Systems biology characterization of lignin metabolism in C4 grasses: looking for new players

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Abstract: Lignin is a complex phenolic polymer mostly deposited in secondary walls of specialized cell types, especially those found in supportive and water-conducting tissues. Given that lignin is recognized as a limiting factor to biomass-to-products conversion in biorefineries, genetic manipulation of lignin deposition is envisaged to tailor plants with optimized biomass processability. Nevertheless, steering the pathway to either reduce lignin content or to modify lignin structure requires a fundamental knowledge of the phenylpropanoid metabolism, including pathway intermediates, biosynthetic enzymes, transporters and regulators. We applied a systems approach based on untargeted metabolomics and large-scale transcriptomics using xylogenic cultures and elongating internodes to gain insights into the phenolic metabolism in C4 grasses, with a major focus on lignin metabolism. In these model systems, we identified groups of metabolites and clusters of genes whose relative abundance/expression correlated with the dynamics of lignin deposition. Co-expression analyses, together with correlation analyses between key metabolites accumulation and gene expression, allowed us to identify candidate genes to play novel functions in lignin metabolism in C4 grasses. Functional characterization of these candidate genes in the model C4 grass Setaria viridis will allow a deeper understanding of their potential functions in lignification. Concomitantly, we are manipulating the phenylpropanoid pathway to produce high-value compounds in sugarcane to convert these plants into "green factories". We aim to generate novel basic knowledge on lignin metabolism in C4 grasses but also to explore novel strategies for steering the shikimate/phenylpropanoid pathway for industrial purposes.