Exogenous melatonin affects kiwifruit growth and quality through hormone signaling pathway

Kiwifruit is popular among consumers owing to its rich nutritional content. Melatonin, a newly recognized plant hormone, has yet to be fully understood in terms of its impact on fruit growth. In this study, we applied different concentrations melatonin to 'Jinyan' kiwifruit at fruit expansion phase. The results showed that melatonin treatments significantly increased the vertical and horizontal diameter and single fruit weight, especially at 200 μM melatonin. Additionally, melatonin treatment increased the soluble sugar concentration and the ratio of sugar and acid, while reducing titrable acid content. Transcriptome analysis showed that DEGs were mainly enriched pathways related to plant hormone and starch sucrose metabolism. The expression of starch degradation genes AcBAMs and sucrose synthesis related genes AcSPSs and AcSUS4 were up-regulated, accompanied by decreased starch content and increased the contents of sucrose, fructose and glucose. Genes involved in auxin, cytokinin and gibberellin signal transduction pathway were also up-regulated at 200 μM melatonin treatment, resulting in increased the contents of 6BA (6-Benzyl aminopurine),ZT (Zeatin) and GA3 (Gibberellin 3). Through weighted co-expression network analysis, four structural genes and two transcription factors were identified as potential key regulators involved in melatonin-mediated regulation on fruit development and starch-sucrose metabolism.