The pH signalling transcription factor PacC modulate growth, development, stress response and pathogenicity of Trichothecium roseum.

pH is one of the important environmental factors that affect the growth, development and pathogenicity of postharvest pathogen. The transcription factor PacC dominates the pH signal pathway. PacC in Trichothecium roseum showed three typical conserved zinc finger domains and closest homology to Fusarium graminearum. T. roseum increased the environmental pH both in vitro and in vivo. Expression patterns of TrpacC under different pH showed that at increasing pH from 3 to 5, the wild-type (WT) strain induced the expression of TrPacC in parallel to increased fungal growth; however, TrPacC expression decline at higer pH than 5, while fungal growth continued to increase. Development of a ΔTrPacC mutant down-regulated the expression of TrbrlA, TrabaA and TrwetA, reduced sporulation and delayed spore germination, resulting in smaller spores and sparse hyphae. ΔTrPacC mutant was sensitive to ionic stress, oxidative stress and cell wall integrity stress compared to the WT strain, especially the ionic stress. In addition, ∆TrPacC mutant showed reduced pathogenicity to muskmelon and tomato fruits. Taken together, T. roseum is an alkalinizing fungus, and the acidic environment could induce TrPacC expression. TrPacC positively regulates fungal growth and development as well as pathogenicity showing effect on fungal response to different stresses.

Alleviation of the degradation of cell wall pectin contributes to the maintenance of integrity of papaya (Carica papaya L.) fruit tissue during abnormal chilling injury behavior.

Papaya (Carica papaya L.) is highly susceptible to chilling injury (CI), which primarily causes surface pitting, black scald, susceptibility to fungal infection, and the breakdown of tissue. It developed rapidly at 6 °C but was significantly suppressed at 1 °C, which indicated that the papaya tissue was more solid at a lower temperature. This rare phenomenon is considered to be abnormal CI behavior. The maintenance of fruit tissue integrity during abnormal CI behavior may be related to modifications of the cell wall pectin. The modifications of cell wall pectin were studied during storage at 1 and 6 °C. Storage at 1 °C alleviated the progression of CI and inhibited the degradation of pectin components and cell wall structure of papaya fruit. The increase in water-soluble pectin and the solubilization of cyclohexanediamine tetraacetic acid- (CDTA-) and sodium carbonate (Na2CO3)-soluble pectin was limited at 1 °C. Storage at this temperature limited the detachment of linear backbone chains and minimized the degradation of branching and reticular structure in the pectin molecules. Storage at this temperature also delayed the decrease in the size of pectin backbone and helped to prevent the loss of neutral sugars from the pectin side chains. Thus, alleviation of the degradation of cell wall pectin contributes to the structural integrity of the cell wall and thus helps the maintenance of the tissue integrity of papaya at 1 °C.

Erg4 Is Involved in Ergosterol Biosynthesis, Conidiation and Stress Response in Penicillium expansum.

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.

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.

The Effect of Environmental pH during Trichothecium roseum (Pers.:Fr.) Link Inoculation of Apple Fruits on the Host Differential Reactive Oxygen Species Metabolism.

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.