A conserved fungal glycosyltransferase facilitates pathogenesis of plants by enabling hyphal growth on solid surfaces

Abstract

Pathogenic fungi must extend filamentous hyphae across solid surfaces to cause diseases of plants. However, the full inventory of genes which support this is incomplete and many may be currently concealed due to their essentiality for the hyphal growth form. During a random T-DNA mutagenesis screen performed on the pleomorphic wheat (Triticum aestivum) pathogen Zymoseptoria tritici, we acquired a mutant unable to extend hyphae specifically when on solid surfaces. In contrast “yeast-like” growth, and all other growth forms, were unaffected. The inability to extend surface hyphae resulted in a complete loss of virulence on plants. The affected gene encoded a predicted type 2 glycosyltransferase (ZtGT2). Analysis of >800 genomes from taxonomically diverse fungi highlighted a generally widespread, but discontinuous, distribution of ZtGT2 orthologues, and a complete absence of any similar proteins in non-filamentous ascomycete yeasts. Deletion mutants of the ZtGT2 orthologue in the taxonomically un-related fungus Fusarium graminearum were also severely impaired in hyphal growth and non-pathogenic on wheat ears. ZtGT2 expression increased during filamentous growth and electron microscopy on deletion mutants (ΔZtGT2) suggested the protein functions to maintain the outermost surface of the fungal cell wall. Despite this, adhesion to leaf surfaces was unaffected in ΔZtGT2 mutants and global RNAseq-based gene expression profiling highlighted that surface-sensing and protein secretion was also largely unaffected. However, ΔZtGT2 mutants constitutively overexpressed several transmembrane and secreted proteins, including an important LysM-domain chitin-binding virulence effector, Zt3LysM. ZtGT2 likely functions in the synthesis of a currently unknown, potentially minor but widespread, extracellular or outer cell wall polysaccharide which plays a key role in facilitating many interactions between plants and fungi by enabling hyphal growth on solid matrices.

The transcription factor Zt107320 affects the dimorphic switch, growth and virulence of the fungal wheat pathogen Zymoseptoria tritici

“…Deletion of ZtAlg2 , which encodes a putative mannosyltransferase in Z. tritici , prevents this switch and results in nonpathogenic strains (Motteram et al , ). In addition, the deletion of a putative glycosyltransferase, ZtGt2, was shown to reduce the ability to extend hyphae on solid surfaces and pathogenicity (King et al , ). Similarly, the deletion of MCC1, which encodes a putative c‐type cyclin, reduces the ability to produce aerial mycelia and impairs pathogenicity (Choi and Goodwin, ).…”

Measurement of infection efficiency of a major wheat pathogen using time‐resolved imaging of disease progress

“…() with Aspergillus fumigatus and King et al . () with Z. tritici . Unlike the previous studies with Z. tritici that used cotton swabs to deploy the inoculum suspension on leaves (Lee et al ., ) or pipetted a well‐defined volume of spore suspension onto the leaf and spread it using a gloved finger (Fones et al ., ), a paint gun sprayer was used in the present study (comparable to Statler & McVey ()) that produced an evenly distributed ‘fog’ of tiny droplets.…”

“…For example, viable spore densities were estimated with reference plates by Chakraborty et al (1990) in a study of anthracnose; a range of inoculum concentrations, including low concentrations, were used by in an investigation of Z. tritici on wheat; attached leaf assays were previously used in the same pathosystem by Keon et al (2007) and Lee et al (2014); and timeresolved imaging has been used before in fungal biology, for example in studies of spore ejection of basidiomycete fungi (Noblin et al, 2009). In addition, numerous studies measured the ability of fungal spores to grow colonies under conducive conditions in vitro to estimate the viability of spores: for example, Valsecchi et al (2017) with Aspergillus fumigatus and King et al (2017) with Z. tritici. Unlike the previous studies with Z. tritici that used cotton swabs to deploy the inoculum suspension on leaves (Lee et al, 2014) or pipetted a well-defined volume of spore suspension onto the leaf and spread it using a gloved finger , a paint gun sprayer was used in the present study (comparable to Statler & McVey (1987)) that produced an evenly distributed 'fog' of tiny droplets.…”

Measurement of infection efficiency of a major wheat pathogen using time-resolved imaging of disease progress

“…In addition, numerous studies measured the ability of fungal spores to grow colonies under conducive conditions in vitro to estimate the viability of spores: for example Valsecchi et al (2017) in Aspergillus fumigatus and King et al (2017) in Z. tritici. Unlike the previous studies in this pathosystem that used cotton swabs to deploy the inoculum suspension on leaves (Lee et al, 2014) or pipetted a well-defined volume of spore suspension onto the leaf and 350 spread it using a gloved finger , we used a paint gun sprayer [comparable to Statler & McVey (1987) for example] that produced an evenly distributed "fog" of tiny droplets.…”