Secondary cell walls account for the majority of total plant biomass and, as mostly composed of polysaccharides, constitute a promising source of fermentable sugars for the production of biofuels and biomaterials. Understanding lignin metabolism became an important goal as this phenolic polymer is recognized as a major bottleneck that hinders the efficient conversion of plant biomass into biofuels and bioproducts in the biorefinery. Despite recent advances, little is known about the transport of monolignols (i.e. lignin monomers) from cytoplasm, where they are synthesized, to the apoplast, where they are oxidized prior to their incorporation into the polymer. So far, only one gene coding for a transporter of a specific monolignol has been functionally characterized in the eudicot model plant Arabidopsis thaliana, belonging to the family of ABC transporters. This major knowledge gap is even more significant in grasses, an economically important group of plants that show specific features regarding lignin structure and composition that are not found in eudicot species. Here, we aim to identify members of the ABC transporters family that are potentially involved in lignin deposition in the model grass Setaria viridis. Accordingly, a three-phase transcriptomic analysis will be employed: I) co-expression analysis using the PLANET plastform; II) analysis of the RNAseq dataset of the single elongating internode of S. viridis; and III) via RT-qPCR using plant tissues contrasting for their lignin content. Genes that are co-expressed with lignin biosynthetic genes, with an expression pattern compatible with lignin deposition in S. viridis internode and with a high expression in tissues undergoing active lignification will be further analyzed I) for their control by master regulators of secondary cell wall deposition, using transactivation assays in S. viridis protoplasts, and II) for their tissue-specific expression pattern using in situ hybridization, to check whether their expression occurs in cell types undergoing lignification. The identification of transporters involved in lignin deposition in S. viridis will contribute not only to fill a knowledge gap on phenolic metabolism in plants but will potentially allow furture biotechnological strategies to produce optimized crops for the biorefinery.
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