The effect of metyrapone on cytokinin ([8-14C]benzylaminopurine) metabolism in mature green tomato pericarp

Cytokinins: activity, biosynthesis, and translocation

Cytokinins (CKs) play a crucial role in various phases of plant growth and development, but the basic molecular mechanisms of their biosynthesis and signal transduction only recently became clear. The progress was achieved by identifying a series of key genes encoding enzymes and proteins controlling critical steps in biosynthesis, translocation, and signaling. Basic schemes for CK homeostasis and root/shoot communication at the whole-plant level can now be devised. This review summarizes recent findings on the relationship between CK structural variation and activity, distinct features in CK biosynthesis between higher plants and Agrobacterium infected plants, CK translocation at whole-plant and cellular levels, and CKs as signaling molecules for nutrient status via root-shoot communication.

Cytokinin oxidase/cytokinin dehydrogenase assay: optimized procedures and applications

Spectrophotometric methods for determining the activity of cytokinin oxidase/cytokinin dehydrogenase (EC 1.5.99.12) were developed and optimized. A sensitive end-point method based on a combination of the electron acceptor 2,6-dichlorophenolindophenol and Schiff base formation of the reaction product with 4-aminophenol under acidic conditions can be applied to crude cell and tissue extracts. The assay was also adapted for other substrates than N6-(2-isopentenyl)adenine, such as zeatin and the aromatic cytokinins, although an enzyme which degrades the latter compounds has not yet been identified. The second novel method is an initial rate method based on the coupled redox reaction of phenazine methosulfate and 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide resulting in the formation of a formazan dye. This method can be used for kinetic studies with purified enzyme and is entirely substrate independent.

An alternative cytokinin biosynthesis pathway

Studies of de novo cytokinin biosynthesis in isopentenyltransferase (ipt)-transformed Arabidopsis thaliana, involving in vivo deuterium labeling and mass spectrometry, showed that the biosynthetic rate of zeatinriboside-5'-monophosphate was around 66-fold higher than that of isopentenyladenosine-5'-monophosphate (iPMP), the proposed primary product of the Agrobacterium ipt. Double tracer analysis, using [(2)H(6)] isopentenyladenosine and deuterium oxide, provided evidence for an alternative, iPMP-independent, biosynthetic pathway for zeatin-type cytokinins, present in both ipt-expressing and wild-type Arabidopsis thaliana. Reduction of the biosynthetic flux in the alternative pathway by use of mevastatin, an inhibitor for 3-hydroxy-3-methylglutaryl CoA reductase, indicated a terpenoid origin for the side-chain precursor of the iPMP independent pathway.

The biochemical mechanisms of the plant activation of promutagenic aromatic amines

Using specific monooxygenase and oxidase inhibitors in a plant cell/microbe coincubation assay, the biochemical mechanisms of the plant activation of two aromatic amines were compared. The biological endpoints included mutation induction, inhibition of mutagenicity, viability of the plant cells (activating system), and viability of the microbial cells (genetic indicator organism). The activation of m-phenylenediamine by TX1 cells was mediated by enzyme systems that were inhibited by diethyldithiocarbamate, potassium cyanide, methimazole, (+)-catechin or acetaminophen. The inhibition by metyrapone was attended by toxicity in the plant cells. These data implicate a TX1 cell peroxidase and a FAD-dependent monooxygenase in the plant activation of m-phenylenediamine. The TX1 cell activation of 2-aminofluorene was inhibited by diethyldithiocarbamate, 7,8-benzoflavone, acetaminophen or (+)-catechin. An additional pathway of the plant cells in the activation of 2-aminofluorene may involve a cytochrome P-448-type N-hydroxylase.