Bringing to light the molecular evolution of gux genes in plants

Hemicellulose and cellulose are essential polysaccharides for plant development and major components of cell wall. They are also an important energy source for the production of ethanol from plant biomass, but their conversion to fermentable sugars is hindered by the complex structure of cell walls. The glucuronic acid substitution of xylan (GUX) enzymes attach glucuronic acid to xylan, a major component of hemicellulose, decreasing the efficiency of enzymes used for ethanol production. Since loss-of-function gux mutants of Arabidopsis thaliana enhance enzyme accessibility and cell wall digestion without adverse phenotypes, GUX genes are potential targets for genetically improving energy crops. However, comprehensive identification of GUX in important species and their evolutionary history are largely lacking. Here, we identified putative GUX proteins using hidden Markov model searches with the GT8 domain and a GUX-specific motif, and inferred the phylogenetic relationship of 18 species with Maximum likelihood and Bayesian approaches. Each species presented a variable number of GUX, and their evolution can be explained by a mixture of divergent, concerted and birth-and-death evolutionary models. This is the first broad insight into the evolution of GUX gene family in plants and will potentially guide genetic and functional studies in species used for biofuel production.