Factors affecting expression of soybean sudden death syndrome: Flooding, oxygen level, and ethylene hormone
Soybean [Glycine max (L.) Merrill] sudden death syndrome (SDS), caused by the soilborne fungus Fusarium virguiforme (Fv), is a very damaging disease in North and South America, with average yield losses in the Unites States estimated at 190 million dollars annually between 1999 and 2004. Major SDS outbreaks have coincided with years of extreme flooding, such as 1993, 2008, and 2010, but there is no information about how and why excessive soil moisture is associated with severe SDS. In this study, the first objective was to investigate the effect of different flood regimes on the development of SDS under greenhouse conditions. Flooding was found to influence SDS disease severity and Fv population density in soil, but the overall effect on SDS development depended on duration of the flooding period. Short-term flooding, such as 3 days of continuous flooding or repeated flooding periods of 8 h a week for 3 weeks, generally predisposed soybean seedlings to SDS, whereas continuous flooding for 5 or 7 days resulted in lower SDS severity and lower Fv population in soil, compared to non-flooded controls.
Flooding conditions cause a decrease in oxygen levels and build-up of carbon dioxide and toxic compounds in the root zone. The second objective of this study was to test the effect of low oxygen levels, similar to those that may occur during flooding conditions, on the soybean-Fv interaction. A hydroponic system was established in a growth chamber where soybean seedlings were exposed to different oxygen levels and tested for the expression of soybean defense-related genes and Fv virulence genes using qPCR. Soybean seedlings exposed to anaerobic conditions for 12 hours exhibited down-regulation of key defense-related genes, including laccase and pathogenesis related proteins, All Fv virulence genes tested, including pectate lyase, pisatin demethylase, and FvTox1also showed down-regulation in soybean roots exposed to anaerobic conditions for 12 hours.
Ethylene is a gaseous hormone involved in multiple plant growth and developmental processes, as well as response to biotic and abiotic stresses. The third objective of this study was to understand the role of ethylene hormone in soybean-Fv interaction. Soybean plants treated with ethephon (ethylene releasing compound) developed less severe SDS symptoms compared to water-treated seedlings, whereas those treated with cobalt chloride (ethylene biosynthesis suppressor) or 1-MCP (ethylene perception suppressor) showed the same or higher SDS foliar severity compared to the water treated control. Ethephon application also resulted in activation of genes involved in ethylene biosynthesis, and genes involved in soybean defense responses, such as phenylpropanoid pathway, pathogenesis related proteins and transcription factors. Cobalt chloride and 1-MCP treatments had little or no effect on these genes. Moreover, ethephon had an inhibitory effect on in-vitro growth of Fv on potato dextrose agar.
The fourth objective of this study was to determine the optimum application timing of ethephon to suppress SDS development in greenhouse and field conditions. In the greenhouse, all ethephon applications significantly reduced SDS foliar symptom severity by 50-60% compared to the untreated control in susceptible cultivar Williams82. In field studies conducted in 2015, ethephon application at planting (in-furrow) or after plant emergence (VE growth stage) reduced SDS foliar severity compared to the untreated control. Plots that received ethephon at VE growth stage showed a 15% increase (P=0.08) in yield compared to untreated plots.
Overall, the results of this research enhance understanding of how flooding and the accompanied anaerobic conditions affect SDS development, and demonstrate that the ethylene-signaling pathway plays an important role in resistance against SDS. Finally, our findings suggest that the use of plant defense inducers, such as ethephon, can suppress SDS and encourage future investigation on their use for SDS management under filed conditions.