Toward the discovery of maize cell wall genes involved in silage quality and capacity to biofuel production.

Silage maize is currently, with grazing, the basis of cattle feeding. In the near future, maize stover and cereal straws will also be a major source of carbohy- drates for sustainable biofuel production. The embedding of cell wall in lignins and the linkages between lignins, p- hydroxycinnamic acids, and arabinoxylans greatly influ- ences cell wall properties, including the enzymatic degradability of structural polysaccharides in animal ru- men or industrial fermenters. Breeding for higher silage quality and biofuel production will thus be based on the discovery of genetic traits involved in each component biosynthesis and deposition in each lignified tissue. Genes involved or putatively involved in the biosynthesis of the grass cell wall were searched for, including pheno- lic compounds, cell wall carbohydrates and regulation factors. While most cytosolic steps of monolignol biosyn- thesis have been identified, most of lignin pathway genes belong to small multigene families which were all identi- fied based on data available in the Maize Genomics Se- quencing Project (MGSP) database. Cell wall carbohydrate genes were identified based on their Arabidopsis or- thologs and previous research in C3 grasses by MITCHELL et al. (2007). Transcription and regulation factors of cell wall genes were similarly identified based their orthologs described both in Arabidopsis and woody species. All these genes were mapped in silico considering their phys- ical position in the MGSP database. Physical positions of previously described QTL for cell wall degradability, lignin and p-hydroxycinnamic acid contents were also searched for based on the position of the flanking marker in the MGSP database and distances between QTL and flanking markers. While only a few lignin pathway genes mapped to QTL positions, several colocalizations were shown between QTL and transcription factor physical po- sitions. This last result is in agreement with expression studies which highlighted that several genes in the lignin pathway are simultaneously under-expressed in lines with higher cell wall degradability, likely corroborating an up- stream regulation rather than co-regulation phenomena. However, none of these possible candidate genes have yet been validated and many QTL still do not have rele- vant candidates. A lot of relevant candidate genes are still to be discovered among those involved in lignin pathway gene regulation, in regulation of lignified tissue assembly, and in cell wall carbohydrate biosynthesis and deposition. In addition, in spite of their critical involvement in maize cell wall assembly and degradability, genes involved in ferulic acid biosynthesis and linkages with other cell wall components are little known.