The Sweet Battlefield: How Fungi Wage Chemical War Inside Tropical Fruits

If you bite into a papaya or a pineapple and think, "Mmm, sweet," you've only experienced about 1% of the story. Deep within the flesh of these tropical fruits, a chemical arms race has been raging for millions of years. On one side stand the fruits' own protease guardians—papain and bromelain—which act as an immune-like defense line, dismantling the proteins of intruders to thwart unwanted guests. On the other side lurk soil-borne pathogenic fungi, chief among them Fusarium. These fungi are no pushovers; they have evolved a sophisticated metabolic arsenal that not only withstands the fruits' protease defenses but also produces compounds that inhibit protease activity, enabling them to successfully colonize the fruit tissue.

In a recent study, a research team from the University of São Paulo, Brazil, isolated 12 fungal strains from papaya and pineapple and used GNPS2 molecular networking to decode their metabolic fingerprints, shedding light on the secrets of this "chemical shadow war."

The researchers isolated a total of 12 fungal strains from papaya and pineapple—dubbed the "twelve emissaries." Five belong to the core Fusarium genus:

The collection also includes a reference laboratory strain F1 (Fusarium guttiforme); two Aspergillus species (A. flavus and A. terreus); one Mucor (Mucor circinelloides); one Talaromyces (Talaromyces funiculosus); one Trichoderma sp.; and one endophytic fungus of particular interest—LMC23006 (Neofusicoccum ribis), isolated from papaya. LMC23006 would later play a pivotal role in co-culture experiments, where it was paired with Fusarium strains to explore whether interspecies interactions could unlock novel chemical potential.

Fungal metabolites are intimately tied to cultivation conditions—essentially, "you are what you eat." To fully mine their metabolic potential, the team selected four distinct solid media:

The results confirmed that the culture medium exerts a profound influence on the metabolic profile. Different substrates awaken different biosynthetic pathways, prompting the fungi to produce strikingly different chemical repertoires.

Even more scientifically intriguing were the co-culture experiments. The team co-inoculated each of the five Fusarium strains with the endophyte LMC23006 on rice medium, mimicking the ecological reality of multiple fungi sharing the same niche.

The outcome: a dramatic shift in the metabolic landscape.

Asperlin (compound 137), previously detected only in LMC23006's sugarcane bagasse monoculture, was now found across all co-culture combinations. This suggests that LMC23006, upon sensing the presence of Fusarium, activates a biosynthetic pathway that normally lies silent—presumably a defensive response mechanism.

Statistically, PLS-DA analysis revealed that co-culture samples clustered distinctly, regardless of which Fusarium partner was involved. This implies that the regulatory effect of interspecies interaction on the metabolome may even outweigh the influence of culture medium type; fungal cross-talk directly reshapes their chemical phenotype.

The study also uncovered a striking pattern: hierarchical clustering of each strain's metabolome on rice medium closely mirrored their phylogenetic relationships. The F. solani complex (LMC23007.2 and LMC23008) clustered together; the Fujikuroi complex (LMC23012, LMC23015, LMC23018, and F1) formed another branch; and LMC23006 (N. ribis) stood alone—exactly as it appears on the phylogenetic tree.

This finding reveals that a fungus's chemical phenotype is governed by its genetic background. Closely related strains produce more similar metabolite profiles; distantly related strains diverge significantly in their chemistry.

Metabolic diversity is only the first step; biological activity is what matters. The team tested all co-culture extracts for papain inhibitory activity at 25 µg/mL.

The top three performers—LMC23012, LMC23015, and LMC23018—all achieved inhibition rates exceeding 85%. PLS-DA showed that their chemical profiles were highly similar, indicating that potent protease inhibition is tightly linked to specific metabolic signatures.

Moreover, co-culture extracts generally outperformed monoculture extracts in activity. Thus, co-culture not only stimulates the production of novel compounds but also enhances the bioactivity of the resulting extracts.

In total, the study resolved 173 metabolites: 134 from the six Fusarium strains, 24 from N. ribis, and 18 from co-culture conditions. Among the structurally characterized compounds were one entirely new compound (81) and one new derivative (73, 10-hydroxyfusaric acid), alongside multiple known compounds documented for the first time in specific species or on specific media.