Abstract
Cytokinins (CKs) are a class of phytohormones affecting many aspects of plant growth and development. In the complex process of CK homeostasis in plants, N-glucosylation represents one of the essential metabolic pathways. Its products, CK N7- and N9-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 N-glucosides across the plant kingdom proceeding from evolutionary older to younger plants with different proportions between N7- and N9-glucosides in the total CK pool. We show dramatic changes in their profiles as well as in expression levels of the UGT76C1 and UGT76C2 genes during Arabidopsis ontogenesis. We also demonstrate specific physiological effects of CK N-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, TCSv2::3XVENUS. Last but not least, we present the considerable impact of CK N7- and N9-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 N7- and N9-glucosides and indicate their involvement in CK evolution while suggesting their unique function(s) in plants.
Highlights
Cytokinins (CKs) are endogenous phytohormones acting at low concentrations as essential players in a wide variety of plant physiological processes [1,2]
Higher concentrations of total CK N-glucosides were found in Angiosperms, and an upward trend was observed, resulting in concentration maxima of total CK N-glucosides in youngest clades represented by Eudicots: Superasterids, Campanulids and Lamiids (Figure 1, Supplemental Table S1)
Our findings argue against the general image of CK N-glucosides as irreversible
Summary
Cytokinins (CKs) are endogenous phytohormones acting at low concentrations as essential players in a wide variety of plant physiological processes (e.g., cell division, de novo organ formation, shoot and root development, growth of auxiliary buds, chlorophyll biosynthesis and nutrients translocation) [1,2]. The primary structural feature of CKs is the presence of N6 -substituted adenine with an isoprenoid or aromatic N6 -side chain that can result in dozens of different CK forms in planta. Derivatives of N6 -benzyladenine (BA) represent the natural aromatic ones [3]. Both isoprenoid and aromatic CK groups substantially differ in terms of biochemical properties, receptor affinities, biological activities, and amounts in plants [4,5,6,7,8]. CK free bases represent the active forms [9]. To regulate a pool of individual CK forms and to maintain CKs at optimal active levels during plant growth, numerous CK interconversions occur in plant tissues
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