Team:UNITN-Trento/Extra/Fruit Info

Plants naturally produce ethylene starting from a common amino acid: methionine (MET), which is transformed in S-adenosyl-methionine (SAM) by a reaction catalyzed by SAM synthetase. The key enzyme of the pathway, ACC synthase, converts SAM to 1-aminocycloprane-1-carboxylic acid (ACC), the immediate precursor of ethylene. Quite recently, the final enzyme of the pathway was identified: ACC oxidase, which converts ACC to ethylene.

All fruits, with a few exceptions (European pears, avocados and bananas), reach their best eating quality when allowed to ripen on the tree or plant. However, some fruits are usually picked mature but unripe so that they can withstand the post-harvest handling system when shipped long-distance. Bananas, for example, are treated with 10-100 ppm of ethylene to initiate their ripening during transport or at destination handling facilities. Treatment of avocados, kiwi fruit, mangos and pears before marketing is increasingly being performed to provide consumers with the choiche of purchasing ready-to-eat, ripe fruits or mature fruits that can be ripened at home. This practice has, in many cases, resulted in increased sales and profits.

Fruit is generally classified in two main physiological groups: climateric and non-climacteric. This is due to their respiratory activity and associated ethylene biosynthesis profiles during ripening. The climateric fruit such as tomatoes, apples and bananas, synthesize ethylene in order to favour the process. Either blocking the synthesis or blocking the perception of this hormone prevents ripening.
Fruit such as strawberries, citruses, peppers and grapes have been classified as non-climacteric, based on the lack of the respiratory burst and on the relative low endogenous production of ethylene. In peppers, for example, some cultivars seem to be ethylene-insensitive, while other pepper cultivars treated with exogenous ethylene were able to stimulate the expression of ripening-specific genes. In strawberries, which has emerged as a prime model of non-climacteric ripening fruit, ethylene is relatively high in green fruits, decreases in white fruits, and finally increases again at the red stage of ripening. Interestingly, this last increase is accompanied by an enhanced respiration rate that resembles the one that occurs in climacteric fruits at the onset of ripening. In fact, recent comparative transcriptome and metabolome studies during the maturation processes of climacteric and non-climacteric fruit (tomatoes and peppers, respectively) suggest that both species have similar ethylene-mediated signaling components. However, in peppers, the regulation of these genes is clearly different and may reflect altered ethylene sensitivity or regulators other than ethylene.