ABSTRACT
Basic helix-loop-helix (bHLH) genes are important regulators of development in plants. SbbHLH1, a Sorghum bicolor bHLH sequence, was isolated from a suppression subtractive hybridization library constructed using 13 independent brown midrib (bmr) mutants as the tester and wild-type sorghum as the driver. The gene was upregulated in at least five of the mutants at the five- to seven-leaf stage. Using a yeast expression system, the N-terminal portion of SbbHLH1 was shown to be required for proper transactivation. Its heterologous expression in Arabidopsis thaliana markedly reduced the plant's lignin content. It downregulated the lignin synthesis genes 4CL1, HCT, COMT, PAL1, and CCR1, and upregulated the transcription factors MYB83, MYB46, and MYB63. The hypothesis is proposed that SbbHLH1 has stronger effect on the regulation of lignin synthesis than the various MYB transcription factors, with a possible feedback mechanism acting on the MYB transcriptional regulators.
Subject(s)
Arabidopsis/metabolism , Lignin/biosynthesis , Plant Proteins/genetics , Plants, Genetically Modified/metabolism , Sorghum/genetics , Transcription Factors/genetics , Arabidopsis/genetics , Arabidopsis Proteins/genetics , Arabidopsis Proteins/metabolism , Down-Regulation , Gene Expression , Gene Expression Regulation, Plant , Helix-Loop-Helix Motifs , Plant Proteins/chemistry , Plant Proteins/metabolism , Plants, Genetically Modified/genetics , Sorghum/chemistry , Transcription Factors/chemistry , Transcription Factors/metabolismABSTRACT
Sorghum, a species able to produce a high yield of biomass and tolerate both drought and poor soil fertility, is considered to be a potential bioenergy crop candidate. The reduced lignin content characteristic of brown midrib (bmr) mutants improves the efficiency of bioethanol conversion from biomass. Suppression subtractive hybridization combined with cDNA microarray profiling was performed to characterize differential gene expression in a set of 13 bmr mutants, which accumulate significantly less lignin than the wild-type plant BTx623. Among the 153 differentially expressed genes identified, 43 were upregulated and 110 downregulated in the mutants. A semi-quantitative RT-PCR analysis applied to 12 of these genes largely validated the microarray analysis data. The transcript abundance of genes encoding l-phenylalanine ammonia lyase and cinnamyl alcohol dehydrogenase was less in the mutants than in the wild type, consistent with the expectation that both enzymes are associated with lignin synthesis. However, the gene responsible for the lignin synthesis enzyme cinnamic acid 4-hydroxylase was upregulated in the mutants, indicating that the production of monolignol from l-phenylalanine may involve more than one pathway. The identity of the differentially expressed genes could be useful for breeding sorghum with improved efficiency of bioethanol conversion from lignocellulosic biomass.
