Arabidopsis cuticular waxes: advances in synthesis, export and regulation

Transcriptomic Profiling of the Maize (Zea mays L.) Leaf Response to Abiotic Stresses at the Seedling Stage

Abiotic stresses, including drought, salinity, heat, and cold, negatively affect maize (Zea mays L.) development and productivity. To elucidate the molecular mechanisms of resistance to abiotic stresses in maize, RNA-seq was used for global transcriptome profiling of B73 seedling leaves exposed to drought, salinity, heat, and cold stress. A total of 5,330 differentially expressed genes (DEGs) were detected in differential comparisons between the control and each stressed sample, with 1,661, 2,019, 2,346, and 1,841 DEGs being identified in comparisons of the control with salinity, drought, heat, and cold stress, respectively. Functional annotations of DEGs suggested that the stress response was mediated by pathways involving hormone metabolism and signaling, transcription factors (TFs), very-long-chain fatty acid biosynthesis and lipid signaling, among others. Of the obtained DEGs (5,330), 167 genes are common to these four abiotic stresses, including 10 up-regulated TFs (five ERFs, two NACs, one ARF, one MYB, and one HD-ZIP) and two down-regulated TFs (one b-ZIP and one MYB-related), which suggested that common mechanisms may be initiated in response to different abiotic stresses in maize. This study contributes to a better understanding of the molecular mechanisms of maize leaf responses to abiotic stresses and could be useful for developing maize cultivars resistant to abiotic stresses.

Calculated Mechanism of Cyanobacterial Aldehyde-Deformylating Oxygenase: Asymmetric Aldehyde Activation by a Symmetric Diiron Cofactor

Cyanobacterial aldehyde-deformylating oxygenase (cADO) is a nonheme diiron enzyme that catalyzes the conversion of aldehyde to alk(a/e)ne, an important transformation in biofuel research. In this work, we report a highly desired computational study for probing the mechanism of cADO. By combining our QM/MM results with the available ⁵⁷Fe Mössbauer spectroscopic data, the gained detailed structural information suggests construction of asymmetry from the symmetric diiron cofactor in an aldehyde substrate and O₂ activation. His₁₆₀, one of the two iron-coordinate histidine residues in cADO, plays a pivotal role in this asymmetric aldehyde activation process by unprecedented reversible dissociation from the diiron cofactor, a behavior unknown in any other nonheme dinuclear or mononuclear enzymes. The revealed intrinsically asymmetric interactions of the substrate/O₂ with the symmetric cofactor in cADO are inspirational for exploring diiron subsite resolution in other nonheme diiron enzymes.

Identification and characterization of wheat drought-responsive MYB transcription factors involved in the regulation of cuticle biosynthesis

A plant cuticle forms a hydrophobic layer covering plant organs, and plays an important role in plant development and protection from environmental stresses. We examined epicuticular structure, composition, and a MYB-based regulatory network in two Australian wheat cultivars, RAC875 and Kukri, with contrasting cuticle appearance (glaucousness) and drought tolerance. Metabolomics and microscopic analyses of epicuticular waxes revealed that the content of β-diketones was the major compositional and structural difference between RAC875 and Kukri. The content of β-diketones remained the same while those of alkanes and primary alcohols were increased by drought in both cultivars, suggesting that the interplay of all components rather than a single one defines the difference in drought tolerance between cultivars. Six wheat genes encoding MYB transcription factors (TFs) were cloned; four of them were regulated in flag leaves of both cultivars by rapid dehydration and/or slowly developing cyclic drought. The involvement of selected MYB TFs in the regulation of cuticle biosynthesis was confirmed by a transient expression assay in wheat cell culture, using the promoters of wheat genes encoding cuticle biosynthesis-related enzymes and the SHINE1 (SHN1) TF. Two functional MYB-responsive elements, specifically recognized by TaMYB74 but not by other MYB TFs, were localized in the TdSHN1 promoter. Protein structural determinants underlying the binding specificity of TaMYB74 for functional DNA cis-elements were defined, using 3D protein molecular modelling. A scheme, linking drought-induced expression of the investigated TFs with downstream genes that participate in the synthesis of cuticle components, is proposed.

Cuticular wax biosynthesis is positively regulated by WRINKLED4, an AP2/ERF-type transcription factor, in Arabidopsis stems

The aerial surfaces of terrestrial plants are covered by a cuticular wax layer, which protects the plants from environmental stresses such as desiccation, high irradiance, and UV radiation. Cuticular wax deposition is regulated in an organ-specific manner; Arabidopsis stems have more than 10-fold higher wax loads than leaves. In this study, we found that WRINKLED4 (WRI4), encoding an AP2/ERF (ethylene-responsive factor) transcription factor (TF), is predominantly expressed in stem epidermis, is upregulated by salt stress, and is involved in activating cuticular wax biosynthesis in Arabidopsis stems. WRI4 harbors a transcriptional activation domain at its N-terminus, and fluorescent signals from a WRI4:eYFP construct were localized to the nuclei of tobacco leaf protoplasts. Deposition of epicuticular wax crystals on stems was reduced in wri4-1 and wri4-3 knockout mutants. Total wax loads were reduced by ~28% in wri4 stems but were not altered in wri4 siliques or leaves compared to the wild type. The levels of 29-carbon long alkanes, ketones, and secondary alcohols, which are the most abundant components of stem waxes, were significantly reduced in wri4 stems relative to the wild type. A transactivation assay in tobacco protoplasts and a chromatin immunoprecipitation (ChIP) assay showed that the expression of long-chain acyl-CoA synthetase1 (LACS1), β-ketoacyl CoA reductase1 (KCR1), PASTICCINO2 (PAS2), trans-2,3-enoyl-CoA reductase (ECR), and bifunctional wax synthase/acyl-CoA: diacylglycerol acyltransferase (WSD1) is positively regulated by direct binding of WRI4 to their promoters. Taken together, these results suggest that WRI4 is a transcriptional activator that specifically controls cuticular wax biosynthesis in Arabidopsis stems.

Enzymes for fatty acid-based hydrocarbon biosynthesis

Surging energy consumption and environmental concerns have stimulated interest in the production of chemicals and fuels through sustainable and renewable approaches. Fatty acid-based hydrocarbons, such as alkanes and alkenes, are of particular interest to directly replace fossil fuels. Towards this effort, understanding of hydrocarbon-producing enzymes is the first indispensable step to bio-production of hydrocarbons. Here, we review recent advances in the discovery and mechanistic study of enzymes capable of converting fatty acid precursors into hydrocarbons, and provide perspectives on the future of this rapidly growing field.

OsHSD1, a hydroxysteroid dehydrogenase, is involved in cuticle formation and lipid homeostasis in rice

Cuticular wax, a hydrophobic layer on the surface of all aerial plant organs, has essential roles in plant growth and survival under various environments. Here we report a wax-deficient rice mutant oshsd1 with reduced epicuticular wax crystals and thicker cuticle membrane. Quantification of the wax components and fatty acids showed elevated levels of very-long-chain fatty acids (VLCFAs) and accumulation of soluble fatty acids in the leaves of the oshsd1 mutant. We determined the causative gene OsHSD1, a member of the short-chain dehydrogenase reductase family, through map-based cloning. It was ubiquitously expressed and responded to cold stress and exogenous treatments with NaCl or brassinosteroid analogs. Transient expression of OsHSD1-tagged green fluorescent protein revealed that OsHSD1 localized to both oil bodies and endoplasmic reticulum (ER). Dehydrogenase activity assays demonstrated that OsHSD1 was an NAD(+)/NADP(+)-dependent sterol dehydrogenase. Furthermore, OsHSD1 mutation resulted in faster protein degradation, but had no effect on the dehydrogenase activity. Together, our data indicated that OsHSD1 plays a specialized role in cuticle formation and lipid homeostasis, probably by mediating sterol signaling. This work provides new insights into oil-body associated proteins involved in wax and lipid metabolism.

Secondary metabolites in plants: transport and self-tolerance mechanisms

Plants produce a host of secondary metabolites with a wide range of biological activities, including potential toxicity to eukaryotic cells. Plants generally manage these compounds by transport to the apoplast or specific organelles such as the vacuole, or other self-tolerance mechanisms. For efficient production of such bioactive compounds in plants or microbes, transport and self-tolerance mechanisms should function cooperatively with the corresponding biosynthetic enzymes. Intensive studies have identified and characterized the proteins responsible for transport and self-tolerance. In particular, many transporters have been isolated and their physiological functions have been proposed. This review describes recent progress in studies of transport and self-tolerance and provides an updated inventory of transporters according to their substrates. Application of such knowledge to synthetic biology might enable efficient production of valuable secondary metabolites in the future.

BODYGUARD is required for the biosynthesis of cutin in Arabidopsis

The cuticle plays a critical role in plant survival during extreme drought conditions. There are, however, surprisingly, many gaps in our understanding of cuticle biosynthesis. An Arabidopsis thaliana T-DNA mutant library was screened for mutants with enhanced transpiration using a simple condensation spot method. Five mutants, named cool breath (cb), were isolated. The cb5 mutant was found to be allelic to bodyguard (bdg), which is affected in an α/β-hydrolase fold protein important for cuticle structure. The analysis of cuticle components in cb5 (renamed as bdg-6) and another T-DNA mutant allele (bdg-7) revealed no impairment in wax synthesis, but a strong decrease in total cutin monomer load in young leaves and flowers. Root suberin content was also reduced. Overexpression of BDG increased total leaf cutin monomer content nearly four times by affecting preferentially C18 polyunsaturated ω-OH fatty acids and dicarboxylic acids. Whole-plant gas exchange analysis showed that bdg-6 had higher cuticular conductance and rate of transpiration; however, plant lines overexpressing BDG resembled the wild-type with regard to these characteristics. This study identifies BDG as an important component of the cutin biosynthesis machinery in Arabidopsis. We also show that, using BDG, cutin can be greatly modified without altering the cuticular water barrier properties and transpiration.

Barrier against water loss: relationship between epicuticular wax composition, gene expression and leaf water retention capacity in banana

In the present study we examined 13 banana (Musa spp.) genotypes belonging to different genomic groups with respect to total leaf cuticular wax concentration, chemical composition, carbon chain length and their relationship with leaf water retention capacity (LWRC). A positive correlation between epicuticular wax content and LWRC clearly indicated that the cuticular wax plays an important role in maintaining banana leaf water content. The classification of hexane soluble cuticular wax components into different classes based on functional group and their association with LWRC showed that alcohol and ester compounds have a positive correlation. Further, the compounds with >C28 carbon chain length had a positive correlation with LWRC, indicating the role of longer carbon chain length in maintaining the water status of banana leaves. Also, the gene expression analysis showed higher expression of the wax biosynthetic genes FATB and KCS11 in higher wax load genotypes whereas lower expression was seen in low wax banana genotypes. Here, we report for the first time the compositional variations of cuticular wax in different banana genotypes, followed by their association with leaf water retention capacity. The results were also supported by variation in gene expression analysis of cuticular wax biosynthetic genes - FATB and KCS11.

Overexpression of the Transcription Factors GmSHN1 and GmSHN9 Differentially Regulates Wax and Cutin Biosynthesis, Alters Cuticle Properties, and Changes Leaf Phenotypes in Arabidopsis

SHINE (SHN/WIN) clade proteins, transcription factors of the plant-specific APETALA 2/ethylene-responsive element binding factor (AP2/ERF) family, have been proven to be involved in wax and cutin biosynthesis. Glycine max is an important economic crop, but its molecular mechanism of wax biosynthesis is rarely characterized. In this study, 10 homologs of Arabidopsis SHN genes were identified from soybean. These homologs were different in gene structures and organ expression patterns. Constitutive expression of each of the soybean SHN genes in Arabidopsis led to different leaf phenotypes, as well as different levels of glossiness on leaf surfaces. Overexpression of GmSHN1 and GmSHN9 in Arabidopsis exhibited 7.8-fold and 9.9-fold up-regulation of leaf cuticle wax productions, respectively. C31 and C29 alkanes contributed most to the increased wax contents. Total cutin contents of leaves were increased 11.4-fold in GmSHN1 overexpressors and 5.7-fold in GmSHN9 overexpressors, mainly through increasing C16:0 di-OH and dioic acids. GmSHN1 and GmSHN9 also altered leaf cuticle membrane ultrastructure and increased water loss rate in transgenic Arabidopsis plants. Transcript levels of many wax and cutin biosynthesis and leaf development related genes were altered in GmSHN1 and GmSHN9 overexpressors. Overall, these results suggest that GmSHN1 and GmSHN9 may differentially regulate the leaf development process as well as wax and cutin biosynthesis.

An ethoxylated surfactant enhances the penetration of the sulfated laminarin through leaf cuticle and stomata, leading to increased induced resistance against grapevine downy mildew

Some β-1,3-glucans and particularly sulfated laminarin (PS3) are known as resistance inducers (RIs) in grapevine against the downy mildew. However, their efficacy in vineyard is still often too low, which might be caused by a limited penetration through the leaf cuticle following spray application. We used (14) C-sucrose uptake experiments with grapevine leaves in order to select a surfactant as saccharide penetration enhancer. Our results showed that although sucrose foliar uptake was low, it was strongly enhanced by Dehscofix CO125 (DE), a highly ethoxylated surfactant. Fluorescent saccharides were then produced and laser scanning microscopy was used to analyze their foliar diffusion pattern in Arabidopsis thaliana and grapevine. Interestingly, sucrose and PS3 were seemingly able to penetrate the leaf cuticle only when formulated with DE. Diffusion could preferentially occur via stomata, anticlinal cell walls and trichomes. In grapevine, PS3 penetration rate was much higher on the stomateous abaxial surface of the leaf than on the adaxial surface. Finally, using DE allowed a higher level of downy mildew control by PS3, which corroborated diffusion observations. Our results have practical consequences for the improvement of treatments with saccharidic inducers on grape. That is, formulation of such RIs plays a critical role for their cuticular diffusion and consequently their efficacy. Also, spray application should preferentially target the abaxial surface of the leaves in order to maximize their penetration.

Fatty Acid and Lipid Transport in Plant Cells

Fatty acids (FAs) and lipids are essential - not only as membrane constituents but also for growth and development. In plants and algae, FAs are synthesized in plastids and to a large extent transported to the endoplasmic reticulum for modification and lipid assembly. Subsequently, lipophilic compounds are distributed within the cell, and thus are transported across most membrane systems. Membrane-intrinsic transporters and proteins for cellular FA/lipid transfer therefore represent key components for delivery and dissemination. In addition to highlighting their role in lipid homeostasis and plant performance, different transport mechanisms for land plants and green algae - in the model systems Arabidopsis thaliana, Chlamydomonas reinhardtii - are compared, thereby providing a current perspective on protein-mediated FA and lipid trafficking in photosynthetic cells.

Identification of β-hydroxy fatty acid esters and primary, secondary-alkanediol esters in cuticular waxes of the moss Funaria hygrometrica

The plant cuticle, a multi-layered membrane that covers plant aerial surfaces to prevent desiccation, consists of the structural polymer cutin and surface-sealing waxes. Cuticular waxes are complex mixtures of ubiquitous, typically monofunctional fatty acid derivatives and taxon-specific, frequently bifunctional specialty compounds. To further our understanding of the chemical diversity of specialty compounds, the waxes on the aerial structures of the leafy gametophyte, sporophyte capsule, and calyptra of the moss Funaria hygrometrica were surveyed. Respective moss surfaces were extracted, and resulting lipid mixtures were analyzed by gas chromatography-mass spectrometry (GC-MS). The extracts contained ubiquitous wax compound classes along with two prominent, unidentified classes of compounds that exhibited some characteristics of bifunctional structures. Microscale transformations led to derivatives with characteristic MS fragmentation patterns suggesting possible structures for these compounds. To confirm the tentative structure assignments, one compound in each of the suspected homologous series was synthesized. Based on GC-MS comparison with the authentic standards, the first series of compounds was identified as containing esters formed by β-hydroxy fatty acids and wax alcohols, with ester chain lengths varying from C42 to C50 and the most prominent homolog being C46. The second series consisted of fatty acid esters of 1,7-alkanediols, linked via the primary hydroxyl group, with ester chain lengths C40-C52 also dominated by the C46 homolog. The β-hydroxy acid esters were restricted to the sporophyte capsule, and the diol esters to the leafy gametophyte and calyptra. Based on their homolog and isomer distributions, and the presence of free 1,7-triacontanediol, possible biosynthetic reactions leading to these compounds are discussed.

Acyl-CoA-Binding Proteins (ACBPs) in Plant Development

Acyl-CoA-binding proteins (ACBPs) play a pivotal role in fatty acid metabolism because they can transport medium- and long-chain acyl-CoA esters. In eukaryotic cells, ACBPs are involved in intracellular trafficking of acyl-CoA esters and formation of a cytosolic acyl-CoA pool. In addition to these ubiquitous functions, more specific non-redundant roles of plant ACBP subclasses are implicated by the existence of multigene families with variable molecular masses, ligand specificities, functional domains (e.g. protein-protein interaction domains), subcellular locations and gene expression patterns. In this chapter, recent progress in the characterization of ACBPs from the model dicot plant, Arabidopsis thaliana, and the model monocot, Oryza sativa, and their emerging roles in plant growth and development are discussed. The functional significance of respective members of the plant ACBP families in various developmental and physiological processes such as seed development and germination, stem cuticle formation, pollen development, leaf senescence, peroxisomal fatty acid β-oxidation and phloem-mediated lipid transport is highlighted.

Developmental and Genotypic Variation in Leaf Wax Content and Composition, and in Expression of Wax Biosynthetic Genes in Brassica oleracea var. capitata

Cuticular waxes act as a protective barrier against environmental stresses. In the present study, we investigated developmental and genotypic variation in wax formation of cabbage lines, with a view to understand the related morphology, genetics and biochemistry. Our studies revealed that the relative expression levels of wax biosynthetic genes in the first-formed leaf of the highest-wax line remained constantly higher but were decreased in other genotypes with leaf aging. Similarly, the expression of most of the tested genes exhibited decrease from the inner leaves to the outer leaves of 5-month-old cabbage heads in the low-wax lines in contrast to the highest-wax line. In 10-week-old plants, expression of wax biosynthetic genes followed a quadratic function and was generally increased in the early developing leaves but substantially decreased at the older leaves. The waxy compounds in all cabbage lines were predominately C₂₉-alkane, -secondary alcohol, and -ketone. Its deposition was increased with leaf age in 5-month-old plants. The high-wax lines had dense, prominent and larger crystals on the leaf surface compared to low-wax lines under scanning electron microscopy. Principal component analysis revealed that the higher expression of LTP2 genes in the lowest-wax line and the higher expression of CER3 gene in the highest-wax line were probably associated with the comparatively lower and higher wax content in those two lines, respectively. This study furthers our understanding of the relationships between the expression of wax biosynthetic genes and the wax deposition in cabbage lines. Highlight: In cabbage, expression of wax-biosynthetic genes was generally decreased in older and senescing leaves, while wax deposition was increased with leaf aging, and C₂₉-hydrocarbon was predominant in the wax crystals.

Plant Cytosolic Acyl-CoA-Binding Proteins

A gene family encoding six members of acyl-CoA-binding proteins (ACBP) exists in Arabidopsis and they are designated as AtACBP1-AtACBP6. They have been observed to play pivotal roles in plant lipid metabolism, consistent to the abilities of recombinant AtACBP in binding different medium- and long-chain acyl-CoA esters in vitro. While AtACBP1 and AtACBP2 are membrane-associated proteins with ankyrin repeats and AtACBP3 contains a signaling peptide for targeting to the apoplast, AtACBP4, AtACBP5 and AtACBP6 represent the cytosolic forms in the AtACBP family. They were verified to be subcellularly localized in the cytosol using diverse experimental methods, including cell fractionation followed by western blot analysis, immunoelectron microscopy and confocal laser-scanning microscopy using autofluorescence-tagged fusions. AtACBP4 (73.2 kDa) and AtACBP5 (70.1 kDa) are the largest, while AtACBP6 (10.4 kDa) is the smallest. Their binding affinities to oleoyl-CoA esters suggested that they can potentially transfer oleoyl-CoA esters from the plastids to the endoplasmic reticulum, facilitating the subsequent biosynthesis of non-plastidial membrane lipids in Arabidopsis. Recent studies on ACBP, extended from a dicot (Arabidopsis) to a monocot, revealed that six ACBP are also encoded in rice (Oryza sativa). Interestingly, three small rice ACBP (OsACBP1, OsACBP2 and OsACBP3) are present in the cytosol in comparison to one (AtACBP6) in Arabidopsis. In this review, the combinatory and distinct roles of the cytosolic AtACBP are discussed, including their functions in pollen and seed development, light-dependent regulation and substrate affinities to acyl-CoA esters.

OsWS1 involved in cuticular wax biosynthesis is regulated by osa-miR1848

Cuticular wax forms a hydrophobic layer covering aerial plant organs and acting as a protective barrier against biotic and abiotic stresses. Compared with well-known wax biosynthetic pathway, molecular regulation of wax biosynthesis is less known. Here, we show that rice OsWS1, a member of the membrane-bound O-acyl transferase gene family, involved in wax biosynthesis and was regulated by an osa-miR1848. OsWS1-tagged green fluorescent protein localized to the endoplasmic reticulum (ER). Compared with wild-type rice, OsWS1 overexpression plants displayed a 3% increase in total wax, especially a 35% increase in very long-chain fatty acids, denser wax papillae around the stoma, more cuticular wax crystals formed on leaf and stem surfaces, pollen coats were thicker and more seedlings survived after water-deficit treatment. In contrast, OsWS1-RNAi and osa-miR1848 overexpression plants exhibited opposing changes. Gene expression analysis showed that overexpression of osa-miR1848 down-regulated OsWS1 transcripts; furthermore, expression profiles of OsWS1 and osa-miR1848 were inversely correlated in the leaf, panicle and stem, and upon water-deficit treatment. These results suggest that OsWS1 is regulated by osa-miR1848 and participates in cuticular wax formation.

W3 Is a New Wax Locus That Is Essential for Biosynthesis of β-Diketone, Development of Glaucousness, and Reduction of Cuticle Permeability in Common Wheat

The cuticle plays important roles in plant development, growth and defense against biotic and abiotic attacks. Crystallized epicuticular wax, the outermost layer of cuticle, is visible as white-bluish glaucousness. In crops like barley and wheat, glaucousness is trait of adaption to the dry and hot cultivation conditions, and hentriacontane-14,16-dione (β-diketone) and its hydroxy derivatives are the major and unique components of cuticular wax in the upper parts of adult plants. But their biosynthetic pathway and physiological role largely remain unknown. In the present research, we identified a novel wax mutant in wheat cultivar Bobwhite. The mutation is not allelic to the known wax production gene loci W1 and W2, and designated as W3 accordingly. Genetic analysis localized W3 on chromosome arm 2BS. The w3 mutation reduced 99% of β-diketones, which account for 63.3% of the total wax load of the wild-type. W3 is necessary for β-diketone synthesis, but has a different effect on β-diketone hydroxylation because the hydroxy-β-diketones to β-diketone ratio increased 11-fold in the w3 mutant. Loss of β-diketones caused failure to form glaucousness and significant increase of cuticle permeability in terms of water loss and chlorophyll efflux in the w3 mutant. Transcription of 23 cuticle genes from five functional groups was altered in the w3 mutant, 19 down-regulated and four up-regulated, suggesting a possibility that W3 encodes a transcription regulator coordinating expression of cuticle genes. Biosynthesis of β-diketones in wheat and their implications in glaucousness formation and drought and heat tolerance were discussed.

De novo transcriptome sequencing and comprehensive analysis of the drought-responsive genes in the desert plant Cynanchum komarovii

BACKGROUND

Cynanchum komarovii Al Iljinski is a xerophytic plant species widely distributing in the severely adverse environment of the deserts in northwest China. At present, the detailed transcriptomic and genomic data for C. komarovii are still insufficient in public databases.

RESULTS

To investigate changes of drought-responsive genes and explore the mechanisms of drought tolerance in C. komarovii, approximately 27.5 GB sequencing data were obtained using Illumina sequencing technology. After de novo assembly 148,715 unigenes were generated with an average length of 604 bp. Among these unigenes, 85,106 were annotated with gene descriptions, conserved domains, gene ontology terms, and metabolic pathways. The results showed that a great number of unigenes were significantly affected by drought stress. We identified 3134 unigenes as reliable differentially expressed genes (DEGs). During drought stress, the regulatory genes were involved in signaling transduction pathways and in controlling the expression of functional genes. Moreover, C. komarovii activated many functional genes that directly protected against stress and improved tolerance to adapt drought condition. Importantly, the DEGs were involved in biosynthesis, export, and regulation of plant cuticle, suggesting that plant cuticle may play a vital role in response to drought stress and the accumulation of cuticle may allow C. komarovii to improve the tolerance to drought stress.

CONCLUSION

This is the first large-scale reference sequence data of C. komarovii, which enlarge the genomic resources of this species. Our comprehensive transcriptome analysis will provide a valuable resource for further investigation into the molecular adaptation of desert plants under drought condition and facilitate the exploration of drought-tolerant candidate genes.

Bridging the gaps: evolution and development of perianth fusion

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

Epidermal Micromorphology and Mesophyll Structure of Populus euphratica Heteromorphic Leaves at Different Development Stages

Leaf epidermal micromorphology and mesophyll structure during the development of Populus euphratica heteromorphic leaves, including linear, lanceolate, ovate, dentate ovate, dentate rhombic, dentate broad-ovate and dentate fan-shaped leaves, were studied by using electron and light microscopy. During development of heteromorphic leaves, epidermal appendages (wax crystals and trichomes) and special cells (mucilage cells and crystal idioblasts) increased in all leaf types while chloroplast ultrastructure and stomatal characters show maximum photosynthetic activity in dentate ovate and rhombic leaves. Also, functional analysis by subordinate function values shows that the maximum adaptability to adverse stress was exhibited in the broad type of mature leaves. The 12 heteromorphic leaf types are classified into three major groups by hierarchical cluster analysis: young, developing and mature leaves. Mature leaves can effectively obtain the highest stress resistance by combining the protection of xerophytic anatomy from drought stress, regulation of water uptake in micro-environment by mucilage and crystal idioblasts, and assistant defense of transpiration reduction through leaf epidermal appendages, which improves photosynthetic activity under arid desert conditions. Our data confirms that the main leaf function is differentiated during the developing process of heteromorphic leaves.

The maize fused leaves1 (fdl1) gene controls organ separation in the embryo and seedling shoot and promotes coleoptile opening

The fdl1-1 mutation, caused by an Enhancer/Suppressor mutator (En/Spm) element insertion located in the third exon of the gene, identifies a novel gene encoding ZmMYB94, a transcription factor of the R2R3-MYB subfamily. The fdl1 gene was isolated through co-segregation analysis, whereas proof of gene identity was obtained using an RNAi strategy that conferred less severe, but clearly recognizable specific mutant traits on seedlings. Fdl1 is involved in the regulation of cuticle deposition in young seedlings as well as in the establishment of a regular pattern of epicuticular wax deposition on the epidermis of young leaves. Lack of Fdl1 action also correlates with developmental defects, such as delayed germination and seedling growth, abnormal coleoptile opening and presence of curly leaves showing areas of fusion between the coleoptile and the first leaf or between the first and the second leaf. The expression profile of ZmMYB94 mRNA-determined by quantitative RT-PCR-overlaps the pattern of mutant phenotypic expression and is confined to a narrow developmental window. High expression was observed in the embryo, in the seedling coleoptile and in the first two leaves, whereas RNA level, as well as phenotypic defects, decreases at the third leaf stage. Interestingly several of the Arabidopsis MYB genes most closely related to ZmMYB94 are also involved in the activation of cuticular wax biosynthesis, suggesting deep conservation of regulatory processes related to cuticular wax deposition between monocots and dicots.

Climate-resilient agroforestry: physiological responses to climate change and engineering of crassulacean acid metabolism (CAM) as a mitigation strategy

Global climate change threatens the sustainability of agriculture and agroforestry worldwide through increased heat, drought, surface evaporation and associated soil drying. Exposure of crops and forests to warmer and drier environments will increase leaf:air water vapour-pressure deficits (VPD), and will result in increased drought susceptibility and reduced productivity, not only in arid regions but also in tropical regions with seasonal dry periods. Fast-growing, short-rotation forestry (SRF) bioenergy crops such as poplar (Populus spp.) and willow (Salix spp.) are particularly susceptible to hydraulic failure following drought stress due to their isohydric nature and relatively high stomatal conductance. One approach to sustaining plant productivity is to improve water-use efficiency (WUE) by engineering crassulacean acid metabolism (CAM) into C3 crops. CAM improves WUE by shifting stomatal opening and primary CO2 uptake and fixation to the night-time when leaf:air VPD is low. CAM members of the tree genus Clusia exemplify the compatibility of CAM performance within tree species and highlight CAM as a mechanism to conserve water and maintain carbon uptake during drought conditions. The introduction of bioengineered CAM into SRF bioenergy trees is a potentially viable path to sustaining agroforestry production systems in the face of a globally changing climate.

Analysis of cuticular wax constituents and genes that contribute to the formation of 'glossy Newhall', a spontaneous bud mutant from the wild-type 'Newhall' navel orange

Navel orange (Citrus sinensis [L.] Osbeck) fruit surfaces contain substantial quantities of cuticular waxes, which have important eco-physiological roles, such as water retention and pathogen defense. The wax constituents of ripe navel orange have been studied in various reports, while the wax changes occurring during fruit development and the molecular mechanism underlying their biosynthesis/export have not been investigated. Recently, we reported a spontaneous bud mutant from the wild-type (WT) 'Newhall' Navel orange. This mutant displayed unusual glossy fruit peels and was named 'glossy Newhall' (MT). In this study, we compared the developmental profiles of the epicuticular and intracuticular waxes on the WT and MT fruit surfaces. The formation of epicuticular wax crystals on the navel orange surface was shown to be dependent on the accumulation of high amounts of aliphatic wax components with trace amounts of terpenoids. In sharp contrast, the underlying intracuticular wax layers have relatively low concentrations of aliphatic wax components but high concentrations of cyclic wax compounds, especially terpenoids at the late fruit developmental stages. Our work also showed that many genes that are involved in wax biosynthesis and export pathways were down-regulated in MT fruit peels, leading to a decrease in aliphatic wax component amounts and the loss of epicuticular wax crystals, ultimately causing the glossy phenotype of MT fruits.

Hydrocarbons, the advanced biofuels produced by different organisms, the evidence that alkanes in petroleum can be renewable

It is generally regarded that the petroleum cannot be renewable. However, in recent years, it has been found that many marine cyanobacteria, some eubacteria, engineered Escherichia coli, some endophytic fungi, engineered yeasts, some marine yeasts, plants, and insects can synthesize hydrocarbons with different carbon lengths. If the organisms, especially some native microorganisms and engineered bacteria and yeasts, can synthesize and secret a large amount of hydrocarbons within a short period, alkanes in the petroleum can be renewable. It has been documented that there are eight pathways for hydrocarbon biosynthesis in different organisms. Unfortunately, most of native microorganisms, engineered E. coli and engineered yeasts, only synthesize a small amount of intracellular and extracellular hydrocarbons. Recently, Aureobasidium pullulans var. melanogenum isolated from a mangrove ecosystem has been found to be able to synthesize and secret over 21.5 g/l long-chain hydrocarbons with a yield of 0.275 g/g glucose and a productivity of 0.193 g/l/h within 5 days. The yeast may have highly potential applications in alkane production.

Position-Specific Gene Expression Analysis Using a Microgram Dissection Method Combined with On-Bead cDNA Library Construction

Gene expression analysis is a key technology that is used to understand living systems. Multicellular organisms, including plants, are composed of various tissues and cell types, each of which exhibits a unique gene expression pattern. However, because of their rigid cell walls, plant cells are difficult to isolate from the whole plant. Although laser dissection has been used to circumvent this problem, the plant sample needs to be fixed beforehand, which presents several problems. In the present study, we developed an alternative method to conduct highly reliable gene expression profiling. First, we assembled a dissection apparatus that used a narrow, sharpened needle to dissect out a microsample of fresh plant tissue (0.1-0.2 mm on each side) automatically from a target site within a short time frame. Then, we optimized a protocol to synthesize a high-quality cDNA library on magnetic beads using a single microsample. The cDNA library was amplified and subjected to high-throughput sequencing. In this way, a stable and reliable system was developed to conduct gene expression profiling in small regions of a plant. The system was used to analyze the gene expression patterns at successive 50 µm intervals in the shoot apex of a 4-day-old Arabidopsis seedling. Clustering analysis of the data demonstrated that two small, adjacent domains, the shoot apical meristem and the leaf primordia, were clearly distinguishable. This system should be broadly applicable in the investigation of the spatial organization of gene expression in various contexts.

Molecular Mechanisms Underlying Hull-Caryopsis Adhesion/Separation Revealed by Comparative Transcriptomic Analysis of Covered/Naked Barley (Hordeum vulgare L.)

The covered/naked caryopsis trait of barley is an important agronomic trait because it is directly linked to dietary use. The formation of covered/naked caryopsis is controlled by an NUD transcription factor, which is involved in pericarp cuticle development. However, the molecular mechanism underlying this trait remains so far largely unknown. In this study, comparative transcriptomes of grains three weeks after anthesis of Tibetan Hulless barley landrace Dulihuang and covered barley Morex were analyzed using RNA-seq technique. A total of 4031 differentially expressed genes (DEGs) were identified. The Nud gene was overexpressed in Morex, with trace expression in Dulihuang. Among seventeen cuticle related DEGs, sixteen were down regulated and one up regulated in Morex. These results suggest that the Nud gene in covered caryopsis might down regulate cuticle related genes, which may cause a permeable cuticle over pericarp, leading to a hull-caryopsis organ fusion. A functional cuticle covering the pericarp of naked caryopsis might be the result of deletion or low expression level of the Nud gene. The functional cuticle defines a perfect boundary to separate the caryopsis from the hull in naked barley.

The biosynthesis, composition and assembly of the outer pollen wall: A tough case to crack

The formation of the durable outer pollen wall, largely composed of sporopollenin, is essential for the protection of the male gametophyte and plant reproduction. Despite its apparent strict conservation amongst land plants, the composition of sporopollenin and the biosynthetic pathway(s) yielding this recalcitrant biopolymer remain elusive. Recent molecular genetic studies in Arabidopsis thaliana (Arabidopsis) and rice have, however, identified key genes involved in sporopollenin formation, allowing a better understanding of the biochemistry and cell biology underlying sporopollenin biosynthesis and pollen wall development. Herein, current knowledge of the biochemical composition of the outer pollen wall is reviewed, with an emphasis on enzymes with characterized biochemical activities in sporopollenin and pollen coat biosynthesis. The tapetum, which forms the innermost sporophytic cell layer of the anther and envelops developing pollen, plays an essential role in sporopollenin and pollen coat formation. Recent studies show that several tapetum-expressed genes encode enzymes that metabolize fatty acid derived compounds to form putative sporopollenin precursors, including tetraketides derived from fatty acyl-CoA starter molecules, but analysis of mutants defective in pollen wall development indicate that other components are also incorporated into sporopollenin. Also highlighted are the many uncertainties remaining in the development of a sporopollenin-fortified pollen wall, particularly in relation to the mechanisms of sporopollenin precursor transport and assembly into the patterned form of the pollen wall. A working model for sporopollenin biosynthesis is proposed based on the data obtained largely from studies of Arabidopsis, and future challenges to complete our understanding of pollen wall biology are outlined.

Micromorphology and development of the epicuticular structure on the epidermal cell of ginseng leaves

BACKGROUND

A leaf cuticle has different structures and functions as a barrier to water loss and as protection from various environmental stressors.

METHODS

Leaves of Panax ginseng were examined by scanning electron microscopy and transmission electron microscopy to investigate the characteristics and development of the epicuticular structure.

RESULTS

Along the epidermal wall surface, the uniformly protuberant fine structure was on the adaxial surface of the cuticle. This epicuticular structure was highly wrinkled and radially extended to the marginal region of epidermal cells. The cuticle at the protuberant positions maintained the same thickness. The density of the wall matrix under the structures was also similar to that of the other wall region. By contrast, none of this structure was distributed on the abaxial surface, except in the region of the stoma. During the early developmental phase of the epicuticular structure, small vesicles appeared on wall-cuticle interface in the peripheral wall of epidermal cells. Some electron-opaque vesicles adjacent to the cuticle were fused and formed the cuticle layer, whereas electron-translucent vesicles contacted each other and progressively increased in size within the epidermal wall.

CONCLUSION

The outwardly projected cuticle and epidermal cell wall (i.e., an epicuticular wrinkle) acts as a major barrier to block out sunlight in ginseng leaves. The small vesicles in the peripheral region of epidermal cells may suppress the cuticle and parts of epidermal wall, push it upward, and consequently contribute to the formation of the epicuticular structure.

Advances in the understanding of cuticular waxes in Arabidopsis thaliana and crop species

The aerial parts of plants are covered with a cuticle, a hydrophobic layer consisting of cutin polyester and cuticular waxes that protects them from various environmental stresses. Cuticular waxes mainly comprise very long chain fatty acids and their derivatives such as aldehydes, alkanes, secondary alcohols, ketones, primary alcohols, and wax esters that are also important raw materials for the production of lubricants, adhesives, cosmetics, and biofuels. The major function of cuticular waxes is to control non-stomatal water loss and gas exchange. In recent years, the in planta roles of many genes involved in cuticular wax biosynthesis have been characterized not only from model organisms like Arabidopsis thaliana and saltwater cress (Eutrema salsugineum), but also crop plants including maize, rice, wheat, tomato, petunia, Medicago sativa, Medicago truncatula, rapeseed, and Camelina sativa through genetic, biochemical, molecular, genomic, and cell biological approaches. In this review, we discuss recent advances in the understanding of the biological functions of genes involved in cuticular wax biosynthesis, transport, and regulation of wax deposition from Arabidopsis and crop species, provide information on cuticular wax amounts and composition in various organs of nine representative plant species, and suggest the important issues that need to be investigated in this field of study.

OsGL1-3 is involved in cuticular wax biosynthesis and tolerance to water deficit in rice

Cuticular wax covers aerial organs of plants and functions as the outermost barrier against non-stomatal water loss. We reported here the functional characterization of the Glossy1(GL1)-homologous gene OsGL1-3 in rice using overexpression and RNAi transgenic rice plants. OsGL1-3 gene was ubiquitously expressed at different level in rice plants except root and its expression was up-regulated under ABA and PEG treatments. The transient expression of OsGL1-3–GFP fusion protein indicated that OsGL1-3 is mainly localized in the plasma membrane. Compared to the wild type, overexpression rice plants exhibited stunted growth, more wax crystallization on leaf surface, and significantly increased total cuticular wax load due to the prominent changes of C₃₀–C₃₂ aldehydes and C₃₀ primary alcohols. While the RNAi knockdown mutant of OsGL1-3 exhibited no significant difference in plant height, but less wax crystallization and decreased total cuticular wax accumulation on leaf surface. All these evidences, together with the effects of OsGL1-3 on the expression of some wax synthesis related genes, suggest that OsGL1-3 is involved in cuticular wax biosynthesis. Overexpression of OsGL1-3 decreased chlorophyll leaching and water loss rate whereas increased tolerance to water deficit at both seedling and late-tillering stages, suggesting an important role of OsGL1-3 in drought tolerance.

Cuticular wax biosynthesis is up-regulated by the MYB94 transcription factor in Arabidopsis

The aerial parts of all land plants are covered with hydrophobic cuticular wax layers that act as the first barrier against the environment. The MYB94 transcription factor gene is expressed in abundance in aerial organs and shows a higher expression in the stem epidermis than within the stem. When seedlings were subjected to various treatments, the expression of the MYB94 transcription factor gene was observed to increase approximately 9-fold under drought, 8-fold for ABA treatment and 4-fold for separate NaCl and mannitol treatments. MYB94 harbors the transcriptional activation domain at its C-terminus, and fluorescent signals from MYB94:enhanced yellow fluorescent protein (eYFP) were observed in the nucleus of tobacco epidermis and in transgenic Arabidopsis roots. The total wax loads increased by approximately 2-fold in the leaves of the MYB94-overexpressing (MYB94 OX) lines, as compared with those of the wild type (WT). MYB94 activates the expression of WSD1, KCS2/DAISY, CER2, FAR3 and ECR genes by binding directly to their gene promoters. An increase in the accumulation of cuticular wax was observed to reduce the rate of cuticular transpiration in the leaves of MYB94 OX lines, under drought stress conditions. Taken together, a R2R3-type MYB94 transcription factor activates Arabidopsis cuticular wax biosynthesis and might be important in plant response to environmental stress, including drought.

Overexpression of Arabidopsis MYB96 confers drought resistance in Camelina sativa via cuticular wax accumulation

Camelina has been highlighted as an emerging oilseed crop. Transgenic Camelina plants overexpressing Arabidopsis MYB96 exhibited drought resistance by activating expression of Camelina wax biosynthetic genes and accumulating wax load. Camelina (Camelina sativa L.) is an oilseed crop in the Brassicaeae family with potential to expand biofuel production to marginal land. The aerial portion of all land plants is covered with cuticular wax to protect them from desiccation. In this study, the Arabidopsis MYB96 gene was overexpressed in Camelina under the control of the CaMV35S promoter. Transgenic Camelina plants overexpressing Arabidopsis MYB96 exhibited normal growth and development and enhanced tolerance to drought. Deposition of epicuticular wax crystals and total wax loads increased significantly on the surfaces of transgenic leaves compared with that of non-transgenic plants. The levels of alkanes and primary alcohols prominently increased in transgenic Camelina plants relative to non-transgenic plants. Cuticular transpiration occurred more slowly in transgenic leaves than that in non-transgenic plants. Genome-wide identification of Camelina wax biosynthetic genes enabled us to determine that the expression levels of CsKCS2, CsKCS6, CsKCR1-1, CsKCR1-2, CsECR, and CsMAH1 were approximately two to sevenfold higher in transgenic Camelina leaves than those in non-transgenic leaves. These results indicate that MYB96-mediated transcriptional regulation of wax biosynthetic genes is an approach applicable to generating drought-resistant transgenic crops. Transgenic Camelina plants with enhanced drought tolerance could be cultivated on marginal land to produce renewable biofuels and biomaterials.

Solvent isotope effects on alkane formation by cyanobacterial aldehyde deformylating oxygenase and their mechanistic implications

The reaction catalyzed by cyanobacterial aldehyde deformylating oxygenase is of interest both because of its potential application to the production of biofuels and because of the highly unusual nature of the deformylation reaction it catalyzes. Whereas the proton in the product alkane derives ultimately from the solvent, the identity of the proton donor in the active site remains unclear. To investigate the proton transfer step, solvent isotope effect (SIE) studies were undertaken. The rate of alkane formation was found to be maximal at pH 6.8 and to be the same in D2O or H2O within experimental error, implying that proton transfer is not a kinetically significant step. However, when the ratio of protium to deuterium in the product alkane was measured as a function of the mole fraction of D2O, a (D2O)SIEobs of 2.19 ± 0.02 was observed. The SIE was invariant with the mole fraction of D2O, indicating the involvement of a single protic site in the reaction. We interpret this SIE as most likely arising from a reactant state equilibrium isotope effect on a proton donor with an inverse fractionation factor, for which Φ = 0.45. These observations are consistent with an iron-bound water molecule being the proton donor to the alkane in the reaction.

Studies on the regulation of lipid biosynthesis in plants: application of control analysis to soybean

Although there is much knowledge of the enzymology (and genes coding the proteins) of lipid biosynthesis in higher plants, relatively little attention has been paid to regulation. We have demonstrated the important role for cholinephosphate cytidylyltransferase in the biosynthesis of the major extra-plastidic membrane lipid, phosphatidylcholine. We followed this work by applying control analysis to light-induced fatty acid synthesis. This was the first such application to lipid synthesis in any organism. The data showed that acetyl-CoA carboxylase was very important, exerting about half of the total control. We then applied metabolic control analysis to lipid accumulation in important oil crops - oilpalm, olive, and rapeseed. Recent data with soybean show that the block of fatty acid biosynthesis reactions exerts somewhat more control (63%) than lipid assembly although both are clearly very important. These results suggest that gene stacks, targeting both parts of the overall lipid synthesis pathway will be needed to increase significantly oil yields in soybean. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Fine mapping and metabolic and physiological characterization of the glume glaucousness inhibitor locus Iw3 derived from wild wheat

This research provided the first view of metabolic and physiological effect of a tissue-specific glaucousness inhibitor in wheat and laid foundation for map-based cloning of the Iw3 locus. Cuticular wax constitutes the outermost layer of plant skin, and its composition greatly impacts plant appearance and plant-environment interaction. Epicuticular wax in the upper part of adult wheat plants can form the glaucousness, which is associated with drought tolerance. In this research, we characterized a glume-specific glaucousness inhibitor, Iw3, by fine mapping, physiological, and molecular approaches. Iw3 inhibits glaucousness formation by altering wax composition. Compared to the wild type, Iw3 eliminated β-diketone, reduced 47 % primary alcohols, but increased aldehyde 400-fold and alkanes fivefold, which led to 30 % reduction of total glume wax load. Loss of the glaucousness increased cuticle permeability, suggesting an important role in drought sensitivity. Genetically, the glaucousness-inhibiting effect by Iw3 is partially dominant in a dosage-dependent manner. We localized the Iw3 locus within a 0.13-cM interval delimited by marker loci Xpsp3000 and XWL3096. Of the 53 wax genes assayed, we detected transcription changes in nine genes by Iw3, downregulation of Cer4-1 and upregulation of other five Cer4 and three KCS homologs. All these results provided initial insights into Iw3-mediated regulation of wax metabolism and paved way for in-depth characterization of the Iw3 locus and the glaucousness-related β-diketone pathway.

Acyl-lipid thioesterase1-4 from Arabidopsis thaliana form a novel family of fatty acyl-acyl carrier protein thioesterases with divergent expression patterns and substrate specificities

Hydrolysis of fatty acyl thioester bonds by thioesterases to produce free fatty acids is important for dictating the diversity of lipid metabolites produced in plants. We have characterized a four-member family of fatty acyl thioesterases from Arabidopsis thaliana, which we have called acyl-lipid thioesterase1 (ALT1), ALT2, ALT3, and ALT4. The ALTs belong to the Hotdog fold superfamily of thioesterases. ALT-like genes are present in diverse plant taxa, including dicots, monocots, lycophytes, and microalgae. The four Arabidopsis ALT genes were found to have distinct gene expression profiles with respect to each other. ALT1 was expressed specifically in stem epidermal cells and flower petals. ALT2 was expressed specifically in root endodermal and peridermal cells as well as in stem lateral organ boundary cells. ALT3 was ubiquitously expressed in aerial and root tissues and at much higher levels than the other ALTs. ALT4 expression was restricted to anthers. All four proteins were localized in plastids via an N-terminal targeting sequence of about 48 amino acids. When expressed in Escherichia coli, the ALT proteins used endogenous fatty acyl-acyl carrier protein substrates to generate fatty acids that varied in chain length (C6-C18), degree of saturation (saturated and monounsaturated), and oxidation state (fully reduced and β-ketofatty acids). Despite their high amino acid sequence identities, each enzyme produced a different profile of lipids in E. coli. The biological roles of these proteins are unknown, but they potentially generate volatile lipid metabolites that have previously not been reported in Arabidopsis.

Linking gene expression and membrane lipid composition of Arabidopsis

Glycerolipid metabolism of plants responds dynamically to changes in light intensity and temperature, leading to the modification of membrane lipid composition to ensure optimal biochemical and physical properties in the new environment. Although multiple posttranscriptional regulatory mechanisms have been reported to be involved in the process, the contribution of transcriptional regulation remains largely unknown. Here, we present an integrative analysis of transcriptomic and lipidomic data, revealing large-scale coordination between gene expression and changes in glycerolipid levels during the Arabidopsis thaliana response to light and temperature stimuli. Using a multivariate regression technique called O2PLS, we show that the gene expression response is strictly coordinated at the biochemical pathway level and occurs in parallel with changes of specific glycerolipid pools. Five interesting candidate genes were chosen for further analysis from a larger set of candidates identified based on their close association with various groups of glycerolipids. Lipidomic analysis of knockout mutant lines of these five genes showed a significant relationship between the coordination of transcripts and glycerolipid levels in a changing environment and the effects of single gene perturbations.

Mechanistic insights from reaction of α-oxiranyl-aldehydes with cyanobacterial aldehyde deformylating oxygenase

The biosynthesis of long-chain aliphatic hydrocarbons, which are derived from fatty acids, is widespread in Nature. The last step in this pathway involves the decarbonylation of fatty aldehydes to the corresponding alkanes or alkenes. In cyanobacteria, this is catalyzed by an aldehyde deformylating oxygenase. We have investigated the mechanism of this enzyme using substrates bearing an oxirane ring adjacent to the aldehyde carbon. The enzyme catalyzed the deformylation of these substrates to produce the corresponding oxiranes. Performing the reaction in D2O allowed the facial selectivity of proton addition to be examined by (1)H NMR spectroscopy. The proton is delivered with equal probability to either face of the oxirane ring, indicating the formation of an oxiranyl radical intermediate that is free to rotate during the reaction. Unexpectedly, the enzyme also catalyzes a side reaction in which oxiranyl-aldehydes undergo tandem deformylation to furnish alkanes two carbons shorter. We present evidence that this involves the rearrangement of the intermediate oxiranyl radical formed in the first step, resulting in aldehyde that is further deformylated in a second step. These observations provide support for a radical mechanism for deformylation and, furthermore, allow the lifetime of the radical intermediate to be estimated based on prior measurements of rate constants for the rearrangement of oxiranyl radicals.

Analyses of tomato fruit brightness mutants uncover both cutin-deficient and cutin-abundant mutants and a new hypomorphic allele of GDSL lipase

The cuticle is a protective layer synthesized by epidermal cells of the plants and consisting of cutin covered and filled by waxes. In tomato (Solanum lycopersicum) fruit, the thick cuticle embedding epidermal cells has crucial roles in the control of pathogens, water loss, cracking, postharvest shelf-life, and brightness. To identify tomato mutants with modified cuticle composition and architecture and to further decipher the relationships between fruit brightness and cuticle in tomato, we screened an ethyl methanesulfonate mutant collection in the miniature tomato cultivar Micro-Tom for mutants with altered fruit brightness. Our screen resulted in the isolation of 16 glossy and 8 dull mutants displaying changes in the amount and/or composition of wax and cutin, cuticle thickness, and surface aspect of the fruit as characterized by optical and environmental scanning electron microscopy. The main conclusions on the relationships between fruit brightness and cuticle features were as follows: (1) screening for fruit brightness is an effective way to identify tomato cuticle mutants; (2) fruit brightness is independent from wax load variations; (3) glossy mutants show either reduced or increased cutin load; and (4) dull mutants display alterations in epidermal cell number and shape. Cuticle composition analyses further allowed the identification of groups of mutants displaying remarkable cuticle changes, such as mutants with increased dicarboxylic acids in cutin. Using genetic mapping of a strong cutin-deficient mutation, we discovered a novel hypomorphic allele of GDSL lipase carrying a splice junction mutation, thus highlighting the potential of tomato brightness mutants for advancing our understanding of cuticle formation in plants.

Transcriptional regulation of cuticle biosynthesis

Plant cuticle is the hydrophobic protection layer that covers aerial plant organs and plays a pivotal role during plant development and interactions of plants with the environment. The mechanical structure and chemical composition of cuticle lipids and other secondary metabolites vary considerably between plant species, and in response to environmental stimuli and stresses. As the cuticle plays an important role in responses of plants to major abiotic stresses such as drought and high salinity, close attention has been paid to molecular processes underlying the stress-induced biosynthesis of cuticle components. This review addresses the genetic networks responsible for cuticle formation and in particular highlights the role of transcription factors that regulate cuticle formation in response to abiotic stresses.

Susceptibility genes 101: how to be a good host

To confer resistance against pathogens and pests in plants, typically dominant resistance genes are deployed. However, because resistance is based on recognition of a single pathogen-derived molecular pattern, these narrow-spectrum genes are usually readily overcome. Disease arises from a compatible interaction between plant and pathogen. Hence, altering a plant gene that critically facilitates compatibility could provide a more broad-spectrum and durable type of resistance. Here, such susceptibility (S) genes are reviewed with a focus on the mechanisms underlying loss of compatibility. We distinguish three groups of S genes acting during different stages of infection: early pathogen establishment, modulation of host defenses, and pathogen sustenance. The many examples reviewed here show that S genes have the potential to be used in resistance breeding. However, because S genes have a function other than being a compatibility factor for the pathogen, the side effects caused by their mutation demands a one-by-one assessment of their usefulness for application.

The cuticle and plant defense to pathogens

The cuticle provides a physical barrier against water loss and protects against irradiation, xenobiotics, and pathogens. Components of the cuticle are perceived by invading fungi and activate developmental processes during pathogenesis. In addition, cuticle alterations of various types induce a syndrome of reactions that often results in resistance to necrotrophs. This article reviews the current knowledge on the role of the cuticle in relation to the perception of pathogens and activation of defenses.

A novel dominant glossy mutation causes suppression of wax biosynthesis pathway and deficiency of cuticular wax in Brassica napus

BACKGROUND

The aerial parts of land plants are covered with cuticular waxes that limit non-stomatal water loss and gaseous exchange, and protect plants from ultraviolet radiation and pathogen attack. This is the first report on the characterization and genetic mapping of a novel dominant glossy mutant (BnaA.GL) in Brassica napus.

RESULTS

Transmission electron microscopy revealed that the cuticle ultrastructure of GL mutant leaf and stem were altered dramatically compared with that of wide type (WT). Scanning electron microscopy corroborated the reduction of wax on the leaf and stem surface. A cuticular wax analysis of the GL mutant leaves further confirmed the drastic decrease in the total wax content, and a wax compositional analysis revealed an increase in aldehydes but a severe decrease in alkanes, ketones and secondary alcohols. These results suggested a likely blockage of the decarbonylation step in the wax biosynthesis pathway. Genetic mapping narrowed the location of the BnaA.GL gene to the end of A9 chromosome. A single-nucleotide polymorphism (SNP) chip assay in combination with bulk segregant analysis (BSA) also located SNPs in the same region. Two SNPs, two single sequence repeat (SSR) markers and one IP marker were located on the flanking region of the BnaA.GL gene at a distance of 0.6 cM. A gene homologous to ECERIFERUM1 (CER1) was located in the mapped region. A cDNA microarray chip assay revealed coordinated down regulation of genes encoding enzymes of the cuticular wax biosynthetic pathway in the glossy mutant, with BnCER1 being one of the most severely suppressed genes.

CONCLUSIONS

Our results indicated that surface wax biosynthesis is broadly affected in the glossy mutant due to the suppression of the BnCER1 and other wax-related genes. These findings offer novel clues for elucidating the molecular basis of the glossy phenotype.

Distinct phyllosphere bacterial communities on Arabidopsis wax mutant leaves

The phyllosphere of plants is inhabited by diverse microorganisms, however, the factors shaping their community composition are not fully elucidated. The plant cuticle represents the initial contact surface between microorganisms and the plant. We thus aimed to investigate whether mutations in the cuticular wax biosynthesis would affect the diversity of the phyllosphere microbiota. A set of four Arabidopsis thaliana eceriferum mutants (cer1, cer6, cer9, cer16) and their respective wild type (Landsberg erecta) were subjected to an outdoor growth period and analysed towards this purpose. The chemical distinctness of the mutant wax phenotypes was confirmed by gas chromatographic measurements. Next generation amplicon pyrosequencing of the bacterial communities showed distinct community patterns. This observation was supported by denaturing gradient gel electrophoresis experiments. Microbial community analyses revealed bacterial phylotypes that were ubiquitously present on all plant lines (termed “core” community) while others were positively or negatively affected by the wax mutant phenotype (termed “plant line-specific“ community). We conclude from this study that plant cuticular wax composition can affect the community composition of phyllosphere bacteria.