Overview – glycobiology
Our lab loves sugars! Not the kind found in candy though (well, not while we are in the lab). Instead we’re interested in how the plant makes and uses complex multi-unit sugar chains (polysaccharides). In particular, we explore the plant cell wall – a structure that surrounds every plant cell, and acts like a skeleton. It provides structural support to the plant, and protection from the environment and predators. The plants cell wall is also really important to humans. It forms the majority of what we call plant “biomass”, and it provides us with dietary fiber, animal feed, fuel, building materials, paper, fabric (cotton) etc. It is also a carbon-sink, since it is a store of carbon fixed during photosynthesis.
Some of the questions we are asking include “how is the plant cell wall made?”, “can we engineer a better cell wall?”, “what else does the plant do with complex sugars?”, “do plants with altered cell walls interact differently with the environment?”.
Within the Joint Bioenergy Institute (JBEI), our goal is to make lignocellulosic biofuels and biochemicals (fuels and chemicals made from plant biomass as a starting feedstock) an economic and environmentally sustainable possibility. It is a complex network of polysaccharides, lignin and proteins that makes up the majority of plant biomass. Our task is to make it easier and more efficient for chemical engineers to deconstruct the cell wall into simple sugars. The sugars will be fed to yeast and other micro-organisms to be fermented into ethanol, methyl-ketones, diesel, rocket fuel, plastics… whatever you and the synthetic biologists can imagine!
JBEI is a DOE bioenergy research center (BRC) funded by the Department of Energy (DOE).
Our current research uses sorghum and switchgrass (promising bionergy crops in the USA), as well as the model plants Arabidopsis and rice.
Some ongoing projects in the lab:
Increase the ratio of C6:C5 sugars
C6 (hexose) sugars are more easily fermented than C5 (pentose) sugars. Cellulose (the major polysaccharide in the cell wall) is composed of only the C6 sugar glucose, albeit in a difficult to access crystalline form. In the angiosperms (flowering plants, which include most crops), the next most abundant polysaccharide, xylan, is composed of C5 sugars (xylose, arabinose) and uronic acids. However, in gymnosperms such as pine trees, the next most abundant polysaccharide is mannan.
Mannan is composed entirely of C6 sugars (mannose, glucose and galactose in varying quantities). It is also found in angiosperms, but at very low levels (<5% of biomass). We would like to increase the mannan content of the plant cell wall, and test whether we can functionally replace xylan with mannan.
To do this, we are trying to understand the mannan biosynthetic pathway using a systems approach. This includes testing mannan synthases (CSLAs) from different species, increasing the pool of nucleotide sugar substrates (pyrophosphorylases, nucleotide sugar transporters) and non-enzymatic protein interactions.
Linking plasma membrane structure with cell wall biosynthesis
Cellulose is synthesized at the plasma membrane, whilst other hemicelluloses and pectins are synthesized in the Golgi. This process is somehow controlled to produce a functional cell wall.
Glycosylinositolphosphoceramides (GIPCs) are sphingolipds which are found in the outer leaflet of the plasma membrane. They have a sugar headgroup which points into the apoplastic space. GIPCs are glycosylated in the Golgi.
To be continued…