An Ammonium Transporter Gene Contributes to the Aggressiveness of the Dutch Elm Disease Pathogen Ophiostoma novo-ulmi

TL;DR

A gene encoding an ammonium transporter in a fungus that causes Dutch elm disease is found to be important for its ability to aggressively infect elm trees.

Contribution

The study identifies and characterizes an ammonium transporter gene (amtA) as a key factor in the aggressiveness of the Dutch elm disease pathogen Ophiostoma novo-ulmi.

Findings

A knockdown mutant of the amtA gene showed significantly reduced aggressiveness in infecting elm saplings.

The amtA gene product has features similar to Mep2-type transporters, including conserved amino acid residues and transmembrane helices.

CRISPR-Cas9 knockout mutants of amtA retained some wild-type traits but were less aggressive towards elm trees.

Abstract

Molecular mechanisms determining pathogenicity of the Dutch elm disease fungus Ophiostoma novo-ulmi are poorly understood. Prior identification of the pathogenicity locus pat1 prompted a chromosome walking approach to elucidate gene function in this region. Among 17 identified genes, ONUg0282 (amtA) was predicted to encode a high-affinity ammonium transporter. In silico analyses confirmed the presence of four additional amt genes (amtB, amtC, amtD, and amtE) in both O. novo-ulmi and the less aggressive O. ulmi and that amtA and amtB belong to the Saccharomyces cerevisiae mep2 clade. The predicted amtA gene product showed features of Mep2-type transceptors, including amino acid residues corresponding to His-168 and His-318 in Escherichia coli AmtB protein, 11 transmembrane helices, and a conserved 22 amino acid motif immediately downstream of the last transmembrane helix. A knockdown amtA mutant with 25% residual expression was significantly less aggressive than wild-type O. novo-ulmi strain H327 when infecting Ulmus americana × U. parvifolia saplings. Predicted AmtA transporters from two CRISPR-Cas9 knockout mutants contained only five intact transmembrane helices. The ΔamtA mutants retained several wild-type phenotypic traits, including yeast–mycelium dimorphism, but were significantly less aggressive than H327 towards U. americana saplings. We concluded that ONUg0282 is an important determinant of aggressiveness in O. novo-ulmi.

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