Populus endo-glucanase 16 localizes to the cell walls of developing tissues.

The hemicelluloses comprise a group of matrix glycans that interact with cellulose microfibrils in plant cell walls and play important roles in establishing wall architecture. The structures of hemicelluloses are determined by carbohydrate-active enzymes (CAZymes) that synthesize, integrate, and break down these polymers. Specifically, endo-glucanase 16 (EG16) enzymes, which are related to the well-known xyloglucan endotransglycosylase/hydrolase ( XTH ) gene products in Glycoside Hydrolase Family 16 (GH16), have been implicated in the degradation of the β(1,4)-linked backbone of mixed-linkage β(1,3);β(1,4)-glucans (MLG) and xyloglucans. EG16 members are single-copy genes found in most plant clades but are absent from many eudicots, including the model plant Arabidopsis thaliana . Until recently, EG16 members had only been characterized in vitro, establishing their substrate specificity, protein structure, and phylogenetic history, but their biological function was unknown. Here we used a hybrid polar, Populus alba  ×  Populus grandidentata (P39), as a model to examine EG16 expression, subcellular localization, and pheno- and chemotypes of EG16 -downregulated P39 plants. Populus EG16 expression is strong in young tissues, but RNAi-mediated downregulation did not impact plant growth nor the fine structure of the hemicellulose xyloglucan, suggesting a restricted or currently unknown role in angiosperm physiology.