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 correlation 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.

Fruit taste is an important component of fruit quality, but its genetic basis is complex, making it difficult to alter by plant breeding. Thaumatin is a sweet‐tasting, flavour‐enhancing protein produced by fruits of the African plant Thaumatococcus daniellii Benth. Agrobac‐terium‐mediated transformation was used to produce two transgenic tomato lines expressing biologically active thaumatin in fruits. Transgenic tomato fruits from the T2 plant generation were sweeter than the controls and possessed a specific aftertaste as determined by sensory evaluation. These results demonstrate that transgenic expression of thaumatin could be useful for modifying tomato fruit taste, especially in breeding lines possessing poor fruit taste, such as those carrying a non‐ripening (nor) mutation.

Fruits of transgenic cucumber lines expressing preprothaumatin II gene were evaluated concerning their aroma. Four homozygous lines, that is, 210 06, 212 01, 224 09, and 225 03 with different levels of transgene expression were selected. Recipient line cv. Borszczagowski, which was formed by inbred line of Cucumis sativus L., was used as a control. The experiment was carried out in a greenhouse and an outdoor experimental plot. The aroma of cucumber fruits was evaluated by GC/MS, as well as GC/MS/TOF in the distillates and by SPME. Irrespective of the isolation/separation technique used, the differences between aroma compounds in transgenic cucumbers and the control were quantitative, and not qualitative. Modified samples showed higher concentrations of volatiles, particularly of the main cucumber fruits odorant (E, Z)-2,6 nonadienal. Transgenic expression of the thaumatin II gene resulted not only in a sweeter taste of fruits in comparison with the control, but also higher aroma acceptability. This was shown by sensory profile analysis. Also electronic nose measurements differentiated between transgenic lines and the control.

The composition of the fruits of non-transgenic or transgenic (GM) cucumbers with genes coding the synthesis of a sweet protein, thaumatin, were compared and effects of feeding the fruits in balanced diets to rats were determined. The transgenic cucumbers contained more protein (20.3 vs 17.9% DM) and less fibre (9.4 vs 11.4% DM) and also had lower Na, K, Ca and Mg contents and higher levels of Fe and Cu in ash than normal cucumbers. Feeding male rats of initial body weight 150 g for 5 weeks on isoprotein diets containing 0 or 15% lyophilized transgenic or non-transgenic cucumbers did not affect weight gain, apparent health status, or relative organ weights of animals. Protein digestibility was slightly but significantly lower (89.2 vs 90.0%), that of crude fibre was higher (28.2 vs 15.0%) in diets containing transgenic than non-transgenic cucumbers, while digestibility of fat and N-free extractives did not differ.

Transgenic plants are commonly used in breeding programs because of the various features that can be introduced. However, unintended effects caused by genetic transformation are still a topic of concern. This makes research on the nutritional safety of transgenic crop plants extremely interesting. Cucumber (Cucumis sativus L.) is a crop that is grown worldwide. The aim of this study was to identify and characterize differentially expressed genes and regulatory miRNAs in transgenic cucumber fruits that contain the thaumatin II gene, which encodes the sweet-tasting protein thaumatin II, by NGS sequencing. We compared the fruit transcriptomes and miRNomes of three transgenic cucumber lines with wild-type cucumber. In total, we found 47 differentially expressed genes between control and all three transgenic lines. We performed the bioinformatic functional analysis and gene ontology classification. We also identified 12 differentially regulated miRNAs, from which three can influence the two targets (assigned as DEGs) in one of the studied transgenic lines (line 224). We found that the transformation of cucumber with thaumatin II and expression of the transgene had minimal impact on gene expression and epigenetic regulation by miRNA, in the cucumber fruits.