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UC Berkeley Researcher Looks to microRNA to Boost Switchgrass Biofuel Potential

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NEW YORK (GenomeWeb) – Switchgrass, a fast-growing perennial native to North America, has long been the focus of bioenergy research given its ability to grow on lands not suitable for food crops. However, difficulties in extracting the sugars that are fermented to create fuel from the plant have hampered such efforts.

But a microRNA associated with the maturation of plants may hold the key to unlocking the full bioenergy potential of switchgrass. And with support from the US Department of Agriculture, University of California, Berkeley investigator Sarah Hake is testing this theory.

To create biofuel, plant biomass is reduced to monosaccharides through a process called saccharification and then converted into fuels such as ethanol. Plant cell walls, however, are made up of cellulose microfibrils linked together with hemicellulosic tethers, creating a network of matrix polysaccharides and copolymerized with lignin that prevents the enzymatic saccharification of cell wall polysaccharides.

Of the various plants used for biofuel production, switchgrass is particularly valuable given its drought and flood tolerance, its relatively high yield, and its ability to grow with minimal agricultural inputs. But saccharification remains an expensive and time-consuming step for the plant.

As part of an effort to address this issue, Hake and colleagues previously reported on the introduction of a maize gene called Corngrass1 (Cg1) into switchgrass to make its cell wall easier to break down.

According to Hake, Cg1 encodes a miRNA called miR-156 known to target transcription factors involved in, among other things, leaf development and plant architecture. Research has shown that miR-156 is highly expressed in maize during its juvenile phase of development, and then gradually disappears as the plant matures.

As described in a 2011 paper in the Proceedings of the National Academy of Sciences, Hake and colleagues introduced Cg1 into switchgrass. Doing so promoted juvenile traits in the plant, particularly the leaves, and made its tissue easier to saccharify. Further, the modified switchgrass yielded more sugar than its wild-type counterpart.

Still, Cg1 expression had some negative effects, with transgenic plants failing to develop as robust a root system as normal ones, Hake told GenomeWeb this week. As a result, she and her collaborators set out to find a way to limit the effects of Cg1 expression to switchgrass leaves.

As part of a USDA Agricultural Research Service project, Hake is currently developing tissue-specific promoters for miR-156 with the goal of being able to induce miR-156 expression only in switchgrass leaves. This project is set to wrap up this summer.

Meanwhile, Hake and her team are also experimenting with overexpressing miR-156 in other grasses, in part to better understand the miRNA's full spectrum of effects. "Is there a universal response by grasses to this microRNA … or do different grasses respond differently?" she said. "We can say right now, of the six grasses we tried, they all have a pretty common suite of responses."

One response to the miRNA that has only been observed in switchgrass, however, was an absence of flowering. Should this be replicated in other grasses not yet tested, Hake said there might be an opportunity to develop a non-flowering turf that can be used, for instance, on lawns or playing fields where plant growth regulators are currently used.

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