Cytokinin -glucosides: Occurrence, Metabolism and Biological Activities in Plants.

Cytokinins (CKs) are a class of phytohormones affecting many aspects of plant growth and development. In the complex process of CK homeostasis in plants, -glucosylation represents one of the essential metabolic pathways. Its products, CK 7- and 9-glucosides, have been largely overlooked in the past as irreversible and inactive CK products lacking any relevant physiological impact. In this work, we report a widespread distribution of CK -glucosides across the plant kingdom proceeding from evolutionary older to younger plants with different proportions between 7- and 9-glucosides in the total CK pool. We show dramatic changes in their profiles as well as in expression levels of the and genes during ontogenesis. We also demonstrate specific physiological effects of CK -glucosides in CK bioassays including their antisenescent activities, inhibitory effects on root development, and activation of the CK signaling pathway visualized by the CK-responsive YFP reporter line, . Last but not least, we present the considerable impact of CK 7- and 9-glucosides on the expression of CK-related genes in maize and their stimulatory effects on CK oxidase/dehydrogenase activity in oats. Our findings revise the apparent irreversibility and inactivity of CK 7- and 9-glucosides and indicate their involvement in CK evolution while suggesting their unique function(s) in plants.