The Aroma Gene: Its Discovery and Use from Rice to Other Economic Crops

Thai Hom Mali rice has outstanding cooking qualities and a unique aroma. The aroma of Hom Mali rice results from the production and accumulation of the aroma compound 2-acetyl-1-pyrroline (2AP), an economically important trait. Because of the importance of this aroma trait, research teams from Kasetsart University and the National Science and Technology Development Agency worked together to find the gene controlling aroma compound production, hoping to use this gene to develop Thai fragrant rice varieties. They began by cooperating with international partners to decode the rice genome. At the start of the project, Her Royal Highness Princess Maha Chakri Sirindhorn graciously provided initial research funding for rice genomics research, which succeeded and ultimately led to the discovery of the aroma gene. The gene was named Os2AP and the rice aroma gene was patented, and is now recognized in 11 countries worldwide: the United States, Australia, Japan, China, Vietnam, India, France, the United Kingdom, the Netherlands, Germany and Switzerland (Vanavichit et al., 2004). The discovery of the aroma gene has clearly benefited Thai fragrant rice breeding today, with new fragrant rice varieties developed using the aroma gene, such as the glutinous rice Thunya Sirin, Hom Mali 80, Hom Chonlasit and Hom Pin Kaset.

The aroma compound 2AP is produced in the polyamine biosynthesis pathway, which depends on the role of the aroma gene Os2AP In non-fragrant rice, this gene produces the enzyme aminoaldehyde dehydrogenase (AMADH), which converts the precursor 4-aminobutanal into gamma-aminobutyric acid (GABA). In contrast, the gene Os2AP in fragrant rice is mutated and cannot function in synthesizing the AMADH enzyme, so the precursor 4-aminobutanal cannot be converted to GABA but is instead converted into the aroma compound 2AP.

Besides using the discovery of the aroma gene in rice breeding, the aroma gene found in rice has also led to the search for aroma genes in other economic crops that can produce aroma compounds. Several research groups have found homologs of the rice aroma gene in other economic crops, such as fragrant soybean (Arikit et al., 2011), fragrant sorghum (Yundaeng et al., 2013), fragrant cucumber (Yundaeng  et al., 2015), fragrant winter melon (Ruangnam et al., submitted), fragrant luffa (Unpublished), fragrant taro (Unpublished), and even perennial plants such as aromatic coconut (Saensuk et al., 2016) and fragrant pomelo (Unpublished), all of which have homologous gene relationships (Figure 1). The discovery of aroma genes in these plants has led to the development of plant varieties and the creation of new agricultural product quality standards, with an overall impact on the public and private sectors as well as the country's agricultural industry. The starting point of discovering the aroma gene in rice is therefore extremely important to Thailand's agriculture today.

References

• Vanavichit, A., 2008. Transgenic rice plants with reduced expression of Os2AP and elevated levels of 2-acetyl-1-pyrroline. 
• U.S. patent No. 7,319,181
• Arikit, S. et al., 2011. Deficiency in the amino aldehyde dehydrogenase encoded by GmAMADH2, the homologue of rice
• Os2AP, enhances 2-acetyl-1-pyrroline biosynthesis in soybeans (Glycine max L.). Plant biotechnology journal, 9(1): 75-87
• Yundaeng C., et al., 2013 Gene discovery and functional marker development for fragrance in sorghum (Sorghum bicolor (L.)
• Moench). Theor Appl Genet. 126(11):2897-906
• Yundaeng C., et al., 2015 A single base substitution in BADH/AMADH is responsible for fragrance in cucumber (Cucumis
• sativus L.), and development of SNAP markers for the fragrance. Theor Appl Genet. 128(9):1881-92
• Saensuk C. et al., 2016 De novo transcriptome assembly and identification of the gene conferring a "pandan-like"  aroma in
• coconut (Cocos nucifera L.). Plant Sci. 252:324-334
• Ruangnam et al., 2017 A deletion of the gene encoding amino aldehyde dehydrogenase enhances the "pandan-like" aroma
• of winter melon (Benincasa hispida) and is a functional marker for the development of the aroma (submitted)