Polymeric quaternary ammonium salt activity against Fusarium oxysporum f. sp. cubense race 4: Synthesis, structure-activity relationship and mode of action

Polymeric quaternary ammonium salts (PQAS) have been widely used to prevent microbial contamination, but little is known about their activity against phytopathogenic fungi. Our previous report described the synthesis of two novel PQAS, namely a homopolymer of (2-methacrylamido) propyltetrabenzyldimethylammonium chloride (PQD-BC) and dimethylaminopropyl benzyl chloride-grafted polysiloxanes (PDMS-g-BC); we demonstrated their structure-activity relationship against phytopathogenic fungi such as R. solani and Fusarium oxysporum f. sp. cubense tropical race 4 (Foc4). Here, we analyzed the structure-activity relationship and toxicity mechanism of these compounds at the molecular level against Foc4 compared with the low molecular-weight quaternary ammonium salt benzalkonium chloride (BC). The results revealed that PQD-BC and PDMS-g-BC application inhibited the growth of Foc4 in a concentration-dependent manner and that PDMS-g-BC exhibited higher activity than PQD-BC. In addition, these polymers were found to induce cell death in Foc4 by disrupting the cellular structure integrity, such as the loss of the cell wall and plasma membrane integrity and oxidative stress (lipid peroxidation), leading to the release of intracellular contents and inducing mitochondrial dysfunction and interference with genomic DNA. The newly elucidated mechanism provides possible applications in which PQAS can be used against phytopathogenic fungi.