Melonet-DB, a Grand RNA-Seq Gene Expression Atlas in Melon (Cucumis melo L.)

Melon (Cucumis melo L.) is an important Cucurbitaceae crop produced worldwide, exhibiting wide genetic variations and comprising both climacteric and non-climacteric fruit types. The muskmelon cultivar 'Earl's favorite Harukei-3 (Harukei-3)' known for its sweetness and rich aroma is used for breeding of high-grade muskmelon in Japan. We conducted RNA sequencing (RNA-seq) transcriptome studies in 30 different tissues of the 'Harukei-3' melon. These included root, stems, leaves, flowers, regenerating callus and ovaries, in addition to the flesh and peel sampled at seven stages of fruit development. The expression patterns of 20,752 genes were determined with fragments per kilobase of transcript per million fragments sequenced (FPKM) > 1 in at least one tissue. Principal component analysis distinguished 30 melon tissues based on the global gene expression profile and, further, the weighted gene correlation network analysis classified melon genes into 45 distinct coexpression groups. Some coexpression groups exhibited tissue-specific gene expression. Furthermore, we developed and published web application tools designated 'Gene expression map viewer' and 'Coexpression viewer' on our website Melonet-DB (http:melonet-db.agbi.tsukuba.ac.jp) to promote functional genomics research in melon. By using both tools, we analyzed melon homologs of tomato fruit ripening regulators such as E8, RIPENING-INHIBITOR (RIN) and NON-RIPENING (NOR). The 'Coexpression viewer' clearly distinguished fruit ripening-associated melon RIN NOR CNR homologs from those expressed in other tissues. In addition, several other MADS-box, NAM ATAF CUC (NAC) and homeobox transcription factor genes were identified as fruit ripening-associated genes. Our tools provide useful information for research not only on melon but also on other fleshy fruit plants.