Endophytic Bacterium Pseudomonas fluorescens RG11 May Transform Tryptophan to Melatonin and Promote Endogenous Melatonin Levels in the Roots of Four Grape Cultivars.

Yaner Ma,Jian Jiao,Chonghuai Liu,Ying Zhang,Jianfu Jiang,Haisheng Sun,Xiucai Fan

doi:10.3389/fpls.2016.02068

Abstract

Endophytes have been verified to synthesize melatonin in vitro and promote abiotic stress-induced production of endogenous melatonin in grape (Vitis vinifera L.) roots. This study aimed to further characterize the biotransformation of tryptophan to melatonin in the endophytic bacterium Pseudomonas fluorescens RG11 and to investigate its capacity for enhancing endogenous melatonin levels in the roots of different grape cultivars. Using ultra performance liquid chromatography-tandem mass spectrometry combined with 15N double-labeled L-tryptophan as the precursor for melatonin, we detected isotope-labeled 5-hydroxytryptophan, serotonin, N-acetylserotonin, and melatonin, but tryptamine was not detected during the in vitro incubation of P. fluorescens RG11. Furthermore, the production capacity of these four compounds peaked during the exponential growth phase. RG11 colonization increased the endogenous levels of 5-hydroxytryptophan, N-acetylserotonin, and melatonin, but reduced those of tryptamine and serotonin, in the roots of the Red Globe grape cultivar under salt stress conditions. Quantitative real-time PCR revealed that RG11 reduced the transcription of grapevine tryptophan decarboxylase and serotonin N-acetyltransferase genes when compared to the un-inoculated control. These results correlated with decreased reactive oxygen species bursts and cell damage, which were alleviated by RG11 colonization under salt stress conditions. Additionally, RG11 promoted plant growth and enhanced the levels of endogenous melatonin in different grape cultivars. Intraspecific variation in the levels of melatonin precursors was found among four grape cultivars, and the associated root crude extracts appeared to significantly induce RG11 melatonin biosynthesis in vitro. Overall, this study provides useful information that enhances the existing knowledge of a potential melatonin synthesis pathway in rhizobacteria, and it reveals plant–rhizobacterium interactions that affect melatonin biosynthesis in plants subjected to abiotic stress conditions.

Highlights

During their evolution, most land plants, developed symbiotic associations with microbes, including fungi, bacteria, and actinomycetes that can grow in the roots and other plant tissues (Compant et al, 2011; Hameed et al, 2015; Wang et al, 2016)
After RG11 was incubated with 15N-tryptophan, no peak was detected at the correct retention time for 15N-tryptamine at any time point of m/z+ 163→145 (Figure 2C)
These findings suggest that 5hydroxytryptophan might be a key intermediate in the melatonin biosynthesis pathway of P. fluorescens RG11

Summary

Introduction

Most land plants, developed symbiotic associations with microbes, including fungi, bacteria, and actinomycetes that can grow in the roots and other plant tissues (Compant et al, 2011; Hameed et al, 2015; Wang et al, 2016). We previously reported that several common endophytic bacteria have the capacity to synthesize the additional plant physiology regulator, melatonin (N-acetyl-5methoxytryptamine), and could promote the abiotic stress-induced production of endogenous melatonin in grape roots (Vitis vinifera L.) (Jiao et al, 2016). This indoleamine molecule was first isolated from the bovine pineal gland (Lerner et al, 1958), and is recognized as a ubiquitous compound among living organisms including humans, animals, plants, bacteria, fungi, and macroalgae (Tilden et al, 1997; Rodriguez-Naranjo et al, 2012; Tan et al, 2012). Abiotic stressors can elevate the levels of endogenous melatonin in plants (Arnao and Hernández-Ruiz, 2013a,b, 2016; Shi et al, 2015), so stress-induced ROS may trigger melatonin accumulation (Arnao and Hernández-Ruiz, 2015)

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