Chapter 21 Cytokinins: a diffusible and water-soluble regulators of cell division autoclaved herring sperm DNA immature endosperm of corn (banana) Common structure: N 6 -substituted aminopurines, the structural requirements for activity No kinetin-induced cell division occurs without auxin in the culture medium.

Cytokinins definition: have biological activities similar to those of trans-zeatin, including ¤ induce cell division in callus cells in the presence of an auxin ¤ promote bud and root formation from callus cultures when in the appropriate molar ratios to auxin ¤ delay senescence of leaves ¤ promote expansion of dicot cotyledons ¤ cell proliferation ¤ chloroplast differentiation ¤ nutrient mobilization

Callus: grow as a mass of disorganized, relatively undifferentiated cells wounding inducible de-differentiated Crown gall: a specific type of callus, the wounding site of plants was invaded by Agrobacterium tumefaciens, it cause the neoplastic (tumor-forming) disease.

Sources: yeast extract, tomato juice, banana extract, coconut milk, transfer-RNA— hypermodified bases

Conjugated cytokinins: ¤ N 3, N 7, N 9 with sugars riboside: N 9 attach sugar ribotide: ribose sugar moiety contains a phosphate group glycoside: even the oxygen of zeatin. ¤ is inactive form ¤ glucosidase remove the conjugation at the side chain ¤ Dormant seeds: high levels of cytokinin glucosides

Witches’ broom on balsam fir, Induced by Corynebacterium fascians Some plant pathogenic bacteria, insects, and nematodes secrete free cytokinins

Isopentenyl transferase Dimethylally diphosphate 1-hydroxy-2-methyl-2- butenyl 4-diphosphate

古森本

The sites of cytokinins biosynthesis: root apical meristems: root-derived cytokinin in shoot development, in general young maize embryos, young developing leaves, young fruits the difference of physiological significance The transported form: zeatin ribosides in the xylem exudate The degradation of cytokinins: cytokinin oxidase but not for O-glucoside derivatives