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The plant-pathogenic fungus Mycosphaerella graminicola (asexual stage: Septoria tritici) causes septoria tritici blotch, a disease that greatly reduces the yield and quality of wheat. This disease is economically important in most wheat-growing areas worldwide and threatens global food production. Control of the disease has been hampered by a limited understanding of the genetic and biochemical bases of pathogenicity, including mechanisms of infection and of resistance in the host. Unlike most other plant pathogens, M. graminicola has a long latent period during which it evades host defenses. Although this type of stealth pathogenicity occurs commonly in Mycosphaerella and other Dothideomycetes, the largest class of plant-pathogenic fungi, its genetic basis is not known. To address this problem, the genome of M. graminicola was sequenced completely. The finished genome contains 21 chromosomes, eight of which could be lost with no visible effect on the fungus and thus are dispensable. This eight-chromosome dispensome is dynamic in field and progeny isolates, is different from the core genome in gene and repeat content, and appears to have originated by ancient horizontal transfer from an unknown donor. Synteny plots of the M. graminicola chromosomes versus those of the only other sequenced Dothideomycete, Stagonospora nodorum, revealed conservation of gene content but not order or orientation, suggesting a high rate of intra-chromosomal rearrangement in one or both species. This observed “mesosynteny” is very different from synteny seen between other organisms. A surprising feature of the M. graminicola genome compared to other sequenced plant pathogens was that it contained very few genes for enzymes that break down plant cell walls, which was more similar to endophytes than to pathogens. The stealth pathogenesis of M. graminicola probably involves degradation of proteins rather than carbohydrates to evade host defenses during the biotrophic stage of infection and may have evolved from endophytic ancestors.

In the phytopathogenic fungus Ustilago maydis the mating-type loci control the transition from yeastlike to filamentous growth required for pathogenic development. In a large REMI (restriction enzyme mediated integration) screen, non-pathogenic mutants were isolated in a haploid strain that had been engineered to be pathogenic. In one of these mutants, which showed a specific morphological phenotype, the tagged gene, glo1, was found to encode a product that is highly homologous to a glyoxal oxidase gene from the woodrot fungus Phanerochaete chrysosporium. Glyoxal oxidase homologues are found in human, plant pathogenic fungi and in plants, but not in other mammals or yeasts. To confirm the function of the glo1 gene, null mutations were generated in compatible haploid U. maydis strains. In crosses null mutants were unable to generate filamentous dikaryons, and were completely non-pathogenic. Using a Glo1-overproducing strain we demonstrated that Glo1 is membrane bound, oxidizes a series of small aldehydes (

Two NADPH:cytochrome P450 oxidoreductases (CPRs) from parsley (Petroselinum crispum) were cloned, and the complete proteins were expressed and functionally identified in yeast. The two enzymes, designated CPR1 and CPR2, are 80% identical in amino acid sequence with one another and about 75% identical with CPRs from several other plant species. The mRNA accumulation patterns for CPR1 and CPR2 in fungal elicitor-treated or UV-irradiated cultured parsley cells and in developing or infected parsley plants were compared with those for cinnamate 4-hydroxylase (C4H), one of the most abundant CPR-dependent P450 enzymes in plants. All treatments strongly induced the mRNAs for C4H and CPR1 but not for CPR2, suggesting distinct metabolic roles of CPR1 and CPR2 and a functional relationship between CPR1 and C4H.

A previously isolated parsley (Petroselinum crispum) cDNA with high sequence similarity to cinnamate 4-hydroxylase (C4H) cDNAs from several plant sources was expressed in yeast (Saccharomyces cerevisiae) containing a plant NADPH:cytochrome P450 oxidoreductase and verified as encoding a functional C4H (CYP73A10). Low genomic complexity and the occurrence of a single type of cDNA suggest the existence of only one C4H gene in parsley. The encoded mRNA and protein, in contrast to those of a functionally related NADPH:cytochrome P450 oxidoreductase, were strictly coregulated with phenylalanine ammonia-lyase mRNA and protein, respectively, as demonstrated by coinduction under various conditions and colocalization in situ in cross-sections from several different parsley tissues. These results support the hypothesis that the genes encoding the core reactions of phenylpropanoid metabolism form a tight regulatory unit.C4H (EC 1.14.13.11) constitutes the CYP73 family of the large group of Cyt P450 monooxygenases (Teutsch et al., 1993). It catalyzes the 4-hydroxylation of trans-cinnamate, the central step in the generation of Phe-derived substrates for the many branches of phenylpropanoid metabolism (Russell, 1971). The first and the last enzymes of this short sequence of closely related reactions, termed the general phenylpropanoid metabolism, are PAL (EC 4.3.1.5) and 4CL (EC 6.2.1.12), respectively (Hahlbrock and Scheel, 1989). A second metabolic link couples C4H to the membrane-localized CPR (EC 1.6.2.4; Durst and O'Keefe, 1995;Schuler, 1996). The resulting pivotal role of C4H at the interface between cytosolic phenylpropanoid pathways and membrane-localized electron-transfer reactions is schematically illustrated in Figure 1.The expression patterns of all three C4H-linked enzymes, PAL, 4CL, and CPR, and of the corresponding mRNAs have recently been analyzed in cell-suspension cultures and various intact tissues of parsley (Petroselinum crispum L.; Logemann et al