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Abstract
The filamentous fungal pathogen Magnaporthe oryzae causes devastating blast diseases on economically important staple crops including rice, wheat, and finger millet. During a plant-pathogen interaction, the pathogen secretes effector proteins to modulate the plant cell structure, function and immune responses to facilitate infection. Expression of effector genes is typically repressed during axenic growth and highly induced during plant infection, yet the mechanism of the concerted expression remains largely unknown. In this study, I systematically investigated epigenetic and transcriptional regulation of effector gene expression in M. oryzae using genetic, molecular, and cell biology approaches. During mycelial growth in axenic culture, I found that the silencing histone modification trimethylated histone H3 lysine 27 (H3K27me3) mainly represses expression of H3K27me3-enriched effector genes. Interestingly, expression levels of many H3K27me3-enriched effector genes are also controlled by the transcription factor MoGti1. MoGti1 overexpression in the absence of H3K27me3 synergistically upregulates expression of some effector genes during mycelial growth. In particular, most (81%) of these synergistically upregulated effector genes are also highly induced during plant infection at 36 hpi by a wild-type M. oryae strain. Thus, these results suggest a double control of effector gene expression: the epigenetic mechanism mediated by H3K27me3 represses expression during mycelial growth, while the transcriptional mechanism mediated by MoGti1 activates expression during plant infection. To better understand effector gene expression and regulation at single cell resolution, I developed a dual-color gene expression reporter of the PWL2 effector gene. Expression of PWL2 is successively upregulated during appressorium-mediated penetration and invasive hyphae cell-to-cell movement. This expression is controlled by the tandem repeats in the PWL2 promoter, specifically, the 12-bp cis-regulatory sequence that is required for promoter activity during biotrophic invasion of rice cells. Although the interaction between M. oryzae and rice is well characterized, little is known about the infection on other plants. Thus, by developing a finger millet live-cell imaging assay, I investigated cytological dynamics of the finger millet-M. oryzae interaction and found the formation of a novel biotrophic interfacial complex (BIC) pattern, multisite BICs, during compatible interaction. Additionally, the rare infection of a M. oryzae isolate from finger millet (MoE) on rice through both microscopic and macroscopic observations demonstrates the host species specificity of MoE toward rice.