Lettuce (Lactuca sativa L.)

Lettuce is a globally important leafy vegetable with the United States being the largest world producers. The crop is susceptible to a number of viruses that are aphid transmitted and also highly vulnerable to post harvest diseases. Although wild species of lettuce are an important source of disease resistance genes, their introgression into commercial lettuce has been limited owing to sexual incompatibilities. Hence, the development of a gene transfer system for lettuce would be extremely valuable both in improving the genetic diversity of the crop and also for the transfer of useful agronomic traits. This chapter describes an Agrobacterium-mediated gene delivery system that is highly adaptable for the production of transgenic plants using a wide range of lettuce germplasms. The system described, commonly referred to as the genotype-independent transformation system, has been used for the transfer of several agriculturally useful traits into commercial varieties of lettuce. In this case, A. tumefaciens strain LBA4404 carrying a binary vector with supervirulent pToK47 was used for infecting excised cotyledonary explants. The plant selectable marker gene neomycin phosphotransferase II (nptII) was used, and transformed plants were selected using kanamycin in the culture medium. The beta-glucuronidase gene with intron (gus-intron) was also used in the gene transfer study to confirm the transgenicity of regenerated plants further.