The DR5 and E8 reporters are suitable systems for studying the application of the <i>Ruby</i> reporter gene in tomato

Tomato (Solanum lycopersicum) fruits can be found in a range of colors, including green, yellow, red, pink and purple. The various colors are determined by the accumulation of molecules such as chlorophyll, carotenoids (including lycopene) and flavonoids. Since the DR5 promoter is induced by auxin, we can infer that the auxin responses differ at different developmental stages in tomato. Fruit-specific promoters can effectively drive gene expression in fruits, and the E8 promoter is one of the most widely used fruit-specific promoters. In this study, DR5::Ruby transgenic tomato plants were used to further clarify the auxin content at different developmental stages in tomato. The results showed that the nonuniformity of Ruby activity was consistent with the natural distribution characteristics of native auxin, and the results were also consistent with the qRT‒PCR results. Furthermore, the accumulation of endogenous auxin at the immature green stage (IMG) was higher than that at the other stages.In addition, E8::Ruby was transformed into different tomato varieties (red, white and yellow) to generate new tomato varieties with fruit of different colors. The results showed that the E8::Ruby-transformed plants exhibited asymmetrical pink coloration in different tomato tissues of plants from different tomato cultivars. In conclusion, all the results showed that DR5 and E8 are suitable systems for studying the application of the Ruby reporter gene in tomato. Moreover, the study will provide insight for the improvement of tomato fruit color via biotechnology.