Phenylalanine 4-Hydroxylase Contributes to Endophytic Bacterium Pseudomonas fluorescens' Melatonin Biosynthesis

Novel melatonin-producing Bacillus safensis EH143 mitigates salt and cadmium stress in soybean

Recently, microorganism and exogenous melatonin application has been recognized as an efficient biological tool for enhancing salt tolerance and heavy metal detoxification in agriculture crops. Thus, the goal of this study was to isolate and evaluate a novel melatonin-producing plant growth promoting bacterium. With high-throughput whole genome sequencing, phytohormone measurements, expression profiling, and biochemical analysis, we can identify a novel PGPB that produces melatonin and unravel how it promotes soybean growth and development and protects against salt and Cd stress. We identify the melatonin synthesis pathway (tryptophan→tryptamine→serotonin melatonin) of the halotolerant (NaCl > 800 mM) and heavy metal-resistant (Cd >3 mM) rhizobacterium Bacillus safensis EH143 and use it to treat soybean plants subjected to Cd and NaCl stresses. Results show that EH143 will highly bioaccumulate heavy metals and significantly improve P and Ca2+ uptake and the K+/Na+ (93%↑under salt stress) ratio while reducing Cd uptake (49% under Cd stress) in shoots. This activity was supported by the expression of the ion regulator HKT1, MYPB67, and the calcium sensors CDPK5 and CaMK1 which ultimately led to increased plant growth. EH143 significantly decreased ABA content in shoots by 13%, 20%, and 34% and increased SA biosynthesis in shoots by 14.8%, 31%, and 48.2% in control, salt, and Cd-treated plants, upregulating CYP707A1 and CYP707A2 and PAL1 and ICS, respectively. The melatonin content significantly decreased along with a reduced expression of ASMT3 following treatment with EH143; moreover, reduced expression of peroxidase (POD) and superoxide dismutase (SOD) by 134.5% and 39% under salt+Cd stress, respectively and increased level of total amino acids were observed. Whole-genome sequencing and annotation of EH143 revealed the presence of the melatonin precursor tryptophan synthase (trpA, trpB, trpS), metal and other ion regulators (Cd: cadA, potassium: KtrA and KtrB, phosphate: glpT, calcium: yloB, the sodium/glucose cotransporter: sgIT, and the magnesium transporter: mgtE), and enzyme activators (including the siderophore transport proteins yfiZ and yfhA, the SOD sodA, the catalase katA1, and the glutathione regulator KefG) that may be involved in programming the plant metabolic system. As a consequence, EH143 treatment significantly reduced the contents of lipid peroxidation (O2-, MDA, and H2O2) up to 69%, 46%, and 29% in plants under salt+Cd stress, respectively. These findings suggest that EH143 could be a potent biofertilizer to alleviate NaCl and Cd toxicity in crops and serve as an alternative substitute for exogenous melatonin application.

Host-adaptive traits in the plant-colonizing Pseudomonas donghuensis P482 revealed by transcriptomic responses to exudates of tomato and maize

Pseudomonads are metabolically flexible and can thrive on different plant hosts. However, the metabolic adaptations required for host promiscuity are unknown. Here, we addressed this knowledge gap by employing RNAseq and comparing transcriptomic responses of Pseudomonas donghuensis P482 to root exudates of two plant hosts: tomato and maize. Our main goal was to identify the differences and the common points between these two responses. Pathways upregulated only by tomato exudates included nitric oxide detoxification, repair of iron-sulfur clusters, respiration through the cyanide-insensitive cytochrome bd, and catabolism of amino and/or fatty acids. The first two indicate the presence of NO donors in the exudates of the test plants. Maize specifically induced the activity of MexE RND-type efflux pump and copper tolerance. Genes associated with motility were induced by maize but repressed by tomato. The shared response to exudates seemed to be affected both by compounds originating from the plants and those from their growth environment: arsenic resistance and bacterioferritin synthesis were upregulated, while sulfur assimilation, sensing of ferric citrate and/or other iron carriers, heme acquisition, and transport of polar amino acids were downregulated. Our results provide directions to explore mechanisms of host adaptation in plant-associated microorganisms.

Improved tolerance of apple plants to drought stress and nitrogen utilization by modulating the rhizosphere microbiome via melatonin and dopamine

This study explored the contributions of melatonin and dopamine to the uptake and utilization of nitrogen and the formation of rhizosphere microbial communities in 'Tianhong 2'/M. hupehensis, with the goal improving plant resistance to drought stress. Drought stress was formed by artificially controlling soil moisture content. And melatonin or dopamine solutions were applied to the soil at regular intervals for experimental treatment. After 60 days of treatment, plant indices were determined and the structure of the rhizosphere microbial community was evaluated using high-throughput sequencing technology. The findings revealed two ways through which melatonin and dopamine alleviate the inhibition of growth and development caused by drought stress by promoting nitrogen uptake and utilization in plants. First, melatonin and dopamine promote the absorption and utilization of nitrogen under drought stress by directly activating nitrogen transporters and nitrogen metabolism-related enzymes in the plant. Second, they promote the absorption of nitrogen by regulating the abundances of specific microbial populations, thereby accelerating the transformation of the soil nitrogen pool to available nitrogen that can be absorbed directly by plant roots and utilized by plants. These findings provide a new framework for understanding how melatonin and dopamine regulate the uptake and utilization of nitrogen in plants and improve their ability to cope with environmental disturbances.

Enzyme Promiscuity in Serotonin Biosynthesis, From Bacteria to Plants and Humans

Serotonin is a phylogenetically ancient compound found in animals, plants, and some bacteria. In eukaryotes, serotonin is synthesized from the aromatic amino acid tryptophan via the key enzymes aromatic amino acid hydroxylase (AAAH) and aromatic amino acid decarboxylase (AAAD). Serotonin is also an intermediate in the melatonin biosynthetic pathway and is involved in several vital functions. In humans, serotonin is produced in the gut and in the brain, is critical in the regulation of multiple body functions, and its depletion has been implicated in multiple neurological disorders including depression and Alzheimer’s disease, as well as other peripheral conditions namely irritable bowel syndrome and fibromyalgia. The serotonin biosynthetic pathway is well described in eukaryotes, but very little is known about this pathway in bacteria. Evidence points to similar pathways since eukaryote-like AAAH and AAAD (and their genes) have been identified in multiple bacteria, even though serotonin production has not yet been detected in most species. Although data on bacterial tryptophan decarboxylase genes are very limited and no bacterial tryptophan hydroxylase genes have been identified to date, evidence suggests that serotonin production in bacteria might occur through different AAAH and AAAD. Substrate promiscuity in these enzymes has been previously reported and seems to be the key aspect in bacterial serotonin synthesis. Considering the human gut microbiota as a potential source of serotonin, further investigation on its biosynthetic pathways in microbes might lead to important discoveries, which may ultimately foster the development of new therapeutic strategies to treat serotonin depletion-related disorders in humans.