A novel bioinformatics approach identifies candidate genes for the synthesis and feruloylation of arabinoxylan

Arabinoxylans (AXs) are major components of graminaceous plant cell walls, including those in the grain and straw of economically important cereals. Despite some recent advances in identifying the genes encoding biosynthetic enzymes for a number of other plant cell wall polysaccharides, the genes encoding enzymes of the final stages of AX synthesis have not been identified. We have therefore adopted a novel bioinformatics approach based on estimation of differential expression of orthologous genes between taxonomic divisions of species. Over 3 million public domain cereal and dicot expressed sequence tags were mapped onto the complete sets of rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana) genes, respectively. It was assumed that genes in cereals involved in AX biosynthesis would be expressed at high levels and that their orthologs in dicotyledonous plants would be expressed at much lower levels. Considering all rice genes encoding putative glycosyl transferases (GTs) predicted to be integral membrane proteins, genes in the GT43, GT47, and GT61 families emerged as much the strongest candidates. When the search was widened to all other rice or Arabidopsis genes predicted to encode integral membrane proteins, cereal genes in Pfam family PF02458 emerged as candidates for the feruloylation of AX. Our analysis, known activities, and recent findings elsewhere are most consistent with genes in the GT43 families encoding b-1,4-xylan synthases, genes in the GT47 family encoding xylan a-1,2- or a-1,3-arabinosyl transferases, and genes in the GT61 family encoding feruloyl-AX b-1,2-xylosyl transferases.