Effect of null mutations in the AbNIK1 gene on saprophytic and parasitic fitness of Alternaria brassicicola isolates highly resistant to dicarboximide fungicides

Abstract

Alternaria brassicicola field isolates highly resistant to dicarboximide fungicides (DCF) and carrying non-sense codons or micro-deletions in the AbNIK1 gene encoding a group III histidine kinase were characterized at the protein level using specific antibodies. These isolates, as well as laboratory mutants expressing the same resistant phenotype, were shown to correspond to null mutants. The fitness of these isolates was compared to that of wild-type isolates by measuring i) mycelial growth, ii) sporulation and iii) conidial germination under standard conditions. None of these factors were affected in the resistant isolates. Mycelial growth was also measured under stress conditions. Unlike suboptimal incubation temperatures or exposure to the superoxide ion generator menadione, osmotic stress generated by high sorbitol concentrations in the culture medium significantly affected the growth of AbNIK1-null mutants compared to wild-types. The effects of these mutations on parasitic fitness were estimated under both controlled and field conditions after inoculation of radish with individual isolates, to measure their aggressiveness, or with mixed conidial suspensions, to measure their competitive ability against wild-type isolates. These tests on radish seedlings revealed that DCF-resistant isolates were still aggressive. Despite this, the data obtained in the competition experiments showed that the loss of the ability to synthesize AbNIK1p in A. brassicicola DCF-resistant mutants was linked to a severe decrease in their competitiveness.

Cell wall integrity and high osmolarity glycerol pathways are required for adaptation of Alternaria brassicicola to cell wall stress caused by brassicaceous indolic phytoalexins

“…Such reduced virulence was interpreted as a consequence of slower development of bos1 mutants in the host because of stresses imposed by the intrinsic osmotic state of the host. Similarly, Iacomi‐Vasilescu et al . (2008) reported that osmosensitive Alternaria brassicicola null mutants for a GIII‐HK have a reduced capacity to colonize radish seeds.…”

Phenylpyrroles: 30 Years, Two Molecules and (Nearly) No Resistance

“…No significant developmental defects could be detected in the A. brassicicola resistant mutants and only moderate osmosensitivity (Avenot et al, 2005; Iacomi-Vasilescu et al, 2008). However, phenylpyrrole resistance seems limited in Alternaria field populations (Avenot and Michailides, 2015; Malandrakis et al, 2015) indicating a potential fitness penalty not detected under controlled laboratory conditions.…”

The Group III Two-Component Histidine Kinase of Filamentous Fungi Is Involved in the Fungicidal Activity of the Bacterial Polyketide Ambruticin

“…All but strain AbraCP were shown to carry mutations at the AbNIK1 locus. Four isolates were previously shown to bear nonsense or frameshift mutations (3) and characterized at the protein level as group III HK-null mutants (13). Immunoblots probed with specific antibodies raised against AbNIK1p (Fig.…”