■ 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

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . 509 MOLECULAR BIOLOGY OF TRANSFORMATION BY A. TUMEFACIENS 51l EVIDENCE FOR PHYTOHORMONE PARTICIPATION IN CROWN GALL TUMOR GROWTH . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . 513 GENES C ODING FOR CYTOKININ BIOSYNTHESIS . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . 514 Advances in Cytokinin Analysis .. . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . .. . 514 Cytokinin Biosynthesis and Metabolism in Untrans/ormed Plants . . . . . . . . . . . . . .. . . . . . . . . 514 Cytokinin Habituation 515 Cytokinin Biosynthetic Genes Within A. tumejaciens T-DNA ...... . .. . ..... . . . 516 Cytokinin Biosynthetic Genes Outside A. tume/adens T-DNA . . . . . . . . . . . . . . . . . .. . . . . . . . . . 519 Cytokinin Biosynthesis in Pseudomonas savastanoi 520 Homology Between Cytokinin Biosynthetic Genes of A. tumejadens and P. savastanoi 521 Cytokinin Production by Rhizobium . . . . . . . . . . . . . . .. . . . . . . . . . . , .. . . . ...... . . . . . . . . . . .. . . . ...... 522 GENES CODING FOR A UXIN BIOSyNTHESIS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523 Auxin Biosynthetic Genes in P. savastanoi 523 Auxin Biosynthetic Genes Within A. tume/adens T-DNA .. .. . ........ ....... . . .. ....... . .. 525 Auxin Biosynthetic Genes Outside A. tume/adens T-DNA 528 Auxin Production by Rhizobium... . . . . . . . . . . . . ... . . . . . . . . . . . .... . . . . . . . . . . ... . . . . . . . .. . . . . . . . . . 529 CYTOKININ PRODUCTION BY OTHER MICROORGANISMS . . . . . . . . . . . . . . . . . . . . . . . . . . 529 S UMMARy . .. . . . . . . . . . ......... ........ 530

Genes Specifying Auxin and Cytokinin Biosynthesis in Phytopathogens

Different aspects of the in vitro somatic embryogenesis regulation are reviewed in this paper.work. A description of g General aspects, such as terminology, uses, stages of development and factors associated with the somatic embryogenesis, are described. is carried out. Although a brief description ofn the effects of the addition of different plant growth regulators to the culture medium wasis given, the article is centereds itself on the effect that the endogenous hormone concentrations in the initial explants and in the tissue cultures derived from them could play oin the induction and expression of somatic embryogenesis. It is significant that few to emphasize the low amount of systematic studies have been conducted, in this subject, in which different species and hormone groups were compared in cultures with and without embryogenic capacity. Moreover, the lack of correlation between the results presented in different studies the distinct works indicates that the hormone content of the cultures is not the only factor involved.

/pdf/regulation-of-in-vitro-somatic-embryogenesis-with-emphasis-1yrl1tv5i4.pdf

Regulation of in vitro somatic embryogenesis with emphasis on to the role of endogenous hormones

Hormones in plants differ from most of those in animals by having pleiotropic effects; that is, they are involved in the control of a wide range of developmental processes. At the same time the effect of a hormone on any developmental process depends on the species. For example, ethylene inhibits growth in dicotyledons and most monocotyledons but is promotory in deepwater rice and other hydrophytes. Moreover, two or more hormones can interact synergistically or antagonistically in many circumstances. Equally, any given hormone may affect the biosynthesis or metabolism of another, thus affecting endogenous levels. The issue is further complicated by the fact that environmental factors e.g. light, water status, wounding, pathogens may modify responses and indeed hormone levels themselves. The reason for this appears to be that hormones (and growth regulators) and environmental factors share many components in their transduction chains (i.e. the very early events which occur after the signal abiotic or biotic is perceived by the plant tissue). These transduction chains interact to produce an integrated response.

Plant Growth Regulators II: Cytokinins, their Analogues and Antagonists

Cytokinins are essential hormones for plant growth and development. They are also of vital importance for in vitro manipulations of plant cells and tissues. The biological activities and chemistry of cytokinins are well defined but very little is known about their mode of action and it is only recently that cytokinin genes have been identified in plants. This review summarizes the current status of knowledge on cytokinin biosynthesis, metabolism and signal transduction, with an emphasis on genes encoding metabolic enzymes and putative receptors, and genes rapidly induced by cytokinins.

Cytokinins: biosynthesis, metabolism and perception.

Tissue cultures of Actinidia kolomikta can be maintained as callus through repeated passages on a nutrient medium devoid of cytokinin but containing inorganic nutrients, sucrose, and other basal organics plus auxin. Under these conditions, actively growing callus contained 2 and 0.5 nmol of the cytokinins zeatin [io6Ade; 6-(4-hydroxy-3-methylbut-2-enylamino)purine] and N6-(Δ2-isopentenyl)adenine (i6Ade), respectively, per gram (fresh weight). When tissues were transferred from cytokininless medium to 30 μM i6Ade, endogenous io6Ade increased linearly to 160 nmol/g (fresh weight) during 8 hr, and i6Ade increased to 5 nmol/g (fresh weight) in 2 hr and then declined. The apparent Km for i6Ade in A. kolomikta and Actinidia chinensis × Actinidia arguta callus and in tissue slices of A. arguta stems was 12 μM. In addition, the reaction(s) converting i6Ade to io6Ade was O2-requiring and specific for i6Ade versus N6-(Δ2-isopentenyl)adenosine (i6A). When A. kolomikta callus was fed 30 μM i6A, io6Ade increased and reached a concentration corresponding to 6 nmol/g (fresh weight) in 8 hr. Ribosylzeatin (io6A) did not increase. Under N2 during i6A feeds, i6A accumulated rather than being metabolized to i6Ade, suggesting that i6A normally may be metabolized via i6AMP and i6Ade to io6Ade. A survey of 30 species of woody plants in 20 families of dicotyledonous angiosperms indicated that the ability to accumulate io6Ade (≥10 nmol/g) in 24-hr feeds with 30 μM i6Ade was restricted to certain systematic groups—e.g., order Ericales, families Oleaceae and Rubiaceae. This suggests that plants may differ in their pathways for io6Ade biosynthesis and that cytokinin biochemistry has potential as a taxonomic character above the species and genus levels.

https://www.pnas.org/content/pnas/83/4/972.full.pdf

Zeatin biosynthesis from N6-(Δ2-isopentenyl)adenine in Actinidia and other woody plants

Conversion of N6-isopentenyladenine to zeatin by Actinidia tissues.

By incubating explants from Actinidia arguta seedlings on a nutrient medium supplemented with 20 to 30 micromolar N(6)-(Delta(2)-isopentenyl)adenine (i(6)Ade) and then measuring zeatin (io(6)Ade) accumulation in tissues, the distribution of i(6)Ade hydroxylase activities in whole plants could be determined. Based on analyses with three entire plants, it is estimated that, as an organ system, roots contain approximately 68% of the plant's hydroxylase, while stems and leaves account for about 26% and 6%, respectively, of the total activity. Depending on the part of the root examined, hydroxylase activities ranged from 20 to 148 nanomoles io(6)Ade accumulated per gram fresh weight per 24 hours of incubation. Stem activities ranged from 17 to 165 nanomoles per gram fresh weight per 24 hours with the lowest activities being found at the tip. Leaf activities were substantially lower (1-10 nanomoles per leaf depending on position) than either root or stem.

Hydroxylation of N6-(Δ2-Isopentenyl)adenine to Zeatin Relative Activities of Organ Systems from Actinidia arguta

In Actinidia arguta (hardy kiwifruit) plants, the potential to accumulate the cytokinin zeatin (io6Ade) during feeding with a precursor, N6-(Δ2-isopentenyl)adenine (i6Ade), varies depending on the tissue. This can be demonstrated by incubating explants for 24 hr on a basal nutrient medium supplemented with 30 μM i6Ade and then extracting the tissues and analyzing cytokinin contents using HPLC methodology. Under these conditions, the potential for io6Ade accumulation in tissue slices from growing roots was 93 nmol/g at the root tip, >200 nmol/g immediately behind the tip (1.0-mm diameter), ≈100 nmol/g in 2-mm diameter root, and progressively lower in older tissues. A similar gradient of io6Ade accumulation was detected in growing stems, with relatively low activity (40 nmol/g) in the terminal 0.5 cm, ≈170 nmol/g in the 5- to 15-cm interval, and about 25 nmol/g in stem tissues taken 15-100 cm from the tip. Growing leaves accumulated little io6Ade (7-26 nmol/g) during feeding, as did fruits (0-8 nmol/g) at various stages of development and maturation. Seasonally, root activity was detected as early as March 1, whereas stem activity did not appear until March 15. Thus, the activation of i6Ade metabolism in both of these organs preceded sap flow (March 29) and bud break (April 16) by several weeks. The results suggest that root and stem tissues may be sites of cytokinin biosynthesis in growing Actinidia plants and that cytokinin production may not be the critical factor controlling the beginning of shoot growth in the spring.

John W. Einset

Papers

Zeatin biosynthesis from N6-(Δ2-isopentenyl)adenine in Actinidia and other woody plants

Conversion of N6-isopentenyladenine to zeatin by Actinidia tissues.

Hydroxylation of N6-(Δ2-Isopentenyl)adenine to Zeatin Relative Activities of Organ Systems from Actinidia arguta

Biosynthesis of zeatin from N6-(Δ2-isopentenyl)adenine in Actinidia: Sites and seasonal changes in activity