The Complex Architecture of Plant Cuticles and Its Relation to Multiple Biological Functions

Formulated hydroxy fatty acids from fruit pomaces reduce apple scab development caused by Venturia inaequalis through a dual mode of action

The outermost hydrophobic layer of plants, i.e. the cuticle, is mainly composed of cutin, a polyester of hydroxy fatty acids with reported eliciting and/or antimicrobial activities for some of them. By-products of the fruit processing industry (fruit pomaces), often strongly enriched in cuticular material, are therefore a potential source of bioactive compounds for crop protection against pathogen attack. We investigated the utilization of tomato and apple pomaces in the development of a cutin-based biocontrol solution against apple scab, a major apple disease caused by Venturia inaequalis. Several cutin monomer extracts obtained through different strategies of depolymerization and purification were first compared for their ability to induce a targeted set of defense genes in apple seedlings after foliar application. After a step of formulation, some extracts were chosen for further investigation in planta and in vitro. Our results show that formulated cutin monomers could trigger a significant transcriptome reprogramming in apple plants and exhibit an antifungal effect on V. inaequalis. Cutin monomers-treated apple seedlings were significantly protected against infection by the apple scab agent. Altogether, our findings suggest that water-dispersed cutin monomers extracted from pomaces are potential new bio-based solutions for the control of apple scab.

Cuticle architecture and mechanical properties: a functional relationship delineated through correlated multimodal imaging

Cuticles are multifunctional hydrophobic biocomposites that protect the aerial organs of plants. During plant development, plant cuticles must accommodate different mechanical constraints combining extensibility and stiffness, and the corresponding relationships with their architecture are unknown. Recent data showed a fine-tuning of cuticle architecture during fruit development, with several chemical clusters which raise the question of how they impact the mechanical properties of cuticles. We investigated the in-depth nanomechanical properties of tomato (Solanum lycopersicum) fruit cuticle from early development to ripening, in relation to chemical and structural heterogeneities by developing a correlative multimodal imaging approach. Unprecedented sharps heterogeneities were evidenced including an in-depth mechanical gradient and a 'soft' central furrow that were maintained throughout the plant development despite the overall increase in elastic modulus. In addition, we demonstrated that these local mechanical areas are correlated to chemical and structural gradients. This study shed light on fine-tuning of mechanical properties of cuticles through the modulation of their architecture, providing new insight for our understanding of structure-function relationships of plant cuticles and for the design of bioinspired material.

Direct surface analysis mass spectrometry uncovers the vertical distribution of cuticle-associated metabolites in plants

The plant cuticle covers the plant's entire aerial surface and acts as the outermost protective layer. Despite being crucial for the survival of plants, surprisingly little is known about its biosynthesis. Conventional analytical techniques are limited to the isolation and depolymerization of the polyester cutin, which forms the cuticular scaffold. Although this approach allows the elucidation of incorporated cutin monomers, it neglects unincorporated metabolites participating in cutin polymerization. The feasibility of a novel approach is tested for in situ analysis of unpolymerized cuticular metabolites to enhance the understanding of cuticle biology. Intact cotyledons of Brassica napus and Arabidopsis thaliana seedlings are immersed in organic solvents for 60 seconds. Extracts are analyzed using high-resolution direct infusion mass spectrometry. A variety of different diffusion routes of plant metabolites across the cuticle are discussed. The results reveal different feasibilities depending on the research question and cuticle permeabilities in combination with the analyte's polarity. Especially hydrophilic analytes are expected to be co-located in the cell wall beneath the cuticle causing systematic interferences when comparing plants with different cuticle permeabilities. These interferences limit data interpretation to qualitative rather than quantitative comparison. In contrast, quantitative data evaluation is facilitated when analyzing cuticle-specific metabolites or plants with similar cuticle permeabilities.

Natural variation and drought-induced differences in metabolite profiles of red oak-leaf and Romaine lettuce play a role in modulating the interaction with Salmonella enterica

Nutrients on produce surfaces are vital for successful enteric pathogen colonisation. In this study, we investigated natural variation in metabolite profiles of Romaine 'Parris Island Cos' and red oak-leaf lettuce 'Mascara' under regular and restricted watering conditions. We also investigated the impact of plant drought stress on the Salmonella - lettuce association. Salmonella Newport and Typhimurium were able to persist at higher levels on regularly watered Romaine than red oak-leaf lettuce. Drought treatment to lettuce impaired epiphytic Salmonella association, with S. Newport and Typhimurium being differentially affected. A higher log reduction of both serotypes was measured on drought-subjected red oak-leaf lettuce plants than controls, but S. Typhimurium was unaffected on water deficit-treated Romaine lettuce (p < 0.05). To assess Salmonella interaction with leaf surface metabolites, leaf washes collected from both cultivars were inoculated and found to be able to support S. Newport growth, with higher levels of Salmonella retrieved from Romaine washes (p < 0.05). The lag phase of S. Newport in washes from water restricted red oak-leaf lettuce was prolonged in relation to regularly-watered controls (p < 0.05). Untargeted plant metabolite profiling using electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) revealed natural variation between Romaine and red oak-leaf lettuce profiles for leaf tissue and leaf washes. Metabolite profile shifts were detected in both lettuce types in response to drought stress, but more unique peaks were detected in red oak-leaf than Romaine lettuce after drought treatment. Variation between the two cultivars was in part attributed to naturally higher levels of flavonoids and anthocyanins in red oak-leaf lettuce compared to Romaine. Moreover, red oak-leaf, but not Romaine lettuce, responded to drought by inducing the accumulation of proline, phenolics, flavonoids and anthocyanins. Drought stress, therefore, enhanced the functional food properties of red oak-leaf lettuce. Salmonella growth dynamics in lettuce leaf washes suggested that natural variation and drought-induced changes in metabolite profiles in lettuce could partly explain the differential susceptibility of various lettuce types to Salmonella, although the primary or secondary metabolites mediating this effect remain unknown. Regulated mild water stress should be investigated as an approach to lower Salmonella contamination risk in suitable lettuce cultivars, while simultaneously boosting the health beneficial quality of lettuce.

3D (x-y-t) Raman imaging of tomato fruit cuticle: Microchemistry during development

The cuticle is a protective extracellular matrix that covers the above-ground epidermis of land plants. Here, we studied the cuticle of tomato (Solanum lycopersicum L.) fruits in situ using confocal Raman microscopy. Microsections from cuticles isolated at different developmental stages were scanned to visualize cuticle components with a spatial resolution of 342 nm by univariate and multivariate data analysis. Three main components, cutin, polysaccharides, and aromatics, were identified, with the latter exhibiting the strongest Raman scattering intensity. Phenolic acids and flavonoids were differentiated within the cuticle, and three schematic cuticle models were identified during development. Phenolic acids were found across the entire cuticle at the earliest stage of development, i.e. during the formation of the procuticle layer. Based on a mixture analysis with reference component spectra, the phenolic acids were identified as mainly esterified p-coumaric acid together with free p-hydroxybenzoic acid. During the cell expansion period of growth, phenolic acids accumulated in an outermost layer of the cuticle and in the middle region of the pegs. In these stages of development, cellulose and pectin were detected next to the inner cuticle region, close to the epidermal cell where flavonoid impregnation started during ripening. In the first ripening stage, chalconaringenin was observed, while methoxylated chalcones were chosen by the algorithm to fit the mature cuticle spectra. The colocation of carbohydrates, esterified p-coumaric acid, and methoxylated chalconaringenin suggests that the latter two link polysaccharide and cutin domains. Elucidating the different distribution of aromatics within the cuticle, suggests important functions: (1) overall impregnation conferring mechanical and thermal functions (2) the outermost phenolic acid layer displaying UV-B protection of the plant tissue.

Bricks out of the wall: polysaccharide extramural functions

Plant polysaccharides are components of plant cell walls and/or store energy. However, this oversimplified classification neglects the fact that some cell wall polysaccharides and glycoproteins can localize outside the relatively sharp boundaries of the apoplastic moiety, where they adopt functions not directly related to the cell wall. Such polysaccharide multifunctionality (or 'moonlighting') is overlooked in current research, and in most cases the underlying mechanisms that give rise to unconventional ex muro trafficking, targeting, and functions of polysaccharides and glycoproteins remain elusive. This review highlights major examples of the extramural occurrence of various glycan cell wall components, discusses the possible significance and implications of these phenomena for plant physiology, and lists exciting open questions to be addressed by future research.

Exploring and exploiting cuticle biosynthesis for abiotic and biotic stress tolerance in wheat and barley

Wheat and barley are widely distributed cereal crops whose yields are adversely affected by environmental stresses such as drought, salinity, extreme temperatures, and attacks of pathogens and pests. As the interphase between aerial plant organs and their environments, hydrophobic cuticle largely consists of a cutin matrix impregnated and sealed with cuticular waxes. Increasing evidence supports that the cuticle plays a key role in plant adaptation to abiotic and biotic stresses, which could be harnessed for wheat and barley improvement. In this review, we highlighted recent advances in cuticle biosynthesis and its multifaceted roles in abiotic and biotic stress tolerance of wheat and barley. Current strategies, challenges, and future perspectives on manipulating cuticle biosynthesis for abiotic and biotic stress tolerance in wheat and barley are discussed.

The cutin polymer matrix undergoes a fine architectural tuning from early tomato fruit development to ripening

The cuticle is a complex polymer matrix that protects all aerial organs of plants, fulfills multiple roles in plant-environment interactions, and is critical for plant development. These functions are associated with the structural features of cuticles, and the architectural modeling of cuticles during plant development is crucial for understanding their physical properties and biological functions. In this work, the in-depth architecture of the cutin polymer matrix during fruit development was investigated. Using cherry tomato fruit (Solanum lycopersicum) as a model from the beginning of the cell expansion phase to the red ripe stage, we designed an experimental scheme combining sample pretreatment, Raman mapping, multivariate data analyses, and biochemical analyses. These approaches revealed clear chemical areas with different contributions of cutin, polysaccharides, and phenolics within the cutin polymer matrix. Besides, we demonstrated that these areas are finely tuned during fruit development, including compositional and macromolecular rearrangements. The specific spatiotemporal accumulation of phenolic compounds (p-coumaric acid and flavonoids) suggests that they fulfill distinct functions during fruit development. In addition, we highlighted an unexpected dynamic remodeling of the cutin-embedded polysaccharides pectin, cellulose, and hemicellulose. Such structural tuning enables consistent adaption of the cutin-polysaccharide continuum and the functional performance of the fruit cuticle at the different developmental stages. This study provides insights into the plant cuticle architecture and in particular into the organization of the epidermal cell wall-cuticle.

Valorization of Agro-Industrial Wastes by Ultrasound-Assisted Extraction as a Source of Proteins, Antioxidants and Cutin: A Cascade Approach

The use of agro-industrial wastes to obtain compounds with a high added-value is increasing in the last few years in accordance with the circular economy concept. In this work, a cascade extraction approach was developed based on ultrasound-assisted extraction (UAE) for tomato, watermelon, and apple peel wastes. The protein and antioxidant compounds were obtained during the first extraction step (NaOH 3 wt.%, 98.6 W, 100% amplitude, 6.48 W/cm², 6 min). The watermelon peels (WP) showed higher proteins and total phenolic contents (857 ± 1 mg BSA/g extract and 107.2 ± 0.2 mg GAE/100 g dm, respectively), whereas the highest antioxidant activity was obtained for apple peels (1559 ± 20 µmol TE/100 g dm, 1767 ± 5 µmol TE/100 g dm, and 902 ± 16 µmol TE/100 g dm for ABTS, FRAP and DPPH assays, respectively). The remaining residue obtained from the first extraction was subsequently extracted to obtain cutin (ethanol 40 wt.%, 58 W, 100% amplitude, 2 W/cm², 17 min, 1/80 g/mL, pH 2.5). The morphological studies confirmed the great efficiency of UAE in damaging the vegetal cell walls. WP showed a higher non-hydrolysable cutin content (55 wt.% of the initial cutin). A different monomers’ profile was obtained for the cutin composition by GC-MS, with the cutin from tomato and apple peels being rich in polyhydroxy fatty acids whereas the cutin extracted from WP was mainly based on unsaturated fatty acids. All of the cutin samples showed an initial degradation temperature higher than 200 °C, presenting an excellent thermal stability. The strategy followed in this work has proved to be an effective valorization methodology with a high scaling-up potential for applications in the food, pharmaceutical, nutraceutical, cosmetics and biopolymer sectors.

Foliar selenium fertilization alters the content of dietary phytochemicals in two rocket species

Biofortification is the process that aims to enrich crops in micronutrients and valuable compounds. Selenium (Se) biofortification has particularly attracted increasing interest in recent times due to the growing number of individuals suffering from Se deficiency. Selenate and selenite are the Se forms most frequently administered to crops. In this study, Se was applied foliarly as selenate at 2.5, 5, or 10 mg per plant to two rocket species, Diplotaxis tenuifolia and Eruca sativa, grown in soil and the effects in terms of Se enrichment and content of primary and secondary metabolites were comparatively analyzed. We also compared our results with those obtained previously when selenate was supplied to the same species in hydroponics by addition to the nutrient solution. In most cases, the results were the opposite. In E. sativa, foliar Se treatment was more effective in promoting Se accumulation, sulfur (S), cysteine, and glucosinolates. No significant effect of Se was evident on total phenolic content, but there were individual phenols. Among amino acids, the content of proline was increased by Se, perhaps to counteract osmotic stress due to high Se accumulation. In D. tenuifolia, the content of S and cysteine decreased under Se treatment, but the amount of glutathione was steady, suggesting a preferred assimilation of cysteine toward the synthesis of this antioxidant. Consistent, the content of methionine and glucosinolates was reduced. The content of total phenolics was enhanced only by the low Se dosage. In both species, selenocysteine (SeCys) was identified, the content of which was higher compared to plants grown hydroponically. Concluding, most metabolic differences between rocket species were observed at high Se supplementation. Low Se foliar fertilization was effective in an enriching rocket in Se without affecting other phytochemicals. However, the Se dosages sufficient for biofortification could be even lower, as the Se concentration in rocket treated with 2.5 mg Se per plant was still very high and the edible part should not be eaten undiluted. Also, a single method of Se supplementation does not appear to be optimal for all plant species or the same species, as the metabolic responses could be very different.

Toward a smart skin: Harnessing cuticle biosynthesis for crop adaptation to drought, salinity, temperature, and ultraviolet stress

Drought, salinity, extreme temperatures, and ultraviolet (UV) radiation are major environmental factors that adversely affect plant growth and crop production. As a protective shield covering the outer epidermal cell wall of plant aerial organs, the cuticle is mainly composed of cutin matrix impregnated and sealed with cuticular waxes, and greatly contributes to the plant adaption to environmental stresses. Past decades have seen considerable progress in uncovering the molecular mechanism of plant cutin and cuticular wax biosynthesis, as well as their important roles in plant stress adaptation, which provides a new direction to drive strategies for stress-resilient crop breeding. In this review, we highlighted the recent advances in cuticle biosynthesis in plant adaptation to drought, salinity, extreme temperatures, and UV radiation stress, and discussed the current status and future directions in harnessing cuticle biosynthesis for crop improvement.

The SlSHN2 transcription factor contributes to cuticle formation and epidermal patterning in tomato fruit

Tomato (Solanum lycopersicum) is an established model for studying plant cuticle because of its thick cuticle covering and embedding the epidermal cells of the fruit. In this study, we screened an EMS mutant collection of the miniature tomato cultivar Micro-Tom for fruit cracking mutants and found a mutant displaying a glossy fruit phenotype. By using an established mapping-by-sequencing strategy, we identified the causal mutation in the SlSHN2 transcription factor that is specifically expressed in outer epidermis of growing fruit. The point mutation in the shn2 mutant introduces a K to N amino acid change in the highly conserved 'mm' domain of SHN proteins. The cuticle from shn2 fruit showed a ~ fivefold reduction in cutin while abundance and composition of waxes were barely affected. In addition to alterations in cuticle thickness and properties, epidermal patterning and polysaccharide composition of the cuticle were changed. RNAseq analysis further highlighted the altered expression of hundreds of genes in the fruit exocarp of shn2, including genes associated with cuticle and cell wall formation, hormone signaling and response, and transcriptional regulation. In conclusion, we showed that a point mutation in the transcriptional regulator SlSHN2 causes major changes in fruit cuticle formation and its coordination with epidermal patterning.