Physiological implications of trehalose in the ectomycorrhizal fungus Pisolithus sp. under thermal stress

Exogenous trehalose increased polysaccharide content and altered their properties and metabolism in Lentinula edodes mycelium

Trehalose promotes polysaccharide synthesis in edible mushrooms, yet its effect on the antioxidant activity and structure of Lentinula edodes mycelium polysaccharides is still unknown. We investigated the effect of trehalose on the antioxidant activity of L. edodes polysaccharides and expression of genes related to L. edodes polysaccharide metabolism. Five g/L trehalose increased the biomass and polysaccharide content of L. edodes mycelium by over 50 % compared to the control. Trehalose increased the superoxide anion radical scavenging rate of L. edodes polysaccharides up to 80.9 %. Compared to the control, the molecular weight of polysaccharides was lower, and their glucuronic acid content was higher in the trehalose treatment. RNA-seq analysis revealed 1045 differentially expressed genes in the trehalose treatment compared to the control. Genes related to glycolysis/gluconeogenesis pathway, starch and sucrose metabolic pathway, and the pentose and glucuronate interconversions pathway were differentially expressed, possibly accounting for the increased polysaccharide synthesis. In summary, exogenous trehalose has potential to increase the biosynthesis and biological activity of L. edodes polysaccharides.

iTRAQ-Based Quantitative Proteomic Analysis Reveals Proteomic Changes in Mycelium of Pleurotus ostreatus in Response to Heat Stress and Subsequent Recovery

High temperature is a key limiting factor for mycelium growth and development in Pleurotus ostreatus. Thermotolerance includes the direct response to heat stress and the ability to recover from heat stress. To better understand the mechanism of thermotolerance in P. ostreatus, we used morphological and physiological analysis combined with an iTRAQ-based proteomics analysis of P. ostreatus subjected to 40°C for 48 h followed by recovery at 25°C for 3 days. High temperature increased the concentrations of thiobarbituric acid reactive substances (TBARS) indicating that the mycelium of P. ostreatus were damaged by heat stress. However, these physiological changes rapidly returned to control levels during the subsequent recovery phase from heat stress. In comparison to unstressed controls, a total of 204 proteins were changed during heat stress and/or the recovery phase. Wherein, there were 47 proteins that responded to both stress and recovery conditions, whereas 84 and 73 proteins were responsive to only heat stress or recovery conditions, respectively. Furthermore, quantitative real-time PCR (qRT-PCR) confirmed differential expression of nine candidate genes revealed that some of the proteins, such as a mitogen-activated protein kinase (MAPK), phenylalanine ammonia-lyase (PAL), and heat shock protein (HSP), were also regulated by heat stress at the level of transcription. These differentially expressed proteins (DEPs) in mycelium of P. ostreatus under heat stress were from 13 biological processes. Moreover, protein-protein interaction analysis revealed that proteins involved in carbohydrate and energy metabolism, signal transduction, and proteins metabolism could be assigned to three heat stress response networks. On the basis of these findings, we proposed that effective regulatory protein expression related to MAPK-pathway, antioxidant enzymes, HSPs, and other stress response proteins, and glycolysis play important roles in enhancing P. ostreatus adaptation to and recovery from heat stress. Of note, this study provides useful information for understanding the thermotolerance mechanism for basidiomycetes.

Trees, fungi and bacteria: tripartite metatranscriptomics of a root microbiome responding to soil contamination

BACKGROUND

One method for rejuvenating land polluted with anthropogenic contaminants is through phytoremediation, the reclamation of land through the cultivation of specific crops. The capacity for phytoremediation crops, such as Salix spp., to tolerate and even flourish in contaminated soils relies on a highly complex and predominantly cryptic interacting community of microbial life.

METHODS

Here, Illumina HiSeq 2500 sequencing and de novo transcriptome assembly were used to observe gene expression in washed Salix purpurea cv. 'Fish Creek' roots from trees pot grown in petroleum hydrocarbon-contaminated or non-contaminated soil. All 189,849 assembled contigs were annotated without a priori assumption as to sequence origin and differential expression was assessed.

RESULTS

The 839 contigs differentially expressed (DE) and annotated from S. purpurea revealed substantial increases in transcripts encoding abiotic stress response equipment, such as glutathione S-transferases, in roots of contaminated trees as well as the hallmarks of fungal interaction, such as SWEET2 (Sugars Will Eventually Be Exported Transporter). A total of 8252 DE transcripts were fungal in origin, with contamination conditions resulting in a community shift from Ascomycota to Basidiomycota genera. In response to contamination, 1745 Basidiomycota transcripts increased in abundance (the majority uniquely expressed in contaminated soil) including major monosaccharide transporter MST1, primary cell wall and lamella CAZy enzymes, and an ectomycorrhiza-upregulated exo-β-1,3-glucanase (GH5). Additionally, 639 DE polycistronic transcripts from an uncharacterised Enterobacteriaceae species were uniformly in higher abundance in contamination conditions and comprised a wide spectrum of genes cryptic under laboratory conditions but considered putatively involved in eukaryotic interaction, biofilm formation and dioxygenase hydrocarbon degradation.

CONCLUSIONS

Fungal gene expression, representing the majority of contigs assembled, suggests out-competition of white rot Ascomycota genera (dominated by Pyronema), a sometimes ectomycorrhizal (ECM) Ascomycota (Tuber) and ECM Basidiomycota (Hebeloma) by a poorly characterised putative ECM Basidiomycota due to contamination. Root and fungal expression involved transcripts encoding carbohydrate/amino acid (C/N) dialogue whereas bacterial gene expression included the apparatus necessary for biofilm interaction and direct reduction of contamination stress, a potential bacterial currency for a role in tripartite mutualism. Unmistakable within the metatranscriptome is the degree to which the landscape of rhizospheric biology, particularly the important but predominantly uncharacterised fungal genetics, is yet to be discovered.

Effect of different stresses on trehalose levels in Rhizopus oryzae

Rhizopus oryzae accumulates and degrades trehalose in response to environmental stress conditions such as heat, osmotic stress, nitrogen starvation and pH. When heat stress was applied to R. oryzae, the trehalose content of the cells was increased from 0.9 to 4.8 mg/g dw and when the stress was relieved it decreased back to 1.1 mg/g dw. Under osmotic stress and nitrogen starvation, trehalose content was increased by 3.5 and 3 fold, respectively. The decrease in external pH increased trehalose level up to 2.6 mg/g dw from 1.1 mg/g dw. Increased trehalose level decreased back to normal after pH stress was relieved. These results indicate that trehalose accumulates and acts as a stress protectant in R. oryzae.

Physiological implication of intracellular trehalose and mannitol changes in response of entomopathogenic fungus Beauveria bassiana to thermal stress

To explore possible role of intracellular trehalose accumulation in fungal tolerance to summer-like thermal stress, 3-day colonies of Beauveria bassiana grown on a glucose-free medium at 25 degrees C were separately exposed to 35, 37.5 and 40 degrees C for 1-18 h, respectively. Trehalose accumulation in stressed mycelia increased from initial 4.2 to 88.3, 74.7 and 65.5 mg g(-1) biomass after 6-h stress at 35, 37.5 and 40 degrees C, respectively, while intracellular mannitol level generally declined with higher temperatures and longer stress time. The stress-enhanced trehalose level was significantly correlated to decreased trehalase activity (r(2) = 0.73) and mannitol content (r(2) = 0.38), which was inversely correlated to the activity of mannitol dehydrogenase (r(2) = 0.41) or mannitol 1-phosphate dehydrogenase (r(2) = 0.30) under the stresses. All stressed cultures were successfully recovered at 25 degrees C but their vigor depended on stressful temperature, time length and the interaction of both (r (2) = 0.98). The highest level of 6-h trehalose accumulation at 35 degrees C was found enhancing the tolerance of the stressed cultures to the greater stress of 48 degrees C. The results suggest that the trehalose accumulation result partially from metabolized mannitol and contribute to the fungal thermotolerance. Trehalase also contributed to the thermotolerance by hydrolyzing accumulated trehalose under the conditions of thermal stress and recovery.