Biological origins of normal-chain hydrocarbons: a pathway model based on cuticular wax analyses of maize silks

Long-chain normal hydrocarbons (e.g. alkanes, alkenes and dienes) are rare biological molecules and their biosynthetic origins are obscure. Detailed analyses of the surface lipids that accumulate on maize silks have revealed that these hydrocarbons constitute a large portion (>90%) of the cuticular waxes that coat this organ, which contrasts with the situation on maize seedling leaves, where the cuticular waxes are primary alcohols and aldehydes. The normal hydrocarbons that occur on silks are part of a homologous series of alkanes, alkenes and dienes of odd-number carbon atoms, ranging between 19 and 33 in number. The alkenes and dienes consist of a homologous series, each of which has double bonds situated at defined positions of the alkyl chains: alkenes have double bonds situated at the sixth, ninth or 12th positions, and dienes have double bonds situated at the sixth and ninth, or ninth and twelfth positions. Finding a homologous series of unsaturated aldehydes and fatty acids suggests that these alkenes and dienes are biosynthesized by a series of parallel pathways of fatty-acid elongation and desaturation reactions, which are followed by sequential reduction and decarbonylation. In addition, the silk cuticular waxes contain metabolically related unsaturated long-chain methylketones, which probably arise via a decarboxylation mechanism. Finally, metabolite profiling analyses of the cuticular waxes of two maize inbred lines (B73 and Mo17), and their genetic hybrids, have provided insights into the genetic control network of these biosynthetic pathways, and that the genetic regulation of these pathways display best-parent heterotic effects.     

Physical, chemical and spectral properties of leaf surfaces of Berberis aquifolium (Pursh.)

Abstract The leaves of Berberis aquifolium (Pursh.) exhibit either diffuse or specular (shiny) reflection, depending on the variety, but in no case are the leaves obviously glaucous. The dull-surfaced leaves were less wettable than the glossy ones. Using scanning electron microscopy it was determined that the diffuse reflection was due to tubular crystals of wax 250 nm in diameter. The crystals were primarily composed of 19-nonacosanol, a 29-carbon secondary alcohol, as determined by gas chromatography-mass spectrometry. The chemical constituents of the wax underlying the tubes appeared to be the same as those of the wax from glossy leaves, with 29-carbon and 31-carbon n-alkanes and n-heptacosanol as major constituents. The reflection spectra of dull-surfaced (diffuse reflection) or glossy (specular reflection) leaves were the same, as were those of leaves with different amounts of epicuticular wax. Removing the epicuticular wax with chloroform did not change the spectrum.     

Effect of leaf age on epicuticular wax alkanes in Rhododendron

Alkane distribution pattern in epicuticular wax of Rhododendron fortunei cv Admiral Piet Hein leaves remains unchanged from the age of 6 weeks to abscission.     

Resistance in sorghum to the shoot fly, Atherigona soccata: epicuticular wax and wetness of the central whorl leaf of young seedlings

Sorghum genotypes known to be resistant or susceptible to shoot fly, Atherigona soccata Rondani were examined by scanning electron microscopy for differences in epicuticular wax structure and wetness of the central leaf whorl. Two major types of wax structures were observed: shoot fly resistant and moderately resistant genotypes were characterised by a smooth amorphous wax layer and sparse wax crystals while susceptible genotypes possessed a dense meshwork of crystalline epicuticular wax. The density of wax crystals decreased from the third leaf to the seventh leaf stage and was related to both seedling age and leaf position. Water droplets on susceptible genotypes with dense wax crystals showed spreading at the edges indicating a tendency to wet easily. In resistant genotypes with less dense wax crystals the droplets remained intact and did not spread.     

Influence of epiphytic micro-organisms on leaf wettability: wetting of the upper leaf surface of Juglans regia and of model surfaces in relation to colonization by micro-organisms

Wetting of the upper leaf surface of Juglans regia L. and of model surfaces colonized by epiphytic micro-organisms was investigated by measuring contact angles of aqueous solutions buffered at different pH values. During June to October 1995, contact angles of aqueous solutions on the leaf surface of J. regia decreased by angles ranging from 12° (low pH values) to 25° at high pH values. At the end of this vegetation period, wetting was strongly dependent on pH showing significantly lower contact angles with alkaline solutions (pH 9·0) than with acidic solutions (pH 3·0). Contact angle titration measured angles on the leaf surface as a function of the pH of buffered aqueous solutions, covering a pH range from 3·0 to 11·0. Titration curves revealed inflection points around 7·5, indicating the existence of ionizable carboxylic groups at the interface of the phylloplane. Altered leaf-surface wetting properties observed on the intact leaf surface could be simulated in model experiments by measuring contact angles on artificial surfaces colonized by Pseudomonas fluorescens and by epiphytic micro-organisms isolated from the phylloplane of J. regia . Strong evidence is provided that interfacial carboxylic groups derive from epiphytic micro-organisms present on the phylloplane. Results suggest that the age-dependent increase in, and pH dependence of, wetting as leaves mature are related to the presence of epiphytic micro-organisms on the phylloplane. Ecological consequences of increased leaf-surface wetting, concerning the structure of the leaf surface as a microhabitat for epiphytic micro-organisms, are discussed.     

Characterization of an alkylresorcinol synthase that forms phenolics accumulating in the cuticular wax on various organs of rye (Secale cereale)

Alkylresorcinols are bioactive compounds produced in diverse plant species, with chemical structures combining an aliphatic hydrocarbon chain and an aromatic ring with characteristic hydroxyl substituents. Here, we aimed to isolate and characterize the enzyme that forms the alkylresorcinols accumulating in the cuticular wax on the surface of all above‐ground organs of rye. Based on sequence homology with other type‐III polyketide synthases, a candidate alkylresorcinol synthase was cloned. Yeast heterologous expression showed that the enzyme, ScARS, is highly specific for the formation of the aromatic resorcinol ring structure, through aldol condensation analogous to stilbene synthases. The enzyme accepts long‐chain and very‐long‐chain acyl‐CoA starter substrates, preferring saturated over unsaturated chains. It typically carries out three rounds of condensation with malonyl‐CoA prior to cyclization, with only very minor activity for a fourth round of malonyl‐CoA condensation and cyclization to 5‐(2′‐oxo)‐alkylresorcinols or 5‐(2′‐hydroxy)‐alkylresorcinols. Like other enzymes involved in cuticle formation, ScARS is localized to the endoplasmic reticulum. ScARS expression patterns were found correlated with alkylresorcinol accumulation during leaf development and across different rye organs. Overall, our results thus suggest that ScARS synthesizes the cuticular alkylresorcinols found on diverse rye organ surfaces.     

Effect of an increased sink demand on the carbon metabolism and export of a maize source leaf

The sink demand was increased on a source maize leaf ( Zea mays L. cv. F7F2) by darkening all the leaves except the fourth, which was maintained under the prevailing irradiance conditions. The parameters of carbon metabolism were measured precisely during the first hours, and then daily during one week. The ambient photosynthetic activity and the maximum photosynthetic capacity were not altered by the treatment but the soluble carbohydrate and starch contents diminished, while ADP-glucose pyrophosphorylase (EC 2.7.7.27) activity increased. The carbon export rate, evaluated by the rate of disappearance of radioactivity after a 1-min ¹⁴CO₂ pulse, was faster than in control leaves. A compartmental analysis of the time course of ¹⁴C export further indicated that the sucrose pool providing the export flux was largely increased by the dark treatment. The darkened leaf 5, taken as an example of the darkened sources, was completely depleted of its carbohydrate content after one day in the dark and remained devoid of carbohydrates during the following week.     

Chemical composition of the epicuticular and intracuticular wax layers on the adaxial side of Ligustrum vulgare leaves

Previous research has shown that cuticular triterpenoids are exclusively found in the intracuticular wax layer of Prunus laurocerasus. To investigate whether this partitioning was species-specific, the intra- and epicuticular waxes were identified and quantified for the glossy leaves of Ligustrum vulgare, an unrelated shrub with similar wax morphology. Epicuticular wax was mechanically stripped from the adaxial leaf surface using the adhesive gum arabic. Subsequently, the organic solvent chloroform was used to extract the intracuticular wax from within the cutin matrix. The isolated waxes were quantified using gas chromatography with flame ionization detection and identified by mass spectrometry. The results were visually confirmed by scanning electron microscopy. The outer wax layer consisted entirely of homologous series of very-long-chain aliphatic compound classes. By contrast, the inner wax layer was dominated (80%) by two cyclic triterpenoids, ursolic and oleanolic acid. The accumulation of triterpenoids in the intracuticular leaf wax of a second, unrelated species suggests that this localization may be a more general phenomenon in smooth cuticles lacking epicuticular wax crystals. The mechanism and possible ecological or physiological reasons for this separation are currently being investigated.     

空气湿度与土壤水分胁迫对紫花苜蓿叶表皮蜡质特性的影响

选用2个抗旱性不同的紫花苜蓿品种,敖汉(强抗旱)和三得利(弱抗旱),设置空气湿度(45%-55%和75%-85%)和土壤水分胁迫(75%和35%田间持水量)处理,分析紫花苜蓿叶表皮蜡质含量、组分及晶体结构、气体交换参数、水势及脯氨酸含量的变化规律。结果表明,单独土壤水分胁迫时,紫花苜蓿叶表皮蜡质晶体结构及蜡质总量无显著变化;敖汉蜡质组分中烷类、酯类含量增加,醇类含量下降;三得利醇类含量下降,烷类、酯类含量变化不显著。低空气湿度胁迫时,两品种蜡质总量无显著变化,烷类和酯类含量显著增加,醇类含量显著下降,叶表皮片状蜡质晶体结构熔融呈弥漫性,扩大了对叶表面积的覆盖,其蒸腾速率显著低于正常湿度。复合胁迫处理时,叶表皮片状蜡质晶体结构继续呈弥漫性,烷类、酯类、未知蜡质组分含量均高于单独胁迫处理,醇类含量最低,而蜡质总量除三得利显著高于对照外,其余均无显著差异。紫花苜蓿叶表皮蜡质各组分含量(除醇类)及蜡质总量与光合速率呈显著负相关,与蒸腾速率无显著相关关系。蜡质总量与叶水势呈显著正相关。总体上,敖汉蜡质总量显著高于三得利,蜡质组分中烷类物质的增加有助于提高植株的抗旱性。在复合胁迫下,强抗旱品种主要通过气孔因素控制水分散失,而弱抗旱品种通过气孔和非气孔因素共同控制植物水分散失。     

Effect of aflatoxin B1 on chlorophyll, nucleic acid and protein contents in maize

Aflatoxin B1 significantly lowered the contents of nucleic acids (DNA, RNA) and protein (both quantititive and qualitative) of germinating seeds of maize cv. Suwan Composite and also the contents of chlorophylls and carotenoids in seedlings. The extent of inhibition depended on the concentration of toxin. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of norflurazon, an inhibitor of carotenogenesis, on abscisic acid and xanthoxin in the caps of gravistimulated maize roots

Maize seeds were germinated in the dark in the presence of the carotenoid synthesis inhibitor norflurazon and the teveis of abscisic acid, xanthoxin and total carotenoids were measured in the root cap and in the adjacent 1.5 mm segment. In norflurazon-treated roots abscisic acid levels were markedly reduced, but an increase occurred in the levels of xanthoxin, a compound structurally and physiologically similar to abscisic acid. In the cultivar of maize ( Zea mays L. cv. Merit) used for this work, brief illumination of the root is required for gravitropic curving. Following illumination both control and norflurazon-treated roots showed normal gravitropic curvature, however, the rate of curvature was delayed in norflurazon-treated roots. Our data from norflurazon-treated roots are consistent with a role for xanthoxin in maize root gravitropism. The increase in xanthoxin in the presence of an inhibitor of carotenoid synthesis suggests that xanthoxin and abscisic acid originate, at least in part, via different metabolic pathways.     

Influence of polishing and abrasion on the diffusive conductance of leaf surface of Festuca arundinacea Schreb.

Abstract Leaf surfaces of Festuca arundinacea Schreb. were subjected to controlled polishing and abrasion on a test-bed designed to simulate components of abrasive wind damage. Both treatments substantially increased the leaf surface conductance, particularly the polishing treatment. Scanning electron micrographs of cryo-fixed leaf surfaces showed displacement and smoothing of the epicuticular waxes, damage to the cuticle, collapse of epidermal cells and fracture of trichomes. The importance of the epicuticular waxes in determining leaf diffusivity and permeability is discussed.     

Morphological factors of the central whorl leaf associated with leaf surface wetness and resistance in sorghum to shoot fly,Atherigona soccata*

Earlier studies showed that leaf surface water on the central whorl leaf of sorghum seedlings is associated with resistance to shoot fly. In this study, the results of an experiment to determine if leaf surface wetness (LSW) originates from atmospheric condensation or from the plant are described. Morphological structures: trichomes, stomata, leaf cuticle and quantity of surface wax of the central whorl leaf were also examined for their role in LSW production. The results suggest that LSW of the central whorl leaf originates from the plant and is not due to condensation of atmospheric moisture. The presence of trichomes was indirectly associated with LSW and resistance to shoot fly but stomatal density was not associated with LSW production. The amount of wax extracted per 100 mg of fresh weight varied significantly between genotypes and seedling age. It was more in susceptible than in resistant genotypes; however, cuticular thickness was not associated with resistance. It is suggested that LSW could be the result of some form of cuticular movement of water to the leaf surface.     

Comparative effects of light during growth on the photosynthetic properties of NADP-ME type C4 grasses from open and shaded habitats. I. Gas exchange,leaf anatomy and ultrastructure*

Abstract Maximum photosynthetic rate (P_max in Zea mays L. was reduced to a much greater extent by neutral shading during growth than in the shade-adapted C₄ grass Paspalum conjugatum Berg., although under a high light regime the P_max of Z. mays was two-fold higher than that of P. conjugatum. In both species the shade-induced reductions of P_max were not of stomatal origin since the intercellular CO₂ concentration (C_i) was not decreased by growth under low light levels. The C_i of P. conjugatum (～200 μPa Pa^?1) measured at air levels of CO₂ and high photon flux densities was 30% greater than that of Z. mays and, concomitantly, leaf water use efficiency was less. As with P_max, specific leaf weight, leaf thickness and chlorenchyma volume were reduced to a greater extent by shading in Z. mays than in P. conjugatum. In contrast to Z. mays, bundle sheath chloroplasts of P. conjugatum contained well-defined stacks of grana. Mesophyll chloroplasts of P. conjugatum developed under a high light regime also contained large amounts of starch. This was not the case with Z. mays.     

Effect of organic fertiliser and Chromolaena odorata residue on the pathogenicity of Meloidogyne incognita on maize

Pot experiments laid out in a complete randomised design were conducted in the screen house of the Department of Crop Protection, University of Agriculture, Abeokuta, Ogun State, Nigeria to determine the effects of organic fertiliser and Chromolaena odorata residue at 1% w/w on the pathogenicity of Meloidogyne incognita infecting maize. M. incognita significantly reduced the plant height, number of leaves per plant, leaf area, cob weight and grain yield of maize by 6.89, 15.18, 20, 63.92 and 56.16% respectively. C. odorata residue and organic fertiliser significantly suppressed M. incognita galling, inhibited the nematode fecundity and reduced the number of eggs and juveniles on maize. A remarkable increase in plant height, number of leaves per plant, leaf area, cob weight and grain yield were observed on maize plants treated with the mixture of C. odorata and organic fertiliser despite the nematode infection. The observation from this study suggests that C. odorata in combination with organic fertiliser is a viable option for the control of M. incognita on maize.