International Journal of Horticulture, 2025, Vol.15, No.4, 185-194 http://hortherbpublisher.com/index.php/ijh 187 MGL activity is very high in durian, showing that it plays a big role in making the smell. There are also enzymes called thiotransferases that help in this process. They move sulfur from one molecule to another. This helps create more complex and stronger-smelling sulfur compounds (Lu et al., 2024). When these enzymes work together, they turn simple substances into the key ingredients that give durian its special aroma. 3.3 Formation of precursors to final aroma Methionine turns into a substance called methyl mercaptan. This thing smells strong and has a lot of sulfur in it. An enzyme called methionine gamma-lyase does this step (Teh et al., 2017). Then, methyl mercaptan keeps getting changed by other enzymes. In the end, it becomes some smell-related compounds we know, like diethyl disulfide and 3,5-dimethyl-1,2,4-trisulfonamide (Belgis et al., 2017). This whole process gives durian its strong and special smell. Besides methionine, cysteine also takes part in this process. Under the work of sulfur transferase and some other enzymes, cysteine breaks down into things like hydrogen sulfide. These also make the durian smell like sulfur (Lu et al., 2024). 4 Genetic Networks and Molecular Regulatory Mechanisms 4.1 Core gene families behind sulfur compound production Durian has a special smell. This is mainly because of a group of genes in its body called the MGL gene family. There are many of these genes in durian. Compared to other similar fruits, durian has a lot more of them. This helps explain why durian has so many volatile sulfur compounds (VSCs) (Figure 1) (Teh et al., 2017). These sulfur compounds are the main reason durian smells so strong and special. The increase in these genes is not very common. But this is what gives durian its unique smell. It is very good at making strong sulfur smells because of this. In addition to MGL gene, ethylene related ACS gene (aminocyclopropane-1-carboxylate synthase) was also up-regulated in durian. There is a potential link between ethylene production and sulfur compound biosynthesis that may be related to the Yang Cycle, which involves methionine cycling, showing how layered and complex this odor-related gene network really is (Teh et al., 2017). 4.2 The Role of Transcription Factors Like MYB and bHLH For genes to work, they need instructions. That’s where transcription factors come in, especially ones from the MYB and bHLH families. These proteins act like switches, turning on genes that help make volatile organic compounds (VOCs), which are responsible for smell. In fruits like durian, MYB and bHLH transcription factors affect genes involved in making terpenes, phenylalanine-related compounds, fatty acids, and sulfur-containing volatiles (Lu et al., 2024; Wang and Zhang, 2024). But in durian, controlling these sulfur compounds is not easy. MYB and bHLH don’t work alone — they team up with other proteins and even epigenetic factors to control gene expression at different levels. This complex setup helps adjust the production of durian’s special smell, depending on its growth stage and the environment (Lu et al., 2024). 4.3 Construction of gene regulatory network The researchers linked some key genes, like MGL and ACS, with the transcription factors that control them. They wanted to make a regulatory network map to see how durian makes its special smell. In this map, the MGL gene plays a main role in making sulfur volatiles. The ACS gene is related to ethylene control. The transcription factors MYB and bHLH affect how MGL and ACS work. The activity of these genes also changes with the plant’s growth and the environment (Teh et al., 2017; Lu et al., 2024). The gene network also looks at how other metabolic paths interact and give feedback. For example, the paths that make terpenoids and phenylalanine. These also add to the durian’s total smell. By showing all these links, we can see how the durian’s genes work together to make this strong and complex scent (Liang, 2024; Lu et al., 2024).
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