Submitted to: Physiologia Plantarum
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/30/2012
Publication Date: 6/20/2012
Citation: Yan, L., Liu, S., Zhao, S., Kang, Y., Wang, D., Gu, T., Xin, Z., Xia, G., Huang, Y. 2012. Identification of differentially expressed genes in sorghum (Sorghum bicolor) brown midrib mutants. Physiologia Plantarum. 146(4):375-387. Interpretive Summary: Development of renewable energy sources is becoming increasingly necessary due to the exhaustion of fossil fuel reserves. Biomass, all plant and plant-derived materials (i.e. the lignocellulosic matter), is among the most promising types of the renewable energy sources because of the most abundant availability and very low cost. Sorghum is one of the most competitive bioenergy crops for biomass production. However, converting lignocellulosic biomass to ethanol is currently far too expensive to be commercially competitive. One of the major problems lies in the fact that polysaccharides in lignocellulosic biomass are embedded in a complex matrix of lignin, which interferes with the conversion process. Removing lignin from lignocellulosic biomass is very costly step in the ethanol conversion process. In order to achieve cost-effective conversion, the feedstock quality needs to be genetically improved, particularly with reduced lignin concentration and increased cellulose content. The brown-midrib (bmr), a kind of genetic mutation found in sorghum and other grass species, is proven to be associated with reduced lignin content in plant. For instance, lignin concentrations in sorghum bmr mutants were decreased by as much as 51% in mature plant stalk when compared to normal genotypes, which offer the great potential for an improved bioethanol production. This study identified differentially expressed genes in sorghum bmr mutants using a complementary approach combining suppression subtractive hybridiza¬tion (SSH) and cDNA microarray expression profiling. Expression of the genes linked to the reduction of lignin production in bmr sorghum was characterized at the molecular level, which contribute a better knowledge on which genes are involved in lignin biosynthesis and how these genes are regulated in bmr sorghum. Furthermore, these newly identified candidate genes offer the new resources for genetic improvement of lignocellulosic biomass with an improved fiber and whole plant digestibility, leading to effective conversion of biomass to bioenthanol at a low cost.
Technical Abstract: Sorghum (Sorghum bicolor L.), with a high biomass yield and excellent tolerance to drought and low nutrition, has been recommended as one of the most competitive bioenergy crops. Brown midrib (bmr) mutant sorghum with reduced lignin content showed a high potential for the improvement of bioethanol conversion efficiency. To dissect genes involved in the cell wall metabolism resulting in the brown midrib phenotypes and their regulatory mechanisms in sorghum, suppression subtractive hybridization (SSH) combined with cDNA microarray profiling was performed to identify the differentially expressed genes in 13 sorghum bmr mutants. Lignin content of all the bmr mutants detected was significantly reduced when compared with wild type plant BTx623. A total of 153 differentially expressed genes were identified, of which 43 genes showed up-regulated expression while the other 110 showed down-regulated expression in the bmr mutants. The expression patterns of 12 candidate genes through quantitive RT-PCR confirmed the accuracy of the data from microarray analysis. The expression of L-phenylalanine ammonia lyase (PAL) and cinnamyl alcohol dehydrogenase (CAD) was repressed in the bmr mutants, which might be responsible for the reduction of lignin content in the bmr mutants. However, the expression of another lignin biosynthetic gene cinnamic acid 4-hydroxylase (C4H) was enhanced in the bmr mutants, which might indicate that monolignol biosynthesis from L-phenylalanine might occur by more than one route. Some of the obtained genes will provide new resources for genetic improvement of lignocellulosic biomass with improved bioethanol production.