Regulatory Effect of Cytokinin on Secondary Xylem Fiber Formation in an In Vivo System

Abstract

The regulatory effect of cytokinin on the formation of secondary xylem fibers was studied in the hypocotyl of young Helianthus annuus L. plants. Positive correlation was found between the kinetin supplied (0.25-0.5 micrograms/gram) to the growth medium and the rate of fiber formation within and between the vascular bundles. Reducing the root originated cytokinin supply, either by root removal or by lowering the transpiration rate, diminished the number of newly formed secondary xylem fibers. This decrease was considerably reversed in the presence of 0.5 microgram/gram kinetin. Early pulse exposure of kinetin had a temporary promoting effect on fiber differentiation at low concentrations and a permanent inhibitory effect at high concentration.Fiber differentiation is dependent on signals originating in the leaves (1, 2, 13). The signals for fiber differentiation flow in a strictly polar fashion from the leaves downward along several internodes and induce fibers along their flow (1, 2). The role of the leaves in primary phloem fiber differentiation in Coleus can be replaced, both qualitatively and quantitatively, by exogenous application of combined IAA and GA3. When various combinations of both hormones are applied, high concentrations of IAA stimulate rapid differentiation of fibers with thick secondary walls, while high levels of GA3 result in long fibers with thin walls (3).Cytokinin is both a limiting and controlling factor in the early stages of secondary xylem fiber differentiation in cultured hypocotyl segments of Helianthus, at the time when many cell divisions occur in the explants. Later stages of fiber differentiation can occur in the absence of cytokinin. At high concentrations, zeatin is more effective than kinetin (4). Kinetin increases the length of secondary xylem fibers in cultured wood of Adhatoda (12). Aloni (4) proposed that the mechanism which controls and determines the early stages of fiber differentiation is based on interaction of three major hormonal signals: IAA plus GA3 from the leaves with zeatin from the root apices. He also suggested that the basis for correlation between the development of the plant body and the differentiation of its supportive tissues is based on their common dependence on the same shoot/root feedback control signals.However, there is no evidence frotn intact plants on the role of cytokinin in fiber differentiation. Torrey et al. (17) suggested that the difficulty in proving the cytokinin involvement in in vivo systems is because most of the tissues and organs of higher plants contain a considerable level of root-supplied cytokinin throughout most of their life cycle. The difficulty in studying cytokinin participation in morphogenetic, as well as other processes in the plant is further increased by the lack ofanticytokinins and by the fact that excised plant organs or tissues are capable of synthesizing cytokinin under certain experimental conditions (8).We were able to show cytokinin involvement in fiber differentiation in intact young plants of Helian...

Aluminum inhibition of shoot lateral branches ofGlycine max and reversal by exogenous cytokinin

“…A potential mode for this enhanced translocation is the effect of cytokinin on xylem formation in soybean (Fosket and Torrey, 1969). Saks et al (1984) observed that partial removal of side roots and root apices inhibited the rate of secondary xylem formation in the hypocotyl of Helianthus. In addition, the exogenous addition of kinetin to the growth medium stimulated the rate of formation of secondary xylem fibers and vascular bundles.…”

Leaf-Induced Gibberellin Signaling Is Essential for Internode Elongation, Cambial Activity, and Fiber Differentiation in Tobacco Stems

“…We show that both auxin and GA are required for cambium proliferation, but each induces different derivatives: Auxin promotes the development of vessel elements only below the site of application, while GA induces xylem fiber formation both above and below the source. Since it is known that cytokinin-regulated xylogenesis is necessary for fiber development (Aloni, 1982;Saks et al, 1984;Matsumoto-Kitano et al, 2008), we assume that both GA and cytokinin signaling are required for the initiation and maturation of fiber cells.…”

Disruption of interfascicular fiber differentiation in an Arabidopsis mutant.

“…Although high concentrations of auxin stimulated rapid differentiation of short fibers with thick secondary walls, high levels of GA resulted in long fibers with thin walls. In addition, both in vivo and in vitro experiments have demonstrated that cytokinin transported from root apices is also required for fiber differentiation in stems (Saks et al, 1984). For example, the number of newly differentiated fibers decreased significantly by removal of root apices or by lowering the transpiration rate, and this decrease could be reversed by the application of cytokinin.…”