Unveiling unknown transcripts and exons: Insights into durian var. D24 (Durio zibethinus Murr.) fruit development and ripening

Durian (Durio zibethinus Murr.) plant fruits are popular among Southeast Asians. This study aimed to characterise the gene expression during the growth and development of durian fruit at its young, mature, and ripening stages. We used a high-throughput RNA Sequencing approach to identify and characterise unknown transcripts (UTs) from durian fruit pulp transcriptomes. The categorisation of UTs relied on assessing their coding potential and conducting sequence analysis, followed by thorough manual curation. After mapping, 110 million high-quality reads were analysed for each of the nine samples, revealing that each contains 76,700 to 89,117 distinct transcripts and 561,211 to 646,291 exons. In the present analysis, we emphasised identifying and classifying UTs, disregarding transcripts with accurate annotations. Differential expression analysis identified 280 significant unknown transcripts, presumably involved in various biological functions in durian growth. The top BLASTn comparative analysis results for 183 unknown transcripts (UTs) primarily showed significant homology, ranging from 80 % to 100 %, with transcripts from closely related species within the Malvaceae family, including Bombax ceiba, Gossypium hirsutum, Gossypium raimondii, Herrania umbratica, and Theobroma cacao. Functional annotation showed that many upregulated UTs may encode protein-coding genes involved in cellular and metabolic activities, catalytic and electron transfer activities, cellular and anatomical entities, and protein-containing complexes. Out of 97 UTs that do not have a BLASTn hit, 2 of them match GO terms, TCONS 00034019 and TCONS 00058246, corresponding to the InterPro GO Names, namely P: positive regulation of organ growth (GO:0046622) and F: calcium ion binding (GO:0005509). Three open reading frame (ORF) sequences longer than 300 nucleotides were identified as potential protein-coding genes through SMARTBLAST analysis, showing sequence similarities with Retropepsins, pepsin-like aspartate proteases, DUF4492 domain-containing protein, and a hypothetical protein CTI12_AA006750 in UniProtKB/Swiss-Prot. In conclusion, this study sheds light on the dynamic gene expression patterns during durian fruit development. It highlights the significance of characterising unknown transcripts and their potential roles in biological processes, thus enhancing our understanding of durian genetics.