https://science.sciencemag.org/content/sci/171/3974/853.full.pdf

Cytokinins: Permissive Role in Seed Germination

Publisher Summary The chapter discusses the hormones, its synthesis, and utilization of ribonucleic acids (RNAs). The chapter also discusses the different ways, in which the hormones affect RNA metabolism, and emphasizes the similarities and differences between different hormones. The possible ways, in which hormones may control the utilization of RNA for protein synthesis, are also discussed in this chapter, especially, in the general context of the regulation of growth and differentiation. Those hormones that regulate RNA synthesis seem to exert a dual action: (1) a selective action on the transcription of molecular ribonucleic acids (mRNA) that could perhaps explain the biological specificity of the hormone and (2) what appears to be a nonspecific stimulation of ribosomal RNA (rRNA) synthesis. While there is no conclusive evidence, regarding the hormonal control of the synthesis of specific mRNA molecules, there is combined evidence, from separate studies, that the cytoplasmic content of total mRNA is influenced by hormones. Finally, the regulation of RNA synthesis and utilization by hormones has provided a new perspective in looking at the action of many hormones, both those that control growth and differentiation and those that regulate the metabolic activity of mature cells. This chapter presents information on the hormonal control of the biosynthetic activities that does not explain the mechanisms of action at the molecular level and may turn out to be extremely diverse.

Hormones and the synthesis and utilization of ribonucleic acids.

Publisher Summary This chapter provides an overview of plant cell-walls. The cell wall is an envelope that encases the plant cell. The wall must be rigid enough to give the plant strength and form, and yet, if necessary, it must yield freely to facilitate growth. The network of cell walls, where adjoining cells have a wall in common, provides in a plant the structural framework analogous to both the skin and the bones of an animal. In some plants, especially those having woody tissues, the strength of this cell-wall network is prodigious. However, despite their apparently tough sheathing, the cells of the growing regions of a plant are able to extend to many times their initial length. The cell wall lies outside the plasma membrane, which defines the boundaries of the cell itself. The wall is freely permeable to most molecules, but the membrane exhibits selective permeability tending to concentrate certain dissolved molecules and ions inside the cell. The presence of such charged components as acidic polysaccharides within the wall imparts ion-exchange properties to the wall. The chapter discusses the concepts related to the types of cell-wall polysaccharides and elaborates the methods used in the elucidation of primary-wall structure. It also presents an overview of cell-wall glycoproteins.

Plant Cell-Walls

Measurements were made over a 4-day period of the effect of added indoleacetic acid (IAA), puromycin, actinomycin D and 5-fluorodeoxyuridine (FUdR) on growth and the levels of total DNA, RNA, protein and cellulase in segments of tissue at the apex of decapitated etiolated epicotyls of Pisum sativum, L. var. Alaska.The hormone induced swelling of parenchyma cells and cell division. By 3 days after IAA application, the amounts of DNA and protein were approximately double, RNA triple and cellulase 12 to 16 times the levels in controls. All of these changes were prevented by both puromycin and actinomycin D. FUdR prevented DNA synthesis and cell division but not swelling or synthesis of RNA, protein and cellulase.It is concluded that IAA-induced RNA synthesis is required for cellulase synthesis and lateral cell expansion, whether or not cell division takes place.

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Massive Synthesis of Ribonucleic Acid and Cellulase in the Pea Epicotyl in Response to Indoleacetic Acid, With and Without Concurrent Cell Division

Culture conditions, allowing the unlimited growth and maintenance in shaken suspensions of a kinetin dependent strain of tobacco pith cells, were determined.



Cell clusters from 1 to 25 cells were selected from the cultures to study the cell multiplication after addition of specific growth factors to the basal medium. Cell division was found to be strictly dependent upon the presence in the medium of both kinetin and auxin. In complete medium the generation period was 45 to 49 hours.



Kinetics of the total protein synthesis in the cell suspensions have been measured either by estimation of protein nitrogen or by 35S incorporation into the proteins. After 3 days of culture, growth was exponential, as expressed by cell number or protein synthesis, which varied proportionally. Evidence was also found that the initial incorporation rate of radioactivity into the proteins was more rapid than expected from the increment of the protein net weight. It seems therefore reasonable to assume that a significant amount of protein turn over does exist during the initial period of growth. This phenomenon was observed even when no cell division occurred.



Kinetin significantly activated protein synthesis, whether or not auxin was present in the medium.

R. Roychoudhury

Papers

The mechanism of action of plant growth substances: the role of nuclear RNA in growth substance action.