Cellulose is the most abundant biopolymer on earth. The great abundance of cellulose places it at the forefront as a primary source of biomass for renewable biofuels and a variety of efforts are underway to improve cellulose degradation. However, little is known about the mechanism by which plant cells make cellulose. Understanding the complex process of cellulose synthesis will be important for optimizing the use of cellulose as a renewable energy source. Cellulose microfibrils are synthesized at the plasma membrane by hexameric protein complexes, also known as cellulose synthase complexes (CSCs). The only known components of CSCs are cellulose synthase (CESA) proteins, first discovered in bacteria in 1990. The principal investigator recently identified a novel plant gene, CSI1, which associates with CESA complexes and are required for normal cellulose biosynthesis. CSI1, as the first non-CESA proteins associated with CSCs, opens up many opportunities. The successful identification of CSI1 prompts us to further explore molecular genetics and biochemical approaches in identification of additional players in CSCs. The cutting-edge live cell imaging will be used to visualize CSCs in living plant cells and to assess individual components’ function in CSCs. Together with biochemical, molecular genetics, and plant genetics approaches, we will pursue the following objectives aimed to unravel the mystery of cellulose biosynthesis in plants: 1) Identification and characterization of novel components in CSCs. 2) Investigate interactions between minimal components in CSCs. 3) Advance our understanding in assembly, delivery, and regulation of CSCs. Together, these studies will substantially increase our knowledge of how plant cells make cellulose and provide unprecedented perspective that aids to increase the efficiency of biomass-based energy production.
Figure 1 Imaging of CESA complexes (CSCs). (A) Hexameric CSCs, also known as rosettes, are observed by freeze fracture electron microscopy in algae, moss, and vascular plants. Images are adapted from Giddings et al. (Giddings et al., 1980). (B) CSCs are thought to be composed of 36 subunits of three types in vascular plants, with a diameter about 30 nm. (C) In vivo imaging of CSCs in Arabidopsis. CSI1 is the first non-CESA protein co-localized with CSCs. Bar = 5 μM.
Figure 2 Hypothetical schematic
diagram of the trafficking of CSCs to and from the plasma membrane. CSCs are presumably synthesized in
ER and delivered to Golgi for assembly. From Golgi to plasma membrane, delivery
may occur directly from the trans-Golgi network (TGN) or through an
intermediate compartment such as the MASC/SmaCC. At the plasma membrane, CESA interactive
proteins, e.g. CSI1, bridge between CSCs and microtubules and enforce the
co-alignment of newly synthesized cellulose microfibrils and cortical
microtubules. Adapted from Lei et al., 2012.