Toward the identification of class III peroxidases potentially involved in lignification in the model C4 grass Setaria viridis

Class III peroxidases are key enzymes involved in the last step of lignin deposition, the oxidation of monolignols prior to polymerization. These enzymes belong to large multigene families with functionally redundant members, but not all of them play a role in the lignification process. Here, we characterized the class III peroxidase gene family in the model C4 grass Setaria viridis and applied a pipeline based on expression analyses to identify family members potentially involved in lignin polymerization. A total of 154 genes encoding class III peroxidases ( SvPRX1 to SvPRX154 ) were found in the S. viridis genome, and 70% of these genes were involved in tandem duplication events. Five SvPRX genes ( SvPRX49 , SvPRX58 , SvPRX119 , SvPRX122 , and SvPRX149 ) were identified as potential candidates to play a role in lignification based on (1) similar expression pattern to that of secondary cell wall (SCW)-related genes in a public transcriptomic database, (2) similar expression pattern to that observed for lignin biosynthetic genes in the S. viridis elongating internode, and (3) high expression in S. viridis tissues considered key-lignification sites. The promoter region of all five genes contained SCW-related cis elements, suggesting that their expression is controlled by the same regulatory network that controls lignin deposition and further supporting a role for these genes in lignin polymerization. Comparative analysis of the primary structure of the selected SvPRXs indicated that only SvPRX58 exhibits all structural motifs characteristic of syringyl (S) peroxidases, suggesting that this peroxidase can oxidize sinapyl alcohol. Finally, in situ hybridization experiments indicated that both SvPRX49 and SvPRX58 are expressed in cell types undergoing active lignification in S. viridis internodes. The identification of peroxidase genes potentially involved in lignin polymerization in S. viridis might help developing lignin bioengineering strategies for the bioeconomy.