# Plant Synthetic Promoters
## 7 Discussion  
The properties of synthetic promoters and their applications have been described in several review articles [256; 257; 258; 259; 260; 261]. However, research in this field is progressing rapidly, fueled by climate change, decreasing arable land area, and a fast-growing global population [1]. The presented processes increase the demand for the genetic modification of plant's complex metabolic routes, which could be supported by synthetic promoters, enabling the precise spatiotemporal regulation of multiple transgenes [153; 154; 155; 178; 179; 180]. Therefore, the presentation of the novel research results, combined with an individual perspective of the area, could add interesting features to the presented studies. Plant genetic modification by synthetic promoters is pivotal for introducing metabolic switches, and for enabling the development of novel, more efficient, or stress-resistant crop variants with outstanding characteristics or an increased biosynthesis rate of valuable metabolites [1; 4; 5; 9; 10]. Genetic modifications of common crop plants require the application of up to 15 different genes [155; 156; 152; 13; 153]. Their activity should be precisely regulated to adjust the right proportions between all transcripts within complex traits. Such a task is realized by synthetic promoter sets of exactly adjusted activities [195; 197; 198; 199; 200; 201; 202; 203]. The precise regulation of co-expressed genes in complex metabolic traits by synthetic promoters of different strengths protects against imbalance in metabolite concentration, feedback regulation of induction and energy losses [27; 178; 194]. However, the repeated use of the same or related DNA native promoter sequences, combined with conventional, multi-round breeding techniques, results in homology-based gene silencing, plasmid instability, complex segregation patterns, and the random integration of multiple transgenes expressed at varied and poorly controlled levels [156; 157; 165; 166]. Therefore, novel sets of synthetic promoters with low homology to those already existing are developed and applied to plant transformation [28; 29; 199; 200; 201]. Moreover, transgenes are generally stacked in a single construct controlled by a minimum number of synthetic promoters that should not be homologous to those already used in plant genetic modification [160; 167; 170; 171; 172]. This reduces the frequency and scale of complex segregation patterns and random integration into the plant genome [168; 169]. This necessity for gene stacking, combined with the decreasing number of available native promoters, is addressed by a more broad development and application of synthetic bidirectional promoters that are relatively short and compact, enabling the expression and precise regulation of up to six-eight genes for each synthetic promoter [173; 174; 175; 176; 177; 238]. A more common application of these promoters is supported by already available multi-stronic vectors [170]. Therefore, the demand for novel synthetic promoters is still increasing, ordered by an increased rate of complex plant genetic modifications, requesting the application of more genes and their promoters for each transgenic plant [152; 153; 154; 155]. They are typically obtained by hybridizing existing cis-active elements with other core promoters or by exchanging entire promoters as well as their domains [195; 215; 216; 217; 222]. Therefore, synthetic promoters fuse different domains or cis-regulatory elements with the core promoter, thus combining both tissue specificity and increased or precisely tuned expression in the same construct [85; 178; 180; 181]. Cis-active elements are introduced according to their positions, copy number, distance, and orientation to promote optimum spacing among the cis-motifs and corresponding trans-factors, which could mutually interact and build functional di- or oligomer complexes [182; 183; 184; 185; 186; 188].