Modifications targeting fruit

Ripening is a genetically regulated developmental process that triggers metabolic changes enhancing the flavor, texture and aroma of fruit but that simultaneously initiates fruit senescence and deterioration. A major goal of tomato genetic engineering has been to manipulate the ripening process in order to delay fruit senescence and deterioration while retaining the beneficial metabolic attributes of the ripening process.

Because of the importance of this developmental process, several approaches have been used to manipulate tomato fruit ripening (Grierson and Fray, 1994). The most general approach has been to modify the expression of regulators of suites of genes that control fruit development or ripening and over-ripening. Ethylene gas is produced by ripening tomato fruit and is the natural hormonal regulator of the ripening process. The amount of ethylene produced has been specifically modified in several lines of transgenic plants as a means to regulate the ripening process (Theologis et al., 1992; 1993). A more specific approach to the regulation of aspects of ripening has been to modify the expression of genes whose products encode enzymes or proteins that are instrumental in a targeted component of the process of ripening. In this regard, modification of the expression of various cell wall hydrolases has been attempted several times to suppress cell wall disassembly and fruit softening while allowing other aspects of ripening to proceed normally (Gray et al., 1992; 1994; Tucker et al., 1999).

	Table 8.2 Promoters used in transgenic tomato
	Specificity	Promoter	Regulation
	Fruit-specific	Tomato2A11
		Tomato PG2	Outer pericarp
		TomatoE8	Ethylene and ripening induced
		TomatoE4
		TomatoLoxA	Outer pericarp
		Apple ACC oxidase	Ripening
		Apple PG	Ripening
		Pepper capsanthin / capsorubin synthase	Drought and ethylene induced
		Pepper fibrillin	Drought and ethylene induced
		Tomato HMG2	Coincides with lycopene synthesis
		Tomato RBCS	Locular specific in developing fruit
	Sucrose sink tissues	Potato ADP-glucose pyrophosphorylase	Sucrose regulated
	Leaf abscission	Soybean cellulase	Ethylene induced
	zones	Tomato abscission zone PG
	Flower-specific	TomatoLAP	Methyl jasmonate induced and induced in wounded leaves
		Tomato LAT 52 and LAT 59	Pollen and late anther specific
	Germinating seeds	Tamoto LoxC
		Tomato LeEXP8	Micropylar region specific
	Vegetative tissue	MaizeSh-1	Not expressed in fruit
	Circadian expression	Tomato Lhc	Clock controlled
	Pathogen induced	Tomato prosystemin	Jasmonate induced insect feeding induced
	Drought induced	Tamoto H1-S histone	ABA induced in all tissues
	Metal regulated	Tamoto metallothionein	Expressed more in leaves than in roots