Unfolded Protein Response (UPR) Regulator Cib1 Controls Expression of Genes Encoding Secreted Virulence Factors in Ustilago maydis

Abstract

The unfolded protein response (UPR), a conserved eukaryotic signaling pathway to ensure protein homeostasis in the endoplasmic reticulum (ER), coordinates biotrophic development in the corn smut fungus Ustilago maydis. Exact timing of UPR activation is required for virulence and presumably connected to the elevated expression of secreted effector proteins during infection of the host plant Zea mays. In the baker’s yeast Saccharomyces cerevisiae, expression of UPR target genes is induced upon binding of the central regulator Hac1 to unfolded protein response elements (UPREs) in their promoters. While a role of the UPR in effector secretion has been described previously, we investigated a potential UPR-dependent regulation of genes encoding secreted effector proteins. In silico prediction of UPREs in promoter regions identified the previously characterized effector genes pit2 and tin1-1, as bona fide UPR target genes. Furthermore, direct binding of the Hac1-homolog Cib1 to the UPRE containing promoter fragments of both genes was confirmed by quantitative chromatin immunoprecipitation (qChIP) analysis. Targeted deletion of the UPRE abolished Cib1-dependent expression of pit2 and significantly affected virulence. Furthermore, ER stress strongly increased Pit2 expression and secretion. This study expands the role of the UPR as a signal hub in fungal virulence and illustrates, how biotrophic fungi can coordinate cellular physiology, development and regulation of secreted virulence factors.

Unfolded protein response and scaffold independent pheromone MAP kinase signalling controlVerticillium dahliaegrowth, development and plant pathogenesis

“…Successful propagation within the host requires fungal spreading within the vascular system by conidiation, which is defective in the HAC1 deletion strain. In addition, the UPR considerably contributes to the necessary adaptation of secretory capacities during host colonization of pathogens and the processing and secretion of fungal effectors in U. maydis (Richie et al ., 2011; Heimel et al ., 2013; Hampel et al ., 2016; Pinter et al ., 2019).…”

“…Necrotrophic plant ascomycetes such as the rice blast fungus Magnaporthe oryzae require Hac1 and the UPR for conidia formation as well as for penetration and invasive hyphae growth during plant infection (Tang et al ., 2015), whereas in Alternaria brassicicola the UPR is required for resistance against plant antimicrobial compounds and virulence, but is not involved in plant penetration and initial colonization (Joubert et al ., 2011; Guillemette et al ., 2014). The UPR pathway regulates virulence specific genes in the dimorphic basidiomycete corn smut fungus Ustilago maydis (Heimel et al ., 2010; Hampel et al ., 2016; Pinter et al ., 2019). In this fungus a cross-regulation of the UPR pathway and the pheromone response mitogen-activated protein kinase (MAPK) pathway was described.…”

When green and red mycology meet: Impressions from an interdisciplinary forum on virulence mechanisms of phyto- and human-pathogenic fungi

“…Sascha Brunke from Jena, Germany, had the kick-off in this vivid session that was chaired by Nir Osherov from Tel Aviv, Israel, by presenting his latest insights about the experimental evolution of pathogenic fungi, to be followed by Kai Heimel from the University of G€ ottingen, Germany, who revealed that the unfolded protein response of U. maydis coordinates development and virulence of this maize smut pathogen. 44 Falk Hillmann from Jena spoke about the virulence determinants of pathogenic fungi that contribute to the defense against mycophagous amoeba. 45 Vera G€ ohre from Cologne described her efforts to establish a novel pathosystem of the smut-fungus Thecaphora thlaspeos and the model plant Arabidopsis thaliana.…”

“…Among the former, the seminal works on Fusarium oxysporum performed in the Di Pietro laboratory clearly stands out by describing the influence of ambient pH and its modulation on virulence, an aspect that has been addressed for Aspergillus infections before extensively. 24,32 Also the UPR system as presented by Kai Heimel in his short talk on U. maydis has been recognized to support pathogenicity of human pathogenic fungi as well, 44,69 thereby representing a common determinant of fungal virulence. The same is obviously true for fungal-specific traits, such as RNA biology and transport elucidated by the Feldbr€ ugge group, 65 or the unique composition and structure of the fungal cell wall, as it has been studied by N. Gow and several others in the field.…”

Conditional gene expression reveals stage‐specific functions of the unfolded protein response in the Ustilago maydis–maize pathosystem

“…Expression of effector‐encoding genes is specifically induced during the fungal–plant interaction (Kämper et al , ; Lanver et al , ), resulting in increased stress imposed on the endoplasmic reticulum (ER). Activation of the unfolded protein response (UPR) is critical to counteract elevated ER stress levels and for efficient secretion of effector proteins (Hampel et al , ; Lo Presti et al , ; Pinter et al , ). The UPR is controlled by a key regulatory basic leucine zipper (bZIP) transcription factor termed Hac1 in Saccharomyces cerevisiae , XBP1 in higher eukaryotes and Cib1 in U. maydis (Cox and Walter, ; Heimel et al , ; Kawahara et al , ; Rüegsegger et al , ).…”