Abstract
Drechslera gigantea Heald & Wolf is a worldwide-spread necrotrophic fungus closely related to the Bipolaris genus, well-known because many member species provoke severe diseases in cereal crops and studied because they produce sesterpenoid phytoxins named ophiobolins which possess interesting biological properties. The unfolded protein response (UPR) is a conserved mechanism protecting eukaryotic cells from the accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER). In plants, consolidated evidence supports the role of UPR in the tolerance to abiotic stress, whereas much less information is available concerning the induction of ER stress by pathogen infection and consequent UPR elicitation as part of the defense response. In this study, the infection process of D. gigantea in Arabidopsis thaliana wild type and UPR-defective bzip28 bzip60 double mutant plants was comparatively investigated, with the aim to address the role of UPR in the expression of resistance to the fungal pathogen. The results of confocal microscopy, as well as of qRT-PCR transcript level analysis of UPR genes, proteomics, microRNAs expression profile and HPLC-based hormone analyses demonstrated that ophiobolin produced by the fungus during infection compromised ER integrity and that impairment of the IRE1/bZIP60 pathway of UPR hampered the full expression of resistance, thereby enhancing plant susceptibility to the pathogen.
Highlights
The endoplasmic reticulum (ER) is a pivotal eukaryotic organelle ensuring a proper folding of de novo synthesized proteins [1]
The upregulation of miR393 was previously shown to improve resistance to bacteria in Arabidopsis, a fact that was linked to the repression of auxin signaling [73]. These results demonstrate that unfolded protein response (UPR) impairment during fungal infection severely affected the expression profile of several defense-related miRNAs; as correlated to the increased susceptibility of the UPR mutant bzip28 bzip60 to the pathogen, this finding strongly suggests that the inositol-requiring enzyme 1 (IRE1)/bZIP60 pathway of UPR may affect basal defense response through a regulatory mechanism involving microRNAs
Arabidopsis plants infected by D. gigantea. qRT-PCR results demonstrated that in WT plants the pathogen elicited a robust activation of the IRE1/bZIP60 pathway of UPR and that the functionality of this pathway was requested to fully activate the defense response
Summary
The endoplasmic reticulum (ER) is a pivotal eukaryotic organelle ensuring a proper folding of de novo synthesized proteins [1]. Assembled, unfolded polypeptides enter in the ER lumen, where they interact with enzymes ensuring N-linked glycosylation, disulfide bond formation and folding [2]. Proteins not properly folded are detected by the ER quality control machinery (ER-QC) and degraded by the ATP-dependent ubiquitinproteasome system, according to the ER-associated protein degradation mechanism [3]. Eukaryotic cells restore the ER homeostasis by increasing the abundance of molecular chaperones and by enhancing ER-associated protein degradation, according to a mechanism known as unfolded protein response (UPR) [4,5]. Eukaryotic cells restore the ER homeostasis by increasing the abundance of molecular chaperones and by enhancing ER-associated protein degradation, according to a mechanism known as unfolded protein response (UPR) [4,5]. 4.0/).
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