Old player, new roles: defining the role of the plastidial phosphorylase

The plastidial phosphorylase (Pho1 or Phs1; E.C. 2.4.1.1) is a ubiquitous enzyme among plants that catalyzes the formation and degradation of glucans. Although the first report connecting Pho1 with starch metabolism came out > 80 years ago, its precise role is still a matter of debate. In this article, we evaluate the catalytic and regulatory mechanisms of Pho1 in the context of known mechanisms in its animal, fungal, and bacteria homologs. We further discuss recent breakthroughs in understanding Pho1's function in initiating starch granule formation. This role is relevant to both photosynthetic and nonphotosynthetic tissues, as loss of Pho1 affects the regulation of the number of transitory starch granules in Arabidopsis leaves under various metabolic contexts, as well as the number of storage starch granules and/or starch granule morphology in wheat endosperm and potato tubers. Our comparison of phosphorylases across kingdoms reveals several regulatory mechanisms that require further investigation in plants. We also discuss emerging research on Pho1 protein interactions that give rise to other metabolic processes, such as photosynthesis. Overall, these multiple emerging roles of phosphorylase emphasize its importance in plant metabolism and its broad potential as a target for crop improvement.