Transformation in Lycopersicon esculentum L. (Tomato)

Abstract

Cellular genetic manipulations offer new possibilities for breeding tomato. Until the last few years, the introduction of favorable characteristics such as resistance to pathogens, modification of plant habit, and improvement of fruit quality have been carried out by crossing with wild species (Rick 1978). Nevertheless, because of the unilateral or interspecific incompatibility between tomato and its wild relatives, a vast reserve of genetic resources remains unexploited (Rick 1982). Protoplast fusion can be an alternative method for mixing genomes and the development of plant genetic transformation offers new possibilities. Transformation of tomato cultivars was achieved using Ti or Ri plasmid vectors and stable transformants were selected on medium containing kanamycin (Koorneef et al. 1986; McCormick et al. 1986; Shahin et al. 1986). This could be achieved because of the relative facility to regenerate tomato plants from primary expiants. Direct gene transfer procedures are useful for inserting new genes into the genome, as it is not necessary to clone the gene to be transferred into vectors derived from Agrobacterium. In addition, they offer the possibility of performing transient gene expression assays that facilitate comparisons between the strength of promotors that regulate the expression of structural genes (Ou-Lee et al. 1986; Boston et al. 1987), and permit investigation into the interactions between several genes in the same cell (Ecker and Davis 1986). This approach requires an efficient procedure of protoplast culture and plant regeneration. Regeneration of plants from tomato protoplasts still presents several difficulties and makes the use of direct gene transfer techniques less attractive. Direct gene transfer in tomato was first achieved using a calcium-phosphate DNA transformation procedure or a PEG treatment (Koorneef et al. 1986; Jongsma et al. 1987). Toyoda et al. (1989) adapted the microinjection technique to tomato callus cells, but did not succeed in regenerating transformed plants. More recently, Tsukuda et al. (1989) compared the efficiency of promoters in the transient expression of foreign genes introduced in tomato protoplasts by electroporation.