Unprecedented detoxification of the cruciferous phytoalexin camalexin by a root phytopathogen

Specialized metabolite modifications in Brassicaceae seeds and plants: diversity, functions and related enzymes

Covering: up to 2023Specialized metabolite (SM) modifications and/or decorations, corresponding to the addition or removal of functional groups (e.g. hydroxyl, methyl, glycosyl or acyl group) to SM structures, contribute to the huge diversity of structures, activities and functions of seed and plant SMs. This review summarizes available knowledge (up to 2023) on SM modifications in Brassicaceae and their contribution to SM plasticity. We give a comprehensive overview on enzymes involved in the addition or removal of these functional groups. Brassicaceae, including model (Arabidopsis thaliana) and crop (Brassica napus, Camelina sativa) plant species, present a large diversity of plant and seed SMs, which makes them valuable models to study SM modifications. In this review, particular attention is given to the environmental plasticity of SM and relative modification and/or decoration enzymes. Furthermore, a spotlight is given to SMs and related modification enzymes in seeds of Brassicaceae species. Seeds constitute a large reservoir of beneficial SMs and are one of the most important dietary sources, providing more than half of the world's intake of dietary proteins, oil and starch. The seed tissue- and stage-specific expressions of A. thaliana genes involved in SM modification are presented and discussed in the context of available literature. Given the major role in plant phytochemistry, biology and ecology, SM modifications constitute a subject of study contributing to the research and development in agroecology, pharmaceutical, cosmetics and food industrial sectors.

Discovery of Phytoalexin Camalexin and Its Derivatives as Novel Antiviral and Antiphytopathogenic-Fungus Agents

In response to the invasion of plant viruses and pathogenic fungi, higher plants produce defensive allelochemicals. Finding candidate varieties of botanical pesticides based on allelochemicals is one of the important ways to create efficient and green pesticides. Here, a series of camalexin derivatives based on a phytoalexin camalexin scaffold were designed, synthesized, and assessed for their antiviral and fungicidal activities systematically. Most of these camalexin derivatives exhibited better antiviral activities against tobacco mosaic virus (TMV) than the control antiviral agent ribavirin. Under the same test conditions, the anti-TMV activities of compounds 3d, 5a, 5d, and 10f-10h were found to be equivalent to or better than that of ningnanmycin, an agricultural cytosine nucleoside antibiotic with excellent protective effect. The antiviral mechanism research showed that compound 5a could cause 20S CP disk fusion and disintegration, thus affecting the assembly of virus particles. The results of molecular docking indicate that there were obvious hydrogen bonds between compounds 3d, 5a, and 10f and TMV CP. The binding constants of compounds 5a and 10f to TMV CP were also calculated using fluorescence titration. These camalexin derivatives also presented broad spectrum fungicidal activities, especially for Rhizoctonia solani and Physalospora piricola. In this work, the design, synthesis, structure optimization, and mode of action of camalexin derivatives were carried out progressively. This work provides a reference for using defensive chemical compounds as novel pesticide lead compounds.

Chemical signaling involved in plant-microbe interactions

Microorganisms are found everywhere, and they are closely associated with plants. Because the establishment of any plant-microbe association involves chemical communication, understanding crosstalk processes is fundamental to defining the type of relationship. Although several metabolites from plants and microbes have been fully characterized, their roles in the chemical interplay between these partners are not well understood in most cases, and they require further investigation. In this review, we describe different plant-microbe associations from colonization to microbial establishment processes in plants along with future prospects, including agricultural benefits.

The isolation and characterization of resident yeasts from the phylloplane of Arabidopsis thaliana

The genetic model plant Arabidopsis thaliana (arabidopsis) has been instrumental to recent advances in our understanding of the molecular function of the plant immune system. However, this work has not yet included plant associated and phytopathogenic yeasts largely due to a lack of yeast species known to interact with arabidopsis. The plant phylloplane is a significant habitat for neutral-residents, plant-growth and health-promoting species, and latent-pathogenic species. However, yeast phylloplane residents of arabidopsis remain underexplored. To address this, resident yeasts from the phyllosphere of wild arabidopsis collected in field conditions have been isolated and characterized. A total of 95 yeast strains representing 23 species in 9 genera were discovered, including potentially psychrophilic and pathogenic strains. Physiological characterization revealed thermotolerance profiles, sensitivity to the arabidopsis phytoalexin camalexin, the production of indolic compounds, and the ability to activate auxin responses in planta. These results indicate a rich diversity of yeasts present in the arabidopsis phylloplane and have created culture resources and information useful in the development of model systems for arabidopsis-yeast interactions.

Disruption of the Bcchs3a chitin synthase gene in Botrytis cinerea is responsible for altered adhesion and overstimulation of host plant immunity

The fungal cell wall is a dynamic structure that protects the cell from different environmental stresses suggesting that wall synthesizing enzymes are of great importance for fungal virulence. Previously, we reported the isolation and characterization of a mutant in class III chitin synthase, Bcchs3a, in the phytopathogenic fungus Botrytis cinerea. We demonstrated that virulence of this mutant is severely impaired. Here, we describe the virulence phenotype of the cell-wall mutant Bcchs3a on the model plant Arabidopsis thaliana and analyze its virulence properties, using a variety of A. thaliana mutants. We found that mutant Bcchs3a is virulent on pad2 and pad3 mutant leaves defective in camalexin. Mutant Bcchs3a was not more susceptible towards camalexin than the wild-type strain but induced phytoalexin accumulation at the infection site on Col-0 plants. Moreover, this increase in camalexin was correlated with overexpression of the PAD3 gene observed as early as 18 h postinoculation. The infection process of the mutant mycelium was always delayed by 48 h, even on pad3 plants, probably because of lack of mycelium adhesion. No loss in virulence was found when Bcchs3a conidia were used as the inoculum source. Collectively, these data led us to assign a critical role to the BcCHS3a chitin synthase isoform, both in fungal virulence and plant defense response.

Phytoalexin accumulation in the interaction between rice and the blast fungus

Blast fungus-induced accumulations of major rice diterpene phytoalexins (PA), momilactones A and B, and phytocassanes A through E were studied, focusing on their biosynthesis and detoxification. In resistant rice, all PA started to accumulate at 2 days postinoculation (dpi), at which hypersensitive reaction (HR)-specific small lesions became visible and increased 500- to 1,000-fold at 4 dpi, while the accumulation was delayed and several times lower in susceptible rice. Expression of PA biosynthetic genes was transiently induced at 2 dpi only in resistant plants, while it was highly induced in both plants at 4 dpi. Fungal growth was severely suppressed in resistant plants by 2 dpi but considerably increased at 3 to 4 dpi in susceptible plants. Momilactone A treatment suppressed fungal growth in planta and in vitro, and the fungus detoxified the PA in vitro. These results indicate that HR-associated rapid PA biosynthesis induces severe restriction of fungus, allowing higher PA accumulation in resistant rice, while in susceptible rice, failure of PA accumulation at the early infection stage allows fungal growth. Detoxification of PA would be a tactic of fungus to invade the host plant, and prompt induction of PA biosynthesis upon HR would be a trait of resistant rice to restrict blast fungus.

Transcriptional responses to exposure to the brassicaceous defence metabolites camalexin and allyl-isothiocyanate in the necrotrophic fungus Alternaria brassicicola

SUMMARY Alternaria brassicicola is the causative agent of black spot disease of Brassicaceae belonging to the genera Brassica and Raphanus. During host infection, A. brassicicola is exposed to high levels of antimicrobial defence compounds such as indolic phytoalexins and glucosinolate breakdown products. To investigate the transcriptomic response of A. brassicicola when challenged with brassicaceous defence metabolites, suppression subtractive hybridization (SSH) was performed to generate two cDNA libraries from germinated conidia treated either with allyl isothiocyanate (Al-ITC) or with camalexin. Following exposure to Al-ITC, A. brassicicola displayed a response similar to that experienced during oxidative stress. Indeed, a substantial subset of differentially expressed genes was related to cell protection against oxidative damage. Treatment of A. brassicicola conidia with the phytoalexin camalexin appeared to activate a compensatory mechanism to preserve cell membrane integrity and, among the camalexin-elicited genes, several were involved in sterol and sphingolipid biosynthesis. The transcriptomic analysis suggested that protection against the two tested compounds also involved mechanisms aimed at limiting their intracellular accumulation, such as melanin biosynthesis (in the case of camalexin exposure only) and drug efflux. From the Al-ITC and the camalexin differentially expressed genes identified here, 25 were selected to perform time-course studies during interactions with brassicaceous hosts. In planta, up-regulation of all the selected genes was observed during infection of Raphanus sativus whereas only a subset were over-expressed during the incompatible interaction with Arabidopsis thaliana ecotype Columbia.

Camalexin

Camalexin (3-thiazol-2'-yl-indole) is the characteristic phytoalexin of Arabidopsis thaliana, which is induced by a great variety of plant pathogens. While particular pathogens, as well as a human tumour cell line, were growth inhibited by camalexin, some fungi show resistance due to active degradation. Camalexin originates from tryptophan and its biosynthesis involves the cytochrome P450 enzymes CYP79B2 and CYP71B15 (PAD3). Camalexin induction is a complex process, for which triggering by reactive oxygen species (ROS), salicylic acid signalling, and the glutathione status are important. Targets of the signalling cascade are the tryptophan and camalexin biosynthetic genes, which are strongly transcriptionally upregulated at the sites of pathogen infection. The important knowledge on camalexin, which is reviewed in this paper, will help to establish camalexin as a model for the investigation of the significance of phytoalexins in response pathogen challenge.

Secondary metabolites influence Arabidopsis/Botrytis interactions: variation in host production and pathogen sensitivity

Numerous studies have suggested that plant/pathogen interactions are partially mediated via plant secondary metabolite production and corresponding pathogen tolerance. However, there are inconsistent reports on the ability of particular compounds to provide resistance to a pathogen. Most of these studies have focused on individual isolates of a given pathogen, suggesting that pathogens vary in their sensitivity to plant-produced toxins. We tested variability in virulence among pathogen isolates, and the impact on this by plant production of, and pathogen tolerance to, secondary metabolites. Botrytis cinerea isolates showed differing sensitivity to purified camalexin, and camalexin-sensitive isolates produced larger lesions on camalexin-deficient Arabidopsis genotypes than on the wild type. In contrast, the camalexin-insensitive isolate produced lesions of similar size on wild-type and camalexin-deficient Arabidopsis. Additional analysis with Arabidopsis secondary metabolite biosynthetic mutants suggests that Botrytis also has variable sensitivity to phenylpropanoids and glucosinolates. Furthermore, Botrytis infection generates a gradient of secondary metabolite responses emanating from the developing lesion, with the Botrytis isolate used determining the accumulation pattern. Collectively, our results indicate that Arabidopsis/Botrytis interactions are influenced at the metabolic level by variations in toxin production in the host and sensitivity in the pathogen.

Metabolism and detoxification of phytoalexins and analogs by phytopathogenic fungi

To date, the many examples reporting that fungal pathogens can efficiently detoxify phytoalexins provide strong evidence that the pathogenicity and/or virulence of some fungi is linked to their ability to detoxify their hosts' phytoalexins. The pathways used by plant pathogenic fungi to metabolize and detoxify phytoalexins are reviewed. Prospects for application of recent findings are discussed.

AWARD LECTURE / CONFÉRENCE D'HONNEUR Prospects for controlling plant fungal diseases — Alternatives based on chemical ecology and biotechnology

Plants produce a diverse array of secondary metabolites associated with important defence and resistance mechanisms. In general, within the same plant family the chemical structures of these metabolites are related and suggest common biogenetic precursors. Crucifers are known to produce constitutive and induced tryptophan derived metabolites. Pathogenic fungi can resist such defences utilizing a variety of processes, as for example, enzymatic detoxification. On the other hand, to facilitate their penetration and colonization of the plant tissues, fungal phytopathogens can produce phytotoxic metabolites, some of which are selectively toxic to host-plants. However, plants may be resistant to these pathogens partly due to their ability to detoxify these selective phytotoxins. Nonetheless, the outcome of these interactions frequently favours the pathogen and can lead to enormous yield losses or even major plant epidemics. An overview of some of the most recent studies of metabolic pathways and stress responses in crucifers and several of their pathogens is presented. Potential strategies to prevent and control plant microbial diseases based on chemical ecology studies and biotechnology will be discussed. Key words: phytoalexin, phytotoxin, chemical defence, metabolic detoxification, destruxin B.

Phytoalexins from crucifers: synthesis, biosynthesis, and biotransformation

Phytoalexins play a significant role in the defense response of plants. These secondary metabolites, which are synthesized de novo in response to diverse forms of stress, including fungal infection, are part of the plants' chemical and biochemical defense mechanisms. Phytoalexins from crucifers are structurally and biogenetically related, but display significantly different biological activities. Here, we review work reporting the chemical structures, synthesis, biosynthesis and metabolism of cruciferous phytoalexins, as well as their biological activity towards different microorganisms.

Biotransformation of the phytoalexin camalexin by the phytopathogen Rhizoctonia solani

The unusual metabolism of the cruciferous phytoalexin camalexin by virulent and weakly virulent isolates of the root rot fungus Rhizoctonia solani Kuhn is reported. This biotransformation proceeded via 5-hydroxycamalexin, which was further biotransformed into more polar metabolites. Importantly, the metabolites resulting from transformation of camalexin were significantly less toxic to the pathogen than camalexin. Thus, it was concluded that R. solani can detoxify camalexin through oxidation of the indole ring. The chemistry involved in the structure determination of the intermediates of this pathway, their synthesis as well as antifungal activity is described.

Fungal resistance to plant antibiotics as a mechanism of pathogenesis

Many plants produce low-molecular-weight compounds which inhibit the growth of phytopathogenic fungi in vitro. These compounds may be preformed inhibitors that are present constitutively in healthy plants (also known as phytoanticipins), or they may be synthesized in response to pathogen attack (phytoalexins). Successful pathogens must be able to circumvent or overcome these antifungal defenses, and this review focuses on the significance of fungal resistance to plant antibiotics as a mechanism of pathogenesis. There is increasing evidence that resistance of fungal pathogens to plant antibiotics can be important for pathogenicity, at least for some fungus-plant interactions. This evidence has emerged largely from studies of fungal degradative enzymes and also from experiments in which plants with altered levels of antifungal secondary metabolites were generated. Whereas the emphasis to date has been on degradative mechanisms of resistance of phytopathogenic fungi to antifungal secondary metabolites, in the future we are likely to see a rapid expansion in our knowledge of alternative mechanisms of resistance. These may include membrane efflux systems of the kind associated with multidrug resistance and innate resistance due to insensitivity of the target site. The manipulation of plant biosynthetic pathways to give altered antibiotic profiles will also be valuable in telling us more about the significance of antifungal secondary metabolites for plant defense and clearly has great potential for enhancing disease resistance for commercial purposes.