The sucrose synthase gene family in Populus: structure, expression, and evolution

Sucrose synthase is a key enzyme in sucrose metabolism in plant cells, and it is involved in the synthesis of cell wall cellulose. Although the sucrose synthase gene (SUS) family in the model plants Arabidopsis thaliana has been characterized, little is known about this gene family in trees. This study reports the identification of two novel SUS genes in the economically important poplar tree. These genes were expressed predominantly in mature xylem. Using molecular cloning and bioinformatics analysis of the Populus genome, we demonstrated that SUS is a multigene family with seven members that each exhibit distinct but partially overlapping expression patterns. Of particular interest, three SUS genes were preferentially expressed in the stem xylem, suggesting that poplar SUSs are involved in the formation of the secondary cell wall. Gene structural and phylogenetic analyses revealed that the Populus SUS family is composed of four main subgroups that arose before the separation of monocots and dicots. Phylogenetic analyses associated with the tissue- and organ-specific expression patterns. The high intraspecific nucleotide diversity of two SUS genes was detected in the natural population, and the pi (nonsyn)/pi (syn) ratio was significantly less than 1; therefore, SUS genes appear to be evolving in Populus, primarily under purifying selection. This is the first comprehensive study of the SUS gene family in woody plants; the analysis includes genome organization, gene structure, and phylogeny across land plant lineages, as well as expression profiling in Populus.