Submitted to: Multicrop Aflatoxin and Fumonisin Elimination and Fungal Genomics Workshop-The Peanut Foundation
Publication Type: Proceedings
Publication Acceptance Date: October 1, 2007
Publication Date: December 31, 2007
Citation: Glenn, A.E. 2007. Panel discussion: Fumonisin Elimination. In: Proceedings of the 2007 Annual Multi-Crop Aflatoxin/Fumonisin Elimination and Fungal Genomics Workshop, October 22-24, 2007, Atlanta, Georgia. p. 42. Technical Abstract: Summary of Presentations: The session was moderated by Scott Averhoff of the Texas Corn Producers Board. Ron Riley and Anthony Glenn from the USDA-ARS Toxicology & Mycotoxin Research Unit in Athens, GA presented a set of reports on the effects of fumonisin on corn seedling development and disease. Fumonisins were shown to accumulate and disrupt ceramide biosynthesis in susceptible corn seedlings, resulting in elevated concentrations of sphingoid bases and their 1-phosphates in plant tissues. The accumulation of these sphingoid bases may represent the basic cellular mechanism of action resulting in inhibited seedling development and disease. Resistant corn genotypes accumulated much less fumonisin in their tissues and had significantly less disruption of ceramide biosynthesis. Studies of seedling disease associated with fumonisin production were facilitated by a population of Fusarium verticillioides from banana which did not produce fumonisins due to a unique deletion of the fumonisin biosynthetic gene cluster. Through transformation-mediated complementation of the gene cluster deletion, molecular genetic evidence was presented indicating that fumonisin-nonproducing strains are not pathogenic on susceptible corn seedlings while fumonisin-producing strains cause necrotic leaf lesions, stunting, and even death. Daren Brown, representing the USDA-ARS Mycotoxin Research Unit in Peoria, IL, then presented phylogenetic data examining the polyketide synthase (PKS) genes from F. verticillioides, F. graminearum, F. oxysporum, and F. solani and specifically addressed the question of whether the distribution of conserved PKS genes might reflect pathogen specialization versus general housekeeping functions. He also presented data indicating that F. verticillioides exhibits a significant degree of alternative splice forms (ASFs) among its mRNA transcripts (~4% of genes). The significance of these ASFs is the subject of additional studies since they could possibly result in multiple functional proteins from the same gene. Summary of Panel Discussion: A main theme within some of the questions concerned the possible selective advantages of fumonisin production and ASFs. What advantage would F. verticillioides gain from maintaining the fumonisin gene cluster and biosynthetic pathway? Dr. Glenn responded that since fumonisin-nonproducing strains are rarely isolated from field samples of corn, this would suggest a selective advantage for fumonisin production, which could include effects on host plant tissues that facilitate infection or nutrient acquisition or even microbial antibiosis to limit niche competition. A contrast was made to Aspergillus flavus since aflatoxin-nonproducing strains are readily isolated from field samples, thus suggesting that aflatoxin may not provide a significant advantage to the life cycle of A. flavus. A similar question of selective advantage was posed to Dr. Brown regarding ASFs. He responded that since ASFs are not specific to any particular class or functional group of genes, their function or advantage wasn’t clearly evident. They may provide a level of temporal or developmental specificity or functionality for the expression of particular genes and therefore may represent another mechanism of gene regulation. Other general questions related to disease pathology, including what is the nominal dose of fumonisin B1 (FB1) for observable effects on corn seedlings? Dr. Riley responded that dose response experiments indicated that 1-5 ppm of FB1 resulted in measurable effects on plant development and increases in phytosphingosine, sphinganine, and their respective 1-phosphates due to disruption of ceramide biosynthesis. He stressed the importance of elevated concentrations of sphingoid bases and sphingoid base 1-phosphates in cells due to their role in numerous cell signaling pathways. The 1-phosphates in particular are emerging as important mediators of cell signaling in eukaryotic cells, including the signal-transduction pathway controlling guard cell turgor regulation and closure of stomatal apertures. Thus, perturbation of ceramide biosynthesis could effect the growth and development of corn plants even if disease symptoms are not evident.