC11, is involved in cuticle development and prevention of organ fusion

Cuticle, including wax and cutin, is the barrier covering plant aerial organs and protecting the inner tissues. The Arabidopsis thaliana ATP-binding cassette (ABC) transporter CER5 (AtWBC12) has been identified as a wax exporter. In agreement with the latest report of another wax exporter, AtWBC11, here we show that atwbc11 mutants displayed organ fusions and stunted growth, and became vulnerable to chlorophyll leaching and toluidine blue staining. Chemical analysis showed that wax and cutin monomers were both reduced in the atwbc11 mutant. AtWBC11 was widely expressed in aerial organs. Interestingly, we found that the expression was light dependent, and the phytohormone ABA up-regulated AtWBC11 expression. We also found that while the AtWBC11 promoter had a broad pattern of activity, the expression was converted to epidermis specific when the reporter gene was fused to AtWBC11 cDNA. Furthermore, RNA blot analysis supported epidermis-specific expression of AtWBC11. Our results support that AtWBC11 is involved in cuticle development.

Gene expression changes and early events in cotton fibre development

BACKGROUND

Cotton is the dominant source of natural textile fibre and a significant oil crop. Cotton fibres, produced by certain species in the genus Gossypium, are seed trichomes derived from individual cells of the epidermal layer of the seed coat. Cotton fibre development is delineated into four distinct and overlapping developmental stages: fibre initiation, elongation, secondary wall biosynthesis and maturation.

SCOPE

Recent advances in gene expression studies are beginning to provide new insights into a better understanding of early events in cotton fibre development. Fibre cell development is a complex process involving many pathways, including various signal transduction and transcriptional regulation components. Several analyses using expressed sequence tags and microarray have identified transcripts that preferentially accumulate during fibre development. These studies, as well as complementation and overexpression experiments using cotton genes in arabidopsis and tobacco, indicate some similar molecular events between trichome development from the leaf epidermis and fibre development from the ovule epidermis. Specifically, MYB transcription factors regulate leaf trichome development in arabidopsis and may regulate seed trichome development in cotton. In addition, transcript profiling and ovule culture experiments both indicate that several phytohormones and other signalling pathways mediate cotton fibre development. Auxin and gibberellins promote early stages of fibre initiation; ethylene- and brassinosteroid-related genes are up-regulated during the fibre elongation phase; and genes associated with calmodulin and calmodulin-binding proteins are up-regulated in fibre initials. Additional genomic data, mutant and functional analyses, and genome mapping studies promise to reveal the critical factors mediating cotton fibre cell development.

Saturated very-long-chain fatty acids promote cotton fiber and Arabidopsis cell elongation by activating ethylene biosynthesis

Fatty acids are essential for membrane biosynthesis in all organisms and serve as signaling molecules in many animals. Here, we found that saturated very-long-chain fatty acids (VLCFAs; C20:0 to C30:0) exogenously applied in ovule culture medium significantly promoted cotton (Gossypium hirsutum) fiber cell elongation, whereas acetochlor (2-chloro-N-[ethoxymethyl]-N-[2-ethyl-6-methyl-phenyl]-acetamide; ACE), which inhibits VLCFA biosynthesis, abolished fiber growth. This inhibition was overcome by lignoceric acid (C24:0). Elongating fibers contained significantly higher amounts of VLCFAs than those of wild-type or fuzzless-lintless mutant ovules. Ethylene nullified inhibition by ACE, whereas C24:0 was inactive in the presence of the ethylene biosynthesis inhibitor (l-[2-aminoethoxyvinyl]-glycine), indicating that VLCFAs may act upstream of ethylene. C24:0 induced a rapid and significant increase in ACO (for 1-aminocyclopropane-1-carboxylic acid oxidase) transcript levels that resulted in substantial ethylene production. C24:0 also promoted Ser palmitoyltransferase expression at a later stage, resulting in increased sphingolipid biosynthesis. Application of C24:0 not only stimulated Arabidopsis thaliana root cell growth but also complemented the cut1 phenotype. Transgenic expression of Gh KCS13/CER6, encoding the cotton 3-ketoacyl-CoA synthase, in the cut1 background produced similar results. Promotion of Arabidopsis stem elongation was accompanied by increased ACO transcript levels. Thus, VLCFAs may be involved in maximizing the extensibility of cotton fibers and multiple Arabidopsis cell types, possibly by activating ethylene biosynthesis.

Characterization of Arabidopsis ABCG11/WBC11, an ATP binding cassette (ABC) transporter that is required for cuticular lipid secretion

ABCG11/WBC11, an ATP binding cassette (ABC) transporter from Arabidopsis thaliana, is a key component of the export pathway for cuticular lipids. Arabidopsis wbc11 T-DNA insertional knock-out mutants exhibited lipidic inclusions inside epidermal cells similar to the previously characterized wax transporter mutant cer5, with a similar strong reduction in the alkanes of surface waxes. Moreover, the wbc11 knock-out mutants also showed defects not present in cer5, including post-genital organ fusions, stunted growth and a reduction in cutin load on the plant surface. A mutant line previously isolated in a forward genetics screen, called permeable leaves 1 (pel1), was identified as an allele of ABCG11/WBC11. The double knock-out wbc11 cer5 exhibited the same morphological and biochemical phenotypes as the wbc11 knock-out. A YFP-WBC11 fusion protein rescued a T-DNA knock-out mutant and was localized to the plasma membrane. These results show that WBC11 functions in secretion of surface waxes, possibly by interacting with CER5. However, unlike ABCG12/CER5, ABCG11/WBC11 is important to the normal process of cutin formation.

Transcriptional analysis of petal organogenesis in Gerbera hybrida

Understanding of the molecular interplay, which determines early steps of flower formation has grown considerably during last years. In contrast, genetic actions responsible for how flower organs acquire their size and shape at later phases of organogenesis are still poorly understood. We have exploited the large and anatomically simple Gerbera (Gerbera hybrida var. Terra regina) ray flower petals to describe transcriptional changes during organogenesis. Gerbera 9 K cDNA microarray was utilized to profile gene expression at six different developmental stages of petal organogenesis, at the earliest stage expansion of petals is starting and at the latest stage petals have reached their final size and shape. Genes potentially participating in petal opening were identified based on the similarity in expression with a known marker gene. Our results showed characteristic sets of genes expressed during the cell division and cell expansion phases of petal development. Interestingly, there was a transition stage during which neither cell division nor cell expansion marker genes were abundantly expressed. Moreover, constitutive expression of late petal specific genes indicates that they participate in petal organogenesis throughout the development and they are not involved in stage specific switch points.

Allelopathic Monoterpenes Interfere with Arabidopsis thaliana Cuticular Waxes and Enhance Transpiration

Exposure to the allelopathic monoterpenes camphor (100 mg/10 L) and menthol (50 mg/10 L) for 24 h enhanced transpiration of Arabidopsis thaliana fully developed rosette leaves similar to de-waxing. As ascertained by ESEM analyses the leaf surfaces were spotted with platelet like structures which seem to be partly mixed with the lipophilic epicuticular layers. The structures are supposed to contain the condensed monoterpenes, which could be identified by GC. Long term exposure (more than 48 h) to 100 mg/50 mg killed the plants by desiccation, a 24 h exposure caused necrotic spots that became visible one to two days after the treatment. Examinations of the stomatal apertures indicated that monoterpenes induced stomatal opening followed by extreme swelling and a final break down of the protoplasts. Exposure of Arabidopsis thaliana to volatiles of Mentha piperita, Lavandula latifolia and Artemisia camphorata resulted in a dramatic increase of the stomata aperture but swelling of the protoplasts was less exhibited.In contrast to de-waxing, expression of the fatty acid condensing enzyme encoding CER6 gene and de novo synthesis of CER6 protein was not induced after 24 h of exposure to the monoterpenes.The aim of the study was to demonstrate that the lipophilic layers of the leaf surface and the stomata are primary targets of monoterpene allelopathic attack. Enhanced transpiration results from a combination of affected lipophilic wax layers and a disturbed stomata function.

Novel receptor-like kinase ALE2 controls shoot development by specifying epidermis inArabidopsis

The epidermis plays crucial roles in the development of various organs and in water retention in both animals and plants. In Arabidopsis thaliana, the subtilase ABNORMAL LEAF SHAPE 1 (ALE1) and the Arabidopsis homolog of the Crinkly4 (ACR4) receptor-like protein kinase (RLK) have been implicated in the intercellular communication that is required for surface functions of the epidermis. We have identified a novel mutant gene in Arabidopsis, ale2, which is associated with various epidermal defects, including disorganization of epidermis-related tissues,defects in the leaf cuticle and the fusion of organs. ALE2 encodes a previously uncharacterized RLK with a cluster of basic amino acid residues followed by a cysteine-containing sequence in the putative extracellular domain. Our genetic investigations suggest that ALE2 and ACR4 function in the same process, whereas ALE1 has a different mode of action, and that these three genes play partially overlapping roles in positively regulating protoderm-specific gene expression and for the formation of leafy organs. We propose that at least two modes of intercellular communication facilitate the specification of epidermis, thereby promoting shoot organogenesis in Arabidopsis.

A genomic approach to suberin biosynthesis and cork differentiation

Cork (phellem) is a multilayered dead tissue protecting plant mature stems and roots and plant healing tissues from water loss and injuries. Cork cells are made impervious by the deposition of suberin onto cell walls. Although suberin deposition and cork formation are essential for survival of land plants, molecular studies have rarely been conducted on this tissue. Here, we address this question by combining suppression subtractive hybridization together with cDNA microarrays, using as a model the external bark of the cork tree (Quercus suber), from which bottle cork is obtained. A suppression subtractive hybridization library from cork tree bark was prepared containing 236 independent sequences; 69% showed significant homology to database sequences and they corresponded to 135 unique genes. Out of these genes, 43.5% were classified as the main pathways needed for cork biosynthesis. Furthermore, 19% could be related to regulatory functions. To identify genes more specifically required for suberin biosynthesis, cork expressed sequence tags were printed on a microarray and subsequently used to compare cork (phellem) to a non-suberin-producing tissue such as wood (xylem). Based on the results, a list of candidate genes relevant for cork was obtained. This list includes genes for the synthesis, transport, and polymerization of suberin monomers such as components of the fatty acid elongase complexes, ATP-binding cassette transporters, and acyltransferases, among others. Moreover, a number of regulatory genes induced in cork have been identified, including MYB, No-Apical-Meristem, and WRKY transcription factors with putative functions in meristem identity and cork differentiation.

Wax-deficient anther1 is involved in cuticle and wax production in rice anther walls and is required for pollen development

In vegetative leaf tissues, cuticles including cuticular waxes are important for protection against nonstomatal water loss and pathogen infection as well as for adaptations to environmental stress. However, their roles in the anther wall are rarely studied. The innermost layer of the anther wall (the tapetum) is essential for generating male gametes. Here, we report the characterization of a T-DNA insertional mutant in the Wax-deficient anther1 (Wda1) gene of rice (Oryza sativa), which shows significant defects in the biosynthesis of very-long-chain fatty acids in both layers. This gene is strongly expressed in the epidermal cells of anthers. Scanning electron microscopy analyses showed that epicuticular wax crystals were absent in the outer layer of the anther and that microspore development was severely retarded and finally disrupted as a result of defective pollen exine formation in the mutant anthers. These biochemical and developmental defects in tapetum found in wda1 mutants are earlier events than those in other male-sterile mutants, which showed defects of lipidic molecules in exine. Our findings provide new insights into the biochemical and developmental aspects of the role of waxes in microspore exine development in the tapetum as well as the role of epicuticular waxes in anther expansion.

CER4 encodes an alcohol-forming fatty acyl-coenzyme A reductase involved in cuticular wax production in Arabidopsis

A waxy cuticle that serves as a protective barrier against uncontrolled water loss and environmental damage coats the aerial surfaces of land plants. It is composed of a cutin polymer matrix and waxes. Cuticular waxes are complex mixtures of very-long-chain fatty acids and their derivatives. We report here the molecular cloning and characterization of CER4, a wax biosynthetic gene from Arabidopsis (Arabidopsis thaliana). Arabidopsis cer4 mutants exhibit major decreases in stem primary alcohols and wax esters, and slightly elevated levels of aldehydes, alkanes, secondary alcohols, and ketones. This phenotype suggested that CER4 encoded an alcohol-forming fatty acyl-coenzyme A reductase (FAR). We identified eight FAR-like genes in Arabidopsis that are highly related to an alcohol-forming FAR expressed in seeds of jojoba (Simmondsia chinensis). Molecular characterization of CER4 alleles and genomic complementation revealed that one of these eight genes, At4g33790, encoded the FAR required for cuticular wax production. Expression of CER4 cDNA in yeast (Saccharomyces cerevisiae) resulted in the accumulation of C24:0 and C26:0 primary alcohols. Fully functional green fluorescent protein-tagged CER4 protein was localized to the endoplasmic reticulum in yeast cells by confocal microscopy. Analysis of gene expression by reverse transcription-PCR indicated that CER4 was expressed in leaves, stems, flowers, siliques, and roots. Expression of a beta-glucuronidase reporter gene driven by the CER4 promoter in transgenic plants was detected in epidermal cells of leaves and stems, consistent with a dedicated role for CER4 in cuticular wax biosynthesis. CER4 was also expressed in all cell types in the elongation zone of young roots. These data indicate that CER4 is an alcohol-forming FAR that has specificity for very-long-chain fatty acids and is responsible for the synthesis of primary alcohols in the epidermal cells of aerial tissues and in roots.

Genetic and biochemical evidence for involvement of HOTHEAD in the biosynthesis of long-chain alpha-,omega-dicarboxylic fatty acids and formation of extracellular matrix

In plants, extracellular matrix polymers built from polysaccharides and cuticular lipids have structural and protective functions. The cuticle is found to be ten times thinner in Arabidopsis thaliana (L.) Heynh than in many other plants, and there is evidence that it is unusual in having a high content of alpha-,omega-dicarboxylic fatty acids (FAs) in its polyesters. We designated the new organ fusion mutant hth-12 after it appeared to be allelic to adhesion of calyx edges (ace) and hothead (hth), upon molecular cloning of the gene by transposon tagging. This mutant is deficient in its ability to oxidize long-chain omega-hydroxy FAs to omega-oxo FAs, which results in leaf polyesters in decreased alpha-,omega-dicarboxylic FAs and increased omega-hydroxy FAs. These chemical phenotypes lead to disorder of the cuticle membrane structure in hth-12. ACE/HTH is a single-domain protein showing sequence similarity to long-chain FA omega-alcohol dehydrogenases from Candida species, and we hypothesize that it may catalyze the next step after cytochrome P450 FA omega-hydroxylases in the omega-oxidation pathway. We show that ACE/HTH is specifically expressed in epidermal cells. It appears very likely therefore that the changes in the amount of alpha-,omega-dicarboxylic FAs in hth-12 reflect the different composition of cuticular polyesters. The ACE/HTH gene is also expressed in root epidermal cells which do not form a polyester membrane on the exterior surface, thereby making it possible that the end products of the pathway, alpha-,omega-dicarboxylic FAs, are generally required for the cross-linking that ensures the integrity of the outer epidermal cell wall.

Substrate specificity of Arabidopsis 3-ketoacyl-CoA synthases

The very long chain fatty acids (VLCFA) incorporated into plant lipids are derived from the iterative addition of C2 units provided by malonyl-CoA to an acyl-CoA by the 3-ketoacyl-CoA synthase (KCS) component of a fatty acid elongase (FAE) complex. Mining of the Arabidopsis genome sequence database revealed 20 genes with homology to seed-specific FAE1 KCS. Eight of the 20 putative KCSs were cloned, expressed in yeast, and isolated as (His)6 fusion proteins. Five of the eight (At1g71160, At1g19440, At1g07720, At5g04530, and At4g34250) had little or no activity with C16 to C20 substrates while three demonstrated activity with C16, C18, and C20 saturated acyl-CoA substrates. At1g01120 KCS (KCS1) and At2g26640 KCS had broad substrate specificities when assayed with saturated and mono-unsaturated C16 to C24 acyl-CoAs while At4g34510 KCS was specific for saturated fatty acyl-CoA substrates.