Transcriptome Profiling of Parthenocarpic Genes in Cucumber (Cucumis sativus L.) Reveals Global Effects of Multiple Phytohormone Signal Pathways

Parthenocarpy leads to un-pollinated/-fertilized fruit set. It helps to maintain a stable production of fruits under adverse environment and widely exists in fleshy fruit plants. Previous studies showed that parthenocarpic fruit set is a complex physiological process that can be induced by various phytohormones and controlled by multiple genes. Global study of the transcriptome of parthenocarpy will help to reveal molecular mechanism of parthenocarpy and screen valuable parthenocarpic genes.

Transcriptome of cucumber fruits was investigated with RNA-Seq technique. Differentially expressed genes were analyzed in parthenocarpic, fertilized and abortive cucumber fruits.

There were about 1710 genes up-regulated and 1311 genes down-regulated in expansive fruits (including parthenocarpy and fertilized fruits) vs. abortive fruits and 1339 genes were found that specifically expressed in parthenocarpic fruits. The endogenous hormones were increased during the expansion of cucumber fruits e.g. auxin, cytokinin, and gibberellin. RNA-Seq data also supported that most of these hormone synthesis genes were up-regulated in expansive fruit. Furthermore, the expression of signal pathway relative genes also dramatically changed during fruit development. However, 23 genes were found that expressed only in parthenocarpic fruits, which distribute nearly all the hormone signal pathways and 9 of them were proved to induce parthenocarpic fruit set through transient expressing experiments.

It is concluded that the initiation and development of cucumber fruit requires multiple effects of various phytohormones. Parthenocarpic fruit set can be stimulated either by master gene level, i.e. hormone synthesis genes, or by regulator genes, i.e. transcriptional factor genes involve in hormone signal pathway.