# The Small of Synthetic Biology: Engineering Strategies for Aroma Compound Production in Yeast
## 5 Future Outlook
Although not studied in yeast, mutants of a sesquiterpenoid synthase (TPS24) from Syrah grape (_Vitis vinifera_) produced significantly higher levels of \(\alpha\)-guaiene, which is a non-enzymatic precursor of rotundone, the active compound of black pepper [91]. There are also non-metabolic engineering-based challenges that exist in order to enhance aroma compound development. The overwhelming majority of interesting aroma compounds are derived from plants and it would thus make sense to explore the genes relevant to the production of the given product and assess their suitability within a recombinant yeast host. Yet, genomic data of many plants are still lacking and even though the enzymes involved in the production of a certain aroma compound are known, the genes that encode these enzymes have still not been identified. The complexities in assembling plant genomes explain the current dearth of publicly available fully-annotated whole genome sequences of plants [92; 93] but more options will open up as more become available. Many recombinant pathways (like with \(p\)-coumaric acid and raspberry ketone) employed _Arabidopsis thaliana_ genes, mainly due its sequence availability as it is a model organism and hardly because it is the 'best' enzyme candidates. Related to this is the mystery of how certain recombinant genes are expressed in high levels whereas similar ones are expressed poorly [94]. This underlines the utility of still employing a trial-and-error approach and exploring many recombinant genes to assess their ability to be expressed in yeast. Non-_Saccharomyces_ yeast have been shown, on many occasions, to be a more suitable host for the production of aroma compounds. However, the tools available to manipulate these so-called \"non-conventional\" yeast are not as extensive. As discussed in [PERSON] and [PERSON] [95] tremendous strides have been made in this field as even CRISPR-based tools have been developed for a number of species. Further development is required, with a possible aim to incorporate large synthetically designed genetic elements in the non-_Saccharomyces_ genomes similar to the SCRaMbLE-set-up developed for _S. cerevisiae._  
As already shown with the development of a hoppy yeast [87], many of the strategies employed and discussed here could easily be used to engineer aroma development in strains involved in the fermented beverage industry [96; 97; 98]. The engineering strategies should not, however, interfere with all other processes involved in fermentation. Strains tailor-made to produce certain aroma compounds could streamline the fermentation process. An ultimate goal would be to have a winemaker or brewmaster use a specific yeast strain to achieve a more predictable outcome in the aroma profile of the fermented product. An example would be to develop a yeast strain that could synthesise the oak lactones found in oak barrels. Its biosynthesis pathway is complicated and fine-tuning within the yeast strain would be necessary but such a strain would eliminate the use of expensive oak barrels in winemaking and dramatically shorten the production time. Recombinant aroma compound development in yeast is still in its infancy especially when compared to chemical synthesis or extraction from plants--both methods in their various incarnations are centuries old. The field of synthetic biology is primed to evolve by aiming to achieve levels that can compete with the status quo. An even newer competitor is that of cell-free enzyme pathways [99] and it would be interesting to witness how each approach would evolve to compete in producing aroma compounds in a cost-effective manner. A revolutionary step, as one could imagine, would be a large yeast fermentation set-up producing a given aroma compound eventually replacing the acres and acres of flower fields and all the required inputs, dedicated for eventual aroma compound extraction. [PERSON] outlined of the review and wrote the biggest part of the review. [PERSON], H.K. and L.S.P. contributed towards the graphical expansion of the text.