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Poster: Biology & Disease Mgmt: Genetics of Resistance


Plant defensins inhibit growth of pathogens in the alfalfa crown rot disease complex
A. SATHOFF (1), D. Samac (2) (1) University of Minnesota, U.S.A.; (2) USDA-ARS, U.S.A.

Alfalfa crown rot is a disease complex that severely limits alfalfa stand density and productivity in all alfalfa-producing areas. Currently, there are no viable methods of control. Plant defensins are small cationic antimicrobial peptides with a conserved signature of cysteines. Defensins have a gamma-core motif, a cluster of positively charged residues, which is essential for antimicrobial activity. The core motifs of five synthetic defensins were tested for antimicrobial activity against the pathogens in the alfalfa crown rot disease complex. In a 96-well microplate, each well contained half strength potato dextrose broth, approximately 2000 spores, and concentrations of defensin peptide up to 30 ug/mL in a total volume of 100 uL. After 48 h of incubation at 25°C in the dark, absorbance of the wells was measured at 595 nm on a microplate reader to quantify the inhibition of fungal growth. The amount of defensin needed to inhibit growth of pathogen strains by 50% (EC50) was calculated. The core motif of MtDef4 was shown to be the most effective peptide with EC50 values of 5.3 uM against Phoma medicaginis and 6.9 uM against Fusarium oxysporum f.sp. medicaginis. In addition, MtDef4 had activity against Pseudomonas syringae pv. syringae and Xanthomonas alfalfa but not the oomycete Aphanomyces euteiches in in vitro assays. These results indicate that transgenic expression of plant defensins in alfalfa has the potential to lead to improved crown rot resistance.