Gene dosage and genetic background are factors that influence transgene expression in transgenic plants. In our previous studies, we produced transgenic tomato plants that accumulate miraculin, a taste-modifying protein, in a genetically stable manner. To elucidate the effects of gene dosage and genetic background on miraculin accumulation in transgenic tomato fruits, we generated hybrid tomato lines between the homozygous transgenic line 56B (background cultivar 'Moneymaker') and the pure cultivars 'Micro-Tom,' 'Moneymaker,' 'Ailsa Craig,' 'M82,' 'Rutgers' and 'Aichi-first' and analyzed them for miraculin mRNA expression and miraculin protein accumulation. The hybrid lines exhibited variation in their fruit structures. Among the hybrid lines heterozygous for the miraculin gene, miraculin accumulation in the fruits varied from 111.0 mg g Ϫ1 fresh weight (FW) to 159.4 mg g Ϫ1 FW. Furthermore, the homozygous line 56B showed higher miraculin accumulation and miraculin mRNA expression than the heterozygous line 56Bϫ'Moneymaker.' These results demonstrate the profound effects of gene dosage and genetic background on miraculin accumulation in transgenic tomato fruits.Key words: Gene dosage, genetic background, miraculin, taste-modifying protein.Plant Biotechnology 27, 333-338 (2010) Original PaperThis article can be found at http://www.jspcmb.jp/ allelic composition and genetic background affect transgene expression and inheritance in white clover. Another study in rice showed that the expression pattern of Xa3/Xa26, which is a rice disease resistance gene, was affected by the genetic background (Cao et al. 2007). The full effects of gene dosage and genetic background on transgene expression and translation remain to be elucidated.In this study, we generated hybrid lines between transgenic line 56B (background cultivar 'Moneymaker'), which is homozygous for the miraculin gene, and the pure line cultivars 'Micro-Tom,' 'Moneymaker,' 'Ailsa Craig,' 'M82,' 'Rutgers' and 'Aichi-first.' Characterization of the hybrids' miraculin mRNA expression and miraculin protein accumulation, revealed the effects of gene dosage and genetic background on miraculin accumulation in transgenic tomato fruits, and in addition we discuss the significance of combining molecular breeding with cross-breeding to produce transgenic tomatoes that accumulate miraculin. Materials and methods Plant material and growth conditionsThe transgenic tomato line 56B was produced in our previous work (Sun et al. 2007). In brief miraculin encoding 660-bp DNA fragment was inserted into the XbaI/SacI sites of the plant transformation vectors pBI121 (Sun et al. 2006). Line 56B possesses a single copy of the miraculin gene driven by the CaMV 35S promoter.In the present study, the miraculin-accumulating transgenic tomato line 56B (background cultivars 'Moneymaker') was crossed with six other cultivars: 'Micro-Tom (accession number TOMJPF00001), ' 'Moneymaker (accession number TOMJPF00002), ' 'Aichi-first (accession number TOMJPF00003), ' 'Ailsa Cra...

The E8 promoter, a tomato fruit-ripening-specific promoter, and the CaMV 35S promoter, a constitutive promoter, were used to express the miraculin gene encoding the taste-modifying protein in tomato. The accumulation of miraculin protein and mRNA was compared among transgenic tomatoes expressing the miraculin gene driven by these promoters. Recombinant miraculin protein predominantly accumulated in transgenic tomato lines using the E8 promoter (E8-MIR) only at the red fruit stage. The accumulations were almost uniform among all fruit tissues. When the 35S promoter (35S-MIR) was used, miraculin accumulation in the exocarp was much higher than in other tissues, indicating that the miraculin accumulation pattern can be regulated by using different types of promoters. We also discuss the potential of the E8-MIR lines for practical use.