Structural Deterioration in Produce: Phospholipase D, Membrane Deterioration, and Senescence

Ripening and senescence are the ultimate phases in the developmental events of fruits that result in the expression of the quality characteristics inherent to the fruit. Although it is difficult to demarcate a dividing point between ripening and senescence, the former can be designated as the quality-developing phase, whereas the latter predominantly involves the loss of quality. Degradation of structural elements such as the cell wall and the plasma membrane is an inherent feature of ripening and senescence. Wilting of flowers also involves similar deteriorative changes. Harvested vegetables, when exposed to abiotic stresses such as heat, cold, and water deficit, undergo deteriorative changes in the cell membrane that result in the loss of quality. Seasonal changes that result in a decrease in light duration and intensity, temperature, etc., are natural cues for the initiation of ripening in fruits. Often such changes are associated with the biosynthesis of ethylene, which is also an initiator of ripening in many fruits and senescence in leaves. At the ultrastructural level, the events that occur during ripening and senescence reflect the deterioration of cellular structures, and in particular the cell membrane, which results in a loss of compartmentalization of ions and metabolites, leading to the loss of tissue structure and ultimately homeostasis. The importance of regulating the deteriorative events that occur during ripening and senescence has been well recognized. Consequently, several scientific approaches and technologies have been developed with the objective of enhancing the shelf life and quality of harvested fruits, vegetables, and flowers. Some of these include attempts to stabilize the cell wall by the application of calcium, biotechnological approaches to inhibit cell wall–degrading enzymes, and modification of the ethylene biosynthetic pathway through inhibition of key enzymes (e.g., inhibition of ACC synthase by the application of aminoethoxyvinylglycine (AVG), commercially available as “ReTain” (Valent Biosciences)), and biotechnological approaches (e.g., antisense transformation of apple, tomato, and melon for ACC synthase or ACC oxidase). Inhibiting ethylene action through the application of the ethylene antagonist 1-methylcyclopropene (SmartFresh, AgroFresh, Inc.) has become a practice of both basic biological and practical interest. Application of 1-MCP has been successfully used to extend the shelf life of fruits (e.g., apples and pears) and flowers. The cell membrane is a key site where senescence-dependent changes are observed to occur earlier than most other