Effect of exogenous cytokinins, auxins and adenine on cytokinin N-glucosylation and cytokinin oxidase/dehydrogenase activity in de-rooted radish seedlings

Abstract

The role of cytokinin N-glucosylation and degradation by cytokinin oxidase/dehydrogenase (CKX, EC 1.5.99.12) in response to application of exogenous auxins (2,4-dichlorophenoxyacetic acid [2,4-D] and a-naphthaleneacetic acid [NAA]) and cytokinins (N 6 -benzyladenine [BA] and trans-zeatin [Z]) was investigated in de-rooted seedlings of Raphanus sativus L. cv. Rampouch. Both auxins applied for 24 h at 1 and 10 lM concentration increased N-glucosylation of exogenously applied [ 3 H]dihydrozeatin (DHZ) by up to 20%. The level of endogenous 7N-glucosides (of Z, isopentenyladenine [iP] and DHZ) was increased by 2,4-D and NAA at 10 lM concentration by 28 and 23%, respectively, the level of Z being decreased by 90 and 59%, respectively. 2,4-D and NAA suppressed CKX activity ca. by half. Exogenous cytokinins Z and BA applied at 1 and 10 lM concentration stimulated 7N-glucosylation of [ 3 H]DHZ (by up to 40%). BA both at 1 and 10 lM, increased the level of endogenous Z by up to 35% and that of 7N-glucosides by up to 27%. BA application also strongly stimulated CKX activity (by up to 180%). Feeding with 1 and 10 lM Z resulted in ca. 100-fold and 2000-fold increase of Z level, respectively. The main metabolite, Z7G, was increased ca. 6-fold and 60-fold, respectively. Levels of Z 9-glucoside (Z9G), trans-zeatin riboside (ZR) and Z O-glucoside (ZOG) were elevated to lesser extent. As compared to BA, Z had only negligible effect on CKX activity. Adenine (1-500 lM) was preferentially 7N-glucosylated inhibiting competitively 7N-glucosylation of [ 3 H]DHZ. At high concentrations (100-500 lM) it increased endogenous levels of active cytokinins, especially of Z, however, it had no effect on CKX activity. Cytokinin N-glucosylation proved to be involved in down-regulation of active cytokinins in response to auxin and in the re-establishment of cytokinin homeostasis following application of exogenous cytokinins.

Cellular response of light/dark-grown green alga Chlorella vulgaris Beijerinck (Chlorophyceae) to exogenous adenine- and phenylurea-type cytokinins

“…Our observations are supported by other examinations where high concentration of cytokinins possessed phytotoxic effect on the vascular plants blocking cell proliferation and inducing programmed cell death (Carimi et al 2004;Synková et al 2004). High dose of exogenous cytokinins also stimulates the processes of cytokinin N-glucosylation, nucleotides formation and cytokinin oxidase/dehydrogenase activity involved in the re-establishment of phytohormone homeostasis (Blagoeva et al 2004). The presence of glucosides and nucleotides of cytokinins in Chlorophyceae suggests that cytokinin metabolism is complex as in the higher plants (Stirk et al The analysis of dose-effect relationships shows that cytokinins at 10 -10 M did not possess statistically important influence on cell division and metabolites accumulation.…”

Biochemical and molecular characterisation of exogenous cytokinin application on grain filling in rice

“…The homeostasis of CKs in cells and tissues is maintained not only by its biosynthesis by the enzymes like IPTs (adenosine phosphate isopentenyl transferases) and CYP735A, but also by their degradation by CKX [ 57 ] and conversion to inactive O -glucosides and N -glucosides [ 31 , 64 ]. The expressions of CKs biosynthesis enzymes are in turn regulated by the phytohormones like ABA and auxin, and CK itself [ 56 ].…”

“…In addition to promoting axillary buds growth, the plant hormone cytokinin is known for its regulation of many cellular and developmental processes, such as induction of cell division and de novo shoot formation [ 22 ], flowers and fruit development, and seed germination [ 23 ], shoot and root branching [ 24 ], photomorphogenic development and vascular differentiation [ 25 , 26 ], and chloroplast biogenesis and leaf senescence [ 27 ]. In these contexts, the role of OsCKX becomes very important because it could regulate the level of the hormone cytokinins in cells and tissues, as is evident from decrease/increase in cytokinin contents in CKX overexpressing/knockdown plants [ 28 , 29 ], in addition to regulation of the cellular levels of the hormone by biosynthetic processes and conversion to functionally inactive forms through N- and O-glycosylation [ 30 , 31 ].…”

Auxin as compère in plant hormone crosstalk

“…For IPT5, this upregulation was shown to function via the degradation of SHOOT HYPO-COTYL2 (SHY2) (Dello Ioio et al 2008). CK homeostasis is also controlled via an inhibitory effect of auxin on cytokinin oxidase/dehydrogenase (CKX), involved in cytokinin degradation (Blagoeva et al 2004). The interplay between auxin and CK in regulating lateral bud outgrowth also reflects crosstalk between auxin and CK homeostasis: auxin suppresses local biosynthesis of CK in the nodal stem of peas by decreasing levels of PsIPT2 mRNA and by increasing PsCKX2 expression (Sato et al 2009; Fig.…”