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