Di Gong

erg4 is a key gene for ergosterol biosynthesis in filamentous fungi, but its function in Penicillium expansum remains unknown. Our results showed that P. expansum contains three erg4 genes, including erg4A, erg4B and erg4C. The expression levels of the three genes showed differences in the wild-type (WT) strain, and the expression level of erg4B was the highest, followed by erg4C. Deletion of erg4A, erg4B or erg4C in the WT strain revealed functional redundancy between them. Compared to the WT strain, erg4A, erg4B or erg4C knockout mutants reduced ergosterol levels, with erg4B deletion having the greatest effect. Furthermore, deletion of the three genes reduced sporulation of the strain, and Δerg4B and Δerg4C mutants showed defective spore morphology. In addition, Δerg4B and Δerg4C mutants were found to be more sensitive to cell wall integrity and oxidative stress. However, deletion of erg4A, erg4B or erg4C had no significant effect on colony diameter, spore germination rate, conidiophore structure of P. expansum or pathogenicity to apple fruit. Taken together, erg4A, erg4B and erg4C have redundant functions and are all involved in ergosterol synthesis and sporulation in P. expansum. In addition, erg4B and erg4C contribute to spore morphogenesis, cell wall integrity and response to oxidative stress in P. expansum.

Antioxidant activity is an important feature for evaluating fruit quality and tolerance to biotic or abiotic stresses. Moreover, antioxidant activity is involved in chilling injury (CI) response and postharvest quality in fruit. Chemical elicitors can induce systemic acquired resistance in fruit against pathogens, which could partially replace synthetic fungicides. Recently, researchers have found that preharvest sprays with chemical elicitors can improve antioxidant activity, reduce CI, and maintain quality in harvested fruit. In this review, we summarize that preharvest elicitors spray improve antioxidant activity in harvested fruit by promoting antioxidant components biosynthesis as well as antioxidant ability in vitro. Moreover, preharvest elicitors spray alleviates CI in fruit by regulation of membrane lipid metabolism and reactive oxygen species metabolism. In addition, preharvest elicitors spray maintains fruit quality by modulation of respiration and ethylene release. Finally, this review points out the issues existing and proposes an outlook on preharvest elicitors spray to maintain postharvest fruit quality.

Penicillium expansum is a necrotrophic pathogen, which actively kills host cells and obtains nutrients from dead cells to achieve infection. However, few reports have elucidated the differential levels of carbon and nitrogen sources over increasing distances of the leading edge in fungal colonized fruit tissues during colonization. Our results showed that the highest consumption of sucrose and fructose, as well as the accumulation of glucose, were found in the decayed region of P. expansum-colonized ‘Delicious’ apple fruit compared with the healthy region at the leading edge and the healthy region 6 mm away from the leading edge. As nitrogen sources, the contents of methionine, glutamate, leucine, valine, isoleucine and serine were the lowest in the decayed region compared with the healthy regions during colonization. In addition, the titratable acidity, oxalic acid, citric acid, succinic acid and malic acid showed the highest accumulation in the decayed region compared with the healthy regions. P. expansum colonization induced the accumulation of saturated fatty acids in the decayed region, while the level of unsaturated fatty acids was the lowest. These changes were not observed in the healthy regions. These results indicated that P. expansum kills cells in advance of its colonization in order to obtain the nutrients of the apple tissue from the distal leading tissue of the colonized apple. It is understood that more carbon and nitrogen sources are required for fungal colonization, and a stronger defense response against colonization occurred in the fruit, causing the transit of nutrients from the distal tissue to the infected sites.

Trichothecium roseum is an important postharvest pathogen, belonging to an alkalizing group of pathogens secreting ammonia during fungal growth and colonization of apple fruits. Fungal pH modulation is usually considered a factor for improving fungal gene expression, contributing to its pathogenicity. However, the effects of inoculation with T. roseum spore suspensions at increasing pH levels from pH 3 up to pH 7, on the reactive oxygen species (ROS) production and scavenging capability of the apple fruits, affecting host susceptibility, indicate that the pH regulation by the pathogens also affects host response and may contribute to colonization. The present results indicate that the inoculation of T. roseum spores at pH 3 caused the lowest cell membrane permeability, and reduced malondialdehyde content, NADPH oxidases activity, O₂^●− and H₂O₂ production in the colonized fruit. Observations of the colonized area on the 9th day after inoculation at pH 3, showed that the rate of O₂^●− production and H₂O₂ content was reduced by 57% and 25%, compared to their activities at pH 7. In contrast, antioxidative activities of superoxide dismutase, catalase and peroxidases of fruit tissue inoculated with spores’ suspension in the presence of a solution at pH 3.0 showed their highest activity. The catalase and peroxidases activities in the colonized tissue at pH 3 were higher by almost 58% and 55.9%, respectively, on the 6th day after inoculation compared to inoculation at pH 7. The activities of key enzymes of the ascorbate-glutathione (AsA-GSH) cycle and their substrates and products by the 9th day after fruit inoculation at pH 3 showed 150%, 31%, 16%, and 110% higher activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase, respectively, compared to pH 7. A similar pattern of response was also observed in the accumulation of ascorbic acid and dehydroascorbate which showed a higher accumulation at pH 3 compared to the colonization at pH 7. The present results indicate that the metabolic regulation of the pH environment by the T. roseum not only modulates the fungal pathogenicity factors reported before, but it induces metabolic host changes contributing both together to fungal colonization.