Cytokinins transport

Pilet PE (1964) Auxin transport in roots of Lens culinaris. Nature 204 561-562 Pilet PE (1965) Polar transport of radioactivity from " C-labelled- -indoleacetic acid in stems of Lens culinaris. Physiol Plant 18 687-702 Pilet PE (1968) In vitro and in vivo auxin and cytokinin transport. In Wightman F, Setterfield G (eds) Biochemistry and physiology of plant growth substances. Runge, Ottawa, pp 993-1004... 

Vonk CR (1979) Origin of cytokinins transported in the phloem. Physiol Plant 46 235-240... 

It became clear, however, that in the whole plant the major site of cytokinin synthesis was in the roots from whence it was transported to the rest of the plant via the xylem transpiration stream. Perhaps kinins were a kind of rhizocaline ... 

Equilibrative-type nucleoside transporters (ENTs) were also characterized in rice451 and Arabidopsis370 in reference to cytokinin nucleoside transport using the yeast system. One of the four rice ENT gene products, OsENT2, mediates the uptake of cytokinin nucleoside as well as that of adenosine451 with higher affinity to iPR... 

A root-derived suppressor negatively regulating the formation of roots has been hypothesized.497 This proposed mechanism, termed root apical dominance, is analogous to the means for apical dominance in shoots. The cytokinin, tZR, in root xylem sap is hypothesized to play the role of the main suppressor, and tZR transported from roots to shoots via the transpiration stream negatively regulates the formation of adventitious roots.365... 

In fact, the enzyme-inhibitor interaction in itself is a chain of complex processes for the inhibitor molecule, including a number of desolvations, collisions with nonspecific sites on the enzyme protein, and resolvations before reaching the specific inhibition site. The complexity is not at all less than those considered for the transport and drug-receptor interaction processes of cytokinins. The situation is analogous to that a set of every rational number between zero and one corresponds in a one-to-one... 

In planta, the CKX activity is affected by molecular components of the local metabolic resource pool that can serve as electron acceptors directly or after oxidative modification [72]. Favorable redox environment in cell or in the apoplast may allow the enzyme to work at maximum rate. However, it is still questionable, how its high catalytic power is used and if really needed for the depletion of active cytokinins in vivo. The most active CKX enzymes, AtCKX2 and AtCKX4 in Arabidopsis, act on the cytokinins in apoplast, where the concentration of cytokinins reflects the balance of accumulation via local synthesis and transport. 

Lejohn, H. B. and Stevenson, R. M., Inhibition of amino acid transport and enhancement of tryptophan binding in a water mold by a range of natural and synthetic cytokinins, Can. J. Microbiol., 28, 1165, 1982. 

Other cytokinin-regulated genes include an invertase and a glucose transporter in Chenopodium rubrum. Cytokinin is known to influence sink-source relations, and cDNAs representing cytokinin induced genes appear to encode activities that may be involved in apoplastic sink-source relations. One is an extracellular isoform of invertase (Cinl), the... 

Conrad and Hepler [61] found that a subclass of voltage-gated calcium channels, L-type channels sensitive to dihydropyridines (DHP), were involved. DHP agonists induced bud initials in the absence of cytokinin while the DHP antagonist blocked bud formation even in the presence of cytokinin. Agonists did not promote complete bud development suggesting that cytokinins have other effects as well. Direct measurements of uptake in moss protoplasts demonstrated the existence of DHP-sensitive calcium transport... 

Two other smdies failed to show any calcium involvement in cytokinin responses. Cytokinins had no detectable effect on calcium transport in protoplasts from wheat leaves... 

However, wheat protoplasts have no other demonstrable responses to cytokinin and, therefore, the lack of a cytokinin effect on calcium transport is not a decisive test. Zhao and Ross [69] used zeatin induced growth and chlorophyll formation in excised cucumber cotyledons as cytokinin responses to examine the effects of calcium chelators, ionophore and several anti-calmodulin drugs. None of these drugs affected the two cytokinin responses unless used at concentrations high enough to cause indirect damage. 

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