**9. Conclusions**

Over the last few decades, studies on plant volatile compounds and their biosynthetic processes have markedly increased. In orchids, volatile compounds play a key role in pollination, which ensures fertilization. To date, the biosynthesis of orchid floral fragrance is not well understood, with only some terpenoid pathways reported. Plant volatiles are generally produced at very low concentrations with low quantity, even in floral tissues. Thus, isolation of each component in the volatile compounds is inefficient and expensive. Despite the presence of studies of floral VOCs, many aspects of their biosynthesis together with transcriptional regulation and function require further studies. Further advances in functional studies on key genes for floral scent may rely on a breakthrough in orchid transformation technology that may lead to more efficient results. The genome sequences of several orchids have now been determined [112,113]. Overall, it is clear that genetic manipulation of orchid volatile compounds may be possible, but requires the selection of the appropriate species. In the future, studies in scent research may focus on orchid floral traits and on increasing phytochemical compounds, flavor, and aroma through the regulation of genes by transcription factors in floriculture crops. This review provides an important theoretical reference for aromatic volatile compound studies in orchids.

**Author Contributions:** M.R., S.J., and P.H.P., drafted the manuscript. M.R., H.-R.A., P.-M.P., and S.J. collected the background information. S.J. and S.-Y.L. revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This review paper was financially supported by the National Institute of Horticulture and Herbal Science, RDA, Korea, under the project grant PJ01183202. Funding for the World Vegetable Center co-author (S.J.) was provided in part by the World Veg Korea Office budget (WKO #10000379) and the long-term strategic donors to the World Vegetable Center: Republic of China (Taiwan). The authors are also grateful for UK aid from the UK government, the United States Agency for International Development (USAID), the Australian Centre for International Agricultural Research (ACIAR), Germany, Thailand, Philippines, Korea, and Japan.

**Conflicts of Interest:** The authors of this paper declare that there is no conflict of interest in relation to the publication of this review paper.
