Suppression of Chorismate Mutase 1 in Hybrid Poplar to Investigate Potential Redundancy in the Supply of Lignin Precursors

Chorismate is an important branchpoint metabolite in the biosynthesis of lignin and a wide array of metabolites in plants. Chorismate mutase (CM), the enzyme responsible for transforming chorismate into prephenate, is a key regulator of metabolic flux towards the synthesis of aromatic amino acids and onwards to lignin. We examined three CM genes in hybrid poplar (<i>Populus alba</i> × <i>grandidentata; P39</i>, abbreviated as <i>Pa×g</i>) and used RNA interference (RNAi) to suppress the expression of <i>Pa</i>×<i>g</i>CM1, the most highly expressed isoform found in xylem tissue. Although this strategy was successful in disrupting <i>Pa×g</i>CM1 transcripts, there was also an unanticipated increase in lignin content, a shift towards guaiacyl lignin units, and more xylem vessels with smaller lumen areas, at least in the most severely affected transgenic line. This was accompanied by compensatory expression of the other two CM isoforms, <i>Pa×g</i>CM2 and <i>Pa×g</i>CM3, as well as widespread changes in gene expression and metabolism. This study investigates potential redundancy within the CM gene family in the developing xylem of poplar and highlights the pivotal role of chorismate in plant metabolism, development, and physiology.