# Plant Synthetic Promoters
## 7 Discussion
The prepared synthetic promoters demonstrate different tissue specificities, as well as strengths, and inducibility properties to precisely control the co-expression of numerous genes in complex biosynthesis traits [189; 190; 191; 192; 193; 194]. In addition to the exchange of domains or cis-elements, novel synthetic promoter variants can also be prepared by the introduction of genetic changes into existing cis-active elements and core promoters [89; 214]. Usually, these approaches are based on error-prone PCR [204; 205; 206; 207; 208; 209; 210; 211]. Larger libraries of synthetic promoters are obtained after repeated applications of directed evolution [213]. The further growth of libraries above \(10^{8}\) elements requires the application of synthetic DNA fragments not related to known DNA genome sequences [46]. However, the analysis of such large libraries usually employs ML or DL to identify relations betweenDNA sequence properties and promoter activity [46; 50; 241; 242; 243; 251; 262; 263]. To improve these ML and DL approaches, they are trained on synthetic, random DNA fragments to test a larger sequence space; models trained on such synthetic data can predict genomic activity better than those solely trained on genome DNA [46; 251]. More complex relationships between synthetic promoter structure, activity and DNA methylation or histone acetylation status are efficiently addressed by multilayered DL algorithms, while the ML models could not grasp these labyrinthine qualities precisely [242; 247; 248; 249; 250]. Usually, information obtained from the ML and DL methods is verified experimentally to test the accuracy of model predictions [213; 264], and the repeated construction of ML/DL models followed by the experimental validation of the promoter properties has yielded stepwise improvement. Synthetic promoter libraries used in ML and DL architecture are not only of a large size but they need to indicate a sufficient dynamic range, gradient strength, and clear sequence profiles [46]. However, the large scale of the applied libraries requires the more broad application of high-throughput analytical tool methods, such as high-capacity protoplast transformation, combined with a fluorescence-inducing laser projector (FLIP) platform equipped with an ultra-low-noise camera; this has been used to discriminate between numerous fluorescent signatures under different stress conditions [253; 254; 255]. Besides the ML- and DL-dependent production and analysis of huge synthetic promoter libraries, synthetic promoters are also developed on the basis of orthogonal expression systems. The main advantage of these systems is the general lack of similarity with existing DNA promoter sequences, resulting in low homology-based gene silencing [28; 29; 186; 198]. Moreover, trans-factors recognizing them are also synthetic, built from elements obtained from yeast, bacteria and plants, enabling the complete decoupling of the transgenes transcriptional regulation from those endogenous to the genome [28]. Transcriptional activity of orthogonal expression systems could be precisely tuned and accustomed to use in complex logic gates to provide a very precise control of complex genetic circuits [28; 29; 201; 202; 203].