Infection Structure-Specific Expression of  -1,3-Glucan Synthase Is Essential for Pathogenicity of Colletotrichum graminicola and Evasion of  -Glucan-Triggered Immunity in Maize

Abstract

,3-Glucan and chitin are the most prominent polysaccharides of the fungal cell wall. Covalently linked, these polymers form a scaffold that determines the form and properties of vegetative and pathogenic hyphae. While the role of chitin in plant infection is well understood, the role of b-1,3-glucan is unknown. We functionally characterized the b-1,3-glucan synthase gene GLS1 of the maize (Zea mays) pathogen Colletotrichum graminicola, employing RNA interference (RNAi), GLS1 overexpression, live-cell imaging, and aniline blue fluorochrome staining. This hemibiotroph sequentially differentiates a melanized appressorium on the cuticle and biotrophic and necrotrophic hyphae in its host. Massive b-1,3-glucan contents were detected in cell walls of appressoria and necrotrophic hyphae. Unexpectedly, GLS1 expression and b-1,3-glucan contents were drastically reduced during biotrophic development. In appressoria of RNAi strains, downregulation of b-1,3-glucan synthesis increased cell wall elasticity, and the appressoria exploded. While the shape of biotrophic hyphae was unaffected in RNAi strains, necrotrophic hyphae showed severe distortions. Constitutive expression of GLS1 led to exposure of b-1,3-glucan on biotrophic hyphae, massive induction of broad-spectrum defense responses, and significantly reduced disease symptom severity. Thus, while b-1,3-glucan synthesis is required for cell wall rigidity in appressoria and fast-growing necrotrophic hyphae, its rigorous downregulation during biotrophic development represents a strategy for evading b-glucantriggered immunity.

Signals that stop the rot: Regulation of secondary metabolite defences in cereals

“…Similarly, β-glucan was shown to elicit production of momilactones as well as small amounts of oryzalexins in rice suspension cells [47]. Upon exposure to β-glucan from Colletotrichum graminicola (responsible for anthracnose), the expression of putative phytoalexin biosynthetic genes in maize were upregulated by more than 150-fold [54]. Interestingly, the same study showed that C. graminicola ostensibly attempts to evade maize PAMP-elicited defence responses by downregulating its β-glucan production during the biotrophic growth phase to establish a compatible interaction [54].…”

“…Upon exposure to β-glucan from Colletotrichum graminicola (responsible for anthracnose), the expression of putative phytoalexin biosynthetic genes in maize were upregulated by more than 150-fold [54]. Interestingly, the same study showed that C. graminicola ostensibly attempts to evade maize PAMP-elicited defence responses by downregulating its β-glucan production during the biotrophic growth phase to establish a compatible interaction [54]. Another study in maize demonstrated phytoalexin accumulation after wounded stems were treated with a pectinase elicitor derived from Rhizopus microsporus [27].…”

Secreted biomolecules in fungal plant pathogenesis

“…Post penetration of the plant surface, these species produce bulbous biotrophic hyphae enveloped by the host plasma membrane that ultimately develop into fastgrowing, thin, necrotrophic hyphae ( Figure 19.5) (Perfect et al, 1999). In addition to an altered appear ance, the cell wall composition of the two hyphal forms differs, with β-1,3-glucan synthase expression and β-1,3-glucan content being reduced in the bulbous hyphae (Oliveira-Garcia and Deising, 2013). Genome and transcriptome analyses have shown the spatiotemporal expression of virulence factors, with the genes coding for effectors and secondary metabolism being upregulated during biotrophic infection, while hydrolase and transporters were expressed during the necrotrophic phase (O'Connell et al, 2012).…”

Conditional gene expression and promoter replacement in Zymoseptoria tritici using fungal nitrate reductase promoters

“…Fungal b-1,3-glucan synthases are involved in the biosynthesis of cell wall glucans. The catalytic sub-unit is encoded by a single gene (BGS1) described as essential for growth in Aspergillus fumigatus, Coccidioides posadasii and Colletotrichum graminicola (Firon and d'Enfert, 2002;Kellner et al, 2005;Oliveira-Garcia and Deising, 2013). Replacement of the native Z. tritici BGS1 promoter with pZtNIA1 and phenotypic screening of resulting transformants suggested that BGS1 is important for growth in Z. tritici and is involved in tolerance to cell wall stress induced by Calcofluor White (CFW).…”