Exogenous nitric oxide induces disease resistance against Monilinia fructicola through activating the phenylpropanoid pathway in peach fruit

The roles of nitric oxide in improving postharvest fruits quality: Crosstalk with phytohormones

Nowadays, improving the quality of postharvest fruits has become a hot research topic. Nitric oxide (NO) is often regarded as a signaling molecule that delays the postharvest senescence of fruits. Moreover, phytohormones affect the postharvest senescence of fruits. This review mainly describes how NO improves the postharvest quality of fruits by delaying postharvest fruit senescence, mitigating fruit cold damage and controlling postharvest diseases. Furthermore, the crosstalk of NO and multiple plant hormones effectively delays the postharvest senescence of fruits, and the major crosstalk mechanisms include (1) mediating phytohormone signaling. (2) inhibiting ETH production. (3) stimulating antioxidant enzyme activity. (4) decreasing membrane lipid peroxidation. (5) maintaining membrane integrity. (6) inhibiting respiration rate. (7) regulating gene expression related to fruit senescence. This review concluded the roles and mechanisms of NO in delaying postharvest fruit senescence. In addition, the crosstalk mechanisms between NO and various phytohormones on the regulation of postharvest fruit quality are also highlighted, which provides new ideas for the subsequent research.

The Impact of Rootstock on "Big Top" Nectarine Postharvest Concerning Chilling Injury, Biochemical and Molecular Parameters

Peaches and nectarines have a short shelf life even when harvested at appropriate physiological maturity. Market life is increased by storage at low temperatures. However, chilling injury symptoms can appear, causing physiological disorders and limiting shipping potential. The rootstock effect on the post-harvest quality has hardly been explored. Thus, the principal aim of this work was to study the influence of seven different Prunus rootstocks on the "Big Top" nectarine cv, considering harvest and post-harvest quality parameters and their correlation with chilling injury disorders. Basic fruit quality traits, individual sugars and organic acids analyzed by HPLC and other biochemical compounds such as relative antioxidant capacity, total phenolics content, flavonoids, anthocyanins, vitamin C and related enzyme activities (PAL, POD, PPO) were considered. In addition, correlations with possible candidate genes for chilling injury (CI) tolerance were searched by qPCR. Although a low susceptibility to CI symptoms has been found in "Big Top", rootstocks "PADAC 9902-01", "PADAC 99-05" and "ReplantPAC" exhibited lower CI symptoms. A statistically significant influence of the evaluated rootstocks was found concerning the parameters of this study. Phenols and anthocyanins seem to be important parameters to be considered in the prevention of chilling injury disorders. Moreover, PAL1, PPO4, PG2 and LDOX genes relative expressions were positively associated with chilling injury susceptibility. This study opens new perspectives for understanding peach fruit adaptation and response to cold storage temperatures during the post-harvest period.

Genome-wide association analysis of Monilinia fructicola lesion in a collection of Spanish peach landraces

Brown rot, caused by the Monilinia spp., is the disease that causes the greatest losses in stone fruit worldwide. Currently, M. fructicola has become the dominant species in the main peach production area in Spain. The fruit cuticle is the first barrier of protection against external aggressions and may have a key role in the susceptibility to brown rot. However, information on the role of skin fruit on the resistance to brown rot in peach is scarce. Previous genetic analyses in peach have demonstrated that brown rot resistance is a complex and quantitative trait in which different fruit parts and resistance mechanisms are involved. To search for genomic areas involved in the control of the cultivar susceptibility to brown rot and to elucidate the role of fruit skin against this infection, we have studied, for two consecutive seasons (2019 and 2020), the fruit susceptibility to M. fructicola, together with fruit cuticle thickness (CT) and density (CD), in a collection of 80 Spanish and 5 foreign peach cultivars from the National Peach Collection at CITA (Zaragoza, Spain). Brown rot incidence, lesion diameter, and severity index were calculated after 5 days of inoculation on non-wounded fruit. The peach collection has also been genotyped using the new peach SNP chip (9 + 9K). Genotypic and phenotypic data have been used to perform a genome-wide association analysis (GWAS). Phenotyping has shown a wide variability on the brown rot susceptibility within the Spanish germplasm as well as on CD and CT. The GWAS results have identified several significant SNPs associated with disease severity index (DSI), CD, and CT, five of which were considered as reliable SNP-trait associations. A wide protein network analysis, using 127 genes within the regions of the reliable SNPs and previously identified candidate genes (169) associated with Monilinia spp. resistance, highlighted several genes involved in classical hypersensitive response (HR), genes related to wax layers as ceramidases and lignin precursors catalyzers, and a possible role of autophagy during brown rot infection. This work adds relevant information on the complexity resistance mechanisms to brown rot infection in peach fruits and the genetics behind them.

Nano-selenium enhances melon resistance to Podosphaera xanthii by enhancing the antioxidant capacity and promoting alterations in the polyamine, phenylpropanoid and hormone signaling pathways

Powdery mildew is one of the main problematic diseases in melon production, requiring the use of chemical pesticides with disease-resistant cultivars for control. However, the often rapid acquisition of fungicidal resistance by mildew pathogens makes this practice unsustainable. The identification of crop treatments that can enhance resistance to powdery mildew resistance is therefore important to reduce melon crop attrition. This study indicates that the application of Nano-Se can reduce the powdery mildew disease index by 21-45%. The Nano-Se treatment reduced reactive oxygen species (ROS) and malondialdehyde (MDA) accumulation, with increases in glutathione (GSH), proline and 1,1-Diphenyl-2-picrylhydrazyl radical (DPPH). Increases were also observed in the activities and transcriptional levels of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and peroxidase (POD). Assays with four different cultivars of melon with differing levels of mildew resistance demonstrated that relative to the control, the Nano-Se treatment resulted in larger responses to mildew infection, including increases in the levels of putrescine (PUT; 43-112%) and spermine (SPM; 36-118%), indoleacetic acid (IAA; 43-172%) and salicylic acid (SA; 24-73%), the activities of phenylalanine ammonium lyase (PAL), trans-cinnamate 4-hydroxylase (C4H) and 4-coumarate: Co A ligase (4CL) of the phenylpropanoid pathway (22-38%, 24-126% and 19-64%, respectively). Key genes in the polyamine and phenylpropanoid pathway were also upregulated. These results indicate that the foliar application of Nano-Se improved melon defenses against powdery mildew infection, with a significant reduction in mildew disease development.

Elevated methylglyoxal levels inhibit tomato fruit ripening by preventing ethylene biosynthesis

Methylglyoxal (MG), a toxic compound produced as a by-product of several cellular processes, such as respiration and photosynthesis, is well known for its deleterious effects, mainly through glycation of proteins during plant stress responses. However, very little is known about its impact on fruit ripening. Here, we found that MG levels are maintained at high levels in green tomato (Solanum lycopersicum L.) fruits and decline during fruit ripening despite a respiratory burst during this transition. We demonstrate that this decline is mainly mediated through a glutathione-dependent MG detoxification pathway and primarily catalyzed by a Glyoxalase I enzyme encoded by the SlGLYI4 gene. SlGLYI4 is a direct target of the MADS-box transcription factor RIPENING INHIBITOR (RIN), and its expression is induced during fruit ripening. Silencing of SlGLYI4 leads to drastic MG overaccumulation at ripening stages of transgenic fruits and interferes with the ripening process. MG most likely glycates and inhibits key enzymes such as methionine synthase and S-adenosyl methionine synthase in the ethylene biosynthesis pathway, thereby indirectly affecting fruit pigmentation and cell wall metabolism. MG overaccumulation in fruits of several nonripening or ripening-inhibited tomato mutants suggests that the tightly regulated MG detoxification process is crucial for normal ripening progression. Our results underpin a SlGLYI4-mediated regulatory mechanism by which MG detoxification controls fruit ripening in tomato.

Ultraviolet radiation promotes the production of hispidin polyphenols by medicinal mushroom Inonotus obliquus

Production of hispidin polyphenols in Inonotus obliquus is a stress-induced response triggered by environmental factors. As one of the important environmental factors, ultraviolet (UV) radiation plays regulatory roles in fungal growth and development. However, whether UV radiation regulates the formation of hispidin polyphenols remains to be established. In this study, we cultivated I. obliquus on solid medium and imposed intermittent UV radiation. We showed that UV exposure inhibited the growth of mycelia but increased the production of polyphenols. Further bioassays revealed that UV radiation also increased the catalytic activities of phenylalanine ammonia-lyase (PAL) and chalcone isomerase (CHI), up-regulated expression of genes related to redox, transcriptional regulation, and metabolism. In addition, the total extracts from the UV-irradiated group were more capable of scavenging DPPH and ABTS⁺ free radicals, especially at the later stage of culture. Thus, UV radiation, acting as one of the environmental factors, stimulated the accumulation of polyphenols in I. obliquus by regulating the activities of enzymes and the expression of genes related to growth and metabolism, and can be tentatively used as a feasible strategy to enhance the production of polyphenols in I. obliquus.

Nitrate–Nitrite–Nitric Oxide Pathway: A Mechanism of Hypoxia and Anoxia Tolerance in Plants

Oxygen (O₂) is the most crucial substrate for numerous biochemical processes in plants. Its deprivation is a critical factor that affects plant growth and may lead to death if it lasts for a long time. However, various biotic and abiotic factors cause O₂ deprivation, leading to hypoxia and anoxia in plant tissues. To survive under hypoxia and/or anoxia, plants deploy various mechanisms such as fermentation paths, reactive oxygen species (ROS), reactive nitrogen species (RNS), antioxidant enzymes, aerenchyma, and adventitious root formation, while nitrate (NO₃⁻), nitrite (NO₂⁻), and nitric oxide (NO) have shown numerous beneficial roles through modulating these mechanisms. Therefore, in this review, we highlight the role of reductive pathways of NO formation which lessen the deleterious effects of oxidative damages and increase the adaptation capacity of plants during hypoxia and anoxia. Meanwhile, the overproduction of NO through reductive pathways during hypoxia and anoxia leads to cellular dysfunction and cell death. Thus, its scavenging or inhibition is equally important for plant survival. As plants are also reported to produce a potent greenhouse gas nitrous oxide (N₂O) when supplied with NO₃⁻ and NO₂⁻, resembling bacterial denitrification, its role during hypoxia and anoxia tolerance is discussed here. We point out that NO reduction to N₂O along with the phytoglobin-NO cycle could be the most important NO-scavenging mechanism that would reduce nitro-oxidative stress, thus enhancing plants’ survival during O₂-limited conditions. Hence, understanding the molecular mechanisms involved in reducing NO toxicity would not only provide insight into its role in plant physiology, but also address the uncertainties seen in the global N₂O budget.

Microorganisms in Plant Growth and Development: Roles in Abiotic Stress Tolerance and Secondary Metabolites Secretion

Crops aimed at feeding an exponentially growing population are often exposed to a variety of harsh environmental factors. Although plants have evolved ways of adjusting their metabolism and some have also been engineered to tolerate stressful environments, there is still a shortage of food supply. An alternative approach is to explore the possibility of using rhizosphere microorganisms in the mitigation of abiotic stress and hopefully improve food production. Several studies have shown that rhizobacteria and mycorrhizae organisms can help improve stress tolerance by enhancing plant growth; stimulating the production of phytohormones, siderophores, and solubilizing phosphates; lowering ethylene levels; and upregulating the expression of dehydration response and antioxidant genes. This article shows the secretion of secondary metabolites as an additional mechanism employed by microorganisms against abiotic stress. The understanding of these mechanisms will help improve the efficacy of plant-growth-promoting microorganisms.

Beneficial Effects of Sodium Nitroprusside on the Aroma, Flavors, and Anthocyanin Accumulation in Blood Orange Fruits

The quality of Tarocco blood orange (Citrus sinensis (L.) Osbeck), which has been cultivated for many years, has degraded substantially. Decreased sugar content, decreased blood color, and increased sour flavor have developed as a result. To improve fruit quality, we studied the effects of bagging and sodium nitroprusside, as a nitric oxide (NO) donor, on the fruit quality of Tarocco blood orange two months before picking. The results showed that NO treatment effectively improved the content of total soluble solids and limonene in the fruit, as well as the color and hardness of the fruit, but reduced the tannin content. It also increased the contents of soluble sugar, fructose, sucrose, vitamin C, amino acids, and mineral elements. NO treatment inhibited the activities of polygalacturonase and pectin esterase, delayed the degradation of protopectin, and promoted the accumulation of anthocyanins, total flavonoids, and flavonoids synthesis. Thus, NO treatment improved the aroma, flavors, and physical properties of blood orange fruit.

An Antifungal Role of Hydrogen Sulfide on Botryosphaeria Dothidea and Amino Acid Metabolism Involved in Disease Resistance Induced in Postharvest Kiwifruit

Botryosphaeria dothidea is a major pathogen responsible for postharvest kiwifruit soft rot. This study aimed to determine the influence of hydrogen sulfide (H₂S) on postharvest resistance to kiwifruit soft rot and the antifungal role of H₂S against B. dothidea. The results indicated that H₂S (20 μl L^-1) restricted the lesion area following inoculation with B. dothidea. H₂S enhanced the production of shikimic acid, tyrosine, tryptophan, and phenylalanine while also increasing the total phenols, flavonoids, and lignin. H₂S upregulated the expression of AcDHQS, AcSDH, AcSK, AcPAL, AcCAD, and AcCHS. Additionally, sodium hydrosulfide (NaHS)-released H₂S inhibited mycelial growth. NaHS concentrations of 20 and 40 mmol L^-1 significantly decreased the mycelial weight and malondialdehyde content (MDA) content while increasing cell membrane conductivity and membrane leakage. The results indicate that H₂S induces resistance in kiwifruit via a microbicidal role and amino acid metabolism involved in postharvest kiwifruit disease resistance.

Cell wall modification and lignin biosynthesis involved in disease resistance against Diaporthe citri in harvested pummelo fruit elicited by carvacrol

BACKGROUND

Phomopsis stem-end rot caused by Diaporthe citri, causes significant commercial postharvest losses of pummelo fruit during storage. Carvacrol (CVR) is a known generally recognized as safe and has the ability to prolong the preservation of harvested fruits. In the present study, the inhibitory effects of CVR treatment at the appropriate concentration on Phomopsis stem-end rot development of harvested pummelo fruit inoculated with D. citri were evaluated by the amounts of cell wall components, the activities and gene expressions of related enzymes involved in cell wall modification and lignin biosynthesis.

RESULTS

Results indicated that CVR completely inhibited D. citri growth in vitro at 200 mg L^-1 and significantly controlled Phomopsis stem-end rot development in harvested pummelo. The CVR treatment delayed peel softening and browning, and retarded electrolyte leakage, superoxide radical (O₂ ^•- ) production, and malondialdehyde content. The CVR-treated fruit maintained higher amounts of cell wall material, protopectin, hemicelluloses, and cellulose, but exhibited lower water-soluble pectin amount. Moreover, in D. citri-inoculated fruit, CVR treatment suppressed the activities and gene expressions of cell wall disassembling-enzymes, including pectin methylesterase, polygalacturonase, cellulase, and β-galactosidase, while the development of cell wall degradation was reduced. Meanwhile, the CVR treatment enhanced the lignin biosynthesis by increasing the activities and up-regulating the gene expressions of phenylalanine ammonialyase, cinnamic alcohol dehydrogenase, and peroxidase accompanied with elevated level of lignin in pummelo fruit.

CONCLUSION

The disease resistance to D. citri in pummelo fruit elicited by CVR treatment is related to delaying cell wall degradation and enhancing lignin biosynthesis. © 2021 Society of Chemical Industry.

N-Acetyl-d-glucosamine Inhibition of Hexokinase Results in Downregulation of the Phenylpropanoid Metabolic Pathway and Decreased Resistance to Brown Rot in Peach Fruit

To explore the role of hexokinase (HXK) on disease resistance in peach fruit, peaches were treated with N-acetyl-d-glucosamine (NAG), a known HXK inhibitor, and then inoculated with Monilinia fructicola. We demonstrate that NAG significantly inhibits HXK activity, which in turn results in significantly reduced resistance to M. fructicola infection. In the HXK-inhibited fruit, the sucrose content was higher and the glucose and fructose contents were lower than in the control fruit. By transcriptome analysis, we found 347 differentially expressed genes (DEGs) between NAG-treated and control peaches, most of which were involved in the mitogen-activated protein kinase signaling pathway in plants, plant-pathogen interaction, plant hormone signal transduction, and the phenylpropanoid biosynthesis pathway. In particular, the DEGs related to phenylpropanoid metabolism, such as peroxidase, flavonoid, and isoflavonoid biosynthesis were significantly downregulated. Nontargeted metabolomic analysis revealed 44 differential metabolites, 9 of which were increased and 35 of which were decreased in the NAG-treated fruit. The decreased metabolites were secondary metabolites, including polyphenols, flavonoids, terpenoids, and glycosides. The relationship between HXK and phenylpropanoid metabolism was further investigated, and we found that in HXK-inhibited fruits the activities of phenylalanine ammonia-lyase, 4-coumarate-CoA ligase, and cinnamate 4-hydroxylase were significantly decreased over the control fruit, as well as the total phenol and total flavone contents were also significantly decreased. These results demonstrate that the inhibition of HXK activity decreases the disease resistance of peach fruits by affecting sugar metabolism and the phenylpropanoid pathway.

Nitric oxide regulation of plant metabolism

Nitric oxide (NO) has emerged as an important signal molecule in plants, having myriad roles in plant development. In addition, NO also orchestrates both biotic and abiotic stress responses, during which intensive cellular metabolic reprogramming occurs. Integral to these responses is the location of NO biosynthetic and scavenging pathways in diverse cellular compartments, enabling plants to effectively organize signal transduction pathways. NO regulates plant metabolism and, in turn, metabolic pathways reciprocally regulate NO accumulation and function. Thus, these diverse cellular processes are inextricably linked. This review addresses the numerous redox pathways, located in the various subcellular compartments that produce NO, in addition to the mechanisms underpinning NO scavenging. We focus on how this molecular dance is integrated into the metabolic state of the cell. Within this context, a reciprocal relationship between NO accumulation and metabolite production is often apparent. We also showcase cellular pathways, including those associated with nitrate reduction, that provide evidence for this integration of NO function and metabolism. Finally, we discuss the potential importance of the biochemical reactions governing NO levels in determining plant responses to a changing environment.

Sodium Hydrosulfide Induces Resistance Against Penicillium expansum in Apples by Regulating Hydrogen Peroxide and Nitric Oxide Activation of Phenylpropanoid Metabolism

As a multifunctional signaling molecule, hydrogen sulfide (H₂S) has been reported to induce plant responses to a variety of abiotic stresses. However, there are no reports on H₂S treatment inducing resistance in apples against Penicillium expansum, a biotic factor, and its possible mechanism of action. In this study, fumigating apples with 5 mM sodium hydrosulfide (NaHS), the exogenous donor of H₂S, for 12 h reduced the diameter of lesions in fruit colonized by P. expansum. NaHS treatment markedly promoted the synthesis of endogenous H₂S, hydrogen peroxide (H₂O₂), and nitrogen oxide (NO). In vivo NaHS treatment enhanced the activities of phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, p-coumarate:coenzyme A ligase isoenzymes, caffeoyl-CoA-O-methyltransferase, caffeic acid-O-methyltransferase, ferulic acid-5-hydroxylase, cinnamyl-CoA reductase, and cinnamyl-alcohol dehydrogenase. The treatment also facilitated the production of specific phenolic acids, such as cinnamic acid, p-coumaric acid, caffeic acid, ferulic acid, and sinapic acid; total phenolic compounds; p-coumaryl alcohol; coniferyl alcohol; sinapyl alcohol; and lignin. NaHS treatment induced resistance against P. expansum in apples through H₂O₂- and NO-mediated activation of phenylpropanoid metabolism.

Effects of nitric oxide treatment on lignin biosynthesis and texture properties at wound sites of muskmelons

Lignin is an important component of the healing tissue in fruits. In this study, we treated muskmelon (Cucumis melo L. cv. "Manao") fruit with exogenous nitric oxide (NO) donor sodium nitroprusside (SNP) to observe and analyze its effect on lignin synthesis and accumulation during healing. Results showed that SNP treatment enhanced the contents of endogenous NO and H₂O₂, increased the activities of phenylalanine ammonia lyase, cinnamate 4 hydroxylase, cinnamyl alcohol dehydrogenase, and peroxidase, and raised the contents of sinapyl alcohol, coniferyl alcohol, coumaryl alcohol, and lignin. SNP augmented the hardness of the healing tissue and decreased its resilience, springiness, and cohesiveness. In addition, SNP treatment effectively reduced the weight loss and disease index of wounded muskmelons. All these results suggest that lignin metabolism mediated by NO play a crucial role in wound healing of muskmelons.

Blue and UV-B light synergistically induce anthocyanin accumulation by co-activating nitrate reductase gene expression in Anthocyanin fruit (Aft) tomato

The tomato accession LA1996, which carries a dominant allele of anthocyanin fruit (Aft) locus, accumulates anthocyanins in the epidermis of fruits when exposed to sunlight. The involvement of blue, UV-A, UV-B and a combination of these wavelengths on anthocyanin accumulation and the molecular mechanism of their regulation was investigated in LA1996. The most effective treatment for inducing anthocyanin biosynthesis in Aft fruits was co-irradiation with blue and UV-B (blue + UV-B) light. Finding the correlated genes is an important approach towards understanding their molecular mechanisms. In the present study, the nitrate reductase (NR) gene SlNIA was isolated using RNA-seq profiling of Aft fruits given different light treatments. The functions of NR-mediated anthocyanin induction by blue + UV-B were confirmed using a series of chemical treatments, followed by assessment of NR activity and nitric oxide (NO) detection. The expression of NR was highly induced by blue + UV-B, and this specificity was also confirmed with the enzyme activity of NR and the NO concentration. The NR inhibitors, which reduce NO generation, the expression levels of anthocyanin related genes and decreased anthocyanin accumulation in LA1996. Our results suggest that NR plays a key role in blue + UV-B-mediated anthocyanin accumulation in LA1996 fruits.

Exogenous Sodium Nitroprusside Mitigates Salt Stress in Lentil (Lens culinaris Medik.) by Affecting the Growth, Yield, and Biochemical Properties

Soil salinity disrupts the physiological and biochemical processes of crop plants and ultimately leads to compromising future food security. Sodium nitroprusside (SNP), a contributor to nitric oxide (NO), holds the potential to alleviate abiotic stress effects and boost tolerance in plants, whereas less information is available on its role in salt-stressed lentils. We examined the effect of exogenously applied SNP on salt-stressed lentil plants by monitoring plant growth and yield-related attributes, biochemistry of enzymes (superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD)) amassing of leaf malondialdehyde (MDA) and hydrogen peroxide (H2O2). Salinity stress was induced by NaCl application at concentrations of 50 mM (moderate salinity) and 100 mM (severe salinity), while it was alleviated by SNP application at concentrations of 50 µM and 100 µM. Salinity stress severely inhibited the length of roots and shoots, the relative water content, and the chlorophyll content of the leaves, the number of branches, pods, seeds, seed yield, and biomass per plant. In addition, MDA, H2O2 as well as SOD, CAT, and POD activities were increased with increasing salinity levels. Plants supplemented with SNP (100 µM) showed a significant improvement in the growth- and yield-contributing parameters, especially in plants grown under moderate salinity (50 mM NaCl). Essentially, the application of 100 µM SNP remained effective to rescue lentil plants under moderate salinity by regulating plant growth and biochemical pathways. Thus, the exogenous application of SNP could be developed as a useful strategy for improving the performance of lentil plants in salinity-prone environments.

The dual interplay of RAV5 in activating nitrate reductases and repressing catalase activity to improve disease resistance in cassava

Cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (Xam) seriously affects cassava yield. Nitrate reductase (NR) plays an important role in plant nitrogen metabolism in plants. However, the in vivo role of NR and the corresponding signalling pathway remain unclear in cassava. In this study, we isolated MeNR1/2 and revealed their novel upstream transcription factor MeRAV5. We also identified MeCatalase1 (MeCAT1) as the interacting protein of MeRAV5. In addition, we investigated the role of MeCatalase1 and MeRAV5-MeNR1/2 module in cassava defence response. MeNRs positively regulates cassava disease resistance against CBB through modulation of nitric oxide (NO) and extensive transcriptional reprogramming especially in mitogen-activated protein kinase (MAPK) signalling. Notably, MeRAV5 positively regulates cassava disease resistance through the coordination of NO and hydrogen peroxide (H2 O2 ) level. On the one hand, MeRAV5 directly activates the transcripts of MeNRs and NO level by binding to CAACA motif in the promoters of MeNRs. On the other hand, MeRAV5 interacts with MeCAT1 to inhibit its activity, so as to negatively regulate endogenous H2 O2 level. This study highlights the precise coordination of NR activity and CAT activity by MeRAV5 through directly activating MeNRs and interacting with MeCAT1 in plant immunity.

Pichia galeiformis Induces Resistance in Postharvest Citrus by Activating the Phenylpropanoid Biosynthesis Pathway

This study aimed to investigate the effect of Pichia galeiformis on disease resistance and elucidate the changes in phenylpropane biosynthesis treated by P. galeiformis in postharvest citrus. The results showed that P. galeiformis reduced the disease incidence and lesion diameters without direct contact with the pathogen Penicillium digitatum. Transcriptome analysis revealed that phenylpropanoid biosynthesis was triggered by P. galeiformis. Genes encoding phenylpropanoid biosynthesis were upregulated, including phenylalanine ammonia-lyase (PAL), 4-coumaroyl-CoA ligase (4CL), cinnamate-4-hydroxylase (C4H), peroxidase (POD), cinnamyl alcohol dehydrogenase (CAD), O-methyltransferase, and hydroxyl cinnamoyl transferase. Moreover, P. galeiformis increased the activity of PAL, 4CL, C4H, POD, polyphenol oxidase, and CAD in citrus pericarp. In addition, P. galeiformis treated citrus displayed higher levels of total phenolic compounds, flavonoid, and lignin and higher amounts of ferulic and sinapic acid. In conclusion, these results suggested that P. galeiformis could induce resistance through modulating the pathway of phenylpropanoid biosynthesis in postharvest citrus.

Multifaceted roles of nitric oxide in tomato fruit ripening: NO-induced metabolic rewiring and consequences for fruit quality traits

Nitric oxide (NO) has been implicated as part of the ripening regulatory network in fleshy fruits. However, very little is known about the simultaneous action of NO on the network of regulatory events and metabolic reactions behind ripening-related changes in fruit color, taste, aroma and nutritional value. Here, we performed an in-depth characterization of the concomitant changes in tomato (Solanum lycopersicum) fruit transcriptome and metabolome associated with the delayed-ripening phenotype caused by NO supplementation at the pre-climacteric stage. Approximately one-third of the fruit transcriptome was altered in response to NO, including a multilevel down-regulation of ripening regulatory genes, which in turn restricted the production and tissue sensitivity to ethylene. NO also repressed hydrogen peroxide-scavenging enzymes, intensifying nitro-oxidative stress and S-nitrosation and nitration events throughout ripening. Carotenoid, tocopherol, flavonoid and ascorbate biosynthesis were differentially affected by NO, resulting in overaccumulation of ascorbate (25%) and flavonoids (60%), and impaired lycopene production. In contrast, the biosynthesis of compounds related to tomato taste (sugars, organic acids, amino acids) and aroma (volatiles) was slightly affected by NO. Our findings indicate that NO triggers extensive transcriptional and metabolic rewiring at the early ripening stage, modifying tomato antioxidant composition with minimal impact on fruit taste and aroma.

Improvement of Postharvest Anthracnose Resistance in Mango Fruit by Nitric Oxide and the Possible Mechanisms Involved

The anthracnose rot of postharvest mango fruit is a devastating fungal disease often resulting in tremendous quality deterioration and postharvest losses. Nitric oxide (NO), as an important signaling molecule, is involved in the responses to postharvest fruit diseases. In the present study, the effectiveness of NO donor sodium nitroprusside (SNP) to prevent anthracnose of "Tainong" mango fruit caused by Colletotrichum gloeosporioides was evaluated through in vivo and in vitro tests. Results from in vivo test showed that SNP treatment effectively inhibited the lesion diameter and disease incidence on inoculated mango fruit during storage. SNP treatment could regulate hydrogen peroxide levels by reinforcing the activities of catalase, peroxidase, superoxide dismutase, and ascorbate peroxidase. Furthermore, SNP elevated the accumulation of lignin, total phenolics, anthocyanin, and flavonoids and the activities of chitinase and β-1,3-glucanase. In addition, in vitro tests indicated that SNP markedly suppressed mycelial growth and spore germination of C. gloeosporioides through damaging plasma membrane integrity and increasing the leakage of soluble sugar and protein. Our results suggested that SNP could suppress anthracnose decay in postharvest mango fruit, possibly by directly suppressing pathogen growth and indirectly triggering host defense responses.

Nitric oxide: A radical molecule with potential biotechnological applications in fruit ripening

Nitric oxide (NO) is a short-life and free radical molecule involved in a wide range of cellular, physiological and stressful processes in higher plants. In recent years it has been observed that exogenous NO application can palliate adverse damages against abiotic and biotic stresses. Conversely, there is accumulating information indicating that endogenous NO participates significantly in the mechanism of modulation of the ripening in climacteric and non-climacteric fruits. Even more, when NO is exogenously applied, it can mediate beneficial effects during ripening and postharvest storage being one of the main effects the increase of antioxidant systems. Consequently, NO could be a promising biotechnological tool to improve crops through ameliorating nutritional indexes and to alleviate damages during fruit ripening and postharvest management. Thus, this approach should be complementary to previous strategies to allow preserving the quality and healthiness of fruits with a view of enhancing their added value. The present mini-review aims to provide an overview of NO biochemistry in plants and updated information on the relevance of NO in fruit ripening and postharvest stages with a view to its biotechnological applications.

Methyl jasmonate induces the resistance of postharvest blueberry to gray mold caused by Botrytis cinerea

BACKGROUND

The effects of postharvest methyl jasmonate (MeJA) treatment (50 μmol L^-1 ) on the control of gray mold caused by Botrytis cinerea in blueberry fruit were evaluated by analyzing (i) the levels of disease resistance signals; (ii) the activity of enzymes involved in antioxidant system, disease resistance and phenylpropanoid pathway, and (iii) the secondary metabolite content.

RESULTS

The results indicated that MeJA treatment significantly restrained the development of gray mold decay in blueberries. The treatment induced a nitric oxide (NO) burst and increased the endogenous hydrogen peroxide (H₂ O₂ ) content in the earlier period of storage. The enhanced NO and H₂ O₂ generation by MeJA treatment might serve as a signal to induce resistance against B. cinerea infection. Furthermore, in inoculated fruit, MeJA treatment significantly promoted antioxidant enzymes and defense-related enzyme activity, which included superoxide dismutase, catalase, ascorbate peroxidase, chitinase, and β-1,3-glucanase, and the degree of membrane lipid peroxidation was reduced. The MeJA treatment enhanced the phenylpropanoid pathway by provoking phenylalanine ammonialyase, cinnamate 4-hydroxylase, and 4-coumarate CoA ligase activity, which was accompanied by elevated levels of phenolics and flavonoids in blueberry fruit.

CONCLUSION

These results suggested that MeJA could induce the disease resistance of blueberries against B. cinerea by regulating the antioxidant enzymes, defense-related enzymes, and the phenylpropanoid pathway through the activation of signaling molecules.

LbNR-Derived Nitric Oxide Delays Lycium Fruit Coloration by Transcriptionally Modifying Flavonoid Biosynthetic Pathway

Anthocyanin-derived fleshy fruit pigmentation has become an excellent system for studying the regulatory network underlying fruit ripening and quality. The transcriptional control of anthocyanin biosynthesis by MYB-bHLH-WDR complexes has been well established, but the intermediate signals through which the environmental or developmental cues regulate these transcription factors remain poorly understood. Here we found that nitric oxide (NO) production during Lycium fruit ripening decreased progressively presenting a negative relationship with anthocyanins. After cloning of the nitric reductase (NR) gene from Lycium barbarum (LbNR) plants, we demonstrated that LbNR-derived NO partially inhibited anthocyanin biosynthesis but enhanced proanthocyanidin (PA) accumulation, and delayed fruit coloration. Application of the NO donor, sodium nitroprusside (SNP), produced a similar effect. The endogenous or exogenous NO downregulated the transcripts both of the regulatory genes and the structural genes that related to anthocyanin biosynthesis, while upregulated both of those genes that related to PA biosynthesis. Given there is a significant negative relationship between the levels of anthocyanins and PAs during Lycium fruit ripening, NO not only inhibited anthocyanin de novo biosynthesis but redirected the flavonoid biosynthetic pathway from anthocyanins to PA production. Two types of LrMYB transcription factors of opposite nature, namely anthocyanin-specific and PA-specific, which belong to the R2R3-MYB subfamily and 1R-MYB subfamily, respectively, were identified from L. ruthenicum fruits. It was further found that NO acts by antagonizing the ABA signaling, a phytohormone we have previously shown playing a positive role in Lycium fruit coloration. Our results provided particularly novel information about NO-ABA-anthocyanin interplay during Lycium fruit development and ripening, which may fill a gap between the developmental cues and the transcriptional regulation of anthocyanin biosynthesis.

Terpinen-4-ol Enhances Disease Resistance of Postharvest Strawberry Fruit More Effectively than Tea Tree Oil by Activating the Phenylpropanoid Metabolism Pathway

This study aimed to reveal the effects and possible mechanism of terpinen-4-ol, the main component of tea tree oil (TTO), on the disease resistance of strawberry fruit. When the effects of TTO and its components were compared on the decay development in fruit inoculated with Botrytis cinerea after treatment, strawberry treated with terpinen-4-ol showed the lowest disease incidence (44.4%) after 48 h and also the smallest lesion diameter during the whole storage. This indicates that terpinen-4-ol induces the highest disease resistance in strawberry compared with TTO and other components. Untargeted metabolomic analysis showed that terpinen-4-ol treatment strongly activated phenylpropanoid biosynthesis and flavonoid metabolism pathway by increasing the accumulation of cinnamaldehyde, coniferyl aldehyde, naringenin, taxifolin, quercetin, and quercitrin in fruit at 12 h after treatment. In addition, terpinen-4-ol treatment also caused the accumulation of total phenolics and lignin by enhancing activities and relative gene expression of key enzymes in the phenylpropanoid metabolism pathway. These results suggest that terpinen-4-ol, as the key component of TTO, is the most important contributor to the effectiveness of TTO in improving disease resistance of strawberry fruit through activating the phenylpropanoid metabolism pathway.

Exogenous β-aminobutyric acid application attenuates Aspergillus decay, minimizes aflatoxin B1 accumulation, and maintains nutritional quality in fresh-in-hull pistachio kernels

BACKGROUND

Pistachio fruits suffer from postharvest decay, caused by Aspergillus flavus. This results in aflatoxin B1 (AFB₁ ) accumulation in kernels, which is hazardous for human health due to its carcinogenic activity. In this study, the mechanism used by exogenous β-aminobutyric acid (BABA) treatment for attenuating Aspergillus decay, minimizing aflatoxin B₁ (AFB₁ ) accumulation, and maintaining nutritional quality in fresh-in-hull pistachio kernels, infected by A. flavus during storage at 25 °C for 18 days, was investigated.

RESULT

Results of an in vivo assay showed that the spore germination and germ tube elongation of A. flavus was repressed by BABA treatment at 7.5 mM. Aspergillus decay accompanied by AFB₁ accumulation was also minimized in fresh-in-hull pistachio kernels treated with BABA at 7.5 mM and infected by A. flavus. Fresh-in-hull pistachio kernels, infected by A. flavus, treated with BABA at 7.5 mM, also exhibited higher phenol and flavonoid accumulation and 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging capacity accompanied by higher phenylalanine ammonia lyase (PAL) enzyme activity.

CONCLUSION

Promoting phenylpropanoid pathway activity with higher PAL enzyme activity in fresh-in-hull pistachio kernels treated with BABA may not only reduce Aspergillus decay in kernels by cell wall fortification but also may be favorable for maintaining the kernels' nutritional quality through its effects on ROS scavenging capacity. As oxidative stress, represented by ROS accumulation, is responsible for A. flavus growth and AFB1 accumulation, higher phenol and flavonoid accumulation in fresh-in-hull pistachio kernels treated with BABA may be beneficial for attenuating Aspergillus decay and minimizing AFB1 accumulation. © 2019 Society of Chemical Industry.

Citrus Postharvest Green Mold: Recent Advances in Fungal Pathogenicity and Fruit Resistance

As the major postharvest disease of citrus fruit, postharvest green mold is caused by the necrotrophic fungus Penicillium digitatum (Pd), which leads to huge economic losses worldwide. Fungicides are still the main method currently used to control postharvest green mold in citrus fruit storage. Investigating molecular mechanisms of plant-pathogen interactions, including pathogenicity and plant resistance, is crucial for developing novel and safer strategies for effectively controlling plant diseases. Despite fruit-pathogen interactions remaining relatively unexplored compared with well-studied leaf-pathogen interactions, progress has occurred in the citrus fruit-Pd interaction in recent years, mainly due to their genome sequencing and establishment or optimization of their genetic transformation systems. Recent advances in Pd pathogenicity on citrus fruit and fruit resistance against Pd infection are summarized in this review.

Physiological Responses to the Foliar Application of Synthetic Resistance Elicitors in Cape Gooseberry Seedlings Infected with Fusarium oxysporum f. sp. physali

Vascular wilt caused by Fusarium oxysporum is the most limiting disease that affects cape gooseberry (Physalis peruviana L.) crops in Colombia. The use of synthetic elicitors for vascular wilt management is still scarce in Andean fruit species. The objective of the present study was to evaluate the effect and number of foliar applications of synthetic elicitors such as jasmonic acid (JA), salicylic acid (SA), brassinosteroids (BR), or a commercial resistance elicitor based on botanical extracts (BE) on disease progress and their effect on the physiology of cape gooseberry plants inoculated with F. oxysporum f. sp. physali. Groups of ten plants were separately sprayed once, twice, or three times with a foliar synthetic elicitor, respectively. Elicitor applications were performed at the following concentrations: JA (10 mL L⁻¹), SA (100 mg L⁻¹), BR (1 mL L⁻¹) and BE (2.5 mL of commercial product (Loker^®) L⁻¹). The results showed that three foliar BR, SA, or BE applications reduced the area under the disease progress, severity index, and vascular browning in comparison to inoculated plants without any elicitor spray. Three BR, SA, or BE sprays also favored stomatal conductance, water potential, growth (total dry weight and leaf area) and fluorescence parameters of chlorophyll compared with inoculated and untreated plants with no elicitor sprays. Three foliar sprays of SA, BR, or BE enhanced photosynthetic pigments (leaf total chlorophyll and carotenoid content) and proline synthesis and decreased oxidative stress in Foph-inoculated plants. In addition, the effectiveness of three foliar BR, SA, or BE sprays was corroborated by three-dimensional plot and biplot analysis, in which it can evidence that stomatal conductance, proline synthesis, and efficacy percentage were accurate parameters to predict Foph management. On the hand, JA showed the lowest level of amelioration of the negative effects of Foph inoculation. In conclusion, the use of the synthetic elicitors BR, SA, or BE can be considered as a tool complementary for the commercial management of vascular wilt in areas where this disease is a limiting factor.

Present knowledge and controversies, deficiencies, and misconceptions on nitric oxide synthesis, sensing, and signaling in plants

After 30 years of intensive work, nitric oxide (NO) has just started to be characterized as a relevant regulatory molecule on plant development and responses to stress. Its reactivity as a free radical determines its mode of action as an inducer of posttranslational modifications of key target proteins through cysteine S-nitrosylation and tyrosine nitration. Many of the NO-triggered regulatory actions are exerted in tight coordination with phytohormone signaling. This review not only summarizes and updates the information accumulated on how NO is synthesized, sensed, and transduced in plants but also makes emphasis on controversies, deficiencies, and misconceptions that are hampering our present knowledge on the biology of NO in plants. The development of noninvasive accurate tools for the endogenous NO quantitation as well as the implementation of genetic approaches that overcome misleading pharmacological experiments will be critical for getting significant advances in better knowledge of NO homeostasis and regulatory actions in plants.

Nitric oxide-dependent regulation of sweet pepper fruit ripening

Ripening is a complex physiological process that involves changes in reactive nitrogen and oxygen species that govern the shelf-life and quality of fruits. Nitric oxide (NO)-dependent changes in the sweet pepper fruit transcriptome were determined by treating fruits at the initial breaking point stage with NO gas. Fruits were also harvested at the immature (green) and ripe (red) stages. Fruit ripening in the absence of NO resulted in changes in the abundance of 8805 transcripts whose function could be identified. Among these, functional clusters associated with reactive oxygen/nitrogen species and lipid metabolism were significantly modified. NO treatment resulted in the differential expression of 498 genes framed within these functional categories. Biochemical analysis revealed that NO treatment resulted in changes in fatty acid profiling, glutathione and proline contents, and the extent of lipid peroxidation, as well as increases in the activity of ascorbate peroxidase and lipoxygenase. These data provide supporting evidence for the crucial role of NO in the ripening of pepper fruit.

Nitric oxide in the physiology and quality of fleshy fruits

Fruits are unique to flowering plants and confer a selective advantage as they facilitate seed maturation and dispersal. In fleshy fruits, development and ripening are associated with numerous structural, biochemical, and physiological changes, including modifications in the general appearance, texture, flavor, and aroma, which ultimately convert the immature fruit into a considerably more attractive and palatable structure for seed dispersal by animals. Treatment with exogenous nitric oxide (NO) delays fruit ripening, prevents chilling damage, promotes disease resistance, and enhances the nutritional value. The ripening process is influenced by NO, which operates antagonistically to ethylene, but it also interacts with other regulatory molecules such as abscisic acid, auxin, jasmonic acid, salicylic acid, melatonin, and hydrogen sulfide. NO content progressively declines during fruit ripening, with concomitant increases in protein nitration and nitrosation, two post-translational modifications that are promoted by reactive nitrogen species. Dissecting the intimate interactions of NO with other ripening-associated factors, including reactive oxygen species, antioxidants, and the aforementioned phytohormones, remains a challenging subject of research. In this context, integrative 'omics' and gene-editing approaches may provide additional knowledge of the impact of NO in the regulatory processes involved in controlling physiology and quality traits in both climacteric and non-climacteric fruits.

Nitric oxide induced modulations in adventitious root growth, lignin content and lignin synthesizing enzymes in the hypocotyls of Vigna radiata

The present study evaluated the role of nitric oxide (NO) in mediating adventitious root (AR) growth, lignification and related enzymatic changes in the hypocotyls of Vigna radiata. To meet the objectives, the changes in AR growth, lignin content, and the activities of enzymes-peroxidases, polyphenol oxidases, and phenylalanine ammonia lyases- with NO donor and its scavenger were monitored. Hypocotyls were cultivated in aqueous solution supplemented with different concentrations of SNP (sodium nitroprusside, NO donor compound) and its scavenging compound (2,4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; cPTIO). Specifically, at low concentrations, SNP induced AR growth, increased the total lignin content and altered the activities of related oxidoreductases- peroxidases, polyphenol oxidases and phenylalanine ammonia lyases- which are involved in lignin biosynthesis pathway. At higher concentrations, a decline in AR growth and lignification was noticed. We analysed the function of NO in AR formation by depleting the endogenous NO using scavenging compound cPTIO. Hypocotyls grown in a medium supplemented with scavenger cPTIO exhibited significant decline in AR growth and the activities of lignin synthesizing enzymes. Application of NO scavenger showed that stimulatory properties on root lignification may be owing to NO itself. In addition, changes in AR growth were significantly correlated with these modified biochemical activities. Our analysis revealed that NO supplementation induces prominent alterations in lignin level during AR formation and this might be due to an alteration in the activity of lignin biosynthetic enzymes, which further affected the polymerization of monolignols and AR growth.

Nitric oxide on/off in fruit ripening

Fruit ripening is a complex physiological process involving significant external and internal modifications. Classic edible fleshy fruits have been classified as climacteric or non-climacteric according to their dependence on the phyto hormone ethylene; however, data have increasingly confirmed the involvement of the free radical nitric oxide (NO) in this process. Moreover, the exogenous application of NO demonstrates its beneficial effects on fruit quality.

Hot Air Treatment Induces Disease Resistance through Activating the Phenylpropanoid Metabolism in Cherry Tomato Fruit

To explore the effects of hot air (HA, 38 °C for 12 h) treatment on the phenylpropanoid metabolism in cherry tomatoes, phenylpropanoid metabolite levels and the activities and expression of key enzymes were analyzed in HA-treated fruit. HA treatment enhanced phenylpropanoid metabolism, as evidenced by elevated levels of phenolics and flavonoids, higher activities of phenylalanine ammonia-lyase and cinnamate-4-hydroxylase, and upregulated expression of LeCHS, LeCHI, LeF3H, and LeFLS. Levels of several phenylpropanoid metabolites were higher after HA treatment, including p-coumaric acid, caffeic acid, chlorogenic acid, isoquercitrin, quercetin, and rutin. These metabolic changes may be related to the reduced disease incidence and smaller lesion diameters observed in HA-treated fruit inoculated with Alternaria alternata (black mold) or Botrytis cinerea (gray mold). The results suggest that HA treatment induces disease resistance by activating the phenylpropanoid pathway in cherry tomato fruit.

Synergistic Effects of l-Arginine and Methyl Salicylate on Alleviating Postharvest Disease Caused by Botrysis cinerea in Tomato Fruit

The effects of l-arginine (Arg, 1 mM) and/or methyl salicylate (MeSA, 0.05 mM) treatment on gray mold caused by Botrytis cinerea in tomato fruit were studied. Results indicated that Arg or MeSA alleviated the incidence and severity of fruit disease caused by B. cinerea, and that both Arg and MeSA (Arg + MeSA) further inhibited the development of fruit decay. Treatment with Arg + MeSA not only enhanced the activities of superoxide dismutase, catalase, and peroxidase but also promoted the expression levels of pathogenesis-related protein 1 gene and the activities of defense-related enzymes of phenylalanine ammonia-lyase, polyphenol oxidase, β-1,3-glucanase, and chitinase during most of the storage periods, which were associated with lower disease incidence and disease index. In addition, the combined treatment elevated the levels of total phenolics, polyamines, especially putrescine, and nitric oxide. These observations suggest that treatment of fruit with Arg + MeSA is an effective and promising way to alleviate postharvest decays on a commercial scale.