Transgene over-expression has fueled many important plant biology discoveries in the last 40 years, as well as the development of economically important transgenic crop varieties. The 35S promoter from the plant pathogen Cauliflower Mosaic Virus (CaMV) has been instrumental in driving constitutive expression of these transgenes, helping elucidate the function of many plant genes and increasing the understanding of plant processes. The CaMV 35S promoter is arguably the most well studied and experimentally used regulatory component with activity in plant cells, with abundant information available on its individual functional domains and their contribution to promoter activity. Besides its use for constitutive expression of genes, the modular architecture has allowed the CaMV 35S promoter to serve as a backbone to develop a variety of transcription control systems for inducible or repressible gene expression in plants. As plant research embarks onto the era of synthetic biology, novel and more complex plant traits will be developed that require multigene circuits and quantitative understanding of the function of each individual genetic part, so that we can better predict their interactions in vivo . Genetic regulatory parts ( e.g. , promoters, terminators) are becoming more standardized regarding their nucleotide sequence composition and length, and quantitative parameters combined with mathematical models are increasingly being used to describe and predict the behavior of biological systems. In this context, it is fitting for plant synthetic biologists to establish the CaMV 35S promoter as a quantitative reference standard for transcription activity in plants — all other promoters would have their activity expressed relative to CaMV 35S. This will provide a reference point for synthetic biology approaches in plants and leverage plant biology and biotechnology for the next 40 years and beyond.
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