# Plant Synthetic Promoters
## 3 Objectives and General Methodology of Synthetic Promoter Creating  
Modern plant biotechnology may require the modification of up to 15 genes [152]. Indeed, for all metabolic and signaling pathways in plants, the engineering of complex regulatory networks requires the coordinated expression of numerous genes [152; 153; 154; 155]. Therefore, sets of different regulatory elements as synthetic promoters are needed. Novel synthetic promoters are also needed to resolve the problems observed after the repeated application of the same synthetic promoters, such as plasmid instability or homology-based gene silencing [156; 157]. Genetically modified plants could produce valuable metabolites, acquire resistance to herbicidal, insect, or fungal expositions, and efficiently grow under abiotic stress conditions [155; 158]. Although the final outcome of such approaches is the modification of metabolic fluxes adopting sessile plants to external conditions, the starting point is the precise and simultaneous expression control of numerous genes [159; 160; 161; 162]. The stacking of numerous genes in transgenic plants requires sequential transformation by single gene vectors or multiple separate gene expression vectors [163; 164]. Conventional breeding techniques require multiple crosses of different transgenic events. These traditional strategies suffer from inherent pitfalls such as complex segregation patterns and the random integration of multiple transgenes, resulting in non-stoichiometric and variable transgene expression [165; 166]. One efficient solution involves using a single transgenic event to allow the coordinated expression of multiple transgenes by the minimum number of regulatory elements [160; 167]. Such stacking is an optimal method of allowing the simultaneous expression of a gene set, as it integrates them into adjacent regions of the genome and reduces the probability of transgene segregation in later generations [168; 169]. Multicistronic plant expression vector systems have been developed for this purpose [170; 171; 172]. One particularly useful approach that would allow the controlled expression of numerous genes involves the use of bidirectional promoter systems [173; 174; 175; 176]. Contrary to tandemly repeated monodirectional expression cassettes, bidirectional promoters can identify optimal promoter contributions for transgene co-expression, available in a single round of cloning, expression, and screening experiments. However, a library of synthetic promoters is required to study their varying expression levels and regulatory profiles in both expression directions [177]. According to [PERSON] et al., the gene stacking approach could potentially be extended to express 6-8 genes with a single bidirectional promoter [173]. Synthetic promoters join different cis-regulatory elements with the core promoter to combine tissue specificity with increased or precisely tuned expression within the same construct [178; 179; 180; 85; 177; 181]. Cis-active elements within synthetic promoters are introduced according to their positions, copy number, spacing, and orientation to promote optimum spacing among cis-motifs and corresponding trans-factors [182; 183; 184; 185; 186; 187; 188]. The obtained promoters possess different tissue-specific, strength, and inducibility properties, enabling the co-expression of numerous genes at different levels, as is required by the complex biosynthesis traits or regulatory circuits responding to various environmental conditions, i.e., biotic, abiotic, tissue-specific, light stress, or hormonal stimulation [184; 185; 189; 190; 191; 184]. To avoid unwanted recombination or homology-dependent gene silencing, the expression of each gene is regulated by a different promoter [189; 195].