DataSheet_1_MLO Differentially Regulates Barley Root Colonization by Beneficial Endophytic and Mycorrhizal Fungi.pdf

Loss-of-function alleles of MLO (Mildew Resistance Locus O) confer broad-spectrum resistance to foliar infections by powdery mildew pathogens. Like pathogens, microbes that establish mutually beneficial relationships with their plant hosts, trigger the induction of some defense responses. Initially, barley colonization by the root endophyte Serendipita indica (syn. Piriformospora indica) is associated with enhanced defense gene expression and the formation of papillae at sites of hyphal penetration attempts. This phenotype is reminiscent of mlo-conditioned immunity in barley leaf tissue and raises the question whether MLO plays a regulatory role in the establishment of beneficial interactions. Here we show that S. indica colonization was significantly reduced in plants carrying mlo mutations compared to wild type controls. The reduction in fungal biomass was associated with the enhanced formation of papillae. Moreover, epidermal cells of S. indica-treated mlo plants displayed an early accumulation of iron in the epidermal layer suggesting increased basal defense activation in the barley mutant background. Correspondingly, the induction of host cell death during later colonization stages was impaired in mlo colonized plants, highlighting the importance of the early biotrophic growth phase for S. indica root colonization. In contrast, the arbuscular mycorrhizal fungus Funneliformis mosseae displayed a similar colonization morphology on mutant and wild type plants. However, the frequency of mycorrhization and number of arbuscules was higher in mlo-5 mutants. These findings suggest that MLO differentially regulates root colonization by endophytic and AM fungi.

MLO（Mildew Resistance Locus O）的功能丧失等位基因可赋予植物针对叶片白粉病病原菌侵染的广谱抗性。与病原菌类似，与植物宿主建立互利共生关系的微生物，同样会诱导部分防御反应的激活。初始阶段，内生根际真菌Serendipita indica（曾用名Piriformospora indica）对大麦的定殖过程伴随防御基因表达上调以及菌丝侵入位点处乳突结构的形成。该表型与大麦叶片组织中mlo介导的免疫反应高度相似，由此提出科学问题：MLO是否在互利共生互作的建立过程中发挥调控作用？本研究显示，与野生型对照相比，携带mlo突变的植株中S. indica的定殖量显著降低。真菌生物量的减少与乳突结构形成增多相关。此外，经S. indica处理的mlo突变体植株的表皮细胞，其表皮层会较早出现铁积累现象，这表明大麦突变体背景下基础防御的激活水平更高。相应地，在定殖后期阶段，宿主细胞死亡的诱导过程在mlo突变体定殖植株中受到损害，这凸显了早期活体营养生长阶段对S. indica根系定殖的重要性。与之形成对比的是，丛枝菌根真菌Funneliformis mosseae在突变体与野生型植株上展现出相似的定殖形态。不过，mlo-5突变体的菌根侵染频率与丛枝数量均更高。上述研究结果表明，MLO对内生真菌与丛枝菌根真菌的根系定殖存在差异化调控。