Abstract

Cytokinin (CK) N-glucosides are the most abundant group of CK metabolites in many species; however, their physiological role in planta was for a long time perceived as irreversible storage CK forms only. Recently, a comprehensive screen showed that only vascular plants form CK N-glucosides in contrast to mosses, algae, and fungi. The formation of CK N-glucosides as biologically inactive CK conjugates thus represents an evolutionarily young mechanism for deactivation of CK bases. Even though CK N-glucosides are not biologically active themselves due to their inability to activate the CK perception system, new data on CK N-glucoside metabolism show that trans-zeatin (tZ) N7- and N9-glucosides are metabolized in vivo, efficiently releasing free CK bases that are most probably responsible for the biological activities observed in a number of bioassays. Moreover, CK N-glucosides’ subcellular localization as well as their abundance in xylem both point to their possible plasma membrane transport and indicate a role also as CK transport forms. Identification of the enzyme(s) responsible for the hydrolysis of tZ N7- and N9-glucosides, as well as the discovery of putative CK N-glucoside plasma membrane transporter, would unveil important parts of the overall picture of CK metabolic interconversions and their physiological importance.

Highlights

  • Specialty section: This article was submitted toPlant Physiology, a section of the journal Frontiers in Plant ScienceReceived: 09 March 2020 Accepted: 08 May 2020 Published: 05 June 2020Citation: Hoyerová K and Hošek P (2020) New Insights Into the Metabolism and Role of Cytokinin N-Glucosides in Plants

  • It was shown that iP derivatives including iPN9G are the least active, which is in agreement with the high levels of biologically inactive iP N-glucosides that are commonly seen soon after iP overabundance – since the iP N-glucosides seem not to be hydrolyzed and accumulate in the tissue with no physiological effects

  • It was demonstrated that the ugt76c2 mutant produces smaller seeds with reduced weights compared to wild type, while no difference was observed in the UGT76C2 overexpressing lines (Wang et al, 2011)

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Summary

INTRODUCTION

Based on the longevity of benzyladenine N7-glucoside (BAPN7G) in plant cells due to its resistance to enzymes that degrade CK molecules by side chain cleavage, CK-N7Gs were suggested to be either active or detoxification or storage forms of CKs more than 40 years ago (Fox et al, 1973) Their role as terminal products of irreversible deactivation was supported by a study showing that N7and N9-glucosides are not efficiently cleaved by β-glucosidase (Brzobohatý et al, 1993), and their possible hydrolysis was further questioned based on their enormous accumulation in comparison to active CKs (Šmehilová et al, 2016).

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