Contrasting roles of fungal siderophores in maintaining iron homeostasis in Epichloë festucae

High-affinity iron uptake is required for optimal Epichloë festucae colonization of Lolium perenne and seed transmission

Epichloë festucae uses a siderophore-mediated system to acquire iron, which is important to maintain endophyte-grass symbioses. Here we investigate the roles of the alternative iron acquisition system, reductive iron assimilation (RIA), via disruption of the fetC gene, which encodes a multicopper ferroxidase, either alone (i.e., ΔfetC) or in combination with disruption of the gene sidA, which encodes a siderophore biosynthesis enzyme (i.e., ΔfetC/ΔsidA). The phenotypic characteristics of these mutants were compared to ΔsidA and wild-type (WT) strains during growth under axenic culture conditions (in culture) and in symbiosis with the host grass, perennial ryegrass (in planta). Under iron deficiency, the colony growth rate of ΔfetC was slightly slower than that of WT, while the growth of ΔsidA and ΔfetC/ΔsidA mutants was severely suppressed. Siderophore analyses indicated that ΔfetC mutants hyperaccumulate ferriepichloënin A (FEA) at low iron concentrations and ferricrocin and FEA at higher iron concentrations. When compared to WT, all mutant strains displayed hyperbranching hyphal structures and a reduced ratio of Epichloë DNA to total DNA in planta. Furthermore, host colonization and vertical transmission through infection of the host seed were significantly reduced in the ΔfetC/ΔsidA mutants, confirming that high-affinity iron uptake is a critical process for Epichloë transmission. Thus, RIA and siderophore iron uptake are complementary systems required for the maintenance of iron metabolism, fungal growth, and symbiosis between E. festucae and perennial ryegrass.

Enhanced mercury phytoremediation by Pseudomonodictys pantanalensis sp. nov. A73 and Westerdykella aquatica P71

Mercury is a non-essential and toxic metal that induces toxicity in most organisms, but endophytic fungi can develop survival strategies to tolerate and respond to metal contaminants and other environmental stressors. The present study demonstrated the potential of mercury-resistant endophytic fungi in phytoremediation. We examined the functional traits involved in plant growth promotion, phytotoxicity mitigation, and mercury phytoremediation in seven fungi strains. The endophytic isolates synthesized the phytohormone indole-3-acetic acid, secreted siderophores, and solubilized phosphate in vitro. Inoculation of maize (Zea mays) plants with endophytes increased plant growth attributes by up to 76.25%. The endophytic fungi stimulated mercury uptake from the substrate and promoted its accumulation in plant tissues (t test, p < 0.05), preferentially in the roots, which thereby mitigated the impacts of metal phytotoxicity. Westerdykella aquatica P71 and the newly identified species Pseudomonodictys pantanalensis nov. A73 were the isolates that presented the best phytoremediation potential. Assembling and annotation of P. pantanalensis A73 and W. aquatica P71 genomes resulted in genome sizes of 45.7 and 31.8 Mb that encoded 17,774 and 11,240 protein-coding genes, respectively. Some clusters of genes detected were involved in the synthesis of secondary metabolites such as dimethylcoprogen (NRPS) and melanin (T1PKS), which are metal chelators with antioxidant activity; mercury resistance (merA and merR1); oxidative stress (PRX1 and TRX1); and plant growth promotion (trpS and iscU). Therefore, both fungi species are potential tools for the bioremediation of mercury-contaminated soils due to their ability to reduce phytotoxicity and assist phytoremediation.

Cross-species transcriptomics identifies core regulatory changes differentiating the asymptomatic asexual and virulent sexual life cycles of grass-symbiotic Epichloë fungi

Fungi from the genus Epichloë form systemic endobiotic infections of cool season grasses, producing a range of host-protective natural products in return for access to nutrients. These infections are asymptomatic during vegetative host growth, with associations between asexual Epichloë spp. and their hosts considered mutualistic. However, the sexual cycle of Epichloë spp. involves virulent growth, characterized by the envelopment and sterilization of a developing host inflorescence by a dense sheath of mycelia known as a stroma. Microscopic analysis of stromata revealed a dramatic increase in hyphal propagation and host degradation compared with asymptomatic tissues. RNAseq was used to identify differentially expressed genes in asymptomatic vs stromatized tissues from 3 diverse Epichloë-host associations. Comparative analysis identified a core set of 135 differentially expressed genes that exhibited conserved transcriptional changes across all 3 associations. The core differentially expressed genes more strongly expressed during virulent growth encode proteins associated with host suppression, digestion, adaptation to the external environment, a biosynthetic gene cluster, and 5 transcription factors that may regulate Epichloë stroma formation. An additional 5 transcription factor encoding differentially expressed genes were suppressed during virulent growth, suggesting they regulate mutualistic processes. Expression of biosynthetic gene clusters for natural products that suppress herbivory was universally suppressed during virulent growth, and additional biosynthetic gene clusters that may encode production of novel host-protective natural products were identified. A comparative analysis of 26 Epichloë genomes found a general decrease in core differentially expressed gene conservation among asexual species, and a specific decrease in conservation for the biosynthetic gene cluster expressed during virulent growth and an unusual uncharacterized gene.

Epichloë Fungal Endophytes-From a Biological Curiosity in Wild Grasses to an Essential Component of Resilient High Performing Ryegrass and Fescue Pastures

The relationship between Epichloë endophytes found in a wide range of temperate grasses spans the continuum from antagonistic to mutualistic. The diversity of asexual mutualistic types can be characterised by the types of alkaloids they produce in planta. Some of these are responsible for detrimental health and welfare issues of ruminants when consumed, while others protect the host plant from insect pests and pathogens. In many temperate regions they are an essential component of high producing resilient tall fescue and ryegrass swards. This obligate mutualism between fungus and host is a seed-borne technology that has resulted in several commercial products being used with high uptake rates by end-user farmers, particularly in New Zealand and to a lesser extent Australia and USA. However, this has not happened by chance. It has been reliant on multi-disciplinary research teams undertaking excellent science to understand the taxonomic relationships of these endophytes, their life cycle, symbiosis regulation at both the cellular and molecular level, and the impact of secondary metabolites, including an understanding of their mammalian toxicity and bioactivity against insects and pathogens. Additionally, agronomic trials and seed biology studies of these microbes have all contributed to the delivery of robust and efficacious products. The supply chain from science, through seed companies and retailers to the end-user farmer needs to be well resourced providing convincing information on the efficacy and ensuring effective quality control to result in a strong uptake of these Epichloë endophyte technologies in pastoral agriculture.

Fungal iron homeostasis with a focus on Aspergillus fumigatus

To maintain iron homeostasis, fungi have to balance iron acquisition, storage, and utilization to ensure sufficient supply and to avoid toxic excess of this essential trace element. As pathogens usually encounter iron limitation in the host niche, this metal plays a particular role during virulence. Siderophores are iron-chelators synthesized by most, but not all fungal species to sequester iron extra- and intracellularly. In recent years, the facultative human pathogen Aspergillus fumigatus has become a model for fungal iron homeostasis of siderophore-producing fungal species. This article summarizes the knowledge on fungal iron homeostasis and its links to virulence with a focus on A. fumigatus. It covers mechanisms for iron acquisition, storage, and detoxification, as well as the modes of transcriptional iron regulation and iron sensing in A. fumigatus in comparison to other fungal species. Moreover, potential translational applications of the peculiarities of fungal iron metabolism for treatment and diagnosis of fungal infections is addressed.

Defects in the Ferroxidase That Participates in the Reductive Iron Assimilation System Results in Hypervirulence in Botrytis Cinerea

The plant-pathogenic fungus B. cinerea causes enormous economic losses, estimated at anywhere between $10 billion and $100 billion worldwide, under both pre- and postharvest conditions. Here, we present the characterization of a loss-of-function mutant in a component involved in iron acquisition that displays hypervirulence. While in different microbial systems iron uptake mechanisms appear to be critical to achieve full pathogenic potential, we found that the absence of the ferroxidase that is part of the reductive iron assimilation system leads to hypervirulence in this fungus. This is an unusual and rather underrepresented phenotype, which can be modulated by iron levels in the plant and provides an unexpected link between iron acquisition, reactive oxygen species (ROS) production, and pathogenesis in the Botrytis-plant interaction.

The plant pathogen Botrytis cinerea is responsible for gray-mold disease, which infects a wide variety of species. The outcome of this host-pathogen interaction, a result of the interplay between plant defense and fungal virulence pathways, can be modulated by various environmental factors. Among these, iron availability and acquisition play a crucial role in diverse biological functions. How B. cinerea obtains iron, an essential micronutrient, during infection is unknown. We set out to determine the role of the reductive iron assimilation (RIA) system during B. cinerea infection. This system comprises the BcFET1 ferroxidase, which belongs to the multicopper oxidase (MCO) family of proteins, and the BcFTR1 membrane-bound iron permease. Gene knockout and complementation studies revealed that, compared to the wild type, the bcfet1 mutant displays delayed conidiation, iron-dependent sclerotium production, and significantly reduced whole-cell iron content. Remarkably, this mutant exhibited a hypervirulence phenotype, whereas the bcftr1 mutant presents normal virulence and unaffected whole-cell iron levels and developmental programs. Interestingly, while in iron-starved plants wild-type B. cinerea produced slightly reduced necrotic lesions, the hypervirulence phenotype of the bcfet1 mutant is no longer observed in iron-deprived plants. This suggests that B. cinerea bcfet1 knockout mutants require plant-derived iron to achieve larger necrotic lesions, whereas in planta analyses of reactive oxygen species (ROS) revealed increased ROS levels only for infections caused by the bcfet1 mutant. These results suggest that increased ROS production, under an iron sufficiency environment, at least partly underlie the observed infection phenotype in this mutant.

VelA and LaeA are Key Regulators of Epichloë festucae Transcriptomic Response during Symbiosis with Perennial Ryegrass

VelA (or VeA) is a key global regulator in fungal secondary metabolism and development which we previously showed is required during the symbiotic interaction of Epichloë festucae with perennial ryegrass. In this study, comparative transcriptomic analyses of ∆velA mutant compared to wild-type E. festucae, under three different conditions (in culture, infected seedlings, and infected mature plants), were performed to investigate the impact of VelA on E. festucae transcriptome. These comparative transcriptomic studies showed that VelA regulates the expression of genes encoding proteins involved in membrane transport, fungal cell wall biosynthesis, host cell wall degradation, and secondary metabolism, along with a number of small secreted proteins and a large number of proteins with no predictable functions. In addition, these results were compared with previous transcriptomic experiments that studied the impact of LaeA, another key global regulator of secondary metabolism and development that we have shown is important for E. festucae–perennial ryegrass interaction. The results showed that although VelA and LaeA regulate a subset of E. festucae genes in a similar manner, they also regulated many other genes independently of each other suggesting specialised roles.

The Role of SreA-Mediated Iron Regulation in Maintaining Epichloë festucae-Lolium perenne Symbioses

In ascomycetes and basidiomycetes, iron-responsive GATA-type transcriptional repressors are involved in regulating iron homeostasis, notably to prevent iron toxicity through control of iron uptake. To date, it has been unknown whether this iron regulator contributes toward mutualistic endosymbiosis of microbes with plants, a system where the endophyte must function within the constraints of an in-host existence, including a dependency on the host for nutrient acquisition. Functional characterization of one such protein, SreA from Epichloë festucae, a fungal endosymbiont of cool-season grasses, indicates that regulation of iron homeostasis processes is important for symbiotic maintenance. The deletion of the sreA gene (ΔsreA) led to iron-dependent aberrant hyphal growth and the gradual loss of endophyte hyphae from perennial ryegrass. SreA negatively regulates the siderophore biosynthesis and high-affinity iron uptake systems of E. festucae, similar to other fungi, resulting in iron accumulation in mutants. Our evidence suggests that SreA is involved in the processes that moderate Epichloë iron acquisition from the plant apoplast, because overharvesting of iron in ΔsreA mutants was detected as premature chlorosis of the host using a hydroponic plant growth assay. E. festucae appears to have a tightly regulated iron management system, involving SreA that balances endophyte growth with its survival and prevents overcompetition with the host for iron in the intercellular niche, thus promoting mutualistic associations. Mutations that interfere with Epichloë iron management negatively affect iron-dependent fungal growth and destabilize mutualistic Epichloë -ryegrass associations.

Ferricrocin, the intracellular siderophore of Trichoderma virens, is involved in growth, conidiation, gliotoxin biosynthesis and induction of systemic resistance in maize

Fungal siderophores are known to be involved in iron acquisition and storage, as well as pathogenicity of mammals and plants. As avirulent plant symbionts, Trichoderma spp. colonize roots and induce resistance responses both locally and systemically. To study the role of intracellular siderophore(s) in Trichoderma-plant interactions, we have obtained mutants in a non-ribosomal peptide synthetase, TvTex10, that was predicted to be involved in intracellular siderophore(s) biosynthesis. This gene has a detectable basal level of expression and is also upregulated under iron-deplete conditions. This is unlike two other siderophore-encoding genes, which are tightly regulated by iron. Disruption of tex10 gene using homologous recombination resulted in mutants with enhanced growth rate, reduced conidiation and hyper-sensitivity to oxidative stress as compared to wildtype strain. The mutants also produced reduced levels of gliotoxin and dimethyl gliotoxin but have enhanced ability to colonize maize seedling roots. The mutants were also impaired in induction of induced systemic resistance (ISR) in maize against the foliar pathogen Cochliobolus heterostrophus.