Resistance to stalk pathogens for bioenergy sorghum

Authors: Funnell-Harris, Deanna; O`Neill, Patrick; Sattler, Scott; Khasin, Maya; Scully, Erin

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/25/2018
Publication Date: 2/25/2018
Citation: Funnell-Harris, D.L., Oneill, P.M., Sattler, S.E., Khasin, M., Scully, E.D. 2018. Resistance to stalk pathogens for bioenergy sorghum. Meeting Abstract. [abstract]. In: Proceedings of 2018 Genomic Sciences Program Annual Principal Investigator (PI) Metting, U.S. Department of Energy, February 25-28,2018 Tysons, Virginia.

Interpretive Summary:

Technical Abstract: Sorghum is a promising bioenergy crop with high yield potentials and significant tolerance to both drought and heat. However, 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 within sorghum plants. When sorghum plants experience water stress, host changes often trigger a developmental switch causing the fungi to become pathogenic. 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 sorghum brown midrib (bmr) 6 and 12 lines with altered lignin content and composition, which were previously demonstrated to have increased resistance or tolerance to sorghum stalk pathogens (1,3,4,5). Lignin, a component of plant cell walls, has been a focus for development of bioenergy sorghums because its presence increases recalcitrance of biomass to cellusolic ethanol conversion, but its presence also increases total energy content of biomass, which is important for thermal conversion technologies. To increase energy content, we have engineered sorghum plants overexpressing a Myb transcription factor that induces synthesis of monolignols, the lignin subunits, and a gene encoding caffeoyl-CoA O-methyltransferase, a monolignol pathway enzyme. Both the transgenic and bmr plants accumulate phenolic intermediates from monolignol biosynthesis that inhibit stalk pathogens in vitro (4). We have identified a procedure to determine pathogen survival in lesions and asymptomatic tissues of sorghum peduncles (top of the stalk, below the head; 3) and have recently developed a controlled-environment, water-stress assay, which reliably induces the developmental switch from endophytic to pathogenic growth of sorghum stalk rot fungi. Our recent research may have identified sources of resistance in bmr6 and bmr12 lines, relative to the wild-type, to two 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 (2). In the current research, there were significant differences in mean lesion lengths resulting on bmr6 and bmr12 plants at 13 dpi with each fungus under adequate water or water deficit conditions as compared to wild-type plants with these treatments. In particular, bmr6 plants under the adequate water treatment and both bmr6 and bmr12 plants under water deficiency had significantly smaller mean lesion lengths than wild-type plants after inoculations with M. phaseolina. No significant differences were apparent after inoculations with F. thapsinum under adequate water, but both bmr lines had significantly smaller mean lesion lengths than wild-type under water deficit conditions. Interestingly, bmr12 plants had significantly smaller mean lesion lengths under water deficit than under adequate water when inoculated with either pathogen, counter to expected response to stalk pathogens under water stress. Across both water conditions, bmr12 plants had reduced F. thapsinum survival within lesions. At 3-cm beyond the lesion border, there was reduced pathogen survival in both bmr6 (F. thapsinum and M. phaseolina) and bmr12 (M. phaseolina) plants as compared with wild-type. These results suggest that reduced survival of the pathogens within bmr6 and b