Regulation of Biosynthesis

Lignin biosynthesis is controlled by developmental and environmental signals (Sarkanen and Hergert, 1971; Wardrop, 1981; Sederoff, 1999; Rogers et al., 2005; Jubany-Mari et al., 2008). 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 (Ye et al., 1994; Yhong et al., 1998; Humphreys et al., 1999; Osakabe et al., 1999; Li et al., 2000, 2001; Dixon et al., 2001; Guo et al., 2001; Higuchi, 2003). Higher level controls, such as by miRNAs (Lu et al., 2005; Lim et al., 2005; Ochando et al., 2006), 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). Proteins of LIM (Kawaoka et al., 2000, 2001), MYB (Tamagnone et al., 1998; Dixon et al., 2002; Li et al., 2003; Goicoechea et al., 2005; Ghiladi et al., 2005; Gomal et al., 2008; Zhou et al., 2009), and KNOX (Mele et al., 2003) genes are known positive and negative TF regulators of PAL, C4H, 4CL, CCoAOMT, CCR, AldOMT, CAD, or PO (Figure, 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 (Figure, green dotted lines; Sato et al., 1982; Bolwell et al., 1986, 1988; Mavandad et al., 1990; Osakabe et al., 1999; Humphreys et al., 1999; Blount et al., 2000; Li et al., 2000, 2001) and gene expression (Figure left, red dotted lines; Rohde et al., 2004) have all been implicated in lignin biosynthesis. Genetic perturbation of any one of eight lignin pathway genes examined (Figure, 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 fluxes (Amthor, 2003). 
 
 
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