Image courtesy of Clint Chapple, Purdue University
Disruption of Mediator rescues growth and fertility in Arabidopsis low lignin mutants. The dwarf phenotype of the lignin-deficient ref8-1 mutant and restoration of near-normal growth of the med5a/5b ref8-1 triple mutant is evident.
By modifying a key regulatory protein complex in plants known as Mediator while simultaneously blocking a key metabolic pathway, researchers produced plants more amenable to cellulosic biofuel production while maintaining normal growth and productivity. Mediator modifications allowed for the structural simplification of an important polymeric component of the plant cell wall called lignin, which in its native form makes the plant cell otherwise resistant to conversion to biofuels.
Simplifying the structure of lignin by modifying both the lignin biosynthetic pathway and Mediator may reduce or eliminate the need for costly pretreatment processes that currently make renewable biofuels derived from bioenergy crops and crop residues more costly than biofuels derived from corn. It also opens up the possibility of using the simplified lignin, or components from it, as a new source material for fuels and other useful chemicals.
Plant biomass, generally referred to as lignocellulose, represents a large source of stored carbon with potential to become an economically viable source of renewable fuels and chemicals. The two major types of materials in lignocellulose are polymerized sugars called polysaccharides (such as cellulose, xylans, pectins, and others) and polymerized aromatic compounds called phenylpropanoids, known as lignins. The three major types of lignin—S, G, and H lignin—are classified based on their specific phenylpropanoid composition. Lignin associated with wall polysaccharides confers strength and rigidity to the plant, allowing it to grow normally. Generally, the more lignin present—especially the more structurally complex G and S forms—the more difficult it is to access and convert the polysaccharides to fuels and other useful materials. Previous strategies to disrupt lignin biosynthesis to improve forage and bioenergy crops have resulted in plants with stunted growth and developmental abnormalities. This DOE-funded research showed that the stunted growth, or dwarf phenotype, of a lignin-deficient Arabidopsis mutant known as ref8 is dependent on a protein complex called Mediator that co-regulates gene transcription. Surprisingly, removing Mediator restored normal growth of the ref8 dwarf plants. Analysis of the plant cell walls from these “restored variants” showed they contained only H lignin, with no G or S lignins normally found in Arabidopsis. Furthermore, the cellulose of these variants could be more easily converted into its component sugars without the need for pretreatment – potentially reducing costs for utilizing these materials. Focusing on Mediator and similar genetic modifications for altering plant productivity and structure opens up new possibilities for improving biomass crops for biofuel production.
Department of Biochemistry, Purdue University
This work was primarily funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DOE) through Grant DE-FG02-07ER15905. Additional funding was derived from a post-doctoral fellowship from the Life Sciences Research Foundation, the U.S. DOE Great Lakes Bioenergy Research Center (DOE BER Office of Science DE-FC02-07ER64944), the Center for Direct Catalytic Conversion of Biomass to Biofuels (C3Bio), an Energy Frontier Research Center funded by the U.S. DOE, Office of Science, Office of Basic Energy Sciences, Award number DE-SC0000997, U.S. DOE through Grant DE-FG02-06ER64301, and the Purdue University Office of Agricultural Research Programs.
Bonawitz, N. D. et al. Disruption of Mediator rescues the stunted growth of a lignin-deficient Arabidopsis mutant. Nature, in press. [DOI:10.1038/nature13084].
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