No abstract

In plants, flowering is triggered by endogenous and environmental signals. CONSTANS (CO) promotes flowering of Arabidopsis in response to day length. Four early target genes of CO were identified using a steroid-inducible version of the protein. Two of these genes, SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1) and FLOWERING LOCUS T (FT), are required for CO to promote flowering; the others are involved in proline or ethylene biosynthesis. The SOC1 and FT genes are also regulated by a second flowering-time pathway that acts independently of CO. Thus, early target genes of CO define common components of distinct flowering-time pathways.

The vascular wilt fungi Verticillium dahliae and V. albo-atrum infect over 200 plant species, causing billions of dollars in annual crop losses. The characteristic wilt symptoms are a result of colonization and proliferation of the pathogens in the xylem vessels, which undergo fluctuations in osmolarity. To gain insights into the mechanisms that confer the organisms' pathogenicity and enable them to proliferate in the unique ecological niche of the plant vascular system, we sequenced the genomes of V. dahliae and V. albo-atrum and compared them to each other, and to the genome of Fusarium oxysporum, another fungal wilt pathogen. Our analyses identified a set of proteins that are shared among all three wilt pathogens, and present in few other fungal species. One of these is a homolog of a bacterial glucosyltransferase that synthesizes virulence-related osmoregulated periplasmic glucans in bacteria. Pathogenicity tests of the corresponding V. dahliae glucosyltransferase gene deletion mutants indicate that the gene is required for full virulence in the Australian tobacco species Nicotiana benthamiana. Compared to other fungi, the two sequenced Verticillium genomes encode more pectin-degrading enzymes and other carbohydrate-active enzymes, suggesting an extraordinary capacity to degrade plant pectin barricades. The high level of synteny between the two Verticillium assemblies highlighted four flexible genomic islands in V. dahliae that are enriched for transposable elements, and contain duplicated genes and genes that are important in signaling/transcriptional regulation and iron/lipid metabolism. Coupled with an enhanced capacity to degrade plant materials, these genomic islands may contribute to the expanded genetic diversity and virulence of V. dahliae, the primary causal agent of Verticillium wilts. Significantly, our study reveals insights into the genetic mechanisms of niche adaptation of fungal wilt pathogens, advances our understanding of the evolution and development of their pathogenesis, and sheds light on potential avenues for the development of novel disease management strategies to combat destructive wilt diseases.

Changes in genes encoding transcriptional regulators can alter development and are important components of the molecular mechanisms of morphological evolution. MADS-box genes encode transcriptional regulators of diverse and important biological functions. In plants, MADS-box genes regulate flower, fruit, leaf, and root development. Recent sequencing efforts in Arabidopsis have allowed a nearly complete sampling of the MADS-box gene family from a single plant, something that was lacking in previous phylogenetic studies. To test the long-suspected parallel between the evolution of the MADS-box gene family and the evolution of plant form, a polarized gene phylogeny is necessary. MEF2 ͉ SRF ͉ homeotic genes ͉ MADS ͉ development C hanges in genes encoding transcriptional regulators may represent the most important determinants of morphological evolution in plants and animals (1), and phylogenetic analyses provide a historical framework to identify such changes. The MADS-box genes encode a eukaryotic family of transcriptional regulators involved in diverse and important biological functions, ranging from cardiac muscle development in animals to pheromone response in yeast (2). In plants, MADS-box genes encode the three floral homeotic functions predicted by the genetic ABC model of flower organ identity (3, 4). In addition, plant MADS-box genes regulate the timing of flower initiation and flower meristem identity, as well as various aspects of ovule, fruit, leaf, and root development (4, 5).Previously identified plant MADS-box genes encode proteins that share a stereotypical MIKC structure (Fig. 1), with the highly conserved DNA-binding MADS domain at the amino terminus. The moderately conserved K domain in the central portion of these proteins has been shown to be important for protein-protein interactions and likely forms a coiled-coil structure. The MADS and K domains are linked to one another by a weakly conserved I domain, whereas a poorly conserved carboxyl-terminal (C) region may function as a trans-activation domain (4). In animals and fungi, two distinct types of MADSbox genes have been identified, the SRF-like and MEF2-like classes (ref. 2; see Fig. 1).This paper provides a hypothesis on the evolutionary history of the eukaryotic MADS-box gene family. Previous studies of eukaryotic MADS-box gene evolution, which included plant and animal sequences, provided unrooted trees useful to infer the phylogenetic relationships of the MADS-box lineages (6). These previous studies suggested that at least one MADS-box gene was present in the common ancestor of plants, animals, and fungi, and that probably the duplication that gave rise to the animal MEF2-and SRF-like genes occurred after animals diverged from plants but before fungi diverged from animals (6). However, previous plant and eukaryotic studies were based on a relatively small sampling of plant MADS-box sequences for a particular species (6-9). To test whether all Arabidopsis MADSbox sequences group in a monophyletic clade distinct from all animal and fungal ...

The fungal pathogen Ustilago maydis exhibits a dimorphic switch from budding to filamentous growth in response to mating interactions and environmental conditions. We have found that disruption of the uacl gene, encoding adenylate cyclase, results in a constitutively filamentous phenotype. Budding is restored to the uacl mutant upon growth in the presence of cAMP or by extragenic suppression because of a mutation in the ubcl gene. The ubcl gene encodes a type II regulatory subunit of cAMP-dependent protein kinase (PKA); defects in this gene attenuate the filamentous growth that normally occurs in response to mating and exposure to air. Growth of wild-type cells in cAMP and mutation of the ubcl gene also cause defects in the separation of mother and daughter cells (cytokinesis) and alter bud site selection. These results indicate a key role for cAMP and PKA in morphogenesis in U. maydis; this role may be common among dimorphic fungal pathogens.

No abstract

SummaryUstilago maydis, the causal agent of corn smut disease, displays dimorphic growth in which it alternates between a budding haploid saprophyte and a ®la-mentous dikaryotic pathogen. We are interested in identifying the genetic determinants of ®lamentous growth and pathogenicity in U. maydis. To do this, we have taken a forward genetic approach. Previously, we showed that haploid adenylate cyclase (uac1) mutants display a constitutively ®lamentous phenotype. Mutagenesis of a uac1 disruption strain allowed the isolation of a large number of budding suppressor mutants. These mutants are named ubc, for Ustilago bypass of cyclase, as they no longer require the production of cAMP to grow in the budding morphology. Complementation of one of these suppressor mutants led to the identi®cation of ubc3, which is required for ®lamentous growth and encodes a MAP kinase most similar to those of the yeast pheromone response pathway. In addition to ®lamentous growth, the ubc3 gene is required for pheromone response and for full virulence. Mutations in the earlier identi®ed fuz7 MAP kinase kinase also suppress the ®lamentous phenotype of the uac1 disruption mutant, adding evidence that both ubc3 and fuz7 are members of this same MAP kinase cascade. These results support an important interplay of the cAMP and MAP kinase signal transduction pathways in the control of morphogenesis and pathogenicity in U. maydis.

Fusarium verticillioides is a fungus of significant economic importance because of its deleterious effects on plant and animal health and on the quality of their products. Corn (Zea mays) is the primary host for F. verticillioides, and we have investigated the impact of the plant's antimicrobial compounds (DIMBOA, DIBOA, MBOA, and BOA) on fungal virulence and systemic colonization. F. verticillioides is able to metabolize these antimicrobials, and genetic analyses indicated two loci, Fdb1 and Fdb2, were involved in detoxification. Mutation at either locus caused sensitivity and no detoxification. In vitro physiological complementation assays resulted in detoxification of BOA and suggested that an unknown intermediate compound was produced. Production of the intermediate compound involved Fdbl, and a lesion in fdb2 preventing complete metabolism of BOA resulted in transformation of the intermediate into an unidentified metabolite. Based on genetic and physiological data, a branched detoxification pathway is proposed. Use of genetically characterized detoxifying and nondetoxifying strains indicated that detoxification of the corn antimicrobials was not a major virulence factor, since detoxification was not necessary for development of severe seedling blight or for infection and endophytic colonization of seedlings. Production of the antimicrobials does not appear to be a highly effective resistance mechanism against F. verticillioides.