Translocation cytokinins

To summarize, cytokinin translocation via the xylem has been studied in three systems, namely, blue lupin, pigeonpea and tobacco. These provide insight into seed development and new metabolites, lateral shoot development, and leaf senescence, respectively. The view that xylem cytokinin level greatly influences the cytokinin status of the shoot and thus affects aspects of shoot development and also senescence is widely accepted although it still lacks adequate validation. The results with pigeonpea and work with soybean explants (see Nooden and et al., this Vol.) indeed support the concept of a regulatory role for xylem cytokinin. However, the tobacco system clearly shows that cytokinin level in leaves is governed by additional mechanisms such as biosynthesis in the leaf per se. 

Davenport TL, Morgan PW, Jordan WR (1980) Reduction of auxin transport capacity with age and internal water deficits in cotton petioles. Plant Physiol 65 1023-1025 Davey JE, van Staden J (1976) Cytokinin translocation Changes in zeatin and zeatin-riboside levels in the root exudate of tomato plants during their development. Planta 130 69-72... 

It is possible that, over these longer periods, protein synthesis is affected. Itai Vaadia (1965, 1971) showed that during water stress cytokinin activity in the root exduate and translocation of cytokinin were reduced. Ben-Zioni, Itai Vaadia (1967) showed that the lower rate of protein synthesis of stressed leaf tissue could be raised by treatment of leaf material with kinetin. We need more information on levels of various proteins after naturally occurring stress. 

Several members of Arabidopsis APETALA2 (AP2) transcription factors, cytokinin response factors (CRFs), are transcriptionally upregulated by cytokinin.546 The CRF proteins rapidly accumulate in the nucleus in response to cytokinin, and this translocation depends on AHKs and AHPs, but is independent of ARRs (Figure 23).546 Comparison of the cytokinin response in type-B arr mutants and erf mutants revealed substantial overlap among the genes regulated by these two families of transcription factors.546 These results suggest that CRFs function in tandem with type-B ARRs as a branch of an evolutionarily ancient two-component system for cytokinin response. 

Singh, S., D.S. Letham, and L.M.S. Palni Cytokinin biochemistry in relation to leaf senescence. VIII. Translocation, metabolism and biosynthesis of cytokinins in relation to sequential leaf senescence of tobacco Physiol. Plant 86 (1992) 398-406. 

C. are synthesized mainly in the roots of higher plants and they are not subject to much translocation. It is interesting that they occur in certain transfer RNA molecules 0.05 to 0.1 % of the purine bases in tRNA have cytokinin activity. In E. coli, the tRNA species for phenylalanine, leucine, serine, tyrosine and tryptophan contain C. N -(y,y -dimethylallyl)-Adenosine (see) also occurs in the free form. The most important C. are Kinetin (see), Zeatin (see) and Dihydrozeatin (see). The synthetic C, A-benzyl-adenine (see 6-Benzylaminopurine) and SD 8339 (Fig. 2) also have high cytokinin activity. 

These results of [ Cj-adenine incubation studies directly demonstrate cytokinin synthesis by germinating seeds, and further indicate that possibly only the embryonic axes have the capacity to synthesize cytokinins, which are then translocated to the cotyledons, apparently accumulating therein to evoke physiological response. This is reflected in experiments with the intact seed where [ Cj incorporation into cytokinins (per g fw) was considerably higher in embryos than in the cotyledons (Table 2), while incorporation per organ is greater for the cotyledons [18]. Translocation experiments carried out with selective application of pH]-(diH) [9R]Z to embryos or cotyledons also indicate a polar movement of cytokinins from the embryonic axes to the cotyledons. The incorporation of [ C]-adenine into (diH) [9R]Z is particularly interesting because (diH)Z-type cytokinins predominate in lupin seed. 

This investigation was based on studies of sequential leaf senescence in tobacco Nicotiana rustica) plants where leaf cytokinin levels are important in control of sequential leaf senescence [24]. The basal yellow leaves had much reduced levels of cytokinins (22 ng/g fw) in comparison to green, fully expanded leaves (65 ng/g fw) and very green, expanding leaves (57 ng/g fw) near the apex Z and [9R]Z were major cytokinins [24]. This difference in cytokinin levels could not be attributed to any differences in the translocation and metabolism of xylem-supplied [ H]-[9R]Z... 

Translocation of Cytokinins in the Xylem of Tobacco in Relation to Sequential Leaf Senescence... 

The differing levels of cytokinins in tobacco leaves of varying age could be due to one or a combination of the following factors (1) differential translocation of xylem cytokinins (2) differential metabolism of xylem cytokinins (3) differential retention of xylem cytokinins and (4) differential biosynthesis of cytokinins in situ in the leaves. These four factors were assessed. By RIA, the principal cytokinins in tobacco xylem exudate were identified as Z, (diH)Z, [9R]Z and (diH)[9R]Z. The distribution and metabolism of the two ribosides in the tobacco shoot were determined after supply via the xylem. The major metabolites of [9R]Z in tobacco leaf laminae were adenine, adenosine and AMP, while the principal metabolite of (diH)[9R]Z was the 7-glucoside of (diH)Z. However, expanded pre-senescent and early senescent laminae did not differ in their cytokinin metabolism, while small expanding laminae showed a higher rate of metabolism. There was no differential distribution of the ribosides to these laminae, and no differential retention in leaves of differing maturity. 

Cytokinins were applied directly to the apical bud to avoid translocation problems. A single application of BA to plants exposed to one suboptimal LD promoted flower initiation at the concentrations of 5X 10 M and 2.5 X lO M, but inhibited it at 5 X lO M and greater (Fig. 1). 

Roots are generally considered as the major site of cytokinin biosynthesis in vegetative plants [23]. There is thus the possibility that the extra-cytokinins found in induced leaves were formed in roots and translocated in the xylem sap. Indeed, analyses showed that (a) the cytokinin levels extracted from root tissue decreased at 16 h (Lejeune, unpublished), and (b) the cytokinin activity (mostly at the Rt of ZR) was increased in the root (xylem) exudate at 9, 16 and 22 h, but not at 12 and 30 h (Fig. 2) [15]. 

Hirose N, Takei K, Kuroha T, Kamada-Nobusada T, Hayashi H, SakaMbara H (2008) Regulation of cytokinin biosynthesis, compartmentalization and translocation. J Exp Bot 59 75-83... 

Endogenous growth regulators may be translocated in the plant s vascular systems. This would seem to hold especially true for gibberellins and cytokinins, since activity has been detected in the xylem sap of many different herbaceous as well as woody species. Obviously these hormones are also exported from photosynthesizing leaves, as indicated by their presence in sieve tube sap. Although with less frequency, also auxin and ABA-like activity have been detected in both xylem and phloem sap (Table 3.1). 

The existence of a closed circulatory system in higher animals provides the organism with an easy and efficient route for the transport of hormones from the site of synthesis to the target tissues. In plants some hormones appear to be transported directly in the vascular tissue for example, cytokinin, GA, and ABA move from the root to the shoot in the xylem GA moves out of young leaves in the phloem and ABA is transported out of wilting leaves in the phloem (Fig. 6.1). However, auxin is not transported directly in the vascular tissue, but instead appears to be transported in cells associated with the phloem (Fig. 6.1). Ethylene poses a special problem in that it is a diffusable gas. However, its precursor, 1-aminocyclopropane-l-carboxylic acid (ACC), is transported from the root to the shoot in the xylem. Therefore, using the traditional concept of a hormone as a translocated chemical messenger, ACC may be more aptly considered to be a hormone than ethylene. 

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