An Integrative Volatile Terpenoid Profiling and Transcriptomics Analysis in <i>Hoya cagayanensis</i>, <i>Hoya lacunosa</i> and <i>Hoya coriacea</i> (Apocynaceae, Marsdenieae)

<i>Hoya’s</i> R.Br. attractive flower shapes and unique scents make it suitable to be exploited as a new source of tropical fragrance. Therefore, this study aims to elucidate the biosynthesis of secondary metabolites using phytochemical and transcriptomic approaches to understand the mechanisms of scents biosynthesis, especially terpenoid in <i>Hoya</i>. Three <i>Hoya</i> flower species were selected in this study: <i>Hoya cagayanensis</i>, <i>Hoya lacunosa</i>, and <i>Hoya coriacea</i>. The secondary metabolite profiles characterizing scents on flowers were performed using head space solid phase microextraction (HS-SPME). Gas chromatography-mass spectrometry (GC-MS) revealed 23 compounds from <i>H. cagayanensis</i>, 14 from <i>H. lacunose</i>, and 36 from <i>H. coriacea</i>. Volatiles from the three species had different fragrance profiles, with β-ocimene and methyl salicylate compounds dominating the odor in <i>H. cagayanensis</i>. The 1-octane-3-ol was found highest in <i>H. lacunosa</i>, and (Z)-acid butyric, 3-hexenyl ester was found highest in <i>H. coriacea</i>. Subsequent studies were conducted to identify the biosynthesis pathway of secondary metabolites responsible for the aroma profile released by <i>Hoya</i> flowers through transcriptome sequencing using the Illumina Hiseq 4000 platform. A total of 109,240 (75.84%) unigenes in <i>H. cagayanensis</i>, 42,479 (69.00%) in <i>H. lacunosa</i> and 72,610 (70.55%) in <i>H. coriacea</i> of the total unigenes were successfully annotated using public databases such as NCBI-Nr, KEGG, InterPro, and Gene Ontology (GO). In conclusion, this study successfully identified the complete outline of terpenoid biosynthesis pathways for the first time in <i>Hoya</i>. This discovery could lead to the exploitation of new knowledge in producing high-value compounds using the synthetic biology approach.