The regulatory functions of oxylipins in fungi: A review

Quorum sensing (QS) is a communication mechanism between microorganisms originally found in bacteria. In recent years, an important QS mechanism has been discovered in the field of fungi, namely, the lipoxygenase compound oxylipin of arachidonic acid acts as a QS molecule in life cycle control, particularly in the sexual and asexual development of fungi. However, the role of oxylipins in mediating eukaryotic communication has not been previously described. In this paper, we review the regulatory role of oxylipins and the underlying mechanisms and discuss the potential for application in major fungi. The role of oxylipin as a fungal quorum-sensing molecule is the main focus of the review. Besides, the quorum regulation of fungal morphological transformation, biofilm formation, virulence factors, secondary metabolism, infection, symbiosis, and other physiological behaviors are discussed. Moreover, future prospectives and applications are elaborated as well.

Fungal and bacterial oxylipins are signals for intra- and inter-cellular communication within plant disease

Lipids are central at various stages of host-pathogen interactions in determining virulence and modulating plant defense. Free fatty acids may act as substrates for oxidizing enzymes [e.g., lipoxygenases (LOXs) and dioxygenases (DOXs)] that synthesize oxylipins. Fatty acids and oxylipins function as modulators of several pathways in cell-to-cell communication; their structural similarity among plant, fungal, and bacterial taxa suggests potential in cross-kingdom communication. We provide a prospect of the known role of fatty acids and oxylipins in fungi and bacteria during plant-pathogen interactions. In the pathogens, oxylipin-mediated signaling pathways are crucial both in development and host infection. Here, we report on case studies suggesting that oxylipins derived from oleic, linoleic, and linolenic acids are crucial in modulating the pathogenic lifestyle in the host plant. Intriguingly, overlapping (fungi-plant/bacteria-plant) results suggest that different inter-kingdom pathosystems use similar lipid signals to reshape the lifestyle of the contenders and occasionally determine the outcome of the challenge.

Fatty Acids and Oxylipins as Antifungal and Anti-Mycotoxin Agents in Food: A Review

Fungal contamination of food, especially by mycotoxigenic fungi, not only reduces the quality of the food, but can also cause serious diseases, thus posing a major food safety challenge to humans. Apart from sound food control systems, there is also a continual need to explore antifungal agents that can inhibit fungal growth and mycotoxin production in food. Many types of fatty acids (FAs) and their oxidized derivatives, oxylipins, have been found to exhibit such effects. In this review, we provide an update on the most recent literature on the occurrence and formation of FAs and oxylipins in food, their effects on fungal growth and mycotoxin synthesis, as well as the genetic and molecular mechanisms of actions. Research gaps in the field and needs for further studies in order to realizing the potential of FAs and oxylipins as natural antifungal preservatives in food are also discussed.

Oxylipins from both pathogen and host antagonize jasmonic acid-mediated defence via the 9-lipoxygenase pathway in Fusarium verticillioides infection of maize

Oxylipins are a newly emerging group of signals that serve defence roles or promote virulence. To identify specific host and fungal genes and oxylipins governing the interactions between maize and Fusarium verticillioides, maize wild-type and lipoxygenase3 (lox3) mutant were inoculated with either F. verticillioides wild-type or linoleate-diol-synthase 1-deleted mutant (ΔFvlds1D). The results showed that lox3 mutants were more resistant to F. verticillioides. The reduced colonization on lox3 was associated with reduced fumonisin production and with a stronger and earlier induction of ZmLOX4, ZmLOX5 and ZmLOX12. In addition to the reported defence function of ZmLOX12, we showed that lox4 and lox5 mutants were more susceptible to F. verticillioides and possessed decreased jasmonate levels during infection, suggesting that these genes are essential for jasmonic acid (JA)-mediated defence. Oxylipin profiling revealed a dramatic reduction in fungal linoleate diol synthase 1 (LDS1)-derived oxylipins, especially 8-HpODE (8-hydroperoxyoctadecenoic acid), in infected lox3 kernels, indicating the importance of this molecule in virulence. Collectively, we make the following conclusions: (1) LOX3 is a major susceptibility factor induced by fungal LDS1-derived oxylipins to suppress JA-stimulating 9-LOXs; (2) LOX3-mediated signalling promotes the biosynthesis of virulence-promoting oxylipins in the fungus; and (3) both fungal LDS1- and host LOX3-produced oxylipins are essential for the normal infection and colonization processes of maize seed by F. verticillioides.

Tramesan, a novel polysaccharide from Trametes versicolor. Structural characterization and biological effects

Mushrooms represent a formidable source of bioactive compounds. Some of these may be considered as biological response modifiers; these include compounds with a specific biological function: antibiotics (e.g. plectasin), immune system stimulator (e,g, lentinan), antitumor agents (e.g. krestin, PSK) and hypolipidemic agents (e.g. lovastatin) inter alia. In this study, we focused on the Chinese medicinal mushroom "yun zhi", Trametes versicolor, traditionally used for (cit.) "replenish essence and qi (vital energy)". Previous studies indicated the potential activity of extracts from culture filtrate of asexual mycelia of T. versicolor in controlling the growth and secondary metabolism (e.g. mycotoxins) of plant pathogenic fungi. The quest of active principles produced by T. versicolor, allowed us characterising an exo-polysaccharide released in its culture filtrate and naming it Tramesan. Herein we evaluate the biological activity of Tramesan in different organisms: plants, mammals and plant pathogenic fungi. We suggest that the bioactivity of Tramesan relies mostly on its ability to act as pro antioxidant molecule regardless the biological system on which it was applied.

Careful with That Axe, Gene, Genome Perturbation after a PEG-Mediated Protoplast Transformation in Fusarium verticillioides

Fusarium verticillioides causes ear rot disease in maize and its contamination with fumonisins, mycotoxins harmful for humans and livestock. Lipids, and their oxidized forms, may drive the fate of this disease. In a previous study, we have explored the role of oxylipins in this interaction by deleting by standard transformation procedures a linoleate diol synthase-coding gene, lds1, in F. verticillioides. A profound phenotypic diversity in the mutants generated has prompted us to investigate more deeply the whole genome of two lds1-deleted strains. Bioinformatics analyses pinpoint significant differences in the genome sequences emerged between the wild type and the lds1-mutants further than those trivially attributable to the deletion of the lds1 locus, such as single nucleotide polymorphisms, small deletion/insertion polymorphisms and structural variations. Results suggest that the effect of a (theoretically) punctual transformation event might have enhanced the natural mechanisms of genomic variability and that transformation practices, commonly used in the reverse genetics of fungi, may potentially be responsible for unexpected, stochastic and henceforth off-target rearrangements throughout the genome.

Evaluating Fumonisin Gene Expression in Fusarium verticillioides

Transcript levels of key genes in a biosynthetic pathway are often taken as a proxy for metabolite production. This is the case of FUM1, encoding the first dedicated enzyme in the metabolic pathway leading to the production of the mycotoxins Fumonisins by fungal species belonging to the genus Fusarium. FUM1 expression can be quantified by different methods; here, we detail a protocol based on quantitative reverse transcriptase polymerase chain reaction (RT-qPCR), by which relative or absolute transcript abundance can be estimated in Fusaria grown in vitro or in planta. As very seldom commercial kits for RNA extraction and cDNA synthesis are optimized for fungal samples, we developed a protocol tailored for these organisms, which stands alone but can be also easily integrated with specific reagents and kits commercially available.

Fumonisins and their modified forms, a matter of concern in future scenario?

Masked mycotoxins are found in grains and derived foods as a result of plant phase II metabolism. Recently, masked mycotoxins senso strictu, together with other covalently or non-covalently conjugated forms, even formed upon processing, have been classified as modified mycotoxins. In this context, the issue of modified fumonisins is of great interest, on account of the wide range of factors affecting their formation and accumulation in maize pre- and postharvest. Fumonisins, indeed, may undergo modification in plants, along the growing season, but also during storage and drying of maize kernels, and upon processing. All these modifications strongly affect the analytical outcome, thus making more difficult the assessment of maize compliance. Since the ratio between free and modified fumonisins is affected by maize composition and environmental factors, a deeper knowledge on the phenomena driving the production and accumulation of free and modified forms in plants may support the selection of resistant hybrids. This review provides a critical picture of the state of the art on this topic, mainly focusing on those events occurring in field, identified as crucial in determining amount and partitioning of contamination. Nevertheless, knowledge on modified fumonisins is still in its dawn, on account of the wide range of factors involved. Anyway, reported results, taking altogether, clearly indicate that modified fumonisins should be included in the monitoring plans to have an overview of the possible contribution to human exposure. Furthermore, next efforts should focus on the events occurring in field and on the cross-talk between the plant and the fungus, to support the identification of resistant hybrids and to provide data for predictive models, the most suitable tool to forecast what is going to happens in the future changing climate.

Menadione-Induced Oxidative Stress Re-Shapes the Oxylipin Profile of Aspergillus flavus and Its Lifestyle

Aspergillus flavus is an efficient producer of mycotoxins, particularly aflatoxin B₁, probably the most hepatocarcinogenic naturally-occurring compound. Although the inducing agents of toxin synthesis are not unanimously identified, there is evidence that oxidative stress is one of the main actors in play. In our study, we use menadione, a quinone extensively implemented in studies on ROS response in animal cells, for causing stress to A. flavus. For uncovering the molecular determinants that drive A. flavus in challenging oxidative stress conditions, we have evaluated a wide spectrum of several different parameters, ranging from metabolic (ROS and oxylipin profile) to transcriptional analysis (RNA-seq). There emerges a scenario in which A. flavus activates several metabolic processes under oxidative stress conditions for limiting the ROS-associated detrimental effects, as well as for triggering adaptive and escape strategies.

Maize lipids play a pivotal role in the fumonisin accumulation

The role of lipids in maize – Fusarium verticillioides interaction and fumonisin production in natural field conditions were investigated. In 2010, ten maize hybrids were grown in fields located in 3 districts in Northern Italy and sampled at 4 growing stages, from early dough to full ripe. Chemical composition, fungal incidence and free and hidden fumonisin contamination were determined in all grain samples. All the hybrids considered within this study showed a strong fungal incidence, with Fusarium section Liseola as prevalent, already at the early dough maturity and along the ripening period. Fumonisins accumulated over the growing season and reached the maximum level at the full ripe stage; hidden fumonisins were found significant in all the considered samples (~57% of the free form at harvest). Hybrid H9 showed more than 50% of kernels infected by Aspergillus flavus and no hidden fumonisins were detected. This finding stresses the relevance of monitoring both free and total fumonisins for a comprehensive assessment of consumer exposure to mycotoxins. Previous studies showed a positive correlation between the content of linoleic acid and fumonisin accumulation into maize kernels infected with Fusarium section Liseola. Hence, an untargeted and targeted lipid analysis of maize kernels along the growing season and at harvest was performed. Results suggested a significant involvement of lipid composition of maize kernels in fungal infection and toxin accumulation. Specifically, mass spectrometry data pinpointed that at least 4 lipid entities might differentiate high-contaminated from low-contaminated samples when the cut-off of 2,000 μg/kg of fumonisins was selected. Among them, the oxylipin 9-HODE and three sphingolipids were identified. These results suggest that sphingolipid and oxylipin metabolism in maize kernels interferes with F. verticillioides growth and fumonisin production in plants growing in field.

LDS1-produced oxylipins are negative regulators of growth, conidiation and fumonisin synthesis in the fungal maize pathogen Fusarium verticillioides

Oxylipins are fatty acid-derived signaling compounds produced by all eukaryotes so far investigated; in mycotoxigenic fungi, they modulate toxin production and interactions with the host plants. Among the many enzymes responsible for oxylipin generation, Linoleate Diol Synthase 1 (LDS1) produces mainly 8-hydroperoxyoctadecenoic acid and subsequently different di-hydroxyoctadecenoic acids. In this study, we inactivated a copy of the putative LDS1 ortholog (acc. N. FVEG_09294.3) of Fusarium verticillioides, with the aim to investigate its influence on the oxylipin profile of the fungus, on its development, secondary metabolism and virulence. LC-MS/MS oxylipin profiling carried out on the selected mutant strain revealed significant quali-quantitative differences for several oxylipins when compared to the WT strain. The Fvlds1-deleted mutant grew better, produced more conidia, synthesized more fumonisins and infected maize cobs faster than the WT strain. We hypothesize that oxylipins may act as regulators of gene expression in the toxigenic plant pathogen F. verticillioides, in turn causing notable changes in its phenotype. These changes could relate to the ability of oxylipins to re-shape the transcriptional profile of F. verticillioides by inducing chromatin modifications and exerting a direct control on the transcription of secondary metabolism in fungi.

Quantitative profiling of oxylipins through comprehensive LC-MS/MS analysis of Fusarium verticillioides and maize kernels

Fusarium verticillioides is one of the most important fungal pathogens causing ear and stalk rot in maize, even if frequently asymptomatic, producing a harmful series of compounds named fumonisins. Plant and fungal oxylipins play a crucial role in determining the outcome of the interaction between the pathogen and its host. Moreover, oxylipins result as signals able to modulate the secondary metabolism in fungi. In keeping with this, a novel, quantitative LC-MS/MS method was designed to quantify up to 17 different oxylipins produced by F. verticillioides and maize kernels. By applying this method, we were able to quantify oxylipin production in vitro - F. verticillioides grown into Czapek-Dox/yeast extract medium amended with 0.2% w/v of cracked maize - and in vivo, i.e. during its growth on detached mature maize ears. This study pinpoints the role of oxylipins in a plant pathogen such as F. verticillioides and sets up a novel tool aimed at understanding the role oxylipins play in mycotoxigenic pathogens during their interactions with respective hosts.

Mycotoxins that affect the North American agri-food sector: state of the art and directions for the future

This paper summarises workshop discussions at the 5th international MYCORED meeting in Ottawa, Canada (June 2012) with over 200 participants representing academics, government and industry scientists, government officials and farming organisations (present in roughly equal proportions) from 27 countries. Workshops centred on how mycotoxins in food and feed affect value chains and trade in the region covered by the North American Free Trade Agreement. Crops are contaminated by one or more of five important mycotoxins in parts of Canada and the United States every year, and when contaminated food and feed are consumed in amounts above tolerable limits, human and animal health are at risk. Economic loss from such contamination includes reduced crop yield, grain quality, animal productivity and loss of domestic and export markets. A systematic effort by grain producers, primary, transfer, and terminal elevators, millers and food and feed processers is required to manage these contaminants along the value chain. Workshops discussed lessons learned from investments in plant genetics, fungal genomics, toxicology, analytical and sampling science, management strategies along the food and feed value chains and methods to ameliorate the effects of toxins in grain on animal production and on reducing the impact of mycotoxins on population health in developing countries. These discussions were used to develop a set of priorities and recommendations.