No abstract

Among the biotic constraints to wheat (Triticum aestivum L.) production, fusarium head blight (FHB), caused by Fusarium graminearum, leaf rust (LR), caused by Puccinia triticina, and stripe rust (SR) caused by Puccinia striiformis are problematic fungal diseases worldwide. Each can significantly reduce grain yield while FHB causes additional food and feed safety concerns due to mycotoxin contamination of grain. Genetic resistance is the most effective and sustainable approach for managing wheat diseases. In the past 20 years, over 500 quantitative trait loci (QTLs) conferring small to moderate effects for the different FHB resistance types have been reported in wheat. Similarly, 79 Lr-genes and more than 200 QTLs and 82 Yr-genes and 140 QTLs have been reported for seedling and adult plant LR and SR resistance, respectively. Most QTLs conferring rust resistance are race-specific generally conforming to a classical gene-for-gene interaction while resistance to FHB exhibits complex polygenic inheritance with several genetic loci contributing to one resistance type. Identification and deployment of additional genes/ QTLs associated with FHB and rust resistance can expedite wheat breeding through marker-assisted and/or genomic selection to combine small-effect QTL in the gene pool. LR disease has been present in the southeast United States for decades while SR and FHB have become increasingly problematic in the past 20 years, with FHB arguably due to increased corn acreage in the region. Currently, QTLs on chromosome 1B from

Acid pH induces the yeast-to-mycelium transition in haploid cells of Ustilago maydis. We tested two signal transduction pathways known to be involved in dimorphism for roles in acid-induced filamentation. In wild-type cells intracellular cAMP levels were reduced under acid growth. A mutant defective in the regulatory subunit of PKA, ubc1, failed to respond to acid induction on solid medium, but in liquid medium showed a mycelial phenotype at acid pH. Mutants in the pheromone-responsive MAP kinase pathway lost the capacity to grow as mycelium at acid pH, while a mutant in the pheromone response-transcriptional regulator, prf1, behaved as wild-type. Filamentation by both ubc1 and prf1 mutants was inhibited by addition of cAMP. A putative MAP kinase cascade adaptor protein gene, ubc2, complemented a previously identified myc mutant strain defective in pH-induced myceliation. These results indicate that pH-dependent dimorphism is regulated by two known signaling pathways but that an effector for cAMP signaling alternative to Ubc1 is present in U. maydis and that Prf1 is not the sole downstream target of MAP kinase signaling.

Proteins involved in the mitogen-activated protein (MAP) kinase pathway controlling mating, morphogenesis, and pathogenicity have been identified previously in the fungus Ustilago maydis. One of these, the Ubc2 adaptor protein, possesses a basidiomycete-specific structure. In addition to containing sterile alpha motif (SAM) and ras association (RA) domains typical of Ste50-like adaptor proteins found in the fungal phylum Ascomycota, Ubc2 also contains two C-terminal SH3 domains. Yeast two-hybrid assays indicated that Ubc2 interacts with the MAP kinase-kinase kinase Ubc4 via the SAM domains at each of their respective N-termini. Site-directed mutagenesis of ubc2 and complementation analyses revealed that the SAM and RA domains of Ubc2 are essential for filamentous growth. These data support a role for the ascomycete-like N-terminus of Ubc2 in regulating pheromone-responsive mating and morphogenesis analogous to the role of Ste50p in Saccharomyces cerevisiae. In contrast, C-terminal deletion mutants were fully capable of filamentous growth and mating. However, surprisingly, these strains were nonpathogenic. Further, directed mutagenesis of the C-terminus revealed that both SH3 domains are required for pathogenicity. These results suggest that the Basidiomycota have retained the mating and morphogenetic functions of Ste50-type proteins in the N-terminal half of their Ubc2-type adaptors but, additionally, have integrated C-terminal SH3 domains that are critical for additional signal transduction mechanisms, including those that lead to pathogenesis.

We have developed conditions which promote the dimorphic transition of haploid cells of Ustilago maydis in vitro by controlling the pH of the media. At low pH (below 5.0) mycelial growth occurs, whereas at neutral pH yeastlike growth takes place. We screened for mutants unable to form mycelium at low pH and obtained 26 mutants. These mutants have been characterized by their cell and colony morphology in different media. Mutations in 18 strains were found to be recessive when these strains were crossed with the wild type. Other crosses indicated that they were affected in genes other than a and b. Crosses between mutants suggest that the mutations fall in at least two complementation groups. In addition, mutants were characterized by their pathogenicity to corn seedlings. Mutations which were recessive for pathogenicity were also recessive for morphogenesis in vitro.

No abstract

The heterobasidiomycetes responsible for plant smuts obligatorily require their hosts for the completion of the sexual cycle. Accordingly, the sexual cycle of these fungi could so far be studied only by infecting host plants. We have now induced Ustilago maydis, the causative agent of corn smut, to traverse the whole life cycle by growing mixtures of mating-compatible strains of the fungus on a porous membrane placed on top of embryogenic cell cultures of its host Zea mays. Under these conditions, mating, karyogamy and meiosis take place, and the fungus induces differentiation of the plant cells. These results suggest that embryogenic maize cells produce diffusible compounds needed for completion of the sexual cycle of U. maydis, as the plant does for the pathogen during infection.

The fungi are the most economically important plant pathogens and continue to be the focus of extensive research with a wide variety of methodologies. Enhancements in microscopy techniques have increased our ability to visualize the intimate interaction of fungi and their host plants. Improving methods allow pharmacological inhibition and genetic dissection of the determinants of fungal pathogenicity in a gene-by-gene approach. Identification and analysis of genes differentially transcribed in ways pertinent to pathogenicity continues to be a frequent research approach. Genome-wide analysis is gaining favor in biological research and fungal plant pathogens are no exception. Several industrial research groups are exploring fungal plant pathogenesis based on genomic sequence data and genome-wide mutagenesis. In March 2001 the first publicly available complete genome of a filamentous fungus (Neurospora crassa) was released. N. crassa is of course a saprophyte and there is no complete sequence available for a plant pathogenic fungus in public databases. However, freely accessible entire genome sequences for both plant pathogenic fungi and their hosts are on the horizon. Sequence availability promises to revolutionize the rate at which data relevant to disease processes will be accrued. In this review we describe approaches currently applied to the study of plant pathogenic fungi and explore developments of potential future benefit with existing technologies not yet applied to this group of important organisms.