A Proteomic-Based Quantitative Analysis Of The Relationship Between Monolignol Biosynthetic Protein Abundance And Lignin Content Using Transgenic Populus Trichocarpa

As part of a long-term project to develop a predictive model of lignin biosynthesis in the stem differentiating xylem of Populus trichocarpa, we explored the quantitative relationships of gene-specific monolignol pathway proteins and the amount of the lignin polymer. We determined the absolute abundance of monolignol pathway proteins in wild-type (Nisqually-1) and 80 transgenic trees, downregulated for the expression of genes in the monolignol pathway. Total lignin content for wildtype and transgenics ranged from 9.4 to 24.2%. Comparison of protein variation with lignin content showed that the proteins are produced in several-fold excess, suggesting that the monolignol pathway is highly homeostatic. Strong downregulation of xylem-specific PtrPAL2, 4, and 5 showed the most direct relationship with lignin content. These results are consistent with the Predictive Kinetic Metabolic Flux (PKMF) model of Wang et al., based on differential equations of mass action k-inetics. Functional redundancy of multiple genes also moderates gene-specific effects. In addition, targeted production of specific proteins may affect the concentration of nontarget monolignol proteins, suggesting a yet-to-be-described level of coordinated control.