RNA Modulation of Chlorogenic Acid and Lignin Deposition in and Insufficient Compensatory Metabolic Cross-Talk.

Chlorogenic acid (CGA) and guaiacyl/syringyl (G/S) lignin formation involves hydroxycinnamoyl ester intermediacy, the latter formed via hydroxycinnamoyl CoA:shikimate hydroxycinnamoyl transferase (HCT) and hydroxycinnamoyl CoA:quinate hydroxycinnamoyl transferase (HQT) activities. and RNA silencing of a commercial tobacco () K326 line was examined herein. gene silencing gave relatively normal plant phenotypes, with CGA levels reduced (down to 1% of wild type) with no effects on lignin. RNA silencing had markedly adverse phenotypes (e.g., stunted, multiple stems, delayed flowering, with senescence delayed by several months). Lignin contents were partially lowered, with a small increase in cleavable hydroxyphenyl (H) monomers; those plants had no detectable CGA level differences relative to wild type. NtHCT kinetic parameters revealed preferential -coumaroyl CoA and shikimate esterification, as compared to other structurally related potential acyl group donors and acceptors. In the presence of coenzyme A, NtHCT catalyzed the reverse reaction. Site-directed mutagenesis of NtHCT (His153Ala) abolished enzymatic activity. NtHQT, by comparison, catalyzed preferential conversion of -coumaroyl CoA and quinic acid to form -coumaroyl quinate, the presumed CGA precursor. In sum, metabolic pathways to CGA and lignins appear to be fully independent, and previous conflicting reports of substrate versatilities and metabolic cross-talk are resolved.