The secretion of organic acids is also regulated by factors other than aluminum

Strigolactones alleviate AlCl3 stress by vacuolar compartmentalization and cell wall blocking in apple

Poor management and excess fertilization of apple (Malus domestica Borkh.) orchards are causing increasingly serious soil acidification, resulting in Al toxicity and direct poisoning of roots. Strigolactones (SLs) are reported to be involved in plant responses to abiotic stress, but their role and mechanism under AlCl3 stress remain unknown. Here, we found that applying 1 μm GR24 (an SL analoge) significantly alleviated AlCl3 stress of M26 apple rootstock, mainly by blocking the movement of Al through cell wall and by vacuolar compartmentalization of Al. RNA-seq analysis identified the core transcription factor gene MdWRKY53, and overexpressing MdWRKY53 enhanced AlCl3 tolerance in transgenic apple plants through the same mechanism as GR24. Subsequently, we identified MdPMEI45 (encoding pectin methylesterase inhibitor) and MdALS3 (encoding an Al transporter) as downstream target genes of MdWRKY53 using chromatin immunoprecipitation followed by sequencing (ChIP-seq). GR24 enhanced the interaction between MdWRKY53 and the transcription factor MdTCP15, further increasing the binding of MdWRKY53 to the MdPMEI45 promoter and inducing MdPMEI45 expression to prevent Al from crossing cell wall. MdWRKY53 also bound to the promoter of MdALS3 and enhanced its transcription to compartmentalize Al in vacuoles under AlCl3 stress. We therefore identified two modules involved in alleviating AlCl3 stress in woody plant apple: the SL-WRKY+TCP-PMEI module required for excluding external Al by blocking the entry of Al3+ into cells and the SL-WRKY-ALS module allowing internal detoxification of Al through vacuolar compartmentalization. These findings lay a foundation for the practical application of SLs in agriculture.

Unveiling the impact of flooding and salinity on iron oxides-mediated binding of organic carbon in the rhizosphere of Scirpus mariqueter

The abundant Fe (hydr-) oxides present in wetland sediments can form stable iron (Fe)-organic carbon (OC) complexes (Fe-OC), which are key mechanisms contributing to the stability of sedimentary OC stocks in coastal wetland ecosystems. However, the effects of increased flooding and salinity stress, resulting from global change, on the Fe-OC complexes in sediments remain unclear. In this study, we conducted controlled experiments in a climate chamber to quantify the impacts of flooding and salinity on the different forms of Fe (hydr-) oxides binding to OC in the rhizosphere sediments of S. mariqueter as well as the influence on Fe redox cycling bacteria in the rhizosphere. The results of this study demonstrated that prolonged flooding and high salinity treatments significantly reduced the content of organo-metal complexes (FePP) in the rhizosphere. Under high salinity conditions, the content of FePP-OC increased significantly, while flooding led to a decrease in FePP-OC content, inhibiting co-precipitation processes. The association of amorphous Fe (hydr-) oxides (FeHH) with OC showed no significant differences under different flooding and salinity treatments. Prolonged flooding significantly increased the relative abundance of Fe-reducing bacteria (FeRB) Deferrisoma and Geothermobacter and decreased polyphenol oxidase in the rhizosphere, while the relative abundance of Fe-oxidizing bacteria (FeOB) Paracoccus and Pseudomonas decreased with increasing salinity and duration of flooding. Overall, short-term water and salinity stress promoted the binding of FeDH to OC in the rhizosphere of S. mariqueter, leading to a reduction in the OC content held by FePP. However, there were no significant differences observed in the OC stocks or the total Fe-OC content in the rhizosphere sediments. The findings suggest a degree of consistency in the Fe-OC of the "plant-soil" complex system within tidal flat wetlands, showing resilience to abrupt shifts in flooding and salinity over short periods.

Impact of the Arbuscular Mycorrhizal Fungus Funneliformis mosseae on the Physiological and Defence Responses of Canna indica to Copper Oxide Nanoparticles Stress

Copper oxide nanoparticles (nano-CuO) are recognized as an emerging pollutant. Arbuscular mycorrhizal fungi (AMF) can mitigate the adverse impacts of various pollutants on host plants. However, AMF’s mechanism for alleviating nano-CuO phytotoxicity remains unclear. The goal of this study was to evaluate how AMF inoculations affect the physiological features of Canna indica seedlings exposed to nano-CuO stress. Compared with the non-AMF inoculated treatment, AMF inoculations noticeably improved plant biomass, mycorrhizal colonization, leaf chlorophyll contents, and the photosynthetic parameters of C. indica under nano-CuO treatments. Moreover, AMF inoculation was able to significantly mitigate nano-CuO stress by enhancing antioxidant enzyme activities and decreasing ROS levels in the leaves and roots of C. indica, thus increasing the expression of genes involved in the antioxidant response. In addition, AMF inoculation reduced the level of Cu in seedlings and was associated with an increased expression of Cu transport genes and metallothionein genes. Furthermore, AMF inoculations increased the expression levels of organic acid metabolism-associated genes while facilitating organic acid secretion, thus reducing the accumulation of Cu. The data demonstrate that AMF–plant symbiosis is a feasible biocontrol approach to remediate nano-CuO pollution.

DA-6 improves sunflower seed vigor under Al3+ stress by regulating Al3+ balance and ethylene metabolic

Aluminum (Al³⁺) stress restricts plant seed germination and seedling growth seriously. Here, the sunflower "S175″ variety was used to explore the technique of improving seed vigor under Al³⁺ stress and investigate the effect of diethyl aminoethyl hexanoate (DA-6) on physiological characteristics in sunflower seeds during germination under Al³⁺ stress. The results showed that 3.0 mmol·L^-1 Al³⁺ treatment significantly suppressed the sunflower seed germination and seedling growth. Al³⁺ stress significantly increased Al³⁺ content and secretion rates of citric and malic acids in sunflower seeds during germination. Besides, endogenous ethylene content was increased in Al³⁺-treated seeds. DA-6 serves as a positive signal to regulate the sunflower seed germination under Al³⁺ stress. Moreover, DA-6 enhanced the activities of malic dehydrogenase, citrate synthase, and isocitrate dehydrogenase, up-regulated the expressions of organic acid transport-related genes (ALMT and MATE), resulting in reduced accumulation of Al³⁺. Furthermore, exogenous DA-6 mitigated excessive accumulation of ethylene by decreasing the 1-aminocyclopropane-1-dihydrodipicolinate synthase activity and related-gene expression. However, DA-6 treatment had no effect on abscisic acid or gibberellin metabolism in sunflower seeds under Al³⁺ stress. These results confirmed that DA-6 application enhanced the germination capacity through induction of the synthesis and transport of malic and citric acids, and suppression of the excessive accumulation of endogenous ethylene, thus contributing to alleviate Al³⁺ toxicity in sunflower seeds.

Aluminum relieves fluoride stress through stimulation of organic acid production in Camellia sinensis

Tea plants (Camellia sinensis O. Kuntze) can hyperaccumulate fluoride (F) in leaves. Although, aluminum (Al) can alleviate F toxicity in C. sinensis, the mechanisms driving this process remain unclear. Here, we measured root length, root activity, soluble proteins content, and levels of peroxidase, superoxide dismutase, catalase, malondialdehyde (MDA), and chlorophyll in tea leaves after treatment with different F concentrations. In addition, we focused on the content of organic acids, the gene transcription of malate dehydrogenase (MDH), glycolate oxidase (GO) and citrate synthase (CS) and the relative enzyme activity involved in the tolerance to F in C. sinensis. We also examined the role of Al in this process by analyzing the content of these physiological indicators in tea leaves treated with F and Al. Our results demonstrate that increased MDA content, together with decreased chlorophyll content and soluble proteins are responsible for oxidative damage and metabolism inhibition at high F concentration. Moreover, increased antioxidant enzymes activity regulates ROS damage to protect tea leaves during F stress. Furthermore, exogenous Al alleviated F stress in tea leaves through the regulation of MDA content and antioxidant enzymes activity. In addition, organic acids in exudate stimulated root growth in tea plants exposed to low F concentrations are regulated by MDH, GO, and CS. In addition, Al can stimulate the exudation of organic acids, and may participate in regulating rhizosphere pH of the roots through the interaction with F, eventually leading to the response to F stress in C. sinensis.

Interacting effect of diclofop-methyl on the rice rhizosphere microbiome and denitrification

A better knowledge of the intertwined effects of herbicides on plant physiology and microbiome as well as nutrient biogeochemical cycles are needed for environmental management. Here we studied the influence of herbicide diclofop-methyl (DM) on the rice root microbiome and its relationship with N cycle. To do so, we exposed rice seedlings to 100 μg/L DM and studied rhizosphere microbiota using MiSeq-pyrosequencing, root exudation by GC-MS, and denitrification activity by ¹⁵N isotope-tracing and qRT-PCR. The richness and diversity of rhizosphere microorganisms, significantly increased after DM exposure combined with an increase in root exudation of amino acids, sugars, and fatty acids. Transcription of denitrification-related gene and denitrification rate increased significantly in the rice rhizosphere. Our results suggest that DM strongly influenced the root exudation of bacteria nutrients, which affected root microbiome community and potentially influenced N cycle in rice rhizosphere.

Recent strategies of increasing metal tolerance and phytoremediation potential using genetic transformation of plants

Avoidance and reduction of soil contamination with heavy metals is one of the most serious global challenges. Nowadays, science offers us new opportunities of utilizing plants to extract toxic elements from the soil by means of phytoremediation. Plant abilities to uptake, translocate, and transform heavy metals, as well as to limit their toxicity, may be significantly enhanced via genetic engineering. This paper provides a comprehensive review of recent strategies aimed at the improvement of plant phytoremediation potential using plant transformation and employing current achievements in nuclear and cytoplasmic genome transformation. Strategies for obtaining plants suitable for effective soil clean-up and tolerant to excessive concentrations of heavy metals are critically assessed. Promising directions in genetic manipulations, such as gene silencing and cis- and intragenesis, are also discussed. Moreover, the ways of overcoming disadvantages of phytoremediation using genetic transformation approachare proposed. The knowledge gathered here could be useful for designing new research aimed at biotechnological improvement of phytoremediation efficiency.

Cell wall pectin methyl-esterification and organic acids of root tips involve in aluminum tolerance in Camellia sinensis

Tea plant (Camellia sinensis (O.) Kuntze) can survive from high levels of aluminum (Al) in strongly acidic soils. However, the mechanism driving its tolerance to Al, the predominant factor limiting plant growth in acid condition, is still not fully understood. Here, two-year-old rooted cuttings of C. sinensis cultivar 'Longjingchangye' were used for Al resistance experiments. We found that the tea plants grew better in the presence of 0.4 mM Al than those grew under lower concentration of Al treatments (0 and 0.1 mM) as well as higher levels treatment (2 and 4 mM), confirming that appropriate Al increased tea plant growth. Hematoxylin staining assay showed that the apical region was the main accumulator in tea plant root. Subsequently, immunolocalization of pectins in the root tip cell wall showed a rise in low-methyl-ester pectin levels and a reduction of high-methyl-ester pectin content with the increasing Al concentration of treatments. Furthermore, we observed the increased expressions of C. sinensis pectin methylesterase (CsPME) genes along with the increasing de-esterified pectin levels during response to Al treatments. Additionally, the levels of organic acids increased steadily after treatment with 0.1, 0.4 or 2 mM Al, while they dropped after treatment with 4 mM Al. The organic acids secretion from root followed a similar trend. Similarly, a gradual increase in malate dehydrogenase (MDH), citrate synthase (CS) and glycolate oxidase (GO) enzyme activities and relevant metabolic genes expression were detected after the treatment of 0.1, 0.4 or 2 mM Al, while a sharp decrease was resulted from treatment with 4 mM Al. These results confirm that both pectin methylesterases and organic acids contribute to Al tolerance in C. sinensis.

Diclofop-methyl affects microbial rhizosphere community and induces systemic acquired resistance in rice

Diclofop-methyl (DM), a widely used herbicide in food crops, may partly contaminate the soil surface of natural ecosystems in agricultural area and exert toxic effects at low dose to nontarget plants. Even though rhizosphere microorganisms strongly interact with root cells, little is known regarding their potential modulating effect on herbicide toxicity in plants. Here we exposed rice seedlings (Xiushui 63) to 100μg/L DM for 2 to 8days and studied the effects of DM on rice rhizosphere microorganisms, rice systemic acquired resistance (SAR) and rice-microorganisms interactions. The results of metagenomic 16S rDNA Illumina tags show that DM increases bacterial biomass and affects their community structure in the rice rhizosphere. After DM treatment, the relative abundance of the bacterium genera Massilia and Anderseniella increased the most relative to the control. In parallel, malate and oxalate exudation by rice roots increased, potentially acting as a carbon source for several rhizosphere bacteria. Transcriptomic analyses suggest that DM induced SAR in rice seedlings through the salicylic acid (but not the jasmonic acid) signal pathway. This response to DM stress conferred resistance to infection by a pathogenic bacterium, but was not influenced by the presence of bacteria in the rhizosphere since SAR transcripts did not change significantly in xenic and axenic plant roots exposed to DM. The present study provides new insights on the response of rice and its associated microorganisms to DM stress.

Pre-sowing Seed Treatment with 24-Epibrassinolide Ameliorates Pesticide Stress in Brassica juncea L. through the Modulation of Stress Markers

The present experiment was designed to assess the effects of seed soaking with 24-epibrassinolide (EBR) on the physiology of Brassica juncea L. seedlings grown under imidacloprid (IMI) toxicity. Application of EBR increased the length of seedlings, dry weight, and pigment contents, polyphenols, total phenols, and organic acids under IMI toxicity. The expression of genes coding key enzymes of pigment, phenols, polyphenols, and organic acid biosynthetic pathways was also studied including CHLASE (chlorophyllase), PSY (phytoene synthase), CHS (chalcone synthase) and PAL (phenylalanine ammonialyase), CS (citrate synthase), SUCLG1 (succinyl Co-A ligase,), SDH (succinate dehydrogenase), FH (fumarate hydratase), MS (malate synthase). Multiple linear regression (MLR) analysis revealed that IMI application regressed negatively on seedling length, dry weight and total chlorophyll content. However, EBR seed treatment regressed positively on all the parameters studied. Moreover, interaction between IMI and EBR showed positive regression for growth parameters, content of pigments, total polyphenol, total phenol and malate, and expression of PSY and PAL. Negative interactions were noticed for the contents of fumarate, succinate and citrate, and expression of CHS and all genes studied related to organic acid metabolism. In conclusion, EBR enhanced the growth and contents of all studied metabolites by regulating the gene expression of B. juncea seedlings under IMI stress.

Tropical soils cultivated with tomato: fractionation and speciation of Al

Soil acidity and the associated problems of aluminum (Al) toxicity and scarce exchangeable bases are typically the most important limiting factors of agricultural yield in wet tropical regions. The goals of this study were to test how soil lime rates affect the forms and distribution of Al in the soil fractions and how different levels of bioavailable Al affect two tomato genotypes grown in wet tropical soils. The tomato genotypes CNPH 0082 and Calabash Rouge were grown in two wet tropical soils in a greenhouse. Soil lime rates of 0, 560, and 2240 mg kg(-1) soil (clay soil) and 0, 280, and 1120 mg kg(-1) soil (sandy soil) were applied to modify Al concentrations. Dry mass production and Al concentrations were determined in shoots and roots. Al was fractionated in the soil, and the soil solution was speciated after cultivation. The Calabash Rouge genotype possesses mechanisms to tolerate Al3+, absorbed less Al, exhibited smaller reduction in growth, and lower Al concentrations in plant parts than the CNPH 0082. Increased soil pH reduced the exchangeable Al fraction and increased the fraction mainly linked to organic matter. Al in the soil in the form of complexes with organic compounds and Al(SO4)+ (at the highest lime rate) did not affect plant development. Soil acidity can be easily neutralized by liming the soil, which transforms toxic Al3+ in the soil into forms that do not harm tomato plants, thereby avoiding oxidative stress in the plants. Al-induced stress in tomatoes varies with genotypes and soil type.

Determination of genotoxic effects of Imazethapyr herbicide in Allium cepa root cells by mitotic activity, chromosome aberration, and comet assay

Imazethapyr (IM) is an imidazolinone herbicide that is currently used for broad-spectrum weed control in soybean and other legume crops. In this study, cytotoxic and genotoxic effects of IM were investigated by using mitotic index (MI), mitotic phases, chromosomal abnormalities (CAs) and DNA damage on the root meristem cells of Allium cepa. In Allium root growth inhibition test, EC50 value was determined as 20 ppm, and 0.5xEC50, EC50 and 2xEC50 concentrations of IM herbicide were introduced to onion tuber roots. Distilled water and methyl methane sulfonate (MMS, 10 mg/L) were used as a negative and positive control, respectively. As A. cepa cell cycle is 24 hours, so, application process was carried out for 24, 48, 72 and 96 hours. All the applied doses decreased MIs compared to control group and these declines were found to be statistically meaningful. Analysis of the chromosomes showed that 10 ppm IM except for 48 h induced CAs but 40 ppm IM except for 72 h decreased CAs. DNA damage was found significantly higher in 20 and 40 ppm of IM compared to the control in comet assay. These results indicated that IM herbicide exhibits cytotoxic activity but not genotoxic activity (except 10 ppm) and induced DNA damage in a dose dependent manner in A. cepa root meristematic cells.

Unraveling the toxicity mechanisms of the herbicide diclofop-methyl in rice: modulation of the activity of key enzymes involved in citrate metabolism and induction of cell membrane anion channels

Residual soil concentrations of the herbicide diclofop-methyl (DM) can be toxic to other nontarget plant species, but the toxicity mechanisms at play are not fully understood. In the present study, we analyzed the toxic effect of DM on root growth and metabolism in the rice species Oryza sativa. The results show that a 48-h exposure to a trace level (5 μg/L) of DM inhibits rice root growth by almost 70%. A 48-h exposure to 5 μg/L DM also leads to an ≈2.5-fold increase in citrate synthase (CS) activity (and CS gene transcription) and an ≈2-fold decrease in the citrate lyase gene transcripts, which lead to an increase in the intracellular concentration of citrate and in citrate exudation rate. Addition of a specific inhibitor of cell membrane anion channel, anthracene-9-carboxylic acid, decreased citrate release in the culture, suggesting that DM-induced citrate loss from the cells is mediated by a specific membrane-bound channel protein. This study brings new insights into the key biochemical mechanisms leading to DM toxicity in rice.