Exogenous calcium improves viability of biocontrol yeasts under heat stress by reducing ROS accumulation and oxidative damage of cellular protein

Hydrogen sulfide and calcium effects on cadmium removal and resistance in the white-rot fungus Phanerochaete chrysosporium

Hydrogen sulfide (H₂S), an emerging gas transmitter, has been shown to be involved in multiple intracellular physiological and biochemical processes. In this study, the effects of hydrogen sulfide coupled with calcium on cadmium removal and resistance in Phanerochaete chrysosporium were examined. The results revealed that H₂S enhanced the uptake of calcium by P. chrysosporium to resist cadmium stress. The removal and accumulation of cadmium by the mycelium was reduced by H₂S and Ca²⁺ pretreatment. Moreover, oxidative damage and membrane integrity were alleviated by H₂S and Ca²⁺. Corresponding antioxidative enzyme activities and glutathione were also found to positively respond to H₂S and Ca²⁺, which played an important role in the resistance to cadmium-induced oxidative stress. The effects of hydroxylamine (HA; a hydrogen sulfide inhibitor) and ethylene glycol-bis-(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA; a calcium chelator) toward H₂S and Ca²⁺ and their cross-interactions confirmed the positive roles and the potential crosstalk of H₂S and Ca²⁺ in cadmium stress resistance. These findings imply that the protective effects of H₂S in P. chrysosporium under cadmium stress may occur through a reduction in the accumulation of cadmium and promotion of the antioxidant system, and the H₂S-regulated pathway may be associated with the intracellular calcium signaling system.Key points• Altered monoterpenoid tolerance mainly related to altered activity of efflux pumps.• Increased tolerance to geranic acid surprisingly caused by decreased export activity.• Reduction of export activity can be beneficial for biotechnological conversions.

Pichia anomala Induced With Chitosan Triggers Defense Response of Table Grapes Against Post-harvest Blue Mold Disease

To study the mechanism by which Pichia anomala induced with chitosan (1% w/v) controls blue mold disease in table grapes caused by Penicillium expansum, this study evaluated alterations in three yeast enzymatic activities. The changes in the five primary disease defense-related enzymes and two non-enzyme activities of table grapes were assayed. The results of the study showed that chitosan (1% w/v) significantly increased the yeast β-1,3-glucanase, catalase (CAT), and malondialdehyde (MDA) activities. Furthermore, P. anomala alone or induced with chitosan (1% w/v) significantly increased the table grapes enzymatic activities of Polyphenol oxidase (PPO), phenylalanine (PAL), peroxidase (POD), and catalase (CAT) compared to the control. The RT-qPCR results also confirmed that the genes of these major disease defense enzymes were up-regulated when the table grapes were treated with P. anomala. The highest results were recorded when the fruit was treated by yeast induced with chitosan (1% w/v). The phenolic compounds, in addition to their nutritional value, can also increase the antimicrobial properties of table grapes. The current experiment determined that the total phenol and flavonoid contents of table grapes showed the highest results for fruits treated by P. anomala induced with chitosan compared with the control. Generally, the increment of these fruit enzymatic and non-enzymatic activities shows improved table grape defense against the pathogenic fungus. The induction of the yeast with chitosan also increases its bio-control efficacy against the pathogen. This study will enable future detailed investigation in the yeast pathogen control mechanisms and the use of yeasts as bio-pesticides.

Effect of stress factors associated with postharvest citrus conditions on the viability and biocontrol activity of Clavispora lusitaniae strain 146

Only quite recently, we have shown that yeast strains Clavispora lusitaniae 146 and Pichia fermentans 27 can act as efficient biocontrol agents for combating postharvest fungal diseases in lemons. During postharvest and storage conditions, microorganisms are subject to different stress factors that could affect both their survival and their protective capacity. Understanding the tolerance of yeasts to environmental stress factors could support the future development and commercial application of biological control formulations based on such organisms. Thus, the impact of different stressors on the viability and protection efficiency of C. lusitaniae strain 146 and P. fermentans strain 27 was evaluated, and the yeasts were subjected to oxidative stress, thermal treatments, exposure to NaOCl, osmotic stress, and ultraviolet irradiation. Candida oleophila strain O served as the reference control. C. lusitaniae 146 was more resistant to H₂O₂ in plate assays; however, in liquid media there was no significant difference to the other strains. Strain 146 was less affected by NaOCl, being able to survive with 300 ppm. P. fermentans 27 was the strain most heavily affected by osmotic pressure, while strains 146 and strain O showed a similar adaptation. UV-B irradiation severely affected C. oleophila and P. fermentans, while C. lusitaniae was the most resistant. Strains 146 and 27 were similarly tolerant to thermal shocks, compared to the reference strain, which was less viable. In in vivo tests, exposure to 10 mM H₂O₂, 45°C or 200 ppm NaOCl prior to fruit inoculation, reduced the antagonistic activity against the pathogen Penicillium digitatum. However, in no case was the biocontrol efficiency reduced to less than 50%. As C. lusitaniae 146 demonstrated a great potential to combat P. digitatum under a wide range of conditions, the organism is a promising candidate as an effective and valuable alternative to toxic fungicides.

Pretreatment of the Antagonistic Yeast, Debaryomyces hansenii, With Mannitol and Sorbitol Improves Stress Tolerance and Biocontrol Efficacy

The effect of exogenous mannitol and sorbitol on the viability of the antagonist yeast, Debaryomyces hansenii, when exposed to oxidative and high-temperature stress was determined. Results indicated that both the 0.1 M mannitol (MT) and 0.1 M sorbitol (ST) treatments improved the tolerance of D. hansenii to subsequent oxidative and high-temperature stress. MT or ST cells had a significantly higher level of cell survival, elevated the gene expression of catalase 1 (CAT1) and copper-zinc superoxide dismutase (SOD1), as well as the corresponding enzyme activity. Treated cells also exhibited a lower accumulation of intracellular reactive oxygen species (ROS), and a higher content of intracellular mannitol and sorbitol relative to non-treated, control yeast cells, when exposed to a subsequent oxidative (30 mM H₂O₂) or heat (40.5°C) stress for 30 min. Additionally, MT and ST yeast exhibited a higher growth rate in kiwifruit wounds, and a greater ability to inhibit postharvest blue mold (Penicillium expansum) and gray mold (Botrytis cinerea) infections. The present study indicates that increased antioxidant response induced by mannitol and sorbitol in D. hansenii can enhance stress tolerance and biocontrol performance.

In vitro antifungal activity and possible mechanisms of action of chelerythrine

Chelerythrine (CHE) possesses broad pharmacological activities. In this study, the extract of Chelidonium majus L. were characterized by high performance liquid chromatography (HPLC), infrared radiation (IR) spectroscopy and nuclear magnetic resonance (NMR). It was proved that the extract was CHE. The antifungal activity of CHE against five fungal pathogens of rice was researched in vitro, revealing that CHE inhibited Ustilaginoidea virens (U. virens) and Cochliobolus miyabeanus (C. miyabeanus) with 50% effective concentrations (EC₅₀) of 6.53 × 10^-3 mg/mL and 5.62 × 10^-3 mg/mL, respectively. When the concentration of CHE was 7.5 × 10^-3 mg/mL, the inhibition rate of U. virens reached 56.1%. Moreover, CHE (4 × 10^-3 mg/mL) exhibited the greatest efficacy in inhibiting spore of U. virens growth with an inhibition rate as high as 86.7%. CHE displayed the best inhibitory activity against U. virens at the concentration of 7.5 × 10^-3 mg/mL, compared with the other two isoquinoline alkaloids and commercial fungicide validamycin. After treating U. virens mycelia with CHE, twisted and atrophied mycelia were observed by optical microscopy. SEM results demonstrated narrow and locally fractured mycelium. TEM observations showed that the cell wall had become thin and broken, and most organelles were difficult to recognize. Furthermore, membrane of mycelia was destroyed and reactive oxygen species (ROS) of spores was accumulated, which induced apoptosis of pathogenic fungi. From these results, our understanding of the mechanisms of antifungal activity of CHE against U. virens was enriched and this research is relevant for developing novel pesticides.

Metabolic Dynamics During Loquat Fruit Ripening and Postharvest Technologies

Loquat is an important fruit widely cultivated worldwide with high commercial value. During loquat fruit development, ripening, and storage, many important metabolites undergo dramatic changes, resulting in accumulation of a diverse mixture of nutrients. Given the value of loquat fruit, significant progresses have been achieved in understanding the metabolic changes during fruit ripening and storage, as well as postharvest technologies applied in loquat fruit in recent years. The objective of the present review is to summarize currently available knowledge and provide new references for improving loquat fruit quality.

Improvement of Multiple-Stress Tolerance and Ethanol Production in Yeast during Very-High-Gravity Fermentation by Supplementation of Wheat-Gluten Hydrolysates and Their Ultrafiltration Fractions

The effects of wheat-gluten hydrolysates (WGH) and their ultrafiltration fractions on multiple-stress tolerance and ethanol production in yeast during very-high-gravity (VHG) fermentation were examined. The results showed that WGH and WHG-ultrafiltration-fraction supplementations could significantly enhance the growth and viability of yeast and further improve the tolerance of yeast to osmotic stress and ethanol stress. The addition of MW < 1 kDa fractions led to 51.08 and 21.70% enhancements in cell-membrane integrity, 30.74 and 10.43% decreases in intracellular ROS accumulation, and 34.18 and 26.16% increases in mitochondrial membrane potential (ΔΨ_m) in yeast under osmotic stress and ethanol stress, respectively. Moreover, WGH and WHG-ultrafiltration-fraction supplementations also improved the growth and ethanol production of yeast during VHG fermentation, and supplementation with the <1 kDa fraction resulted in a maximum biomass of 16.47 g/L dry cell and an ethanol content of 18.50% (v/v) after VHG fermentation.

Effects of cadmium on calcium homeostasis in the white-rot fungus Phanerochaete chrysosporium

Due to the widespread application of white-rot fungi for the treatment of pollutants, it's crucial to exploit the special effects of pollutants on the microbes. Here, we studied the effects of cadmium on calcium homeostasis in the most studied white-rot fungus Phanerochaete chrysosporium. The response of P. chrysosporium to cadmium stress is concentration-dependent. A high concentration of cadmium caused the release of calcium from P. chrysosporium, while a hormesis effect was observed at a lower cadmium concentration (10 μM), which resulted in a significant increase in calcium uptake and reversed the decrease in cell viability. Calcium (50 μM) promoted cell viability (127.2% of control), which reflects that calcium can protect P. chrysosporium from environmental stress. Real-time changes in the Ca²⁺ and Cd²⁺ fluxes of P. chrysosporium were quantified using the noninvasive microtest technique. Ca²⁺ influx decreased significantly under cadmium exposure, and the Ca²⁺ channel was involved in Ca²⁺ and Cd²⁺ influx. The cadmium and/or calcium uptake results coupled with the real-time Ca²⁺ and Cd²⁺ influxes microscale signatures can enhance our knowledge of the homeostasis of P. chrysosporium with respect to cadmium stress, which may provide useful information for improving the bioremediation process.

TeA is a key virulence factor for Alternaria alternata (Fr.) Keissler infection of its host

A toxin-deficient mutant strain, HP001 mutant of Alternaria alternata, whose mycelium is unable to infect its host, produces little tenuazonic acid (TeA) toxin. How TeA plays a role in initiating host infection by A. alternata remains unclear. In this research we use Imaging-PAM based on chlorophyll fluorescence parameters and transmission electron microscopy to explore the role of TeA toxin during the infection process of A. alternata. Photosystem II damage began even before wild type mycelium infected the leaves of its host, croftonweed (Ageratina adenophora). Compared with the wild type, HP001 mutant produces morphologically different colonies, hyphae with thinner cell walls, has higher reactive oxygen species (ROS) content and lower peroxidase activity, and fails to form appressoria on the host surface. Adding TeA toxin allows the mutant to partially recover these characters and more closely resemble the wild type. Additionally, we found that the mutant is able to elicit disease symptoms when its mycelium is placed on leaves whose epidermis has been manually removed, which indicates that TeA may be determinant in the fungus recognition of its plant host. Lack of TeA toxin appears responsible for the loss of pathogenicity of the HP001 mutant. As a key virulence factor, TeA toxin not only damages the host plant but also is involved in maintaining ROS content, host recognition, inducing appressoria to infect the host and for allowing completion of the infection process.

Calcium Supplementation Abates the Inhibition Effects of Acetic Acid on Saccharomyces cerevisiae

The toxic level of acetic acid could be released during the pretreatment of lignocellulosic biomass, and an economical method was reported to minimize the acidic stress on the fermentation of Saccharomyces cerevisiae by cation supplementation. A dose-dependent protection of Ca²⁺ was monitored, and the optimal concentration of Ca²⁺ was 8 mM under 4.5 g/L acetic acid stress. The activities of catalase and superoxide dismutase of yeast cells supplemented with optimal Ca²⁺ increased by 18.6 and 27.3 %, respectively, coupling with an obvious decrease of reactive oxygen species content. Cell viability also performed a significant increase from 52.4 % (without Ca²⁺ addition) to 73.56 % (with 8 mM Ca²⁺ addition). No significant improvements were found in the bioethanol yields by Ca²⁺ supplementation; however, the fermentation time was shortened by about 8 h obviously. Our results illustrated that the Ca²⁺ supplementation could be an economical method to make the bioethanol production more efficient and cost-effective.

Responses of yeast biocontrol agents to environmental stress

Biological control of postharvest diseases, utilizing wild species and strains of antagonistic yeast species, is a research topic that has received considerable attention in the literature over the past 30 years. In principle, it represents a promising alternative to chemical fungicides for the management of postharvest decay of fruits, vegetables, and grains. A yeast-based biocontrol system is composed of a tritrophic interaction between a host (commodity), a pathogen, and a yeast species, all of which are affected by environmental factors such as temperature, pH, and UV light as well as osmotic and oxidative stresses. Additionally, during the production process, biocontrol agents encounter various severe abiotic stresses that also impact their viability. Therefore, understanding the ecological fitness of the potential yeast biocontrol agents and developing strategies to enhance their stress tolerance are essential to their efficacy and commercial application. The current review provides an overview of the responses of antagonistic yeast species to various environmental stresses, the methods that can be used to improve stress tolerance and efficacy, and the related mechanisms associated with improved stress tolerance.

Evaluation of yeasts from Tibetan fermented products as agents for biocontrol of blue mold of Nashi pear fruits

A total of 20 strains of yeast isolated from Tibetan fermented products were screened for antagonism against blue mold of pear caused by Penicillium expansum. Six isolates that inhibited incidence of postharvest decay by 35% or more were selected for further screening. Among them, the most effective was Rhodotorula mucilaginosa. The results showed that washed cell suspensions of R. mucilaginosa yielded better antagonistic efficacy than unwashed cell-culture mixtures, cell-free culture filtrates, and autoclaved cell cultures. Biocontrol activity improved with increasing concentrations of incubated cells. The best concentration was 1×10(8) cells/ml, at which the incidence of decay was only 16.7% after 6 d of incubation. The germination of conidia of P. expansum in vitro was significantly inhibited by both washed cell-suspensions and unwashed cell-culture mixtures. Rapid colonization by yeast at different concentrations showed a relationship between yeast-cell concentration and biocontrol activity. Although the titratable acidity of pear fruits increased after treatment, R. mucilaginosa did not affect the total soluble solids or ascorbic acid content. This is the first study to report that the yeast R. mucilaginosa from Tibet Autonomous Region of China may have potential as an antagonist to control the postharvest decay of pear fruits.

Enhanced anti-oxidative activity and lignocellulosic ethanol production by biotin addition to medium in Pichia guilliermondii fermentation

Commercialization of lignocellulosic ethanol fermentation requires its high titer, but the reactive oxygen species (ROS) accumulation during the bioprocess damaged the cells and compromised this goal. To improve the cellular anti-oxidative activity during non-detoxified corncob residue hydrolysate fermentation, seed cells were prepared to possess a higher level of intracellular biotin pool (IBP), which facilitated the biosyntheses of catalase and porphyrin. As a result, the catalase activity increased by 1.3-folds compared to control while the ROS level reduced by 50%. Cell viability in high-IBP cells was 1.7-folds of control and the final ethanol titer increased from 31.2 to 41.8 g L(-1) in batch fermentation. The high-IBP cells were further used for repeated-batch fermentation in the non-detoxified lignocellulosic hydrolysate, and the highest titer and average productivity of ethanol reached 63.7 g L(-1) and 1.2 g L(-1)h(-1). The results were favorable to future industrial application of this lignocellulosic bioethanol process.

Different effects of sodium chloride preincubation on cadmium tolerance of Pichia kudriavzevii and Saccharomyces cerevisiae

Application of growing microorganisms for cadmium removal is restricted by high cadmium toxicity. The effects of sodium chloride (NaCl) preincubation on the cadmium tolerance and removal ability of Pichia kudriavzevii and Saccharomyces cerevisiae were investigated in this study. NaCl preincubation improved the biomass of P. kudriavzevii under cadmium stress, while no obvious effect was observed in S. cerevisiae. The improved activities of peroxidase (POD) and catalase (CAT) after NaCl preincubation might be an important reason for the decrease of the reactive oxygen species (ROS) accumulation, cell death, and oxidative damage of proteins and lipids induced by cadmium, contributing to the improvement of the yeast growth. The cadmium bioaccumulation capacity of P. kudriavzevii decreased significantly after NaCl preincubation, which played an important role in mitigating the cadmium toxicity to the yeast. The cadmium removal rate of P. kudriavzevii was obviously higher than S. cerevisiae and was significantly enhanced after NaCl preincubation. The results suggested that NaCl preincubation improved the cadmium tolerance and removal ability of P. kudriavzevii.

Ca(2+)-CaM regulating viability of Candida guilliermondii under oxidative stress by acting on detergent resistant membrane proteins

Reactive oxygen species (ROS) play a vital role in reducing viability of yeast cells. The Ca(2+)-CaM signaling pathways are involved in regulating the intracellular ROS level in yeast cells under stress. Detergent resistant membranes (DRMs), the sterol-rich microdomains, participate in a wide range of cellular processes including growth, trafficking and death in yeast cells. In the present study, we found that Trifluoperazine (TFP), an antagonist of CaM, could increase the viability of Candida guilliermondii cells under H2O2 stress. Based on comparative analysis of DRM sub proteomics, a total number of 29 differentially expressed protein spots were identified, among which 8 protein spots belong to the electron transport chain and 7 protein spots belong to transporters. It is suggested that TFP treatment could modulate the intracellular ROS generation in yeast cells. We additionally ascertained that TFP treatment could effectively alleviate the ROS accumulation and protein damage in C. guilliermondii cells under H2O2 stress, via investigating the intracellular ROS levels and protein oxidative damage in yeast cells. These findings firstly revealed that the Ca(2+)-CaM signaling pathway is related to the viability of yeast cells under H2O2 stress, and provide novel evidences for exploring Ca(2+)-CaM's role in regulating this viability via acting on DRM proteins.

BIOLOGICAL SIGNIFICANCE

Detergent-resistant membranes (DRMs), which are more resistant to extraction with cold non-ionic detergents, have been considered the functional microdomains in the plasma membrane. In yeast, DRMs are involved in a wide range of additional cellular processes including cell growth and death. The Ca(2+)-CaM signaling pathways could regulate the stress tolerance of yeast cells by modulating the intracellular ROS generation. In this study, we found that trifluoperazine (TFP), a calmodulin antagonist, could increase the viability of C. guilliermondii under H2O2 stress. Based on comparative analysis of DRM sub proteomics, electron transport chain proteins and transporters were identified to be associated with the Ca(2+)-CaM transduction. We proved that TFP treatment decreases the intracellular ROS accumulation and alleviates oxidative damage to cellular proteins. These results ascertain that Ca(2+)-CaM is involved in regulating the viability of C. guilliermondii under oxidative stress via acting on the DRM proteins.

Survival of the biological control agent Candida sake CPA-1 on grapes under the influence of abiotic factors

AIMS

As reliability of preharvest applications of biological control agents (BCAs) to control fruit pathogens is highly dependent on the survival of the selected organism, this study aimed to describe the population dynamics of the yeast-BCA Candida sake (Saito & Ota) CPA-1 on grape berries under the effect of abiotic factors such as temperature, relative humidity, sunlight and rainfall.

METHODS AND RESULTS

Candida sake (5 × 10(7) CFU ml(-1)), combined with different concentrations of the food additive Fungicover(®), was applied on grape berry clusters. Treated clusters were then exposed to abiotic factors in field or laboratory conditions, recovering populations to describe C. sake population dynamics. The addition of Fungicover significantly increased C. sake multiplication under optimal growth conditions and improved survival under fluctuating abiotic factors. After field applications, significant differences in populations on grape bunches exposed or covered by fine foliage were detected. Simulated rainfall washed off C. sake populations by 0·6-0·9 log units after 20 mm of rain volume. Allowing populations to establish for 24 h or more, prior to a rain event, persistence on grape berries significantly increased and the effect of rain intensity was not observable.

CONCLUSIONS

Candida sake demonstrated high survival ability under unfavourable environmental conditions and persistence under intense rain. The study evidenced the importance of the first period just after application for C. sake survival on grape tissues and also the protective effect of the additive Fungicover.

SIGNIFICANCE AND IMPACT OF THE STUDY

This research provides new information on the survival of C. sake under field conditions and its practical implications for recommending timing of spray with this antagonist. Our results could be useful for other yeast antagonists applied before harvest. This work, for the first time, defines population dynamics of a yeast BCA using simulated rainfall.

Heat-induced oxidative injury contributes to inhibition of Botrytis cinerea spore germination and growth

The inhibitory effect of heat treatment (HT) on Botrytis cinerea, a major postharvest fungal pathogen, and the possible mode of action were investigated. Spore germination and germ tube elongation of B. cinerea were both increasingly and significantly inhibited by HT (43 °C) for 10, 20 or 30 min. HT-induced gene expression of NADPH oxidase A, resulted in the intracellular accumulation of reactive oxygen species. HT-treated B. cinerea spores exhibited higher levels of oxidative damage to proteins and lipids, compared to the non-HT control. These findings indicate that HT resulted in oxidative damage which then played an important role in the inhibitory effect on B. cinerea. In the current study, HT was effective in controlling gray mold, caused by B. cinerea, in pear fruits. Understanding the mode of action by which HT inhibits fungal pathogens will help in the application of HT for management of postharvest fungal diseases of fruits and vegetables.

A 'fragile cell' sub-population revealed during cytometric assessment of Saccharomyces cerevisiae viability in lipid-limited alcoholic fermentation

AIMS

To show that in anaerobic fermentation with limiting lipid nutrients, cell preparation impacts the viability assessment of yeast cells, and to identify the factors involved.

METHODS AND RESULTS

Saccharomyces cerevisiae viability was determined using propidium iodide staining and the flow cytometry. Analyses identified intact cells, dead cells and, under certain conditions, the presence of a third subpopulation of apparently damaged cells. This intermediate population could account for up to 40% of the entire cell population. We describe, analyse and discuss the effects of different solutions for cell resuspension on the respective proportion of these three populations, in particular that of the intermediate population. We show that this intermediate cell population forms in the absence of Ca(2+)/Mg(2+).

CONCLUSIONS

Cell preparation significantly impacts population viability assessment by FCM. The intermediate population, revealed under certain conditions, could be renamed as 'fragile cells'. For these cells, Ca(2+) and Mg(2+) reduce cell membrane permeability to PI.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study that analyses and discusses the factors influencing the formation of an intermediate population when studying viability in yeast alcoholic fermentation. With a wider application in biological research, this study provides important support to the relatively new questioning of propidium iodide staining as a universal cell death indicator.