Abstract
Alternaria alternata, the casual agent of black rot of pear fruit, can sense and respond to the physicochemical cues from the host surface and form infection structures during infection. To evaluate the role of cyclic AMP-dependent protein kinase (cAMP-PKA) signaling in surface sensing of A. alternata, we isolated and functionally characterized the cyclic adenosine monophosphate-dependent protein kinase A catalytic subunit gene (AaPKAc). Gene expression results showed that AaPKAc was strongly expressed during the early stages of appressorium formation on hydrophobic surfaces. Knockout mutants ΔAaPKAc were generated by replacing the target genes via homologous recombination events. We found that intracellular cAMP content increased but PKA content decreased in ΔAaPKAc mutant strain. Appressorium formation and infection hyphae were reduced in the ΔAaPKAc mutant strain, and the ability of the ΔAaPKAc mutant strain to recognize and respond to high hydrophobicity surfaces and different surface waxes was lower than in the wild type (WT) strain. In comparison with the WT strain, the appressorium formation rate of the ΔAaPKAc mutant strain on high hydrophobicity and fruit wax extract surface was reduced by 31.6 and 49.3% 4 h after incubation, respectively. In addition, AaPKAc is required for the hypha growth, biomass, pathogenicity, and toxin production of A. alternata. However, AaPKAc negatively regulated conidia formation, melanin production, and osmotic stress resistance. Collectively, AaPKAc is required for pre-penetration, developmental, physiological, and pathological processes in A. alternata.
Highlights
Alternaria alternata, as a typical latent infectious fungus that can cause different diseases in more than 100 plant species (Wang et al, 2019), results in severe losses for the economy and the industry and poses a potential safety hazard to animals and human beings due to the production of mycotoxins during the growth and development (Andersen et al, 2015; Estiarte et al, 2016)
AaPKAc was found to contain a 1571-bp open reading frame interrupted by two small introns of 59 and 49 bp, which encodes a polypeptide of 488 amino acids
The deduced amino acid sequences of AaPKAc indicated that AaPKAc possesses conserved recognition sequence of catalytic subunits Arg (R)-X-Ser/Thr (S/T)-Y, among which Arg plays an important role in the substrate recognition of kinases (Supplementary Figure 3B)
Summary
Alternaria alternata, as a typical latent infectious fungus that can cause different diseases in more than 100 plant species (Wang et al, 2019), results in severe losses for the economy and the industry and poses a potential safety hazard to animals and human beings due to the production of mycotoxins during the growth and development (Andersen et al, 2015; Estiarte et al, 2016). Yoshida et al (2015) suggested that the conidial germination and the formation of appressoria of Blumeria graminis, Colletotrichum dematium, and Magnaporthe grisea were significantly decreased due to reduced hydrophobicity of solid surfaces. The C28 aldehyde was involved in infection structure differentiation of Puccinia graminis (Reisige et al, 2006). These studies showed that chemical components and hydrophobicity of plant cuticular wax might activate the formation of the fungal pathogen infection structure
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