School of Fundamental Sciences and the BioProtection Research Centre, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand; Research and Development Center, Mayekawa Mfg. Co., Ltd., 2000 Tatsuzawa, Moriya, Ibaraki 302-0118, Japan(1).
School of Fundamental Sciences and the BioProtection Research Centre, Massey University, Private Bag 11 222, Palmerston North 4442, New Zealand.
Fungal Genet Biol. 2019 Apr;125:71-83. doi: 10.1016/j.fgb.2019.02.001. Epub 2019 Feb 4.
Epichloë festucae forms mutualistic symbiotic interactions with grasses of the Lolium and Festuca genera. Protection from insect and mammalian herbivory are the best-documented host benefits of these associations. The two main classes of anti-mammalian alkaloids synthesized by E. festucae are the ergot alkaloids and indole diterpenes, of which ergovaline and lolitrems are the principal terminal products. Synthesis of both metabolites require multiple gene products encoded by clusters of 11 genes located at the subtelomeric regions of chromosomes I and III respectively. These loci are essentially unexpressed in axenic culture but among the most highly expressed genes in planta. We show here that heterochromatin 1 protein (HepA) is an important component of the regulatory machinery that maintains these loci in a silent state in culture. Deletion of this gene led to derepression of eas and ltm gene expression under non-symbiotic culture conditions. Although there was no obvious culture phenotype, RNAseq analysis revealed that around 1000 genes were differentially expressed in the ΔhepA mutant compared to wild type with just one-third upregulated. Inoculation of the ΔhepA mutants into seedlings of Lolium perenne led to a severe host interaction phenotype characterized by a reduction in tiller length but an increase in tiller number. Hyphae within the leaves of these associations were much more abundant in the intercellular spaces of the leaves and aberrantly colonized the vascular bundles. This physiological change was accompanied by a dramatic change in the transcriptome with around 900 genes differentially expressed, with two thirds of these upregulated. This major physiological change was accompanied by a decrease in ltm gene expression and loss of the ability to synthesize lolitrems. These results show that HepA has an important role in controlling the chromatin state of these sub-telomeric secondary metabolite genes, including their symbiosis-specific regulation.
内生真菌 Epichloë festucae 与 Lolium 和 Festuca 属的禾本科植物形成互利共生关系。这些共生体对昆虫和哺乳动物食草动物的保护是其宿主受益的最好记录。内生真菌 E. festucae 合成的两种主要类型的抗哺乳动物生物碱是麦角生物碱和吲哚二萜，其中麦角炔醇和麦角流胺是主要的末端产物。这两种代谢物的合成都需要分别位于染色体 I 和 III 端粒区域的 11 个基因簇编码的多个基因产物。这些基因座在无菌培养中基本上不表达，但在植物中是表达水平最高的基因之一。我们在这里表明，异染色质 1 蛋白（HepA）是维持这些基因座在培养中沉默状态的调控机制的重要组成部分。该基因缺失导致 eas 和 ltm 基因在非共生培养条件下的表达去抑制。尽管没有明显的培养表型，但 RNAseq 分析显示，与野生型相比，ΔhepA 突变体中约有 1000 个基因差异表达，只有三分之一上调。将ΔhepA 突变体接种到黑麦草幼苗中，导致严重的宿主相互作用表型，其特征是分蘖长度减少，但分蘖数增加。这些共生体中的菌丝在叶片的细胞间隙中更加丰富，并且异常地定植在维管束中。这种生理变化伴随着转录组的显著变化，约有 900 个基因差异表达，其中三分之二上调。这种主要的生理变化伴随着 ltm 基因表达的降低和失去合成麦角流胺的能力。这些结果表明，HepA 在控制这些端粒次生代谢基因的染色质状态方面发挥着重要作用，包括它们的共生特异性调控。
