Abstract
BackgroundSorghum (Sorghum bicolor L. Moench) is a rich source of natural phytochemicals. We performed massive parallel sequencing of mRNA to identify differentially expressed genes after sorghum BTx623 had been infected with Bipolaris sorghicola, a necrotrophic fungus causing a sorghum disease called target leaf spot.ResultSeventy-six-base-pair reads from mRNAs of mock- or pathogen-infected leaves were sequenced. Unannotated transcripts were predicted on the basis of the piling-up of mapped short reads. Differentially expressed genes were identified statistically; particular genes in tandemly duplicated putative paralogs were highly upregulated. Pathogen infection activated the glyoxylate shunt in the TCA cycle; this changes the role of the TCA cycle from energy production to synthesis of cell components. The secondary metabolic pathways of phytoalexin synthesis and of sulfur-dependent detoxification were activated by upregulation of the genes encoding amino acid metabolizing enzymes located at the branch point between primary and secondary metabolism. Coordinated gene expression could guide the metabolic pathway for accumulation of the sorghum-specific phytochemicals 3-deoxyanthocyanidin and dhurrin. Key enzymes for synthesizing these sorghum-specific phytochemicals were not found in the corresponding region of the rice genome.ConclusionPathogen infection dramatically changed the expression of particular paralogs that putatively encode enzymes involved in the sorghum-specific metabolic network.
Highlights
Pathogen infection dramatically changed the expression of particular paralogs that putatively encode enzymes involved in the sorghum-specific metabolic network
Pathogen-treated or control sorghum leaves were collected, and 76base-pair reads from mRNAs were sequenced by using Illumina mRNA-Seq technology
As the ratios of unmapped reads were almost the same in the control and infected samples, we considered that few reads were derived from fungal transcripts
Summary
Plants synthesize low-molecular-weight phytoalexins via secondary metabolic pathways to protect themselves from pathogens such as fungi [1]. Phytoalexins of sorghum such as 3-deoxyanthocyanidins first appear in the cells under fungal attack, where they accumulate in cytoplasmic inclusion bodies. Moench) is the fifth most commonly grown cereal in the world and is a rich source of sorghum-specific natural products. In response to pathogen infection, sorghum synthesizes a unique class of flavonoid phytoalexins, namely 3-deoxyanthocyanidins [2,8]. These are structurally similar to anthocyanins except that they lack C-3 hydroxylation. The enzymes required for phytoalexin synthesis have not been fully identified, and the nature of the coordinated gene expression for the production of these enzymes remains to be elucidated
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