From yeast to hypha: defining transcriptomic signatures of the morphological switch in the dimorphic fungal pathogen Ophiostoma novo-ulmi.

M Nigg,L Bernier

doi:10.1186/s12864-016-3251-8

Abstract

BackgroundYeast-to-hypha transition is a major morphological change in fungi. Molecular regulators and pathways that are involved in this process have been extensively studied in model species, including Saccharomyces cerevisiae. The Mitogen-Actived Protein Kinase (MAPK) cascade, for example, is known to be involved in the yeast-to-pseudohypha switch. Yet the conservation of mechanisms regulating such morphological changes in non-model fungi is still poorly understood. Here, we investigate cell remodeling and transcriptomic modifications that occur during this morphological switch in the highly aggressive ascomycete fungus Ophiostoma novo-ulmi, the causal agent of Dutch elm disease.ResultsUsing a combination of light microscopy, scanning electron microscopy and flow cytometry, we demonstrate that the morphological switch occurs in less than 27 h, with phenotypic cell modifications being detected within the first 4 h. Using RNAseq, we found that over 22% of the genome of O. novo-ulmi is differentially expressed during the transition. By performing clustering analyses of time series gene expression data, we identified several sets of genes that are differentially expressed according to distinct and representative temporal profiles. Further, we found that several genes that are homologous to S. cerevisiae MAPK genes are regulated during the yeast-to-hypha transition in O. novo-ulmi and mostly over-expressed, suggesting convergence in gene expression regulation.ConclusionsOur results are the first report of a time-course experiment monitoring the morphological transition in a non-model Sordariomycota species and reveal many genes of interest for further functional investigations of fungal dimorphism.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-3251-8) contains supplementary material, which is available to authorized users.

Highlights

Yeast-to-hypha transition is a major morphological change in fungi
Scanning electron micrographs (SEM) that were taken at each time point confirm the Y-to-H transition and highlight cell shape modification within the first 4 h, as cells may switch from an ovoid (OY, Fig. 2d) to a spherical shape (SY)
Identification and expression analysis of S. cerevisiae Mitogen-Actived Protein Kinase (MAPK), Protein Kinase A (PKA), and RIM pathway homologs S. cerevisiae MAPK protein homologs We identified genes coding for potential proteins that were homologous to S. cerevisiae MAPK proteins

Summary

Introduction

Yeast-to-hypha transition is a major morphological change in fungi. Molecular regulators and pathways that are involved in this process have been extensively studied in model species, including Saccharomyces cerevisiae. Ophiostoma novo-ulmi (Ascomycota, Sordariomycetes) is the highly aggressive dimorphic pathogen that is responsible for the ongoing pandemic of Dutch elm disease (DED) [1]. This fungus is capable of assuming two distinct forms: a unicellular yeast stage and a multicellular hyphal form. Few studies have been devoted to the identification of key factors that regulate the yeast-to-hypha (Y-to-H) transition within this genus Nutritional factors, such as nitrogen sources [7, 8], pyridoxine [9] or linoleic acid [10], have been shown to be involved, together with other molecular factors, such as Ca2+-calmodulin interaction [11, 12] or cyclic Adenosine MonoPhosphate (cAMP) [13]. Pathways and genes regulating the Y-to-H transition in O. novo-ulmi are still largely unknown

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