Potencial herbicida e antifúngico de compostos isolados de fungos obtidos em ecossistemas tropicais e antárticos

Abstract

With the increase in the world's population and, consequently, the rising demand for food, agricultural production has been overloaded worldwide. The shortage of effective pesticides, coupled with the continuous evolution and spread of species resistant to these products, presents a potential problem for global food security that needs to be urgently addressed. Intensive use of pesticides can have adverse effects on terrestrial and aquatic ecosystems, consequently contaminating the food chain. Numerous studies have highlighted the negative impacts of pesticides on human health, both in short-term and long-term exposures. Thus, the search for safer pesticide substances becomes crucial to mitigate adverse impacts on the environment and human health. Recent research has evidenced the potential of secondary metabolites produced by fungi as a promising source for developing new pesticides. These natural products exhibit a range of biological activities, making them attractive targets for studies aiming at the development of more efficient and environmentally friendly pesticides. Based on the above, the goal of this study was to select fungi capable of producing metabolites with herbicidal potential, isolate and characterize the substances responsible for their phytotoxic activity and evaluate the antifungal activity of these substances against the agricultural pathogen Colletotrichum fragariae. Initially, dichloromethane extracts were produced through solid fermentation from 47 fungal isolates originating from tropical and Antarctic ecosystems, which were tested at a concentration of 1 mg mL-1 on seeds of the monocotyledon model (Allium schoenoprasum - chives) and the dicotyledon model (Lactuca sativa - lettuce) to evaluate their phytotoxic potential. Twelve fungi showed activity in the assay by inhibiting the germination of at least one of the target models; six demonstrated selective activity against A. schoenoprasum; and six were active against both tested models. The active fungi were identified as belonging to the genera Xylaria, Fusarium, Penicillium, Lasiodiplodia, and Diaporthe. These fungi were cultivated on large scale and then subjected to a biomonitored chemical fractionation process to isolate the phytotoxic metabolites. In this process, nine substances were identified: patulin (1), penienone (2), palitantin (3) grammicin (4), methyl ester of xylaric acid (5), anhydrofusarubin (6), 5,10-dihydroxy-1,7-dimethoxy-3-methyl-1H-benzo[g]isocoumarin-6,9-dione (7), javanicin (8), and lasiodiplodin (9). All substances, except (9), exhibited phytotoxic activity. The 50% inhibitory concentrations (IC50) of these substances were evaluated against the plant Lemna paucicostata, revealing values ranging from 28 to 88 µM; on the other hand, the herbicide glyphosate demonstrated a value of 388 µM in the same assay. Regarding the antifungal activity against the phytopathogenic fungus Colletotrichum fragariae, compound (2) exhibited activity with inhibition zones of 12 mm at 100 µg spot-1 and 6.3 mm at 10 µg spot-1; (7) showed activity with inhibition zones of 12 mm at 100 µg spot-1 and 5 mm at 10 µg spot-1, while (8) exhibited activity with inhibition zones of 8 mm at 100 µg spot-1 and 4 mm at 10 µg spot-1. Among the isolated metabolites, grammicin (4) stood out as the most promising candidate for the development of a natural herbicide. It demonstrated significant phytotoxic activity against L. paucicostata, with an IC50 of 88 µM. Furthermore, it exhibited cytotoxic activity with an effective concentration (EC50) of 170 µM for HepG2, 200 µM for LLC-PK1, and did not exhibit relevant cytotoxicity in the VERO cell line up to a concentration of 250 µM. The notable stability of grammicin, its high yield in the crude extract, potent phytotoxic activity, and low cytotoxicity highlight it as a substance of great interest. Consequently, investigations aimed at maximizing its production in the crude extract and elucidating its mechanism of action become extremely relevant for its use as a bioherbicide. The results of this study highlight the importance of exploring secondary metabolites produced by fungi, as they represent sources of substances with high biotechnological and industrial value. Moreover, they emphasize the versatility of fungi, as a single isolate can produce metabolites with different activities, such as phytotoxic and antifungal.