Active Transport of Lignin Precursors into Membrane Vesicles from Lignifying Tissues of Bamboo

Lignin is the second most abundant natural polymer on Earth and is a major cell wall component in vascular plants. Lignin biosynthesis has three stages: biosynthesis, transport, and polymerization of its precursors. However, there is limited knowledge on lignin precursor transport, especially in monocots. In the present study, we aimed to elucidate the transport mode of lignin monomers in the lignifying tissues of bamboo (Phyllostachys pubescens). The growth manners and lignification processes of bamboo shoots were elucidated, which enabled us to obtain the lignifying tissues reproducibly. Microsomal membrane fractions were prepared from tissues undergoing vigorous lignification to analyze the transport activities of lignin precursors in order to show the ATP-dependent transport of coniferin and p-glucocoumaryl alcohol. The transport activities for both precursors depend on vacuolar type H⁺-ATPase and a H⁺ gradient across the membrane, suggesting that the electrochemical potential is the driving force of the transport of both substrates. These findings are similar to the transport properties of these lignin precursors in the differentiating xylem of poplar and Japanese cypress. Our findings suggest that transport of coniferin and p-glucocoumaryl alcohol is mediated by secondary active transporters energized partly by the vacuolar type H⁺-ATPase, which is common in lignifying tissues. The loading of these lignin precursors into endomembrane compartments may contribute to lignification in vascular plants.