Cellular Senescence, Radiation Damage to Mitochondria, and the Compensatory Response in Ripening Pear Fruits

Abstract

It is generally accepted that the climacteric phase of ripening fruits is well suited for the study of cellular senescence (1). Since a surge in respiratory activity is the dominant, readily measurable phenomenon, mitochondria are implicated as the sites of causal events. This view was embodied in the early work of Millerd et al. (19) with the resulting postulate that uncoupling of oxidative phosphorylation permitted an increased respiratory rate. Along similar lines, Hulme (7) and Pearson and Robertson (21) attributed the rise in respiration to the availability of phosphate acceptor (ADP) resulting from increased protein synthesis and accelerated demands for energy. However, with specific reference to mitochondrial protein, data on the yield of particulate protein (27) and on the incorporation of amino acids into mitochondrial protein (28) indicate a decline in synthetic activities as fruit reach the climacteric peak.Richmond and Biale (23), Sacher (37), and especially Young and Biale (44) have questioned the prevalence of "uncoupling" or "ADP control" in ripening fruit cells. Moreover, the work of Lance et al. (10), Hulme et al. (8,9) and the experiments reported below, reveal that mitochondria remain functionally sound and "cotipled" throughout the climacteric phase.

The Ribosomes of Pear Fruit

“…However, repair reactions are difficult to assess in fully developed, slowly metabolizing fruit cells unless induced by an imposed stress. Ionizing radiation is a useful tool for such purposes, as demonstrated with the mitochondrial system in ripening pears (36). The mitochondria remained active and coupled at all stages of the climacteric except for a marked agedependent loss in capacity to recover from radiation damage.…”

Acid Phosphatase Development during Ripening of Avocado

“…Ripening of fleshy fruits is an irreversible process of senescence (1,6,8,(16)(17)(18) that is dependent on synthesis of both RNA and protein (2,8,15). Although there is no consistent correlation among different kinds of fruit between changes in the levels of total RNA and protein and the ripening process, there is ample evidence for an increase in activity of manv enzymes during ripening (12,14,19 (22 C), in a large desiccator, at high relative humidity, with an opening to allow for gas exchange.…”

“…The increase in enzvme activity that occurs in slices ripening in moist air was inhibited when tissue sections were infiltrated with solutions, by aspiration for 2 minutes or by soaking for 2 hours, anytime 22 hours or more after addition of ethylene. This inhibition was independent of the presence or absence of cycloheximide or sucrose (0.3-0.5M).However, the large decline in enzyme activity in the presence of cycloheximide, as compared with the controls, indicated that synthesis of acid phosphatase was occurring at all stages of ripening.Ripening of fleshy fruits is an irreversible process of senescence (1,6,8,(16)(17)(18) that is dependent on synthesis of both RNA and protein (2,8,15). Although there is no consistent correlation among different kinds of fruit between changes in the levels of total RNA and protein and the ripening process, there is ample evidence for an increase in activity of manv enzymes during ripening (12,14,19 (22 C), in a large desiccator, at high relative humidity, with an opening to allow for gas exchange.…”

Mitochondrial Longevity In Vitro : The Retention of Respiratory Control

“…Ionizing radiation, for example, will cause a loss of mitochondrial protein and uncoupling with subsequent quantitative and functional recovery dependent upon the extent of initial damage and physiological age of the cells. 12 To the extent that long-lived, isolated mitochondria stimulate mitochondrial functions in vivo, some variability in their survival may be expected. Conversely, longer-lived organelles should be useful in assessing the equivalence, or difference, between the functions of isolated mitochondria and those in their natural environment.…”