Metabolism of the Host-Selective Toxins Destruxin B and Homodestruxin B: Probing a Plant Disease Resistance Trait

Plant Resistance to Fungal Pathogens: Bibliometric Analysis and Visualization

Plants are susceptible to fungal pathogen infection, threatening plant growth and development. Researchers worldwide have conducted extensive studies to address this issue and have published numerous articles on the subject, but they lack a scientometric evaluation. This study analyzed international research on the topic "Plant resistance to fungal pathogens" between 2008 and 2021, using the core database of the Web of Science (WoS). By searching the subject words "Plants", "Disease Resistance", and "Fungal Pathogens", we received 6687 articles. Bibliometric visualization software analyzes the most published countries, institutions, journals, authors, the most cited articles, and the most common keywords. The results show that the number of articles in the database has increased year by year, with the United States and China occupying the core positions, accounting for 46.16% of the total published articles worldwide. The United States Department of Agriculture (USDA) is the main publishing organization. Wang Guoliang is the author with the most published articles, and the Frontiers in Plant Science ranks first in published articles. The research on plant anti-fungal pathogens is booming, and international exchanges and cooperation need to be further strengthened. This paper summarizes five possible research ideas, from fungal pathogens, gene editing technology, extraction of secondary metabolites from plants as anti-fungal agents, identification of related signal pathways, fungal molecular databases, and development of nanomaterials, to provide data for related research.

Phytotoxic Secondary Metabolites from Fungi

Fungal phytotoxic secondary metabolites are poisonous substances to plants produced by fungi through naturally occurring biochemical reactions. These metabolites exhibit a high level of diversity in their properties, such as structures, phytotoxic activities, and modes of toxicity. They are mainly isolated from phytopathogenic fungal species in the genera of Alternaria, Botrytis, Colletotrichum, Fusarium, Helminthosporium, and Phoma. Phytotoxins are either host specific or non-host specific phytotoxins. Up to now, at least 545 fungal phytotoxic secondary metabolites, including 207 polyketides, 46 phenols and phenolic acids, 135 terpenoids, 146 nitrogen-containing metabolites, and 11 others, have been reported. Among them, aromatic polyketides and sesquiterpenoids are the main phytotoxic compounds. This review summarizes their chemical structures, sources, and phytotoxic activities. We also discuss their phytotoxic mechanisms and structure-activity relationships to lay the foundation for the future development and application of these promising metabolites as herbicides.

Alternaria host-specific (HSTs) toxins: An overview of chemical characterization, target sites, regulation and their toxic effects

Alternaria causes pathogenic disease on various economically important crops having saprophytic to endophytic lifecycle. Pathogenic fungi of Alternaria species produce many primary and secondary metabolites (SMs). Alternaria species produce more than 70 mycotoxins. Several species of Alternaria produce various phytotoxins that are host-specific (HSTs) and non-host-specific (nHSTs). These toxins have various negative impacts on cell organelles including chloroplast, mitochondria, plasma membrane, nucleus, Golgi bodies, etc. Non-host-specific toxins such as tentoxin (TEN), Alternaric acid, alternariol (AOH), alternariol 9-monomethyl ether (AME), brefeldin A (dehydro-), Alternuene (ALT), Altertoxin-I, Altertoxin-II, Altertoxin-III, zinniol, tenuazonic acid (TeA), curvularin and alterotoxin (ATX) I, II, III are known toxins produced by Alternaria species. In other hand, Alternaria species produce numerous HSTs such as AK-, AF-, ACT-, AM-, AAL- and ACR-toxin, maculosin, destruxin A, B, etc. are host-specific and classified into different family groups. These mycotoxins are low molecular weight secondary metabolites with various chemical structures. All the HSTs have different mode of actions, biochemical reactions, and signaling mechanisms to causes diseases in the host plants. These HSTs have devastating effects on host plant tissues by affecting biochemical and genetic modifications. Host-specific mycotoxins such as AK-toxin, AF-toxin, and AC-toxin have the devastating effect on plants which causes DNA breakage, cytotoxic, apoptotic cell death, interrupting plant physiology by mitochondrial oxidative phosphorylation and affect membrane permeability. This article will elucidate an understanding of the disease mechanism caused by several Alternaria HSTs on host plants and also the pathways of the toxins and how they caused disease in plants.

Structural Diversity and Biological Activities of Cyclic Depsipeptides from Fungi

Cyclic depsipeptides (CDPs) are cyclopeptides in which amide groups are replaced by corresponding lactone bonds due to the presence of a hydroxylated carboxylic acid in the peptide structure. These peptides sometimes display additional chemical modifications, including unusual amino acid residues in their structures. This review highlights the occurrence, structures and biological activities of the fungal CDPs reported until October 2017. About 352 fungal CDPs belonging to the groups of cyclic tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-, and tridecadepsipeptides have been isolated from fungi. These metabolites are mainly reported from the genera Acremonium, Alternaria, Aspergillus, Beauveria, Fusarium, Isaria, Metarhizium, Penicillium, and Rosellina. They are known to exhibit various biological activities such as cytotoxic, phytotoxic, antimicrobial, antiviral, anthelmintic, insecticidal, antimalarial, antitumoral and enzyme-inhibitory activities. Some CDPs (i.e., PF1022A, enniatins and destruxins) have been applied as pharmaceuticals and agrochemicals.

Alternaria Toxins: Potential Virulence Factors and Genes Related to Pathogenesis

Alternaria is an important fungus to study due to their different life style from saprophytes to endophytes and a very successful fungal pathogen that causes diseases to a number of economically important crops. Alternaria species have been well-characterized for the production of different host-specific toxins (HSTs) and non-host specific toxins (nHSTs) which depend upon their physiological and morphological stages. The pathogenicity of Alternaria species depends on host susceptibility or resistance as well as quantitative production of HSTs and nHSTs. These toxins are chemically low molecular weight secondary metabolites (SMs). The effects of toxins are mainly on different parts of cells like mitochondria, chloroplast, plasma membrane, Golgi complex, nucleus, etc. Alternaria species produce several nHSTs such as brefeldin A, tenuazonic acid, tentoxin, and zinniol. HSTs that act in very low concentrations affect only certain plant varieties or genotype and play a role in determining the host range of specificity of plant pathogens. The commonly known HSTs are AAL-, AK-, AM-, AF-, ACR-, and ACT-toxins which are named by their host specificity and these toxins are classified into different family groups. The HSTs are differentiated on the basis of bio-statistical and other molecular analyses. All these toxins have different mode of action, biochemical reactions and signaling mechanisms to cause diseases. Different species of Alternaria produced toxins which reveal its biochemical and genetic effects on itself as well as on its host cells tissues. The genes responsible for the production of HSTs are found on the conditionally dispensable chromosomes (CDCs) which have been well characterized. Different bio-statistical methods like basic local alignment search tool (BLAST) data analysis used for the annotation of gene prediction, pathogenicity-related genes may provide surprising knowledge in present and future.

Metabolites from Alternaria fungi and their bioactivities

Alternaria is a cosmopolitan fungal genus widely distributing in soil and organic matter. It includes saprophytic, endophytic and pathogenic species. At least 268 metabolites from Alternaria fungi have been reported in the past few decades. They mainly include nitrogen-containing metabolites, steroids, terpenoids, pyranones, quinones, and phenolics. This review aims to briefly summarize the structurally different metabolites produced by Alternaria fungi, as well as their occurrences, biological activities and functions. Some considerations related to synthesis, biosynthesis, production and applications of the metabolites from Alternaria fungi are also discussed.

The destruxins: synthesis, biosynthesis, biotransformation, and biological activity

Destruxins, secondary metabolites first reported in 1961, are cyclic hexadepsipeptides composed of an alpha-hydroxy acid and five amino acid residues. The name "destruxin" is derived from "destructor" from the species Oospora destructor, the entomopathogenic fungus from which these metabolites were first isolated. Individual destruxins differ on the hydroxy acid, N-methylation, and R group of the amino acid residues; where established, the configurations of the amino acid residues are S, and those of the hydroxy acids are R. Destruxins exhibit a wide variety of biological activities, but are best known for their insecticidal and phytotoxic activities. The great interest in destruxins derives from their potential role as virulence factors in fungi, whether such microorganisms are useful insect biocontrol agents or detrimental, causing great plant disease epidemics. Reports on isolation, chemical structure determination, total synthesis, transformation by diverse organisms, and biological activity of destruxins and related metabolites are reviewed for the first time.

In planta sequential hydroxylation and glycosylation of a fungal phytotoxin: Avoiding cell death and overcoming the fungal invader

To facilitate plant colonization, some pathogenic fungi produce phytotoxic metabolites that damage tissues; plants may be resistant to a particular pathogen if they produce an enzyme(s) that catalyzes detoxification of this metabolite(s). Alternaria blackspot is one of the most damaging and significant fungal diseases of brassica crops, with no source of resistance known within the Brassica species. Destruxin B is the major phytotoxin produced by the blackspot-causing fungus, Alternaria brassicae (Berkley) Saccardo. We have established that a blackspot-resistant species (Sinapis alba) metabolized (14)C-labeled destruxin B to a less toxic product substantially faster than any of the susceptible species. The first metabolite, hydroxydestruxin B ((14)C-labeled), was further biotransformed to the beta-d-glucosyl derivative at a slower rate. The structures of hydroxydestruxin B and beta-d-glucosyl hydroxydestruxin B were deduced from their spectroscopic data [NMR, high resolution (HR)-MS, Fourier transform infrared (FTIR)] and confirmed by total chemical synthesis. Although these hydroxylation and glucosylation reactions occurred in both resistant (S. alba) and susceptible (Brassica napus, Brassica juncea, and Brassica rapa) species, hydroxylation was the rate limiting step in the susceptible species, whereas glucosylation was the rate limiting step in the resistant species. Remarkably, it was observed that the hydroxydestruxin B induced the biosynthesis of phytoalexins in blackspot-resistant species but not in susceptible species. This appears to be a unique example of phytotoxin detoxification and simultaneous phytoalexin elicitation by the detoxification product. Our studies suggest that S. alba can overcome the fungal invader through detoxification of destruxin B coupled with production of phytoalexins.

Sinalbins A and B, phytoalexins from Sinapis alba: elicitation, isolation, and synthesis

The chemical structure and synthesis of sinalbin A is described. This cruciferous phytoalexin is produced by white mustard (Sinapis alba) after treatment with biotic and abiotic elicitors. In addition, a related metabolite, named sinalbin B, is present in extracts from elicited plants, but not in those from non-elicited controls. Sinalbin B, which was also synthesized, appears to be both a phytoalexin and a biosynthetic precursor of sinalbin A.

Comparison of the phytotoxic activity of the phytotoxin destruxin B and four natural analogs

A quantitative bioassay utilizing staining of plant cell suspension cultures of Sinapis alba was employed to establish a structure-phytotoxic activity correlation among destruxin B, homodestruxin B, and desmethyldestruxin B, toxins produced by Alternaria brassicae (Berk.) Sacc., the causative agent of Alternaria blackspot of brassicas. In addition, the phytotoxicity of destruxin B, homodestruxin B, and their respective metabolites hydroxydestruxin B and hydroxyhomodestruxin B were tested on resistant and susceptible plant species utilizing in planta leaf assays and leaf uptake of toxin solutions. Overall, the results obtained from punctured leaf and cell staining assays indicated that homodestruxin B (EC(50) 3x10(-4) M) was the most toxic of the five compounds, followed by destruxin B (EC(50) 5x10(-4) M), and desmethyldestruxin B (EC(50)&z.Gt;5x10(-4) M). On the other hand, the hydroxylated destruxins (hydroxydestruxin B EC(50)&z.Gt;5x10(-4) M) were significantly less phytotoxic than the parent toxins.