Heterologous Expression of the Melatonin-Related Gene HIOMT Improves Salt Tolerance in Malus domestica.

Kexin Tan,Jiangzhu Zheng,Xianghan Liu,Fengwang Ma,Cheng Liu,Zhiwei Wei,Tengteng Gao,Xiaomin Liu,Xinyang Song,Chao Li

doi:10.3390/ijms222212425

Kexin Tan, Jiangzhu Zheng + Show 8 more

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https://doi.org/10.3390/ijms222212425

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Abstract

Melatonin, a widely known indoleamine molecule that mediates various animal and plant physiological processes, is formed from N-acetyl serotonin via N-acetylserotonin methyltransferase (ASMT). ASMT is an enzyme that catalyzes melatonin synthesis in plants in the rate-determining step and is homologous to hydroxyindole-O-methyltransferase (HIOMT) melatonin synthase in animals. To date, little is known about the effect of HIOMT on salinity in apple plants. Here, we explored the melatonin physiological function in the salinity condition response by heterologous expressing the homologous human HIOMT gene in apple plants. We discovered that the expression of melatonin-related gene (MdASMT) in apple plants was induced by salinity. Most notably, compared with the wild type, three transgenic lines indicated higher melatonin levels, and the heterologous expression of HIOMT enhanced the expression of melatonin synthesis genes. The transgenic lines showed reduced salt damage symptoms, lower relative electrolyte leakage, and less total chlorophyll loss from leaves under salt stress. Meanwhile, through enhanced activity of antioxidant enzymes, transgenic lines decreased the reactive oxygen species accumulation, downregulated the expression of the abscisic acid synthesis gene (MdNCED3), accordingly reducing the accumulation of abscisic acid under salt stress. Both mechanisms regulated morphological changes in the stomata synergistically, thereby mitigating damage to the plants’ photosynthetic ability. In addition, transgenic plants also effectively stabilized their ion balance, raised the expression of salt stress–related genes, as well as alleviated osmotic stress through changes in amino acid metabolism. In summary, heterologous expression of HIOMT improved the adaptation of apple leaves to salt stress, primarily by increasing melatonin concentration, maintaining a high photosynthetic capacity, reducing reactive oxygen species accumulation, and maintaining normal ion homeostasis.

Highlights

Melatonin Concentration in Apple Leaves In GL-3 (Malus domestica Borkh.) plants, salinity induced the heterologous expression of the HIOMT homolog MdASMT; its expression was highest at 9 h, when it was upregulated 5.39-fold relative to unstressed controls (Figure 1A)
All leaves of the wild type (WT) exhibited poorly wilting and necrosis, while only the upper leaves of the transgenic lines exhibited brown spots or chlorosis (Figure 1B). These findings demonstrated that heterologous expression of HIOMT
Our results show that MdNCED3 was significantly upregulated under salt stress, but heterologous expression of HIOMT inhibited its expression, compliance with the discoveries of Li et al [45], in which exogenous melatonin restrained the expression of the abscisic acid (ABA) synthesis gene MdNCED3 under drought

Summary

Introduction

Apple (Malus domestica Borkh.) is one of the main fruits in the world, the cultivation in China is mainly concentrated in bohai Bay and Loess Plateau, which are two dominant producing areas Soil salinization in these areas is one of the main obstacles to the expansion of apple eugenic cultivation areas [1]. The resulting disturbances in most physiological processes include growth inhibition, reduction in photosynthetic capacity, destruction of membranes, changes in enzymatic activities, and ionic imbalances [5]. These phenomena emphasize the urgency of improving apple plants salt stress tolerance

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