The Role of Phytohormones (Cytokinins) in Chloroplast Development

A cytokinin binding protein from higher plant ribosomes.

Intact wheat seedlings (Triticum aestivum L.) were induced to senesce by placing them in the dark and at various stages of senescence were placed back in the light and their recoverability observed. Seedlings demonstrated complete recovery of chlorophyll, protein, and rate of photosynthesis after 2 days in the dark, but were unable to recover fully after 4 days. This suggests the onset of an irreversible stage in senescence by day 4. Foliar applied cytokinins delayed senescence, and zeatin at 0.1 mm delayed the onset of the irreversible stage for 6 days. In addition to delaying the loss of total soluble protein, zeatin maintained the net protein recovery capacity of the tissue. Control seedlings, however, lost their potential for net protein recovery at a rate similar to their loss of total soluble protein. Treatment with zeatin had no apparent effect on dark respiration during senescence, and although treatment did delay the loss of membrane permeability to substrate, the change in permeability occurred too late to have a causal role in senescence.

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Induced Senescence of Intact Wheat Seedlings and Its Reversibility

INTRODUCTION ...... . . . ...... ..... .... .. ......... .. .. ........ . 538 HORMONAL MODIFICATION OF RNA METABOLISM . 538 Auxin 538 Receptors and mediators 538 Regulation of transcription (isolated nuclei, chromatin, and DNA) 540 Polymerases ....... ... ...... ... .... 542 Regulation of RNA metabolism in vivo ......... 543 Gibberellins .. ..... ....... ..... ......... .. .. ... 545 Receptors 545 Regulation of transcription (isolated nuclei, chromatin, and DNA) 545 Regulation of RNA metabolism in vivo 546 Cytokinins . ..... ... 549 Receptors ... ..... ... 549

Regulation of Ribonucleic Acid Metabolism by Plant Hormones

Publisher Summary This chapter discusses the molecular biology of wound healing. The necessity of a wound for tumorigenesis was initially conceived of in terms of an entrance site for the invading bacteria. Wounding clearly removes mechanical barriers for the approach of the bacteria. It normally destroys a number of cells, the residues of which accumulate at the wound site and could be a factor in wound response of the adjacent living cells as well as in bacterial metabolism. Treatment of small wounds with wound sap appears to stimulate the development of tumors. Although the cells of the various plant species cytologically react quite differently to wounding, their metabolic reactions are quite similar and rather uniform. A series of primary events after wounding leads to a transient disorganization of the membrane systems of the cell. As a consequence, some blocked metabolic pathways are unblocked and reactions are set into motion that induces the switching on of genes and the synthesis of various enzymes.

Molecular Biology of Wound Healing: The Conditioning Phenomenon

The synthetic cytokinins kinetin and 6-benzylaminopurine exhibit equilibrium-type binding to purified chinese-cabbage leaf ribosomes. At 23mum and 4 degrees C one molecule of kinetin and 1.34 molecules of 6-benzylaminopurine are bound per ribosome. Adenine and adenine derivatives that are inactive as cytokinins showed much less affinity for ribosomes. Pretreatment of ribosomes with 0.5m-ammonium chloride or Triton X-100 did not decrease the extent of cytokinin binding. Binding appeared to be to the 83S ribosome species. A positive correlation between the extent of binding and the biological effect of various cytokinin analogues was demonstrated. These results are discussed in terms of cytokinin control of growth processes at the ribosomal level.

The binding of kinetin to plant ribosomes.

Considerable experimental evidence supports the view that phytohormones affect transcription (see Letham, 1969). However certain other observations suggest that cytokinins may also modify transiational processes. For example cytokinins maintain the level of ribosomes in excised leaf tissue (Shaw and Manocha, 1965; Srivastava and Arglebe, 1968; Berridge and Ralph, 1969). Although the conclusion that cytokinins promote protein synthesis in excised leaf tissue undergoing senescence (Osborne, 1962) no longer seems valid (see Tavares and Kende, 1970; Kuraishi, 1968), cytokinins markedly increase the specific activity of protein in purified nuclei, mitochondria and plastids incubated with labelled amino acids in vitro (Datta and Sen, 1965; Bhattacharyya and Roy, 1969; Davies and Cocking, 1967). The rapidity of the response to cytokinin in plastids and mitochondria (lag period only a few minutes) suggests that the hormone may promote protein synthesis in a direct way, possibly by interaction with the ribosome.

D. S. Letham

Papers

The binding of kinetin to plant ribosomes.

On the Significance of Cytokinin Binding to Plant Ribosomes