Factors Affecting the Production of Elsinochrome Phytotoxin by the Citrus Scab Pathogen, Elsinoe fawcettii

Biotechnological production and potential applications of hypocrellins

Hypocrellins (HYPs), a kind of natural perylenequinones (PQs) with an oxidized pentacyclic core, are important natural compounds initially extracted from the stromata of Hypocrella bambusae and Shiraia bambusicola. They have been widely concerned for their use as anti-microbial, anti-cancers, and anti-viral photodynamic therapy agents in recent years. Considering the restrictions of natural stromal resources, submerged fermentation with Shiraia spp. has been viewed as a promising alternative biotechnology for HYP production, and great efforts have been made to improve HYP production over the past decade. This article reviews recent publications about the mycelium fermentation production of HYPs, and their bioactivities and potential applications, and especially summarizes the progresses toward manipulation of fermentation conditions. Also, their chemical structure and analytic methods are outlined. Herein, it is worth mentioning that the gene arrangement in HYP gene cluster is revised; previous unknown genes in HYP and CTB gene clusters with correct function annotation are deciphered; the homologous sequences of HYP, CTB, and elc are systematically aligned, and especially the biosynthetic pathway of HYPs is full-scale proposed. KEY POINTS: • The mycelial fermentation process and metabolic regulation of hypocrellins are reviewed. • The bioactivities and potential applications of hypocrellins are summarized. • The biosynthesis pathway and regulatory mechanisms of hypocrellins are outlined.

Nitrogen nutrition modifies the susceptibility of Arabidopsis thaliana to the necrotrophic fungus, Alternaria brassicicola

The impact of the form of nitrogen (N) source (nitrate versus ammonium) on the susceptibility to Alternaria brassicicola, a necrotrophic fungus, has been examined in Arabidopsis thaliana at the rosette stage. Nitrate nutrition was found to increase fungal lesions considerably. There was a similar induction of defence gene expression following infection under both N nutritions, except for the phytoalexin deficient 3 gene, which was overexpressed with nitrate. Nitrate also led to a greater nitric oxide production occurring in planta during the saprophytic growth and lower nitrate reductase (NIA1) expression 7 days after inoculation. This suggests that nitrate reductase-dependent nitric oxide production had a dual role, whereby, despite its known role in the generic response to pathogens, it affected plant metabolism, and this facilitated fungal infection. In ammonium-grown plants, infection with A. brassicicola induced a stronger gene expression of ammonium transporters and significantly reduced the initially high ammonium content in the leaves. There was a significant interaction between N source and inoculation (presence versus absence of the fungus) on the total amino acid content, while N nutrition reconfigured the spectrum of major amino acids. Typically, a higher content of total amino acid, mainly due to a stronger increase in asparagine and glutamine, is observed under ammonium nutrition while, in nitrate-fed plants, glutamate was the only amino acid which content increased significantly after fungal inoculation. N nutrition thus appears to control fungal infection via a complex set of signalling and nutritional events, shedding light on how nitrate availability can modulate disease susceptibility.

Current status and future prospects of grapevine anthracnose caused by Elsinoe ampelina: An important disease in humid grape-growing regions

Anthracnose, caused by Elsinoe ampelina, is one of the most destructive diseases of grapevines worldwide, especially in humid areas. E. ampelina mainly infects young tissues starting from shoots to berries and affects vine vigour and berry yield. The occurrence and the role of the sexual stage in the disease cycle and the grapevine-E. ampelina interaction remain poorly understood. However, the recent genome sequence data of E. ampelina provides the basis for further studies to understand its evolution, pathogenicity mechanisms, and effector repertoire. New studies on E. ampelina have been conducted in recent years. In this pathogen profile, we present a comprehensive literature review of E. ampelina to summarize the findings on its aetiology, infection mechanisms, genome, pathogenicity, and host resistance.

TAXONOMY

Elsinoe ampelina Shear; Kingdom Fungi; Phylum Ascomycota; Subphylum Pezizomycotina; Class Dothideomycetes; Subclass Dothideomycetidae; Order Myriangiales Starbäck; Family Elsinoaceae Höhnel; Genus Elsinoe Racib.

HOST RANGE

E. ampelina only infects Vitis species and hybrids.

DISTRIBUTION

The grapevine anthracnose is distributed worldwide but is most prevalent in Argentina, Australia, Brazil, Canada, China, India, Japan, Korea, New Zealand, South Africa, Thailand, USA, and Uruguay.

DISEASE SYMPTOMS

E. ampelina causes slightly abundant depressed spots on young leaves, petioles, stems, tendrils, rachises, and berries. Under severe infection conditions, early defoliation, berry dropping, and delayed berry development and ripening may occur.

GENOME

The genomes of two E. ampelina isolates, YL-1 and CECT 20119, are publicly released with 8,057 and 10,207 predicted genes, respectively.

Unravelling the Roles of Nitrogen Nutrition in Plant Disease Defences

Nitrogen (N) is one of the most important elements that has a central impact on plant growth and yield. N is also widely involved in plant stress responses, but its roles in host-pathogen interactions are complex as each affects the other. In this review, we summarize the relationship between N nutrition and plant disease and stress its importance for both host and pathogen. From the perspective of the pathogen, we describe how N can affect the pathogen’s infection strategy, whether necrotrophic or biotrophic. N can influence the deployment of virulence factors such as type III secretion systems in bacterial pathogen or contribute nutrients such as gamma-aminobutyric acid to the invader. Considering the host, the association between N nutrition and plant defence is considered in terms of physical, biochemical and genetic mechanisms. Generally, N has negative effects on physical defences and the production of anti-microbial phytoalexins but positive effects on defence-related enzymes and proteins to affect local defence as well as systemic resistance. N nutrition can also influence defence via amino acid metabolism and hormone production to affect downstream defence-related gene expression via transcriptional regulation and nitric oxide (NO) production, which represents a direct link with N. Although the critical role of N nutrition in plant defences is stressed in this review, further work is urgently needed to provide a comprehensive understanding of how opposing virulence and defence mechanisms are influenced by interacting networks.

Elsinochrome phytotoxin production and pathogenicity of Elsinoë arachidis isolates in China

Peanut scab caused by Elsinoë arachidis is found throughout China's peanut-growing areas. Elsinochrome produced by E. arachidis is a perylenequinone photosensitive mycotoxin vital to the pathogenic process of the pathogen. In this study, the complex mechanism underlying the regulation of elsinochrome biosynthesis by E. arachidis was investigated based on various nutritional and environmental factors. The initiation of elsinochrome biosynthesis depends on light. E. arachidis produced substantially more quantities of elsinochrome when grown on a semi-synthetic medium (PDA) than when grown on synthetic media with defined ingredients in the presence of light. Elsinochrome accumulation decreased when adjusted with either citrate or phosphate buffers and changing pH suppressed the radical growth. At temperatures ranging from 10°C to 25°C, the production of elsinochrome increased, peaking at 28°C, and it decreased slightly at 30°C. 63 field-collected isolates from China were assessed for the level of elsinochrome production, and pathogenicity analysis was conducted by selecting 12 strains from each 3 of the 4 groups with different levels of elsinochrome production. A direct correlation was observed between elsinochrome production and pathogenicity among the isolates. The results showed elsinochrome biosynthesis to be controlled by E. arachidis and showed elsinochrome to be a vital virulence factor of E. arachidis, required for disease severity.

Reactive oxygen species in plant pathogenesis: the role of perylenequinone photosensitizers

SIGNIFICANCE

Reactive oxygen species (ROS) play multiple roles in interactions between plants and microbes, both as host defense mechanisms and as mediators of pathogenic and symbiotic associations. One source of ROS in these interactions are photoactivated, ROS-generating perylenequinone pigments produced via polyketide metabolic pathways in plant-associated fungi. These natural products, including cercosporin, elsinochromes, hypocrellins, and calphostin C, are being utilized as medicinal agents, enzyme inhibitors, and in tumor therapy, but in nature, they play a role in the establishment of pathogenic associations between fungi and their plant hosts.

RECENT ADVANCES

Photoactivated perylenequinones are photosensitizers that use light energy to form singlet oxygen (¹O₂) and free radical oxygen species which damage cellular components based on localization of the perylenequinone molecule. Production of perylenequinones during infection commonly results in lipid peroxidation and membrane damage, leading to leakage of nutrients from cells into the intercellular spaces colonized by the pathogen. Perylenequinones show almost universal toxicity against organisms, including plants, mice, bacteria, and most fungi. The producing fungi are resistant, however, and serve as models for understanding resistance mechanisms.

CRITICAL ISSUES

Studies of resistance mechanisms by perylenequinone-producing fungi such as Cercospora species are leading to an understanding of cellular resistance to ¹O₂ and oxidative stress. Recent studies show commonalities between resistance mechanisms in these fungi with extensive studies of ¹O₂ and oxidative stress responses in photosynthetic organisms.

FUTURE DIRECTIONS

Such studies hold promise both for improved medical use and for engineering crop plants for disease resistance.

Elsinoë fawcettii and Elsinoë australis: the fungal pathogens causing citrus scab

Elsinoë fawcettii and E. australis are important pathogens of citrus. Both species are known to produce red or orange pigments, called elsinochrome. Elsinochrome is a nonhost-selective phytotoxin and is required for full fungal virulence and lesion formation. This article discusses the taxonomy, epidemiology, genetics and pathology of the pathogens. It also provides a perspective on the cellular toxicity, biosynthetic regulation and pathological role of elsinochrome phytotoxin.

TAXONOMY

Elsinoë fawcettii (anamorph: Sphaceloma fawcettii) and E. australis (anamorph: S. australis) are classified in the Phylum Ascomycota, Class Dothideomycetes, Order Myriangiales and Family Elsinoaceae.

HOST RANGE

Elsinoë fawcettii causes citrus scab (formerly sour orange scab and common scab) on various species and hybrids in the Rutaceae family worldwide, whereas E. australis causes sweet orange scab, primarily on sweet orange and some mandarins, and has a limited geographical distribution.

DISEASE SYMPTOMS

Citrus tissues infested with Elsinoë often display erumpent scab pustules with a warty appearance. TOXIN PRODUCTION: Elsinochrome and many perylenequinone-containing phytotoxins of fungal origin are grouped as photosensitizing compounds that are able to absorb light energy, react with oxygen molecules and produce reactive oxygen species, such as superoxide and singlet oxygen. Elsinochrome has been documented to cause peroxidation of cell membranes and to induce rapid electrolyte leakage from citrus tissues. Elsinochrome biosynthesis and conidiation are coordinately regulated in E. fawcettii, and the environmental and physiological inducers commonly involved in both processes have begun to be elucidated.

Transcriptional regulation of elsinochrome phytotoxin biosynthesis by an EfSTE12 activator in the citrus scab pathogen Elsinoë fawcettii

Elsinochrome (ESC), produced by the citrus pathogen Elsinoë fawcettii, is a nonhost-selective, light-dependent, polyketide-derived phytotoxin and plays a crucial role for full virulence. The biosynthesis of ESC is regulated by a wide array of environmental stimuli and is primarily governed by the pathway-specific TSF1 transcription regulator whose coding gene is clustered with the EfPKS1 gene encoding a polyketide synthase and other biosynthetic genes in the genome. In this report, an EfSTE12 gene, encoding a polypeptide resembling the yeast STE12 transcription factor, was cloned and characterized to play a role, independent of TSF1, for ESC production in E. fawcettii. The loss-of-function mutant, specifically disrupted at the EfSTE12 locus, displays reduced ESC accumulation, elevated activities for pectinase and proteolytic enzymes but unaltered in conidiation and fungal pathogenicity. Impairment of the EfSTE12 gene decreased the abundance of the EfPKS1 but not the TSF1 gene transcript. In contrast, expression of the EfSTE12 gene appears normal in the EfPKS1 or TSF1 disruptants. The results indicate that EfSTE12 is functioning for ESC biosynthesis by directly activating the biosynthetic genes without regulating the pathway-specific TSF1 regulator. The defective phenotypes were fully reverted when a functional copy of EfSTE12 was re-introduced into the disrupted mutant. A hypothetical model underlying intertwined regulatory pathways via TSF1, EfSTE12, and other potent transcriptional activators led to the ESC biosynthesis and conidiation is described.