Biopolyester Membranes of Plants: Cutin and Suberin

Exploring candidate genes for pericarp russet pigmentation of sand pear (Pyrus pyrifolia) via RNA-Seq data in two genotypes contrasting for pericarp color

Sand pear (Pyrus pyrifolia) russet pericarp is an important trait affecting both the quality and stress tolerance of fruits. This trait is controlled by a relative complex genetic process, with some fundamental biological questions such as how many and which genes are involved in the process remaining elusive. In this study, we explored differentially expressed genes between the russet- and green-pericarp offspring from the sand pear (Pyrus pyrifolia) cv. 'Qingxiang' × 'Cuiguan' F1 group by RNA-seq-based bulked segregant analysis (BSA). A total of 29,100 unigenes were identified and 206 of which showed significant differences in expression level (log2fold values>1) between the two types of pericarp pools. Gene Ontology (GO) analyses detected 123 unigenes in GO terms related to 'cellular_component' and 'biological_process', suggesting developmental and growth differentiations between the two types. GO categories associated with various aspects of 'lipid metabolic processes', 'transport', 'response to stress', 'oxidation-reduction process' and more were enriched with genes with divergent expressions between the two libraries. Detailed examination of a selected set of these categories revealed repressed expressions of candidate genes for suberin, cutin and wax biosynthesis in the russet pericarps.Genes encoding putative cinnamoyl-CoA reductase (CCR), cinnamyl alcohol dehydrogenase (CAD) and peroxidase (POD) that are involved in the lignin biosynthesis were suggested to be candidates for pigmentation of sand pear russet pericarps. Nine differentially expressed genes were analyzed for their expressions using qRT-PCR and the results were consistent with those obtained from Illumina RNA-sequencing. This study provides a comprehensive molecular biology insight into the sand pear pericarp pigmentation and appearance quality formation.

The transcription factor SlSHINE3 modulates defense responses in tomato plants

The cuticle plays an important role in plant interactions with pathogens and with their surroundings. The cuticle acts as both a physical barrier against physical stresses and pathogens and a chemical deterrent and activator of the plant defense response. Cuticle production in tomato plants is regulated by several transcription factors, including SlSHINE3, an ortholog of the Arabidopsis WIN/SHN3. Here we used a SlSHINE3-overexpressing (SlSHN3-OE) and silenced (Slshn3-RNAi) lines and a mutant in SlCYP86A69 (Slcyp86A69)--a direct target of SlSHN3--to analyze the roles of the leaf cuticle and cutin content and composition in the tomato plant's defense response to the necrotrophic foliar pathogen Botrytis cinerea and the biotrophic bacterial pathogen Xanthomonas campestris pv. vesicatoria. We showed that SlSHN3, which is predominantly expressed in tomato fruit epidermis, also affects tomato leaf cuticle, as morphological alterations in the SlSHN3-OE leaf tissue resulted in shiny, stunted and permeable leaves. SlSHN3-OE leaves accumulated 38% more cutin monomers than wild-type leaves, while Slshn3-RNAi and Slcyp86A69 plants showed a 40 and 70% decrease in leaf cutin monomers, respectively. Overexpression of SlSHN3 resulted in resistance to B. cinerea infection and to X. campestris pv. vesicatoria, correlated with cuticle permeability and elevated expression of pathogenesis-related genes PR1a and AOS. Further analysis revealed that B. cinerea-infected Slshn3-RNAi plants are more sensitive to B. cinerea and produce more hydrogen peroxide than wild-type plants. Cutin monomer content and composition differed between SlSHN3-OE, Slcyp86A69, Slshn3-RNAi and wild-type plants, and cutin monomer extracted from SlSHN3-OE plants altered the expression of pathogenesis-related genes in wild-type plants.

Suberin-associated fatty alcohols in Arabidopsis: distributions in roots and contributions to seed coat barrier properties

Suberin is found in a variety of tissues, such as root endoderms and periderms, storage tuber periderms, tree cork layer, and seed coats. It acts as a hydrophobic barrier to control the movement of water, gases, and solutes as well as an antimicrobial barrier. Suberin consists of polymerized phenolics, glycerol, and a variety of fatty acid derivatives, including primary fatty alcohols. We have conducted an in-depth analysis of the distribution of the C18:0 to C22:0 fatty alcohols in Arabidopsis (Arabidopsis thaliana) roots and found that only 20% are part of the root suberin polymer, together representing about 5% of its aliphatic monomer composition, while the remaining 80% are found in the nonpolymeric (soluble) fraction. Down-regulation of Arabidopsis FATTY ACYL REDUCTASE1 (FAR1), FAR4, and FAR5, which collectively produce the fatty alcohols found in suberin, reduced their levels by 70% to 80% in (1) the polymeric and nonpolymeric fractions from roots of tissue culture-grown plants, (2) the suberin-associated root waxes from 7-week-old soil-grown plants, and (3) the seed coat suberin polymer. By contrast, the other main monomers of suberin were not altered, indicating that reduced levels of fatty alcohols did not influence the suberin polymerization process. Nevertheless, the 75% reduction in total fatty alcohol and diol loads in the seed coat resulted in increased permeability to tetrazolium salts and a higher sensitivity to abscisic acid. These results suggest that fatty alcohols and diols play an important role in determining the functional properties of the seed coat suberin barrier.

Characterization and ectopic expression of a populus hydroxyacid hydroxycinnamoyltransferase

Cutinized and suberized cell walls in plants constitute physiologically important environment interfaces. They act as barriers limiting the loss of water and nutrients and protecting against radiation and invasion of pathogens. The roles of cutin- and suberin polyesters are often attributed to their dominant aliphatic components, but the contribution of aromatic composition to their physiological function remains unclear. By functionally screening a subset of Populus trichocarpa BAHD/HXXXD acyltransferases, we identified a hydroxycinnamoyltransferase that shows specific transacylation activity on ω-hydroxyacids using both feruloyl- and p-coumaroyl- CoA as the acyl donors. We named this enzyme P. trichocarpa hydroxyacid/fatty alcohol hydroxycinnamoyltransferase 1 (PtFHT1). The ectopic expression of the PtFHT1 gene in Arabidopsis increased the incorporation of ferulate in root and seed suberins and in leaf cutin, but not that of p-coumarate, while the aliphatic load in both suberin and cutin polyesters essentially remained unaffected. The overaccumulation of ferulate in lipophilic polyester significantly increased the tolerance of transgenic plants to salt stress treatment; under sub-lethal conditions of salt stress, the ratios of their seed germination and seedling establishment were 50% higher than those of wild-type plants. Our study suggests that, although aromatics are the minor component of polyesters, they play important role in the sealing function of lipidic polymers in planta.

Fungal diversity of saprotrophic litter fungi in a Mediterranean maquis environment

Monospecific and mixed-leaf litters from plant species of Mediterranean maquis (Quercus ilex, Phillyrea angustifolia, Pistacia lentiscus, Cistus spp.) in an undisturbed area in southern Italy were studied with respect to the structure and composition of their decomposer fungal community over an incubation period of 403 d. The data matrix structure was analyzed by means of detrended correspondence analysis (DCA), while indicator species analysis (ISA) was used to determine the preferential association of species with a substrate, a succession phase and monospecific/mixed experimental conditions. The ecological nature of the gradient expressed by the DCA axes was investigated by means of experimental and main chemical leaf-litter variables. The litter mixture had non-additive effects on the decomposition process even though the fungal species richness of the mixed litter was considerably higher than that of the monospecific litter. Our findings highlight the occurrence of shifts in the fungal community during decomposition in response to changes in the substrate, such as those related to the cellulose content and lignin/N ratio.

Oligomerization of 10,16-dihydroxyhexadecanoic acid and methyl 10,16-dihydroxyhexadecanoate catalyzed by lipases

The main monomer of tomato cuticle, 10,16-dihydroxyhexadecanoic acid (10,16-DHPA) and its methyl ester derivative (methyl-10,16-dihydroxyhexadecanote; methyl-10,16-DHHD), were used to study their oligomerization reactions catalyzed by five lipases: Candida antarctica lipase B (CAL-B), Rhizomucor miehei lipase (RM), Thermomyces lanuginosus lipase (TL), Pseudomonas cepacia lipase (PCL) and porcine pancreatic lipase (PPL). For 10,16-DHPA, optimum yields were obtained at 60 °C using toluene and 2-methyl-2-butanol (2M2B) as solvent, while for methyl-10,16-DHHD the bests yields were obtained in toluene and acetonitrile. Both reactions leaded to linear polyesters according to the NMR and FT-IR analysis, and there was no data indicating the presence of branched polymers. Using optimized conditions, poly(10,16-DHPA) and poly(methyl-10,16-DHHD) with Mw = 814 and Mn = 1,206 Da, and Mw = 982 and Mn = 860 Da, respectively, were formed according to their MALDI-TOF MS and ESI-MS data. The self-assembly of the polyesters obtained were analyzed by AFM.

Overexpression of AtSHN1/WIN1 provokes unique defense responses

The plant cell cuticle serves as the first barrier protecting plants from mechanical injury and invading pathogens. The cuticle can be breached by cutinase-producing pathogens and the degradation products may activate pathogenesis signals in the invading pathogens. Cuticle degradation products may also trigger the plant’s defense responses. Botrytis cinerea is an important plant pathogen, capable of attacking and causing disease in a wide range of plant species. Arabidopsis thaliana shn1-1D is a gain-of-function mutant, which has a modified cuticular lipid composition. We used this mutant to examine the effect of altering the whole-cuticle metabolic pathway on plant responses to B. cinerea attack. Following infection with B. cinerea, the shn1-1D mutant discolored more quickly, accumulated more H₂O₂, and showed accelerated cell death relative to wild-type (WT) plants. Whole transcriptome analysis of B. cinerea-inoculated shn1-1D vs. WT plants revealed marked upregulation of genes associated with senescence, oxidative stress and defense responses on the one hand, and genes involved in the magnitude of defense-response control on the other. We propose that altered cutin monomer content and composition of shn1-1D plants triggers excessive reactive oxygen species accumulation and release which leads to a strong, unique and uncontrollable defense response, resulting in plant sensitivity and death.

Cork suberin molecular structure: stereochemistry of the C18 epoxy and vic-diol ω-hydroxyacids and α,ω-diacids analyzed by NMR

Suberin is the biopolyester that protects the secondary tissues of plants against environmental variability and aggressions. Cork suberin is composed mostly of C18 ω-hydroxyacids and α,ω-diacids, 9,10-substituted with an unsaturation, an epoxide ring, or a vic-diol group. Although determinant for suberin macromolecular structure, the stereochemistry of these monomers is poorly studied, sometimes with contradictory results. An NMR technique was used here to assign the configuration of the 9,10-epoxy and 9,10-diol groups in C18 suberin acids, comparing the chemical shifts of diagnostic (1)H and (13)C signals with the ones of model compounds, before and after conversion of the vic-diol group into benzylidene acetal derivatives. The relative configuration was proved to be cis in the C18 9,10-epoxy and threo in the C18 9,10-diol suberin acids. These monomers were present in suberin probably as racemic mixtures, as shown by polarimetry. The revealed stereochemistry allows the suberin macromolecule to be built as an ordered array of midchain kinked C18 acids, reinforced by intramolecular hydrogen bonding.

Comparison of whole-cell fatty acid (MIDI) or phospholipid fatty acid (PLFA) extractants as biomarkers to profile soil microbial communities

The whole-cell lipid extraction to profile microbial communities on soils using fatty acid (FA) biomarkers is commonly done with the two extractants associated with the phospholipid fatty acid (PLFA) or Microbial IDentification Inc. (MIDI) methods. These extractants have very different chemistry and lipid separation procedures, but often shown a similar ability to discriminate soils from various management and vegetation systems. However, the mechanism and the chemistry of the exact suite of FAs extracted by these two methods are poorly understood. Therefore, the objective was to qualitatively and quantitatively compare the MIDI and PLFA microbial profiling methods for detecting microbial community shifts due to soil type or management. Twenty-nine soil samples were collected from a wide range of soil types across Oregon and extracted FAs by each method were analyzed by gas chromatography (GC) and GC-mass spectrometry. Unlike PLFA profiles, which were highly related to microbial FAs, the overall MIDI-FA profiles were highly related to the plant-derived FAs. Plant-associated compounds were quantitatively related to particulate organic matter (POM) and qualitatively related to the standing vegetation at sampling. These FAs were negatively correlated to respiration rate normalized to POM (RespPOM), which increased in systems under more intensive management. A strong negative correlation was found between MIDI-FA to PLFA ratios and total organic carbon (TOC). When the reagents used in MIDI procedure were tested for the limited recovery of MIDI-FAs from soil with high organic matter, the recovery of MIDI-FA microbial signatures sharply decreased with increasing ratios of soil to extractant. Hence, the MIDI method should be used with great caution for interpreting changes in FA profiles due to shifts in microbial communities.

Detection of potential suberinase-encoding genes in Streptomyces scabiei strains and other actinobacteria

Streptomyces scabiei causes common scab, an economically important disease of potato tubers. Some authors have previously suggested that S. scabiei penetration into host plant tissue is facilitated by secretion of esterase enzymes degrading suberin, a lipidic biopolymer of the potato periderm. In the present study, S. scabiei EF-35 showed high esterase activity in suberin-containing media. This strain also exhibited esterase activity in the presence of other biopolymers, such as lignin, cutin, or xylan, but at a much lower level. In an attempt to identify the esterases involved in suberin degradation, translated open reading frames of S. scabiei 87-22 were examined for the presence of protein sequences corresponding to extracellular esterases of S. scabiei FL1 and of the fungus Coprinopsis cinerea VTT D-041011, which have previously been shown to be produced in the presence of suberin. Two putative extracellular suberinase genes, estA and sub1, were identified. The presence of these genes in several actinobacteria was investigated by Southern blot hybridization, and both genes were found in most common-scab-inducing strains. Moreover, reverse transcription - polymerase chain reaction performed with S. scabiei EF-35 showed that estA was expressed in the presence of various biopolymers, including suberin, whereas the sub1 gene appeared to be specifically expressed in the presence of suberin and cutin.

Genetic interactions underlying the biosynthesis and inhibition of β-diketones in wheat and their impact on glaucousness and cuticle permeability

Cuticular wax composition greatly impacts plant responses to dehydration. Two parallel pathways exist in Triticeae for manipulating wax composition: the acyl elongation, reduction, and decarbonylation pathway that is active at the vegetative stage and yields primary alcohols and alkanes, and the β-diketone pathway that predominates at the reproductive stage and synthesizes β-diketones. Variation in glaucousness during the reproductive stage of wheat is mainly controlled by the wax production genes, W1 and W2, and wax inhibitor genes, Iw1 and Iw2. Little is known about the metabolic and physiological effects of the genetic interactions among these genes and their roles in shifting wax composition during plant development. We characterized the effect of W1, W2, Iw1, and Iw2 and analyzed their interaction using a set of six near-isogenic lines (NILs) by metabolic, molecular and physiological approaches. Loss of functional alleles of both W genes or the presence of either Iw gene depletes β-diketones and results in the nonglaucous phenotype. Elimination of β-diketones is compensated for by an increase in aldehydes and primary alcohols in the Iw NILs. Accordingly, transcription of CER4-6, which encodes an alcohol-forming fatty acyl-CoA reductase, was elevated 120-fold in iw1Iw2. CER4-6 was transcribed at much higher levels in seedlings than in adult plants, and showed little difference between the glaucous and nonglaucous NILs, suggesting that Iw2 counteracts the developmental repression of CER4-6 at the reproductive stage. While W1 and W2 redundantly function in β-diketone biosynthesis, a combination of both functional alleles led to the β-diketone hydroxylation. Consistent with this, transcription of MAH1-9, which encodes a mid-chain alkane hydroxylase, increased seven-fold only in W1W2. In parallel with the hydroxyl-β-diketone production patterns, glaucousness was intensified and cuticle permeability was reduced significantly in W1W2 compared to the other NILs. This suggests that both W1 and W2 are required for enhancing drought tolerance.

Estimation of the solubility parameters of model plant surfaces and agrochemicals: a valuable tool for understanding plant surface interactions

Background

Most aerial plant parts are covered with a hydrophobic lipid-rich cuticle, which is the interface between the plant organs and the surrounding environment. Plant surfaces may have a high degree of hydrophobicity because of the combined effects of surface chemistry and roughness. The physical and chemical complexity of the plant cuticle limits the development of models that explain its internal structure and interactions with surface-applied agrochemicals. In this article we introduce a thermodynamic method for estimating the solubilities of model plant surface constituents and relating them to the effects of agrochemicals.

Results

Following the van Krevelen and Hoftyzer method, we calculated the solubility parameters of three model plant species and eight compounds that differ in hydrophobicity and polarity. In addition, intact tissues were examined by scanning electron microscopy and the surface free energy, polarity, solubility parameter and work of adhesion of each were calculated from contact angle measurements of three liquids with different polarities. By comparing the affinities between plant surface constituents and agrochemicals derived from (a) theoretical calculations and (b) contact angle measurements we were able to distinguish the physical effect of surface roughness from the effect of the chemical nature of the epicuticular waxes. A solubility parameter model for plant surfaces is proposed on the basis of an increasing gradient from the cuticular surface towards the underlying cell wall.

Conclusions

The procedure enabled us to predict the interactions among agrochemicals, plant surfaces, and cuticular and cell wall components, and promises to be a useful tool for improving our understanding of biological surface interactions.

Growth-dependent chemical and mechanical properties of cuticular membranes from leaves of Sonneratia alba

Chemical and mechanical properties of the leaf cuticular membranes (CMs) of a mangrove, Sonneratia alba J. Smith, were analysed at various leaf development stages to evaluate their tolerance to environmental stress. Our analyses demonstrate that the CMs from leaves of S. alba at different growth stages are generally rich in wax (21.5-25.7%) and cutin (52.4-63.4%) which rapidly accumulate at the early stages of leaf growth, while cutan (4.3-10.3%) and polysaccharide (2.3-7.7%) continuously accumulate throughout growth. Immature CMs are physically weak and highly viscoelastic. However, CMs become strengthened and stiffened during leaf expansion and maturation (by factors of about 1.5 and 2.4, respectively) while their flexibility decreases (68-83% decrease). Finally, the CMs lose their strength at the senescent stage (30-43% decreasement). Correlation analysis between chemical composition and mechanical properties revealed that the cutin matrix is mainly responsible for the high viscoelastic properties of CMs, while wax, cutan and polysaccharide contributed to their elasticity. Wax also affected the strength of the CMs, whereas cutan and polysaccharide showed rigidizing effect. Rapid accumulation of wax and cutin in the CMs after bud burst followed by the mechanical supports of cutan and polysaccharide in an isolateral manner contributed to the remarkable environmental tolerance of S. alba.

Identification of putative candidate genes involved in cuticle formation in Prunus avium (sweet cherry) fruit

BACKGROUND AND AIMS

The cuticular membrane (CM) of Prunus avium (sweet cherry) and other fleshy fruit is under stress. Previous research indicates that the resultant strain promotes microscopic cuticular cracking. Microcracks impair the function of the CM as a barrier against pathogens and uncontrolled water loss/uptake. Stress and strain result from a cessation of CM deposition during early development, while the fruit surface continues to expand. The cessation of CM deposition, in turn, may be related to an early downregulation of CM-related genes. The aims of this study were to identify genes potentially involved in CM formation in sweet cherry fruit and to quantify their expression levels.

METHODS

Fruit growth and CM deposition were quantified weekly from anthesis to maturity and rates of CM deposition were calculated. Sequences of genes expressed in the sweet cherry fruit skin (exocarp) were generated using high-throughput sequencing of cDNA and de novo assembly and analysed using bioinformatics tools. Relative mRNA levels of selected genes were quantified in the exocarp and fruit flesh (mesocarp) weekly using reverse transcriptase-quantitative real-time PCR and compared with the calculated CM deposition rate over time.

KEY RESULTS

The rate of CM deposition peaked at 93 (±5) μg per fruit d(-1) about 19 d after anthesis. Based on sequence analyses, 18 genes were selected as potentially involved in CM formation. Selected sweet cherry genes shared up to 100 and 98 % similarity with the respective Prunus persica (peach) and Arabidopsis thaliana genes. Expression of 13 putative CM-related genes was restricted to the exocarp and correlated positively with the CM deposition rate.

CONCLUSIONS

The results support the view that the cessation of CM deposition during early sweet cherry fruit development is accounted for by a downregulation of genes involved in CM deposition. Genes that merit further investigation include PaWINA, PaWINB, PaLipase, PaLTPG1, PaATT1, PaLCR, PaGPAT4/8, PaLACS2, PaLACS1 and PaCER1.

Pleiotropic phenotypes of the sticky peel mutant provide new insight into the role of CUTIN DEFICIENT2 in epidermal cell function in tomato

Plant epidermal cells have evolved specialist functions associated with adaptation to stress. These include the synthesis and deposition of specialized metabolites such as waxes and cutin together with flavonoids and anthocyanins, which have important roles in providing a barrier to water loss and protection against UV radiation, respectively. Characterization of the sticky peel (pe) mutant of tomato (Solanum lycopersicum) revealed several phenotypes indicative of a defect in epidermal cell function, including reduced anthocyanin accumulation, a lower density of glandular trichomes, and an associated reduction in trichome-derived terpenes. In addition, pe mutant fruit are glossy and peels have increased elasticity due to a severe reduction in cutin biosynthesis and altered wax deposition. Leaves of the pe mutant are also cutin deficient and the epicuticular waxes contain a lower proportion of long-chain alkanes. Direct measurements of transpiration, together with chlorophyll-leaching assays, indicate increased cuticular permeability of pe leaves. Genetic mapping revealed that the pe locus represents a new allele of CUTIN DEFICIENT2 (CD2), a member of the class IV homeodomain-leucine zipper gene family, previously only associated with cutin deficiency in tomato fruit. CD2 is preferentially expressed in epidermal cells of tomato stems and is a homolog of Arabidopsis (Arabidopsis thaliana) ANTHOCYANINLESS2 (ANL2). Analysis of cuticle composition in leaves of anl2 revealed that cutin accumulates to approximately 60% of the levels observed in wild-type Arabidopsis. Together, these data provide new insight into the role of CD2 and ANL2 in regulating diverse metabolic pathways and in particular, those associated with epidermal cells.

Morphological preservation of carbonaceous plant fossils in blueschist metamorphic rocks from New Zealand

Morphological and chemical evidence of ancient life is widespread in sedimentary rocks retrieved from shallow depths in the Earth's crust. Metamorphism is highly detrimental to the preservation of biological information in rocks, thus limiting the geological record in which traces of life might be found. Deformation and increasing pressure/temperature during deep burial may alter the morphology as well as the composition and structure of both the organic and mineral constituents of fossils. However, microspore fossils have been previously observed in intensely metamorphosed rocks. It has been suggested that their small size, and/or the nature of the polymer composing their wall, and/or the mineralogy of their surrounding matrix were key parameters explaining their exceptional preservation. Here, we describe the remarkable morphological preservation of plant macrofossils in blueschist metamorphic rocks from New Zealand containing lawsonite. Leaves and stems can be easily identified at the macroscale. At the microscale, polygonal structures with walls mineralized by micas within the leaf midribs and blades may derive from the original cellular ultrastructure or, alternatively, from the shrinkage during burial of the gelified remnants of the leaves in an abiotic process. Processes and important parameters involved in the remarkable preservation of these fossils during metamorphism are discussed. Despite the excellent morphological preservation, the initial biological polymers have been completely transformed to graphitic carbonaceous matter down to the nanometer scale. This occurrence demonstrates that plant macrofossils may experience major geodynamic processes such as metamorphism and exhumation involving deep changes and homogenization of their carbon chemistry and structure but still retain their morphology with remarkable integrity even if they are not shielded by any hard-mineralized concretion.

Arabidopsis Deficient in Cutin Ferulate encodes a transferase required for feruloylation of ω-hydroxy fatty acids in cutin polyester

The cuticle is a complex aliphatic polymeric layer connected to the cell wall and covers surfaces of all aerial plant organs. The cuticle prevents nonstomatal water loss, regulates gas exchange, and acts as a barrier against pathogen infection. The cuticle is synthesized by epidermal cells and predominantly consists of an aliphatic polymer matrix (cutin) and intracuticular and epicuticular waxes. Cutin monomers are primarily C(16) and C(18) unsubstituted, ω-hydroxy, and α,ω-dicarboxylic fatty acids. Phenolics such as ferulate and p-coumarate esters also contribute to a minor extent to the cutin polymer. Here, we present the characterization of a novel acyl-coenzyme A (CoA)-dependent acyl-transferase that is encoded by a gene designated Deficient in Cutin Ferulate (DCF). The DCF protein is responsible for the feruloylation of ω-hydroxy fatty acids incorporated into the cutin polymer of aerial Arabidopsis (Arabidopsis thaliana) organs. The enzyme specifically transfers hydroxycinnamic acids using ω-hydroxy fatty acids as acyl acceptors and hydroxycinnamoyl-CoAs, preferentially feruloyl-CoA and sinapoyl-CoA, as acyl donors in vitro. Arabidopsis mutant lines carrying DCF loss-of-function alleles are devoid of rosette leaf cutin ferulate and exhibit a 50% reduction in ferulic acid content in stem insoluble residues. DCF is specifically expressed in the epidermis throughout all green Arabidopsis organs. The DCF protein localizes to the cytosol, suggesting that the feruloylation of cutin monomers takes place in the cytoplasm.

Quantification of lateral heterogeneity in carbohydrate permeability of isolated plant leaf cuticles

In phyllosphere microbiology, the distribution of resources available to bacterial colonizers of leaf surfaces is generally understood to be very heterogeneous. However, there is little quantitative understanding of the mechanisms that underlie this heterogeneity. Here, we tested the hypothesis that different parts of the cuticle vary in the degree to which they allow diffusion of the leaf sugar fructose to the surface. To this end, individual, isolated cuticles of poplar leaves were each analyzed for two properties: (1) the permeability for fructose, which involved measurement of diffused fructose by gas chromatography and flame ionization detection (GC–FID), and (2) the number and size of fructose-permeable sites on the cuticle, which was achieved using a green-fluorescent protein (GFP)-based bacterial bioreporter for fructose. Bulk flux measurements revealed an average permeance P of 3.39 × 10⁻⁹ ms⁻¹, while the bioreporter showed that most of the leaching fructose was clustered to sites around the base of shed trichomes, which accounted for only 0.37% of the surface of the cuticles under study. Combined, the GC–FID and GFP measurements allowed us to calculate an apparent rate of fructose diffusion at these preferential leaching sites of 9.15 × 10⁻⁷ ms⁻¹. To the best of our knowledge, this study represents the first successful attempt to quantify cuticle permeability at a resolution that is most relevant to bacterial colonizers of plant leaves. The estimates for P at different spatial scales will be useful for future models that aim to explain and predict temporal and spatial patterns of bacterial colonization of plant foliage based on lateral heterogeneity in sugar permeability of the leaf cuticle.

A comparison of suberin monomers from the multiseriate exodermis of Iris germanica during maturation under differing growth conditions

Iris germanica roots develop a multiseriate exodermis (MEX) in which all mature cells contain suberin lamellae. The location and lipophilic nature of the lamellae contribute to their function in restricting radial water and solute transport. The objective of the current work was to identify and quantify aliphatic suberin monomers, both soluble and insoluble, at specific stages of MEX development and under differing growth conditions, to better understand aliphatic suberin biosynthesis. Roots were grown submerged in hydroponic culture, wherein the maturation of up to three exodermal layers occurred over 21 days. In contrast, when roots were exposed to a humid air gap, MEX maturation was accelerated, occurring within 14 days. The soluble suberin fraction included fatty acids, alkanes, fatty alcohols, and ferulic acid, while the suberin poly(aliphatic) domain (SPAD) included fatty acids, α,ω-dioic acids, ω-OH fatty acids, and ferulic acid. In submerged roots, SPAD deposition increased with each layer, although the composition remained relatively constant, while the composition of soluble components shifted toward increasing alkanes in the innermost layers. Air gap exposure resulted in two significant shifts in suberin composition: nearly double the amount of SPAD monomers across all layers, and almost three times the alkane accumulation in the first layer. The localized and abundant deposition of C18:1 α,ω-dioic and ω-OH fatty acids, along with high accumulation of intercalated alkanes in the first mature exodermal layer of air gap-exposed roots indicate its importance for water retention under drought compared with underlying layers and with entire layers developing under water.

Chemical composition and antimicrobial activity of volatiles from Degenia velebitica, a European stenoendemic plant of the Brassicaceae family

Free and glucosidic bound leaf volatiles of Degenia velebitica were isolated and fractionated simultaneously into H(2)O-soluble, H(2)O-insoluble, and highly volatile compounds by hydrodistillation-adsorption (HDA) and analyzed by GC/MS. Among the 24 constituents identified, the main compounds obtained by the HDA method were S- and/or N-atom containing compounds, i.e., 6-(methylsulfanyl)hexanenitrile (10; 26.78%), dimethyl trisulfide (6; 26.35%), 3,4,5-trimethylpyrazole (17; 13.33%), hex-5-enenitrile (2; 10.11%), dimethyl tetrasulfide (8; 4.93%), and pent-4-enyl isothiocyanate (7; 4.45%). In addition, O-glycosidically bound volatiles and free volatiles were isolated by solvent extraction. Sixteen volatile O-aglycones and twelve free volatile components were identified. The main O-aglycones were eugenol (19; 24.15%), 2-methoxy-4-vinylphenol (11; 11.50%), and benzyl alcohol (20; 9.49%), and the main free volatiles were (9Z,12Z)-octa-9,12-dienic acid (38.35%), hexadecanoic acid (22.64%), and phytol (5.80%). The H(2)O-soluble volatile fraction obtained by HDA, containing mostly glucosinolate degradation products and 3,4,5-trimethylpyrazole (17), was evaluated for antimicrobial activity by determining inhibition zones with the diffusion method as well as minimal inhibitory concentrations (MIC) and minimal microbicidal concentrations (MMC) with the micro-dilution method. The fraction expressed activity against the tested Gram-positive and Gram-negative bacteria as well as against yeast, with MIC values equal to or lower than 16.7 μg/ml.

Water and solute permeabilities of Arabidopsis roots in relation to the amount and composition of aliphatic suberin

Although it is implied that suberized apoplastic barriers of roots negatively correlate with water and solute permeabilities, direct transport measurements across roots with altered amounts and compositions of aliphatic suberin are scarce. In the present study, hydroponically grown Arabidopsis wild types (Col8 and Col0) and different suberin mutants with altered amounts and/or compositions (horst, esb1-1, and esb1-2) were used to test this hypothesis. Detailed histochemical studies revealed late development of Casparian bands and suberin lamellae in the horst mutant compared with wild types and esb mutants. Suberin analysis with gas chromatography and mass spectrometry (GC-MS) showed that the horst mutant had ∼33% lower amounts of aliphatic monomers than Col8 and Col0. In contrast, enhanced suberin mutants (esb1-1 and esb1-2) had twice the amount of suberin as the wild types. Correlative permeability measurements, which were carried out for the first time with a root pressure probe for Arabidopsis, revealed that the hydraulic conductivity (Lp(r)) and NaCl permeability (P(sr)) of the whole root system of the horst mutant were markedly greater than in the respective wild types. This was reflected by the total amounts of aliphatic suberin determined in the roots. However, increased levels of aliphatic suberin in esb mutants failed to reduce either water or NaCl permeabilities below those of the wild types. It was concluded that the simple view and the conventional assumption that the amount of root suberin negatively correlates with permeability may not always be true. The aliphatic monomer arrangement in the suberin biopolymer and its microstructure also play a role in apoplastic barrier formation.

Basis of a humeomics science: chemical fractionation and molecular characterization of humic biosuprastructures

We propose a mild stepwise fractionation of molecular components of a humic acid (HA) suprastructure and their structural identification by advanced analytical methods. This procedure may be the basis of a "Humeomics" approach to characterize natural humic molecules and clarify their relations with ecosystems functions. Sequential fractionation included: (1) organic solvent extraction, (2) transesterification with boron trifluoride in methanol (BF(3)-CH(3)OH), (3) methanolic alkaline hydrolysis (KOH-CH(3)OH), and (4) cleavage of ether and glycosidic bonds with HI. Structural identification of initial and final material, separated organo-soluble and hydrosoluble fractions, and subfractions was conducted by GC-MS, HPSEC-ESI-MS (high-resolution, Orbitrap), and solid- and liquid-state NMR. GC-MS revealed in organosoluble unbound fractions the presence of both saturated and unsaturated, linear and branched, alkanoic, hydroxyalkanoic and alkandioic acids, n-alkanes, and n-alkanols. These components decreased progressively in fractions obtained after weak and strong ester cleavage. Unsubstituted alkanoic acids with variable chain length were ubiquitously detected in all fractions, thereby suggesting their fundamental function in the architecture of humic suprastructures. An important role in differentiating supramolecular associations should also be attributed to substituted alkanoic acids that were detected in variable amounts in different fractions. The content of aromatic acids and steroids was only noticed in the latter fractions. HPSEC-ESI-MS of initial and final solid fractions showed similar compounds, as indicated by GC-MS, whereas the hydrosoluble fraction after transesterification revealed fewer of these compounds but noticeable nitrogen-containing acids. A large amount of "cyclic" acids were identified by MS empirical formula in initial HA, and, to a lesser extent, in the final fractionation residue as well as in the hydrosoluble fraction. The predominant alkyl NMR signals in organosoluble extracts and those of CH-N, CH-O, and O-CH-O groups in hydrosoluble fraction confirmed mass spectrometry results. Homo- and heterocorrelated liquid-state NMR spectra indicated spin systems interactions varying with separated fractions. Solid-state and dipolar-dephasing NMR spectra of final residue showed predominance of sp(2) carbons, 66% of which were quaternary carbons, and a significant increase in conformational rigidity with respect to initial HA. Separated fractions accounted for 60% of initial HA weight, and losses were attributed to hydration water, liberated volatile compounds, and decarboxylation. Quantization of analytes showed that the sum of compound classes in separated fractions was greater than that for the initial HA, thereby showing that stepwise fractionation increased significantly the analytical identification of humic molecules. Our results suggest this "Humeomics" approach as a valid path for mapping humic molecular composition and assess humus origin and formation.

Determination of hydroxylated fatty acids from the biopolymer of tomato cutin and their fate during incubation in soil

INTRODUCTION

The plant cuticle is a thin, predominantly lipid layer that covers all primary aerial surfaces of vascular plants. The monomeric building blocks of the cutin biopolymer are mainly ω-hydroxy fatty acids.

OBJECTIVE

Analysis of ω-hydroxy fatty acids from cutin isolated from tomato fruits at different stages of decomposition in soil. Different derivatives and mass spectrometric techniques were used for peak identification and evaluation.

METHODOLOGY

Preparation of purified cutin involving dewaxing and HCl treatment. Incubation of purified cutin for 20 months in soil. Pentafluorobenzoyl derivatives were used for GC/MS operated in the electron capture negative ion (ECNI) mode and trimethylsilyl ethers for GC/MS operated in the electron ionisation (EI) mode for analysis of ω-hydroxy fatty acids.

RESULTS

Six ω-hydroxy fatty acids were detected in the purified cutin, three of which were identified as degradation products of 9,16-dihydroxyhexadecanoic acid as a consequence of the HCl treatment involved in the purification step. Incubation of the isolated cutin in soil was accompanied with decrease in concentration of all hydroxyl fatty acids.

CONCLUSION

We produced evidence that the HCl treatment only affected free hydroxyl groups and thus could be used for proportioning free and bound OH-groups on cutin fatty acids. The method enabled a direct quantification of the ω-hydroxy fatty acids throughout the incubation phase.

Mechanism by which contact with plant cuticle triggers cutinase gene expression in the spores of Fusarium solani f. sp. pisi

Spores of the phytopathogenic fungus Fusarium solani f. sp. pisi were shown to produce the extracellular enzyme, cutinase, only when cutin or cutin hydrolysate was added to the spore suspension. Dihydroxy-C(16) acid and trihydroxy-C(18) acid, which are unique cutin monomers, showed the greatest cutinase-inducing activity. Experiments with several compounds structurally related to these fatty acids suggested that both a omega-hydroxyl and a midchain hydroxyl are required for cutinase-inducing activity. Cutinase appeared in the medium 30-45 min after the addition of the inducers to the spore suspension, and the activity level increased for 6 hr. Addition of cycloheximide (5 mug/ml) completely inhibited cutinase production, suggesting that protein synthesis was involved in the increase of cutinase activity. Immunoblot analysis with rabbit antibodies prepared against cutinase showed that cutinase protein increased in parallel with the increase in enzyme activity. Measurement of cutinase-specific RNA levels by dot-blot hybridization with (32)P-labeled cutinase cDNA showed that the cutinase gene transcripts could be detected within 15 min after addition of the inducers. Addition of exogenous cutinase greatly enhanced the level of cutinase gene transcripts induced by cutin. These results strongly suggest that the fungal spore senses that it is in contact with the plant by the production of small amounts of cutin monomers catalyzed by the low level of cutinase carried by the spore and that these monomers induce the synthesis of cutinase needed for penetration of the fungus into the plant.

Two Arabidopsis 3-ketoacyl CoA synthase genes, KCS20 and KCS2/DAISY, are functionally redundant in cuticular wax and root suberin biosynthesis, but differentially controlled by osmotic stress

Very-long-chain fatty acids (VLCFAs) are essential precursors of cuticular waxes and aliphatic suberins in roots. The first committed step in VLCFA biosynthesis is condensation of C(2) units to an acyl CoA by 3-ketoacyl CoA synthase (KCS). In this study, two KCS genes, KCS20 and KCS2/DAISY, that showed higher expression in stem epidermal peels than in stems were isolated. The relative expression of KCS20 and KCS2/DAISY transcripts was compared among various Arabidopsis organs or tissues and under various stress conditions, including osmotic stress. Although the cuticular waxes were not significantly altered in the kcs20 and kcs2/daisy-1 single mutants, the kcs20 kcs2/daisy-1 double mutant had a glossy green appearance due to a significant reduction of the amount of epicuticular wax crystals on the stems and siliques. Complete loss of KCS20 and KCS2/DAISY decreased the total wax content in stems and leaves by 20% and 15%, respectively, and an increase of 10-34% was observed in transgenic leaves that over-expressed KCS20 or KCS2/DAISY. The stem wax phenotype of the double mutant was rescued by expression of KSC20. In addition, the kcs20 kcs2/daisy-1 roots exhibited growth retardation and abnormal lamellation of the suberin layer in the endodermis. When compared with the single mutants, the roots of kcs20 kcs2/daisy-1 double mutantss exhibited significant reduction of C(22) and C(24) VLCFA derivatives but accumulation of C(20) VLCFA derivatives in aliphatic suberin. Taken together, these findings indicate that KCS20 and KCS2/DAISY are functionally redundant in the two-carbon elongation to C(22) VLCFA that is required for cuticular wax and root suberin biosynthesis. However, their expression is differentially controlled under osmotic stress conditions.

Genetic and physiological determinants of Streptomyces scabies pathogenicity

SUMMARY Common scab is a severe disease worldwide affecting tap root crops and potato tubers. It is caused by soil-borne filamentous bacteria belonging to the genus Streptomyces. Streptomycetes usually are saprophytic microorganisms, but a few species have acquired the ability to infect underground plant tissues. The predominant causal agent of potato scab worldwide is Streptomyces scabies. The production of phytotoxins called thaxtomins is essential for the virulence of common scab-causing agents. The genes involved in the biosynthetic pathway of thaxtomins and other virulence genes are clustered on a large pathogenicity island. The pathogenicity island can be mobilized and transferred to nonpathogenic relatives, leading to the emergence of new pathogenic streptomycetes. In most pathogenic Streptomyces species, thaxtomin A is the predominant form found. The regulation of thaxtomin A synthesis is complex. Although the plant-derived compound cellobiose is now recognized as the inducer of thaxtomin A synthesis at a genetic level, other molecules (including aromatic amino acids and some secondary metabolites) show inhibitory effects on the production of the toxin. This paper is an overview of common scab with a focus on S. scabies and its virulence mechanisms.

TAXONOMY

Streptomyces scabies (Thaxt.) Lambert and Loria; Kingdom Bacteria; Phylum Actinobacteria; Class Actinomycetes; Order Actinomycetales; Family Streptomycetaceae; genus Streptomyces; species scabies or scabiei.

HOST RANGE

Streptomyces scabies (syn. S. scabiei) has a broad host range comprising tuber vegetables and most tap root crops. Streptomyces scabies causes common scab on potato (Solanum tuberosum), beet (Beta vulgaris), carrot (Daucus carota), parsnip (Pastinaca sativa), radish (Raphanus sativus), rutabaga (Brassica napobrassica) and turnip (Brassica rapa). Disease symptoms: Common scab symptoms appear as randomly distributed shallow, raised or deep-pitted corky lesions. Their size and colour are quite variable, but lesions typically are brown with a diameter of a few millimetres. No above-ground symptoms disclose the presence of the disease as aerial tissues of scab-infected plants remain healthy. Streptomyces scabies also inhibits the growth of seedlings in monocot and dicot plants.

USEFUL WEBSITES

http://www.sanger.ac.uk/Projects/S_scabies, http://www.potatodiseases.org/scab.html, http://www.uri.edu/ce/factsheets/sheets/potatoscab.html.

Environmental effects on the maturation of the endodermis and multiseriate exodermis of Iris germanica roots

BACKGROUND AND AIMS

Most studies of exodermal structure and function have involved species with a uniseriate exodermis. To extend this work, the development and apoplastic permeability of Iris germanica roots with a multiseriate exodermis (MEX) were investigated. The effects of different growth conditions on MEX maturation were also tested. In addition, the exodermises of eight Iris species were observed to determine if their mature anatomy correlated with habitat.

METHODS

Plants were grown in soil, hydroponics (with and without a humid air gap) or aeroponics. Roots were sectioned and stained with various dyes to detect MEX development from the root apical meristem, Casparian bands, suberin lamellae and tertiary wall thickenings. Apoplastic permeability was tested using dye (berberine) and ionic (ferric) tracers.

KEY RESULTS

The root apical meristem was open and MEX development non-uniform. In soil-grown roots, the exodermis started maturing (i.e. Casparian bands and suberin lamellae were deposited) 10 mm from the tip, and two layers had matured by 70 mm. In both hydro- and aeroponically grown roots, exodermal maturation was delayed. However, in areas of roots exposed to an air gap in the hydroponic system, MEX maturation was accelerated. In contrast, maturation of the endodermis was not influenced by the growth conditions. The mature MEX had an atypical Casparian band that was continuous around the root circumference. The MEX prevented the influx and efflux of berberine, but had variable resistance to ferric ions due to their toxic effects. Iris species living in well-drained soils developed a MEX, but species in water-saturated substrates had a uniseriate exodermis and aerenchyma.

CONCLUSIONS

MEX maturation was influenced by the roots' growth medium. The MEX matures very close to the root tip in soil, but much further from the tip in hydro- and aeroponic culture. The air gap accelerated maturation of the second exodermal layer. In Iris, the type of exodermis was correlated with natural habitat suggesting that a MEX may be advantageous for drought tolerance.