Phenylpropanoids following wounding and infection of sweet sorghum lines differing in responses to stalk pathogens

Authors: Khasin, Maya; Bernhardson, Lois; O`Neill, Patrick; Palmer, Nathan - Nate; Scully, Erin; Sattler, Scott; Sarath, Gautam; Funnell-Harris, Deanna

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2023
Publication Date: 2/13/2024
Citation: Khasin, M., Bernhardson, L.F., O'Neill, P.M., Palmer, N.A., Scully, E.D., Sattler, S.E., Sarath, G., Funnell-Harris, D.L. 2024. Phenylpropanoids following wounding and infection of sweet sorghum lines differing in responses to stalk pathogens. Phytopathology. 114:177-192. https://doi.org/10.1094/PHYTO-12-22-0459-R.
DOI: https://doi.org/10.1094/PHYTO-12-22-0459-R

Interpretive Summary: Sweet sorghum is a source of sugars that are used to make molasses syrup, bioethanol, and value-added products. It has advantages over sugarcane in that it can be grown in more areas of the United States. However, sweet sorghum production is threatened by diseases like charcoal rot and Fusarium stalk rot. These diseases are particularly destructive after flowering and can affect sugar quality. The best way to control these stalk rots is by plant resistance. Two sweet sorghum lines, the stalk rot resistant line M81-E and the susceptible line Colman, were selected for a detailed analysis of gene expression to identify candidate resistance genes for breeding purposes and to contribute knowledge that could be useful in other systems. At 13 days after inoculation (DAI), M81-E had shorter lesions in response to charcoal rot, but not to Fusarium stalk rot. An analysis of gene regulation showed that stress-responsive compounds, as well as plant hormones involved in plant defense signaling, were components of early responses to wounding, whereas the cell processes that affect the production of gene products dominated later responses to wounding. M81-E may have a more effective wound response against charcoal rot as evidenced by stronger activation of defense. Genes involved in wound responses are potential sources of resistance to these pathogens.

Technical Abstract: Sweet sorghum [Sorghum bicolor (L.) Moench] lines M81-E and Colman were previously shown to differ in responses to Fusarium thapsinum and Macrophomina phaseolina, stalk rot pathogens that can reduce yields and quality of biomass and extracted sugars. Inoculated tissues were compared for transcriptomic, phenolic metabolite, and enzymatic activity during disease development 3 and 13 days after inoculation (DAI). At 13 DAI M81-E had shorter mean lesion lengths than Colman when inoculated with either pathogen. Transcripts encoding monolignol biosynthetic and modification enzymes were associated with transcriptional wound (control) responses of both lines at 3 DAI. Monolignol biosynthetic genes were differentially coexpressed with transcriptional activator SbMyb76 in all Colman inoculations, but only following M. phaseolina inoculation in M81-E, suggesting that SbMyb76 is associated with lignin biosynthesis during pathogen responses. In control inoculations, defense-related genes were expressed at higher levels in M81-E than Colman. Line, treatment, and timepoint differences observed in phenolic metabolite and enzyme activities did not account for observed differences in lesions. However, generalized additive models were able to relate metabolites, but not enzyme activities, to lesion length, for quantitatively modeling disease progression: in M81-E, but not Colman, sinapic acid levels positively predicted lesion length at 3 DAI when cell wall-bound syringic acid was low, soluble caffeic acid was high, and lactic acid was high, suggesting that sinapic acid may contribute to responses at 3 DAI. These results provide potential gene targets for development of sweet sorghum varieties with increased stalk rot resistance to ensure biomass and sugar quality.