Response of sorghum stalk pathogens to brown midrib plants and soluble phenolic extracts from near isogenic lines

Authors: Funnell-Harris, Deanna; O`Neill, Patrick; Sattler, Scott; Gries, Tammy; Berhow, Mark; PEDERSEN, JEFFREY - Retired ARS Employee

Submitted to: European Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2017
Publication Date: 1/18/2017
Publication URL: http://handle.nal.usda.gov/10113/5729153
Citation: Funnell-Harris, D.L., O'Neill, P.M., Sattler, S.E., Gries, T.L., Berhow, M.A., Pedersen, J.F. 2017. Response of sorghum stalk pathogens to brown midrib plants and soluble phenolic extracts from near isogenic lines. European Journal of Plant Pathology. 148(4):941-953. doi:10.1007/s10658-017-1148-2.

Interpretive Summary: Sorghum is a promising potential feedstock for ethanol production from biomass. However, lignin, a component of plant cell walls, can prevent efficient conversion of biomass into sugars for ethanol production. Reducing lignin increases conversion efficiency, but lignin is important for plant defenses against pathogens. This study investigated fungi that cause the stalk diseases Fusarium stalk rot and charcoal rot in sorghum plants that have mutations in two genes required to make lignin. The bmr6 and bmr12 plants have reduced levels of lignin in their cell walls. In this study, plants with single mutations, bmr6 or bmr12 and plants with two mutations, bmr6 bmr12 were shown to respond similar to the normal plants, when infected with three stalk pathogens. The bmr plants were even more resistant to one of the pathogens, called Fusarium proliferatum, than the normal plants. Surprisingly, another stalk pathogen, Fusarium thapsinum, grew faster under conditions containing the compounds extracted from bmr6 stalks than when grown on compounds from normal stalks. Therefore, other factors contribute to increased resistance in bmr6 plants than compounds that can be extracted from stalks.

Technical Abstract: Sorghum [Sorghum bicolor (L.) Moench] has drawn attention as potential feedstock for lignocellulosic biofuels production, and reducing lignin is one way to increase conversion efficiency. Little research has been previously conducted to assess the response of reduced lignin sorghum lines to the Fusarium stalk rot pathogens Fusarium verticillioides and Fusarium proliferatum and the charcoal rot pathogen, Macrophomina phaseolina. Loss of function mutations in either the Brown midrib (Bmr) 6 or 12 gene that both encode a monolignol biosynthetic enzyme in the pathway that produces subunits of the lignin polymer, results in reduced lignin content. Near-isogenic bmr6, bmr12, and bmr6 bmr12 lines had previously been developed, which were shown to have significantly reduced lignin content and increased levels of soluble phenolics. In the current study, these lines in two backgrounds were shown to not be more susceptible to F. verticillioides, F. proliferatum and M. phaseolina inoculations, and some bmr lines exhibited increased resistance to F. proliferatum and M. phaseolina, compared to wild-type lines. When the Fusarium stalk rot pathogen, Fusarium thapsinum, was grown on methanol soluble stalk extracts from bmr6 and wild-type plants, it grew significantly faster on medium with bmr6 extract than on wild-type extract or controls. This result suggested that factors other than soluble phenolics from the extract, such as cell wall bound phenolics or inducible defense compounds, contributed to increased resistance observed in bmr6 plants.