This study investigates the relationship between laccase enzyme activity and rhizomorph formation in Armillaria species, pathogenic fungi that cause root rot in trees. Rhizomorphs are root-like structures that enable fungal spread through soil. Using Armillaria mellea as the primary model, researchers demonstrated that ethanol and other substances inducing rhizomorph formation also triggered laccase production. Laccase activity appeared approximately two days before visible rhizomorphs emerged and peaked during maximum rhizomorph growth before declining as growth ceased. In surveys of 32 isolates across four Armillaria species, laccase activity and rhizomorph production showed strong positive correlation (r=0.85). Enzyme inhibitors specific to laccase substantially reduced rhizomorph formation while leaving mycelial growth largely unaffected. Reduced oxygen conditions delayed rhizomorph development but not mycelial growth, an effect correlated with decreased laccase activity. Laccase localized primarily in rhizomorphs rather than mycelium and was undetectable in uninduced cultures. The researchers propose that laccase plays a dual role: facilitating early rhizomorph morphogenesis and development, and later catalyzing formation of melanin-like pigments that protect and mature developing rhizomorphs in soil environments.

Key findings

• Laccase activity and rhizomorph formation are tightly coordinated, with laccase appearing 2 days before rhizomorphs and declining as growth ceases
• Strong positive correlation (r=0.85) between laccase activity and rhizomorph production across 32 Armillaria isolates representing four species
• Laccase-specific inhibitors substantially reduced rhizomorph production while leaving mycelial growth unaffected, suggesting a morphogenetic role for laccase
• Laccase localizes primarily in rhizomorphs rather than central mycelium and is produced only in induced cultures
• Reduced oxygen availability inhibits both rhizomorph formation and laccase activity while permitting continued mycelial growth