Regulation of Biosynthesis

Lignin biosynthesis is controlled by developmental and environmental signals.^9,68-71 Regulation affecting lignin content, composition and linkage structure is known at the level of (1) transcription factors (TFs), (2) specific expression of gene family members, and (3) control by pathway intermediates.^19-22,26-30 Higher level controls, such as by miRNAs,^72-74 may have an indirect effect that has not yet been validated. Lignin polymerization is irreversible, providing an absolute end point for the pathway. The major flux through the pathway can be described in a linear form (Figure 4). Proteins of LIM^75,76, MYB^77-84 and KNOX⁸⁵ genes are known positive and negative TF regulators of PAL, C4H, 4CL, CCoAOMT, CCR, COMT, CAD, or PO (Figure 4, black dotted lines), affecting transcript abundance, enzyme activity, lignin content or S/G ratio. Enzyme functional redundancy, pathway metabolite mediated feedback and feed-forward regulation of enzyme function^27-30,86-90 (Figure 4, green dotted lines) and gene expression⁹¹ (Figure 4, red dotted line) have all been implicated in lignin biosynthesis. Genetic perturbation of any one of eight lignin pathway genes examined (Figure 4, brown dotted lines) in Populus species alters lignin content or structure, in spite of redundancy, feedback and feed-forward controls that could buffer such perturbations. Maximizing and balancing the production of G and S is an apparent objective of regulated fluxes⁹².