Cytokinin meristem activity

Kurakawa T, Ueda N, Maekawa M, Kobayashi K, Kojima M, Nagato Y, Sakakibara H, Kyozuka J (2007) Direct control of shoot meristem activity by a cytokinin-activating enzyme. 

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... 

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 The nature of this signal is unknown, but bark-ringing experiments indicate that it moves in living tissues (Lejeune, unpublished).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). 

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