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Artificial Positive Feedback Loop for Increasing Production of a Biosynthetic Product in Specific Plant Tissues


The JBEI has developed a technology that can be used to fine-tune desirable biomass traits in plants. A key feature of the invention is the design of an artificial positive feedback loop whereby a transcription factor induces increased transcription of itself. Gene promoters are selected according to the desired outcome, for example, to improve saccharification efficiency or to raise the level of desirable hexose sugars in relation to hard-to-ferment pentoses. Some promoters can boost secondary cell wall deposition of cellulose; others can decrease deposition of lignin or hemicellulose (xylan). With similar promoter engineering, increased wax production can be directed to the epidermal layers of a plant, improving drought tolerance and efficient water use while preserving energy for increased production of biomass. This versatile technology can be used to improve crops used for biofuels and paper production; provide livestock with more digestible forage; extend the range of crops to marginal land; or produce stronger timber for construction, among other applications. Unlike other genetic engineering methods, when applied to increasing secondary cell wall deposition, the JBEI technologies alter biosynthesis in plant fibers but not in vascular tissue or leaves. Thus they do not adversely affect growth, fertility, or the fruit- or grain-bearing capacity of the plants. Because this new method involves dominant traits and uses genetic promoters that are part of conserved pathways, it will be applicable across many species, including polyploids and sterile plants. Moreover, its application does not require sequencing of the entire genome of the target plant or the presence of a particular variety or cultivar. To date, the technology has been applied to three applications: 1. Controlling Lignin Deposition: To fine-tune lignin deposition, the scientists started with a mutant of Arabidopsis that under-produces lignin in all tissues. The JBEI scientists then selectively restored lignin biosynthesis to vascular tissue but not fiber cells by expressing a wild-type allele under the regulation of a promoter that is expressed only in vascular tissue. The engineered plants were morphologically and developmentally identical to the wild type, but they had a total lignin level that was approximately 33% less. When tested with several different pretreatment methods, biomass from the engineered plants had a saccharification efficiency 1.5-2.3 times greater than that of wild types. 2. Controlling Xylan Deposition: Using a method similar to that described above, the scientists started with a mutant with a defective allele for a key gene in xylan biosynthesis. They then selectively restored expression of a normal allele to vascular tissue only. The resulting plants have a reduced amount of hemicellulose relative to cellulose. Thus, compared to wild types, these plants can be pretreated more easily for biofuel production, yield more glucose per unit of biomass, and produce fewer low-value byproducts such as pentose from biofuels production or black liquor from pulping. 3. Increasing Wood Density and Drought Tolerance: In this application, promoters are used in a positive feedback loop to increase traits such as wood density or drought tolerance. To boost wood density, JBEI scientists upregulated a transcription factor that induces the expression of genes involved in secondary cell wall synthesis in native tissues. This upregulation occurs only in fiber cells and in a manner that does not interfere with growth, cell expansion, or nutrient transport. When this technology was combined with the fine-tuning of lignin deposition, stem density was increased by almost 20% and the saccharification efficiency was two- to three-times greater than that of wild types. While boosting yields, the technology can also decrease the cost of transporting biomass from the field to the biorefinery.
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