Physiological and transcription analyses reveal the regulatory mechanism of melatonin in inducing drought resistance in loquat (Eriobotrya japonica Lindl.) seedlings

Drought limits the growth and development of loquat (Eriobotrya japonica Lindl.) seedlings. Although melatonin (MT) is a naturally occurring small molecule in plants and has been linked with response to drought stress, the underlying mechanism remains largely unknown. The protective effects and regulatory mechanisms of MT on the regulation of drought stress in grafted loquat seedlings were analyzed in this study. The physiological results showed that MT effectively mitigated chlorophyll degradation and malondialdehyde (MDA) accumulation in the loquat leaves under drought stress. In addition, MT improved the photosynthetic efficiency and starch content of grafted loquat seedlings. Transcriptome results revealed that MT regulated genes involved in the Ca2+ signal transduction (cyclic nucleotide gated channel, CNGCs and cam modulin/ calmodulin-like protein, CAM/CMLs), starch and sucrose metabolism (sucrose synthase, SuS5 and SuS6), and plant hormone signal transduction (ABA receptor, PYL4; protein phosphatase 2C, PP2Cs; auxin/indole acetic acid protein, AUX/IAAs and ethylene-responsive transcription factor 1B, ERF1B), and the transcription factors such as WRKYs, NACs, ERFs, and bHLHs. We highlighted the analysis on phytohormone signaling pathways and their possible crosstalk. The results also demonstrated that MT increased the endogenous MT, IAA and CTK contents, while decreasing the ABA content under drought condition. Taken together, these results provided a new perspective on the role of MT in drought stress and a network for gene function analysis in MT-mediated signaling pathways.