Loading and translocation of various cytokinins in phloem and xylem of the seedlings of Ricinus communis L.

Abstract

Abstract. The phloem sap of Ricinus seedlings was analyzed for cytokinins and the concentration was compared with that in cotyledons and xylem sap. The dominant cytokinin in the phloem sap was isopentenyladenine (70 nM) when the endosperm was attached to the cotyledons; zeatin, dihydrozeatin and cytokinin-ribosides were present at relatively low concentrations (1-2 nM). Removal of the endosperm and incubation of the cotyledons in buffer led to a sharp decrease in the level of isopentenyladenine in the phloem sap, down to the value for zeatin, namely 1-2 nM. Similar low cytokinin concentrations were found in the xylem sap, too, whereas in the cotyledons the cytokinin content was at least 10-fold higher. Incubation of the cotyledons with various cytokinins (isopentenyladenine, zeatin and their ribosides) led to an increase of each of the applied cytokinins in the phloem sap, including also the metabolically closely related cytokinins. Zeatin was especially well loaded. It is concluded that the phloem translocates most free bases and ribosides of the various cytokinin species, if they are offered to the phloem. The data also show that the cytokinin levels in the phloem, which may be far higher than in the xylem, are subject to strong fluctuations depending on the physiological situation.

Leaf-to-shoot apex movement of symplastic tracer is restricted coincident with flowering in Arabidopsis

“…Commitment to flowering can be assayed by placing plants that have been exposed to flower-promoting light treatments back into noninductive light conditions (28). Mature vegetative Arabidopsis plants can become committed to flower within 2 days of LD16 photoinduction treatment (14,26). The amount of light perceived by the plant determines when the plant will start to flower, and leaves will no longer be produced on the plant's primary axis.…”

“…Because small hydrophilic molecules such as those mentioned above are expected to move to the shoot apex via the symplast (7,13,14), potential changes in the flow of hydrophilic symplastic tracer molecules to the shoot apex before, during, and after the induction of flowering in Arabidopsis were investigated (15). Because the induction of flowering has been correlated with increases in the movement of sucrose and cytokinin in phloem exudate (16,17), and because the number of plasmodesmata found within the shoot apex increases during the induction of flowering (18), we expected to find increases in the leaf-to-apex movement of symplastic tracers.…”

Characterization of Cytokinin and Adenine Transport in Arabidopsis Cell Cultures

“…In addition, a high-affinity transport system was identified in the Arabidopsis cell cultures, which is characterized by a K m of approximately 200 nM for tZ. The affinity of the high-and medium-affinity systems is compatible with the observed range for cytokinins in plants (Komor et al, 1993;Weiler and Ziegler, 1981;Beck and Wagner, 1994;Takei et al, 2001). In contrast, it is not clear whether the observed systems with an affinity of 100 mM will be relevant in planta.…”

Effects of Elevated [CO2] and Nitrogen Nutrition on Cytokinins in the Xylem Sap and Leaves of Cotton

“…As the xylem stream may only reach the transpiring plant parts and not the apical shoot tips and other major sites of cell division, the phloem may also be involved in the delivery of CKs at some stage in mediating growth regulation (Ziegler, 1975;Vonk, 1979;Komor et al, 1993;Lejeune et al, 1994). Future work that aims to resolve this question should involve xylem and phloem estimation of CK fluxes, cellular growth analysis (kinematic, anatomical, and anisotropy), and carbon translocation studies.…”