# Plant Synthetic Promoters
## 1 Introduction  
Global climate change has elevated the role of sustainable agriculture; there is a growing demand for novel plant genotypes or genetically modified plants with increased tolerance to biotic and abiotic stresses or secondary metabolite production [1; 2; 3; 4; 5; 6; 7]. This can be enabled by the expression of foreign genes that increase resistance to insects, herbicides as glyphosate, or by facilitating biosynthesis, such as carotenes or astaxanthin in rice [8; 9; 10; 11; 12; 13; 14; 15; 16]. Usually, these modifications use the constitutive CAMV35S or native plant promoters to drive transgene expression, often resulting in the silencing, toxicity, or decreased viability of modified plants [17; 18; 19; 20; 21; 22; 23]. Plant genetic modifications support other methods in increasing plant resistance to biotic and abiotic stresses, such as crop breeding and priming [24; 25]. However, to avoid unnecessary feedback regulation or energy loss, there is a need forprecise, simultaneous spatiotemporal or development-stage-dependent regulation of numerous transgenes [26; 27]. Such fine-tuning of transgene expression could be enabled by orthogonal synthetic activators, repressors, and promoter systems containing the introduced cis-active elements, as these would allow better control of transcription regulator binding and gene expression [28; 29]. Traditionally, promoters could be recognized as 5\({}^{\prime}\)-terminal gene fragments that control expression through the coordinated recruitment of particular proteins known as transfactors, recognizing cis-active oligonucleotide sequences in the DNA [30; 31; 32; 33; 34; 35]. The transfactor is organized around the promoter; it interacts with a particular cis-active element to build up a protein complex with RNA polymerase II, resulting in the creation of the RNA polymerase II preinitiation complex [36; 37; 38; 39; 40; 41; 42]. However, relatively little is known of the precise nature of the cis-elements. Although their position weight matrices (PWM) have been characterized, their real, in vivo occurring regulatory functions are usually performed by imperfectly matched, weak, and cooperative DNA-protein interactions dependent on DNA or protein modification [43; 44; 45; 46; 47]. Interactions between cis-active elements and trans-factors are also strongly regulated by the local 3D shape of DNA [48]. In the broader context, gene expression is regulated by various coding and non-coding regions encompassing far-localized elements; these include enhancers, silencers, insulators, gene-embedded promoters, introns, 3\({}^{\prime}\) and 5\({}^{\prime}\) untranslated regions (UTRs), and terminators. These affect not only the chromatin state or assembly of the RNA polymerase II preinitiation complex, but also the 3\({}^{\prime}\)-end processing, stability, translation efficiency, and nuclear-to-cytoplasmic export of mRNAs [49; 50; 51; 52; 53; 54]. Trans-factors can be further regulated through various post-translational modifications such as ubiquitination, acetylation, and phosphorylation; as such, the gene expression rate is dependent on a plethora of signaling events occurring within the plant cell after exposure to biotic or abiotic stresses [43; 55; 56; 57; 58; 59; 60; 61]. Therefore, the changes in promoter activity influenced by trans-factors alter gene expression, influencing current plant cell metabolic demand or external regulatory events [62; 63; 64]. Coordinated gene expression is stimulated by numerous signaling routes. This is reflected in the promoter structure, typically consisting of cis-active motifs forming evolution-conserved clusters known as cis-regulatory modules (CRM); these contain _cis_-active elements assembled closely enough to support the interaction between recognizing trans-factors [65; 66; 67; 68; 69; 70; 71; 72]. Such promoter-dependent dimerization or oligomerization of trans-factors could further increase the complexity of the biological response to changes within plant cells [31; 67].