ANTIFUNGAL ACTIVITY OF NATURAL AND NATURAL-INSPIRED BIOACTIVE COMPOUNDS TO CONTROL THE RICE BLAST FUNGUS

Fungal pathogens pose a major threat to global agriculture and food security, with intensive monoculture and resistance to single-site fungicides exacerbating the challenge of effective crop protection. Among these pathogens, Pyricularia oryzae, the causal agent of rice blast and of other cereals, is particularly devastating due to its rapid spread and ability to infect multiple cereal crops. This thesis explores the discovery and development of natural and nature-inspired compounds as multitarget biofungicides, aiming to interfere with essential cellular processes and fungal virulence mechanisms. A series of natural stilbenoids, including monomeric (deoxyrhapontigenin, pinostilbene, DMHS) and dimeric compounds were synthesized and evaluated against both wild-type and strobilurin-resistant P. oryzae strains. Deoxyrhapontigenin exhibited the highest inhibitory activity on mycelial growth (60–80%), demonstrating potential as an environmentally friendly antifungal agent. Stilbene derivatives merged with strobilurin pharmacophores showed activity comparable to commercial fungicides, though their low efficacy against resistant strains suggested retention of strobilurin-like mechanisms. Ferroptosis, an iron-dependent form of regulated cell death, was investigated for its role in appressorium maturation. A series of benzamides containing chelating catechol moieties effectively suppressed appressorium formation, with activity partially reversible by adding exogenous Fe 3 ⁺, highlighting the critical link between iron homeostasis and fungal virulence. Additionally, phenylamides (PAs) derived from ferulic acid were synthesized and assessed for antimicrobial potential. While these compounds modestly inhibited fungal growth, they significantly impaired P. oryzae appressorium formation (up to 94%) and exhibited antibacterial activity against foodborne pathogens, suggesting a dual role in plant protection and food safety. Phlorotannins, polyphenolic compounds from brown algae, were synthesized and evaluated as antifungal agents. Polymethylated diphenyl ether derivatives showed moderate mycelial growth inhibition (20–45%) across multiple fungal strains, indicating that structural features such as methylation patterns influence bioactivity. In vivo and in vitro studies of the most promising compounds—stilbenoids (deoxyrhapontigenin, DMHS), siderophores (16a), phenylamides (feruloyl-agmatine 3f, feruloyl-tyramine 3b), and phlorotannins (15)—demonstrated their capacity to impair spore germination, appressorium formation, and fungal infection. Confocal microscopy revealed that feruloyl-agmatine 3f partially inhibited autophagy-mediated nuclear degradation, a process essential for appressorium maturation. Overall, this thesis identifies natural and synthetic compounds that target multiple aspects of P. oryzae pathogenicity, including spore germination and mycelial growth, or processes indispensable for appressorium formation and maturation, such as ferroptosis and autophagy. The findings provide a foundation for the development of environmentally friendly biofungicides with novel molecular targets that interfere with until now little explored biological processes involved in pathogenesis, and highlight innovative strategies for durable rice blast management, offering significant potential for enhancing agricultural productivity and food security.