Physiological and Biochemical Mechanism of Allelopathy of Secalonic Acid F on Higher Plants

phenolic acids. All of these have inhibitory activity, and most of them produce different biological lesions (EinOur previous work indicated that secalonic acid F (SAF) was the hellig, 1995). major allelochemical produced by Aspergillus japonicus. Studies showed that SAF markedly inhibited the seedling growth of sorghum Secalonic acids, a series of ergochrome pigments, exist (Sorghum vulgare Pers.), hairy beggarticks (Bidens pilosa L.), and in a group of food-born fungal metabolites (Betina, barnyardgrass (Echinochloa crus-galli (L.) Beauv.). It significantly 1984). These fungi produce one or more secalonic acids reduced the activities of superoxide dismutase (SOD) and peroxidase when they grow on rice (Oryza sativa L.), corn (Zea (POD) at a concentration of 0.3 mM. Secalonic acid F enhanced mays L.), and rye (Secale cereale L.) (Kurobane et al., malondialdehyde (MDA) contents, but it lowered the content of chlo1979). Secalonic acid A is a highly potent phytotoxic rophyll (CHL) a and b as well as the photosynthetic rates of tested compound that is isolated from several fungi such as plants. Respiration, membrane permeability, and abscisic acid (ABA) Aspergillus aculeatus (Andersen et al., 1977) and Pyrecontent increased after treatment with SAF, but the reduction activity nochaeta terrestris (Steffens and Robeson, 1987). Secaof the root system was lowered. There is no remarkable change in lonic acid D, a metabolite produced by Penicillium oxthe soluble proteins of plants that are treated with SAF. Transmission electron microscope (TEM) observations showed that treated plants aium, causes storage rot of cucumber (Cucumis sativus exhibited amorphous mitochondria without integral membranes and L.) and tomato (Lycopersicon lycopersicum L.) (Jarvis swelling chloroplasts without membranes in a disorderly arrangement. et al., 1990). It is an inhibitor of protein kinase C and The SAF treatment also damaged the stratiform structure of the cyclic AMP-dependent protein kinase (Wang et al., chloroplasts and the membranes and structure of the nuclei. These 1996). Secalonic acid F was originally isolated from results suggest that SAF may weaken the protective ability of plant Claviceps purpurea (Aberhart et al., 1965). Our previous tissues against membrane lipid peroxidation and damage the whole work indicated that SAF was a major allelochemical membrane system of plants, resulting in the ultrastructure destruction produced by A. japonicus (Zeng et al., 2001). Details of chloroplasts, mitochondria, and nuclei. Cell ultrastructure destrucare needed about the mode of action of secalonic acids tion causes a reduction of photosynthesis and root activities and an increase in respiration. These abnormal physiological processes conagainst higher plants to deepen the understanding of tribute to the inhibition of plant growth. the chemical interaction between fungi and higher plants. Our objective was to determine the physiological and biochemical mechanism of allelopathy of SAF. A is mediated by many types of compounds with different sites and modes of biochemiMATERIALS AND METHODS cal action (Rice, 1984). It was originally defined as the Secalonic Acid F biochemical interactions between plants of all kinds, including the microorganisms that are typically placed Secalonic acid F was isolated as yellow needles from the in the plant kingdom (Molisch, 1937). Einhellig (1986) fermentative hyphae of A. japonicus. Its purity was deterpointed out that a clear insight into the precise physiomined by H nuclear magnetic resonance data recorded on logical perturbations caused by allelochemicals had not a Bruker AC-P200 spectrometer (200 MHz) using cadmium chloride (CDCl3) as a solvent and tetramethyl silicane as an been obtained, and it might be emphasized that much internal standard. additional information is needed. One of the most signifSecalonic acid F was dissolved in a 1.5 g L2 dimethyl icant limitations that have curtailed attempts to investiformamide (DMF) solution. Ultraviolet absorption of SAF gate how allelochemicals alter growth is the lack of dissolved in 10 g L2 DMF did not change compared with sufficient quantities of a compound necessary to study SAF dissolved in a 950 g L2 ethanol (C2H5OH) solution. the effects on physiological processes and cellular mechA preliminary study showed that DMF did not affect the anisms (Einhellig, 1995). Several modes of action for phytotoxicity of SAF (Zeng et al., 2001). allelochemicals are involved in the inhibition and modification of plant growth and development (Einhellig, Plant Materials 1986). Allelochemicals may be selective in their action, or plants may be selective in their responses. These The seeds of rape (Brassica campestris L.), cucumber, corn, considerations are complicated further by the presence and sorghum were obtained from a local market in Guangof more than one active compound from a single plant zhou, China. The seeds of hairy beggarticks and barnyardgrass were collected from the campus of South China Agriculor fungus. For example, Sorghum species contain cyanotural University. genic glycosides, tannins, flavonoids, and a series of Inst. of Tropical & Subtropical Ecology, S. China Agric. Univ., WuAbbreviations: ABA, abscisic acid; DMF, dimethyl formamide; EC, shan, Guangzhou, 510642. P.R. China. Received 29 Nov. 1999. *Correelectric conductivity; ELISA, enzyme-linked immunosorbent assay; sponding author ([email protected]). MDA, malondialdehyde; POD, peroxidase; SAF, secalonic acid F; SOD, superoxide dismutase; TEM, transmission electron microscope. Published in Agron. J. 93:72–79 (2001).