Root meristems cytokinin

The growth retardant CCC increases cytokinins in the bleeding sap of grape (Skene 1970). Skene has interpreted this as an indication that CCC acts directly on the root meristem to increase cytokinin production. However, since the effect of CCC is counteracted by GA, it is possible that the enhancement of cytokinin production by CCC is mediated by a CCC-induced reduction in GA. This indicates that GA may act to suppress cytokinin production in roots (Woolley and Wareing 1972 a), a possibility supported by Teltscherova (1970) who noted that CCC causes an increase in the cytokinin content of apical buds of Chenopodium rubrum and that GA can overcome this effect. lAA also reduces cytokinins in buds of Chenopodium, indicating that both GA and lAA may be involved in the regulation of cytokinin production in this material. 

Cytokinins are reported to affect both the and G2 phases of the cell cycle of root meristems, causing a reduction in G (MacLeod 1968) and a lengthening of G2 (MacLeod 1968, Van t Hof 1968). This indicates that even though cytokinins may act most directly on a particular step of the cell cycle... 

Van Staden J, Wareing PF (1972) The effect of light on endogenous cytokinin levels in seeds of Rumex obtusifolius. Planta 104 126-133 Van Staden J, Olatoye ST, Hall MA (1973) Effect of light and ethylene upon cytokinin levels in seed of Spergula arvensis. J Exp Bot 24 662-666 Van t Hof J (1965) Relationships between mitotic cycle duration, S period duration and the average rate of DNA synthesis in the root meristem cells of several plants. Exp Cell Res 39 48-58... 

In animals hormones are produced either in special glands (cf. Table 64) or are formed in tissues in which the production of hormones is a biochemical side activity only (so-called tissue hormones). In plants and microorganisms, special hormone-producing glands are absent, but also in plants hormone synthesis is unequal in the different types of cells. Cytokinins, for instance, are produced predominantly in roots and 3-indoleacetic acid in apical meristems. 

So far the Sinapis story can be interpreted as follows The shoot apical meristem of 2-month-old plants is presumably cytokinin-limited. Exposure to LD causes the production, in the leaves, of a signal which is then transported to the root system. There it alters the course of cytokinin metabolism and/or release. The increase in cytokinin levels in the transpiration stream (xylem) causes an increase in leaf cytokinin levels by 16 h. Some of the leaf cytokinins are then re-exported in the phloem sap to the apical meristem, where they cause a mitotic activation at 26-30 h. Since the cytokinin level in root exudate is altered as early as h 9 (Fig. 2), the initial leaf-to-root signal is apparently produced and transported extremely rapidly, i.e. within the first hour of the photo-extension period of the LD. The nature of this signal is unknown, but bark-ringing experiments indicate that it moves in living tissues (Lejeune, unpublished). 

Webster and Langenauer (1973) proposed an alternative to the concept that cytokinin or some other substance produced in the QC maintains active division in meristematic cells. They noted that, after release of cultured root tips from starvation-induced inhibition of division, DNA synthesis and division occur in the QC as well as in the rest of the meristem. After resumption of division in the meristem, however, the quiescence of the QC is re-established, indicating that the activity of the meristem may control the QC rather than vice versa. Other methods of temporarily suppressing meristematic activity also allow resumed DNA synthesis and division in the QC until division activity in the remainder of the meristem recovers (Webster and Langenauer 1973, MacLeod 1976). 

In apical meristems of Vicia faba roots, lAA increases the duration of S with no change in the initial rate of DNA synthesis (Webster 1967). This indicates that the effects of lAA on S, as with the previously-mentioned effects of cytokinins on S, are not by an overall modification of DNA synthesis. This is supported by the data of Nagl (1972) showing that total suppression of mitosis by lAA in onion root tips occurs with no inhibition of DNA replication. 

Cytokinins occur in roots and there is evidence that they are synthesized there (Skene 1975, Feldman 1979, Wareing etal. 1977). Cytokinin can inhibit root elongation (Darimont et al. 1971, Scott 1972, Svensson 1972) especially when applied to roots growing in the dark (Darimont etal. 1971, Svensson 1972). Although cytokinins may regulate meristem activity in roots (see Sect. 4) their... 

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