Cytokinin <i>N</i>-glucosides: Occurrence, Metabolism and Biological Activities in Plants
2020
Eva Pokorná | Tomáš Hluska | Petr Galuszka | H. Tucker Hallmark | Petre I. Dobrev | Lenka Záveská Drábková | Tomáš Filipi | Katarína Holubová | Ondřej Plíhal | Aaron M. Rashotte | Roberta Filepová | Jiří Malbeck | Ondřej Novák | Lukáš Spíchal | Břetislav Brzobohatý | Pavel Mazura | Lenka Zahajská | Václav Motyka
Cytokinins (CKs) are a class of phytohormones affecting many aspects of plant growth and development. In the complex process of CK homeostasis in plants, <i>N</i>-glucosylation represents one of the essential metabolic pathways. Its products, CK <i>N</i>7- and <i>N</i>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 <i>N</i>-glucosides across the plant kingdom proceeding from evolutionary older to younger plants with different proportions between <i>N</i>7- and <i>N</i>9-glucosides in the total CK pool. We show dramatic changes in their profiles as well as in expression levels of the <i>UGT76C1</i> and <i>UGT76C2</i> genes during <i>Arabidopsis</i> ontogenesis. We also demonstrate specific physiological effects of CK <i>N</i>-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, <i>TCSv2::3XVENUS</i>. Last but not least, we present the considerable impact of CK <i>N</i>7- and <i>N</i>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 <i>N</i>7- and <i>N</i>9-glucosides and indicate their involvement in CK evolution while suggesting their unique function(s) in plants.
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