The HOG Pathway Plays Different Roles in Conidia and Hyphae During Virulence of Alternaria alternata

Area of Science:

• Plant Pathology
• Mycology
• Molecular Biology

Background:

• Alternaria alternata causes significant agricultural losses through postharvest spoilage and mycotoxin production.
• Fungal signaling pathways are vital for plant host colonization.
• Mitogen-activated protein kinase HogA is implicated in stress-associated gene expression.

Purpose of the Study:

• To investigate the role of the HogA protein in the pathogenicity of Alternaria alternata.
• To determine the importance of HogA in plant colonization and stress response.

Main Methods:

• Generated a ∆hogA-deletion strain of Alternaria alternata.
• Assessed pathogenicity using conidial and hyphal inocula on tomato and apple.
• Evaluated fungal growth on artificial media under stress conditions.
• Analyzed the expression of stress-related genes (catalase, superoxide dismutase) using quantitative methods.

Main Results:

• The ∆hogA-deletion strain showed impaired colonization of tomato and apple when using conidia, but not hyphae, indicating HogA's role in initial infection stages.
• Wild-type conidia exhibited stress tolerance, while ∆hogA conidia failed to grow under stress.
• Hyphal inoculum of both wild-type and ∆hogA strains showed similar growth patterns.
• Hydrogen peroxide stress induced upregulation of catalase and superoxide dismutase genes in germlings, dependent on the high osmolarity glycerol (HOG) pathway and HogA.
• In mycelia, certain stress-related genes were upregulated independently of HogA.

Conclusions:

• HogA is essential for the initial stages of plant colonization by Alternaria alternata, particularly when using conidial inoculum.
• HogA plays a critical role in the stress response of conidia, but its function in hyphae differs.
• The HOG pathway, involving HogA, regulates the expression of key antioxidant genes in response to oxidative stress.