Proton Gradient-Dependent Transport of p-Glucocoumaryl Alcohol in Differentiating Xylem of Woody Plants

Lignin is a cell wall component of vascular plants crucial for survival in terrestrial environments. While p-hydroxyphenyl lignin is minor, it is considered to be localised in the outermost part of the cell wall providing strong adhesion between cells, which determines cell shape. Transport of the lignin precursor from the cytosol to the cell wall is critical to regulate temporal and spatial lignin deposition; however, little information on the transport step is available. Here, we report transport activity of p-glucocoumaryl alcohol, a precursor of p-hydroxyphenyl lignin, in a broad-leaved tree (hybrid poplar, Populus sieboldii x P. grandidentata) and a coniferous tree (Japanese cypress, Chamaecyparis obtusa). Membrane vesicles of both trees were prepared from differentiating xylem with vigorous lignification and used for transport assays. Several inhibition assays indicated that not ABC transporters but the proton gradient and V-ATPase are involved in p-glucocoumaryl alcohol transport depending on ATP. These results support the hypothesis that p-glucocoumaryl alcohol is loaded into the secretory vesicles and delivered to the cell wall by exocytosis. Furthermore, this transport mechanism was common in both poplar and Japanese cypress, strongly suggesting that p-glucocoumaryl alcohol transport in the differentiating xylem is conserved within woody plants.