Rerouting of lignin biosynthetic pathway without reduction of lignin content by inhibition of cytosolic shikimate recycling in transgenic hybrid aspen. Populus tremula x Populus tremuloides strain:T89

In this study, a gene for a shikimate kinase of Helicobacter pylori (HpSK) with a low Michaelis constant against shikimate compared to other known SKs has been selected to express in hybrid aspen. HpSK is one of the most characterized bacterial SKs because of its medical importance and the Km value of HpSK against shikimate is significantly lower than that of a Populus HCT against the same compound. A coding sequence of the gene has been overexpressed in hybrid aspen under the control of xylem-preferential promoter to reduce the level of cytosolic shikimate. The transgenic plants with a successful expression of HpSK exhibited an elevated H unit in lignin as in the HCT-, C3H-, and CSE-downregulated plants. However, no significant difference of lignin content could be detected in the transgenic plants compared to wild type. Our present data indicate that cytosolic shikimate recycling is a crucial step for determination of monomeric composition of lignin rather than for quantitative determination of lignin deposition in cell wall.