The role of fumonisins in maize seedling disease and the ecological interaction between fusarium verticillioides, soil and plants
Williams, Lonnie Dwayne
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Fumonisins (FB1, FB2, and FB3) are mycotoxins produced by Fusarium verticillioides(Sacc.) Nireberg (synonym=F. moniliforme; telemorph Gibberella moniliformis). They inhibitceramide synthase, a key enzyme in de novo sphingolipid biosynthesis. While fumonisins arenot known to cause plant disease under field conditions, they are found in the ear, roots, andstalks of maize and can enter the soil intact where they can be tightly bound and under certainconditions may be released. We hypothesized that fumonisin production in soils, inhibition ofceramide synthase, and disruption of sphingolipid metabolism modulate disease symptoms in F.verticillioides infected maize seedling.To evaluate the ability of F. verticillioides to produce fumonisins in synthetic and naturalsoils, maize seeds were inoculated with a fumonisin producing strain of F. verticillioides andplanted in potting soil and three different natural soils. The fungus produced fumonisins in eachsoil type, disrupted sphingolipid metabolism in the roots and induced symptoms of maizeseedling diseaseTo investigate the role of fumonisin-induced inhibition of ceramide synthase in F.verticillioides pathogenicity on maize seedlings, maize seeds were inoculated with fumonisinproducing strains or fumonisin non-producing strains of F. verticillioides and planted in sterilepotting soil. Only the seedlings grown from seeds inoculated with the fumonisin producingstrains of F. verticillioides developed symptoms of F. verticillioidesmaize seedling disease. Inaddition, the concentration of fumonisin and free sphingoid bases and their 1-phosphatemetabolites were correlated with the severity of the pathology.To further understand the role of fumonisins in maize seedling disease associated with F.verticillioides, un-inoculated maize seeds were watered with 1, 5, 10, or 20 µg/ml aqueousfumonisin B1. There was a dose-dependent reduction in root mass at 5, 10, and 20 µg/mlfumonisin B1, and there was a dose-dependent elevation in fumonisin B1, sphingoid bases andsphingoid base 1-phosphates in the root tissues. In addition, there was an increase in leaf lesionsand reduced growth. These results show that under laboratory conditions fumonisin B1 inpotting soil can: i) affect root growth and alter seedling performance, ii) be taken up by roots,iii) cause marked dose-dependent elevation in sphingoid bases and their 1-phosphates that arelikely to contribute to reduced root growth and other symptoms of seedling disease. Thesefindings support the hypothesis that fumonisins are phytotoxins and pathogenicity factors thatcontribute to F. verticillioides maize seedling disease in both synthetic and natural soils.