PI: Björn Sundberg
Staff: Manoj Kumar, postdoc; Lorenz Gerber, PhD student; David Öhman, PhD student; Kjell Olofsson, technician
The amount and texture of cellulose microfibrils in plant cell walls is highly important for growth and development, as well as for the industrial use of plant fibers. Plant cellulose will be critical for future production of not only pulp and paper, but also biofuels and biomaterials. An understanding of cellulose biosynthesis and the control of its texture are important for biotechnological application using this highly abundant polymer. Cellulose microfibrils (CMFs), are crystalline aggregates of linear polymers of D-glucopyranosyl residues, linked in the beta-(1-4) conformation. The polymerisation takes place in rosette complex consisting of cellulose synthases (CesA) bound to the plasma membrane. The CesA proteins seem to be specific for primary and secondary walls. Some other proteins have been identified to be required for proper cellulose biosynthesis, e.g. KORREGAN and COBRA, but their exact function is yet unknown. The angle in which CMF are laid down in primary and secondary walls is strictly patterned. In primary walls CMF angle is important in determining the degree of anisotropic/isotropic expansion. In secondary wood cell walls CMF are normally laid down in helical spirals. It has long been observed that cortical microtubules (MT) are oriented in parallel to the microfibrils. A widely accepted model states that the CMFs angle is guided by MT patterning, although this hypothesis is recently strongly challenged.
Secondary cell walls in Populus xylem cells are composed of an S1, S2 and sometimes an S3 layer, with a transverse microfibril angle and cellulose content of ca 40%. In response to gravity, however, secondary wall biosynthesis on the upper side of leaning stems undergoes a remarkable switch by the formation of tension wood (TW). In TW, the S2 layer is only partly formed, and instead an inner gelatinous cell wall layer is synthesized that is composed of 95% crystalline cellulose and has a microfibril angle that is parallel to the cell axis. TW formation is unique to trees and can be induced experimentally by leaning the stem.
We aim to identify and functionally analyze genes/proteins involved in the biosynthesis and orientation of cellulose microfibrils. We will take advantage of the TW response. High-resolution microarray analysis across the developmental switch from S2 to G layer gives us a unique opportunity to filter out cellulose biosynthesis genes from other wall biosynthesis genes. To circumvent problems with undesirable, or even lethal, effects on plant growth when silencing cellulose biosynthesis we will use a promoter system induced by the TW response. Thus gene silencing will specifically take place in the wood formed after leaning. We will focus on some genes already identified to have a role in cellulose biosynthesis to understand their role for wood properties, but perhaps most exciting will be to target unknown genes identified using the TW response system. Phenotyping will use established platforms at Umeå Plant Science Centre, and technology under development within FuncFiber.