Department of Botany, University of British Columbia, Vancouver, BC, Canada
Annually, trees deposit billions of tons of cellulose and lignin, macromolecules essential to the strength of the plant vascular system. The dynamic cellular mechanisms underlying cellulose and lignin deposition in secondary walls have remained elusive, as the xylem cells are deep inside developing plant tissues. Using an inducible master transcription factor controlling xylem cell fate, the processes of cell wall synthesis, such as cellulose deposition, can be directly examined in developing xylem cells with live-cell imaging. The high density, rapid velocity, and spatial concentration of cellulose synthase complexes in the cell membrane accounts for the rapid and spatially well defined secondary cell wall production. In addition to polysaccharides such as cellulose, the polyphenolic polymer lignin is essential for maintaining the structural integrity and, therefore, the function of vascular cells. Lignin precursors, monolignols, are made in the cytoplasm and exported by an unknown mechanism to the cell wall, where they are oxidized by enzymes such as laccases and peroxidases, leading to random combinatorial coupling into the lignin polymer. Using plants that overproduce monolignols, we have tested candidate ATP-binding cassette (ABC) transporters in monolignol export, as well as testing the effect of overexpression of laccases. These data challenge current paradigms of active transport, and support a model of monolignol export where laccase-mediated polymerization in the cell wall creates a concentration gradient leading to monolignol export by diffusion.