Inhibitory mechanisms of cinnamic acid on the growth of Geotrichum citri-aurantii

Inhibitory Effect of L-Methionine on Alternaria alternata Based on Metabolomics Analysis

Alternaria alternata is the main pathogenic fungus of postharvest black spots in fruits and vegetables. This study aimed to explore the antifungal activity of methionine on A. alternata in vitro and to reveal related antifungal mechanisms through a metabolomics analysis. The results showed that the inhibitory effects of L-methionine (Met) treatment on mycelium growth, spore germination, and the germ tube elongation of A. alternata were enhanced with an increase in the Met concentration, but the inhibitory effects decreased when the Met concentration was higher than 50 mmolL-1. The results of propidium iodide staining and scanning electron microscopy showed that the Met treatment damaged the plasma membrane integrity of the A. alternata spores and caused an irreversible deformation of mycelium. In addition, after the Met treatment, the leakage of electrolytes, nucleic acid, and proteins in the A. alternata cells was significantly higher than that in the control group, indicating that the Met treatment increased the permeability of the cell membranes. Eighty-one different metabolites, divided into seven categories, were identified through the metabolomics analysis, including forty-three downregulated metabolites and thirty-eight upregulated metabolites. Among them, these differential metabolites were mainly involved in amino acid synthesis and metabolism, the pentose phosphate pathway, and the TCA cycle. Therefore, the antifungal effect of the Met treatment on A. alternata was mainly to damage the integrity of the cell membranes, make nucleic acid and protein contents leak, and affect the TCA cycle, carbohydrate metabolism, amino acid synthesis metabolism, and the metabolic pathways associated with cell membrane biosynthesis. Thus, the growth and development of A. alternata were inhibited. The research enriched the investigation of the effect of the antifungal mechanism of Met treatment on A. alternata and provided a theoretical basis for the application of Met to prevent and treat postharvest black spots in fruits and vegetables.

Role of gum Arabic combined with cinnamic acid coating on quality and cell wall degradation enzymes of mango fruits at low temperature

Mango exhibits a short shelf life when kept under ambient conditions. Therefore, this study was carried out to assess the synergistic effect of gum Arabic (GA) and cinnamic acid (CA) coatings on mango fruits stored for 28 days at 12 ± 1 °C. The treatments included GA 10 and 20 % sole and its formulation with CA 0.2 % while the uncoated fruits were used as control. The results revealed that GA + CA coatings exhibited reduced weight loss, flesh firmness, delayed respiratory peak, retained soluble solids content (SSC), titratable acidity (TA) and skin colour, retarded spoilage, and decreased electrolyte leakage of fruit as compared to uncoated fruit during storage. Among all, the coating GA 20 % + CA 0.2 % maintained higher firmness (16.6 N), SSC (20.1 %), TA (0.71 %), total flavonoid content (152 mg/kg) and total carotenoids content (45.1 mg/kg) and retained the higher sensory quality of mango fruit at the end of storage. Additionally, the GA 20 % + CA 0.2 % coating suppressed activities of polygalacturonase (PG), cellulase (CL) and pectin methyl esterase (PME) in stored mango fruits.

PdStuA Is a Key Transcription Factor Controlling Sporulation, Hydrophobicity, and Stress Tolerance in Penicillium digitatum

Penicillium digitatum has become one of the main pathogens in citrus due to its high spore production and easy spread. In this study, the function of the APSES transcription factor StuA in P. digitatum was characterized, and the results indicated that it was involved in conidium and conidiophore development. No conidiophores were observed in the mycelium of the ∆PdStuA mutant that had grown for two days, while an abnormal conidiophore was found after another two days of incubation, and only small thin phialides as well as a very small number of spores were formed at the top of the hyphae. Moreover, it was observed that the ∆PdStuA mutant showed various defects, such as reduced hydrophobicity and decreased tolerance to cell wall inhibitors and H2O2. Compared to the original P. digitatum, the colony diameter of the ∆PdStuA mutant was not significantly affected, but the growth of aerial hyphae was obviously induced. In in vivo experiments, the spore production of the ∆PdStuA mutant grown on citrus fruit was remarkably decreased; however, there was no significant difference in the lesion diameter between the mutant and original strain. It could be inferred that less spore production might result in reduced spread in citrus, thereby reducing the green mold infection in citrus fruit during storage. This study provided a gene, PdStuA, which played key role in the sporulation of P. digitatum, and the results might provide a reference for the molecular mechanisms of sporulation in P. digitatum.

Inhibitory Mechanisms of trans-2-Hexenal on the Growth of Geotrichum citri-aurantii

Geotrichum citri-aurantii (G. citri-aurantii) is one of the most important postharvest pathogens leading to a postharvest loss of citrus by causing sour rot. In this study, the antifungal activity of trans-2-hexenal, a natural component of essential oil, against G. citri-aurantii was evaluated. Trans-2-hexenal treatment inhibited the mycelia growth of G. citri-aurantii with a minimum inhibitory concentration and minimum fungicidal concentration of trans-2-hexenal at 0.50 and 1.00 μL/mL, respectively. Moreover, trans-2-hexenal efficiently reduced the incidence of sour rot of Satsuma fruit inoculated with G. citri-aurantii. Ultrastructural observations and Fourier transform infrared (FT-IR) results showed that trans-2-hexenal treatment affected the cell wall and cell membrane instructions of G. citri-aurantii. The content of β-1,3-glucan was significantly decreased after trans-2-hexenal treatment, but the cell wall permeability was not changed. The decrease in lipid and ergosterol contents might be responsible for this antifungal activity. Several important genes, FKS1, ERG1, ERG7, and ERG11, showed decreasing expression levels after trans-2-hexenal treatment. Molecule-docking results also indicated that trans-2-hexenal could join with the protein of FKS1, ERG1, ERG7, and ERG11 to impact enzyme activities. These results demonstrated that trans-2-hexenal is a promising fungicide for controlling sour rot of harvested citrus fruit by damaging the membrane integrity of G. citri-aurantii.

The GRAS Salts of Na2SiO3 and EDTA-Na2 Control Citrus Postharvest Pathogens by Disrupting the Cell Membrane

Sodium silicate (Na₂SiO₃) and ethylenediaminetetraacetic acid disodium salt (EDTA-Na₂) are inorganic salts classified as 'Generally Recognized as Safe' (GRAS) compounds with great advantages in controlling various pathogens of postharvest fruits and vegetables. Here, we determined the median effective concentration (EC₅₀) of Na₂SiO₃ (0.06%, 0.05%, 0.07% and 0.08%) and EDTA-Na₂ (0.11%, 0.08%, 0.5%, and 0.07%) against common pathogens affecting postharvest citrus fruit, including Penicillium digitatum, Penicillium italicum, Geotrichum citri-aurantii, and Colletotrichum gloeosporioides. Na₂SiO₃ and EDTA-Na₂ treatments at the EC₅₀ decreased the spore germination rate, visibly disrupted the spore cell membrane integrity, and significantly increased the lipid droplets (LDs) of the four postharvest pathogens. Moreover, both treatments at EC₅₀ significantly reduced the disease incidence of P. italicum (by 60% and 93.335, respectively) and G. citri-aurantii (by 50% and 76.67%, respectively) relative to the control. Furthermore, Na₂SiO₃ and EDTA-Na₂ treatment resulted in dramatically lower disease severity of the four pathogens, while also demonstrating no significant change in citrus fruit quality compared with the control. Therefore, Na₂SiO₃ and EDTA-Na₂ present a promising approach to control the postharvest diseases of citrus fruit.

Screening of Oxygenated Aromatic Compounds for Potential Antifungal Activity against Geotrichum citri-aurantii through Structure-Activity Relationship Analysis

Sour rot caused by Geotrichum citri-aurantii (G. citri-aurantii) is responsible for huge economic losses during citrus fruit storage. However, the availability of chemical fungicides for controlling this disease is rather limited. In the present study, the antifungal activities of 25 oxygenated aromatic compounds against the mycelial growth of G. citri-aurantii were determined, and their corresponding structure-activity relationships were illustrated. Salicylaldehyde (pMIC = 2.689) possessed the strongest inhibitory effect on G. citri-aurantii growth, followed by thymol (pMIC = 2.478) and o-phthalaldehyde (pMIC = 2.429). Molecular electrostatic potential and molecular orbital analysis showed that the antifungal efficiency of test compounds was determined by the number and location of hydroxyl and aldehyde groups and the length of the ester chain. All compounds were selected for quantitative structure-antifungal activity relationship (QSAR) analysis. A three-dimensional-QSAR model of G. citri-aurantii inhibitors was established and demonstrated good predictive capability [comparative molecular field analysis, q² = 0.532, optimum number of components (ONC) =10, R² = 0.996, F = 560.325, standard error of estimation (SEE) = 0.034, and two descriptors; comparative similarity index analysis, q² = 0.675, ONC = 6, R² = 0.989, F = 263.354, SEE = 0.054, and five descriptors]. QSAR analysis showed that substitution at position 1 with hydrophilic and electron-withdrawing groups produced a hydrogen donor and thus improved the antifungal activity. In contrast, substitution at positions 4 or 5 with hydrophilic and electron-donating groups decreased its antifungal activity. These findings can provide theoretical guidance for preparing effective antifungal drugs for controlling sour rot in citrus.

Dimethyl Dicarbonate as a Food Additive Effectively Inhibits Geotrichum citri-aurantii of Citrus

Dimethyl dicarbonate (DMDC), a food additive, can be added to a variety of foods as a preservative. This study aimed to evaluate the inhibitory effects of DMDC on Geotrichum citri-aurantii in vitro and in vivo, as well as the potential antifungal mechanism. In vitro experiments showed that 250 mg/L DMDC completely inhibited the growth of G. citri-aurantii and significantly inhibited spore germination by 96.33%. The relative conductivity and propidium iodide (PI) staining results showed that DMDC at 250 mg/L increased membrane permeability and damaged membrane integrity. Malondialdehyde (MDA) content and 2, 7-Dichlorodihydrofluorescein diacetate (DCHF-DA) staining determination indicated that DMDC resulted in intracellular reactive oxygen species (ROS) accumulation and lipid peroxidation. Scanning electron microscopy (SEM) analysis found that the mycelia were distorted and the surface collapsed after DMDC treatment. Morphological changes in mitochondria and the appearance of cavities were observed by transmission electron microscopy (TEM). In vivo, 500 mg/L DMDC and G. citri-aurantii were inoculated into the wounds of citrus. After 7 days of inoculation, DMDC significantly reduced the disease incidence and disease diameter of sour rot. The storage experiment showed that DMDC treatment did not affect the appearance and quality of fruits. In addition, we found that DMDC at 500 mg/L significantly increased the activity of citrus defense-related enzymes, including peroxidase (POD) and phenylalanine ammonia-lyase (PAL). Therefore, DMDC could be used as an effective method to control citrus sour rot.

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.