Endoplasmic Reticulum Glucosidases and Protein Quality Control Factors Cooperate to Establish Biotrophy inUstilago maydis 

Abstract

Secreted fungal effectors mediate plant-fungus pathogenic interactions. These proteins are typically N-glycosylated, a common posttranslational modification affecting their location and function. N-glycosylation consists of the addition, and subsequent maturation, of an oligosaccharide core in the endoplasmic reticulum (ER) and Golgi apparatus. In this article, we show that two enzymes catalyzing specific stages of this pathway in maize smut (Ustilago maydis), glucosidase I (Gls1) and glucosidase II b-subunit (Gas2), are essential for its pathogenic interaction with maize (Zea mays). Gls1 is required for the initial stages of infection following appressorium penetration, and Gas2 is required for efficient fungal spreading inside infected tissues. While U. maydis Dgls1 cells induce strong plant defense responses, Dgas2 hyphae are able to repress them, showing that slight differences in the N-glycoprotein processing can determine the extent of plant-fungus interactions. Interestingly, the calnexin protein, a central element of the ER quality control system for N-glycoproteins in eukaryotic cells, is essential for avoiding plant defense responses in cells with defective N-glycoproteins processing. Thus, N-glycoprotein maturation and this conserved checkpoint appear to play an important role in the establishment of an initial biotrophic state with the plant, which allows subsequent colonization.

A conserved co‐chaperone is required for virulence in fungal plant pathogens

“…In eukaryotes, N ‐linked glycoproteins undergo a quality control cycle, known as the calnexin cycle, which ensures that only correctly folded glycoproteins are secreted (Ruddock & Molinari, ). Calnexin (Cne1), a membrane‐bound chaperone and central component of this cycle, is dispensable for virulence in U. maydis (Fernández‐Álvarez et al ., ). To test whether Dnj1 has a partially redundant function with Cne1, we generated single ∆ cne1 and double ∆ dnj1 ∆ cne1 mutants in SG200.…”

“…In plant infection assay, SG200∆cne1 was only slightly reduced in virulence (Fig. S3d), confirming previous observations (Fernández‐Álvarez et al ., ). Conversely, presumably because of the severe growth defect, SG200∆dnj1∆cne1 failed to cause any disease symptoms (Fig.…”

“…The specific sensitivity of the dnj1 mutants to tunicamycin suggests that Dnj1 might be involved in the folding of N ‐glycosylated proteins. N ‐glycosylation is a common post‐transcriptional modification for secreted proteins (Apweiler et al ., ; Schwarz & Aebi, ), such as effectors, and has been shown to be required for virulence in U. maydis , as well as in M. oryzae and Zymoseptoria tritici (Schirawski et al ., ; Motteram et al ., ; Fernández‐Álvarez et al ., ; Chen et al ., ). The lectin chaperone calnexin ( cne1 ) ensures that only correctly folded N ‐glycosylated proteins exit the ER for the Golgi, a process also known as ER quality control (Ruddock & Molinari, ).…”

N-Glycosylation of Effector Proteins by an α-1,3-Mannosyltransferase Is Required for the Rice Blast Fungus to Evade Host Innate Immunity

“…However, the role of N-glycosylation in the pathogenesis of filamentous fungi remains largely unexplored. To date, there are only three reports with glycosylation-related mutants in Ustilago maydis and Mycosphaerella graminicola that implicate N-glycosylation in the pathogenesis of plant pathogenic fungi (Schirawski et al, 2005;Motteram et al, 2011;Fernández-Álvarez et al, 2013). However, the underlying mechanism was not characterized in those studies; therefore, it is not clear why N-glycosylation is important for fungal pathogenesis.…”

“…Three earlier reports have shown that N-glycosylation plays an important role in the pathogenesis of plant pathogenic fungi. In U. maydis, the ER glucosidase I gene, Gls1, is required for the initial stages of infection following appressorium penetration (Fernández-Álvarez et al, 2013) and deletion of the ER glucosidase II gene, GAS1, resulted in arrested growth of IH in the epidermal cell layer of maize (Zea mays) (Schirawski et al, 2005), and in M. graminicola, the a-1,2-mannosyltransferase Alg2 was shown to be important for switching from the yeast-like phase to the hyphal form (Motteram et al, 2011). These studies suggested the importance of N-glycosylation of proteins in the pathogenesis of plant fungal pathogens but did not offer mechanistic insight into why N-glycosylation is important for pathogenesis in plant pathogens.…”