Inhibitory effects of furanone metabolites of a rhizobacterium, Pseudomonas jessenii, on phytopathogenic Aphanomyces cochlioides and Pythium aphanidermatum

An antagonistic rhizobacterium, Pseudomonas jessenii EC-S101, isolated from the rhizosphere of spinach, produces two related secondary metabolites, 3-[(1R)-hydroxyoctyl]-5-methylene-2(5H)-furanone (4,5-didehydroacaterin) (1) and 3-[(1R)-hydroxyhexyl]-5-methylene-2(5H)-furanone (2). This study demonstrated their in vitro inhibitory effects, in particular those of (1), against Aphanomyces cochlioides AC-5 and Pythium aphanidermatum PA-5. The compounds inhibited radial growth and induced morphological abnormalities characterized by hyperbranching and periodic swelling in AC-5 and PA-5 hyphae, respectively. Staining with rhodamine-phalloidin, which binds to plasma-membrane-associated filamentous-actin (F-actin), revealed that tip-specific actin filaments were remodelled into a plaque-like form at an early stage of encounter (up to 24 h) with (1) or (2), whereas at later stages of encounter (48 h), the plaques were eliminated, reflecting the disorganization of actin arrays in the morphologically abnormal AC-5 and PA-5 hyphae. A similar response of actin disorganization was observed in AC-5 and PA-5 hyphae upon treatment with latrunculin B (3), an actin-assembly inhibitor produced by a sea sponge. It is suggested that (1) and (2) caused actin disorganization and their inhibitory activities were comparable to that of (3). Further ultrastructural observations substantiated abnormal functioning and delocalization of F-actin-linked cell organelles.