Concurrent metabolism of the phytoalexins phaseollin, kievitone and phaseollinisoflavan by Fusarium solani f. sp. phaseoli

The Fusarium solani species complex: ubiquitous pathogens of agricultural importance

Members of the Fusarium solani species complex (FSSC) are capable of causing disease in many agriculturally important crops. The genomes of some of these fungi include supernumerary chromosomes that are dispensable and encode host-specific virulence factors. In addition to genomics, this review summarizes the known molecular mechanisms utilized by members of the FSSC in establishing disease.

TAXONOMY

Kingdom Fungi; Phylum Ascomycota; Class Sordariomycetes; Order Hypocreales; Family Nectriaceae; Genus Fusarium.

HOST RANGE

Members of the FSSC collectively have a very broad host range, and have been subdivided previously into formae speciales. Recent phylogenetic analysis has revealed that formae speciales correspond to biologically and phylogenetically distinct species.

DISEASE SYMPTOMS

Typically, FSSC causes foot and/or root rot of the infected host plant, and the degree of necrosis correlates with the severity of the disease. Symptoms on above-ground portions of the plant can vary greatly depending on the specific FSSC pathogen and host plant, and the disease may manifest as wilting, stunting and chlorosis or lesions on the stem and/or leaves.

CONTROL

Implementation of agricultural management practices, such as crop rotation and timing of planting, can reduce the risk of crop loss caused by FSSC. If available, the use of resistant varieties is another means to control disease in the field.

USEFUL WEBSITES

http://genome.jgi-psf.org/Necha2/Necha2.home.html.

Metabolism and resistance of Fusarium spp. to the manzamine alkaloids via a putative retro pictet-spengler reaction and utility of the rational design of antimalarial and antifungal agents

As a part of our continuing investigation of the manzamine alkaloids we studied the in vitro activity of the β-carboline containing manzamine alkaloids against Fusarium solani, Fusarium oxysporium, and Fusarium proliferatum by employing several bioassay techniques including one-dimensional direct bioautography, dilution, and plate susceptibility, and microtiter broth assays. In addition, we also studied the metabolism of the manzamine alkaloids by Fusarium spp. in order to facilitate the redesign of the compounds to prevent resistance of Fusarium spp. through metabolism. The present research reveals that the manzamine alkaloids are inactive against Fusarium spp. and the fungi transform manzamines via hydrolysis, reduction, and a retro Pictet-Spengler reaction. This is the first report to demonstrate an enzymatically retro Pictet-Spengler reaction. The results of this study reveal the utility of the rational design of metabolically stable antifungal agents from this class and the development of manzamine alkaloids as antimalarial drugs through the utilization of Fusarium's metabolic products to reconstruct the molecule.

Cloning and characterization of naringenin 8-prenyltransferase, a flavonoid-specific prenyltransferase of Sophora flavescens

Prenylated flavonoids are natural compounds that often represent the active components in various medicinal plants and exhibit beneficial effects on human health. Prenylated flavonoids are hybrid products composed of a flavonoid core mainly attached to either 5-carbon (dimethylallyl) or 10-carbon (geranyl) prenyl groups derived from isoprenoid (terpenoid) metabolism, and the prenyl groups are crucial for their biological activity. Prenylation reactions in vivo are crucial coupling processes of two major metabolic pathways, the shikimate-acetate and isoprenoid pathways, in which these reactions are also known as a rate-limiting step. However, none of the genes responsible for the prenylation of flavonoids has been identified despite more than 30 years of research in this field. We have isolated a prenyltransferase gene from Sophora flavescens, SfN8DT-1, responsible for the prenylation of the flavonoid naringenin at the 8-position, which is specific for flavanones and dimethylallyl diphosphate as substrates. Phylogenetic analysis shows that SfN8DT-1 has the same evolutionary origin as prenyltransferases for vitamin E and plastoquinone. The gene expression of SfN8DT-1 is strictly limited to the root bark where prenylated flavonoids are solely accumulated in planta. The ectopic expression of SfN8DT-1 in Arabidopsis thaliana resulted in the formation of prenylated apigenin, quercetin, and kaempferol, as well as 8-prenylnaringenin. SfN8DT-1 represents the first flavonoid-specific prenyltransferase identified in plants and paves the way for the identification and characterization of further genes responsible for the production of this large and important class of secondary metabolites.

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.

sti35, a stress-responsive gene in Fusarium spp

A stress-induced mRNA was identified in the phytopathogenic fungus Fusarium oxysporum f. sp. cucumerinum. Treatment of the fungus with ethanol resulted in the induction of a major mRNA species encoding a protein of approximate Mr 37,000. A full-length cDNA clone of the induced message was obtained. RNA blot analysis indicated that the mRNA was induced by various other stresses, including treatment with copper(II) chloride and heat (37 degrees C). However, it was not greatly induced by treatment with phaseollinisoflavan, an antifungal isoflavonoid produced by Phaseolus vulgaris (French bean). In contrast, phaseollinisoflavan induced the homologous mRNA in the related bean pathogen Fusarium solani f. sp. phaseoli. A genomic clone of the F. solani f. sp. phaseoli gene was obtained, and both this and the cDNA clone from F. oxysporum f. sp. cucumerinum were sequenced. The latter indicated an open reading frame of 320 codons encoding a 34,556-dalton polypeptide. The corresponding reading frame in F. solani f. sp. phaseoli was 324 codons, 89% identical to the F. oxysporum f. sp. cucumerium sequence, and was interrupted by a short intron. The gene was designated sti35 (stress-inducible mRNA). Although computer homology searches were negative, the cloned gene was observed to cross-hybridize to DNAs of other filamentous fungi, Saccharomyces cerevisiae, and soybean. Thus, sti35 appears to be a common gene among a variety of eucaryotes.