The Current Status of Cytokinin-Binding Moieties

Spatial and temporal changes in endogenous cytokinins in developing pea roots

Germination and seedling establishment follows a distinct pattern which is partly controlled by hormones. Roots have high levels of cytokinins. By quantifying the fluctuations in endogenous cytokinins over time, further insight may be gained into the role of cytokinins during germination and seedling establishment. Radicles were excised from sterile Pisum sativum L. seeds after 30 min and 5 h imbibition. Seedlings germinated on agar were harvested after 1, 3, 6 and 9 days. The roots were divided into the root tip, root free zone, secondary root zone and from day 6, the secondary roots. Samples were purified by various chromatographic methods and endogenous cytokinins detected by LC(+)ES-MS. Benzyladenine levels doubled after 5 h imbibition and then gradually decreased over time. Low concentrations of cis-Zeatin (cZ) type cytokinins were detected in the radicle after 30 min imbibition. After 5 h imbibition, cis-zeatin riboside-5'-monophosphate had greatly increased. The total cytokinin content of the roots increased over time with the ribotides being the predominant conjugates. From day 3 onwards, there was a gradual increase in the free bases, O-glucosides and their ribosylated forms. Mainly N ( 6 )-(2-isopentenyl)adenine (iP)-type cytokinins were detected in the root tip, whereas trans-zeatin- (tZ), dihyrozeatin- (DHZ) and iP-type cytokinins were found in the secondary roots and root zone. Cytokinin biosynthesis was only detected after day 6. Biosynthesis of iP and tZ derivatives was quite rapid, whereas biosynthesis of cZ derivatives remained at a low basal level. These fluctuations in cytokinin types and concentrations suggest the cytokinins may be synthesized from various pathways in pea roots.

Comparison of cytokinin-binding proteins from wheat and oat grains

Cytokinin-binding proteins (CBPs) isolated from mature grains of oat (Avena sativa L.) and wheat (Triticum aestivum L.) by acid precipitation, ion-exchange and affinity chromatography had similar characteristics, although they differed somewhat in apparent molecular weight of the native protein as determined by gel filtration (109 and 133 kDa, respectively) and subunit size as estimated by SDS-polyacrylamide gel electrophoresis (47 and 55 kDa, respectively). Highly purified oat CBP showed very weak but distinct immunochemical cross-reactivity with anti-wheat CBP IgG, indicating different immunogenic properties of the two CBPs. Nevertheless, both CBPs exhibited very similar binding of different cytokinins and were characterized by high affinity for N6-benzyladenine (BA)-type and by low affinity for zeatin-type cytokinins to both wheat and oat CBPs and by somewhat higher binding activities of oat CBP compared to wheat CBP (Kds for BA: 4.6 x 10-7 M and 6.8 x 10-7 M, respectively). The potential role of CBPs in regulating free BA-type cytokinin levels during cereal grain development and germination is discussed.

Kinetin-induced stimulation of electrogenic pumping in soybean suspension cultures is unrelated to signal transduction

Primary modes of action of cytokinins have been thought to involve stimulation of the electrogenic H(+) pump and-or opening of plasmamembrane Ca(2+) channels. In order to test these hypotheses, rapid changes in membrane transport in response to cytokinin application were studied in heterotrophic suspension-cultured callus of soybean (Glycine max (L.) Merr.) using electrophysiological techniques. Kinetin (N(6)-furfurylaminopurine; 2 μM) elicited membrane hyperpolarization of 13±1 mV. This effect occurred even at membrane poteintials more negative than the most negative ionic equilibrium potential, and therefore might have resulted either from stimulation of the electrogenic pump, or from closure of ionic channels. The former mechanism of action appears most likely because (i) kinetin-induced membrane hyperpolarization is not accompanied by a significant change in plasma-membrane resistivity and (ii) hyperpolarization is abolished by cyanide, which inhibits electrogenic pump activity by depletion of cellular ATP.Electrogenic pumping is also activated by two other cytokinins: N(6)-(benzyl)adenine and trans-zeatin. However, it is unlikely that the hormonal effect on electrogenesis is directly related to transduction of the cytokinin signal, for the following reasons: (i) hormonally inactive, but chemically related compounds (cis-zeatin, adenine) also elicited membrane hyperpolarization; (ii) hormonally active, N(9)-substituted cytokinins failed to stimulate electrogenesis; (iii) the chemically unrelated cytokinin N,N'-diphenylurea also failed to stimulate electrogenesis.The results imply that the kinetin effect on electrogenic pumping is related to adenine, or its metabolism, and not hormonal action. Adenine was absorbed by soybean cells, but not in sufficient quantities to have a significant effect on adeninenucleotide pools. It appears likely that the control of electrogenesis requires either the presence of a purine free base (i.e. no substituents at the N(9) position) or phosphoribosylation of the free base. No evidence was found for cytokinin-induced Ca(2+)-channel opening, though it is argued that such an event might be physiologically relevant, yet undetectable with the methods employed. It is essential that future studies on cytokinin signal transduction - especially as they relate to membrane transport - take into account the possibility that metabolic effects unrelated to hormone action are dominant.