Abstract
Brown midrib (bmr) mutants in sorghum (Sorghum bicolor (L.) Moench) and several other C4 grasses are associated with reduced lignin concentration, altered lignin composition and improved cell wall digestibility, which are desirable properties in biomass development for the emerging lignocellulosic biofuel industry. Studying bmr mutants has considerably expanded our understanding of the molecular basis underlying lignin biosynthesis and perturbation in grasses. In this study, we performed quantitative trait locus (QTL) analysis, identified and cloned a novel cinnamyl alcohol dehydrogenase allele (SbCAD2) that has an 8-bp deletion in its 5′-untranslated region (UTR), conferring the spontaneous brown midrib trait and lignin reduction in the sorghum germplasm line PI 595743. Complementation test and gene expression analysis revealed that this non-coding region alteration is associated with the significantly reduced expression of the SbCAD2 in PI 595743 throughout its growth stages. Moreover, a promoter-GUS fusion study with transgenic Arabidopsis thaliana plants found that SbCAD2 promoter is functionally conserved, driving a specific expression pattern in lignifying vascular tissues. Taken together, our results revealed the genetic basis of bmr occurrence in this spontaneous sorghum mutant and suggested the regulatory region of the SbCAD2 can be a target site for optimizing lignin modification in sorghum and other bioenergy crops.
Highlights
Sorghum is one of the premier biomass feedstocks for biofuel production because of its high biomass yield, outstanding drought tolerance and efficient nutrient usage[1]
We found that the function of SbCAD2 promoter is conserved in both sorghum and Arabidopsis and this deletion region is implicated in the transcriptional regulation of SbCAD2 expression in lignifying tissue
These results provided quantitative evidence that lignin biosynthesis was impaired in PI 595743 plants
Summary
Sorghum is one of the premier biomass feedstocks for biofuel production because of its high biomass yield, outstanding drought tolerance and efficient nutrient usage[1]. We used QTL mapping, DNA sequencing and complementation test to identify that an 8-bp deletion in the 5′ UTR of SbCAD2 is responsible for this novel spontaneous bmr phenotype This deletion results in the down-regulation of SbCAD2 expression in PI 595743 in a mechanism that is different from that underlying the reduced expression of SbCAD2 in the bmr[6] lines. We found that the function of SbCAD2 promoter is conserved in both sorghum and Arabidopsis and this deletion region is implicated in the transcriptional regulation of SbCAD2 expression in lignifying tissue We expect that this alternative allele of SbCAD2 gene will expand the repertoire of genetic resources for biomass improvement in sorghum, and more importantly, provide new insights into the complex underlying regulatory mechanism of the lignin biosynthetic pathway in grasses
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