Role, function and crosstalk of Kmt6 in Fusarium spp.

Fusaria represent one of the most important group of plant-pathogenic fungi. They are widely distributed and infect various economically important crops resulting in huge annual losses. The Fusarium fujikuroi species complex (FFC), consisting of about 50 monophyletic, highly related Fusarium spp. comprises important members of these plant pathogens. Members of this group are known to produce a broad spectrum of small molecular weight compounds, so-called secondary metabolites (SMs). Some have toxic or carcinogenic properties (i.e. mycotoxins), while others are applied in agriculture or medicine (e.g. antibiotics). A crucial step to combat mycotoxin contaminations but also to induce the biosynthesis of novel substances with putative health- promoting properties is to understand SM gene regulation at the molecular level. Morphological changes in chromatin structure play a key role in regulating fungal SM biosynthesis. The chromatin structure is highly dynamic and driven by changes in posttranslational modifications (PTMs) of histones deposited on the genome. In F. fujikuroi, trimethylation of histone 3 lysine 27 (H3K27me3) a hallmark of facultative heterochromatin functions in SM gene silencing. In contrast to published filamentous fungi, loss of H3K27me3 is lethal in this fungus. Knock-down of FfKMT6 by RNA Interference resulted in reduced H3K27me3 levels accompanied by crippled growth, abolished conidiation and increased SM biosynthesis. Intriguingly, reversion phenotypes occurred that showed elevated KMT6 expression and restored wild type-like growth and conidiation. The goal of EpiVit is to gain deeper knowledge on PRC2-mediated gene silencing in members of the FFC. In the focus of this project is the better understanding of the mode of action of Kmt6. Based on this, the following objectives were defined: (1) Evaluate whether Kmt6 vitality is conserved within members of the FFC; (2) Unravel the cause that sets F. fujikuroi (or the FFC) apart from other fungal species; and (3) Exploitation of the relationship between H3K27me3 and other relevant histone PTMs (histone crosstalk). Obtained knowledge will thus provide foundational knowledge to advance the understanding of epigenetic mechanisms ruling fungal life traits, but also for novel drug-discovery strategies

H3K27me3 is a hallmark of facultative heterochromatin, and as such tightly connected with gene silencing. In the genus Fusarium, a large proportion of the genome is allocated with this histone mark. Not surprisingly its removal is detrimental and even lethal for some fusaria e.g., the plant pathogens Fusarium fujikuroi and Fusarium mangiferae for still unknown reasons. In this project, we revealed that Kmt6, the histone methyltransferase involved in establishing H3K27me3, is vital but not essential in the closely related Fusarium proliferatum, allowing for the first time a comprehensive analysis of Kmt6 loss-of-function in a member of the Fusarium fujikuroi species complex (FFSC). By an "omics"-oriented approach, we analyzed the genome-wide distribution of activating and silencing histone marks in the wild-type as well as the fpkmt6 strain, mapped on a newly annotated genome assembled to near chromosome level. Next to Kmt6, we showed that Kmt1 involved in H3K9me3 a hallmark of constitutive heterochromatin, is dispensable from F. proliferatum and F. mangiferae. This stands in marked contrast to F. fujiikuroi in which loss of H3K9me3 is also lethal. Availability of both mutants, fpkmt1 and fpkmt6, in one genetic background, allowed us to uncover, for the first time, a crucial crosstalk between H3K27me3 and H3K9me3 involved in facultative- and constitutive heterochromatin, respectively, in this genus. These results may explain the subtle phenotype of KMT1 deletion in F. proliferatum and F. mangiferae which stands in marked contrast to other fungal species. The data obtained during this project point towards an important role of H3K27me3 in the genus Fusarium, while H3K9me3, though possible essential, can be substituted by H3K27me3.