Exogenous Melatonin Reprograms the Rhizosphere Microbial Community to Modulate the Responses of Barley to Drought Stress

Exogenous Melatonin Reinforces Photosynthesis, Antioxidant Defense and Gene Expression to Ameliorate Na2CO3 Stress in Maize

Salt stress can seriously affect the growth and development of maize (Zea mays L.), resulting in a great yield loss. Melatonin (MT), an indole hormone, is a potential enhancer of plant tolerance against salt stress. However, the complex mechanisms of MT application in enhancing maize salt tolerance are still unclear. Herein, three-leaf seedlings of salt-susceptible P138 and its salt-resistant ethyl methane sulfonate (EMS)-104 mutant were cultured with or without 150 μM MT application under 0 and 100 mM Na2CO3 treatments for seven days, to systematically explore the response mechanisms of exogenous MT in improving the salt tolerance of maize. The results showed that salt stress triggered an escalation in reactive oxygen species production, enhanced multiple antioxidant enzymes' activities, impaired cellular membrane permeability, inhibited photosynthetic pigment accumulation, and ultimately undermined the vigor and photosynthetic prowess of the seedlings. While suitable MT application counteracted the detrimental impacts of Na2CO3 on seedlings' growth and photosynthetic capacity, the seedling length and net photosynthetic rate of P138 and EMS-104 were increased by 5.5% and 18.7%, and 12.7% and 54.5%, respectively. Quantitative real-time PCR (qRT-PCR) analysis further showed that MT application activated the expression levels of antioxidant enzyme-related genes (Zm00001d025106, Zm00001d031908, Zm00001d027511, and Zm00001d040364) and pigment biosynthesis-related genes (Zm00001d011819 and Zm00001d017766) in both maize seedlings under Na2CO3 stress; they then formed a complex interaction network of gene expression, multiple physiological metabolisms, and phenotype changes to influence the salt tolerance of maize seedlings under MT or Na2CO3 stress. To sum up, these observations underscore that 150 μM MT can alleviate salt injury of maize seedlings, which may provide new insights for further investigating MT regulation mechanisms to enhance maize seedlings' salt resistance.

Unravelling the secrets of soil microbiome and climate change for sustainable agroecosystems

The soil microbiota exhibits an important function in the ecosystem, and its response to climate change is of paramount importance for sustainable agroecosystems. The macronutrients, micronutrients, and additional constituents vital for the growth of plants are cycled biogeochemically under the regulation of the soil microbiome. Identifying and forecasting the effect of climate change on soil microbiomes and ecosystem services is the need of the hour to address one of the biggest global challenges of the present time. The impact of climate change on the structure and function of the soil microbiota is a major concern, explained by one or more sustainability factors around resilience, reluctance, and rework. However, the past research has revealed that microbial interventions have the potential to regenerate soils and improve crop resilience to climate change factors. The methods used therein include using soil microbes' innate capacity for carbon sequestration, rhizomediation, bio-fertilization, enzyme-mediated breakdown, phyto-stimulation, biocontrol of plant pathogens, antibiosis, inducing the antioxidative defense pathways, induced systemic resistance response (ISR), and releasing volatile organic compounds (VOCs) in the host plant. Microbial phytohormones have a major role in altering root shape in response to exposure to drought, salt, severe temperatures, and heavy metal toxicity and also have an impact on the metabolism of endogenous growth regulators in plant tissue. However, shelf life due to the short lifespan and storage time of microbial formulations is still a major challenge, and efforts should be made to evaluate their effectiveness in crop growth based on climate change. This review focuses on the influence of climate change on soil physico-chemical status, climate change adaptation by the soil microbiome, and its future implications.

Drought Stress Alleviator Melatonin Reconfigures Water-Stressed Barley (Hordeum vulgare L.) Plants' Photosynthetic Efficiency, Antioxidant Capacity, and Endogenous Phytohormone Profile

The production of crops is severely limited by water scarcity. We still do not fully understand the underlying mechanism of exogenous melatonin (MT)-mediated water stress tolerance in barley. This study is the first of its kind to show how MT can potentially mitigate changes in barley's physio-biochemical parameters caused by water deficiency. Barley was grown under three irrigation levels (100%, 70%, and 30% of field capacity) and was foliar sprayed with 70 μM MT. The results showed that exogenously applied MT protected the photosynthetic apparatus by improving photosynthetic pigment content, photochemical reactions of photosynthesis, Calvin cycle enzyme activity, gas exchange capacity, chlorophyll fluorescence system, and membrane stability index. Furthermore, the increased levels of salicylic acid, gibberellins, cytokinins, melatonin, and indole-3-acetic acid, as well as a decrease in abscisic acid, indicated that foliar-applied MT greatly improved barley water stress tolerance. Additionally, by increasing the activity of antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase and decreasing hydrogen peroxide content, lipid peroxidation, and electrolyte leakage, MT application lessened water stress-induced oxidative stress. According to the newly discovered data, MT application improves barley water stress tolerance by reprogramming endogenous plant hormone production and antioxidant activity, which enhances membrane stability and photosynthesis. This study unraveled MT's crucial role in water deficiency mitigation, which can thus be applied to water stress management.

First insights into diversity and potential metabolic pathways of bacterial and fungal communities in the rhizosphere of Argemonemexicana L. (Papaveraceae) from the water-level-fluctuation zone of Wudongde Reservoir of the upper Yangtze river, China

The water-level fluctuation zone (WLFZ) of Wudongde reservoir of the upper Yangtze river is a completely new aquatic-terrestrial transitional zone, and its plant degenerate issue is attracting global concerns. Uncovering the unknown rhizosphere microbiome of dominant plants of this zone is helpful in understanding the plant-microbe interactions and their growth under the largely varying environment. Here, a first exploration of the rhizosphere bacterial and fungal communities of wilted (JB) and unwilted (JA) Argemonemexicana L. individuals from the WLFZ of Wudongde reservoir was carried out using high-throughput sequencing and MetaCyc metabolic pathway analyses. The results showed that rhizosphere of wilted A.mexicana L individuals exhibited a higher microbial richness and diversity than the unwilted ones, irrespective of the bacterial and fungal communities. It was noted that 837 common bacterial amplicon sequence variants (ASV) and 92 common fungal ASV were presented in both JA and JB with 3108 bacteria and 212 fungi unique to JA, and 3569 bacteria and 693 fungi unique to JB. Linear discriminant analysis effect Size (LEfSe) analyses indicated that the taxa that had the most contribution to observed differences between both JA and JB was Proteobacteria, Actinobacteria and Ascomycota for JA, and Bacteroidetes, Firmicutes, Verrucomicrobia, Basidiomycota and Ascomycota for JB. Organic compound conversion pathway (degradation/reduction/oxidation) was consistently highly represented in the rhizosphere microbiomes of both JA and JB. Overall, this study provides insights into the rhizosphere microbiome composition, diversity and metabolic pathways of both wilted and unwilted A.mexicana L. individuals in the WLFZ of Wudongde reservoir, and the results give valuable clues for manipulating microbes to support plant growth in such a recently-formed WLFZ under a dry-hot valley environment.

Integrative transcriptome and metabolome analysis reveals the mechanism of exogenous melatonin alleviating drought stress in maize roots

Melatonin (MT) is essential for plant development and drought adaptation. However, the molecular and metabolic mechanisms underlying MT-induced drought tolerance in maize roots remain largely unclear. Herein, we investigated the effects of MT on drought tolerance in maize roots using integrated transcriptomic and metabolomic analyses, and identified MT-induced genes and metabolites associated with drought resistance. Compared with the untreated control plants, MT application alleviated the deleterious effects of drought on roots, by decreasing the malondialdehyde level and increasing the solute potential, eventually promoting root growth. Transcriptome and metabolome analysis demonstrated that MT significantly upregulates the expression of genes related to flavonoid biosynthesis (PAL, C4H, 4CL, HCT, CHS, CHI, F3'5'H, and DFR), activates drought-responsive transcription factors (ERFs, NACs, MYBs, and bHLHs), and regulates hormone signaling-related genes, especially ethylene response factors (ERF4, ERF81, and ERF110). Moreover, MT increased the accumulation of flavonoid metabolites, particularly apigenin, luteolin, and quercetin, under drought-stress conditions. These findings were further supported by quantitative real-time polymerase chain reaction analysis and total flavonoid measurements. Altogether, our findings suggest that MT promotes maize root growth during drought by regulating flavonoid synthesis pathways, transcription factors, and plant hormone signals. This study provides new insights into the complex mechanisms by which MT enhances crop resistance to drought damage.

Abiotic Stress in Crop Production

The vast majority of agricultural land undergoes abiotic stress that can significantly reduce agricultural yields. Understanding the mechanisms of plant defenses against stresses and putting this knowledge into practice is, therefore, an integral part of sustainable agriculture. In this review, we focus on current findings in plant resistance to four cardinal abiotic stressors—drought, heat, salinity, and low temperatures. Apart from the description of the newly discovered mechanisms of signaling and resistance to abiotic stress, this review also focuses on the importance of primary and secondary metabolites, including carbohydrates, amino acids, phenolics, and phytohormones. A meta-analysis of transcriptomic studies concerning the model plant Arabidopsis demonstrates the long-observed phenomenon that abiotic stressors induce different signals and effects at the level of gene expression, but genes whose regulation is similar under most stressors can still be traced. The analysis further reveals the transcriptional modulation of Golgi-targeted proteins in response to heat stress. Our analysis also highlights several genes that are similarly regulated under all stress conditions. These genes support the central role of phytohormones in the abiotic stress response, and the importance of some of these in plant resistance has not yet been studied. Finally, this review provides information about the response to abiotic stress in major European crop plants—wheat, sugar beet, maize, potatoes, barley, sunflowers, grapes, rapeseed, tomatoes, and apples.

Role of Melatonin and Nitrogen Metabolism in Plants: Implications under Nitrogen-Excess or Nitrogen-Low

Melatonin is a new plant hormone involved in multiple physiological functions in plants such as germination, photosynthesis, plant growth, flowering, fruiting, and senescence, among others. Its protective role in different stress situations, both biotic and abiotic, has been widely demonstrated. Melatonin regulates several routes in primary and secondary plant metabolism through the up/down-regulation of many enzyme/factor genes. Many of the steps of nitrogen metabolism in plants are also regulated by melatonin and are presented in this review. In addition, the ability of melatonin to enhance nitrogen uptake under nitrogen-excess or nitrogen-low conditions is analyzed. A model that summarizes the distribution of nitrogen compounds, and the osmoregulation and redox network responses mediated by melatonin, are presented. The possibilities of using melatonin in crops for more efficient uptake, the assimilation and metabolization of nitrogen from soil, and the implications for Nitrogen Use Efficiency strategies to improve crop yield are also discussed.

Rhizosphere melatonin application reprograms nitrogen-cycling related microorganisms to modulate low temperature response in barley

Rhizospheric melatonin application has a positive effect on the tolerance of plants to low temperature; however, it remains unknown whether the rhizosphere microorganisms are involved in this process. The aim of this study was to investigate the effect of exogenous melatonin on the diversity and functioning of fungi and bacteria in rhizosphere of barley under low temperature. The results showed that rhizospheric melatonin application positively regulated the photosynthetic carbon assimilation and redox homeostasis in barley in response to low temperature. These effects might be associated with an altered diversity of microbial community in rhizosphere, especially the species and relative abundance of nitrogen cycling related microorganisms, as exemplified by the changes in rhizosphere metabolites in the pathways of amino acid synthesis and metabolism. Collectively, it was suggested that the altered rhizospheric microbial status upon melatonin application was associated with the response of barley to low temperature. This suggested that the melatonin induced microbial changes should be considered for its application in the crop cold-resistant cultivation.