Resistance to stalk pathogens for bioenergy sorghum

Authors: Funnell-Harris, Deanna; Khasin, Maya; Sattler, Scott

Submitted to: Genomic Sciences Program Annual Principal Investigator (PI) Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 2/24/2019
Publication Date: 2/25/2019
Citation: Funnell-Harris, D.L., Khasin, M., Sattler, S.E. 2019. Resistance to stalk pathogens for bioenergy sorghum [abstract]. Genomic Sciences Program Annual Principal Investigator (PI) Meeting.

Interpretive Summary:

Technical Abstract: Sorghum is a promising bioenergy crop with high yield potentials and significant tolerance to both drought and heat. However, under water or heat stress, sorghum is prone to stalk rots, which can significantly limit sorghum biomass yield through growth reductions and lodging. Stalk rot-causing fungi normally grow endophytically (asymptomatically) within sorghum plants. When sorghum plants experience water stress, host changes often trigger a developmental switch causing the fungi to become pathogenic, resulting in decayed stalk tissue. The underlying plant molecular circuits that either limit or exacerbate this fungal transition from endophytic to pathogenic growth are not known and are the focus of this proposal. Several publicly available lines have previously demonstrated resistance or tolerance to sorghum stalk pathogens, including lines with post-flowering drought tolerance (nonsenscence), which appears to suppress pathogenic growth, or a variety of lines that have exhibited increased resistance under field conditions. We have developed several near-isogenic brown midrib (bmr) 6 and 12 lines with altered lignin content and composition, which were previously demonstrated to have increased resistance or tolerance to stalk pathogens (2,3,5). Lignin, a component of plant cell walls, has been a focus for development of bioenergy sorghums because its presence increases recalcitrance of biomass to cellulosic ethanol conversion, but its presence also increases total energy content of biomass, which is important for thermal conversion technologies. The bmr lines have reduced lignin and increased ethanol conversion efficiency (1) due to single mutations in enzymes in the monolignol (lignin subunits) biosynthesis pathway, cinnamyl alchohol dehydrogenase (bmr6) or caffeic acid O-methyltransferase (bmr12). To increase energy content, we have engineered sorghum plants overexpressing a Myb transcription factor that induces synthesis of monolignols and a gene encoding caffeoyl-CoA O-methyltransferase, a monolignol pathway enzyme. Both transgenic and bmr plants accumulate phenolic intermediates from monolignol biosynthesis that inhibit stalk pathogens in vitro (5). We have developed an assay in a controlled environment, with applied water-stress, which reliably induces the developmental switch from endophytic to pathogenic growth of sorghum stalk rot fungi (2). Our research may have identified sources of resistance in bmr6 and bmr12 lines, relative to wild-type, to the stalk rot pathogens, Fusarium thapsinum and Macrophomina phaseolina. We have previously shown that following inoculation of peduncles with each of these fungi, a visible lesion is first apparent at 3 days post inoculation (dpi) and lesion expansion is first apparent at 13 dpi (4). In the current research, mean lesion lengths at 3 dpi were not significantly different between near-isogenic wild-type, bmr6 and bmr12 lines under either well-watered (100% field capacity) or reduced-water (25% field capacity) conditions. However, at 13 dpi, bmr12 had significantly reduced lesion lengths, but only under reduced water, as compared with the wild-type; reductions in mean lesion lengths resulting on bmr6 plants under this condition were not significant. Global gene expression of bmr6, bmr12 and wild-type plants at 3 dpi under both watering conditions was conducted to identify early abiotic and biotic stress response genes. The relative expression profile of infected tissues from the three plant genotypes under both watering conditions suggested common and unique host genetic responses influenced by genotype and watering condition. Gene expression analysis suggested that the reduced water condition primed bmr12 for resistance to infection. Inoculated well-watered bmr12 plants exhibited similar expression profiles to each fungus and to control reduced-water bmr12, but not control well-w