Coordinated activation of the metabolic pathways induced by LED blue light in citrus fruit

Hormones metabolism as affected by LED blue light in citrus fruit

The LED Blue Light (LBL) (450 nm) effect on hormones levels and on jasmonates (JAs) metabolism in oranges was investigated. The quantum flux (2 days, 60 μmol m-2. s-1) was chosen for its efficacy in reducing postharvest rot caused by this crop's main postharvest phytopathogenic fungus (Penicillium digitatum). The analysis of abscisic (ABA), salicylic (SA) and indole-3-acetic (IAA) acids, and of JAs-related metabolites, revealed that LBL modifies all studied metabolites and had major effects on JAs levels, mainly on jasmonic acid (JA) and its precursor cis-(+)-12-oxo-phytodienoic acid (OPDA). This agrees with the up-regulation of the genes participating in their synthesis. Results highlight the relevance of CsLOX1 and CsLOX5, and the contribution of CsAOC3, in the LBL-induced OPDA biosynthesis, whereas CsOPR2, CsACX1 and CsACX3 would play a part in the synthesis of JA from OPDA. Data also suggest that the applied LBL quantum flux favors fruit JA perception by increasing the expression of the coronatine insensitive 1 (COI1) receptor; and signaling by down-regulating abundant CsJAZ negative regulators. Differences in OPDA and JA between the LBL-treated oranges and their control fruit left in the dark disappeared after shifting the LBL-treated oranges to darkness for 3 more days. However, the LBL and darkness combination slightly increased IAA and SA contents.

Deciphering the Effect of Light Wavelengths in Monilinia spp. DHN-Melanin Production and Their Interplay with ROS Metabolism in M. fructicola

Pathogenic fungi are influenced by many biotic and abiotic factors. Among them, light is a source of information for fungi and also a stress factor that triggers multiple biological responses, including the activation of secondary metabolites, such as the production of melanin pigments. In this study, we analyzed the melanin-like production in in vitro conditions, as well as the expression of all biosynthetic and regulatory genes of the DHN-melanin pathway in the three main Monilinia species upon exposure to light conditions (white, black, blue, red, and far-red wavelengths). On the other hand, we analyzed, for the first time, the metabolism related to ROS in M. fructicola, through the production of hydrogen peroxide (H₂O₂) and the expression of stress-related genes under different light conditions. In general, the results indicated a clear importance of black light on melanin production and expression in M. laxa and M. fructicola, but not in M. fructigena. Regarding ROS-related metabolism in M. fructicola, blue light highlighted by inhibiting the expression of many antioxidant genes. Overall, it represents a global description of the effect of light on the regulation of two important secondary mechanisms, essential for the adaptation of the fungus to the environment and its survival.

The Role of ABA in the Interaction between Citrus Fruit and Penicillium digitatum

Abscisic acid (ABA) protects citrus fruit against Penicillium digitatum infection. The global mechanisms involved in the role of ABA in the P. digitatum-citrus fruit interaction are unknown. Here, we determine the transcriptome differences between the Navelate (Citrus sinensis (L.) Osbeck) orange and its ABA-deficient mutant Pinalate, which is less resistant to infection. Low ABA levels may affect both the constitutive mechanisms that protect citrus fruit against P. digitatum and early responses to infection. The repression of terpenoid, phenylpropanoid and glutation metabolism; of oxidation-reduction processes; and of processes related to the defense response to fungus and plant hormone signal transduction may be one part of the constitutive defense reduced in the mutant against P. digitatum. Our results also provide potential targets for developing P. digitatum-citrus fruit-resistant varieties. Of those up-regulated by ABA, a thaumatin protein and a bifunctional inhibitor/LTP, which are relevant in plant immunity, were particularly remarkable. It is also worth highlighting chlorophyllase 1 (CLH1), induced by infection in Pinalate, and the OXS3 gene, which was down-regulated by ABA, because the absence of OXS3 activates ABA-responsive genes in plants.

Blue light combined with salicylic acid treatment maintained the postharvest quality of strawberry fruit during refrigerated storage

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Blue light and salicylic acid combination delayed fruit water loss and decay.

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BL + SA treatment maintained the sensory and nutritional qualities of strawberries.

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BL + SA treatment preserved strawberry bioactive components and antioxidant capacity.

Strawberry is a high economic and nutritional value fruit, but marketing is limited by a short postharvest life. The objective of this work is to assess the influence of blue light (BL) and salicylic acid (SA, 2 mM) on strawberry postharvest quality during cold storage. The results showed that the combination of BL and SA noticeably delayed weight loss, prevented decay, improved fruit skin brightness, and increased soluble protein. Strawberries treated with BL + SA had lower total soluble solids and titratable acidity contents among treatments but had no significant change during the entire storage. Additionally, contents of total flavonoids, phenolics, anthocyanins and proanthocyanidins, activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) and total antioxidant capacities in BL + SA-treated fruit were kept at stable levels throughout the entire storage. Collectively, these findings suggest that BL + SA treatment exhibits a high potential in maintaining postharvest fruit quality of strawberry fruit.

Metabolic changes in cucumber leaves are enhanced by blue light but differentially affected by UV interactions with light signalling pathways in the visible spectrum

Ultraviolet radiation (UV, 280-400 nm) as an environmental signal triggers metabolic acclimatory responses. However, how different light qualities affect UV acclimation during growth is poorly understood. Here, cucumber plants (Cucumis sativus) were grown under blue, green, red, or white light in combination with UV. Their effects on leaf metabolites were determined using untargeted metabolomics. Blue and white growth light triggered increased levels of compounds related to primary and secondary metabolism, including amino acids, phenolics, hormones, and compounds related to sugar metabolism and the TCA cycle. In contrast, supplementary UV in a blue or white light background decreased leaf content of amino acids, phenolics, sugars, and TCA-related compounds, without affecting abscisic acid, auxin, zeatin, or jasmonic acid levels. However, in plants grown under green light, UV induced increased levels of phenolics, hormones (auxin, zeatin, dihydrozeatin-7-N-dihydrozeatin, jasmonic acid), amino acids, sugars, and TCA cycle-related compounds. Plants grown under red light with UV mainly showed decreased sugar content. These findings highlight the importance of the blue light component for metabolite accumulation. Also, data on interactions of UV with green light on the one hand, and blue or white light on the other, further contributes to our understanding of light quality regulation of plant metabolism.

The potential contribution of the WRKY53 transcription factor, gamma-aminobutyric acid (GABA) transaminase, and histone deacetylase in regulating growth, organogenesis, photosynthesis, and transcriptional responses of tomato to different light-emitting diodes (LEDs)

Impressive progress in developing light-emitting diodes (LEDs) offers a new dimension for meeting agricultural and biological expectations. The present study addresses how tomato (Solanum lycopersicum) seedlings respond to the different spectral qualities of LEDs (white, red, blue, and blue + red). The light treatments in a wavelength-dependent manner contributed to the variations in biomass accumulation, morphology, and organogenesis pattern. Light quality epigenetically contributed to the transcriptional regulation of the histone deacetylase (HDA3) gene. The expression of WRKY53 transcription factor and gamma-aminobutyric acid transaminase (GABA-TP1) genes displayed a similar upward trend in response to the blue wavelength. On the contrary, the sole red light downregulated the WRKY53 and GABA-TP1 genes. The blue irradiation was associated with the upregulation in the glycolate oxidase (GLO2) and ribulose-1,5-bisphosphate carboxylase‑oxygenase large subunit (rbcL) genes, while the red wavelength down-regulated the GLO2 and rbcL genes. Moreover, rbcL statistically correlated with GLO2, referring to the balanced regulation of photorespiration and the Calvin cycle. The blue wavelengths were more capable of improving the concentrations of photosynthetic pigments and proline. The seedlings grown under the white LEDs displayed the maximum activity of the catalase enzyme. The cultivation of tomato seedlings under the blue lights enhanced the activities of the superoxide dismutase and ascorbate peroxidase enzymes. The light treatments were associated with the variation in the nutritional status of K⁺ and Ca²⁺ in both leaves and roots. The presented findings and inferences support the potential contribution of WRKY53, HDA3, and GABA signaling in modulating plant responses to light quality.

Ethylene-driven changes in epicuticular wax metabolism in citrus fruit

Epicuticular waxes are important natural compounds that influence cuticle properties and can protect fruit from factors that harm its external quality. We demonstrated that, at a dose that reduces postharvest citrus fruit quality loss (4 d 2 µL L^-1), ethylene redirected epicuticular wax metabolism towards the synthesis of primary alcohols, mostly behenyl alcohol, by favouring the acyl-reduction pathway. This treatment also reduced the synthesis of terpenoids by redirecting the mevalonate pathway towards farnesol accumulation to the detriment of the accumulation of most triterpenoids, but not of their precursor squalene. Moreover, the 4 d ethylene treatment sharply increased the synthesis of docosane and lignoceric acid and lowered that of cerotic acid. Longer ethylene exposure (8 d) reversed some of these effects by lowering the contents of most alcohols, lignoceric acid and squalene, while increasing that of its derivative sitosterol. The 8 d ethylene treatment also increased farnesol and docosane contents.

Albedo- and Flavedo-Specific Transcriptome Profiling Related to Penicillium digitatum Infection in Citrus Fruit

Penicillium digitatum is the main postharvest pathogen of citrus fruit. Although the inner fruit peel part (albedo) is less resistant than the outer part (flavedo) to P. digitatum, the global mechanisms involved in their different susceptibility remain unknown. Here, we examine transcriptome differences between both tissues at fruit harvest and in their early responses to infection. At harvest, not only was secondary metabolism, involving phenylpropanoids, waxes, and terpenoids, generally induced in flavedo vs. albedo, but also energy metabolism, transcription factors (TFs), and biotic stress-related hormones and proteins too. Flavedo-specific induced responses to infection might be regulated in part by ERF1 TF, and are related to structural plant cell wall reinforcement. Other induced responses may be related to H₂O₂, the synthesis of phenylpropanoids, and the stress-related proteins required to maintain basal defense responses against virulent pathogens, whereas P. digitatum represses some hydrolase-encoding genes that play different functions and auxin-responsive genes in this peel tissue. In infected albedo, the repression of transport and signal transduction prevail, as does the induction of not only the processes related to the synthesis of flavonoids, indole glucosinolates, cutin, and oxylipins, but also the specific genes that elicit plant immunity against pathogens.

Control of postharvest diseases caused by Penicillium spp. with myrtle leaf phenolic extracts: in vitro and in vivo study on mandarin fruit during storage

BACKGROUND

In the postharvest handling of horticultural commodities, plant extracts with fungicidal activity are a valid alternative to synthetic fungicides. The fungicidal activity of myrtle leaf extracts from eight cultivars was studied in vitro against Penicillium digitatum, Penicillium italicum, and Penicillium expansum and on artificially inoculated mandarins with green and blue molds during storage for 12 days at 20 °C and 90% RH.

RESULTS

Hydroxybenzoic acids, hydrolysable tannins, and flavonols were identified by high-performance liquid chromatography (HPLC). Despite sharing the same phenolic profile, extracts of eight myrtle cultivars significantly differed in the concentration of phenolics. Hydrolysable tannins are the principal subclass representing nearly 44.9% of the total polyphenols, whereas myricitrin was the most abundant flavonol in all cultivars. Myrtle extracts strongly inhibited conidial germination of the pathogens tested, although the greatest efficacy was observed against P. digitatum. At a concentration of 20 g L^-1 , all the extracts completely inhibited fungi growth; only 'Angela', 'Tonina' and 'Grazia' extracts were effective at lower concentrations (15 g L^-1 ). On inoculated fruit, myrtle extracts significantly controlled rot development. As a preventive treatment, 'Ilaria' and 'Maria Rita' extracts significantly reduced the rate of fruit with green mold decay lesions. When applied as a curative treatment, all the exacts decreased the incidence of decay. Against P. italicum, all the extracts applied as preventive treatments controlled decay effectively, while as curative treatment some of the extracts were not effective. All the extracts reduced the size of the infected areas.

CONCLUSION

The results propose myrtle extracts as a possible natural alternative to synthetic fungicides. © 2021 Society of Chemical Industry.