■ Abstract Cytokinins are structurally diverse and biologically versatile. The chemistry and physiology of cytokinin have been studied extensively, but the regulation of cytokinin biosynthesis, metabolism, and signal transduction is still largely undefined. Recent advances in cloning metabolic genes and identifying putative receptors portend more rapid progress based on molecular techniques. This review centers on cytokinin metabolism with connecting discussions on biosynthesis and signal transduction. Important findings are summarized with emphasis on metabolic enzymes and genes. Based on the information generated to date, implications and future research directions are presented.

Cytokinin metabolism and action

Publisher Summary This chapter explains the growth and behavior pattern of roots and shoots And discusses the generation of hormone like messages due to environmental changes in detail. Hormonal messages are recycled between roots and shoots. Roots import hormones and shoots acts as active hormone sinks. Various techniques to measure hormone levels and activities are described, the primary being the samples taken from the sap. Hormone traffic between roots and shoots forms the basis for the morphological and functional control that roots can exert on shoots. Regulation of root: shoot ratio by roots with mineral supply is explained in detail. Hormone like action on roots is responsible for regulation of protein levels in leaves and flowering in plants. The influence of hormone like messages by roots for development of shoots is presented. The influence of plant hormones cytokinins, gibberellins, abscisic acid, auxin and ethylene are explained. Cytokinins are main source of hormones in plants. They are carried by transpiration from the root tips present on a root system to recipient shoot tissues. Gibberellins are important in delaying leaf senescence. The output of gibberellins under stress is discussed. Abscisic acid is an important regulator of stomatal closure. Ethylene–mediated responses in the shoot are explained.

Are Plant Hormones Involved in Root to Shoot Communication

Present evidence indicates that only the root systems of plants have been shown conclusively to synthesize cytokinins. Although most of these compounds are apparently exported to the shoot via the xylem, there are indications that more attention should be given to the possibility of translocation through the phloem. Within mature leaves the cytokinins derived from the roots are converted to inactive or storage forms by means of glucosylation. While it would appear that glucosylation could occur in all living plant cells whenever the cytokinins are no longer required for active growth, and could provide the plant with a potential reservoir of cytokinins, very little is known with regard to the transport and reutilisation of these compounds.

The synthesis, transport and metabolism of endogenous cytokinins

Biosynthesis and Metabolism of Plant Hormones

Single senescent leaves attached to decapitated shoots of Nicotiana rustica L. regreened, especially when treated with cytokinin. Regreening caused an increase in leaf thickness, due to cell expansion. Senescent leaf plastids (gerontoplasts) were smaller than green chloroplasts, with degenerated membrane systems and stroma, and larger plastoglobuli. At advanced senescence, micrographs showed disintegrating gerontoplasts, reduced numbers of plastids were counted, and regreening became variable. The redevelopment of grana and stroma in regreening plastids was accelerated by cytokinin. All plastids in regreening leaves were identifiable as redifferentiating gerontoplasts because of their content of plastoglobuli and starch. Immunogold labelling showed significant association of POR with etioplasts in cotyledons, but with mature plastids in regreening leaves. No proplastids or dividing chloroplasts were observed in regreening leaves. Plastid numbers declined during senescence and did not increase again during regreening. It is concluded that the chloroplasts of regreening leaves arose by redifferentiation of gerontoplasts.

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Regreening of senescent Nicotiana leaves II. Redifferentiation of plastids

Phaseolus vulgaris plants decapitated above the primary leaves accumulate high cytokinin activity. The major cytokinin in these leaves was identified by Sephadex LH 20 chromatography, sensitivity to β-glucosidase and permanganate oxidation, and by combined gas chromatography-mass spectrometry as 6-(4-O-β-D-glucosyl-3-methylbutylamino) purine, (dihydrozeatin-O-β-D-glucoside). A possible reason for the persistance of this compound in the primary leaves is discussed.

A cytokinin glucoside from the leaves of Phaseolus vulgaris L.

Dihydrozeatin riboside has been identified in the leaves of decapitated bean plants by Sephadex LH20 chromatography and combined gas chromatography-mass spectrometry. The relationship between the cytokinins isolated and identified from this system and those previously reported in Phaseolus is discussed.

T. L. Wang

Papers

A cytokinin glucoside from the leaves of Phaseolus vulgaris L.

Dihydrozeatin riboside, a minor cytokinin from the leaves of Phaseolus vulgaris L.