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.

CYTOKININS: Activity, Biosynthesis, and Translocation

■ 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

Cytokinin hormones are important regulators of development and environmental responses of plants that execute their action via the molecular machinery of signal perception and transduction. The limiting step of the whole process is the availability of the hormone in suitable concentrations in the right place and at the right time to interact with the specific receptor. Hence, the hormone concentrations in individual tissues, cells, and organelles must be properly maintained by biosynthetic and metabolic enzymes. Although there are merely two active cytokinins, isopentenyladenine and its hydroxylated derivative zeatin, a variety of conjugates they may form and the number of enzymes/isozymes with varying substrate specificity involved in their biosynthesis and conversion gives the plant a variety of tools for fine tuning of the hormone level. Recent genome-wide studies revealed the existence of the respective coding genes and gene families in plants and in some bacteria. This review summarizes present knowledge on the enzymes that synthesize cytokinins, form cytokinin conjugates, and carry out irreversible elimination of the hormones, including their phylogenetic analysis and possible variations in different organisms.

Evolution of cytokinin biosynthesis and degradation

Leaf senescence is an active process involving remobilization of nutrients from senescing leaves to other parts of the plant. Whereas senescence is accompanied by a decline in leaf cytokinin content, supplemental cytokinin delays senescence. Plants that overexpress isopentenyl transferase (ipt), a cytokinin-producing gene, or knotted1 (kn1), a homeobox gene, have many phenotypes in common. Many of these phenotypes are characteristic of altered cytokinin physiology. The effect of kn1 on leaf senescence was tested by driving its expression using the promoter of the senescence-associated gene SAG12. SAG:kn1 tobacco plants showed a marked delay in leaf senescence but otherwise developed normally. The delay in senescence was revealed by an increase in chlorophyll content in SAG:kn1 leaves relative to leaves of the control plants and by a decrease in the number of dead leaves. Senescence was also delayed in detached leaves of SAG:kn1 plants. Delayed senescence was accompanied by increased leaf cytokinin content in older leaves expressing kn1. These experiments extend the current understanding of kn1 function and suggest that in addition to mediating meristem maintenance, kn1 is capable of regulating the onset of senescence in leaves.

Leaf senescence is delayed in tobacco plants expressing the maize homeobox gene knotted1 under the control of a senescence-activated promoter.

The view that plant cell division is chemically controlled is not new, indeed it can be traced back to the last century, but it was Haberlandt in 1913 (31) who provided the first experimental evidence for this concept. He showed that phloem diffusates could stimulate parenchymatous potato tuber cells to revert to a meristematic state. However the identity of the active substance/s in these diffusates was not established.

Cytokinin Biosynthesis and Metabolism

In a number of cytokinin bioassays, the activities of the following compounds were compared: 3-, 7-, and 9-glucosides of 6-benzylaminopurine (BAP); 7- and 9-glucosides of zeatin; O-glucosides of zeatin, dihydrozeatin, and their ribosides; 9-alanine conjugates of zeatin, and BAP The bioassays included the radish cotyledon, theAmaranthus betacyanin, the oat leaf senescence, and the tobacco pith callus Cytokinin activity was markedly reduced by 7- and 9-glucosylation in nearly all bioassays, but 3-glucosylation of BAP and O-glucosylation of the zeatin sidechain usually had little effect on activity However, there were two notable exceptions to this generalization: the activity of O-glucosylzeatin markedly exceeded that of zeatin in the oat leaf senescence assay; 9-glucosyl-BAP and free BAP were similarly active in retarding the senescence of radish leaf discs The 9-alanine conjugate of zeatin (lupinic acid) and of BAP were markedly less active than zeatin and BAP, respectively, in all bioassays, but the responses evoked by these conjugates at high concentrations in theAmaranthus bioassay approached those caused by the corresponding base The activities of several new compounds related to the alanine conjugate of BAP were also assessed To serve as a guide in the selection of the most suitable bioassay for detection of the above-mentioned cytokinin conjugates, the lowest detectable amounts in selected bioassays have been compared

C. M. Bates

Papers

Regulators of cell division in plant tissues. XXIX. The activities of cytokinin glucosides and alanine conjugates in cytokinin bioassays