Switchgrass ( L.) promoters for green tissue-specific expression of the transcription factor for reduced-recalcitrance transgenic switchgrass.

Background: Genetic engineering of switchgrass ( L.) for reduced cell wall recalcitrance and improved biofuel production has been a long pursued goal. Up to now, constitutive promoters have been used to direct the expression of cell wall biosynthesis genes toward attaining that goal. While generally sufficient to gauge a transgene's effects in the heterologous host, constitutive overexpression often leads to undesirable plant phenotypic effects. Green tissue-specific promoters from switchgrass are potentially valuable to directly alter cell wall traits exclusively in harvestable aboveground biomass while not changing root phenotypes.

Results: We identified and functionally characterized three switchgrass green tissue-specific promoters and assessed marker gene expression patterns and intensity in stably transformed rice ( L.), and then used them to direct the expression of the switchgrass () transcription factor gene in transgenic switchgrass to endow reduced recalcitrance in aboveground biomass. These promoters correspond to photosynthesis-related light-harvesting complex II chlorophyll-a/b binding gene (), phosphoenolpyruvate carboxylase (), and the photosystem II 10 kDa R subunit (). Real-time RT-PCR analysis detected their strong expression in the aboveground tissues including leaf blades, leaf sheaths, internodes, inflorescences, and nodes of switchgrass, which was tightly up-regulated by light. Stable transgenic rice expressing the reporter under the control of each promoter (756-2005 bp in length) further confirmed their strong expression patterns in leaves and stems. With the exception of the serial promoter deletions of , all marker patterns under the control of each 5'-end serial promoter deletion were not different from that conveyed by their respective promoters. All of the shortest promoter fragments (199-275 bp in length) conveyed strong green tissue-specific expression in transgenic rice. is a master repressor of lignin biosynthesis. The green tissue-specific expression of via each promoter in transgenic switchgrass led to significant gains in saccharification efficiency, decreased lignin, and decreased S/G lignin ratios. In contrast to constitutive overexpression of , which negatively impacts switchgrass root growth, plant growth was not compromised in green tissue-expressed switchgrass plants in the current study.

Conclusions: Each of the newly described green tissue-specific promoters from switchgrass has utility to change cell wall biosynthesis exclusively in aboveground harvestable biomass without altering root systems. The truncated green tissue promoters are very short and should be useful for targeted expression in a number of monocots to improve shoot traits while restricting gene expression from roots. Green tissue-specific expression of is an effective strategy for improvement of transgenic feedstocks.