Seminars

The survival of land plants is dependent upon transport of water and minerals through the xylem tracheary elements and upon fibers to provide structural support.  To strengthen and maintain the continuous network of xylem during development, tracheary elements and fibers deposit thick secondary cell walls, which are reinforced with lignin.  During xylem development, rearrangements of the cytoskeleton and the endomembrane system support the production of polysaccharide-rich secondary cell wall thickenings.  Using live cell imaging of cells undergoing secondary cell wall formation and rapid-freezing to prepare these cells for electron microscopy, we can follow these developmental events in high resolution. Following polysaccharide secretion, lignin is synthesized from monolignols, which in turn are derived from phenylalanine precursors. Monolignols are secreted out of the cell by an unknown mechanism and subsequently polymerized into lignin by peroxidative dehydrogenation in t
he seco
ndary cell wall.  These lignin precursors can be localized during lignification using a combination of feeding radioactive precursors to lignifying cells, cryofixation of developing xylem and autoradiography on light and electron microscopy sections. Elucidation of transport mechanisms, including characterization of putative monolignol ABC transporters and assessment of the contribution of neighboring cells, is currently underway. The removal of lignin from the cellulose of the cell wall has been identified as a barrier to enzymatic degradation of cellulosic feedstock for biofuels, so there is strong interest in understanding lignified secondary cell walls.