Mg deficiency induces photo-oxidative stress primarily by limiting CO2 assimilation and not by limiting photosynthetic light utilization

Integrative transcriptomic analysis reveals the molecular responses of tobacco to magnesium deficiency

Introduction

Magnesium (Mg) is a crucial macronutrient for plants. Understanding the molecular responses of plants to different levels of Mg supply is important for improving cultivation practices and breeding new varieties with efficient Mg utilization.

Methods

In this study, we conducted a comprehensive transcriptome analysis on tobacco (Nicotiana tabacum L.) seedling leaves to investigate changes in gene expression in response to different levels of Mg supply, including Mg-deficient, 1/4-normal Mg, normal Mg, and 4×-normal Mg, with a particular focus on Mg deficiency at 5, 15 and 25 days after treatment (DAT), respectively.

Results

A total of 11,267 differentially expressed genes (DEGs) were identified in the Mg-deficient, 1/4-normal Mg, and/or 4×-normal Mg seedlings compared to the normal Mg seedlings. The global gene expression profiles revealed potential mechanisms involved in the response to Mg deficiency in tobacco leaves, including down-regulation of genes-two DEGs encoding mitochondria-localized NtMGT7 and NtMGT9 homologs, and one DEG encoding a tonoplast-localized NtMHX1 homolog-associated with Mg trafficking from the cytosol to mitochondria and vacuoles, decreased expression of genes linked to photosynthesis and carbon fixation at later stages, and up-regulation of genes related to antioxidant defenses, such as NtPODs, NtPrxs, and NtGSTs.

Discussion

Our findings provide new insights into the molecular mechanisms underlying how tobacco responds to Mg deficiency.

Physiological and biomolecular interventions in the bio-decolorization of Methylene blue dye by Salvinia molesta D. Mitch

Methylene blue, a cationic dye as a pollutant is discharged from industrial effluent into aquatic bodies. The dye is biomagnified through the food chain and is detrimental to the sustainability of aquatic flora. Despite of number of physico-chemical techniques of dye removal, the use of aquatic flora for bio-adsorption is encouraged. Thus, we used Salvinia molesta D. Mitch in bio-reduction of methylene blue on concentrations of 0, 10, 20, and 30 mg L-1 through 5 days with biosorption kinetics. The dye removal was concentration-dependent, maximized at 2 days with 30 mg L-1 which altered the relative growth rate (44%) of plants. Biosorption recorded 71% capacity at optimum pH (8.0), 24 h reducing major bond energies of amide, hydroxyl groups, etc. Bioaccumulation of dye changed potassium content (446%) under maximum dye concentration modifying tissues for dye sequestration. Reactive oxygen species were altered on dye reduction by oxidase (33%) with redox homeostasis by enzymes. Plants altered the metabolism with over accumulation of polyamines (51%), abscisic acids (448%), and phosphoenolpyruvate carboxylase (83%) on dye reduction. Thus, this study is rationalized with a sustainable approach where aquatic ecosystems can be decontaminated from dye toxicity with the exercise of bioresources like Salvinia molesta D. Mitch as herein.

Mg2+ limitation leads to a decrease in chlorophyll, resulting in an unbalanced photosynthetic apparatus in the cyanobacterium Synechocytis sp. PCC6803

Mg2+, the most abundant divalent cation in living cells, plays a pivotal role in numerous enzymatic reactions and is of particular importance for organisms performing oxygenic photosynthesis. Its significance extends beyond serving as the central ion of the chlorophyll molecule, as it also acts as a counterion during the light reaction to balance the proton gradient across the thylakoid membranes. In this study, we investigated the effects of Mg2+ limitation on the physiology of the well-known model microorganism Synechocystis sp. PCC6803. Our findings reveal that Mg2+ deficiency triggers both morphological and functional changes. As seen in other oxygenic photosynthetic organisms, Mg2+ deficiency led to a decrease in cellular chlorophyll concentration. Moreover, the PSI-to-PSII ratio decreased, impacting the photosynthetic efficiency of the cell. In line with this, Mg2+ deficiency led to a change in the proton gradient built up across the thylakoid membrane upon illumination.

Polyethylene microplastics alter root functionality and affect strawberry plant physiology and fruit quality traits

Strawberry, a globally popular crop whose fruit are known for their taste and health benefits, were used to evaluate the effects of polyethylene microplastics (PE-MPs) on plant physiology and fruit quality. Plants were grown in 2-L pots with natural soil mixed with PE-MPs at two concentrations (0.2% and 0.02%; w/w) and sizes (⌀ 35 and 125 µm). Plant physiological responses, root histochemical and anatomical analyses as well as fruit biometric and quality features were conducted. Plants subjected to ⌀ 35 µm/0.2% PE-MPs exhibited the most severe effects in terms of CO2 assimilation due to stomatal limitations, along with the highest level of oxidative stress in roots. Though no differences were observed in plant biomass, the impact on fruit quality traits was severe in ⌀ 35 µm/0.2% MPs treatment resulting in a drop in fruit weight (-42%), soluble solid (-10%) and anthocyanin contents (-25%). The smallest sized PE-MPs, adsorbed on the root surface, impaired plant water status by damaging the radical apparatus, which finally resulted in alteration of plant physiology and fruit quality. Further research is required to determine if these alterations also occur with other MPs and to understand more deeply the MPs influence on fruit physio-chemistry.

Biological remediation treatments improve the health of a mixed contaminated soil before significantly reducing contaminant levels

The remediation of mixed contaminated soil is challenging as it often requires actions to minimize metal-induced risks while degrading organic contaminants. Here, the effectiveness of different bioremediation strategies, namely, rhizoremediation with native plant species, mycoremediation with Pleurotus ostreatus spent mushroom substrate, and biostimulation with organic by-products (i.e., composted sewage sludge and spent mushroom substrate), for the recovery of a mixed contaminated soil from an abandoned gravel pit was studied. The combination of biostimulation and rhizoremediation led to the most significant increase in soil health, according to microbial indicator values. The application of composted sewage sludge led to the highest reduction in anthracene and polychlorinated biphenyls concentrations. None of the strategies managed to decrease contamination levels below regulatory limits, but they did enhance soil health. It was concluded that the biological remediation treatments improved soil functioning in a short time, before the concentration of soil contaminants was significantly reduced.

Photosynthetic plasticity aggravates the susceptibility of magnesium-deficient leaf to high light in rapeseed plants: the importance of Rubisco and mesophyll conductance

Plants grown under low magnesium (Mg) soils are highly susceptible to encountering light intensities that exceed the capacity of photosynthesis (A), leading to a depression of photosynthetic efficiency and eventually to photooxidation (i.e., leaf chlorosis). Yet, it remains unclear which processes play a key role in limiting the photosynthetic energy utilization of Mg-deficient leaves, and whether the plasticity of A in acclimation to irradiance could have cross-talk with Mg, hence accelerating or mitigating the photodamage. We investigated the light acclimation responses of rapeseed (Brassica napus) grown under low- and adequate-Mg conditions. Magnesium deficiency considerably decreased rapeseed growth and leaf A, to a greater extent under high than under low light, which is associated with higher level of superoxide anion radical and more severe leaf chlorosis. This difference was mainly attributable to a greater depression in dark reaction under high light, with a higher Rubisco fallover and a more limited mesophyll conductance to CO2 (gm ). Plants grown under high irradiance enhanced the content and activity of Rubisco and gm to optimally utilize more light energy absorbed. However, Mg deficiency could not fulfill the need to activate the higher level of Rubisco and Rubisco activase in leaves of high-light-grown plants, leading to lower Rubisco activation and carboxylation rate. Additionally, Mg-deficient leaves under high light invested more carbon per leaf area to construct a compact leaf structure with smaller intercellular airspaces, lower surface area of chloroplast exposed to intercellular airspaces, and CO2 diffusion conductance through cytosol. These caused a more severe decrease in within-leaf CO2 diffusion rate and substrate availability. Taken together, plant plasticity helps to improve photosynthetic energy utilization under high light but aggravates the photooxidative damage once the Mg nutrition becomes insufficient.

Chlorophyll decomposition is accelerated in banana leaves after the long-term magnesium deficiency according to transcriptome analysis

Magnesium (Mg) is an essential macronutrient for plant growth and development. Physiological and transcriptome analyses were conducted to elucidate the adaptive mechanisms to long-term Mg deficiency (MD) in banana seedlings at the 6-leaf stage. Banana seedlings were irrigated with a Mg-free nutrient solution for 42 days, and a mock control was treated with an optimum Mg supply. Leaf edge chlorosis was observed on the 9th leaf, which gradually turned yellow from the edge to the interior region. Accordingly, the total chlorophyll content was reduced by 47.1%, 47.4%, and 53.8% in the interior, center and edge regions, respectively, and the net photosynthetic rate was significantly decreased in the 9th leaf. Transcriptome analysis revealed that MD induced 9,314, 7,425 and 5,716 differentially expressed genes (DEGs) in the interior, center and edge regions, respectively. Of these, the chlorophyll metabolism pathway was preferentially enriched according to Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. The expression levels of the five candidate genes in leaves were consistent with what is expected during chlorophyll metabolism. Our results suggest that changes in the expression of genes related to chlorophyll synthesis and decomposition result in the yellowing of banana seedling leaves, and these results are helpful for understanding the banana response mechanism to long-term MD.

Physiological Essence of Magnesium in Plants and Its Widespread Deficiency in the Farming System of China

Magnesium (Mg) is an essential nutrient for a wide array of fundamental physiological and biochemical processes in plants. It largely involves chlorophyll synthesis, production, transportation, and utilization of photoassimilates, enzyme activation, and protein synthesis. As a multifaceted result of the introduction of high-yielding fertilizer-responsive cultivars, intensive cropping without replenishment of Mg, soil acidification, and exchangeable Mg (Ex-Mg) leaching, Mg has become a limiting nutrient for optimum crop production. However, little literature is available to better understand distinct responses of plants to Mg deficiency, the geographical distribution of soil Ex-Mg, and the degree of Mg deficiency. Here, we summarize the current state of knowledge of key plant responses to Mg availability and, as far as possible, highlight spatial Mg distribution and the magnitude of Mg deficiency in different cultivated regions of the world with a special focus on China. In particular, ~55% of arable lands in China are revealed Mg-deficient (< 120 mg kg-1 soil Ex-Mg), and Mg deficiency literally becomes increasingly severe from northern (227-488 mg kg-1) to southern (32-89 mg kg-1) China. Mg deficiency primarily traced back to higher depletion of soil Ex-Mg by fruits, vegetables, sugarcane, tubers, tea, and tobacco cultivated in tropical and subtropical climate zones. Further, each unit decline in soil pH from neutral reduced ~2-fold soil Ex-Mg. This article underscores the physiological importance of Mg, potential risks associated with Mg deficiency, and accordingly, to optimize fertilization strategies for higher crop productivity and better quality.

Infection by phloem-limited phytoplasma affects mineral nutrient homeostasis in tomato leaf tissues

Phytoplasmas are sieve-elements restricted wall-less, pleomorphic pathogenic microorganisms causing devastating damage to over 700 plant species worldwide. The invasion of sieve elements by phytoplasmas has several consequences on nutrient transport and metabolism, anyway studies about changes of the mineral-nutrient profile following phytoplasma infections are scarce and offer contrasting results. Here, we examined changes in macro- and micronutrient concentration in tomato plant upon 'Candidatus Phytoplasma solani' infection. To investigate possible effects of 'Ca. P. solani' infection on mineral element allocation, the mineral elements were separately analysed in leaf midrib, leaf lamina and root. Moreover, we focused our analysis on the transcriptional regulation of genes encoding trans-membrane transporters of mineral nutrients. To this aim, a manually curated inventory of differentially expressed genes encoding transporters in tomato leaf midribs was mined from the transcriptional profile of healthy and infected tomato leaf midribs. Results highlighted changes in ion homeostasis in the host plant, and significant modulations at transcriptional level of genes encoding ion transporters and channels.

A Comparison of the Effects of Several Foliar Forms of Magnesium Fertilization on ‘Superior Seedless’ (Vitis vinifera L.) in Saline Soils

Magnesium (Mg) is the most essential element constituent in chlorophyll molecules that regulates photosynthesis processes. The physiological response of ‘Superior Seedless’ grapes was evaluated under different foliar magnesium fertilization such as sulfate magnesium (MgSO4·7 H2O), magnesium disodium EDTA (Mg-EDTA), and magnesium nanoparticles (Mg-NPs) during the berry development stages (flowering, fruit set, veraison, and harvest). In general, the ‘Superior Seedless’ vine had a higher performance in photosynthesis with Mg-NPs application than other forms. The Fy/Fm ratio declined rapidly after the fruit set stage; then, it decreased gradually up until the harvesting stage. However, both MgSO4 and Mg-EDTA forms showed slight differences in Fv/Fm ratio during the berry development stages. The outcomes of this research suggest that the Fv/Fm ratio during the growth season of the ‘Superior Seedless’ vine may be a good tool to assess magnesium fertilization effects before visible deficiency symptoms appear. Mg-NPs are more effective at improving ‘Superior Seedless’ berry development than the other magnesium forms. These findings suggest that applying foliar Mg-NPs to vines grown on salinity-sandy soil alleviates the potential Mg deficiency in ‘Superior Seedless’ vines and improves bunches quality.

Differences in morphological and physiological features of citrus seedlings are related to Mg transport from the parent to branch organs

BACKGROUND

In this study, we aimed to test the hypothesis that magnesium (Mg) remobilization in citrus plants is regulated by Mg supply and contributes to differences in the growth of the parent and branch organs. Citrus seedlings were grown in sand under Mg deficient (0 mmol Mg²⁺ L^-1, -Mg) and Mg sufficient (2 mmol Mg²⁺ L^-1, + Mg) conditions. The effects on biomass, Mg uptake and transport, gas exchange and chlorophyll fluorescence, as well as related morphological and physiological parameters were evaluated in different organs.

RESULTS

Mg deficiency significantly decreased plant biomass, with a decrease in total plant biomass of 39.6%, and a greater than twofold decrease in the branch organs compared with that of the parent organs. Reduced photosynthesis capacity was caused by a decreased in pigment levels and photosynthetic electron transport chain disruption, thus affecting non-structural carbohydrate accumulation and plant growth. However, the adaptive responses of branch leaves to Mg deficiency were greater than those in parent leaves. Mg deficiency inhibited plant Mg uptake but enhanced Mg remobilization from parent to branch organs, thus changing related growth variables and physiological parameters, including protein synthesis and antioxidant enzyme activity. Moreover, in the principal components analysis, these variations were highly clustered in both the upper and lower parent leaves, but highly separated in branch leaves under the different Mg conditions.

CONCLUSIONS

Mg deficiency inhibits the growth of the parent and branch organs of citrus plants, with high Mg mobility contributing to differences in physiological metabolism. These findings suggest that Mg management should be optimized for sustainable citrus production.

Magnesium Foliar Supplementation Increases Grain Yield of Soybean and Maize by Improving Photosynthetic Carbon Metabolism and Antioxidant Metabolism

(1) Background: The aim of this study was to explore whether supplementary magnesium (Mg) foliar fertilization to soybean and maize crops established in a soil without Mg limitation can improve the gas exchange and Rubisco activity, as well as improve antioxidant metabolism, converting higher plant metabolism into grain yield. (2) Methods: Here, we tested foliar Mg supplementation in soybean followed by maize. Nutritional status of plants, photosynthesis, PEPcase and Rubisco activity, sugar concentration on leaves, oxidative stress, antioxidant metabolism, and finally the crops grain yields were determined. (3) Results: Our results demonstrated that foliar Mg supplementation increased the net photosynthetic rate and stomatal conductance, and reduced the sub-stomatal CO₂ concentration and leaf transpiration by measuring in light-saturated conditions. The improvement in photosynthesis (gas exchange and Rubisco activity) lead to an increase in the concentration of sugar in the leaves before grain filling. In addition, we also confirmed that foliar Mg fertilization can improve anti-oxidant metabolism, thereby reducing the environmental stress that plants face during their crop cycle in tropical field conditions. (4) Conclusions: Our research brings the new glimpse of foliar Mg fertilization as a strategy to increase the metabolism of crops, resulting in increased grain yields. This type of biological strategy could be encouraged for wide utilization in cropping systems.