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Localized Melanization of Appressoria Is Required for Pathogenicity of Venturia inaequalis
Abstract:
ABSTRACT During formation of appressoria produced from conidia and ascospores of Venturia inaequalis, a dark brown ring structure was detected at the base of appressoria. This melanized appressorial ring structure (MARS) was attached to the leaf surface like a sealing ring and formed the fungus-plant interface; it is believed to be required for pathogen penetration of the cuticle. Neither germ tubes nor infection structures beneath the cuticle were found to be visibly melanized. MARS were formed not only on apple leaves but also on leaves of nonhost plants and artificial surfaces differing in hydrophobicity; the formation of appressoria and MARS was confined to hard surfaces. The melanin nature of the ring was confirmed by using melanin biosynthesis inhibitors. Applications prior to inoculation largely inhibited the melanization and reduced infection rate by 45 to 80%; curative applications were not effective. Transmission electron microscopy verified a localized melanization of the cell wall around the penetration pore, and melanin was incorporated into all layers of the fungal cell wall. Appressoria without MARS were not able to infect the plant, suggesting that this structure can be considered to be a pathogenicity factor in V. inaequalis.
Insights
A melanized appressorial ring structure (MARS) is crucial for Venturia inaequalis to infect apple leaves. Inhibiting MARS formation significantly reduces infection rates, highlighting its role as a key pathogenicity factor.
Area of Science:
- Plant Pathology
- Mycology
- Biochemistry
Background:
- Venturia inaequalis causes apple scab, a significant disease in apple cultivation.
- Appressoria are specialized infection structures fungi use to penetrate host plants.
- The precise role of structural components during appressoria formation and plant invasion is not fully understood.
Purpose of the Study:
- To investigate the nature and function of a newly detected melanized appressorial ring structure (MARS) in Venturia inaequalis.
- To determine if MARS is essential for the pathogenicity of Venturia inaequalis.
- To assess the potential of targeting MARS formation for disease control.
Main Methods:
- Microscopic observation of appressoria formation on various surfaces.
- Application of melanin biosynthesis inhibitors.
- Transmission electron microscopy (TEM) for ultrastructural analysis.
- Fungal infection assays on host and nonhost plants.
Main Results:
- A distinct melanized appressorial ring structure (MARS) was consistently observed at the base of Venturia inaequalis appressoria on hard surfaces.
- MARS formation was inhibited by melanin biosynthesis inhibitors, leading to a 45-80% reduction in infection rates when applied preventatively.
- TEM confirmed localized melanization within the appressorial cell wall, forming a seal at the fungus-plant interface.
- Appressoria lacking MARS were unable to infect apple leaves.
Conclusions:
- The melanized appressorial ring structure (MARS) is a critical, localized fungal cell wall component required for Venturia inaequalis pathogenicity.
- MARS formation is essential for successful cuticle penetration and infection of apple plants.
- Targeting MARS formation presents a promising strategy for developing novel fungicides against apple scab.
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