Variable properties of transgenic cucumber plants containing the thaumatin II gene from Thaumatococcus daniellii

Thaumatin represents a unique class of the sweet-tasting plant proteins. Transgenic cucumber (Cucumis sativus L.) plants with stable integrated constructs consisting of the cauliflower mosaic virus 35S promoter and thaumatin II cDNA were produced. Transformed cucumber plants were obtained using Agrobacterium tumefaciens, with one, two or five integration sites in diploid cucumber and with inheritance confirmed by a 3:1 Mendelian ratio and normal morphologies and viable seeds. Inter- and intra-transformant variabilities in the expression of the thaumatin II gene were observed. The variability was independent of integrated copy number of the T-DNA. Variation in thaumatin II protein accumulation levels in the ripe fruits and the lack of correlation between protein and mRNA levels were observed, suggesting that thaumatin may be controlled at the levels of both transcription and translation. Transgenic fruits accumulating thaumatin II protein exhibited sweet phenotype and positive correation between thaumatin accumulation levels and sweet taste intensity was noticed. Thaumatin II belongs to the pathogenesis-related (PR) proteins family. Some of the T2 progeny plants expressing thaumatin II protein did not exhibit tolerance for pathogenic fungus Pseudoperonospora cubensis. These results, together with previously reported results, suggest no relationship between transgenic protein levels and the increased tolerance phenotype.