Cytokinins degradation

Figure 22 Current model of cytokinin degradation in higher plants. CKX utilizes also iPR, iPRMP, iP9G, tZ, tZR, and tZ9G in Arabidopsis. The second step of the reaction is likely nonenzymatic.

Cytokinin degradation via side chain cleavage is another process regulating levels of biologically active cytokinins in plant cells. Unlike other metabolic steps the cleavage of the side chain from the cytokinin molecule results in an irreversible destruction of cytokinin structure, which is of course associated with a complete loss of biological activity. 

The existence of an enzyme activity catalysing cytokinin degradation in plants was first demonstrated in crude homogenates from cultured tobacco cells [103]. Subsequently, the enzyme was characterised in a number of higher plants (reviewed in [104,105]) and named cytokinin oxidase [106]. The presence of cytokinin oxidase activity was also reported in moss protonema [107], cellular slime moulds [108] and yeast [109]. 

Cytokinin degradation seems to be a very important tool for regulation of the active cytokinin pool in plant cells. Cytokinin oxidase activity in plant cells is subject to multiple control (reviewed in [104,155]). Most of the control mechanisms depend directly on the concentration and/or compartmentation of the cytokinins in the cell. 

Cytokinin degradation in plant cells is significantly enhanced in vivo after their exposure to exogenous cytokinins [156,157]. This phenomenon is probably mediated via... 

The biosynthetic pathway producing isoprenoid cytokinins has been identified, whereas that of aromatic cytokinins is poorly characterized.363 385 Two distinct pathways for isoprenoid cytokinin biosynthesis have been described, and each pathway employs a different type of isopentenyltransferase at the initial step. The major pathway in higher plants, which is catalyzed by IPT, is conjugation of adenine nucleotide and DMAPP or 4-hydroxy-3-methyl-2-( )-butenyl diphosphate (HMBDP) (Figure 14). In the less frequently used pathway, cytokinins are formed by degradation of prenylated tRNAs. The initial prenylation reaction of tRNA is catalyzed by tRNA-isopentenyltransferase (tRNA-IPT) (Figure 14). 

Leaf senescence is characterized by loss of chlorophyll, leaf yellowing, degradation of proteins, membrane lipids, and RNA, and recycling to young tissues.453 Delay of leaf senescence by exogenous application of cytokinins has been confirmed by numerous studies,454 suggesting that cytokinins are key components in the regulation of leaf senescence. 

Chloro-3-methyl-4-nitro-lH-pyrazole (Release) has no auxin-, gib-berellin-, or cytokinin-like activity, yet it is an ejffective abscission agent. At the present, there is no evidence to indicate that Release retards auxin, gibberellin, or cytokinin activity. It does stimulate and enhance the tissue production of ethylene (105). Release is fairly stable, and there is no indication that it is degraded by the tissue to ethylene per se (106). 

Dimethylallylpyrophosphate (DMAPP) originating from cytosolic mevalonate (JVIVA) pathway probably serves as a precursor of tRNA isoprenylation [80]. Release of cytokinins via tRNA degradation could at least serve as a possible source of cA-zeatin, which is much more abundant in RNA than the tran5-isomer [89] and further de novo biosynthesis of this cytokinin species [90] as well as its isomerization from trans-xQ-dim have not been proved yet. 

The irreversible degradation of free cytokinin bases and their ribosides is catalyzed by a flavoprotein that has been known for three decades as cytokinin oxidase [138]. It has been shown however, that compounds other than oxygen are much more efficient for cofactor re-oxidation [139-... 

---