Evaluation of transgenic cucumbers expressing the thaumatin gene

We have analyzed the expression of the structural gene of the sweet-tasting plant protein (preprothaumatin II) from Thaumatococcus daniellii in transgenic cucumber (Cucumis sativus L.) plants, obtained from Agobacterium-mediated leaf microexplant transformation, using RNA and protein blot and fruit flavour analysis. Thaumatin is initially translated in a precursor form preprothaumatin II with amino and carboxyterminal peptides that are cleaved of during the protein's maturation [1]. The naturally occuring thaumatin II represents a processed form. Our data demonstrate that a DNA copy of preprothaumatin II-encoding mRNA is expressed in cucumber leaf and fruit tissues under transcriptional control of the CaMV35S promoter. Biologically active recombinant thaumatin (r-thaumatin) is produced in fruits of transgenic cucumbers of levels that induce the sweet-taste phenomenon. Leaf discs of transgenic cucumber plants exhibited inhibited development of disease symptoms after inoculation with the spore suspension of fungus Pseudoperonospora cubensis which is the cause of downy mildew, a disease of cucumber [2].