Exogenous Melatonin Application Enhances Rhizophagus irregularis Symbiosis and Induces the Antioxidant Response of Medicago truncatula Under Lead Stress.

Huijuan Zhang,Ming Tang,Xiangyu Zhang,Haoqiang Zhang

doi:10.3389/fmicb.2020.00516

Abstract

Melatonin is a new kind of plant growth regulator. The aim of this study was to figure out the effect of melatonin on arbuscular mycorrhizal (AM) symbiosis and heavy metal tolerance. A three-factor experiment was conducted to determine the effect of melatonin application on the growth, AM symbiosis, and stress tolerance of Medicago truncatula. A two-factor (AM inoculation and Pb stress) experiment was conducted to determine the effect of AM fungus on melatonin accumulation under Pb stress. AM plants under Pb stress had a higher melatonin accumulation than non-mycorrhizal (NM) plants under Pb stress. Acetylserotonin methyltransferase (ASMT) is the enzymatic reaction of the last step in melatonin synthesis. The accumulation of melatonin may be related to the expression of MtASMT. Melatonin application increased the relative expression of MtPT4 and AM colonization in AM plants. Melatonin application decreased Pb uptake with and without AM inoculation. Both melatonin application and AM inoculation improved M. truncatula growth and increased antioxidant response with Pb stress. These results indicated that melatonin application has positive effects on AM symbiosis and Pb stress tolerance under Pb stress. AM inoculation improve melatonin synthesis capacity under Pb stress. Melatonin application may improve AM plant growth by enhancing AM symbiosis, stimulating antioxidant response, and inhibiting Pb uptake.

Highlights

With social and economic development, soil heavy metal pollution has produced a series of problems in China (Li et al, 2014)
Pb stress stimulated the accumulation of melatonin in roots, indicating that Pb-stressed plants may increase their antioxidant capacity by increasing melatonin accumulation
arbuscular mycorrhizal (AM) inoculation promoted melatonin synthesis under Pb stress. This result suggested that AM plants can become melatonin-rich under Pb stress to cope with Pb toxicity

Summary

Introduction

With social and economic development, soil heavy metal pollution has produced a series of problems in China (Li et al, 2014). Anthropogenic activities such as mining and smelting are the primary sources of soil heavy metal pollution (Yang et al, 2018). Melatonin synthesis in plants is primarily divided into two steps that involve four enzymes, tryptophan decarboxylase (TDC), tryptamine 5hydroxylase (T5H), serotonin N-acetyltransferase (SNAT), and acetylserotonin methyltransferase (ASMT) (Back et al, 2016; Lee et al, 2017). The first step of melatonin synthesis in plants is the transformation of tryptophan to serotonin through TDC and T5H activity. Melatonin-rich plants or plants to which melatonin has been exogenously applied have a higher potential for the improvement of plant growth and stress tolerance (Tan and Reiter, 2015; Tang et al, 2018)

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