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Kinetin an artificial cytokinin.

CYTOKININS History of discovery from tissue culture promotion. Kinetin an artificial cytokinin. Natural cytokinins then found in plants - zeatin and isopentenyl adenine , their ribosides and riboside phosphates (ribotides). Synthesis: Site of production -- roots and seeds. Pathway:

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Kinetin an artificial cytokinin.

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  1. CYTOKININS • History of discovery from tissue culture promotion. • Kinetin an artificial cytokinin. • Natural cytokinins then found in plants -zeatin and isopentenyl adenine, their ribosides and riboside phosphates (ribotides).

  2. Synthesis: Site of production -- roots and seeds. Pathway: 1) adenosine monophosphate. 2) isopentenyl pyrophosphate (side chain +P) formed from acetyl-CoA via mevalonate. 3) above two components react together to give isopentenyl adenosine phosphate. Regulated by isopentenyl transferase. 4) hydroxylation of the side chain gives zeatin riboside phosphate. 5) the phosphate, then ribose is removed to give free zeatin.

  3. Cytokinin effects • Initiates cell division when present with auxin. • Causes differentiation in callus. High CK shoots;

  4. Totipotency First elucidated at Cornell by F.C. Steward in the early 1960’s

  5. Cytokinin Action in Cell Division • Cytokinin promotes the transcription of genes that remove blocks in the cell cycle of plants.

  6. Auxin C2H4 Promotion of lateral bud growth. Breaks apical dominance. Cytokinin

  7. Cytokinin effects • Promotes: • Leaf expansion • Branching. • Inhibits: • Root growth • Prevents: • Leaf senescence.

  8. Genetic engineering of cytokinin content • Isopentenyl transferase gene from Agrobacterium cloned into tobacco behind constitutive promoter. • Plants had: • More branching • Stunted roots. • Isopentenyl transferase gene from Agrobacterium with insertedtransposon (jumping gene)cloned into tobacco behind constitutive promoter. • When transposon jumps out cytokinin synthesis turns on. • Leaf sectors where this happens are much larger.

  9. Cytokinins prevent leaf senescence • Genes turned on in senescence identified. • Gene from cytokinin biosynthesis (Isopentenyl transferase gene from Agrobacterium)cloned into tobacco behind promoter of a gene triggered during senescence. • Leaves remained green

  10. Cytokinin Action in Cell Division AHP1 A histidine protein kinase In membrane Cytokinin signaling intermediate Moves to nucleus • Promoter of a gene turned on by cytokinin was fused to the firefly luciferase (luc) gene and used to transform protoplasts. • Sequences of suspected signaling relay proteins put into protoplasts and light output measured. • Relay genes turned on caused light output in the absence of cytokinin (Hwang and Sheen, Nature 2001)

  11. Ectopic expression of CKI1 (and other signaling intermediates) is sufficient to promote cytokinin responses in transgenic tissues. Vector CKI1 +IAA, no added cytokinin (Hwang and Sheen, Nature 2001)

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