Physiological Effects of Surface Waxes: I. Light Reflectance for Glaucous and Nonglaucous Picea pungens

Transcriptome and Physiological Analyses of a Navel Orange Mutant with Improved Drought Tolerance and Water Use Efficiency Caused by Increases of Cuticular Wax Accumulation and ROS Scavenging Capacity

Drought is one of the main abiotic stresses limiting the quality and yield of citrus. Cuticular waxes play an important role in regulating plant drought tolerance and water use efficiency (WUE). However, the contribution of cuticular waxes to drought tolerance, WUE and the underlying molecular mechanism is still largely unknown in citrus. 'Longhuihong' (MT) is a bud mutant of 'Newhall' navel orange with curly and bright leaves. In this study, significant increases in the amounts of total waxes and aliphatic wax compounds, including n-alkanes, n-primary alcohols and n-aldehydes, were overserved in MT leaves, which led to the decrease in cuticular permeability and finally resulted in the improvements in drought tolerance and WUE. Compared to WT leaves, MT leaves possessed much lower contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), significantly higher levels of proline and soluble sugar, and enhanced superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities under drought stress, which might reduce reactive oxygen species (ROS) damage, improve osmotic regulation and cell membrane stability, and finally, enhance MT tolerance to drought stress. Transcriptome sequencing results showed that seven structural genes were involved in wax biosynthesis and export, MAPK cascade, and ROS scavenging, and seven genes encoding transcription factors might play an important role in promoting cuticular wax accumulation, improving drought tolerance and WUE in MT plants. Our results not only confirmed the important role of cuticular waxes in regulating citrus drought resistance and WUE but also provided various candidate genes for improving citrus drought tolerance and WUE.

An ecological perspective on water shedding from leaves

Water shedding from leaves is a complex process depending on multiple leaf traits interacting with rain, wind, and air humidity, and with the entire plant and surrounding vegetation. Here, we synthesize current knowledge of the physics of water shedding with implications for plant physiology and ecology. We argue that the drop retention angle is a more meaningful parameter to characterize the water-shedding capacity of leaves than the commonly measured static contact angle. The understanding of the mechanics of water shedding is largely derived from laboratory experiments on artificial rather than natural surfaces, often on individual aspects such as surface wettability or drop impacts. In contrast, field studies attempting to identify the adaptive value of leaf traits linked to water shedding are largely correlative in nature, with inconclusive results. We make a strong case for taking the hypothesis-driven experimental approach of biomechanical laboratory studies into a real-world field setting to gain a comprehensive understanding of leaf water shedding in a whole-plant ecological and evolutionary context.

The ARRE RING-Type E3 Ubiquitin Ligase Negatively Regulates Cuticular Wax Biosynthesis in Arabidopsis thaliana by Controlling ECERIFERUM1 and ECERIFERUM3 Protein Levels

The outer epidermal cell walls of plant shoots are covered with a cuticle, a continuous lipid structure that provides protection from desiccation, UV light, pathogens, and insects. The cuticle is mostly composed of cutin and cuticular wax. Cuticular wax synthesis is synchronized with surface area expansion during plant development and is associated with plant responses to biotic and abiotic stresses. Cuticular wax deposition is tightly regulated by well-established transcriptional and post-transcriptional regulatory mechanisms, as well as post-translationally via the ubiquitin-26S proteasome system (UPS). The UPS is highly conserved in eukaryotes and involves the covalent attachment of polyubiquitin chains to the target protein by an E3 ligase, followed by the degradation of the modified protein by the 26S proteasome. A large number of E3 ligases are encoded in the Arabidopsis genome, but only a few have been implicated in the regulation of cuticular wax deposition. In this study, we have conducted an E3 ligase reverse genetic screen and identified a novel RING-type E3 ubiquitin ligase, AtARRE, which negatively regulates wax biosynthesis in Arabidopsis. Arabidopsis plants overexpressing AtARRE exhibit glossy stems and siliques, reduced fertility and fusion between aerial organs. Wax load and wax compositional analyses of AtARRE overexpressors showed that the alkane-forming branch of the wax biosynthetic pathway is affected. Co-expression of AtARRE and candidate target proteins involved in alkane formation in both Nicotiana benthamiana and stable Arabidopsis transgenic lines demonstrated that AtARRE controls the levels of wax biosynthetic enzymes ECERIFERUM1 (CER1) and ECERIFERUM3 (CER3). CER1 has also been confirmed to be a ubiquitination substrate of the AtARRE E3 ligase by an in vivo ubiquitination assay using a reconstituted Escherichia coli system. The AtARRE gene is expressed throughout the plant, with the highest expression detected in fully expanded rosette leaves and oldest stem internodes. AtARRE gene expression can also be induced by exposure to pathogens. These findings reveal that wax biosynthesis in mature plant tissues and in response to pathogen infection is controlled post-translationally.

The Tea Plant Leaf Cuticle: From Plant Protection to Tea Quality

Camellia sinensis (tea tree) is a perennial evergreen woody crop that has been planted in more than 50 countries worldwide; its leaves are harvested to make tea, which is one of the most popular nonalcoholic beverages. The cuticle is the major transpiration barrier to restrict nonstomatal water loss and it affects the drought tolerance of tea plants. The cuticle may also provide molecular cues for the interaction with herbivores and pathogens. The tea-making process almost always includes a postharvest withering treatment to reduce leaf water content, and many studies have demonstrated that withering treatment-induced metabolite transformation is essential to shape the quality of the tea made. Tea leaf cuticle is expected to affect its withering properties and the dynamics of postharvest metabolome remodeling. In addition, it has long been speculated that the cuticle may contribute to the aroma quality of tea. However, concrete experimental evidence is lacking to prove or refute this hypothesis. Even though its relevance to the abiotic and biotic stress tolerance and postharvest processing properties of tea tree, tea cuticle has long been neglected. Recently, there are several studies on the tea cuticle regarding its structure, wax composition, transpiration barrier organization, environmental stresses-induced wax modification, and structure-function relations. This review is devoted to tea cuticle, the recent research progresses were summarized and unresolved questions and future research directions were also discussed.

Quantitative trait locus mapping combined with variant and transcriptome analyses identifies a cluster of gene candidates underlying the variation in leaf wax between upland and lowland switchgrass ecotypes

Mapping combined with expression and variant analyses in switchgrass, a crop with complex genetics, identified a cluster of candidate genes for leaf wax in a fast-evolving region of chromosome 7K. Switchgrass (Panicum virgatum L.) is a promising warm-season candidate energy crop. It occurs in two ecotypes, upland and lowland, which vary in a number of phenotypic traits, including leaf glaucousness. To initiate trait mapping, two F₂ mapping populations were developed by crossing two different F₁ sibs derived from a cross between the tetraploid lowland genotype AP13 and the tetraploid upland genotype VS16, and high-density linkage maps were generated. Quantitative trait locus (QTL) analyses of visually scored leaf glaucousness and of hydrophobicity of the abaxial leaf surface measured using a drop shape analyzer identified highly significant colocalizing QTL on chromosome 7K (Chr07K). Using a multipronged approach, we identified a cluster of genes including Pavir.7KG077009, which encodes a Type III polyketide synthase-like protein, and Pavir.7KG013754 and Pavir.7KG030500, two highly similar genes that encode putative acyl-acyl carrier protein (ACP) thioesterases, as strong candidates underlying the QTL. The lack of homoeologs for any of the three genes on Chr07N, the relatively low level of identity with other switchgrass KCS proteins and thioesterases, as well as the organization of the surrounding region suggest that Pavir.7KG077009 and Pavir.7KG013754/Pavir.7KG030500 were duplicated into a fast-evolving chromosome region, which led to their neofunctionalization. Furthermore, sequence analyses showed all three genes to be absent in the two upland compared to the two lowland accessions analyzed. This study provides an example of and practical guide for trait mapping and candidate gene identification in a complex genetic system by combining QTL mapping, transcriptomics and variant analysis.

The genome of the warm-season turfgrass African bermudagrass (Cynodon transvaalensis)

Cynodon species can be used for multiple purposes and have high economic and ecological significance. However, the genetic basis of the favorable agronomic traits of Cynodon species is poorly understood, partially due to the limited availability of genomic resources. In this study, we report a chromosome-scale genome assembly of a diploid Cynodon species, C. transvaalensis, obtained by combining Illumina and Nanopore sequencing, BioNano, and Hi-C. The assembly contains 282 scaffolds (~423.42 Mb, N50 = 5.37 Mb), which cover ~93.2% of the estimated genome of C. transvaalensis (~454.4 Mb). Furthermore, 90.48% of the scaffolds (~383.08 Mb) were anchored to nine pseudomolecules, of which the largest was 60.78 Mb in length. Evolutionary analysis along with transcriptome comparison provided a preliminary genomic basis for the adaptation of this species to tropical and/or subtropical climates, typically with dry summers. The genomic resources generated in this study will not only facilitate evolutionary studies of the Chloridoideae subfamily, in particular, the Cynodonteae tribe, but also facilitate functional genomic research and genetic breeding in Cynodon species for new leading turfgrass cultivars in the future.

The Surfaces of the Ceratonia siliqua L. (Carob) Leaflet: Insights from Physics and Chemistry

The production of superhydrophobic coatings inspired by the surface of plant leaves is a challenging goal. Such coatings hold a bright technological future in niche markets of the aeronautical, space, naval, building, automobile, and biomedical sectors. This work is focused on the adaxial (top) and abaxial (bottom) surfaces of the leaflet of the Ceratonia silique L. (carob), a high-commercial-value Mediterranean tree cultivated in many regions of the world. The adaxial and abaxial surfaces feature hydrophobic and superhydrophobic behaviors, respectively. Their chemical composition is very simple: monopalmitin ester and palmitic acid are protuberant in the epicuticular and intracuticular wax layers of the adaxial surface, respectively, whereas 1-octacosanol dominates in the abaxial wax layers. In both surfaces, epicuticular wax is organized along a randomly oriented and intricate network of nanometer-thick and micrometer-long plates, whose density and degree of interconnection are significantly higher in the abaxial surface. The measured tilting angles for the abaxial surface (12-70°) reveal unusual variable density and water adhesion of the nanostructured plate-based texture. Optical measurements demonstrate that light reflectance/absorbance of the glaucous abaxial surface is significantly higher/lower than that of the nonglaucous adaxial surface. In both surfaces, diffuse reflectance is dominant, and the absorbance is weakly dependent on the light incidence angle. We show that the highly dense nanostructured platelike texture of the epicuticular abaxial layer of the C. siliqua leaflet works as a sophisticated light and water management system: it reflects solar radiation diffusely to lower the surface temperature, and it has superhydrophobic character to keep the surface dry. Such attributes enable efficient gas exchange (photosynthesis and respiration), transpiration, and evaporation. To mimic for the first time the abaxial surface, a templation approach was adopted using poly(dimethylsiloxane) (PDMS)/poly(methylphenylsiloxane) (PMPS) positive/negative replicas and a soft polymer/siloxane negative replica produced by the sol-gel process. Because high topographical variations of the biotemplate and wax adhesion to the biohybrid film affected the replication quality, the reproduction of the wax texture via the synthesis of 1-octacosanol-grafted siloxane-based hybrid materials is proposed as a suitable route to duplicate the abaxial surface with high fidelity. The natural chemical/physical strategy adopted by the C. siliqua leaflet to face the harsh Mediterranean climate is a powerful source of bioinspiration for the development of diffuse reflecting and superhydrophobic material systems with foreseen applications as dual-functional antiglare and water-repelling coatings.

Yellow nutsedge WRI4-like gene improves drought tolerance in Arabidopsis thaliana by promoting cuticular wax biosynthesis

BACKGROUND

Cuticular wax plays important role in protecting plants from drought stress. In Arabidopsis WRI4 improves drought tolerance by regulating the biosynthesis of fatty acids and cuticular wax. Cyperus esculentus (yellow nutsedge) is a tough weed found in tropical and temperate zones as well as in cooler regions. In the current study, we report the molecular cloning of a WRI4-like gene from Cyperus esculentus and its functional characterization in Arabidopsis.

RESULTS

Using RACE PCR, full-length WRI-like gene was amplified from yellow nutsedge. Phylogenetic analyses and amino acid comparison suggested it to be a WRI4-like gene. According to the tissue-specific expression data, the highest expression of WRI4-like gene was found in leaves, followed by roots and tuber. Transgenic Arabidopsis plants expressing nutsedge WRI4-like gene manifested improved drought stress tolerance. Transgenic lines showed significantly reduced stomatal conductance, transpiration rate, chlorophyll leaching, water loss and improved water use efficiency (WUE). In the absence of drought stress, expression of key genes for fatty acid biosynthesis was not significantly different between transgenic lines and WT while that of cuticular wax biosynthesis genes was significantly higher in transgenic lines than WT. The PEG-simulated drought stress significantly increased expression of key genes for fatty acid as well as wax biosynthesis in transgenic Arabidopsis lines but not in WT plants. Consistent with the gene expression data, cuticular wax load and deposition was significantly higher in stem and leaves of transgenic lines compared with WT under control as well as drought stress conditions.

CONCLUSIONS

WRI4-like gene from Cyperus esculentus improves drought tolerance in Arabidopsis probably by promoting cuticular wax biosynthesis and deposition. This in turn lowers chlorophyll leaching, stomatal conductance, transpiration rate, water loss and improves water use efficiency under drought stress conditions. Therefore, CeWRI4-like gene could be a good candidate for improving drought tolerance in crops.

Predicting the Potential Distribution of Two Varieties of Litsea coreana (Leopard-Skin Camphor) in China under Climate Change

Climate change considerably affects vegetation growth and may lead to changes in vegetation distribution. Leopard-skin camphor is an endangered species, and the main raw material for hawk tea, and has various pharmacodynamic functions. Studying the potential distribution of two leopard-skin camphor varieties under climate change should assist in the effective protection of these species. We collected the distribution point data for 130 and 89 Litsea coreana Levl. var. sinensis and L. coreana Levl. var. lanuginosa, respectively, and data for 22 environmental variables. We also predicted the potential distribution of the two varieties in China using the maximum entropy (MaxEnt) model and analyzed the key environmental factors affecting their distribution. Results showed that the two varieties are mainly located in the subtropical area south of the Qinling Mountains–Huai River line in the current and future climate scenarios, and the potentially suitable area for L. coreana Levl. var. lanuginosa is larger than that of L. coreana Levl. var. sinensis. Compared with current climatic conditions, the potentially suitable areas of the two leopard-skin camphor varieties will move to high-latitude and -altitude areas and the total suitable area will increase slightly, while moderately and highly suitable areas will be significantly reduced under future climatic scenarios. For example, under a 2070-RCP8.5 (representative of a high greenhouse gas emission scenario in the 2070s) climatic scenario, the highly suitable areas of L. coreana Levl. var. sinensis and L. coreana Levl. var. lanuginosa are 6900 and 300 km2, and account for only 10.27% and 0.21% of the current area, respectively. Temperature is the key environmental factor affecting the potential distribution of the two varieties, especially the mean daily diurnal range (Bio2) and the min temperature of the coldest month (Bio6). The results can provide a reference for relevant departments in taking protective measures to prevent the decrease or extinction of the species under climate change.

Precise Editing of the OsPYL9 Gene by RNA-Guided Cas9 Nuclease Confers Enhanced Drought Tolerance and Grain Yield in Rice (Oryza sativa L.) by Regulating Circadian Rhythm and Abiotic Stress Responsive Proteins

Abscisic acid (ABA) is involved in regulating drought tolerance, and pyrabactin resistance-like (PYL) proteins are known as ABA receptors. To elucidate the role of one of the ABA receptors in rice, OsPYL9 was mutagenized through CRISPR/Cas9 in rice. Homozygous and heterozygous mutant plants lacking any off-targets and T-DNA were screened based on site-specific sequencing and used for morpho-physiological, molecular, and proteomic analysis. Mutant lines appear to accumulate higher ABA, antioxidant activities, chlorophyll content, leaf cuticular wax, and survival rate, whereas a lower malondialdehyde level, stomatal conductance, transpiration rate, and vascular bundles occur under stress conditions. Proteomic analysis found a total of 324 differentially expressed proteins (DEPs), out of which 184 and 140 were up and downregulated, respectively. The OsPYL9 mutants showed an increase in grain yield under both drought and well watered field conditions. Most of the DEPs related to circadian clock rhythm, drought response, and reactive oxygen species were upregulated in the mutant plants. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that DEPs were only involved in circadian rhythm and Gene Ontology (GO) analysis showed that most of the DEPs were involved in response to abiotic stimulus, and abscisic acid-activated signaling pathways. Protein GIGANTEA, Adagio-like, and Pseudo-response regulator proteins showed higher interaction in protein–protein interaction (PPI) network. Thus, the overall results showed that CRISPR/Cas9-generated OsPYL9 mutants have potential to improve both drought tolerance and the yield of rice. Furthermore, global proteome analysis provides new potential biomarkers and understandings of the molecular mechanism of rice drought tolerance.

Comparative sequence analysis across Brassicaceae, regulatory diversity in KCS5 and KCS6 homologs from Arabidopsis thaliana and Brassica juncea, and intronic fragment as a negative transcriptional regulator

Intra- and epicuticular-waxes primarily comprising of very long chain aliphatic lipid (VLCFA), terpenoids and secondary metabolites such as sterol and flavonoids played a major role in successful colonization of terrestrial ecosystem by aquatic plants and are thus considered as a key evolutionary innovation. The key rate limiting step of Fatty Acid (FA) biosynthesis of condensation/elongation are catalyzed by the enzyme, β-ketoacyl coenzyme A synthase (KCS), part of FAE (Fatty Acid Elongase) complex. KCS6 has been shown to be responsible for elongation using C22 fatty acid as substrate and is considered essential for synthesis of VLCFA for cuticular waxes. Earlier studies have established KCS5 as a close paralog of KCS6 in Arabidopsis thaliana, albeit with non-redundant function. We subsequently established segmental duplication responsible for origin of KCS6-KCS5 paralogy which is exclusive to Brassicaceae. In the present study, we aim to understand impact of duplication on regulatory diversification and evolution, through sequence and functional analysis of cis-regulatory element of KCS5 and KCS6. High level of sequence variation leading to conservation of only the proximal end of the promoter corresponding to the core promoter was observed among Brassicaceae members; such high diversity was also revealed when sliding window analysis revealed only two to three phylogenetic footprints. Profiling of transcription factor binding sites (TFBS) across Brassicaceae shows presence of light, hormone and stress responsive motifs; a few motifs involved in tissue specific expression (Skn-1; endosperm) were also detected. Functional characterization using transcriptional fusion constructs revealed regulatory diversification when promoter activity of homologs from A. thaliana and Brassica juncea were compared. When subjected to 5-Azacytidine, altered promoter activity was observed, implying role of DNA methylation in transcriptional regulation. Finally, investigation of the role of an 87 bp fragment from first intron that is retained in a splice variant, revealed it to be a transcriptional repressor. This is a first report on comparative sequence and functional analysis of transcriptional regulation of KCS5 and KCS6; further studies are required before manipulation of cuticular waxes as a strategy for mitigating stress.

Genome wide association mapping of epi-cuticular wax genes in Sorghum bicolor

Sorghum accumulates epi-cuticular wax (EW) in leaves, sheaths, and culms. EW reduces the transpirational and nontranspirational (nonstomatal) water loss and protects the plant from severe drought stress in addition to imparting resistance against insect pests. Results presented here are from the analysis of EW content of 387 diverse sorghum accessions and its genome-wide association study (GWAS). EW content in sorghum leaves ranged from 0.1 to 29.7 mg cm^-2 with a mean value of 5.1 mg cm^-2. GWAS using 265,487 single nucleotide polymorphisms identified thirty-seven putative genes associated (P < 9.89E-06) with EW biosynthesis and transport in sorghum. Major EW biosynthetic genes identified included 3-Oxoacyl-[acyl-carrier-protein (ACP)] synthase III, an Ankyrin repeat protein, a bHLH-MYC, and an R2R3-MYB transcription factor. Genes involved in EW regulation or transport included an ABC transporter, a Lipid exporter ABCA1, a Multidrug resistance protein, Inositol 1, 3, 4-trisphosphate 5/6-kinase, and a Cytochrome P450. This GWA study thus demonstrates the potential for genetic manipulation of EW content in sorghum for better adaptation to biotic and abiotic stress.

Estimating Peanut Leaf Chlorophyll Content with Dorsiventral Leaf Adjusted Indices: Minimizing the Impact of Spectral Differences between Adaxial and Abaxial Leaf Surfaces

Relatively little research has assessed the impact of spectral differences among dorsiventral leaves caused by leaf structure on leaf chlorophyll content (LCC) retrieval. Based on reflectance measured from peanut adaxial and abaxial leaves and LCC measurements, this study proposed a dorsiventral leaf adjusted ratio index (DLARI) to adjust dorsiventral leaf structure and improve LCC retrieval accuracy. Moreover, the modified Datt (MDATT) index, which was insensitive to leaves structure, was optimized for peanut plants. All possible wavelength combinations for the DLARI and MDATT formulae were evaluated. When reflectance from both sides were considered, the optimal combination for the MDATT formula was ( R 723 − R 738 ) / ( R 723 − R 722 ) with a cross-validation R2cv of 0.91 and RMSEcv of 3.53 μg/cm2. The DLARI formula provided the best performing indices, which were ( R 735 − R 753 ) / ( R 715 − R 819 ) for estimating LCC from the adaxial surface (R2cv = 0.96, RMSEcv = 2.37 μg/cm2) and ( R 732 − R 754 ) / ( R 724 − R 773 ) for estimating LCC from reflectance of both sides (R2cv = 0.94, RMSEcv = 2.81 μg/cm2). A comparison with published vegetation indices demonstrated that the published indices yielded reliable estimates of LCC from the adaxial surface but performed worse than DLARIs when both leaf sides were considered. This paper concludes that the DLARI is the most promising approach to estimate peanut LCC.

Seasonal changes in morphology govern wettability of Katsura leaves

Deciduous broad-leaf trees survive and prepare for winter by shedding their leaves in fall. During the fall season, a change in a leaf’s wettability and its impact on the leaf-fall are not well understood. In this study, we measure the surface morphology and wettability of Katsura leaves from the summer to winter, and reveal how leaf structural changes lead to wettability changes. The averaged contact angle of leaves decreases from 147° to 124° while the contact-angle hysteresis significantly increases by about 35°, which are attributed to dehydration and erosion of nano-wax. Due to such wettability changes, fall brown leaves support approximately 17 times greater water volume than summer leaves.

Leaf dorsoventrality as a paramount factor determining spectral performance in field-grown wheat under contrasting water regimes

The effects of leaf dorsoventrality and its interaction with environmentally induced changes in the leaf spectral response are still poorly understood, particularly for isobilateral leaves. We investigated the spectral performance of 24 genotypes of field-grown durum wheat at two locations under both rainfed and irrigated conditions. Flag leaf reflectance spectra in the VIS-NIR-SWIR (visible-near-infrared-short-wave infrared) regions were recorded in the adaxial and abaxial leaf sides and at the canopy level, while traits providing information on water status and grain yield were evaluated. Moreover, leaf anatomical parameters were measured in a subset of five genotypes. The spectral traits studied were more affected by the leaf side than by the water regime. Leaf dorsoventral differences suggested higher accessory pigment content in the abaxial leaf side, while water regime differences were related to increased chlorophyll, nitrogen, and water contents in the leaves in the irrigated treatment. These variations were associated with anatomical changes. Additionally, leaf dorsoventral differences were less in the rainfed treatment, suggesting the existence of leaf-side-specific responses at the anatomical and biochemical level. Finally, the accuracy in yield prediction was enhanced when abaxial leaf spectra were employed. We concluded that the importance of dorsoventrality in spectral traits is paramount, even in isobilateral leaves.

Leaf aging of Amazonian canopy trees as revealed by spectral and physiochemical measurements

Leaf aging is a fundamental driver of changes in leaf traits, thereby regulating ecosystem processes and remotely sensed canopy dynamics. We explore leaf reflectance as a tool to monitor leaf age and develop a spectra-based partial least squares regression (PLSR) model to predict age using data from a phenological study of 1099 leaves from 12 lowland Amazonian canopy trees in southern Peru. Results demonstrated monotonic decreases in leaf water (LWC) and phosphorus (P_mass ) contents and an increase in leaf mass per unit area (LMA) with age across trees; leaf nitrogen (N_mass ) and carbon (C_mass ) contents showed monotonic but tree-specific age responses. We observed large age-related variation in leaf spectra across trees. A spectra-based model was more accurate in predicting leaf age (R²  = 0.86; percent root mean square error (%RMSE) = 33) compared with trait-based models using single (R²  = 0.07-0.73; %RMSE = 7-38) and multiple (R²  = 0.76; %RMSE = 28) predictors. Spectra- and trait-based models established a physiochemical basis for the spectral age model. Vegetation indices (VIs) including the normalized difference vegetation index (NDVI), enhanced vegetation index 2 (EVI2), normalized difference water index (NDWI) and photosynthetic reflectance index (PRI) were all age-dependent. This study highlights the importance of leaf age as a mediator of leaf traits, provides evidence of age-related leaf reflectance changes that have important impacts on VIs used to monitor canopy dynamics and productivity and proposes a new approach to predicting and monitoring leaf age with important implications for remote sensing.

A β-Ketoacyl-CoA Synthase Is Involved in Rice Leaf Cuticular Wax Synthesis and Requires a CER2-LIKE Protein as a Cofactor

Cuticular waxes are complex mixtures of very-long-chain fatty acids (VLCFAs) and their derivatives, forming a natural barrier on aerial surfaces of terrestrial plants against biotic and abiotic stresses. In VLCFA biosynthesis, β-ketoacyl-CoA synthase (KCS) is the key enzyme, catalyzing the first reaction in fatty acid elongation and determining substrate specificity. We isolated a rice (Oryza sativa) wax crystal-sparse leaf 4 (WSL4) gene using a map-based cloning strategy. WSL4 is predicted to encode a KCS, a homolog of Arabidopsis (Arabidopsis thaliana) CER6. Complementation of the mutant wsl4-1 with WSL4 genomic DNA rescued the cuticular wax-deficient phenotype, confirming the function of WSL4 The load of wax components longer than 30 carbons (C30) and C28 were reduced markedly in wsl4-1 and wsl4-2 mutants, respectively. Overexpression of WSL4 increased the cuticular wax load in rice leaves. We further isolated a cofactor of WSL4, OsCER2, a homolog of Arabidopsis CER2, by coimmunoprecipitation and confirmed their physical interaction by split-ubiquitin yeast two-hybrid experiments. Expression of WSL4 alone in elo3 yeast cells resulted in increased C24 but did not produce VLCFAs of greater length, whereas expressing OsCER2 alone showed no effect. Coexpression of WSL4 and OsCER2 in elo3 yeast cells yielded fatty acids up to C30. OsCER2 with a mutated HxxxD motif (H172E, D176A, and D176H) interrupted its interaction with WSL4 and failed to elongate VLCFAs past C24 when expressed with WSL4 in elo3 yeast cells. These results demonstrated that WSL4 was involved in VLCFA elongation beyond C22 and that elongation beyond C24 required the participation of OsCER2.

A comparison of methods to estimate photosynthetic light absorption in leaves with contrasting morphology

Accurate temporal and spatial measurements of leaf optical traits (i.e., absorption, reflectance and transmittance) are paramount to photosynthetic studies. These optical traits are also needed to couple radiative transfer and physiological models to facilitate the interpretation of optical data. However, estimating leaf optical traits in leaves with complex morphologies remains a challenge. Leaf optical traits can be measured using integrating spheres, either by placing the leaf sample in one of the measuring ports (External Method) or by placing the sample inside the sphere (Internal Method). However, in leaves with complex morphology (e.g., needles), the External Method presents limitations associated with gaps between the leaves, and the Internal Method presents uncertainties related to the estimation of total leaf area. We introduce a modified version of the Internal Method, which bypasses the effect of gaps and the need to estimate total leaf area, by painting the leaves black and measuring them before and after painting. We assess and compare the new method with the External Method using a broadleaf and two conifer species. Both methods yielded similar leaf absorption estimates for the broadleaf, but absorption estimates were higher with the External Method for the conifer species. Factors explaining the differences between methods, their trade-offs and their advantages and limitations are also discussed. We suggest that the new method can be used to estimate leaf absorption in any type of leaf independently of its morphology, and be used to study further the impact of gap fraction in the External Method.

Cucumis sativus L. WAX2 Plays a Pivotal Role in Wax Biosynthesis, Influencing Pollen Fertility and Plant Biotic and Abiotic Stress Responses

Cuticular waxes play an important part in protecting plant aerial organs from biotic and abiotic stresses. In previous studies, the biosynthetic pathway of cuticular waxes and relative functional genes has been researched and understood; however, little is known in cucumber (Cucumis sativus L.). In this study, we cloned and characterized an AtWAX2 homolog, CsWAX2, in cucumber and found that it is highly expressed in the epidermis, where waxes are synthesized, while subcellular localization showed that CsWAX2 protein is localized to the endoplasmic reticulum (ER). The transcriptional expression of CsWAX2 was found to be induced by low temperature, drought, salt stress and ABA, while the ectopic expression of CsWAX2 in an Arabidopsis wax2 mutant could partially complement the glossy stem phenotype. Abnormal expression of CsWAX2 in transgenic cucumbers specifically affected both very long chain (VLC) alkanes and cutin biosynthesis. Furthermore, transgenic cucumber plants of CsWAX2 showed significant changes in pollen viability and fruit resistance to water loss and pathogens compared with the wild type. Collectively, these results indicated that CsWAX2 plays a pivotal role in wax biosynthesis, influencing pollen fertility and the plant's response to biotic and abiotic stresses.

Distribution and variability of n-alkanes in epicuticular waxes of sedum species from the central Balkan Peninsula: chemotaxonomic importance

For the first time, the n-alkane distribution and variability of the epicuticular waxes within 22 Sedum taxa was reported with focus on the chemotaxonomy of native Sedum representatives from the central Balkan Peninsula, compared to their relations with four other species of the Crassulaceae family. By GC/MS and GC-FID identification and quantification, it was established that n-alkanes C27 , C29 , C31 , C33 , and C35 were the dominant constituents of the examined epicuticular wax samples. Applying multivariate statistical analyses including agglomerative hierarchical clustering (AHC) and principal component analysis (PCA), the relation according to the n-alkane composition between the examined samples was established. It was shown that the n-alkane variability of the central Balkan Sedum species was considerable and that n-alkanes might not be very reliable taxonomic markers for these species.

Evaluation of the surface free energy of plant surfaces: toward standardizing the procedure

Plant surfaces have been found to have a major chemical and physical heterogeneity and play a key protecting role against multiple stress factors. During the last decade, there is a raising interest in examining plant surface properties for the development of biomimetic materials. Contact angle measurement of different liquids is a common tool for characterizing synthetic materials, which is just beginning to be applied to plant surfaces. However, some studies performed with polymers and other materials showed that for the same surface, different surface free energy values may be obtained depending on the number and nature of the test liquids analyzed, materials' properties, and surface free energy calculation methods employed. For 3 rough and 3 rather smooth plant materials, we calculated their surface free energy using 2 or 3 test liquids and 3 different calculation methods. Regardless of the degree of surface roughness, the methods based on 2 test liquids often led to the under- or over-estimation of surface free energies as compared to the results derived from the 3-Liquids method. Given the major chemical and structural diversity of plant surfaces, it is concluded that 3 different liquids must be considered for characterizing materials of unknown physico-chemical properties, which may significantly differ in terms of polar and dispersive interactions. Since there are just few surface free energy data of plant surfaces with the aim of standardizing the calculation procedure and interpretation of the results among for instance, different species, organs, or phenological states, we suggest the use of 3 liquids and the mean surface tension values provided in this study.

Wettability, polarity, and water absorption of holm oak leaves: effect of leaf side and age

Plant trichomes play important protective functions and may have a major influence on leaf surface wettability. With the aim of gaining insight into trichome structure, composition, and function in relation to water-plant surface interactions, we analyzed the adaxial and abaxial leaf surface of holm oak (Quercus ilex) as a model. By measuring the leaf water potential 24 h after the deposition of water drops onto abaxial and adaxial surfaces, evidence for water penetration through the upper leaf side was gained in young and mature leaves. The structure and chemical composition of the abaxial (always present) and adaxial (occurring only in young leaves) trichomes were analyzed by various microscopic and analytical procedures. The adaxial surfaces were wettable and had a high degree of water drop adhesion in contrast to the highly unwettable and water-repellent abaxial holm oak leaf sides. The surface free energy and solubility parameter decreased with leaf age, with higher values determined for the adaxial sides. All holm oak leaf trichomes were covered with a cuticle. The abaxial trichomes were composed of 8% soluble waxes, 49% cutin, and 43% polysaccharides. For the adaxial side, it is concluded that trichomes and the scars after trichome shedding contribute to water uptake, while the abaxial leaf side is highly hydrophobic due to its high degree of pubescence and different trichome structure, composition, and density. Results are interpreted in terms of water-plant surface interactions, plant surface physical chemistry, and plant ecophysiology.

Ultrastructural studies on the natural leaf senescence of Cinnamomum camphora

The process of natural leaf senescence of Cinnamomum camphora (C. camphora)-a commercial tree in Asia, was investigated, focusing on changes in cellular ultrastructure, epicuticular wax, and stoma. The changes to mesophyll cells in a senescing leaf predominantly include degradation of the following cellular components: cytoplasm, the central vacuole, small vacuoles, and vesicles with a diameter smaller than 400 nm, which are involved in the degradation of chloroplasts. The sequence of change in epicuticular wax during leaf senescence was different from those in herbaceous plants by atomic force microscope and scanning electron microscopic analysis. Comparing with maturation leaves, senescing leaves develop a wider aperture in their stoma, which would delay the leaf senescence of C. camphora.

The inhibitor of wax 1 locus (Iw1) prevents formation of β- and OH-β-diketones in wheat cuticular waxes and maps to a sub-cM interval on chromosome arm 2BS

Glaucousness is described as the scattering effect of visible light from wax deposited on the cuticle of plant aerial organs. In wheat, two dominant genes lead to non-glaucous phenotypes: Inhibitor of wax 1 (Iw1) and Iw2. The molecular mechanisms and the exact extent (beyond visual assessment) by which these genes affect the composition and quantity of cuticular wax is unclear. To describe the Iw1 locus we used a genetic approach with detailed biochemical characterization of wax compounds. Using synteny and a large number of F2 gametes, Iw1 was fine-mapped to a sub-cM genetic interval on wheat chromosome arm 2BS, which includes a single collinear gene from the corresponding Brachypodium and rice physical maps. The major components of flag leaf and peduncle cuticular waxes included primary alcohols, β-diketones and n-alkanes. Small amounts of C19-C27 alkyl and methylalkylresorcinols that have not previously been described in wheat waxes were identified. Using six pairs of BC2 F3 near-isogenic lines, we show that Iw1 inhibits the formation of β- and hydroxy-β-diketones in the peduncle and flag leaf blade cuticles. This inhibitory effect is independent of genetic background or tissue, and is accompanied by minor but consistent increases in n-alkanes and C24 primary alcohols. No differences were found in cuticle thickness and carbon isotope discrimination in near-isogenic lines differing at Iw1.

Arabidopsis 3-ketoacyl-coenzyme a synthase9 is involved in the synthesis of tetracosanoic acids as precursors of cuticular waxes, suberins, sphingolipids, and phospholipids

Very-long-chain fatty acids (VLCFAs) with chain lengths from 20 to 34 carbons are involved in diverse biological functions such as membrane constituents, a surface barrier, and seed storage compounds. The first step in VLCFA biosynthesis is the condensation of two carbons to an acyl-coenzyme A, which is catalyzed by 3-ketoacyl-coenzyme A synthase (KCS). In this study, amino acid sequence homology and the messenger RNA expression patterns of 21 Arabidopsis (Arabidopsis thaliana) KCSs were compared. The in planta role of the KCS9 gene, showing higher expression in stem epidermal peels than in stems, was further investigated. The KCS9 gene was ubiquitously expressed in various organs and tissues, including roots, leaves, and stems, including epidermis, silique walls, sepals, the upper portion of the styles, and seed coats, but not in developing embryos. The fluorescent signals of the KCS9::enhanced yellow fluorescent protein construct were merged with those of BrFAD2::monomeric red fluorescent protein, which is an endoplasmic reticulum marker in tobacco (Nicotiana benthamiana) epidermal cells. The kcs9 knockout mutants exhibited a significant reduction in C24 VLCFAs but an accumulation of C20 and C22 VLCFAs in the analysis of membrane and surface lipids. The mutant phenotypes were rescued by the expression of KCS9 under the control of the cauliflower mosaic virus 35S promoter. Taken together, these data demonstrate that KCS9 is involved in the elongation of C22 to C24 fatty acids, which are essential precursors for the biosynthesis of cuticular waxes, aliphatic suberins, and membrane lipids, including sphingolipids and phospholipids. Finally, possible roles of unidentified KCSs are discussed by combining genetic study results and gene expression data from multiple Arabidopsis KCSs.

RDR1 and SGS3, components of RNA-mediated gene silencing, are required for the regulation of cuticular wax biosynthesis in developing inflorescence stems of Arabidopsis

The cuticle is a protective layer that coats the primary aerial surfaces of land plants and mediates plant interactions with the environment. It is synthesized by epidermal cells and is composed of a cutin polyester matrix that is embedded and covered with cuticular waxes. Recently, we have discovered a novel regulatory mechanism of cuticular wax biosynthesis that involves the ECERIFERUM7 (CER7) ribonuclease, a core subunit of the exosome. We hypothesized that at the onset of wax production, the CER7 ribonuclease degrades an mRNA specifying a repressor of CER3, a wax biosynthetic gene whose protein product is required for wax formation via the decarbonylation pathway. In the absence of this repressor, CER3 is expressed, leading to wax production. To identify the putative repressor of CER3 and to unravel the mechanism of CER7-mediated regulation of wax production, we performed a screen for suppressors of the cer7 mutant. Our screen resulted in the isolation of components of the RNA-silencing machinery, RNA-DEPENDENT RNA POLYMERASE1 and SUPPRESSOR OF GENE SILENCING3, implicating RNA silencing in the control of cuticular wax deposition during inflorescence stem development in Arabidopsis (Arabidopsis thaliana).

Rice OsGL1-1 is involved in leaf cuticular wax and cuticle membrane

Cuticular wax forms a hydrophobic barrier on aerial plant organs; it plays an important role in protecting a plant from damage caused by many forms of environmental stress. In the present study, we characterized a rice leaf wax-deficient mutant osgl1-1 derived from a spontaneous mutation, which exhibited a wax-deficient and highly hydrophilic leaf phenotype. We cloned the OsGL1-1 gene by the map-based cloning method and performed a complementation test to confirm the function of the candidate gene. Molecular studies revealed that OsGL1-1 was a member of the OsGL1 family, and contained regions that were homologous to some regions in sterol desaturases and short-chain dehydrogenases/reductases. Compared to the wild-type, the osgl1-1 mutant showed decreased cuticular wax deposition, thinner cuticular membrane, decreased chlorophyll leaching, increased rate of water loss, and enhanced sensitivity to drought. OsGL1-1 is expressed ubiquitously in rice. The transient expression of OsGL1-1-green fluorescent protein fusion protein indicated that OsGL1-1 is localized in the cytoplasm, plasma membrane, and nucleus.

New insights into the properties of pubescent surfaces: peach fruit as a model

The surface of peach (Prunus persica 'Calrico') is covered by a dense indumentum, which may serve various protective purposes. With the aim of relating structure to function, the chemical composition, morphology, and hydrophobicity of the peach skin was assessed as a model for a pubescent plant surface. Distinct physicochemical features were observed for trichomes versus isolated cuticles. Peach cuticles were composed of 53% cutan, 27% waxes, 23% cutin, and 1% hydroxycinnamic acid derivatives (mainly ferulic and p-coumaric acids). Trichomes were covered by a thin cuticular layer containing 15% waxes and 19% cutin and were filled by polysaccharide material (63%) containing hydroxycinnamic acid derivatives and flavonoids. The surface free energy, polarity, and work of adhesion of intact and shaved peach surfaces were calculated from contact angle measurements of water, glycerol, and diiodomethane. The removal of the trichomes from the surface increased polarity from 3.8% (intact surface) to 23.6% and decreased the total surface free energy chiefly due to a decrease on its nonpolar component. The extraction of waxes and the removal of trichomes led to higher fruit dehydration rates. However, trichomes were found to have a higher water sorption capacity as compared with isolated cuticles. The results show that the peach surface is composed of two different materials that establish a polarity gradient: the trichome network, which has a higher surface free energy and a higher dispersive component, and the cuticle underneath, which has a lower surface free energy and higher surface polarity. The significance of the data concerning water-plant surface interactions is discussed within a physiological context.

The MYB96 transcription factor regulates cuticular wax biosynthesis under drought conditions in Arabidopsis

Drought stress activates several defense responses in plants, such as stomatal closure, maintenance of root water uptake, and synthesis of osmoprotectants. Accumulating evidence suggests that deposition of cuticular waxes is also associated with plant responses to cellular dehydration. Yet, how cuticular wax biosynthesis is regulated in response to drought is unknown. We have recently reported that an Arabidopsis thaliana abscisic acid (ABA)-responsive R2R3-type MYB transcription factor, MYB96, promotes drought resistance. Here, we show that transcriptional activation of cuticular wax biosynthesis by MYB96 contributes to drought resistance. Microarray assays showed that a group of wax biosynthetic genes is upregulated in the activation-tagged myb96-1D mutant but downregulated in the MYB96-deficient myb96-1 mutant. Cuticular wax accumulation was altered accordingly in the mutants. In addition, activation of cuticular wax biosynthesis by drought and ABA requires MYB96. By contrast, biosynthesis of cutin monomers was only marginally affected in the mutants. Notably, the MYB96 protein acts as a transcriptional activator of genes encoding very-long-chain fatty acid-condensing enzymes involved in cuticular wax biosynthesis by directly binding to conserved sequence motifs present in the gene promoters. These results demonstrate that ABA-mediated MYB96 activation of cuticular wax biosynthesis serves as a drought resistance mechanism.

Identification of salt responsive genes using comparative microarray analysis in Upland cotton (Gossypium hirsutum L.)

Salinity negatively impacts plant growth and productivity, and little is known about salt responsive genes in cotton. In this study, an intra-specific backcross population of cotton (Gossypium hirsutum L.) was treated with 200 mM NaCl after which differentially expressed genes were identified by comparison between salt tolerant and susceptible segregant bulks using comparative microarray analysis. Microarray analysis identified 720 salt-responsive genes, of which 695 were down-regulated and only 25 were up-regulated in the salt tolerant bulk. Gene ontology of annotated genes revealed that at least some of the identified salt responsive transcripts belong to pathways known to be associated with salt stress including osmolyte and lipid metabolism, cell wall structure, and membrane synthesis. About 48% of all salt-responsive genes were functionally unknown. Quantitative RT-PCR was used to validate 17 selected salt responsive genes. This work represents the first study in employing microarray to investigate the possible mechanisms of the salt response in cotton. Further analysis of salt-responsive genes associated with salt tolerance in cotton will assist in laying a foundation for molecular manipulation in development of new cultivars with improved salt tolerance.

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.

In situ analysis by microspectroscopy reveals triterpenoid compositional patterns within leaf cuticles of Prunus laurocerasus

The cuticular waxes on the leaves of Prunus laurocerasus are arranged in distinct layers differing in triterpenoid concentrations (Jetter et al., Plant Cell Environ 23:619-628, 2000). In addition to this transversal gradient, the lateral distribution of cuticular triterpenoids must be investigated to fully describe the spatial distribution of wax components on the leaf surfaces. In the present investigation, near infrared (NIR) Raman microspectroscopy, coherent anti-Stokes Raman scattering (CARS) microscopy, and third harmonic generation (THG) spectroscopy were employed to map the triterpenoid distribution in isolated cuticles from adaxial and abaxial sides of P. laurocerasus leaves. The relative concentrations of ursolic acid and oleanolic acid were calculated by treating the cuticle spectra as linear combinations of reference spectra from the major compounds found in the wax. Raman maps of the adaxial cuticle showed that the triterpenoids accumulate to relatively high concentrations over the periclinal regions of the pavement cells, while the very long chain aliphatic wax constituents are distributed fairly evenly across the entire adaxial cuticle. In the analysis of the abaxial cuticles, the triterpenoids were found to accumulate in greater amounts over the guard cells relative to the pavement cells. The very long chain aliphatic compounds accumulated in the cuticle above the anticlinal cell walls of the pavement cells, and were found at low concentrations above the periclinals and the guard cells.

Chemical composition of the epicuticular and intracuticular wax layers on adaxial sides of Rosa canina leaves

BACKGROUND AND AIMS

The waxy cuticle is the first point of contact for many herbivorous and pathogenic organisms on rose plants. Previous studies have reported the average composition of the combined wax extract from both sides of rose leaves. Recently, the compositions of the waxes on the adaxial and abaxial surfaces of Rosa canina leaves were determined separately. In this paper, a first report is made on the compositions of the epicuticular and intracuticular wax layers of Rosa canina leaves. The methods described enable the determination of which compounds are truly available at the surface for plant-organism interactions.

METHODS

An adhesive was used to mechanically strip the epicuticular wax from the adaxial leaf surface and the removal was visually confirmed using scanning electron microscopy. After the epicuticular wax had been removed, the intracuticular wax was then isolated using standard chemical extraction. Gas chromatography, flame ionization detection and mass spectrometry were used to identify and quantify compounds in the separated wax mixtures.

KEY RESULTS

The epicuticular wax contained higher concentrations of alkanes and alkyl esters but lower concentrations of primary alcohols and alkenols when compared to the intracuticular wax. In addition, the average chain lengths of these compound classes were higher in the epicuticular wax. Secondary alcohols were found only in the epicuticular layer while triterpenoids were restricted mainly to the intracuticular wax.

CONCLUSIONS

A gradient exists between the composition of the epi- and intracuticular wax layers of Rosa canina leaves. This gradient may result from polarity differences, in part caused by differences in chain lengths. The outer wax layer accessible to the phyllosphere showed a unique composition of wax compounds. The ecological consequences from such a gradient may now be probed.

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.

Characterization of two GL8 paralogs reveals that the 3-ketoacyl reductase component of fatty acid elongase is essential for maize (Zea mays L.) development

Prior analyses established that the maize (Zea mays L.) gl8a gene encodes 3-ketoacyl reductase, a component of the fatty acid elongase required for the biosynthesis of very long chain fatty acids (VLCFAs). A paralogous gene, gl8b, has been identified that is 96% identical to gl8a. The gl8a and gl8b genes map to syntenic chromosomal regions, have similar, but not identical, expression patterns, and encode proteins that are 97% identical. Both of these genes are required for the normal accumulation of cuticular waxes on seedling leaves. The chemical composition of the cuticular waxes from gl8a and gl8b mutants indicates that these genes have at least overlapping, if not redundant, functions in cuticular wax biosynthesis. Although gl8a and gl8b double mutant kernels have endosperms that cannot be distinguished from wild-type siblings, these kernels are non-viable because their embryos fail to undergo normal development. Double mutant kernels accumulate substantially reduced levels of VLCFAs. VLCFAs are components of a variety of compounds, for example, cuticular waxes, suberin, and sphingolipids. Consistent with their essential nature in yeast, the accumulation of the ceramide moiety of sphingolipids is substantially reduced and their fatty acid composition altered in gl8a and gl8b double mutant kernels relative to wild-type kernels. Hence, we hypothesize that sphingolipids or other VLCFA-containing compounds are essential for normal embryo development.

Significance of the expression of the CER6 condensing enzyme for cuticular wax production in Arabidopsis

To learn more about the role of the CER6 condensing enzyme in Arabidopsis surface wax production, we determined CER6 transcription domains and the timing of CER6 transcription in vegetative and reproductive structures from juvenile, mature, and senescing tissues. We found that CER6 is highly transcribed throughout development, exclusively in the epidermal cells in all tissues examined. The only exception to the epidermal expression was observed in anthers nearing maturity, in which CER6 mRNA was localized in the tapetum. To determine if environmental factors such as light and water deficit, which are known to stimulate wax accumulation, induce CER6 transcription, we examined the effects of these factors on CER6 transcript abundance. Our results demonstrate that light is essential for CER6 transcription, and that osmotic stress and the presence of abscisic acid enhance CER6 transcript accumulation. CER6 promoter-directed expression of the beta-glucuronidase reporter gene in transgenic plants demonstrated that the CER6 promoter was highly effective in directing epidermis-specific expression in Arabidopsis and tobacco (Nicotiana tabacum). Furthermore, CER6 promoter-driven CER6 overexpression resulted in increased wax deposition in Arabidopsis stems. These experiments indicate that the expression level of CER6 in the epidermis is one of the factors controlling wax accumulation on Arabidopsis stems.

Estimating near-infrared leaf reflectance from leaf structural characteristics

The relationship between near-infrared reflectance at 800 nm (NIRR) from leaves and characteristics of leaf structure known to affect photosynthesis was investigated in 48 species of alpine angiosperms. This wavelength was selected to discriminate the effects of leaf structure vs. chemical or water content on leaf reflectance. A quantitative model was first constructed correlating NIRR with leaf structural characteristics for six species, and then validated using all 48 species. Among the structural characteristics tested in the reflectance model were leaf trichome density, the presence or absence of both leaf bicoloration and a thick leaf cuticle (>1 μm), leaf thickness, the ratio of palisade mesophyll to spongy mesophyll thickness (PM/SM), the proportion of the mesophyll occupied by intercellular air spaces (%IAS), and the ratio of mesophyll cell surface area exposed to IAS (A(mes)) per unit leaf surface area (A), or A(mes)/A. Multiple regression analysis showed that measured NIRR was highly correlated with A(mes)/A, leaf bicoloration, and the presence of a thick leaf cuticle (r = 0.93). In contrast, correlations between NIRR and leaf trichome density, leaf thickness, the PM/SM ratio, or %IAS were relatively weak (r < 0.25). A model incorporating A(mes)/A, leaf bicoloration, and cuticle thickness predicted NIRR accurately for 48 species (r = 0.43; P < 0.01) and may be useful for linking remotely sensed data to plant structure and function.

CUT1, an Arabidopsis gene required for cuticular wax biosynthesis and pollen fertility, encodes a very-long-chain fatty acid condensing enzyme

Land plants secrete a layer of wax onto their aerial surfaces that is essential for survival in a terrestrial environment. This wax is composed of long-chain, aliphatic hydrocarbons derived from very-long-chain fatty acids (VLCFAs). Using the Arabidopsis expressed sequence tag database, we have identified a gene, designated CUT1, that encodes a VLCFA condensing enzyme required for cuticular wax production. Sense suppression of CUT1 in transgenic Arabidopsis plants results in waxless (eceriferum) stems and siliques as well as conditional male sterility. Scanning electron microscopy revealed that this was a severe waxless phenotype, because stems of CUT1-suppressed plants were completely devoid of wax crystals. Furthermore, chemical analyses of waxless plants demonstrated that the stem wax load was reduced to 6 to 7% of wild-type levels. This value is lower than that reported for any of the known eceriferum mutants. The severe waxless phenotype resulted from the downregulation of both the decarbonylation and acyl reduction wax biosynthetic pathways. This result indicates that CUT1 is involved in the production of VLCFA precursors used for the synthesis of all stem wax components in Arabidopsis. In CUT1-suppressed plants, the C24 chain-length wax components predominate, suggesting that CUT1 is required for elongation of C24 VLCFAs. The unique wax composition of CUT1-suppressed plants together with the fact that the location of CUT1 on the genetic map did not coincide with any of the known ECERIFERUM loci suggest that we have identified a novel gene involved in wax biosynthesis. CUT1 is currently the only known gene with a clearly established function in wax production.

Effects of the ultraviolet-B radiation (UV-B) on conifers: a review

The current knowledge on conifer responses to enhanced ultraviolet-B (UV-B) radiation is mainly based on greenhouse or growth chamber experiments of one growing season in duration. However, the biomass losses observed in greenhouses do not occur in field-grown trees in their natural habitats. Moreover, the majority of the 20 conifer species studied have been 1-year-old seedlings, and no studies have been undertaken on mature trees. Fully grown needles, with their glaucous waxy surfaces and thick epidermal cells with both soluble and wall-bound UV-B screening metabolites, are well protected against UV-B radiation. However, it is not known whether these are sufficient protectants in young emerging needles or during the early spring period of high UV-B levels reflected from snow. In order to understand all the mechanisms that result in the protection of conifer needles against UV-B radiation, future research should focus on the epidermal layer, separating the waxes, cuticle and epidermal and hypodermal cells. Parallel studies should consist of wall-bound and soluble secondary metabolite analysis, antioxidant measurements and microscopic observations.

Leaf Epicuticular Waxes of the Eceriferum Mutants in Arabidopsis

Wild-type Arabidopsis leaf epicuticular wax (EW) occurs as a smooth layer over the epidermal surface, whereas stem EW has a crystalline microstructure. Wild-type EW load was more than 10-fold lower on leaves than on stems. Compared with the EW on wild-type stems, EW on wild-type leaves had a much higher proportion of their total EW load in the form of alkanes and 1-alcohols; a large reduction in secondary alcohols, ketones, and esters; and a chain-length distribution for major EW classes that was skewed toward longer lengths. The eceriferum (cer) mutations often differentially affected leaf and stem EW chemical compositions. For example, the cer2 mutant EW phenotype was expressed on the stem but not on the leaf. Compared to wild type, the amount of primary alcohols on cer9 mutants was reduced on leaves but elevated on stems, whereas an opposite differential effect for primary alcohols was observed on cer16 leaves and stems. Putative functions for CER gene products are discussed. The CER4 and CER6 gene products may be involved in fatty aldehyde reduction and C26 fatty acylcoenzyme A elongation, respectively. CER1, CER8, CER9, and CER16 gene products may be involved in EW substrate transfer. The CER3 gene product may be involved in release of fatty acids from elongase complexes. CER2 gene product may have regulatory functions.

Effects of green light on biological systems

Green light (510-565 nm) constitutes a significant portion of the visible spectrum impinging on biological systems. It plays many different roles in the biochemistry, physiology and structure of plants and animals. In only a relatively small number of responses to green light is the photoreceptor known with certainty or even provisionally and in even fewer systems has the chain of events leading from perception to response been examined experimentally. This review provides a detailed view of those biological systems shown to respond to green light, an evaluation of possible photoreceptors and a review of the known and postulated mechanisms leading to the responses.