Researchers isolated two diterpenes, sclareol and 13-epi-sclareol, from leaves of Nicotiana glutinosa (a tobacco species). These compounds were present as a eutectic mixture in liquid droplets on leaf surfaces at concentrations around 10% by weight. The diterpenes did not prevent fungal spore germination but significantly inhibited radial colony growth on agar for 16 out of 18 fungal species tested at concentrations as low as 20 μg/ml. Microscopic examination revealed the inhibition was caused by altered fungal morphology: while normal hyphae grew primarily from the apex with minimal branching, hyphae exposed to sclareol showed reduced apical growth but greatly increased lateral branching. This morphological change, which resembles a hormonal effect, explains the reduced radial extension across agar surfaces. The compounds had minimal effect on mycelial biomass in liquid culture, suggesting the inhibition is specific to colony expansion on solid substrates. The authors suggest these diterpenes may influence fungal growth-regulatory processes and could potentially affect fungal growth on plant leaf surfaces, though further work is needed to determine their role in plant defense.

Key findings

• Sclareol and 13-epi-sclareol were isolated from N. glutinosa leaves as a eutectic mixture present in surface liquid droplets at ~10% concentration
• These diterpenes did not inhibit fungal spore germination but reduced radial growth of 16 of 18 fungal species on agar at 20 μg/ml
• Sclareol caused altered fungal morphology characterized by reduced apical dominance and excessive lateral hyphal branching, similar to effects of griseofulvin
• The compounds had minimal effect on mycelial biomass in liquid culture, indicating selective effects on surface colony expansion