Analysis of Extracellular Cell Wall Lipids: Wax, Cutin, and Suberin in Leaves, Roots, Fruits, and Seeds

Extracellular lipids of plants can be analyzed using gas chromatography and mass spectrometry. Soluble waxes are extracted with chloroform and thus separated from the extracellular polymers cutin and suberin. Cutin and suberin have to be depolymerized using boron trifluoride-methanol or methanolic HCl before analysis. The released monomeric hydroxylated fatty acids are then extracted with chloroform or hexane. Prior to gas chromatography, all free polar functional groups (alcohols and carboxylic acids) are derivatized by trimethylsilylation. Internal standards, that is, long chain alkanes, are used for the quantification of wax molecules and cutin or suberin monomers. Lipids are quantified using gas chromatography coupled to flame ionization detection. Qualitative analysis is carried out by gas chromatography coupled to mass spectrometry. Thus, all wax molecules of chain lengths from C₁₆ to C₆₀ and different substance classes (fatty acids, alcohols, esters, aldehydes, alkanes, etc.) or all cutin or suberin monomers of chain lengths from C₁₆ to C₃₂ and different substance classes (hydroxylated fatty acids, diacids, etc.) can be analyzed from one sample.

High-resolution spectral information enables phenotyping of leaf epicuticular wax in wheat

BACKGROUND

Epicuticular wax (EW) is the first line of defense in plants for protection against biotic and abiotic factors in the environment. In wheat, EW is associated with resilience to heat and drought stress, however, the current limitations on phenotyping EW restrict the integration of this secondary trait into wheat breeding pipelines. In this study we evaluated the use of light reflectance as a proxy for EW load and developed an efficient indirect method for the selection of genotypes with high EW density.

RESULTS

Cuticular waxes affect the light that is reflected, absorbed and transmitted by plants. The narrow spectral regions statistically associated with EW overlap with bands linked to photosynthetic radiation (500 nm), carotenoid absorbance (400 nm) and water content (~ 900 nm) in plants. The narrow spectral indices developed predicted 65% (EWI-13) and 44% (EWI-1) of the variation in this trait utilizing single-leaf reflectance. However, the normalized difference indices EWI-4 and EWI-9 improved the phenotyping efficiency with canopy reflectance across all field experimental trials. Indirect selection for EW with EWI-4 and EWI-9 led to a selection efficiency of 70% compared to phenotyping with the chemical method. The regression model EWM-7 integrated eight narrow wavelengths and accurately predicted 71% of the variation in the EW load (mg·dm-2) with leaf reflectance, but under field conditions, a single-wavelength model consistently estimated EW with an average RMSE of 1.24 mg·dm-2 utilizing ground and aerial canopy reflectance.

CONCLUSIONS

Overall, the indices EWI-1, EWI-13 and the model EWM-7 are reliable tools for indirect selection for EW based on leaf reflectance, and the indices EWI-4, EWI-9 and the model EWM-1 are reliable for selection based on canopy reflectance. However, further research is needed to define how the background effects and geometry of the canopy impact the accuracy of these phenotyping methods.

Compositional, structural and functional cuticle analysis of Prunus laurocerasus L. sheds light on cuticular barrier plasticity

Barrier properties of the hydrophobic plant cuticle depend on its physicochemical composition. The cuticular compounds vary considerably among plant species but also among organs and tissues of the same plant and throughout developmental stages. As yet, these intraspecific modifications at the cuticular wax and cutin level are only rarely examined. Attempting to further elucidate cuticle profiles, we analysed the adaxial and abaxial surfaces of the sclerophyllous leaf and three developmental stages of the drupe fruit of Prunus laurocerasus, an evergreen model plant native to temperate regions. According to gas chromatographic analyses, the cuticular waxes contained primarily pentacyclic triterpenoids dominated by ursolic acid, whereas the cutin biopolyester mainly consisted of 9/10,ω-dihydroxy hexadecanoic acid. Distinct organ- and side-specific patterns were found for cuticular lipid loads, compositions and carbon chain length distributions. Compositional variations led to different structural and functional barrier properties of the plant cuticle, which were investigated further microscopically, infrared spectroscopically and gravimetrically. The minimum water conductance was highlighted at 1 × 10^-5 m s^-1 for the perennial, hypostomatous P. laurocerasus leaf and at 8 × 10^-5 m s^-1 for the few-month-living, stomatous fruit suggesting organ-specific cuticular barrier demands.

Relationship between the amount and composition of epicuticular wax and tolerance of Ipomoea biotypes to glyphosate

Ipomoea species are troublesome weeds in crop systems through Brazil. Drought stress typically reduces glyphosate efficacy by reducing the foliar uptake of herbicides and their translocation. Using both glyphosate tolerant (GT) and sensitive (GS) plants from Ipomoea grandifolia, I. indivisa and I. purpurea species, this research aimed to (a) correlate amounts of epicuticular wax and tolerance to glyphosate in plants and (b) determine the effect of drought stress (DStress) on changes in the quantity and chemical composition of plant epicuticular waxes. The dose that causes 50% inhibition of growth (GR50) of the biotypes varied between 62 and 1208 (I. grandifolia), 159 and 913 (I. indivisa), and 389 and 1925 g a.e. ha-1 of glyphosate (I. purpurea). There was low inverse correlation (-0.46) between the amount of epicuticular wax and the sensitivity to glyphosate. GT biotypes of the species presented greater plastic capacities than GS biotypes for increasing the amount of epicuticular wax under DStress. The three Ipomoea species exhibited different chemical profiles of waxes supported by IR spectra, which allows for their differentiation. For I. grandifolia and I. purpurea, there was an increase in the polar components in the state without DStress, while for the species I. indivisa, no differences in infrared spectra were detected between the two water conditions.

In situ detection of rice leaf cuticle responses to nitrogen supplies by depth-profiling Fourier transform photoacoustic spectroscopy

Plant cuticle is an important interface on the outmost region of plant and will make the response to environmental changes. However, research about how the variable nutritional status affect plant cuticle is limited. This was the first report about the manners of rice leaf cuticle in answer to different nutritional circumstances of nitrogen detected by the Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) which with a main superiority for in situ and depth-profiling in mid-infrared range. Rice leaves from the seedlings treated with three nitrogen levels designed as low (22N1), medium (N2) and high (N3) concentration were scanned by three moving mirror velocities (0.32 cm s^-1, 0.47 cm s^-1, and 0.63 cm s^-1) at 900-4000 cm^-1 to acquire the spectra of leaf surfaces. Well-resolved peaks had been detected at 3400, 2800, 1650, 1520 and 1050 cm^-1. Combining with the structures and compositions of cuticle, the spectra recorded with 0.63 cm s^-1 were identified to be from cuticle, and were used to analyze the responses of cuticle. Through curve-fitting, the absorption ratio of the peaks at (cm^-1) 1050/3400, 1050/2800 and 1650/2800 shown regular changes，which were suggested to corresponded with ν(CO)/ν(OH), ν(CO)/ν(CH) and ν(C=C)/ν(CH) mainly. These ratios were supported to reflect the amount or variation of cuticle components, such as cutin, fatty alcohols, acids and unsaturated compounds. It provided insights about how nitrogen affected cuticles and showed great potentials to utilized FTIR-PAS for detecting cuticle variations.

Apparent penetration depth in attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy of Allium cepa L. epidermis and cuticle

Over the past decades, ATR-FTIR has emerged as promising tool for the identification of plants at the genus and (sub-) species level through surface measurements of intact leaves. Theoretical considerations regarding the penetration depth of the evanescent wave into the sample and the thickness of plant leaf cuticles suggest that the structure and composition of the cuticle represent universal taxonomic markers. However, experimental evidence for this hypothesis is scarce. In the current contribution, we present results of a series of simple experiments on epidermal monolayers derived from the bulbs of Allium cepa L. (Amaryllidaceae) as a model system to study the effect of an IR active probe located beyond the theoretical penetration depth of the evanescent wave. We found that this probe had a significant influence on the ATR-FTIR spectra for up to 4 epidermal layers stacked on top of each other corresponding to a total thickness of around 60 μm, exceeding the theoretical penetration depth of the evanescent wave by a factor of around 20. Altogether, our data indicate a major discrepancy between theory and practice in ATR-FTIR spectroscopy in general and provide strong evidence that in general plant leaf spectra cannot be fully explained by the structure and composition of the cuticle alone.

A simple technique for assessing the cuticular diffusion of humic acid biostimulants

BACKGROUND

Experimental determination of the extent and rate of transport of liquid humates supplied to plants is critical in testing physiological effects of such biostimulants which are often supplied as foliar sprays. Therefore, an original experimental method for the qualitative investigation and quantitative description of the penetration of humates through plant cuticles is proposed, tested, and evaluated.

RESULTS

The proposed method involves the isolation of model plant leaf cuticles and the subsequent in vitro evaluation of cuticular humate transport. The employed novel methodology is based on a simple diffusion couple arrangement involving continuous spectrophotometric determination of the amount of penetrated humate in a hydrogel diffusion medium. Prunus laurocerasus leaf cuticles were isolated by chemical and enzymatic treatment and the rate of cuticular penetration of a commercial humate (lignohumate) was estimated over time in quantitative and qualitative terms. Different rates of lignohumate transport were determined for abaxial and adaxial leaf cuticles also in relation to the different cuticular extraction methods tested.

CONCLUSIONS

The proposed methodology represents a simple and cheap experimental tool for the study on the trans-cuticular penetration of humic-based biostimulants.

Genotypic and heat stress effects on leaf cuticles of field pea using ATR-FTIR spectroscopy

MAIN CONCLUSION

ATR-FTIR spectroscopy in combination with uni- and multivariate analysis was used to quantify the spectral-chemical composition of the leaf cuticle of pea, investigating the effects of variety and heat stress. Field pea (Pisum sativum L.) is sensitive to heat stress and our goal was to improve canopy cooling and flower retention by investigating the protective role of lipid-related compounds in leaf cuticle, and to use results in the future to identify heat resistant genotypes. The objective was to use Attenuated Total Reflection (ATR)-Fourier Transform Infrared (FTIR) spectroscopy, a non-invasive technique, to investigate and quantify changes in adaxial cuticles of fresh leaves of pea varieties that were subjected to heat stress. Eleven varieties were grown under control (24/18 °C day/night) and heat stress conditions (35/18 °C day/night, for 5 days at the early flowering stage). These 11 had significant spectral differences in the integrated area of the main lipid region, CH2 region, CH3 peak, asymmetric and symmetric CH2 peaks, ester carbonyl peak, and the peak area ratio of CH2 to CH3 and ester carbonyl to CH2 asymmetric peak, indicating that cuticles had spectral-chemical diversity of waxes, cutin, and polysaccharides. Results indicated considerable diversity in spectral-chemical makeup of leaf cuticles within commercially available field pea varieties and they responded differently to high growth temperature, revealing their diverse potential to resist heat stress. The ATR-FTIR spectral technique can, therefore, be further used as a medium-throughput approach for rapid screening of superior cultivars for heat tolerance.

Epicuticular wax on leaf cuticles does not establish the transpiration barrier, which is essentially formed by intracuticular wax

It is well established that waxes built up the barrier properties of cuticles, since their extraction in organic solvent e.g. chloroform increases diffusion of water and organic compounds by 1-2 orders of magnitude. Leaf surface waxes can be divided in epicuticular (on the surface of the cuticular membrane) and intracuticular (embedded in the cutin polymer) waxes. Until today there are only limited investigations dealing with the question to what extent epi- or intracuticular waxes contribute to the formation of the transpiration barrier. For Prunus laurocerasus previous studies have shown that epicuticular waxes do not contribute to the formation of the transpiration barrier. This approach successfully established for P. laurocerasus was applied to further species in order to check whether this finding also applies to a broader spectrum of species. Epicuticular wax was mechanically removed using collodion from the surface of either isolated cuticular membranes or intact leaf discs of ten further plant species differing in total wax amounts, wax compositions and transport properties. Scanning electron microscopy, which was performed to independently verify the successful removal of the surface waxes, indicated that two consecutive treatments with collodion were sufficient for a complete removal of epicuticular wax. The treated surfaces appeared smooth after removal. The total wax amounts removed with the two collodion treatments and the residual amount of waxes after collodion treatment were quantified by gas chromatography and mass spectrometry. This showed that epicuticular waxes essentially consisted of long-chain aliphatic molecules (e.g. alkanes, primary alcohols, fatty acids), whereas intracuticular wax was composed of both, triterpenoids and long-chain aliphatic molecules. Cuticular transpiration using combined replicates was measured before and after removal of surface wax. Results clearly indicated that two consecutive collodion treatments, or the corresponding solvent treatments (diethyl ether:ethanol) serving as control, did not increase cuticular transpiration of the ten further leaf species investigated. Our results lead to the conclusion that epicuticular wax does not contribute to the formation of the transpiration barrier of leaves.

Application of wide selected-ion monitoring data-independent acquisition to identify tomato fruit proteins regulated by the CUTIN DEFICIENT2 transcription factor

We describe here the use of label-free wide selected-ion monitoring data-independent acquisition (WiSIM-DIA) to identify proteins that are involved in the formation of tomato (Solanum lycopersicum) fruit cuticles and that are regulated by the transcription factor CUTIN DEFICIENT2 (CD2). A spectral library consisting of 11 753 unique peptides, corresponding to 2338 tomato protein groups, was used and the DIA analysis was performed at the MS1 level utilizing narrow mass windows for extraction with Skyline 2.6 software. We identified a total of 1140 proteins, 67 of which had expression levels that differed significantly between the cd2 tomato mutant and the wild-type cultivar M82. Differentially expressed proteins including a key protein involved in cutin biosynthesis, were selected for validation by target SRM/MRM and by Western blot analysis. In addition to confirming a role for CD2 in regulating cuticle formation, the results also revealed that CD2 influences pathways associated with cell wall biology, anthocyanin biosynthesis, plant development, and responses to stress, which complements findings of earlier RNA-Seq experiments. Our results provide new insights into molecular processes and aspects of fruit biology associated with CD2 function, and demonstrate that the WiSIM-DIA is an effective quantitative approach for global protein identifications.

Epicuticular wax on cherry laurel (Prunus laurocerasus) leaves does not constitute the cuticular transpiration barrier

Epicuticular wax of cherry laurel does not contribute to the formation of the cuticular transpiration barrier, which must be established by intracuticular wax. Barrier properties of cuticles are established by cuticular wax deposited on the outer surface of the cuticle (epicuticular wax) and in the cutin polymer (intracuticular wax). It is still an open question to what extent epi- and/or intracuticular waxes contribute to the formation of the transpiration barrier. Epicuticular wax was mechanically removed from the surfaces of isolated cuticles and intact leaf disks of cherry laurel (Prunus laurocerasus L.) by stripping with different polymers (collodion, cellulose acetate and gum arabic). Scanning electron microscopy showed that two consecutive treatments with all three polymers were sufficient to completely remove epicuticular wax since wax platelets disappeared and cuticle surfaces appeared smooth. Waxes in consecutive polymer strips and wax remaining in the cuticle after treatment with the polymers were determined by gas chromatography. This confirmed that two treatments of the polymers were sufficient for selectively removing epicuticular wax. Water permeability of isolated cuticles and cuticles covering intact leaf disks was measured using (3)H-labelled water before and after selectively removing epicuticular wax. Cellulose acetate and its solvent acetone led to a significant increase of cuticular permeability, indicating that the organic solvent acetone affected the cuticular transpiration barrier. However, permeability did not change after two subsequent treatments with collodion and gum arabic or after treatment with the corresponding solvents (diethyl ether:ethanol or water). Thus, in the case of P. laurocerasus the epicuticular wax does not significantly contribute to the formation of the cuticular transpiration barrier, which evidently must be established by the intracuticular wax.

Water loss from litchi (Litchi chinensis) and longan (Dimocarpus longan) fruits is biphasic and controlled by a complex pericarpal transpiration barrier

In litchi and longan fruits, a specialised pericarp controls water loss by a protective system consisting of two resistances in series and two water reservoirs separated by a barrier. In the fruits of litchi (Litchi chinensis) and longan (Dimocarpus longan), the pericarp is solely a protective structure lacking functional stomata and completely enclosing the aril that is the edible part. Maintaining a high water content of the fruits is crucial for ensuring the economic value of these important fruit crops. The water loss rates from mature fruits were determined and analysed in terms of the properties of the pericarps. Water loss kinetics and sorption isotherms were measured gravimetrically. The pericarps were studied with microscopy, and cuticular waxes and cutin were analysed with gas chromatography and mass spectrometry. The kinetics of fruit water loss are biphasic with a high initial rate and a lower equilibrium rate lasting for many hours. The outer and inner surfaces of the pericarps are covered with cuticles. Litchi and longan fruits have a unique type of transpiration barrier consisting of two resistances in series (endo- and exocarp cuticles) and two reservoirs of water (aril and mesocarp). The exocarp permeability controls the water loss from fresh fruits while in fruits kept for an extended time at low relative humidity it is determined by the endo- and exocarp permeabilities. Permeances measured are within the range for typical fruit cuticles. The findings may be used to design optimal postharvest storage strategies for litchi and longan fruits.