Functional Characterization of an Organ Specific Effector See1 of Ustilago Maydis
Ustilago maydis is the causative agent of the corn smut. This basidiomycetous fungus is a biotophic plant pathogen that succeeds by colonizing living tissue and establishes a biotrophic interaction which results in the formation of enormous tumors. This tumor formation is a result of efficient host...
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|Summary:||Ustilago maydis is the causative agent of the corn smut. This basidiomycetous fungus is a biotophic plant pathogen that succeeds by colonizing living tissue and establishes a biotrophic interaction which results in the formation of enormous tumors. This tumor formation is a result of efficient host immune suppression and nutrient efflux during disease progression. The fungus secretes several hundreds of effector proteins which are expressed at various stages of colonization to modulate the host. Previous studies have revealed that the effector proteins of U. maydis are acting in an organ specific manner and deletion of one organ specific effector does not hamper the symptom formation in non-target organ (Skibbe et al., 2010; Schilling et al., 2014).
The previous study of Schilling et al., 2014 identified leaf specific effectors, which are induced in juvenile leaves. An interesting candidate among these that showed a perfect organ specificity was see1 (Seedling efficient effector 1, um02239), which is required in the colonized leaves. Deletion mutants for see1 are able to penetrate and colonize the seedling but fail to induce expansion of tumors. The deletion mutant is seen to be actively blocked in mesophyll and vascular cell layers of the leaf, which may indicate that the effector function may be confined to a specific cell or tissue type. In contrast, see1 deletion does not affect tumor formation in the floral parts of the host. Aim of this thesis was the functional characterization of See1. Monitoring of the DNA synthesis in host, showed that See1 is specifically required to induce DNA synthesis in colonized host cells and re-direct them to form tumors. Yeast-two-hybrid analysis showed that See1 interacts with a nucleo-cytoplasmic host protein SGT1, which is a cell cycle and immune response modulator and which also shows a leaf specific transcriptional regulation. Constitutive overexpression of see1 caused tassel base abnormality specifically showing tumors in the vegetative base of the tassel pointing towards an active role of see1 in inducing tumor in vegetative maize tissues. Electron microscopy showed that See1 is translocated to the plant cell and is localized in the cytoplasm and nucleus of the host cell.
Furthermore, it was demonstrated that See1 blocks the phosphorylation of maize SGT1 at a monocot specific site which is necessary to activate the signaling cascade upon pathogen perception. Experiments indicate that see1 specifically activates the host cell cycle release thereby activating the colonized cells to undergo a tumor pathway. Hence organ specific effectors like see1, not only manipulate the defense responses, but also the metabolic state of the host cell leading to tumor development.|