Functions of rice beta-glucosidases and transglucosidases

Continued improvement of yields of rice, a primary nutrient for much of humanity, in the future to feed a growing population will benefit from greater knowledge of the plant’s function at the molecular level, including the roles of various enzymes and how their gene expression levels contribute to traits that can increase crop yields. One group of enzymes of interest are the β-glucosidases, which release single glucose residues from glucosides and oligosaccharides, due to their multiplicity and involvement in various functions. Known β-glucosidases fall in 4 sequence-similarity-based glycoside hydrolase (GH) families in plants, GH1, GH3, GH5 and GH116. Of these the GH1 family encodes the most known β-glucosidases and related enzymes, with 34–35 active genes in rice. These enzymes contribute to cell wall recycling, lignification, glycolipid recycling, release of phytohormones and their precursors, including response to biotic and abiotic stress, defense, and metabolic homeostasis, among other roles. In the past 20 years, numerous studies have teased out various aspects of rice β-glucosidase function using biochemical, cell biological and genetic approaches. Clear roles have been elucidated for certain isoenzymes, although most remain poorly understood. Efficient transglycosylation activities of certain enzymes affords production of novel glucoconjugates, including phytohormone glucosyl esters for identification of enzymes catalyzing related functions. Here, we provide an overview of what has been learned about rice β-glucosidase functions and what remains to be clearly understood.