Surface lipids and plant defenses

Exploring lipid signaling in plant physiology: From cellular membranes to environmental adaptation

Lipids have evolved as versatile signaling molecules that regulate a variety of physiological processes in plants. Convincing evidence highlights their critical role as mediators in a wide range of plant processes required for survival, growth, development, and responses to environmental conditions such as water availability, temperature changes, salt, pests, and diseases. Understanding lipid signaling as a critical process has helped us expand our understanding of plant biology by explaining how plants sense and respond to environmental cues. Lipid signaling pathways constitute a complex network of lipids, enzymes, and receptors that coordinate important cellular responses and stressing plant biology's changing and adaptable traits. Plant lipid signaling involves a wide range of lipid classes, including phospholipids, sphingolipids, oxylipins, and sterols, each of which contributes differently to cellular communication and control. These lipids function not only as structural components, but also as bioactive molecules that transfer signals. The mechanisms entail the production of lipid mediators and their detection by particular receptors, which frequently trigger downstream cascades that affect gene expression, cellular functions, and overall plant growth. This review looks into lipid signaling in plant physiology, giving an in-depth look and emphasizing its critical function as a master regulator of vital activities.

Unveiling physiological responses and modulated accumulation patterns of specialized metabolites in Mentha rotundifolia acclimated to sub-tropical environment

Mints are aromatic plants of Lamiaceae, globally known for the phytochemical-rich essential oils. Most of the cultivated mints are menthol-rich, whereas spearmint being the only dominant carvone-rich species. In this study, another carvone-rich mint Mentha rotundifolia (L.) Huds., a native of temperate region was assessed for its acclimation in sub-tropical environment to see any possible changes in specialized metabolite accumulation. Plants grown under open environment was compared with glasshouse grown plants where, temperature, humidity and photoperiods were uniformly maintained. Thickened leaves with increased cuticular wax load (2.82 folds) and anthocyanin accumulation (202.97 µg/g) in the widened stems were observed in plants grown in open environment, while higher chlorophyll contents were exhibited by the glasshouse-grown plants. Enhanced antioxidant capacity in open environment, correlated with elevated concentration (86.4% increase for caffeic acid) of wall-bound phenolics was observed. Increased proline, hydrogen peroxide and malondialdehyde contents in open environment indicated the plant's ability to cope up with abiotic stress. Higher amounts of terpenes like (-)-carvone (2.68 folds) and D-limonene (1.35 folds) were found in both internal volatile pool and essential oil of glasshouse-grown plants. Histochemical study of glandular trichomes also supported this finding. In conclusion, glasshouse-grown plants showed relatively better growth and higher terpene contents, nevertheless the plant survived well in warmer environment, with increased antioxidant capacities and phenolic contents. Future study includes mass propagation of this species in different geographical locations with distinct climatic variations to determine the suitable sub-tropical locations for cultivation as a potential alternative to spearmint for commercial-scale (-)-carvone production.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01489-8.

Genome-Wide Transcriptomic and Metabolomic Analyses Unveiling the Defence Mechanisms of Populus tremula against Sucking and Chewing Insect Herbivores

Plants and insects coevolved as an evolutionarily successful and enduring association. The molecular arms race led to evolutionary novelties regarding unique mechanisms of defence and detoxification in plants and insects. While insects adopt mechanisms to conquer host defence, trees develop well-orchestrated and species-specific defence strategies against insect herbivory. However, current knowledge on the molecular underpinnings of fine-tuned tree defence responses against different herbivore insects is still restricted. In the current study, using a multi-omics approach, we unveiled the defence response of Populus tremula against aphids (Chaitophorus populialbae) and spongy moths (Lymantria dispar) herbivory. Comparative differential gene expression (DGE) analyses revealed that around 272 and 1203 transcripts were differentially regulated in P. tremula after moth and aphid herbivory compared to uninfested controls. Interestingly, 5716 transcripts were differentially regulated in P. tremula between aphids and moth infestation. Further investigation showed that defence-related stress hormones and their lipid precursors, transcription factors, and signalling molecules were over-expressed, whereas the growth-related counterparts were suppressed in P. tremula after aphid and moth herbivory. Metabolomics analysis documented that around 37% of all significantly abundant metabolites were associated with biochemical pathways related to tree growth and defence. However, the metabolic profiles of aphid and moth-fed trees were quite distinct, indicating species-specific response optimization. After identifying the suitable reference genes in P. tremula, the omics data were further validated using RT-qPCR. Nevertheless, our findings documented species-specific fine-tuning of the defence response of P. tremula, showing conservation on resource allocation for defence overgrowth under aphid and moth herbivory. Such findings can be exploited to enhance our current understanding of molecular orchestration of tree responses against herbivory and aid in developing insect pest resistance P. tremula varieties.

Plant Immunity: At the Crossroads of Pathogen Perception and Defense Response

Plants are challenged by different microbial pathogens that affect their growth and productivity. However, to defend pathogen attack, plants use diverse immune responses, such as pattern-triggered immunity (PTI), effector-triggered immunity (ETI), RNA silencing and autophagy, which are intricate and regulated by diverse signaling cascades. Pattern-recognition receptors (PRRs) and nucleotide-binding leucine-rich repeat (NLR) receptors are the hallmarks of plant innate immunity because they can detect pathogen or related immunogenic signals and trigger series of immune signaling cascades at different cellular compartments. In plants, most commonly, PRRs are receptor-like kinases (RLKs) and receptor-like proteins (RLPs) that function as a first layer of inducible defense. In this review, we provide an update on how plants sense pathogens, microbe-associated molecular patterns (PAMPs or MAMPs), and effectors as a danger signals and activate different immune responses like PTI and ETI. Further, we discuss the role RNA silencing, autophagy, and systemic acquired resistance as a versatile host defense response against pathogens. We also discuss early biochemical signaling events such as calcium (Ca2+), reactive oxygen species (ROS), and hormones that trigger the activation of different plant immune responses. This review also highlights the impact of climate-driven environmental factors on host-pathogen interactions.

Evaluation of Lipid Quality in Fruit: Utilizing Lipidomic Approaches for Assessing the Impact of Biotic Stress on Pecans (Carya illinoinensis)

There is a scarcity of data on how the lipid composition of oily seeds changes in response to biotic stress. Yellow peach moth (Conogethes punctiferalis) has caused massive economic losses on the pecan (Carya illinoinensis) industry. Lipidomics is used in this study to determine the lipid composition of pecan and how it changes in response to insect attack. Pecan had 167 lipids, including 34 glycerolipids (GL), 62 glycerophospholipids (GP), 17 fatty acyls (FA), 41 sphingolipids (SP), and 13 saccharolipids (SL). The effects of biotic stress on lipids, particularly GL and GP, were significant. Biotic stress significantly reduced the lipid content of chains longer than 48. Forty-four significantly different lipids were discovered as potential biomarkers for distinguishing non-infected pecans from infested pecans. In addition, we used bioinformatics to identify the five most important metabolic pathways in order to investigate the processes underlying the changes. Our discoveries may offer valuable insights for enhancing pecan production in the future and contribute novel perspectives towards enhancing the nutritional value of pecans.

Integrated Transcriptome and Metabolome Analysis Reveals the Molecular Mechanism of Rust Resistance in Resistant (Youkang) and Susceptive (Tengjiao) Zanthoxylum armatum Cultivars

Chinese pepper rust is a live parasitic fungal disease caused by Coleosporium zanthoxyli, which seriously affects the cultivation and industrial development of Z. armatum. Cultivating and planting resistant cultivars is considered the most economical and environmentally friendly strategy to control this disease. Therefore, the mining of excellent genes for rust resistance and the analysis of the mechanism of rust resistance are the key strategies to achieve the targeted breeding of rust resistance. However, there is no relevant report on pepper rust resistance at present. The aim of the present study was to further explore the resistance mechanism of pepper by screening the rust-resistant germplasm resources in the early stage. Combined with the analysis of plant pathology, transcriptomics, and metabolomics, we found that compared with susceptible cultivar TJ, resistant cultivar YK had 2752 differentially expressed genes (DEGs, 1253 up-, and 1499 downregulated) and 321 differentially accumulated metabolites (DAMs, 133 up- and 188 down-accumulated) after pathogen infection. And the genes and metabolites related to phenylpropanoid metabolism were highly enriched in resistant varieties, which indicated that phenylpropanoid metabolism might mediate the resistance of Z. armatum. This finding was further confirmed by a real-time quantitative polymerase chain reaction analysis, which revealed that the expression levels of core genes involved in phenylpropane metabolism in disease-resistant varieties were high. In addition, the difference in flavonoid and MeJA contents in the leaves between resistant and susceptible varieties further supported the conclusion that the flavonoid pathway and methyl jasmonate may be involved in the formation of Chinese pepper resistance. Our research results not only help to better understand the resistance mechanism of Z. armatum rust but also contribute to the breeding and utilization of resistant varieties.

GDSL Esterase/Lipase GELP1 Involved in the Defense of Apple Leaves against Colletotrichum gloeosporioides Infection

GDSL esterases/lipases are a subclass of lipolytic enzymes that play critical roles in plant growth and development, stress response, and pathogen defense. However, the GDSL esterase/lipase genes involved in the pathogen response of apple remain to be identified and characterized. Thus, in this study, we aimed to analyze the phenotypic difference between the resistant variety, Fuji, and susceptible variety, Gala, during infection with C. gloeosporioides, screen for anti-disease-associated proteins in Fuji leaves, and elucidate the underlying mechanisms. The results showed that GDSL esterase/lipase protein GELP1 contributed to C. gloeosporioides infection defense in apple. During C. gloeosporioides infection, GELP1 expression was significantly upregulated in Fuji. Fuji leaves exhibited a highly resistant phenotype compared with Gala leaves. The formation of infection hyphae of C. gloeosporioides was inhibited in Fuji. Moreover, recombinant His:GELP1 protein suppressed hyphal formation during infection in vitro. Transient expression in Nicotiana benthamiana showed that GELP1-eGFP localized to the endoplasmic reticulum and chloroplasts. GELP1 overexpression in GL-3 plants increased resistance to C. gloeosporioides. MdWRKY15 expression was upregulated in the transgenic lines. Notably, GELP1 transcript levels were elevated in GL-3 after salicylic acid treatment. These results suggest that GELP1 increases apple resistance to C. gloeosporioides by indirectly regulating salicylic acid biosynthesis.

Use of Arabidopsis thaliana as a model to understand specific carcinogenic events: Comparison of the molecular machinery associated with cancer-hallmarks in plants and humans

Model organisms are fundamental in cancer research given that they rise the possibility to characterize in a quantitative-objective fashion the organisms as a whole in ways that are infeasible in humans. From this perspective, model organisms with short generation times and established protocols for genetic manipulation allow the understanding of basic biology principles that might guide carcinogenic onset. The cancer-hallmarks (CHs) approach, a modular perspective for cancer understanding, stands that underlying the variability among different cancer types, critical events support the carcinogenic origin and progression. Thus, CHs as interconnected genetic circuitry, have a causal effect over cancer biogenesis and might represent a comparison scaffold among model organisms to identify and characterize evolutionarily conserved modules to understand cancer. Nevertheless, the identification of novel cancer regulators by comparative genomics approaches relies on selecting specific biological processes or related signaling cascades that limit the type of detected regulators, even more, holistic analysis from a systemic perspective is absent. Similarly, although the plant Arabidopsis thaliana has been used as a model organism to dissect specific disease-associated mechanisms, given the evolutionary distance between plants and humans, a general concern about the utility of using A. thaliana as a cancer model persists. In the present research, we take advantage of the CHs paradigm as a framework to establish a functional systemic comparison between plants and humans, that allowed the identification not only of specific novel key genetic regulators, but also, biological processes, metabolic systems, and genetic modules that might contribute to the neoplastic transformation. We propose five cancer-hallmarks that overlapped in conserved mechanisms and processes between Arabidopsis and human and thus, represent mechanisms which study can be prioritized in A. thaliana as an alternative model for cancer research. Additionally, derived from network analyses and machine learning strategies, a new set of potential candidate genes that might contribute to neoplastic transformation is described. These findings postulate A. thaliana as a suitable model to dissect, not all, but specific cancer properties, highlighting the importance of using alternative complementary models to understand carcinogenesis.

SHINE clade of ERF transcription factors: A significant player in abiotic and biotic stress tolerance in plants

SHINE (SHN) clade transcription factors (TFs) represents a subfamily of APETALA2/ethylene-responsive factor (AP2/ERF) proteins. The latter, is characterized by its responsiveness to the phytohormone ethylene and the presence of AP2 DNA-binding domain. They are involved in many biological processes and in responses to different environmental constraints. SHN TFs were among the first identified regulators of cuticle formation. Cuticle plays crucial role in plant tolerance to drought, salinity and high temperature as well as in defense against pathogens. In addition, SHN were shown to be involved in the regulation of stomatal development which influences resistance to drought and diseases. Interestingly, recent studies have also shown that SHN TFs are involved in mediating the beneficial effects of arbuscular mycorrhizal fungi (AMF) as well as disease resistance conferred by nanoparticles. To fulfill their roles, SHN TFs are controlled upstream by other TFs and they control, in their turn, different downstream genes. In this review, we highlight the role of SHN TFs in different abiotic and biotic stresses through their involvement in cuticle biosynthesis, stomatal development and molecular regulation of biochemical and physiological traits. In addition, we discuss the regulation of SHN TFs by plant hormones and their influence on hormone biosynthesis and signaling pathways. Knowledge of this complex regulation can be put into contribution to increase multiple abiotic stress tolerances through transgenesis, gene editing and classical breeding.

Genome-Wide Characterization and Functional Analysis of ABCG Subfamily Reveal Its Role in Cutin Formation in Cotton

ATP-binding cassette transporter G (ABCG) has been shown to be engaged in export of broad-spectrum compounds with structural differences, but little is known concerning its role in cutin formation of cotton (Gossypium spp.). In this study, we conduct a genome-wide survey and detected 69, 71, 124 and 131 ABCG genes within G. arboretum, G. raimondii, G. hirsutum and G. barbadense, separately. The above ABCGs could be divided into four groups (Ia, Ib, Ic, II). Some ABCG genes such as GhABCG15, whose homologous gene transports cuticular lipid in Arabidopsis, was preferentially expressed in the development of fiber. A weighted gene co-expression network analysis (WGCNA) demonstrated that GhABCG expression was significantly associated with the amount of 16-Hydroxypalmitate (a main component of cutin precursor) in cotton fibers. Further, silencing of GhABCG15 by virus-induced gene silencing (VIGS) in cotton generated brightened and crinkled leaves as well as reduced thickness of cuticle and increased permeability. Chemical composition analysis showed the cutin content in GhABCG15-silenced leaves had decreased while the wax content had increased. Our results provide an insight for better understanding of the role of the Gossypium ABCG family and revealed the essential role of GhABCGs in cotton cutin formation.

Cuticle structure and chemical composition of waxes in Phaeoceros laevis (L.) Prosk (Notothyladaceae, Anthocerotophyta)

The development of a hydrophobic cuticle covering the epidermis was a crucial evolutionary novelty ensuring the establishment of land plants. However, there is little information about its structure and chemical composition, as well as its functional implications in avascular lineages such as Anthocerotophyta. The main goal of the present study was to compare the gametophyte and sporophyte cuticles of Phaeoceros laevis. Semithin sections were analyzed through light microscopy (LM), cuticle structure was evaluated by transmission electron microscopy (TEM) and epicuticular wax morphology was analyzed by scanning electron microscopy (SEM). Total waxes were analyzed by CG/MS, and the components were identified based on the mass spectra. A thin lipophilic layer was detected on the sporophyte surface, structured as a stratified cuticular layer, similar to the well-known structure described for vascular plants. On the other hand, the gametophyte cuticle was observed only with TEM as a thin osmiophilic layer. SEM analyses showed a film-type wax on the surface of both life phases. The wax layer was eight-fold thicker on the sporophyte (0.8 µg cm^-2) than on gametophyte (0.1 µg cm^-2). Possible mechanical and/or drought protection are discussed. Fatty acids, primary alcohols, and steroids were identified in both life phases, while the kauren-16-ene diterpene (3%) was detected only on the sporophyte. Although no alkanes were detected in P. laevis, our findings unveil great similarity of the sporophyte cuticle of this hornwort species with the general data described for vascular plants, reinforcing the conservative condition of this character and supporting the previous idea that the biosynthetic machinery involved in the synthesis of wax compounds is conserved since the ancestor of land plants.

ALDH2C4 regulates cuticle thickness and reduces water loss to promote drought tolerance

In Arabidopsis thaliana, ALDH2C4 encodes coniferaldehyde dehydrogenase, which oxidizes coniferaldehyde to ferulic acid. Drought stress is one of the important abiotic stresses affecting plant growth. However, the role of ferulic acid in drought resistance is unknown. To investigate the contribution of ferulic acid to cuticle composition and drought resistance, we used two Arabidopsis aldh2c4 mutant lines. Compared with wild-type (WT) leaves, ferulic acid contents were significantly lower (by more than 50 %) in mutants. The mutants also had lower amounts of cutin and wax, primarily due to reductions in C_18:2 dioic acid and alkanes, respectively. Furthermore, the leaves of the mutant plants exhibited greater rates of water loss and released chlorophyll faster than WT leaves when immersed in 80 % ethanol, indicating a defective cuticle barrier. The growth of aldh2c4 mutants was severely inhibited, and their leaves showed a higher degree of wilting relative to the WT plants under drought conditions. In aldh2c4 complementation lines, the growth inhibition of the mutant plants under drought stress was alleviated. Taken together, our results demonstrate that ferulic acid plays an important role in the composition and structural properties of the cuticle and that a ferulic acid deficiency in the cutin leads to reduced drought tolerance.

Design of a biomimetic, small-scale artificial leaf surface for the study of environmental interactions

The cuticle with its superimposed epicuticular waxes represents the barrier of all aboveground parts of higher plant primary tissues. Epicuticular waxes have multiple effects on the interaction of plants with their living and non-living environment, whereby their shape, dimension, arrangement, and chemical composition play significant roles. Here, the ability of self-assembly of wax after isolation from the leaves was used to develop a small-scale wax-coated artificial leaf surface with the chemical composition and wettability of wheat (Triticum aestivum) leaves. By thermal evaporation of extracted plant waxes and adjustment of the evaporated wax amounts, the wettability and chemical character of the microstructure of the surface of wheat leaves were transferred onto a technical surface. For the use of these artificial leaves as a test system for biotic (e.g., germination of fungal pathogens) and non-biotic (e.g., applied surfactants) interactions on natural leaf surfaces, the chemical composition and the wetting behavior should be the same in both. Therefore, the morphology, chemistry, and wetting properties of natural and artificial surfaces with recrystallized wax structures were analyzed by scanning electron microscopy, gas chromatography-mass spectrometry, and by the determination of water contact angles, contact angle hysteresis, and tilting angles. Wheat leaves of different ages were covered exclusively with wax platelets. The extracted wheat wax was composed of alcohols, aldehydes, esters, and acids. The main component was 1-octacosanol. The waxes recrystallized as three-dimensional structures on the artificial surfaces. The three tested wetting parameters resembled the ones of the natural surface, providing an artificial surface with the chemical information of epicuticular waxes and the wetting properties of a natural leaf surface.

Diffusion of volatile organics and water in the epicuticular waxes of petunia petal epidermal cells

Plant cuticles are a mixture of crystalline and amorphous waxes that restrict the exchange of molecules between the plant and the atmosphere. The multicomponent nature of cuticular waxes complicates the study of the relationship between the physical and transport properties. Here, a model cuticle based on the epicuticular waxes of Petunia hybrida flower petals was formulated to test the effect of wax composition on diffusion of water and volatile organic compounds (VOCs). The model cuticle was composed of an n-tetracosane (C₂₄ H₅₀ ), 1-docosanol (C₂₂ H₄₅ OH), and 3-methylbutyl dodecanoate (C₁₇ H₃₄ O₂ ), reflecting the relative chain length, functional groups, molecular arrangements, and crystallinity of the natural waxes. Molecular dynamics simulations were performed to obtain diffusion coefficients for compounds moving through waxes of varying composition. Simulated VOC diffusivities of the model system were found to highly correlate with in vitro measurements in isolated petunia cuticles. VOC diffusivity increased up to 30-fold in completely amorphous waxes, indicating a significant effect of crystallinity on cuticular permeability. The crystallinity of the waxes was highly dependent on the elongation of the lattice length and decrease in gap width between crystalline unit cells. Diffusion of water and higher molecular weight VOCs were significantly affected by alterations in crystalline spacing and lengths, whereas the low molecular weight VOCs were less affected. Comparison of measured diffusion coefficients from atomistic simulations and emissions from petunia flowers indicates that the role of the plant cuticle in the VOC emission network is attributed to the differential control on mass transfer of individual VOCs by controlling the composition, amount, and dynamics of scent emission.

Russeting of Fruits: Etiology and Management

The skin of a fruit protects the vulnerable, nutrient-rich flesh and seed(s) within from the hostile environment. It is also responsible for the fruit’s appearance. In many fruitcrop species, russeting compromises fruit appearance and thus commercial value. Here, we review the literature on fruit russeting, focusing on the factors and mechanisms that induce it and on the management and breeding strategies that may reduce it. Compared with a primary fruit skin, which is usually distinctively colored and shiny, a secondary fruit skin is reddish-brown, dull and slightly rough to the touch (i.e., russeted). This secondary skin (periderm) comprises phellem cells with suberized cell walls, a phellogen and a phelloderm. Russeted (secondary) fruit skins have similar mechanical properties to non-russeted (primary) ones but are more plastic. However, russeted fruit skins are more permeable to water vapor, so russeted fruits suffer higher postharvest water loss, reduced shine, increased shrivel and reduced packed weight (most fruit is sold per kg). Orchard factors that induce russeting include expansion-growth-induced strain, surface wetness, mechanical damage, freezing temperatures, some pests and diseases and some agrochemicals. All these probably act via an increased incidence of cuticular microcracking as a result of local concentrations of mechanical stress. Microcracking impairs the cuticle’s barrier properties. Potential triggers of russeting (the development of a periderm), consequent on cuticular microcracking, include locally high concentrations of O2, lower concentrations of CO2 and more negative water potentials. Horticulturists sometimes spray gibberellins, cytokinins or boron to reduce russeting. Bagging fruit (to exclude surface moisture) is also reportedly effective. From a breeding perspective, genotypes having small and more uniform-sized epidermal cells are judged less likely to be susceptible to russeting.

Translocation of Foliar Absorbed Zn in Sunflower (Helianthus annuus) Leaves

Foliar zinc (Zn) fertilization is an important approach for overcoming crop Zn deficiency, yet little is known regarding the subsequent translocation of this foliar-applied Zn. Using synchrotron-based X-ray fluorescence microscopy (XFM) and transcriptome analysis, the present study examined the translocation of foliar absorbed Zn in sunflower (Helianthus annuus) leaves. Although bulk analyses showed that there had been minimal translocation of the absorbed Zn out of the leaf within 7 days, in situ analyses showed that the distribution of Zn in the leaf had changed with time. Specifically, when Zn was applied to the leaf for 0.5 h and then removed, Zn primarily accumulated within the upper and lower epidermal layers (when examined after 3 h), but when examined after 24 h, the Zn had moved to the vascular tissues. Transcriptome analyses identified a range of genes involved in stress response, cell wall reinforcement, and binding that were initially upregulated following foliar Zn application, whereas they were downregulated after 24 h. These observations suggest that foliar Zn application caused rapid stress to the leaf, with the initial Zn accumulation in the epidermis as a detoxification strategy, but once this stress decreased, Zn was then moved to the vascular tissues. Overall, this study has shown that despite foliar Zn application causing rapid stress to the leaf and that most of the Zn stayed within the leaf over 7 days, the distribution of Zn in the leaf had changed, with Zn mostly located in the vascular tissues 24 h after the Zn had been applied. Not only do the data presented herein provide new insight for improving the efficiency of foliar Zn fertilizers, but our approach of combining XFM with a transcriptome methodological system provides a novel approach for the study of element translocation in plants.

Microbial interaction mediated programmed cell death in plants

Food demand of growing population can only be met by finding solutions for sustaining the crop yield. The understanding of basic mechanisms employed by microorganisms for the establishment of parasitic relationship with plants is a complex phenomenon. Symbionts and biotrophs are dependent on living hosts for completing their life cycle, whereas necrotrophs utilize dead cells for their growth and establishment. Hemibiotrophs as compared to other microbes associate themselves with plants in two phase's, viz. early bio-phase and later necro-phase. Plants and microbes interact with each other using receptors present on host cell surface and elicitors (PAMPs and effectors) produced by microbes. Plant-microbe interaction either leads to compatible or incompatible reaction. In response to various biotic and abiotic stress factors, plant undergoes programmed cell death which restricts the growth of biotrophs or hemibiotrophs while necrotrophs as an opportunist starts growing on dead tissue for their own benefit. PCD regulation is an outcome of plant-microbe crosstalk which entirely depends on various biochemical events like generation of reactive oxygen species, nitric oxide, ionic efflux/influx, CLPs, biosynthesis of phytohormones, phytoalexins, polyamines and certain pathogenesis-related proteins. This phenomenon mostly occurs in resistant and non-host plants during invasion of pathogenic microbes. The compatible or incompatible host-pathogen interaction depends upon the presence or absence of host plant resistance and pathogenic race. In addition to host-pathogen interaction, the defense induction by beneficial microbes must also be explored and used to the best of its potential. This review highlights the mechanism of microbe- or symbiont-mediated PCD along with defense induction in plants towards symbionts, biotrophs, necrotrophs and hemibiotrophs. Here we have also discussed the possible use of beneficial microbes in inducing systemic resistance in plants against pathogenic microbes.

Analysis of Wheat Wax Regulation Mechanism by Liposome and Transcriptome

As a barrier for plants to contact with the outside world, epidermal wax plays an important role in resisting biotic and abiotic stresses. In this study, we analyzed the effect of wax content on leaf permeability by measuring the wax loss rate in the leaf. To further clarify the wax composition of the wheat epidermis and its molecular regulation mechanism, we applied untargeted lipidomic and transcriptome analysis on the leaf epidermis wax of Jimai 22 low-wax mutant (waxless) and multi-wax mutant (waxy). Our research showed that the mutant waxy has a slow loss rate, which can maintain higher leaf water content. 31 lipid subclasses and 1,367 lipid molecules were identified. By analyzing the wax differences of the two mutants, we found that the main lipid components of leaf epidermis wax in Jimai 22 were WE (C19-C50), DG (C27-C53), MG (C31-C35), and OAHFA (C31-C52). Carbon chain length analysis showed that, in wheat epidermis wax, WE was dominated by C44 molecules, DG was mainly concentrated in C47, C45, C37, and C31 molecules, C48 played a leading role in OAHFA, and C35 and C31 played a major role in MG. Among them, DG, MG, and OAHFA were detected in wheat leaf wax for the first time, and they were closely related to stress resistance. Compared with the waxy, 6,840 DEGs were detected in the mutant waxless, 3,181 DEGs were upregulated, and 3,659 DEGs were downregulated. The metabolic pattern of main waxy components in the wheat epidermis was constructed according to KEGG metabolic pathway and 46 related genes were screened, including KSC, TER, FAR, WSD1, CER1, MAH1, ALDH7A1, CYP704B1, ACOT1_2_4, CYP86, MGLL, GPAT, ALDH, DPP1, dgkA, plsC, and E2.3.1.158 related genes. The screened wax-related genes were confirmed to be highly reliable by qRT-PCR. In addition, we found TER gene TraesCS6B03G1132900LC in wheat mutant waxless leaves for the first time, which inhibited the synthesis of long-chain acyl-CoA (n+2) by downregulating its expression. These results provide valuable reference information for further study of wheat epidermis wax heredity and molecular regulation.

Combined Transcriptome and Metabolome Analysis of Alfalfa Response to Thrips Infection

Thrips (Thysanoptera: Thripidae) is a major insect pest for alfalfa which can result in decreased plant nutrients, low yields, and even plant death. To identify the differentially expressed genes and metabolites in response to thrips in alfalfa, a combination of metabolomics and transcriptomics was employed using alfalfa (Caoyuan No. 2) with and without thrips infestation. The results showed that the flavonoid biosynthesis and isoflavonoid biosynthesis pathways were the most significantly enriched pathways in response to thrips infection, as shown by the combined transcriptome and metabolome analysis. The transcriptome results showed that SA and JA signal transduction and PAPM-triggered immunity and the MAPK signaling pathway–plant pathways played a crucial role in thrips-induced plant resistance in alfalfa. In addition, we found that thrips infestation could also induce numerous changes in plant primary metabolism, such as carbohydrate and amino acid metabolism as compared to the control. Overall, our results described here should improve fundamental knowledge of molecular responses to herbivore-inducible plant defenses and contribute to the design of strategies against thrips in alfalfa.

QTL dissection and mining of candidate genes for Ascochyta fabae and Orobanche crenata resistance in faba bean (Vicia faba L.)

BACKGROUND

Ascochyta blight caused by Ascochyta fabae Speg. and broomrape (Orobanche crenata) are among the economically most significant pathogens of faba bean. Several QTLs conferring resistance against the two pathogens have been identified and validated in different genetic backgrounds. The aim of this study was to saturate the most stable QTLs for ascochyta and broomrape resistance in two Recombinant Inbred Line (RIL) populations, 29H x Vf136 and Vf6 x Vf136, to identify candidate genes conferring resistance against these two pathogens.

RESULTS

We exploited the synteny between faba bean and the model species Medicago truncatula by selecting a set of 219 genes encoding putative WRKY transcription factors and defense related proteins falling within the target QTL intervals, for genotyping and marker saturation in the two RIL populations. Seventy and 50 of the candidate genes could be mapped in 29H x Vf136 and Vf6 x Vf136, respectively. Besides the strong reduction of the QTL intervals, the mapping process allowed replacing previous dominant and pedigree-specific RAPD flanking markers with robust and transferrable SNP markers, revealing promising candidates for resistance against the two pathogens.

CONCLUSIONS

Although further efforts in association mapping and expression studies will be required to corroborate the candidate genes for resistance, the fine-mapping approach proposed here increases the genetic resolution of relevant QTL regions and paves the way for an efficient deployment of useful alleles for faba bean ascochyta and broomrape resistance through marker-assisted breeding.

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.

Transcriptional regulation of wound suberin deposition in potato cultivars with differential wound healing capacity

Wounding during mechanical harvesting and post-harvest handling results in tuber desiccation and provides an entry point for pathogens resulting in substantial post​-harvest crop losses. Poor wound healing is a major culprit of these losses. Wound tissue in potato (Solanum tuberosum) tubers, and all higher plants, is composed of a large proportion of suberin that is deposited in a specialized tissue called the wound periderm. However, the genetic regulatory pathway controlling wound-induced suberization remains unknown. Here, we implicate two potato transcription factors, StMYB102 (PGSC0003DMG400011250) and StMYB74 (PGSC0003DMG400022399), as regulators of wound suberin biosynthesis and deposition. Using targeted metabolomics and transcript profiling from the wound healing tissues of two commercial potato cultivars, as well as heterologous expression, we provide evidence for the molecular-genetic basis of the differential wound suberization capacities of different potato cultivars. Our results suggest that (i) the export of suberin from the cytosol to the apoplast and ligno-suberin deposition may be limiting factors for wound suberization, (ii) StMYB74 and StMYB102 are important regulators of the wound suberization process in tubers, and (iii) polymorphisms in StMYB102 may influence cultivar-specific wound suberization capacity. These results represent an important step in understanding the regulated biosynthesis and deposition of wound suberin and provide a practical foundation for targeted breeding approaches aimed at improving potato tuber storage life.

Fight Hard or Die Trying: Current Status of Lipid Signaling during Plant-Pathogen Interaction

Plant diseases pose a substantial threat to food availability, accessibility, and security as they account for economic losses of nearly $300 billion on a global scale. Although various strategies exist to reduce the impact of diseases, they can introduce harmful chemicals to the food chain and have an impact on the environment. Therefore, it is necessary to understand and exploit the plants' immune systems to control the spread of pathogens and enable sustainable agriculture. Recently, growing pieces of evidence suggest a functional myriad of lipids to be involved in providing structural integrity, intracellular and extracellular signal transduction mediators to substantial cross-kingdom cell signaling at the host-pathogen interface. Furthermore, some pathogens recognize or exchange plant lipid-derived signals to identify an appropriate host or development, whereas others activate defense-related gene expression. Typically, the membrane serves as a reservoir of lipids. The set of lipids involved in plant-pathogen interaction includes fatty acids, oxylipins, phospholipids, glycolipids, glycerolipids, sphingolipids, and sterols. Overall, lipid signals influence plant-pathogen interactions at various levels ranging from the communication of virulence factors to the activation and implementation of host plant immune defenses. The current review aims to summarize the progress made in recent years regarding the involvement of lipids in plant-pathogen interaction and their crucial role in signal transduction.

Speaking the language of lipids: the cross-talk between plants and pathogens in defence and disease

Lipids and fatty acids play crucial roles in plant immunity, which have been highlighted over the past few decades. An increasing number of studies have shown that these molecules are pivotal in the interactions between plants and their diverse pathogens. The roles played by plant lipids fit in a wide spectrum ranging from the first physical barrier encountered by the pathogens, the cuticle, to the signalling pathways that trigger different immune responses and expression of defence-related genes, mediated by several lipid molecules. Moreover, lipids have been arising as candidate biomarkers of resistance or susceptibility to different pathogens. Studies on the apoplast and extracellular vesicles have been highlighting the possible role of lipids in the intercellular communication and the establishment of systemic acquired resistance during plant-pathogen interactions. From the pathogen perspective, lipid metabolism and specific lipid molecules play pivotal roles in the pathogen's life cycle completion, being crucial during recognition by the plant and evasion from the host immune system, therefore potentiating infection. Studies conducted in the last years have contributed to a better understanding of the language of lipids during the cross-talk between plants and pathogens. However, it is essential to continue exploring the knowledge brought up to light by transcriptomics and proteomics studies towards the elucidation of lipid signalling processes during defence and disease. In this review, we present an updated overview on lipids associated to plant-pathogen interactions, exploiting their roles from the two sides of this battle.

Evaluation of microalgae polysaccharides as biostimulants of tomato plant defense using metabolomics and biochemical approaches

Microalgal polysaccharides (PSs) may be an effective elicitor agent that can efficiently protect plants against biotic stresses. In this study, wee investigates, the effect of PS obtained from microalgae and cyanobacteria (D. salina MS002, P. tricorontum MS023, Porphyridium sp. MS081, Desmodesmus sp., D. salina MS067 and A. platensis MS001) on the biochemical and metabolomics markers linked to defense pathways in tomato plants. The phenylalanine ammonia lyase (PAL), chitinase, 1,3-beta-glucanase and peroxidase (POX) activities have been improved in tomato plants leaves treated by polysaccharides extracted from P. triocnutum (238.26%); Desmodesmus sp. (19.95%); P. triocnutum (137.50%) and Porphyridium sp. (47.28%) respectively. For proteins, polyphenols and H2O2, the maximum effect was induced by D. salina 067 (55.01%), Porphyridium sp. (3.97%) and A. platensis (35.08%) respectively. On the other hand, Gas Chromatography-mass spectrometry (GC-MS) metabolomics analysis showed that PSs induced the modification of metabolite profile involved in the wax construction of tomato leaves, such as fatty acids, alkanes, alkenes and phytosterol. PS treatments improved the accumulation of fatty acids C16:3, C18:2 and C18:3 released from the membrane lipids as precursors of oxylipin biosynthesis which are signaling molecules of plant defense. In addition, PS treatment induced the accumulation of C18:0 and Azelaic acid which is a regulator of salicylic acid-dependent systemic acquired resistance. However, molecular and metabolic studies can determine more precisely the mode of action of microalgal polysaccharides as biostimulants/elicitors plant defense.

Fight hard or die trying: when plants face pathogens under heat stress

In their natural environment, plants are exposed to biotic or abiotic stresses that occur sequentially or simultaneously. Plant responses to these stresses have been studied widely and have been well characterised in simplified systems involving single plant species facing individual stress. Temperature elevation is a major abiotic driver of climate change and scenarios have predicted an increase in the number and severity of epidemics. In this context, here we review the available data on the effect of heat stress on plant-pathogen interactions. Considering 45 studies performed on model or crop species, we discuss the possible implications of the optimum growth temperature of plant hosts and pathogens, mode of stress application and temperature variation on resistance modulations. Alarmingly, most identified resistances are altered under temperature elevation, regardless of the plant and pathogen species. Therefore, we have listed current knowledge on heat-dependent plant immune mechanisms and pathogen thermosensory processes, mainly studied in animals and human pathogens, that could help to understand the outcome of plant-pathogen interactions under elevated temperatures. Based on a general overview of the mechanisms involved in plant responses to pathogens, and integrating multiple interactions with the biotic environment, we provide recommendations to optimise plant disease resistance under heat stress and to identify thermotolerant resistance mechanisms.

Cuticular waxes of nectarines during fruit development in relation to surface conductance and susceptibility to Monilinia laxa

The cuticle is composed of cutin and cuticular waxes, and it is the first protective barrier to abiotic and biotic stresses in fruit. In this study, we analysed the composition of and changes in cuticular waxes during fruit development in nectarine (Prunus persica L. Batsch) cultivars, in parallel with their conductance and their susceptibility to Monilinia laxa. The nectarine waxes were composed of triterpenoids, mostly ursolic and oleanolic acids, phytosterols, and very-long-chain aliphatics. In addition, we detected phenolic compounds that were esterified with sugars or with triterpenoids, which are newly described in cuticular waxes. We quantified 42 compounds and found that they changed markedly during fruit development, with an intense accumulation of triterpenoids during initial fruit growth followed by their decrease at the end of endocarp lignification and a final increase in very-long-chain alkanes and hydroxylated triterpenoids until maturity. The surface conductance and susceptibility to Monilinia decreased sharply at the beginning of endocarp lignification, suggesting that triterpenoid deposition could play a major role in regulating fruit permeability and susceptibility to brown rot. Our results provide new insights into the composition of cuticular waxes of nectarines and their changes during fruit development, opening new avenues of research to explore brown rot resistance factors in stone fruit.

Phytophthora nicotianae Infection of Citrus Leaves and Host Defense Activation Compared to Root Infection

Currently, little is known about the host pathogen interaction between Phytophthora spp. and citrus roots versus leaves. Therefore, we compared the molecular events occurring in citrus roots and leaves after zoospore inoculation with Phytophthora nicotianae. We analyzed the physical characteristics and genetic responses to P. nicotianae infection of leaves and roots for susceptible and tolerant citrus rootstocks to examine the potential for leaves to model root responses to P. nicotianae infection. Leaves responded faster and stronger to P. nicotianae infection than roots, and leaves showed greater differential response than roots. In addition to differences in hormonal responses, sugar, phospholipase D (PLD), and phospholipase A (PLA) involvement in the interaction between citrus and P. nicotianae was identified. This work, for the first time, creates a solid P. nicotianae zoospore infection protocol, reports P. nicotianae infection on citrus leaves through stomata, and provides evidence that different host organs respond to the pathogen differentially in timing and magnitude. This work identifies the hormones, sugars, pathogenesis-related genes, PLDs, and PLAs that are involved in the molecular events occurring in citrus under infection of P. nicotianae zoospore, and advances our understanding of the mechanisms underlying the interaction.

Exogenous hormones influence Brassica napus leaf cuticular wax deposition and cuticle function

Background

Cuticular waxes cover plant surface and play important roles in protecting plants from abiotic and biotic stresses. The variations of wax deposition and chemical compositions under changing environments have been shown to be related to plant adaptations. However, it is still not clear whether the wax depositions could be adjusted to increase plant adaptations to stressed conditions.

Methods

In this study, exogenous methyl jasmonate (MeJA), the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) and salicylic acid (SA) were applied to test their effects on cuticular wax deposition in two Brassica napus cultivars, Zhongshuang 9 (ZS9, low wax coverage ) and Yuyou 19 (YY19, high wax coverage). Next, we measured the water loss rate and the transcriptional expression of genes involved in wax biosynthesis as well as genes related to disease defense.

Results

Seven wax compound classes, including fatty acids, aldehydes, alkanes, secondary alcohols, ketones, and unbranched as well as branched primary alcohols, were identified in B. napus leaf wax mixtures. MeJA, SA and ACC treatments had no significant effect on total wax amounts in YY19, whereas ACC reduced total wax amounts in ZS9. Overall, hormone treatments led to an increase in the amounts of aldehydes and ketones, and a decrease of secondary alcohol in ZS9, whereas they led to a decrease of alkane amounts and an increase of secondary alcohol amounts in YY19. Concomitantly, both cultivars also exhibited different changes in cuticle permeability, with leaf water loss rate per 15 min increased from 1.57% (averaged across treatments) at 1.57% (averaged across treatments) at 15 min to 3.12% at 30 min for ZS9 (except for ACC treated plant) and decreased for YY19. MeJA-treated plants of both cultivars relatively had higher water loss rate per 15 min when compared to other treatments. Conclusion. Our findings that B. napus leaf wax composition and cuticle permeability are altered by exogenous SA, MeJA and ACC suggest that the hormone treatments affect wax composition, and that the changes in wax profiles would cause changes in cuticle permeability.

The semidominant mutation w5 impairs epicuticular wax deposition in common wheat (Triticum aestivum L.)

The semidominant EMS-induced mutant w5 affects epicuticular wax deposition and mapped to an approximately 194-kb region on chromosome 7DL. Epicuticular wax is responsible for the glaucous appearance of plants and protects against many biotic and abiotic stresses. In wheat (Triticum aestivum L.), β-diketone is a major component of epicuticular wax in adult plants and contributes to the glaucousness of the aerial organs. In the present study, we identified an ethyl methanesulfonate-induced epicuticular wax-deficient mutant from the elite wheat cultivar Jimai22. Compared to wild-type Jimai22, the mutant lacked β-diketone and failed to form the glaucous coating on all aerial organs. The mutant also had significantly increased in cuticle permeability, based on water loss and chlorophyll efflux. Genetic analysis indicated that the mutant phenotype is controlled by a single, semidominant gene on the long arm of chromosome 7D, which was not allelic to the known wax gene loci W1-W4, and was therefore designated W5. W5 was finely mapped to an ~ 194-kb region (flanked by the molecular markers SSR2 and STARP11) that harbored four annotated genes according to the reference genome of Chinese Spring (RefSeq v1.0). Collectively, these data will broaden the knowledge of the genetic basis underlying epicuticular wax deposition in wheat.

TaCER1-1A is involved in cuticular wax alkane biosynthesis in hexaploid wheat and responds to plant abiotic stresses

To protect above-ground plant organs from excessive water loss, their surfaces are coated by waxes. The genes involved in wax formation have been investigated in detail in Arabidopsis but scarcely in crop species. Here, we aimed to isolate and characterize a CER1 enzyme responsible for formation of the very long-chain alkanes present in high concentrations especially during late stages of wheat development. On the basis of comparative wax and transcriptome analyses of various wheat organs, we selected TaCER1-1A as a primary candidate and demonstrated that it was located to the endoplasmic reticulum, the subcellular compartment for wax biosynthesis. A wheat nullisomic-tetrasomic substitution line lacking TaCER1-1A had significantly reduced amounts of C₃₃ alkane, whereas rice plants overexpressing TaCER1-1A showed substantial increases of C₂₅ -C₃₃ alkanes relative to wild type control. Similarly, heterologous expression of TaCER1-1A in Arabidopsis wild type and the cer1 mutant resulted in increased levels of unbranched alkanes, iso-branched alkanes and alkenes. Finally, the expression of TaCER1-1A was found activated by abiotic stresses and abscisic acid treatment, resulting in increased production of alkanes in wheat. Taken together, our results demonstrate that TaCER1-1A plays an important role in wheat wax alkane biosynthesis and involved in responding to drought and other environmental stresses.

Genetic and environmental variation impact the cuticular hydrocarbon metabolome on the stigmatic surfaces of maize

BACKGROUND

Simple non-isoprenoid hydrocarbons accumulate in discrete regions of the biosphere, including within bacteria and algae as a carbon and/or energy store, and the cuticles of plants and insects, where they may protect against environmental stresses. The extracellular cuticular surfaces of the stigmatic silks of maize are rich in linear hydrocarbons and therefore provide a convenient system to study the biological origins and functions of these unique metabolites.

RESULTS

To test the hypotheses that genetics and environment influence the accumulation of surface hydrocarbons on silks and to examine the breadth of metabolome compositions across diverse germplasm, cuticular hydrocarbons were analyzed on husk-encased silks and silks that emerged from the husk leaves from 32 genetically diverse maize inbred lines, most of which are commonly utilized in genetics experiments. Total hydrocarbon accumulation varied ~ 10-fold among inbred lines, and up to 5-fold between emerged and husk-encased silks. Alkenes accounted for 5-60% of the total hydrocarbon metabolome, and the majority of alkenes were monoenes with a double bond at either the 7th or 9th carbon atom of the alkyl chain. Total hydrocarbon accumulation was impacted to similar degrees by genotype and husk encasement status, whereas genotype predominantly impacted alkene composition. Only minor differences in the metabolome were observed on silks that were emerged into the external environment for 3- versus 6-days. The environmental influence on the metabolome was further investigated by growing inbred lines in 2 years, one of which was warmer and wetter. Inbred lines grown in the drier year accumulated up to 2-fold more hydrocarbons and up to a 22% higher relative abundance of alkenes. In summary, the surface hydrocarbon metabolome of silks is primarily governed by genotype and husk encasement status, with smaller impacts of environment and genotype-by-environment interactions.

CONCLUSIONS

This study reveals that the composition of the cuticular hydrocarbon metabolome on silks is affected significantly by genetic factors, and is therefore amenable to dissection using quantitative genetic approaches. Such studies will clarify the genetic mechanisms responsible for the accumulation of these metabolites, enabling detailed functional investigations of the diverse and complex protective roles of silk surface lipids against environmental stresses.

Membrane phospholipids remodeling upon imbibition in Brassica napus L. seeds

Successful seed germination depends on the rapid repair of cell membrane damaged by dry storage. However, little is known about the reorganization of lipids during this process. In this study, the changes of intracellular redox environment, cell membrane integrity, lipid composition, and expression of genes related to phospholipid metabolism were assessed during imbibition of Brassica napus seeds. A total number of 443 lipids belonging to 7 categories were detected by ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS/MS). In the 24 h-imbibed seeds, the relative content of triacylglycerol was lower than in dry seeds, while the relative content of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), and phosphatidylserine (PS), especially PC (36:2, number of carbons in the acyl chains: number of double bonds), PC (36:3), and PE (36:3) were higher than those in dry seeds. Meanwhile, the content and unsaturation levels of phospholipids increased, indicating membrane lipids remodeling during seed imbibition. The plasma membrane integrity, which was measured by the relative electrolyte leakage (REL) of the membrane and FM4-64 fluorescent dye, was improved upon imbibition, confirming that cell membrane was repaired after 24 h-imbibition. The reduction of H₂O₂ content, redox potential, and malondialdehyde (MDA) content indicated that the degree of membrane lipid peroxidation was significantly decreased upon imbibition. Gene expression analysis showed that the differential expression of genes for key enzymes occurred in the plateau phase of the imbibition curve, i.e. after 8 h-to 24 h-imbibition. Moreover, the differential expression of genes such as those encoding phospholipase C (PLC), phospholipase D (PLD), triacylglycerol lipase (TAG lipase), choline/ethanolamine phosphotransferase (CEPT), and phosphatidylserine synthase (PTDSS2) during imbibition indicated that membrane lipid remodeling was related to complex metabolic pathways, among which the degradation of triacylglycerol and the synthesis of phospholipids using diacylglycerol might play an important role during membrane remodeling.

Developmental and Environmental Regulation of Cuticular Wax Biosynthesis in Fleshy Fruits

The aerial parts of land plants are covered by a hydrophobic layer called cuticle that limits non-stomatal water loss and provides protection against external biotic and abiotic stresses. The cuticle is composed of polymer cutin and wax comprising a mixture of very-long-chain fatty acids and their derivatives, while also bioactive secondary metabolites such as triterpenoids are present. Fleshy fruits are also covered by the cuticle, which has an important protective role during the fruit development and ripening. Research related to the biosynthesis and composition of cuticles on vegetative plant parts has largely promoted the research on cuticular waxes in fruits. The chemical composition of the cuticular wax varies greatly between fruit species and is modified by developmental and environmental cues affecting the protective properties of the wax. This review focuses on the current knowledge of the cuticular wax biosynthesis during fleshy fruits development, and on the effect of environmental factors in regulation of the biosynthesis. Bioactive properties of fruit cuticular waxes are also briefly discussed, as well as the potential for recycling of industrial fruit residues as a valuable raw material for natural wax to be used in food, cosmetics and medicine.

Lipids as Key Markers in Maize Response to Fumonisin Accumulation

The present field study offers new insights into the role played by plant lipid pathways in the modulation of fumonisin accumulation in maize. Untargeted metabolomics was applied to better understand the multifactorial plant-pathogen-interaction mechanisms, including host resistance. Our results showed a significant influence from the hybrid genotype and the environmental growing conditions on fumonisin accumulation. A total of 25 significant metabolites have been identified, with glycerophospholipid and linoleic acid metabolism as the main pathways affected by the plant-pathogen interactions. This evidence highlighted the crucial role played by lipid signaling as an integrated part of the complex regulatory network in plants.

Arabidopsis HSP70-16 is required for flower opening under normal or mild heat stress temperatures

Sepals play important roles in protecting inner floral organs from various stresses and in guaranteeing timely flower opening. However, the exact role of sepals in coordinating interior and exterior signals remains elusive. In this study, we functionally characterized a heat shock protein gene, Arabidopsis HSP70-16, in flower opening and mild heat stress response, using combined genetics with anatomic, physiological, chemical, and molecular analyses. We showed that HSP70-16 is required for flower opening and mild heat response. Mutation of HSP70-16 led to a significant reduction in seed setting rate under 22°C, which was more severe at 27°C. Mutation of HSP70-16 also caused postgenital fusion at overlapping tips of two lateral sepals, leading to failed flower opening, abnormal floral organ formation, and impaired fertilization and seed setting. Chemical and anatomic analyses confirmed specific chemical and morphological changes of cuticle property in mutant lateral sepals, and qRT-PCR data indicated that expression levels of different sets of cuticle regulatory and biosynthetic genes were altered in mutants grown at both 22°C and 27°C temperatures. This study provides a link between thermal and developmental perception signals and expands the understanding of the roles of sepal in plant development and heat response.

Plant defense against aphids, the pest extraordinaire

Aphids are amongst the most damaging pests of plants that use their stylets to penetrate the plant tissue to consume large amounts of phloem sap and thus deprive the plant of photoassimilates. In addition, some aphids vector important viral diseases of plants. Plant defenses targeting aphids are broadly classified as antibiosis, which interferes with aphid growth, survival and fecundity, and antixenosis, which influences aphid behavior, including plant choice and feeding from the sieve elements. Here we review the multitude of steps in the infestation process where these defenses can be exerted and highlight the progress made on identifying molecular factors and mechanisms that contribute to host defense, including plant resistance genes and signaling components, as well as aphid-derived effectors that elicit or attenuate host defenses. Also discussed is the impact of aphid-vectored plant viruses on plant-aphid interaction and the concept of tolerance, which allows plant to withstand or recover from damage resulting from the infestation.

MicroR408 regulates defense response upon wounding in sweet potato

MiRNAs play diverse roles in plant development and defense responses by binding to their mRNA targets based on sequence complementarity. Here, we investigated a wound-related miR408 and its target genes in sweet potato (Ipomoea batatas) by small RNA deep sequencing and transcriptome analysis. The expression patterns of miR408 and the miR408 precursor were significantly repressed by wounding and jasmonate (JA). In contrast, expression of the putative target genes IbKCS (3-ketoacyl-CoA synthase 4), IbPCL (plantacyanin), and IbGAUT (galacturonosyltransferase 7-like) of miR408 was increased following wounding, whereas only IbKCS was increased after JA treatment. Target cleavage site mapping and Agrobacterium-mediated transient assay demonstrated that IbKCS, IbPCL, and IbGAUT were the targets of miR408. The expression of miR408 target genes was repressed in transgenic sweet potatoes overexpressing miR408. These data indicated a relationship between miR408 and its target genes. Notably, miR408-overexpressing plants showed a semi-dwarf phenotype and attenuated resistance to insect feeding, while transgenic plants overexpressing IbKCS exhibited more insect resistance than plants overexpressing only the empty vector. Collectively, sweet potato reduces the abundance of miR408 upon wounding to elevate the expression of IbKCS, IbPCL, and IbGAUT. The expression of IbKCS enhances the defense system against herbivore wounding.

The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) and Related Family: Mechanistic Insights in Plant Disease Resistance

The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) and related NPR1-like proteins are a functionally similar, yet surprisingly diverse family of transcription co-factors. Initially, NPR1 in Arabidopsis was identified as a positive regulator of systemic acquired resistance (SAR), paralogs NPR3 and NPR4 were later shown to be negative SAR regulators. The mechanisms involved have been the subject of extensive research and debate over the years, during which time a lot has been uncovered. The known roles of this protein family have extended to include influences over a broad range of systems including circadian rhythm, endoplasmic reticulum (ER) resident proteins and the development of lateral organs. Recently, important advances have been made in understanding the regulatory relationship between members of the NPR1-like protein family, providing new insight regarding their interactions, both with each other and other defense-related proteins. Most importantly the influence of salicylic acid (SA) on these interactions has become clearer with NPR1, NPR3, and NPR4 being considered bone fide SA receptors. Additionally, post-translational modification of NPR1 has garnered attention during the past years, adding to the growing regulatory complexity of this protein. Furthermore, growing interest in NPR1 overexpressing crops has provided new insights regarding the role of NPR1 in both biotic and abiotic stresses in several plant species. Given the wealth of information, this review aims to highlight and consolidate the most relevant and influential research in the field to date. In so doing, we attempt to provide insight into the mechanisms and interactions which underly the roles of the NPR1-like proteins in plant disease responses.

Overexpression of the TaSHN1 transcription factor in bread wheat leads to leaf surface modifications, improved drought tolerance, and no yield penalty under controlled growth conditions

Transcription factors regulate multiple networks, mediating the responses of organisms to stresses, including drought. Here, we investigated the role of the wheat transcription factor TaSHN1 in crop growth and drought tolerance. TaSHN1, isolated from bread wheat, was characterized for molecular interactions and functionality. The overexpression of TaSHN1 in wheat was followed by the evaluation of T₂ and T₃ transgenic lines for drought tolerance, growth, and yield components. Leaf surface changes were analysed by light microscopy, SEM, TEM, and GC-MS/GC-FID. TaSHN1 behaves as a transcriptional activator in a yeast transactivation assay and binds stress-related DNA cis-elements, determinants of which were revealed using 3D molecular modelling. The overexpression of TaSHN1 in transgenic wheat did not result in a yield penalty under the controlled plant growth conditions of a glasshouse. Transgenic lines had significantly lower stomatal density and leaf water loss and exhibited improved recovery after severe drought, compared with control plants. The comparative analysis of cuticular waxes revealed an increased accumulation of alkanes in leaves of transgenic lines. Our data demonstrate that TaSHN1 may operate as a positive modulator of drought stress tolerance. Positive attributes could be mediated through an enhanced accumulation of alkanes and reduced stomatal density.

Structural and Biochemical Properties of Duckweed Surface Cuticle

The plant cuticle, which consists of cutin and waxes, forms a hydrophobic coating covering the aerial surfaces of all plants. It acts as an interface between plants and their surrounding environment whilst also protecting them against biotic and abiotic stresses. In this research, we have investigated the biodiversity and cuticle properties of aquatic plant duckweed, using samples isolated from four different locations around Hongze lake in Jiangsu province, China. The samples were genotyped using two chloroplast markers and nuclear ribosomal DNA markers, which revealed them as ecotypes of the larger duckweed, Spirodela polyrhiza. Duckweed cuticle properties were investigated by compositional analysis using Gas Chromatography coupled with Mass Spectroscopy (GC-MS) Flame Ionization Detector (GC-FID), and ultrastructural observation by cryo-Scanning Electron Microscopy (cryo-SEM). Cuticle compositional analysis indicated that fatty acids and primary alcohols, the two typical constituents found in many land plant cuticle, are the major duckweed wax components. A large portion of the duckweed wax fraction is composed of phytosterols, represented by campesterol, stigmasterol, sitosterol and their common precursor squalene. The cryo-SEM observation uncovered significant differences between the surface structures of the top air-facing and bottom water-facing sides of the plant fronds. The top side of the fronds, containing multiple stomata complexes, appeared to be represented by a rather flat waxy film sporadically covered with wax crystals. Underneath the waxy film was detected a barely distinguished nanoridge net, which became distinctly noticeable after chloroform treatment. On the bottom side of the fronds, the large epidermal cells were covered by the well-structured net, whose sections became narrower and sharper under cryo-SEM following chloroform treatment. These structural differences between the abaxial and adaxial sides of the fronds evidently relate to their distinct physiological roles in interacting with the contrasting environments of sunlight/air and nutrients/water. The unique structural and biochemical features of Spirodela frond surfaces with their rapid reproductive cycle and readily availability genome sequence, make duckweed an attractive monocot model for studying the fundamental processes related to plant protection against ultraviolet irradiation, pathogens and other environmental stresses.

The Cuticle Mutant eca2 Modifies Plant Defense Responses to Biotrophic and Necrotrophic Pathogens and Herbivory Insects

We isolated previously several Arabidopsis thaliana mutants with constitutive expression of the early microbe-associated molecular pattern-induced gene ATL2, named eca (expresión constitutiva de ATL2). Here, we further explored the interaction of eca mutants with pest and pathogens. Of all eca mutants, eca2 was more resistant to a fungal pathogen (Botrytis cinerea) and a bacterial pathogen (Pseudomonas syringae) as well as to a generalist herbivorous insect (Spodoptera littoralis). Permeability of the cuticle is increased in eca2; chemical characterization shows that eca2 has a significant reduction of both cuticular wax and cutin. Additionally, we determined that eca2 did not display a similar compensatory transcriptional response, compared with a previously characterized cuticular mutant, and that resistance to B. cinerea is mediated by the priming of the early and late induced defense responses, including salicylic acid- and jasmonic acid-induced genes. These results suggest that ECA2-dependent responses are involved in the nonhost defense mechanism against biotrophic and necrotrophic pathogens and against a generalist insect by modulation and priming of innate immunity and late defense responses. Making eca2 an interesting model to characterize the molecular basis for plant defenses against different biotic interactions and to study the initial events that take place in the cuticle surface of the aerial organs.

NPR1 as a transgenic crop protection strategy in horticultural species

The NPR1 (NONEXPRESSOR OF PATHOGENESIS RELATED GENES1) gene has a central role in the long-lasting, broad-spectrum defense response known as systemic acquired resistance (SAR). When overexpressed in a transgenic context in Arabidopsis thaliana, this gene enhances resistance to a number of biotic and abiotic stresses. Its position as a key regulator of defense across diverse plant species makes NPR1 a strong candidate gene for genetic engineering disease and stress tolerance into other crops. High-value horticultural crops face many new challenges from pests and pathogens, and their emergence exceeds the pace of traditional breeding, making the application of NPR1-based strategies potentially useful in fruit and vegetable crops. However, plants overexpressing NPR1 occasionally present detrimental morphological traits that make its application less attractive. The practical utility of NPR-based approaches will be a balance of resistance gains versus other losses. In this review, we summarize the progress on the understanding of NPR1-centered applications in horticultural and other crop plants. We also discuss the effect of the ectopic expression of the A. thaliana NPR1 gene and its orthologs in crop plants and outline the future challenges of using NPR1 in agricultural applications.

Multifunctional Roles of Plant Cuticle During Plant-Pathogen Interactions

In land plants the cuticle is the outermost layer interacting with the environment. This lipophilic layer comprises the polyester cutin embedded in cuticular wax; and it forms a physical barrier to protect plants from desiccation as well as from diverse biotic and abiotic stresses. However, the cuticle is not merely a passive, mechanical shield. The increasing research on plant leaves has addressed the active roles of the plant cuticle in both local and systemic resistance against a variety of plant pathogens. Moreover, the fruit cuticle also serves as an important determinant of fruit defense and quality. It shares features with those of vegetative organs, but also exhibits specific characteristics, the functions of which gain increasing attention in recent years. This review describes multiple roles of plant cuticle during plant-pathogen interactions and its responses to both leaf and fruit pathogens. These include the dynamic changes of plant cuticle during pathogen infection; the crosstalk of cuticle with plant cell wall and diverse hormone signaling pathways for plant disease resistance; and the major biochemical, molecular, and cellular mechanisms that underlie the roles of cuticle during plant-pathogen interactions. Although research developments in the field have greatly advanced our understanding of the roles of plant cuticle in plant defense, there still remain large gaps in our knowledge. Therefore, the challenges thus presented, and future directions of research also are discussed in this review.

GDSL lipases modulate immunity through lipid homeostasis in rice

Lipids and lipid metabolites play important roles in plant-microbe interactions. Despite the extensive studies of lipases in lipid homeostasis and seed oil biosynthesis, the involvement of lipases in plant immunity remains largely unknown. In particular, GDSL esterases/lipases, characterized by the conserved GDSL motif, are a subfamily of lipolytic enzymes with broad substrate specificity. Here, we functionally identified two GDSL lipases, OsGLIP1 and OsGLIP2, in rice immune responses. Expression of OsGLIP1 and OsGLIP2 was suppressed by pathogen infection and salicylic acid (SA) treatment. OsGLIP1 was mainly expressed in leaf and leaf sheath, while OsGLIP2 showed high expression in elongating internodes. Biochemical assay demonstrated that OsGLIP1 and OsGLIP2 are functional lipases that could hydrolyze lipid substrates. Simultaneous down-regulation of OsGLIP1 and OsGLIP2 increased plant resistance to both bacterial and fungal pathogens, whereas disease resistance in OsGLIP1 and OsGLIP2 overexpression plants was significantly compromised, suggesting that both genes act as negative regulators of disease resistance. OsGLIP1 and OsGLIP2 proteins mainly localize to lipid droplets and the endoplasmic reticulum (ER) membrane. The proper cellular localization of OsGLIP proteins is indispensable for their functions in immunity. Comprehensive lipid profiling analysis indicated that the alteration of OsGLIP gene expression was associated with substantial changes of the levels of lipid species including monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). We show that MGDG and DGDG feeding could attenuate disease resistance. Taken together, our study indicates that OsGLIP1 and OsGLIP2 negatively regulate rice defense by modulating lipid metabolism, thus providing new insights into the function of lipids in plant immunity.

Lipases are a large family of enzymes conferring lipid metabolism. Lipids and their metabolites play diverse roles in plant growth as well as response to environmental stimuli. Accumulating evidence implicates lipids as signaling molecules mediating plant immunity. Therefore, lipases are presumed to be actively involved in plant defense responses. Based on gene expression profiling, we have identified two functional GDSL lipases, encoded by OsGLIP1 and OsGLIP2, whose expression was suppressed by pathogen infection in the model cereal rice. Both OsGLIP1 and OsGLIP2 proteins localize to lipid droplets and the endoplasmic reticulum (ER) membrane, and they likely coordinate lipid metabolism with differential but complementary expression patterns in tissues and developmental stages. Consequently, alteration of OsGLIP gene expression was associated with substantial changes of lipid abundance and plant disease resistance. Our work identifies and characterizes two lipases that function as negative regulators of plant immune responses, strengthening the understanding of lipid metabolism in plant-microbe interactions.

Breeding for cuticle-associated traits in crop species: traits, targets, and strategies

Improving crop productivity and quality while promoting sustainable agriculture have become major goals in plant breeding. The cuticle is a natural film covering the aerial organs of plants and consists of lipid polyesters covered and embedded with wax. The cuticle protects plants against water loss and pathogens and affects traits with strong impacts on crop quality such as, for horticultural crops, fruit brightness, cracking, russeting, netting, and shelf life. Here we provide an overview of the most important cuticle-associated traits that can be targeted for crop improvement. To date, most studies on cuticle-associated traits aimed at crop breeding have been done on fleshy fruits. Less information is available for staple crops such as rice, wheat or maize. Here we present new insights into cuticle formation and properties resulting from the study of genetic resources available for the various crop species. Our review also covers the current strategies and tools aimed at exploiting available natural and artificially induced genetic diversity and the technologies used to transfer the beneficial alleles affecting cuticle-associated traits to commercial varieties.

The intimate talk between plants and microorganisms at the leaf surface

The plant epidermis or cuticle is constantly exposed to external and internal environmental factors, including an enriched and diverse community of bacteria, yeast, fungi, viruses, and mites. It is not only where the plant has its first physical barrier, but also where organisms can be recognized and potentially where the plant defense responses can be triggered. The plant cuticle is a polymeric composite formed by an array of structurally and chemically heterogeneous compounds, including cutin and wax. A few studies have shown that cuticular components are essential and important drivers of the structure and size of the bacterial community. On the other hand, cuticular components are also important for both pathogens and plants, to initiate the pre-invasion and infection process and to activate the innate immune response, respectively. In this review, we explore current knowledge on the role of the cuticle during the intimate interactions between plants and microorganisms, in particular pathogenic and non-pathogenic bacteria and fungi. Finally, we propose new perspectives on the potential use of this information for agriculture.