Senescence of Pear Fruit Cells Cultured in a Continuously Renewed, Auxin-deprived Medium

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Abstract
Auxins and cytokinins support cell division in tissue and cell cultures. In cytokinin-independent pear (P. communis cv. Passe crassane) cells, omission of 2,4-D from the medium for 2 successive transfers leads to rapid cell lysis, unless the osmolarity is raised to 0.4 M with mannitol. Use of this system (nutrients plus mannitol minus 2,4-D) for the study of cell senescence was explored both in batch culture and in a system designed to permit medium renewal without withdrawal of live cells. In both systems, an initial period (1-6 days) of limited increase in cell number is characterized by a continuous decrease in the respiratory activity and in protein and RNA synthesis to very low basal rates. In batch culture, cell death occurs after 13-15 days with little or no change in metabolic activity, or in protein and RNA synthesis. With renewal of cell medium, death is slightly delayed and is preceded by a burst in RNA synthesis followed by a notable increase in protein synthesis. Cycloheximide inhibition of protein synthesis is transient and its effect on cell longevity variable. Nonetheless, in all instances cell death is preceded by a burst in protein synthesis. Actinomycin D (1.6 .mu.M) did not significantly affect protein synthesis but delayed RNA synthesis and cell death. The possible roles of auxin, osmoticum and macromolecular synthesis in cellular senescence and death are discussed.