Tomato Fruit Ripening: Regulation of Ethylene Production and its Response

Climacteric ripening behaviour of tomato (Solanum lycopersicum L.) fruit has direct implications on its consumption, nutritional status, processing aspects and postharvest losses. Among the category of climacteric fruits, ripening of tomato is recognised as a model system. The plant hormone ethylene is a prime regulator of ripening in tomato fruit. Recent developments in the areas like; factors regulating the endogenous levels of ethylene, interaction of ethylene with other components, sensitivity of tissue towards ethylene and the regulatory mechanisms that operate at perception, signaling and ethylene-mediated response are presented here. The review emphasises the role and regulation of two key enzymes of ethylene production (i.e., ACC-synthase and ACC-oxidase) and system 1 and system 2 of ethylene production at biochemical and molecular levels. The change over from system 1 to system 2 of ethylene production is highlighted in view of newly proposed route that is governed by developmental regulation/factors (independent of ethylene) besides the known route that works via system 1 (dependent on ethylene) of ethylene production. Significance of internal microenvironment of fruit (with respect to the levels of O₂, CO₂ and non-ethylene volatiles such as; acetaldehyde and ethanol) is discussed in view of its regulatory role on biosynthesis and response of ethylene. Various approaches that have been targeted to alter either the endogenous levels of ethylene or its response by following molecular manipulations are presented. Finally, a few front line areas of research have been suggested to get better insight into the regulatory aspects of ethylene for its broader and accurate application in postharvest management.