Epigenetic control of effector gene expression in the plant pathogenic fungus Leptosphaeria maculans.
Epigenetic control of effector gene expression in the plant pathogenic fungus Leptosphaeria maculans.
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DOI：
10.1371/journal.pgen.1004227
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发表时间：
2014-03
期刊：
影响因子：
4.5
通讯作者：
Fudal I
中科院分区：
文献类型：
--
作者：
Soyer JL;El Ghalid M;Glaser N;Ollivier B;Linglin J;Grandaubert J;Balesdent MH;Connolly LR;Freitag M;Rouxel T;Fudal I
植物病原菌分泌大量的小分泌蛋白(SSP)，作为效应器调节宿主免疫以促进感染。SSP编码基因通常位于特定的基因组环境中，并在植物侵染的不同阶段表现出协同表达的波动。到目前为止，人们对它们的表达调控知之甚少。子囊菌Leptosphaeria maculans的基因组由富含基因的GC同工酶和基因贫乏的AT同工酶交替组成。AT-等中心含有转座元件的嵌合体，覆盖了基因组的三分之一，并富含可能的效应基因，呈现出类似的表达模式，即在无菌培养期间不表达或低水平表达，而在初侵染油菜(Brassica Napus)时强诱导表达。在这里，我们研究了一种特殊的组蛋白修饰，组蛋白H3赖氨酸9甲基化(H3K9me3)，参与了斑点杆菌协同效应基因表达的表观遗传调控。为此，我们通过RNAi沉默了异染色质组装和维护中的两个关键角色HP1和DIM-5的表达。通过使用HP1-GFP作为异染色质标记，我们观察到在RNAi沉默LmDIM5的菌株中几乎没有可见的染色质凝集。通过全基因组寡核苷酸芯片，我们观察到沉默的LmHP1和-LmDIM5转化子在无菌培养过程中分别有369和390个基因的过度表达，明显有利于编码SSP的基因在AT-等中心内的表达。在无菌培养中，4个效应基因在GC-等轴线中的异位整合导致了它们在生长过程中的过度表达。这些数据有力地表明，在无菌培养的生长过程中，由HP1和DIM-5介导的表观遗传控制抑制了位于AT-等轴点的至少部分效应基因的表达。我们的假设是，生活方式的改变和向发病机制的转变提高了染色质介导的抑制，允许对新的环境条件做出快速反应。效应器是微生物对植物致病的关键因素。在植物侵染过程中，效应基因通常表现出协同表达，但这种协同表达是如何产生的，在很大程度上仍然是一个未被探索的研究课题。表观遗传机制涉及基因组的维持和完整性，但越来越多地被认为是调控众多和多样化生物中基因表达的重要机制。在这里，我们发现基因组环境对黑斑钩端螺旋体效应基因的表达有影响，并且依赖于参与异染色质形成和维持的两种蛋白质HP1和DIM-5的表观遗传机制调节这种表达，导致在无菌培养中的生长抑制。组蛋白H3、赖氨酸9甲基化和/或HP1的去除组蛋白H3、赖氨酸9甲基化和/或HP1引起的染色质解缩可能是油菜初次侵染过程中效应基因表达的先决条件。因此，我们展示了基于染色质的转录调控，它可以作用于真菌中的效应器基因表达。我们的研究强调了异色景观的重要性，不仅对于基因组的维护，而且对于生物体对不断变化的环境情况的快速和有效的适应。
Plant pathogens secrete an arsenal of small secreted proteins (SSPs) acting as effectors that modulate host immunity to facilitate infection. SSP-encoding genes are often located in particular genomic environments and show waves of concerted expression at diverse stages of plant infection. To date, little is known about the regulation of their expression. The genome of the Ascomycete Leptosphaeria maculans comprises alternating gene-rich GC-isochores and gene-poor AT-isochores. The AT-isochores harbor mosaics of transposable elements, encompassing one-third of the genome, and are enriched in putative effector genes that present similar expression patterns, namely no expression or low-level expression during axenic cultures compared to strong induction of expression during primary infection of oilseed rape (Brassica napus). Here, we investigated the involvement of one specific histone modification, histone H3 lysine 9 methylation (H3K9me3), in epigenetic regulation of concerted effector gene expression in L. maculans. For this purpose, we silenced the expression of two key players in heterochromatin assembly and maintenance, HP1 and DIM-5 by RNAi. By using HP1-GFP as a heterochromatin marker, we observed that almost no chromatin condensation is visible in strains in which LmDIM5 was silenced by RNAi. By whole genome oligoarrays we observed overexpression of 369 or 390 genes, respectively, in the silenced-LmHP1 and -LmDIM5 transformants during growth in axenic culture, clearly favouring expression of SSP-encoding genes within AT-isochores. The ectopic integration of four effector genes in GC-isochores led to their overexpression during growth in axenic culture. These data strongly suggest that epigenetic control, mediated by HP1 and DIM-5, represses the expression of at least part of the effector genes located in AT-isochores during growth in axenic culture. Our hypothesis is that changes of lifestyle and a switch toward pathogenesis lift chromatin-mediated repression, allowing a rapid response to new environmental conditions. Effectors are key players in pathogenicity of microbes toward plants. Effector genes usually show concerted expression during plant infection but how this concerted expression is generated remains a largely unexplored research topic. Epigenetic mechanisms are involved in genome maintenance and integrity but are increasingly considered as important for regulation of gene expression in numerous and diverse organisms. Here we show that the genomic environment has impact on expression of Leptosphaeria maculans effector genes, and that an epigenetic mechanism that relies on two proteins involved in heterochromatin formation and maintenance, HP1 and DIM-5, modulates this expression, leading to repression during growth in axenic culture. Chromatin decondensation by removal of histone H3 lysine 9 methylation and/or HP1 is presumably a prerequisite for effector gene expression during primary infection of oilseed rape. Thus we show chromatin-based transcriptional regulation that can act on effector gene expression in fungi. Our study highlights the importance of heterochromatic landscapes, not only for genome maintenance but also in rapid and efficient adaptation of organisms to changing environmental situations.
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