Differential gene expression in filamentous cells of Ustilago maydis

Coordinate regulation of Ustilago maydis ammonium transporters and genes involved in mating and pathogenicity

The dimorphic switch from budding to filamentous growth is an essential morphogenetic transition many fungi utilize to cause disease in the host. Although different environmental signals can induce filamentous growth, the developmental programs associated with transmitting these different signals may differ. Here, we explore the relationship between filamentation and expression levels of ammonium transporters (AMTs) that also sense low ammonium for Ustilago maydis, the pathogen of maize. Overexpression of the high affinity ammonium transporter, Ump2, under normally non-inducing conditions, results in filamentous growth. Furthermore, ump2 expression levels are correlated with expression of genes involved in the mating response pathway and in pathogenicity. Ump1 and Ump2 transcription levels also tracked expression of genes normally up-regulated during either filamentous growth or during growth of the fungus inside the host. Interestingly, haploid strains deleted for the b mating-type locus, like those deleted for ump2, failed to filament on low ammonium; they also shared some alterations in gene expression patterns with cells deleted for ump2 or over-expressing this gene. Deletion of ump2 either in both mating partners or in a solopathogenic haploid strain resulted in a dramatic reduction in disease severity for infected plants, suggesting some importance of this transceptor in the pathogenesis program.

Phytohormone Involvement in the Ustilago maydis- Zea mays Pathosystem: Relationships between Abscisic Acid and Cytokinin Levels and Strain Virulence in Infected Cob Tissue

Ustilago maydis is the causative agent of common smut of corn. Early studies noted its ability to synthesize phytohormones and, more recently these growth promoting substances were confirmed as cytokinins (CKs). Cytokinins comprise a group of phytohormones commonly associated with actively dividing tissues. Lab analyses identified variation in virulence between U. maydis dikaryon and solopathogen infections of corn cob tissue. Samples from infected cob tissue were taken at sequential time points post infection and biochemical profiling was performed using high performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI MS/MS). This hormone profiling revealed that there were altered levels of ABA and major CKs, with a marked reduction in CK glucosides, increases in methylthiol CKs and a particularly dramatic increase in cisZ CK forms, in U. maydis infected tissue. These changes were more pronounced in the more virulent dikaryon relative to the solopathogenic strain suggesting a role for cytokinins in moderating virulence during biotrophic infection. These findings highlight the fact that U. maydis does not simply mimic a fertilized seed but instead reprograms the host tissue. Results underscore the suitability of the Ustilago maydis- Zea mays model as a basis for investigating the control of phytohormone dynamics during biotrophic infection of plants.

The cell end marker Tea4 regulates morphogenesis and pathogenicity in the basidiomycete fungus Ustilago maydis

Positional cues localized to distinct cell domains are critical for the generation of cell polarity and cell morphogenesis. These cues lead to assembly of protein complexes that organize the cytoskeleton resulting in delivery of vesicles to sites of polarized growth. Tea4, an SH3 domain protein, was first identified in fission yeast, and is a critical determinant of the axis of polarized growth, a role conserved among ascomycete fungi. Ustilago maydis is a badiomycete fungus that exhibits a yeast-like form that is nonpathogenic and a filamentous form that is pathogenic on maize and teozintle. We are interested in understanding how positional cues contribute to generation and maintenance of these two forms, and their role in pathogenicity. We identified a homologue of fission yeast tea4 in a genetic screen for mutants with altered colony and cell morphology and present here analysis of Tea4 for the first time in a basidiomycete fungus. We demonstrate that Tea4 is an important positional marker for polarized growth and septum location in both forms. We uncover roles for Tea4 in maintenance of cell and neck width, cell separation, and cell wall deposition in the yeast-like form, and in growth rate, formation of retraction septa, growth reversal, and inhibition of budding in the filamentous form. We show that Tea4::GFP localizes to sites of polarized or potential polarized growth in both forms, as observed in ascomycete fungi. We demonstrate an essential role of Tea4 in pathogencity in the absence of cell fusion. Basidiomycete and ascomycete Tea4 homologues share SH3 and Glc7 domains. Tea4 in basidiomycetes has additional domains, which has led us to hypothesize that Tea4 has novel functions in this group of fungi.

Investigating the Ustilago maydis/Zea mays pathosystem: transcriptional responses and novel functional aspects of a fungal calcineurin regulatory B subunit

The sustainable control of basidiomycete biotrophic plant pathogenesis requires an understanding of host responses to infection, as well as the identification and functional analysis of fungal genes involved in disease development. The creation and analysis of a suppressive subtractive hybridization (SSH) cDNA library from Ustilago maydis-infected Zea mays seedlings enabled the identification of fungal and plant genes expressed during disease development, and uncovered new insights into the interactions of this model system. Candidate U. maydis pathogenesis genes were identified by using the current SSH cDNA library analysis, and by knowledge generated from previous cDNA microarray and comparative genomic analyses. These identifications were supported by the independent determination of transcript level changes in different cell-types and during pathogenic development. The basidiomycete specific um01632, the highly in planta expressed um03046 (zig1), and the calcineurin regulatory B subunit (um10226, cnb1), were chosen for deletion experiments. um01632 and zig1 mutants showed no difference in morphology and did not have a statistically significant impact on pathogenesis. cnb1 mutants had a distinct cell division phenotype and reduced virulence in seedling assays. Infections with reciprocal wild-type×Δcnb1 haploid strain crosses revealed that the wild-type allele was unable to fully compensate for the lack of a second cnb1 allele. This haploinsufficiency was undetected in other fungal cnb1 mutational analyses. The reported data improves U. maydis genome annotation and expands on the current understanding of pathogenesis genes in this model basidiomycete.

Diploids in the Cryptococcus neoformans serotype A population homozygous for the alpha mating type originate via unisexual mating

The ubiquitous environmental human pathogen Cryptococcus neoformans is traditionally considered a haploid fungus with a bipolar mating system. In nature, the α mating type is overwhelmingly predominant over a. How genetic diversity is generated and maintained by this heterothallic fungus in a largely unisexual α population is unclear. Recently it was discovered that C. neoformans can undergo same-sex mating under laboratory conditions generating both diploid intermediates and haploid recombinant progeny. Same-sex mating (α-α) also occurs in nature as evidenced by the existence of natural diploid αADα hybrids that arose by fusion between two α cells of different serotypes (A and D). How significantly this novel sexual style contributes to genetic diversity of the Cryptococcus population was unknown. In this study, ∼500 natural C. neoformans isolates were tested for ploidy and close to 8% were found to be diploid by fluorescence flow cytometry analysis. The majority of these diploids were serotype A isolates with two copies of the α MAT locus allele. Among those, several are intra-varietal allodiploid hybrids produced by fusion of two genetically distinct α cells through same-sex mating. The majority, however, are autodiploids that harbor two seemingly identical copies of the genome and arose via either endoreplication or clonal mating. The diploids identified were isolated from different geographic locations and varied genotypically and phenotypically, indicating independent non-clonal origins. The present study demonstrates that unisexual mating produces diploid isolates of C. neoformans in nature, giving rise to populations of hybrids and mixed ploidy. Our findings underscore the importance of same-sex mating in shaping the current population structure of this important human pathogenic fungus, with implications for mechanisms of selfing and inbreeding in other microbial pathogens.

Although sex typically involves partners of opposite mating type (sexuality), it can also occur with just one mating type and even single individuals (parthenogenesis, homothallism). However, from a population perspective, sexual reproduction occurs by either outcrossing or inbreeding regardless of the partners' sexuality. Here the impact of sex was studied for Cryptococcus neoformans, a pathogen that causes fungal meningitis. While sex in the laboratory is known to occur via opposite-sex-mating, the population is largely unisexual (α) in nature. Recently, an unusual α-α unisexual mating process involving only mating type α was discovered in the lab, but the impact of unisexual mating in nature was unknown. The global survey of this typically haploid organism reveals ∼8% diploids in the population produced by unisexual α-α mating. Some diploids result from fusion of two genetically distinct parents while other diploids arise via sibling mating or genome duplication. Although hybrid fitness is well-documented, how sex between identical isolates benefits the population is a conundrum. The diploid state may confer growth advantages or serve as a capacitor for evolution, allowing mutations to arise that would be deleterious on their own in the haploid, and then releasing these in novel combinations by meiosis and sporulation.

Genetics of morphogenesis and pathogenic development of Ustilago maydis

Ustilago maydis has emerged as an important model system for the study of fungi. Like many fungi, U. maydis undergoes remarkable morphological transitions throughout its life cycle. Fusion of compatible, budding, haploid cells leads to the production of a filamentous dikaryon that penetrates and colonizes the plant, culminating in the production of diploid teliospores within fungal-induced plant galls or tumors. These dramatic morphological transitions are controlled by components of various signaling pathways, including the pheromone-responsive MAP kinase and cAMP/PKA (cyclic AMP/protein kinase A) pathways, which coregulate the dimorphic switch and sexual development of U. maydis. These signaling pathways must somehow cooperate with the regulation of the cytoskeletal and cell cycle machinery. In this chapter, we provide an overview of these processes from pheromone perception and mating to gall production and sporulation in planta. Emphasis is placed on the genetic determinants of morphogenesis and pathogenic development of U. maydis and on the fungus-host interaction. Additionally, we review advances in the development of tools to study U. maydis, including the recently available genome sequence. We conclude with a brief assessment of current challenges and future directions for the genetic study of U. maydis.

Genome multiplication as adaptation to tissue survival: evidence from gene expression in mammalian heart and liver

To elucidate the functional significance of genome multiplication in somatic tissues, we performed a large-scale analysis of ploidy-associated changes in expression of non-tissue-specific (i.e., broadly expressed) genes in the heart and liver of human and mouse (6585 homologous genes were analyzed). These species have inverse patterns of polyploidization in cardiomyocytes and hepatocytes. The between-species comparison of two pairs of homologous tissues with crisscross contrast in ploidy levels allows the removal of the effects of species and tissue specificity on the profile of gene activity. The different tests performed from the standpoint of modular biology revealed a consistent picture of ploidy-associated alteration in a wide range of functional gene groups. The major effects consisted of hypoxia-inducible factor-triggered changes in main cellular processes and signaling pathways, activation of defense against DNA lesions, acceleration of protein turnover and transcription, and the impairment of apoptosis, the immune response, and cytoskeleton maintenance. We also found a severe decline in aerobic respiration and stimulation of sugar and fatty acid metabolism. These metabolic rearrangements create a special type of metabolism that can be considered intermediate between aerobic and anaerobic. The metabolic and physiological changes revealed (reflected in the alteration of gene expression) help explain the unique ability of polyploid tissues to combine proliferation and differentiation, which are separated in diploid tissues. We argue that genome multiplication promotes cell survival and tissue regeneration under stressful conditions.