EVIDENCE FOR A FUSARIUM VERTICILLIOIDES SEEDLING PATHOGENICITY FACTORS: ALL ROADS TRAVELED LEAD TO FUMONISIN.

Authors: Glenn, Anthony - Tony; Williams, Lonnie; Riley, Ronald

Submitted to: Mycopathologia
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2002
Publication Date: 1/1/2003
Citation: Glenn, A.E., Williams, L.D., Riley, R.T. 2003. Evidence for a fusarium verticillioides seedling pathogenicity factors: all roads traveled lead to fumonisin. Mycopathologia. Preface. p. 409.

Interpretive Summary: Infection of corn by the fungus Fusarium verticillioides poses a risk to animal and human health due to the production of toxins by the fungus. These toxins, known as fumonisins, can contaminate corn and corn products and cause severe diseases in animals, including possibly humans. Corn and F. verticillioides have a very close association. Throughout the world, corn is infected with this fungus, which hampers attempts to control or prevent infection of corn crops or to limit the contamination of corn with fumonisins. The fungus often infects the plant without any symptoms of disease, yet under some conditions plant disease can occur. We are interested in what factors contribute to symptomless infection of corn and also the factors that result in expression of disease symptoms. We have previously reported that F. verticillioides produces an apparent seedling pathogenicity factor that results in necrotic leaf lesions and abnormal development. Genetic analysis of field isolates indicated a single genetic locus segregated for ability to cause disease. Strains carrying the non-pathogenic allele did not cause any disease symptoms, yet still infected and colonized the corn seedlings. We have also utilized mutant strains that were greatly attenuated in their ability to infect corn seedlings yet nonetheless caused severe disease symptoms, suggesting the pathogenicity factor may be a translocated toxic compound. Further analysis of the pathogenic strains indicated only they produced fumonisins since only they had the necessary genes responsible for fumonisin production. To test whether fumonisins were responsible for the disease symptoms, seedlings were watered with a solution of fumonisin B1 (FB1). Symptoms of toxicity were seen on those seedlings watered with FB1 at the 10 ppm concentration. Thus far, data suggest that FB1 may be the toxin causing the seedling disease symptoms, and that production of FB1 may be a F. verticillioides pathogenicity factor. Further experiments will examine the dynamics of FB1 production in soils as well as the plant and fungal genes differentially expressed under the various symptomatic and asymptomatic associations between F. verticillioides and corn.

Technical Abstract: We have previously reported that Fusarium verticillioides produces an apparent seedling pathogenicity factor that results in necrotic leaf lesions and abnormal development. Genetic analysis of field isolates indicated a single locus segregated for ability to cause disease. Strains carrying the non-pathogenic allele did not cause any disease symptoms, yet still infected and endophytically colonized the corn seedlings. We have also utilized mutant strains that were greatly attenuated in their ability to infect corn seedlings yet nonetheless caused severe disease symptoms, suggesting the pathogenicity factor may be a translocated phytotoxin. Fumonisin B1 (FB1) production was assessed among the parental and progeny strains and also was found to segregate as a single locus. Linkage between pathogenicity and fumonisin production was supported since only the pathogenic strains produced FB1; non-pathogenic strains did not produce any detectable FB1. PCR targeting FUM1 and FUM9 indicated a possible deletion within the fumonisin biosynthetic gene cluster in non-producing strains, providing further support for the inability of these strains to produce FB1. Watering seedlings with solutions of FB1 showed a stimulatory effect on seedling growth at the 1 ppm concentration, yet showed small leaf lesions and obvious stunting effects on seedling development at the 10 ppm concentration. Thus far, data suggest that fumonisin is the phytotoxin causing the seedling disease symptoms, and that production of FB1 is a F. verticillioides pathogenicity factor. Further experiments will examine the dynamics of FB1 production in soils as well as the plant and fungal genes differentially expressed under the various symptomatic and asymptomatic associations between F. verticillioides and corn.