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Submitted to: Meeting Abstract
Publication Type: Abstract Only Publication Acceptance Date: 2/5/2013 Publication Date: 6/3/2013 Citation: Dowd, P.F., Sattler, S.E. 2013. Reduced lignin sorghum lines for bioenergy conversion can have greater insect resistance as compared to normal lignin lines. Meeting Abstract. xx. Interpretive Summary: Technical Abstract: To reduce our reliance on dwindling petroleum supplies and mitigate global carbon dioxide emissions, the use of renewable biological materials to produce liquid transportation fuels is being examined. In the U.S., fermentation of maize starch has lead to enhanced ethanol production, but this use has driven the price up and impacted other industries that also use maize, such as livestock production. Plant biomass can also be used to produce ethanol or other biofuels, and grass species including maize, switchgrass, Miscanthus and sorghum are being considered. Sorghum has advantages over the other grass species, because seed production and cultivation are well established, and sorghum is more drought tolerant than maize. However, like the other grasses being considered for biomass production, lignin levels in normal lines of sorghum prevent efficient conversion to ethanol. Low lignin sorghum lines have been derived from brown midrib (bmr) mutants, which have reddish or brown midribs of their leaves. Bmr2, bmr6 and bmr12 loci all encode enzymes in monolignol biosynthesis: a 4-coumarate coenzyme A ligase (4CL), a cinnamyl alcohol dehydrogenase (CAD) and a caffeic acid O-methyltransferase (COMT), respectively. The mutants examined are deficient in the corresponding enzymatic activity. The bmr6 and bmr12 have been shown to increased efficiency of saccharification and ethanol conversion. However, lignin has been implicated as an important defense mechanism against insects and plant pathogens, so reduced lignin levels could result in decreased pest resistance that would interfere with sustainable bioenergy feedstock production. We have used combinations of laboratory, small plot and field assays to examine insect resistance of low sorghum lignin lines to the major insect pests corn earworm Helicoverpa zea, and fall armyworm, Spodoptera frugiperda (both leaf feeding insects), and European corn borer, Ostrinia nubilalis (which feeds on leaves and also bores in stalks). In laboratory assays under controlled growth conditions using mutant lines bmr2, bmr6 and bmr12, leaves examined from young plants generally were damaged to an equal or sometimes lower extent than leaves from normal lignin near isogenic line Tx623. Leaves from older plants were often more resistant to feeding by corn earworms and fall armyworms, especially bmr6. Pith from the upper stalks was more toxic to both insect species, causing up to 47% mortality to corn earworms when from bmr6, compared to 6% mortality for pith from normal lignin near isogenic Tx623. In small plot studies, assays of leaf material exhibited results similar to the laboratory studies, which included increased toxicity of the bmr stalk pith. When planted in a 1 acre plot, although insect incidence was similar, bmr6 plants generally had significantly reduced leaf damage and stalk tunneling by endemic populations of European corn borers. Recent results of laboratory assays with transgenic sorghum materials up or down regulating enzymes important in lignin biosynthesis will also be presented and compared to those with mutant lines. |
