Structural and biochemical characteristics of citrus flowers associated with defence against a fungal pathogen

Floral scents, specialized metabolites and stress-response activities in Heritiera fomes and Bruguiera gymnorrhiza from Sundarban mangrove ecosystem

Floral biochemistry and stress physiology is an underexplored aspect of mangroves, which should be investigated as part of preservation and restoration efforts. A thriving true mangrove tree (Bruguiera gymnorrhiza (L.) Lamk.) and a threatened mangrove-associate species (Heritiera fomes Buch. Ham.) were studied in the Sundarban region of India for seasonal variations in floral odours, non-volatile phytochemicals, antioxidant enzyme activities, and surface water chemistry in surrounding habitat. Both species were found to exhibit significant differences in floral volatilomes, protein contents, antioxidant enzyme activities, total flavonoids, and total phenolic contents between spring and autumn blooms. The bird-pollinated flowers of B. gymnorrhiza also showed considerable seasonal differences in floral anthocyanin and proline contents, indicating vulnerability of the post-anthesis open flowers to environmental factors. Contrarily to previous findings, B. gymnorrhiza floral bouquet appeared to be enriched in various classes of volatiles - dominated by sulphurous compounds in bud stage and terpenoids in open stage. Floral anthocyanins, contributing to the striking colouration of the calyx, were found to comprise cyanidin and delphinidin derivatives. Other glycosides of cyanidin and delphinidin were detected in H. fomes flowers, contributing to visual guides to potential food rewards for pollinating insects. Floral tissue in H. fomes was found to be protected by densely overlapping layers of stellate trichomes containing sesquiterpenoids as phytoprotectants. Comparison of the two floral species suggested that H. fomes flowering is optimized to oligohaline (but not freshwater) vernal conditions; whereas B. gymnorrhiza blooms are adapted for biologically enriched (including abundant herbivores and microbial growth), mesohaline forest habitats.

Seed biopriming with Ochrobactrum ciceri mediated defense responses in Zea mays (L.) against Fusarium rot

Seed bio-priming is a simple and friendly technique to improve stress resilience against fungal diseases in plants. An integrated approach of maize seeds biopriming with Ochrobactrum ciceri was applied in Zn-amended soil to observe the response against Fusarium rot disease of Zea mays (L.) caused by Fusarium verticillioides. Initially, the pathogen isolated from the infected corn was identified as F. verticillioides based on morphology and sequences of the internally transcribed spacer region of the ribosomal RNA gene. Re-inoculation of maize seed with the isolated pathogen confirmed the pathogenicity of the fungus on the maize seeds. In vitro, the inhibitory potential of O. ciceri assessed on Zn-amended/un-amended growth medium revealed that antifungal potential of O. ciceri significantly improved in the Zn-amended medium, leading to 88% inhibition in fungal growth. Further assays with different concentrations (25, 50, and 75%) of cell pellet and the cultural filtrate of O. ciceri (with/without the Zn-amendment) showed a dose-dependent inhibitory effect on mycelial growth of the pathogen that also led to discoloration, fragmentation, and complete disintegration of the fungus hyphae and spores at 75% dose. In planta, biopriming of maize seeds with O. ciceri significantly managed disease, improved the growth and biochemical attributes (up to two-fold), and accelerated accumulation of lignin, polyphenols, and starch, especially in the presence of basal Zn. The results indicated that bioprimed seeds along with Zn as the most promising treatment for managing disease and improving plant growth traits through the enhanced accumulation of lignin, polyphenols, and starch, respectively.

Effect of Pathogenic Fungal Infestation on the Berry Quality and Volatile Organic Compounds of Cabernet Sauvignon and Petit Manseng Grapes

The effect of pathogenic fungal infestation on berry quality and volatile organic compounds (VOCs) of Cabernet Sauvignon (CS) and Petit Manseng (PM) were investigated by using biochemical assays and gas chromatography-ion mobility spectrometry. No significant difference in diseases-affected grapes for 100-berry weight. The content of tannins and vitamin C decreased significantly in disease-affected grapes, mostly in white rot-affected PM, which decreased by 71.67% and 66.29%. The reduced total flavonoid content in diseases-affected grape, among which the least and most were anthracnose-affected PM (1.61%) and white rot-affected CS (44.74%). All diseases-affected CS had much higher titratable acid, a maximum (18.86 g/100 ml) was observed in the gray mold-affected grapes, while only anthracnose-affected grapes with a higher titratable acid level (21.8 g/100 mL) were observed in PM. A total of 61 VOCs were identified, including 14 alcohols, 13 esters, 12 aldehydes, 4 acids, 4 ketones, 1 ether, and 13 unknown compounds, which were discussed from different functional groups, such as C6-VOCs, alcohols, ester acetates, aldehydes, and acids. The VOCs of CS changed more than that of Petit Manseng's after infection, while gray mold-affected Cabernet Sauvignon had the most change. C6-VOCs, including hexanal and (E)-2-hexenal were decreased in all affected grapes. Some unique VOCs may serve as hypothetical biomarkers to help us identify specific varieties of pathogenic fungal infestation.

Effect of Altitude on Volatile Organic and Phenolic Compounds of Artemisia brevifolia Wall ex Dc. From the Western Himalayas

Adaptation to changing environmental conditions is a driver of plant diversification. Elevational gradients offer a unique opportunity for investigating adaptation to a range of climatic conditions. The use of specialized metabolites as volatile and phenolic compounds is a major adaptation in plants, affecting their reproductive success and survival by attracting pollinators and protecting themselves from herbivores and other stressors. The wormseed Artemisia brevifolia can be found across multiple elevations in the Western Himalayas, a region that is considered a biodiversity hotspot and is highly impacted by climate change. This study aims at understanding the volatile and phenolic compounds produced by A. brevifolia in the high elevation cold deserts of the Western Himalayas with the view to understanding the survival strategies employed by plants under harsh conditions. Across four sampling sites with different elevations, polydimethylsiloxane (PDMS) sampling and subsequent GCMS analyses showed that the total number of volatile compounds in the plant headspace increased with elevation and that this trend was largely driven by an increase in compounds with low volatility, which might improve the plant’s resilience to abiotic stress. HPLC analyses showed no effect of elevation on the total number of phenolic compounds detected in both young and mature leaves. However, the concentration of the majority of phenolic compounds decreased with elevation. As the production of phenolic defense compounds is a costly trait, plants at higher elevations might face a trade-off between energy expenditure and protecting themselves from herbivores. This study can therefore help us understand how plants adjust secondary metabolite production to cope with harsh environments and reveal the climate adaptability of such species in highly threatened regions of our planet such as the Himalayas.

Histopathological changes in root and stem of mungbean exposed to Macrophomina phaseolina and dry biomass of Chenopodium quinoa

Mungbean production is affected by a fungal pathogen Macrophomina phaseolina. A pot experiment was carried out to check the effect of quinoa (Chenopodium quinoa Willd.) dry biomass on the histopathological features of mungbean exposed to M. phaseolina. For this, 1%, 2% and 3% (wt/wt) of C. quinoa dry biomass was mixed in the soil inoculated with M. phaseolina. The highest disease incidence (36%) was recorded in the positive control (only M. phaseolina). Different treatments of quinoa dry mass reduced disease incidence to 4-7%. After 4 weeks of germination, stem and root sections were stained in phloroglucinol-HCl, ferric chloride and Lugol's iodine stains for the detection of lignin, polyphenols and starch granules, respectively, and studied under light microscope. Plants of positive control showed damaged cells, and heavy deposition of lignin, phenolics and starch granules as compared to plants of the negative control and those grown in the soil amended with different doses of dry biomass of quinoa. For better understanding, plant root and stem sections were studied under a scanning electron microscope. Plant sections from positive control exhibited the presence of M. phaseolina sclerotial bodies and hyphal growth, whereas in negative control normal cell structures were observed. However, C. quinoa amended stem and root sections revealed the presence of high gel deposition with normal cell structures and no pathogen establishment. This study concludes that application of C. quinoa is an effective and natural remedy to activate the resistance mechanism in plants and to combat the adverse effects of M. phaseolina.

Exploiting Plant Volatile Organic Compounds (VOCs) in Agriculture to Improve Sustainable Defense Strategies and Productivity of Crops

There is an urgent need for new sustainable solutions to support agriculture in facing current environmental challenges. In particular, intensification of productivity and food security needs require sustainable exploitation of natural resources and metabolites. Here, we bring the attention to the agronomic potential of volatile organic compounds (VOCs) emitted from leaves, as a natural and eco-friendly solution to defend plants from stresses and to enhance crop production. To date, application of VOCs is often limited to fight herbivores. Here we argue that potential applications of VOCs are much wider, as they can also protect from pathogens and environmental stresses. VOCs prime plant's defense mechanisms for an enhanced resistance/tolerance to the upcoming stress, quench reactive oxygen species (ROS), have potent antimicrobial as well as allelopathic effects, and might be important in regulating plant growth, development, and senescence through interactions with plant hormones. Current limits and drawbacks that may hamper the use of VOCs in open field are analyzed, and solutions for a better exploitation of VOCs in future sustainable agriculture are envisioned.

Osmophores and floral fragrance in Anacardium humile and Mangifera indica (Anacardiaceae): an overlooked secretory structure in Sapindales

Flowers of Anacardiaceae and other Sapindales typically produce nectar, but scent, often associated with a reward for pollinators, has surprisingly been mentioned only rarely for members of the family and order. However, flowers of Anacardium humile and Mangifera indica produce a strong sweet scent. The origin and composition of these floral scents is the subject of this study. Screening of potential osmophores on the petals and investigations of their anatomy were carried out by light, scanning and transmission electron microscopy. The composition of the floral fragrance was characterized by gas chromatography-mass spectrometry. In both species, the base of the adaxial side of each petal revealed specialized secretory epidermal cells which are essentially similar in structure and distinct from all other neighbouring cells. These cells also showed evidence of granulocrine secretory mechanisms and slight specific variations in their subcellular apparatus coinciding with the respective composition of the floral fragrance, predominantly composed of sesquiterpenes in A. humile and monoterpenes in M. indica. This study reports the presence of osmophores for the first time in flowers of Anacardiaceae and confirms the link between the ultrastructural features of their secretory cells and the volatiles produced by the flowers. The flowers of most Sapindales, including Anacardiaceae, are nectariferous. However, the presence of osmophores has only been described for very few genera of Rutaceae and Sapindaceae. Both the occurrence of osmophores and fragrance may have largely been overlooked in Anacardiaceae and Sapindales until now. Further studies are needed to better understand the nature and diversity of the interactions of their nectariferous flowers with their pollinators.

Genomic Analysis of Terpene Synthase Family and Functional Characterization of Seven Sesquiterpene Synthases from Citrus sinensis

Citrus aroma and flavor, chief traits of fruit quality, are derived from their high content in essential oils of most plant tissues, including leaves, stems, flowers, and fruits. Accumulated in secretory cavities, most components of these oils are volatile terpenes. They contribute to defense against herbivores and pathogens, and perhaps also protect tissues against abiotic stress. In spite of their importance, our understanding of the physiological, biochemical, and genetic regulation of citrus terpene volatiles is still limited. The availability of the sweet orange (Citrus sinensis L. Osbeck) genome sequence allowed us to characterize for the first time the terpene synthase (TPS) family in a citrus type. CsTPS is one of the largest angiosperm TPS families characterized so far, formed by 95 loci from which just 55 encode for putative functional TPSs. All TPS angiosperm families, TPS-a, TPS-b, TPS-c, TPS-e/f, and TPS-g were represented in the sweet orange genome, with 28, 18, 2, 2, and 5 putative full length genes each. Additionally, sweet orange β-farnesene synthase, (Z)-β-cubebene/α-copaene synthase, two β-caryophyllene synthases, and three multiproduct enzymes yielding β-cadinene/α-copaene, β-elemene, and β-cadinene/ledene/allo-aromandendrene as major products were identified, and functionally characterized via in vivo recombinant Escherichia coli assays.

Ultrastructural changes in the epidermis of petals of the sweet orange infected by Colletotrichum acutatum

Postbloom fruit drop (PFD) is an important disease caused by the fungus Colletotrichum acutatum. PFD is characterised by the formation of necrotic lesions on the petals and stigmas of flowers as well as premature abscission of the fruit in Citrus spp. We compare the ultrastructure of the epidermis of uninoculated Citrus sinensis petals with that of petals inoculated with the fungus to understand the changes that occur upon C. acutatum infection. Healthy petals have a cuticle with parallel striations covering the uniseriate epidermis. This pattern consists of vacuolated parietal cells whose cytoplasm contains mitochondria, plastids with an undeveloped endomembrane system and a slightly dense stroma, a poorly developed rough endoplasmic reticulum, polysomes, few lipid droplets, and a nucleus positioned near the inner periclinal wall. In damaged regions, the cytoplasm of some cells is densely packed with well-developed endoplasmic reticulum, a large number of hyperactive dictyosomes, numerous mitochondria, and many lipid droplets. The plastids have an electron-dense stroma, starch grains, and a large amount of electron-dense lipid droplets, which can be released into vacuoles or the endoplasmic reticulum. Multivesicular bodies and myelin bodies are frequently observed in the vacuole, cytoplasm, and periplasmic space. Vesicles migrate through the cell wall and are involved in the deposition of cuticular material. In the later stages of infection, there is deposition of new cuticle layers in plaques. The outer periclinal walls can be thick. These observations indicate that epidermal cells respond to the pathogen, resulting in cuticular and parietal changes, which may limit further infection.

Synthesis of Oxylipin Mimics and Their Antifungal Activity against the Citrus Postharvest Pathogens

Nine oxylipin mimics were designed and synthesized starting from d-mannose. Their antifungal activity against three citrus postharvest pathogens was evaluated by spore germination assay. The results indicated that all the compounds significantly inhibited the growth of Penicillium digitatum, Penicillium italicum and Aspergillus niger. The compound (3Z,6Z,8S,9R,10R)-octadeca-3,6-diene-8,9,10-triol (3) exhibited excellent inhibitory effect on both Penicillium digitatum (IC₅₀ = 34 ppm) and Penicillium italicum (IC₅₀ = 94 ppm). Their in vivo antifungal activities against citrus postharvest blue mold were tested with fruit inoculated with the pathogen Penicillium italicum. The compound (3R,4S)-methyl 3,4-dihydroxy-5-octyltetrahydrofuran-2-carboxylate (9) demonstrated significant efficacy by reducing the disease severity to 60%. The antifungal mechanism of these oxylipin mimics was postulated in which both inhibition of pathogenic mycelium and stimuli of the host oxylipin-mediated defense response played important roles.

Arabidopsis flower specific defense gene expression patterns affect resistance to pathogens

We investigated whether the Arabidopsis flower evolved protective measures to increase reproductive success. Firstly, analyses of available transcriptome data show that the most highly expressed transcripts in the closed sepal (stage 12) are enriched in genes with roles in responses to chemical stimuli and cellular metabolic processes. At stage 15, there is enrichment in transcripts with a role in responses to biotic stimuli. Comparative analyses between the sepal and petal in the open flower mark an over-representation of transcripts with a role in responses to stress and catalytic activity. Secondly, the content of the biotic defense-associated phytohormone salicylic acid (SA) in sepals and petals is significantly higher than in leaves. To understand whether the high levels of stress responsive transcripts and the higher SA content affect defense, wild-type plants (Col-0) and transgenic plants defective in SA accumulation (nahG) were challenged with the biotrophic fungus Golovinomyces cichoracearum, the causal agent of powdery mildew, and the necrotrophic fungus Botrytis cinerea. NahG leaves were more sensitive than those of Col-0, suggesting that in leaves SA has a role in the defense against biotrophs. In contrast, sepals and petals of both genotypes were resistant to G. cichoracearum, indicating that in the flower, resistance to the biotrophic pathogen is not critically dependent on SA, but likely dependent on the up-regulation of stress-responsive genes. Since sepals and petals of both genotypes are equally susceptible to B. cinerea, we conclude that neither stress-response genes nor increased SA accumulation offers protection against the necrotrophic pathogen. These results are interpreted in the light of the distinctive role of the flower and we propose that in the early stages, the sepal may act as a chemical defense barrier of the developing reproductive structures against biotrophic pathogens.