Melatonin and Its Homologs Induce Immune Responses via Receptors trP47363-trP13076 in Nicotiana benthamiana

Mengmeng Kong,Jian Chen,Jia Liu,Qurban Ali,Xuewen Gao,Tao Sheng,Jing Liang,Qin Gu,Huijun Wu

doi:10.3389/fpls.2021.691835

Abstract

Melatonin (N-acetyl-5-methoxytryptamine), a naturally occurring small molecule, can protect plants against abiotic stress after exogenous treatmenting with it. It is not known if melatonin homologs, such as 5-methoxytryptamine and 5-methoxyindole, that are easy and more cost-effective to synthesize can stimulate the plant immune system in the same manner as melatonin. In the present study, we assessed the biological activity of the melatonin homologs, 5-methoxytryptamin and 5-methoxyindole. The results showed that melatonin and its homologs all induced disease resistance against Phytophthora nicotianae in Nicotiana benthamiana plants. The application of all three compounds also induced stomatal closure and the production of reactive oxygen species. Gene expression analysis indicated that the expression of genes involved in hydrogen peroxide (H2O2), nitric oxide (NO) production, and salicylic acid (SA) biosynthesis was significantly upregulated by all three compounds. Four homologs of the melatonin receptors were identified by blasting search with the phytomelatonin receptor in Arabidopsis. Molecular docking studies were also used to identify four putative melatonin receptors in N. benthamiana. Further experimentation revealed that silencing of the melatonin receptors trP47363 and trP13076 in N. benthamiana compromised the induction of stomatal closure, PR-1a gene expression and SA accumulation by all three compounds. Collectively, our data indicate that the induction of defense responses in N. benthamiana by melatonin, 5-methoxytryptamine, and 5-methoxyindole involves the melatonin receptors trP47363 and trP13076.

Highlights

Plants have evolved two innate immunity systems that respond to a broad range of microorganisms (Zuppini et al, 2004; Zhang et al, 2010; Jian et al, 2021)
Results indicated that the pretreatment of N. benthamiana plants with MT, 5-methoxytryptamine, and 5-methoxyindole significantly inhibited (p < 0.01) lesion size in inoculated leaves of N. benthamiana by 16.56–23.31%
Since MT, 5-methoxytryptamine, and 5-methoxyindole are based on a 5-methoxyindole backbone, while N-acetyltryptamine, tryptamine, and indole are based on indole, we postulated that 5-methoxyindole represents the functional backbone of MT

Summary

Introduction

Plants have evolved two innate immunity systems that respond to a broad range of microorganisms (Zuppini et al, 2004; Zhang et al, 2010; Jian et al, 2021). Elicitors may be secreted by the microorganism attempting to infect a host plant or substances released by hydrolytic enzymes produced by the pathogen or plant. Plant cells respond to the elicitors by an influx in calcium ions, as well as the production of active oxygen species (AOS) and nitric oxide (NO). These induced molecules regulate many processes and interconnect different pathways to amplify and generate a physiological response by the host plant, such as a hypersensitive response and stomatal closure, through the induction of transcriptional and metabolic changes (Garcia-Brugger et al, 2006; Zhang et al, 2010). The various downstream responses that are triggered are not independent but rather overlap, including the induced expression of pathogenesis-related (PR) genes, phytohormone homeostasis regulation, synthesis of reactive oxygen species (ROS), and secondary metabolite accumulation (La Camera et al, 2004; Pieterse et al, 2009; Feng and Shan, 2014)

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Results

Conclusion