Manganese Toxicity Inhibited Root Growth by Disrupting Auxin Biosynthesis and Transport in Arabidopsis

Methylisothiazolinone pollution inhibited root stem cells and regeneration through auxin transport modification in Arabidopsis thaliana

Methylisothiazolinone (MIT) is a widely used preservative and biocide to prevent product degradation, yet its potential impact on plant growth remains poorly understood. In this study, we investigated MIT's toxic effects on Arabidopsis thaliana root growth. Exposure to MIT significantly inhibited Arabidopsis root growth, associated with reduced root meristem size and root meristem cell numbers. We explored the polar auxin transport pathway and stem cell regulation as key factors in root meristem function. Our findings demonstrated that MIT suppressed the expression of the auxin efflux carrier PIN1 and major root stem cell regulators (PLT1, PLT2, SHR, and SCR). Additionally, MIT hindered root regeneration by downregulating the quiescent center (QC) marker WOX5. Transcriptome analysis revealed MIT-induced alterations in gene expression related to oxidative stress, with physiological experiments confirming elevated reactive oxygen species (ROS) levels and increased cell death in root tips at concentrations exceeding 50 μM. In summary, this study provides critical insights into MIT's toxicity on plant root development and regeneration, primarily linked to modifications in polar auxin transport and downregulation of genes associated with root stem cell regulation.

Nitrate Starvation Induces Lateral Root Organogenesis in Triticum aestivum via Auxin Signaling

The lateral root (LR) is an essential component of the plant root system, performing important functions for nutrient and water uptake in plants and playing a pivotal role in cereal crop productivity. Nitrate (NO3-) is an essential nutrient for plants. In this study, wheat plants were grown in 1/2 strength Hoagland's solution containing 5 mM NO3- (check; CK), 0.1 mM NO3- (low NO3-; LN), or 0.1 mM NO3- plus 60 mg/L 2,3,5-triiodobenzoic acid (TIBA) (LNT). The results showed that LN increased the LR number significantly at 48 h after treatment compared with CK, while not increasing the root biomass, and LNT significantly decreased the LR number and root biomass. The transcriptomic analysis showed that LN induced the expression of genes related to root IAA synthesis and transport and cell wall remodeling, and it was suppressed in the LNT conditions. A physiological assay revealed that the LN conditions increased the activity of IAA biosynthesis-related enzymes, the concentrations of tryptophan and IAA, and the activity of cell wall remodeling enzymes in the roots, whereas the content of polysaccharides in the LRP cell wall was significantly decreased compared with the control. Fourier-transform infrared spectroscopy and atomic microscopy revealed that the content of cell wall polysaccharides decreased and the cell wall elasticity of LR primordia (LRP) increased under the LN conditions. The effects of LN on IAA synthesis and polar transport, cell wall remodeling, and LR development were abolished when TIBA was applied. Our findings indicate that NO3- starvation may improve auxin homeostasis and the biological properties of the LRP cell wall and thus promote LR initiation, while TIBA addition dampens the effects of LN on auxin signaling, gene expression, physiological processes, and the root architecture.

Lithium ore tailings harm the vegetative development, photosynthetic activity, and nutrition of tree species

Lithium (Li) exploitation promotes socioeconomic advances but may result in harmful environmental impacts. Thus, species selection for recovering environments degraded by Li mining is essential. We investigated the tolerance and early growth of four tree species to Li ore tailings (LOT), Enterolobium contortisiliquum and Handroanthus impetiginosus with wide geographic distribution and Hymenaea courbaril and H. stigonocarpa with restricted geographic distribution. The plants grew in LOT and soil for 255 days to evaluate photosynthesis, growth, and mineral nutrition. LOT negatively affected species growth, reducing the length of stems, roots, and biomass through structural and nutritional impoverishment. LOT favored the accumulation of Mg and decreased the absorption of K. The species presented a reduction in potential quantum efficiency and the chlorophyll index (b and total). E. contortisiliquum was the least tolerant species to LOT, and H. courbaril and H. stigonocarpa maintained their mass production in LOT, indicating greater tolerance to tailings. Furthermore, H. courbaril presented a translocation factor > 1 for Li and Mn, indicating the potential for phytoextraction of these metals. Our results offer first-time insights into the impacts of LOT on the early development of tree species with different geographic distribution ranges. This study may help in the tree species selection with a phytoremediation role, aiming at the recovery of areas affected by Li's mining activity.

The StPti5 ethylene response factor acts as a susceptibility factor by negatively regulating the potato immune response to pathogens

Ethylene response factors (ERFs) have been associated with biotic stress in Arabidopsis, while their function in non-model plants is still poorly understood. Here we investigated the role of potato ERF StPti5 in plant immunity. We show that StPti5 acts as a susceptibility factor. It negatively regulates potato immunity against potato virus Y and Ralstonia solanacearum, pathogens with completely different modes of action, and thereby has a different role than its orthologue in tomato. Remarkably, StPti5 is destabilised in healthy plants via the autophagy pathway and accumulates exclusively in the nucleus upon infection. We demonstrate that StEIN3 and StEIL1 directly bind the StPti5 promoter and activate its expression, while synergistic activity of the ethylene and salicylic acid pathways is required for regulated StPti expression. To gain further insight into the mode of StPti5 action in attenuating potato defence responses, we investigated transcriptional changes in salicylic acid deficient potato lines with silenced StPti5 expression. We show that StPti5 regulates the expression of other ERFs and downregulates the ubiquitin-proteasome pathway as well as several proteases involved in directed proteolysis. This study adds a novel element to the complex puzzle of immune regulation, by deciphering a two-level regulation of ERF transcription factor activity in response to pathogens.

R3 strain and Fe-Mn modified biochar reduce Cd absorption capacity of roots and available Cd content of soil by affecting rice rhizosphere and endosphere key flora

Microorganisms have a significant role in regulating the absorption and transportation of Cd in the soil-plant system. However, the mechanism by which key microbial taxa play a part in response to the absorption and transportation of Cd in rice under Cd stress requires further exploration. In this study, the cadmium-tolerant endophytic bacterium Herbaspirillum sp. R3 (R3) and Fe-Mn-modified biochar (Fe-Mn) were, respectively, applied to cadmium-contaminated rice paddies to investigate the effects of key bacterial taxa in the soil-rice system on the absorption and transportation of Cd in rice under different treatments. The results showed that both R3 and Fe-Mn treatments considerably decreased the content of cadmium in roots, stems and leaves of rice at the peak tillering stage by 17.24-49.28% in comparison to the control (CK). The cadmium content reduction effect of R3 treatment is better than that of Fe-Mn treatment. Further analysis revealed that the key bacterial taxa in rice roots under R3 treatment were Sideroxydans and Actinobacteria, and that their abundance showed a substantial positive correlation and a significant negative correlation with the capacity of rice roots to assimilate Cd from the surroundings, respectively. The significant increase in soil pH under Fe-Mn treatment, significant reduction in the relative abundances of Acidobacteria, Verrucomicrobia, Subdivision3 genera incertae sedis, Sideroxydans, Geobacter, Gp1, and Gp3, and the significant increase in the relative abundance of Thiobacillus among the soil bacterial taxa may be the main reasons for the decrease in available Cd content of the soil. In addition, both the R3 and Fe-Mn treatments showed some growth-promoting effects on rice, which may be related to their promotion of transformations of soil available nutrients. This paper describes the possible microbial mechanisms by which strain R3 and Fe-Mn biochar reduce Cd uptake in rice, providing a theoretical basis for the remediation of Cd contamination in rice and soil by utilizing key microbial taxa.

Regulatory Mechanism through Which Old Soybean Leaves Respond to Mn Toxicity Stress

Manganese (Mn) is a heavy metal that can cause excessive Mn poisoning in plants, disrupting microstructural homeostasis and impairing growth and development. However, the specific response mechanisms of leaves to Mn poisoning have not been fully elucidated. This study revealed that Mn poisoning of soybean plants resulted in yellowing of old leaves. Physiological assessments of these old leaves revealed significant increases in the antioxidant enzymes activities (peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), and catalase (CAT)) and elevated levels of malondialdehyde (MDA), proline, indoleacetic acid (IAA), and salicylic acid (SA), under 100 μM Mn toxicity. Conversely, the levels of abscisic acid (ABA), gibberellin 3 (GA3), and jasmonic acid (JA) significantly decreased. The Mn content in the affected leaves significantly increased, while the levels of Ca, Na, K, and Cu decreased. Transcriptome analysis revealed 2258 differentially expressed genes in the Mn-stressed leaves, 744 of which were upregulated and 1514 were downregulated; these genes included genes associated with ion transporters, hormone synthesis, and various enzymes. Quantitative RT-PCR (qRT-PCR) verification of fifteen genes confirmed altered gene expression in the Mn-stressed leaves. These findings suggest a complex gene regulatory mechanism under Mn toxicity and stress, providing a foundation for further exploration of Mn tolerance-related gene regulatory mechanisms in soybean leaves. Using the methods described above, this study will investigate the molecular mechanism of old soybean leaves' response to Mn poisoning, identify key genes that play regulatory roles in Mn toxicity stress, and lay the groundwork for cultivating high-quality soybean varieties with Mn toxicity tolerance traits.

Physiological and transcriptomic response reveals new insight into manganese tolerance of Celosia argentea Linn

Celosia argentea is a manganese (Mn) hyperaccumulator with high ornamental value and strong stress resistance. It is important to understand the molecular mechanism of tolerance to heavy metals of hyperaccumulators to improve the efficiency of phytoremediation. In this study, the effects of different Mn concentrations (0, 0.8, 3, and 10 mM) on physiological characteristics and molecular changes were determined. Low concentrations of Mn increased the growth of C. argentea, while high concentrations of Mn suppressed its growth, A concentration up to 3 mM did not affect the growth of C. argentea, and the highest transfer factor (TF) was 6.16. Oxidative damage of different Mn level treatments in C. argentea was verified through relative water content, electrolyte leakage, MDA content, H2O2 content and superoxide contents. With an increase in Mn concentration, the contents of chlorophyll a, chlorophyll b, and carotenoids decreased. Our results indicated that low-concentration manganese treatment can reduce the reactive oxygen burst and MDA, soluble sugar and proline, making C. argentea have strong abiotic stress tolerance. The molecular mechanism of C. argentea after 10 mM Mn treatment was analysed through transcriptome analysis, and differentially expressed genes (DEGs) in these pathways were further verified by qRTPCR. Plantpathogen interactions, plant hormone signal transduction, the MAPK signalling pathway and the phenylpropanoid biosynthesis pathway were important in the response to Mn stress, and the heavy metal-associated isoprenylated plant protein, metal transporter Nramp, and zinc transporter play key roles in the strong ability of C. argentea to tolerate heavy metals. These results suggest that C. argentea exhibits strong manganese tolerance and provide new insight into the molecular mechanisms of plant responses to heavy metal stress.

Physiological Mechanism through Which Al Toxicity Inhibits Peanut Root Growth

Al (Aluminum) poisoning is a significant limitation to crop yield in acid soil. However, the physiological process involved in the peanut root response to Al poisoning has not been clarified yet and requires further research. In order to investigate the influence of Al toxicity stress on peanut roots, this study employed various methods, including root phenotype analysis, scanning of the root, measuring the physical response indices of the root, measurement of the hormone level in the root, and quantitative PCR (qPCR). This research aimed to explore the physiological mechanism underlying the reaction of peanut roots to Al toxicity. The findings revealed that Al poisoning inhibits the development of peanut roots, resulting in reduced biomass, length, surface area, and volume. Al also significantly affects antioxidant oxidase activity and proline and malondialdehyde contents in peanut roots. Furthermore, Al toxicity led to increased accumulations of Al and Fe in peanut roots, while the contents of zinc (Zn), cuprum (Cu), manganese (Mn), kalium (K), magnesium (Mg), and calcium (Ca) decreased. The hormone content and related gene expression in peanut roots also exhibited significant changes. High concentrations of Al trigger cellular defense mechanisms, resulting in differentially expressed antioxidase genes and enhanced activity of antioxidases to eliminate excessive ROS (reactive oxygen species). Additionally, the differential expression of hormone-related genes in a high-Al environment affects plant hormones, ultimately leading to various negative effects, for example, decreased biomass of roots and hindered root development. The purpose of this study was to explore the physiological response mechanism of peanut roots subjected to aluminum toxicity stress, and the findings of this research will provide a basis for cultivating Al-resistant peanut varieties.

Effect of Melissa officinalis L. leaf extract on manganese-induced cyto-genotoxicity on Allium cepa L

Although the antioxidant properties of Melissa officinalis extract (Mox) are widely known, little work has focused on its protective capacity against heavy metal stress. The primary objective of this study was to determine the potential of Mox to mitigate manganese (II) chloride (MnCI2)-induced cyto-genotoxicity using the Allium and comet assays. Physiological, genotoxic, biochemical and anatomical parameters as well as the phenolic composition of Mox were examined in Allium cepa (L.). Application of 1000 µM MnCl2 reduced the rooting percentage, root elongation, weight gain, mitotic index and levels of chlorophyll a and chlorophyll b pigments compared to the control group. However, it increased micronuclei formation, chromosomal abnormality frequencies, tail DNA percentage, proline amount, lipid peroxidation level and meristematic damage severity. The activities of superoxide dismutase and catalase also increased. Chromosomal aberrations induced by MnCl2 were fragment, sticky chromosome, vagrant chromosome, unequal distribution of chromatin and bridge. Application of 250 mg/L Mox and 500 mg/L Mox along with MnCl2 significantly alleviated adverse effects dose dependently. The antioxidant activity bestowed by the phenolic compounds in Mox assisted the organism to combat MnCl2 toxicity. Consequently, Mox exerted remarkable protection against MnCl2 toxicity and it needs to be investigated further as a potential therapeutic option.

Auxin homeostasis in plant responses to heavy metal stress

Expeditious industrialization and anthropogenic activities have resulted in large amounts of heavy metals (HMs) being released into the environment. These HMs affect crop yields and directly threaten global food security. Therefore, significant efforts have been made to control the toxic effects of HMs on crops. When HMs are taken up by plants, various mechanisms are stimulated to alleviate HM stress, including the biosynthesis and transport of auxin in the plant. Interestingly, researchers have noted the significant potential of auxin in mediating resistance to HM stress, primarily by reducing uptake of metals, promoting chelation and sequestration in plant tissues, and mitigating oxidative damage. Both exogenous administration of auxin and manipulation of intrinsic auxin status are effective strategies to protect plants from the negative consequences of HMs stress. Regulation of genes and transcription factors related to auxin homeostasis has been shown to be related to varying degrees to the type and concentration of HMs. Therefore, to derive the maximum benefit from auxin-mediated mechanisms to attenuate HM toxicities, it is essential to gain a comprehensive understanding of signaling pathways involved in regulatory actions. This review primarily emphases on the auxin-mediated mechanisms participating in the injurious effects of HMs in plants. Thus, it will pave the way to understanding the mechanism of auxin homeostasis in regulating HM tolerance in plants and become a tool for developing sustainable strategies for agricultural growth in the future.

PIN2/3/4 auxin carriers mediate root growth inhibition under conditions of boron deprivation in Arabidopsis

The mechanistic basis by which boron (B) deprivation inhibits root growth via the mediation of root apical auxin transport and distribution remains elusive. This study showed that B deprivation repressed root growth of wild-type Arabidopsis seedlings, which was related to higher auxin accumulation (observed with DII-VENUS and DR5-GFP lines) in B-deprived roots. Boron deprivation elevated the auxin content in the root apex, coinciding with upregulation of the expression levels of auxin biosynthesis-related genes (TAA1, YUC3, YUC9, and NIT1) in shoots, but not in root apices. Phenotyping experiments using auxin transport-related mutants revealed that the PIN2/3/4 carriers are involved in root growth inhibition caused by B deprivation. B deprivation not only upregulated the transcriptional levels of PIN2/3/4, but also restrained the endocytosis of PIN2/3/4 carriers (observed with PIN-Dendra2 lines), resulting in elevated protein levels of PIN2/3/4 in the plasma membrane. Overall, these results suggest that B deprivation not only enhances auxin biosynthesis in shoots by elevating the expression levels of auxin biosynthesis-related genes but also promotes the polar auxin transport from shoots to roots by upregulating the gene expression levels of PIN2/3/4, as well as restraining the endocytosis of PIN2/3/4 carriers, ultimately resulting in auxin accumulation in root apices and root growth inhibition.

Transcriptome Sequencing Analysis of Root in Soybean Responding to Mn Poisoning

Manganese (Mn) is among one of the essential trace elements for normal plant development; however, excessive Mn can cause plant growth and development to be hindered. Nevertheless, the regulatory mechanisms of plant root response to Mn poisoning remain unclear. In the present study, results revealed that the root growth was inhibited when exposed to Mn poisoning. Physiological results showed that the antioxidase enzyme activities (peroxidase, superoxide dismutase, ascorbate peroxidase, and catalase) and the proline, malondialdehyde, and soluble sugar contents increased significantly under Mn toxicity stress (100 μM Mn), whereas the soluble protein and four hormones' (indolebutyric acid, abscisic acid, indoleacetic acid, and gibberellic acid 3) contents decreased significantly. In addition, the Mn, Fe, Na, Al, and Se contents in the roots increased significantly, whereas those of Mg, Zn, and K decreased significantly. Furthermore, RNA sequencing (RNA-seq) analysis was used to test the differentially expressed genes (DEGs) of soybean root under Mn poisoning. The results found 45,274 genes in soybean root and 1430 DEGs under Mn concentrations of 5 (normal) and 100 (toxicity) μM. Among these DEGs, 572 were upregulated and 858 were downregulated, indicating that soybean roots may initiate complex molecular regulatory mechanisms on Mn poisoning stress. The results of quantitative RT-PCR indicated that many DEGs were upregulated or downregulated markedly in the roots, suggesting that the regulation of DEGs may be complex. Therefore, the regulatory mechanism of soybean root on Mn toxicity stress is complicated. Present results lay the foundation for further study on the molecular regulation mechanism of function genes involved in regulating Mn tolerance traits in soybean roots.

Influence of bio fabricated manganese oxide nanoparticles for effective callogenesis of Moringa oleifera Lam

The use of nanoscale fertilizers to boost crop output has increased in recent years. Nanoparticles (NPs) can stimulate the biosynthesis of bioactive compounds in plants. It is the first report on biosynthesized manganese oxide nanoparticles (MnO-NPs) that mediate in-vitro callus induction of Moringa oleifera. To achieve better biocompatibility the leaf extract of Syzygium cumini was used to synthesize MnO-NPs. Scanning electron microscope SEM revealed spherical shaped morphology of MnO-NPs with an average diameter of 36 ± 0.3 nm. Energy-dispersive X-ray spectroscopy (EDX) depicted the formation of pure MnO-NPs. X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) authenticate the crystalline structure. UV-visible absorption spectroscopy depicted the activity of MnO-NPs under visible light. The biosynthesized MnO-NPs were concentration-dependent and revealed promising results in callus induction of Moringa oleifera. It was found that MnO-NPs enhance callus production of Moringa oleifera and keep the callus infection free by providing an optimum environment for rapid growth and development. Therefore MnO-NPs synthesized through the green process can be utilized in tissue culture studies. This study concludes that MnO is one of the essential plant nutrients that have tailored nutritive properties at a nano scale.

Boron supply restores aluminum-blocked auxin transport by the modulation of PIN2 trafficking in the root apical transition zone

The supply of boron (B) alleviates the toxic effects of aluminum (Al) on root growth; however, the mechanistic basis of this process remains elusive. This study filled this knowledge gap, demonstrating that boron modifies auxin distribution and transport in Al-exposed Arabidopsis roots. In B-deprived roots, treatment with Al induced an increase in auxin content in the root apical meristem zone (MZ) and transition zone (TZ), whereas in the elongation zone (EZ) the auxin content was decreased beyond the level required for adequate growth. These distribution patterns are explained by the fact that basipetal auxin transport from the TZ to the EZ was disrupted by Al-inhibited PIN-FORMED 2 (PIN2) endocytosis. Experiments involving the modulation of protein biosynthesis by cycloheximide (CHX) and transcriptional regulation by cordycepin (COR) demonstrated that the Al-induced increase of PIN2 membrane proteins was dependent upon the inhibition of PIN2 endocytosis, rather than on the transcriptional regulation of the PIN2 gene. Experiments reporting on the profiling of Al³⁺ and PIN2 proteins revealed that the inhibition of endocytosis of PIN2 proteins was the result of Al-induced limitation of the fluidity of the plasma membrane. The supply of B mediated the turnover of PIN2 endosomes conjugated with indole-3-acetic acid (IAA), and thus restored the Al-induced inhibition of IAA transport through the TZ to the EZ. Overall, the reported results demonstrate that boron supply mediates PIN2 endosome-based auxin transport to alleviate Al toxicity in plant roots.

Screening the toxicity profile and genotoxicity mechanism of excess manganese confirmed by spectral shift

In this study, the toxicity induced by excessive doses of manganese (MnCl2), which is one of the essential trace elements for the continuation of the metabolic activities of the organisms, was investigated with the help of the Allium test. Toxicity was investigated by using physiological (percent germination, root length, weight gain), cytogenetic [mitotic index (MI), micronucleus (MN), chromosomal abnormalities (CAs)], biochemical [malondialdehyde (MDA), superoxide dismutase (SOD) catalase (CAT)] and anatomical (root tip meristematic cell damage) parameters. Allium cepa L. bulbs were divided into four groups as one control and three treatments. The control group was germinated with tap water, and the treatment groups were germinated with 250, 500 and 1000 µM doses of MnCl2. The germination process was continued for 72 h without interruption. At the end of the period, the root tips were collected, washed in distilled water and made ready for microscopic and spectrophotometric analyzes with the help of routine preparation techniques. As a result, the highest germination percentage, root length, weight gain and MI, and the lowest MN frequency, CAs numbers, MDA level, SOD and CAT enzyme activities were determined in the control group (group I). MnCl2 exposure caused a decrease in physiological parameter values and an increase in cytogenetic (except MI) and biochemical parameter values, depending on the dose. MnCl2 exposure induced MN and CAs such as fragment, sticky chromosome, vagrant chromosome, unequal distribution of chromatin and bridge. This genotoxic effect of MnCl2 was associated with DNA-MnCl2 interaction, and this interaction was also confirmed by bathochromic and hypochromic shifts in spectral analysis. Anatomical damages such as epidermis cell damage, flattened cell nucleus, cortex cell damage and cortex cell wall thickening were observed after MnCl2 treatment. As a result, it has been determined that excessive doses of the trace element Mn cause physiological, cytogenetic, biochemical and anatomical toxicity and A. cepa test material is a reliable bio-indicator in determining this toxicity.

Plant Hormone and Inorganic Ion Concentrations in the Xylem Exudate of Grafted Plants Depend on the Scion-Rootstock Combination

In grafted plants, inorganic ions and plant hormones in the xylem exudate transported from the rootstock to the scion directly or indirectly affect the scion, thereby improving the traits. Therefore, the concentration of these components in the xylem exudate of grafted plants may be an indicator for rootstock selection. On the other hand, few reports have presented a comprehensive analysis of substances transferred from the rootstock to the scion in plants grafted onto different rootstocks, primarily commercial cultivars. In this study, we measured inorganic ions and plant hormones in the xylem exudate from the rootstock to the scion in various grafted plants of tomato and eggplant. The results revealed that the concentrations of inorganic ions and plant hormones in the xylem exudate significantly differed depending on the type of rootstock. In addition, we confirmed the concentration of the inorganic ions and plant hormones in the xylem exudate of plants grafted onto the same tomato rootstock cultivars as rootstock with tomato or eggplant as the scions. As a result, the concentrations of inorganic ions and plant hormones in the xylem exudate were significantly different in the grafted plants with eggplant compared with tomato as the scion. These results suggest that signals from the scion (shoot) control the inorganic ions and plant hormones transported from the rootstock (root).

Regulatory role of microRNAs (miRNAs) in the recent development of abiotic stress tolerance of plants

MicroRNAs (miRNAs) are a distinct groups of single-stranded non-coding, tiny regulatory RNAs approximately 20-24 nucleotides in length. miRNAs negatively influence gene expression at the post-transcriptional level and have evolved considerably in the development of abiotic stress tolerance in a number of model plants and economically important crop species. The present review aims to deliver the information on miRNA-mediated regulation of the expression of major genes or Transcription Factors (TFs), as well as genetic and regulatory pathways. Also, the information on adaptive mechanisms involved in plant abiotic stress responses, prediction, and validation of targets, computational tools, and databases available for plant miRNAs, specifically focus on their exploration for engineering abiotic stress tolerance in plants. The regulatory function of miRNAs in plant growth, development, and abiotic stresses consider in this review, which uses high-throughput sequencing (HTS) technologies to generate large-scale libraries of small RNAs (sRNAs) for conventional screening of known and novel abiotic stress-responsive miRNAs adds complexity to regulatory networks in plants. The discoveries of miRNA-mediated tolerance to multiple abiotic stresses, including salinity, drought, cold, heat stress, nutritional deficiency, UV-radiation, oxidative stress, hypoxia, and heavy metal toxicity, are highlighted and discussed in this review.

Metal crossroads in plants: modulation of nutrient acquisition and root development by essential trace metals

The metals iron, zinc, manganese, copper, molybdenum, and nickel are essential for the growth and development of virtually all plant species. Although these elements are required at relatively low amounts, natural factors and anthropogenic activities can significantly affect their availability in soils, inducing deficiencies or toxicities in plants. Because essential trace metals can shape root systems and interfere with the uptake and signaling mechanisms of other nutrients, the non-optimal availability of any of them can induce multi-element changes in plants. Interference by one essential trace metal with the acquisition of another metal or a non-metal nutrient can occur prior to or during root uptake. Essential trace metals can also indirectly impact the plant's ability to capture soil nutrients by targeting distinct root developmental programs and hormone-related processes, consequently inducing largely metal-specific changes in root systems. The presence of metal binding domains in many regulatory proteins also enables essential trace metals to coordinate nutrient uptake by acting at high levels in hierarchical signaling cascades. Here, we summarize the known molecular and cellular mechanisms underlying trace metal-dependent modulation of nutrient acquisition and root development, and highlight the importance of considering multi-element interactions to breed crops better adapted to non-optimal trace metal availabilities.

Metal tolerance protein MTP6 is involved in Mn and Co distribution in poplar

With the booming demand of the electric vehicle industry, the concentration of manganese (Mn) and cobalt (Co) flowing into land ecosystems has also increased significantly. While these transition metals can promote the growth and development of plants, they may become toxic under high concentrations. It is thus important to understand how Mn and Co are distributed in plants to develop novel germplasms for the remediation of these heavy metals in contaminated soils. Here, an MTP gene that encodes the CDF (cation diffusion facilitator) protein in Populus trichocarpa, PtrMTP6, was screened as the key gene involved in the distribution of both Mn and Co in poplar. The PtrMTP6-GFP fusion protein was co-localized with the mRFP-VSR2, showing that PtrMTP6 proteins are present at the pre-vacuolar compartment (PVC). Yeast mutant complementation assays further identified that PtrMTP6 serves as a Mn and Co transporter, reducing yeast cell toxicity after exposure to excessive Mn or Co. Histochemical analyses showed that PtrMTP6 was mainly expressed in phloem, suggesting that PtrMTP6 probably involved in the Mn and Co transport via phloem in plants. Under excess Co, PtrMTP6 overexpressing poplar lines were more severely damaged than the control due to higher Co accumulations in young tissue. PtrMTP6 overexpressing lines showed little change in their tolerance to excess Mn, although young tissues also accumulated more Mn. PtrMTP6 play important roles in Mn and Co distribution in poplar and further research on its regulation will be important to increase bioremediation in Mn and Co polluted ecosystems.

Conditions Affecting the Release of Heavy and Rare Earth Metals from the Mine Tailings Kola Subarctic

In the Kola Subarctic, a mining industry has developed, which is a source of environmental pollution with heavy metals. The objects of study were the tailings of three large mining enterprises in the region: apatite-nepheline, complex and loparite ores. The geotechnical characteristics were studied, and the granulometric composition of the samples was established. The main minerals that make up the material of ore dressing tailings have been determined. Using inductively coupled plasma mass spectrometry, the content of trace elements, in particular heavy metals and rare earth elements, has been established. The enrichment factor, the geoaccumulation indexes, the potential ecological risk index factor and the potential environmental hazard index have been calculated. Priority pollutants characteristics for specific objects have been identified. It is noted that the finely dispersed material of the tailings of loparite and complex ores is 1.5–3 times enriched in heavy and rare earth metals in comparison with the total material of the tailings. In laboratory conditions, experiments were carried out to simulate the process of interaction of dust particles with soil solutions containing different amounts of dissolved organic matter and at average seasonal temperatures. It was found that a decrease in the pH of the solution and an increase in the amount of organic carbon and temperature lead to the mobilization of heavy and rare earth metals from the tailings.

Carbon dots inhibit root growth by disrupting auxin biosynthesis and transport in Arabidopsis

Carbon dots (CDs) possess considerable potentials in fields like biomarker and cell imaging due to its good fluorescence properties. Nevertheless, the molecular mechanism concerning influences of CDs on plant growth still remains unknown. In this study, the subcellular localization of CDs in Arabidopsis and the molecular mechanism of CDs toxicity to plants were investigated. Results demonstrate that CDs tend to accumulate in meristematic nucleus of root tips. CDs can inhibit growth of meristem zone of primary root (PR) of Arabidopsis seedlings significantly. The transcription level of auxin biosynthesis related genes decreases and the abundance of auxin efflux carriers PIN1 and PIN2 declines after 40 mg/L CDs treatment, thus lowering the auxin level in root tips. Moreover, CDs weaken activity of cell division in meristem zone by disturbing expressions of DNA damage repair genes and cell cycle regulation genes, thus enabling to inhibit growth of the meristem zone. To sum up, CDs inhibit growth of Arabidopsis seedlings through above pathways. These results provide useful information to elaborate potential toxicity mechanism of CDs on terrestrial plants.

Root secondary growth: an unexplored component of soil resource acquisition

BACKGROUND AND AIMS

Despite recent progress in elucidating the molecular basis of secondary growth (cambial growth), the functional implications of this developmental process remain poorly understood. Targeted studies exploring how abiotic and biotic factors affect this process, as well as the relevance of secondary growth to fitness of annual dicotyledonous crop species under stress, are almost entirely absent from the literature. Specifically, the physiological role of secondary growth in roots has been completely neglected yet entails a unique array of implications for plant performance that are distinct from secondary growth in shoot tissue.

SCOPE

Since roots are directly responsible for soil resource capture, understanding of the fitness landscape of root phenotypes is important in both basic and applied plant biology. Interactions between root secondary growth, edaphic conditions and soil resource acquisition may have significant effects on plant fitness. Our intention here is not to provide a comprehensive review of a sparse and disparate literature, but rather to highlight knowledge gaps, propose hypotheses and identify opportunities for novel and agriculturally relevant research pertaining to secondary growth of roots. This viewpoint: (1) summarizes evidence from our own studies and other published work; (2) proposes hypotheses regarding the fitness landscape of secondary growth of roots in annual dicotyledonous species for abiotic and biotic stress; and (3) highlights the importance of directing research efforts to this topic within an agricultural context.

CONCLUSIONS

Secondary growth of the roots of annual dicots has functional significance with regards to soil resource acquisition and transport, interactions with soil organisms and carbon sequestration. Research on these topics would contribute significantly toward understanding the agronomic value of secondary growth of roots for crop improvement.

Manganese in Plants: From Acquisition to Subcellular Allocation

Manganese (Mn) is an important micronutrient for plant growth and development and sustains metabolic roles within different plant cell compartments. The metal is an essential cofactor for the oxygen-evolving complex (OEC) of the photosynthetic machinery, catalyzing the water-splitting reaction in photosystem II (PSII). Despite the importance of Mn for photosynthesis and other processes, the physiological relevance of Mn uptake and compartmentation in plants has been underrated. The subcellular Mn homeostasis to maintain compartmented Mn-dependent metabolic processes like glycosylation, ROS scavenging, and photosynthesis is mediated by a multitude of transport proteins from diverse gene families. However, Mn homeostasis may be disturbed under suboptimal or excessive Mn availability. Mn deficiency is a serious, widespread plant nutritional disorder in dry, well-aerated and calcareous soils, as well as in soils containing high amounts of organic matter, where bio-availability of Mn can decrease far below the level that is required for normal plant growth. By contrast, Mn toxicity occurs on poorly drained and acidic soils in which high amounts of Mn are rendered available. Consequently, plants have evolved mechanisms to tightly regulate Mn uptake, trafficking, and storage. This review provides a comprehensive overview, with a focus on recent advances, on the multiple functions of transporters involved in Mn homeostasis, as well as their regulatory mechanisms in the plant's response to different conditions of Mn availability.

The mechanism of root growth inhibition by the endocrine disruptor bisphenol A (BPA)

Bisphenol A (BPA) is a harmful environmental contaminant acting as an endocrine disruptor in animals, but it also affects growth and development in plants. Here, we have elucidated the functional mechanism of root growth inhibition by BPA in Arabidopsis thaliana using mutants, reporter lines and a pharmacological approach. In response to 10 ppm BPA, fresh weight and main root length were reduced, while auxin levels increased. BPA inhibited root growth by reducing root cell length in the elongation zone by suppressing expansin expression and by decreasing the length of the meristem zone by repressing cell division. The inhibition of cell elongation and cell division was attributed to the enhanced accumulation/redistribution of auxin in the elongation zone and meristem zone in response to BPA. Correspondingly, the expressions of most auxin biosynthesis and transporter genes were enhanced in roots by BPA. Taken together, it is assumed that the endocrine disruptor BPA inhibits primary root growth by inhibiting cell elongation and division through auxin accumulation/redistribution in Arabidopsis. This study will contribute to understanding how BPA affects growth and development in plants.

Phytoremediation potential of Cd and Pb-contaminated soils by Paspalum fasciculatum Willd. ex Flüggé

The phytoremediation capacity of Paspalum fasciculatum Willd. ex Flüggé, was evaluated in soils from a gold mine contaminated with cadmium (Cd) and lead (Pb), using three concentration levels of each metal (15, 30, and 50 mg kg^-1). Their ability to assimilate Cd and Pb in its different tissues was evaluated during 90 days of exposure. Plant growth behavior, accumulation of Cd and Pb, and translocation (TF) and bioaccumulation (BAF) factors were also determined. During the first 60 days of exposure, Cd had an inductive effect on the growth of P. fasciculatum; however, after 90 days, this metal had begun to show toxic effects. Plants showed a similar pattern of accumulating Cd and Pb in their tissues with concentrations decreasing in the order roots > leaves > stem. However, the accumulated concentrations of Cd were generally higher than those of Pb with the highest metal uptakes being observed during the first 30 days of exposure. P. fasciculatum was shown to have a phytostabilization effect with regard to Cd, high concentrations of metals in tissues and little translocation, whereas it showed phytoextraction capacity for Pb. In addition, it can increase pH and organic matter in the soil rhizosphere.

Deletion in the Promoter of PcPIN-L Affects the Polar Auxin Transport in Dwarf Pear (Pyrus communis L.)

Dwarf cultivars or dwarfing rootstocks enable high-density planting and are therefore highly desirable in modern pear production. Previously, we found that the dwarf growth habit of pear is controlled by a single dominant gene PcDw. In this study, PcPIN-L (PCP021016) was cloned from dwarf-type and standard-type pears. PcPIN-L expression was significantly lower in the dwarf-type pears than in standard-type pears, which was caused by the CT repeat deletion in the promoter of dwarf-type pears. PcPIN-L overexpression in tobacco plants enhanced the growth of the stems and the roots. Notably, the indole acetic acid (IAA) content decreased in the shoot tips and increased in the stems of transgenic lines compared with wild type, which is consistent with the greater IAA content in the shoot tips and lower IAA content in the stems of dwarf-type pears than in standard-type pears. The CT repeat deletion in the promoter that causes a decrease in promoter activity is associated with lower PcPIN-L expression in the dwarf-type pears, which might limit the polar auxin transport and in turn result in the dwarf phenotype. Taken together, the results provide a novel dwarfing molecular mechanism in perennial woody plants.

Potential of PM-selected components to induce oxidative stress and root system alteration in a plant model organism

Over the last years, various acellular assays have been used for the evaluation of the oxidative potential (OP) of particular matter (PM) to predict PM capacity to generate reactive oxygen (ROS) and nitrogen (RNS) species in biological systems. However, relationships among OP and PM toxicological effects on living organisms are still largely unknown. This study aims to assess the effects of atmospheric PM-selected components (brake dust - BD, pellet ash - PA, road dust - RD, certified urban dust NIST1648a - NIST, soil dust - S, coke dust - C and Saharan dust - SD) on the model plant A. thaliana development, with emphasis on their capacity to induce oxidative stress and root morphology alteration. Before growing A. thaliana in the presence of the PM-selected components, each atmospheric dust has been chemically characterized and tested for the OP through dithiothreitol (DTT), ascorbic acid (AA) and 2',7'-dichlorofluorescin (DCFH) assays. After the exposure, element bioaccumulation in the A. thaliana seedlings, i.e., in roots and shoots, was determined and both morphological and oxidative stress analyses were performed in roots. The results indicated that, except for SD and S, all the tested dusts affected A. thaliana root system morphology, with the strongest effects in the presence of the highest OPs dusts (BD, PA and NIST). Principal component analysis (PCA) revealed correlations among OPs of the dusts, element bioaccumulation and root morphology alteration, identifying the most responsible dust-associated elements affecting the plant. Lastly, histochemical analyses of NO and O₂^- content and distribution confirmed that BD, PA and NIST induce oxidative stress in A. thaliana, reflecting the high OPs of these dusts and ultimately leading to cell membrane lipid peroxidation.

Negative Modulation of Bystander DNA Repair Potential by X-Ray Targeted Tissue Volume in Arabidopsis thaliana

Radiation-induced bystander effects (RIBE) entail a cascade of bystander signals produced by the hit cells to the neighboring cells to regulate various biological processes including DNA damage repair. However, there is little clarity regarding the effect of radiation-targeted volume (hit cell amount) on the DNA repair potential of the bystander cells. This is especially important to understand in the context of the whole organism, where the target usually consists of multiple types of cells/tissues. To address this question, model plant Arabidopsis thaliana was locally irradiated, and the DNA repair potential of bystander root-tip cells was assessed based on their radioresistance to subsequent high-dose radiation, i.e. radioadaptive responses (RAR). We found that X-ray irradiation of the aerial parts (AP) of A. thaliana seedlings (5 Gy) initiated RAR in the root-tip cells, which exhibited an alleviated repression of root growth and root cell division, and reduced amount of DNA strand breaks. We also observed an improvement in the repair efficiency of the homologous recombination (HR) and non-homologous end joining (NHEJ) pathways in the bystander root tip cells. We further expanded the X-ray targeted volume to include the aerial parts with upper parts of the primary root and compared it with X-ray irradiated aerial parts alone. Comparative analysis revealed that RAR for these end points either disappeared or decreased; specifically, the repair efficiency of HR was significantly reduced, indicating that radiation-targeted volume negatively modulates the bystander DNA repair potential. In contrast, X-ray irradiation of upper part of the primary root alone did not induce RAR of the root tip cells. Thus, we propose that additional X-ray irradiation of upper part of the primary root reduces the bystander DNA repair potential, possibly by selectively disturbing the transport of bystander signals responsible for HR repair.

Assessing germination characteristics of Australian native plant species in metal/metalloid solution

This study investigated tolerance of Australian native grass species Astrebla lappacea, Themeda australis, and Austrostipa scabra and a tree species Acacia harpophylla to different concentrations of arsenic As(V) (13.34-667.36 μM), Cu²⁺ (0.5-200 μM), Zn²⁺ (9-500 μM), Mn²⁺ (8-10240 μM) and Pb²⁺ (240-9600 μM) in single solutions in germination experiments. Metal/loid tolerance indicators used were maximum germination percentage (G_max), mean germination time (MGT), radicle and shoot tolerance indexes (RTI & STI). Radicle tolerance index was the most sensitive indicator of metal tolerance in germinating seeds. All native species were highly tolerant to the metal/loids tested, however, they showed different metal toxicity thresholds and levels of tolerance based on RTI as a metal tolerance indicator during germination. Overall, all four species could be classified as metallophytes, confirming their current suitability for and established use in mine site rehabilitation. This work may also serve as a basis for future studies on metal/loid tolerance of other plant species during germination.

Heavy metal induced oxidative damage and root morphology alterations of maize (Zea mays L.) plants and stress mitigation by metal tolerant nitrogen fixing Azotobacter chroococcum

Heavy metals are one of the major abiotic stresses that adversely affect the quantity and nutritive value of maize. Microbial management involving the use of plant growth promoting rhizobacteria (PGPR) is a promising inexpensive strategy for metal clean up from polluted soils. Considering these, metal tolerant plant growth promoting nitrogen fixing rhizobacterial strain CAZ3 identified by 16SrRNA gene sequence analysis as Azotobacter chroococcum was recovered from metal polluted chilli rhizosphere. When exposed to varying levels of metals, A. chroococcum survived up to 1400 and 2000 µg mL^-1 of Cu and Pb, respectively and expressed numerous plant growth promoting activities even under metal stress. Strain CAZ3 secreted 65.5 and 60.8 µg mL^-1 IAA at 400 µg mL^-1 each of Cu and Pb, respectively and produced siderophores, ammonia and ACC deaminase under metal pressure. The melanin extracted from A. chroococcum revealed metal chelating ability under EDX. Following application, strain CAZ3 enhanced growth and yield of maize grown both in the presence of Cu and Pb. The dry biomass of roots of inoculated plants grown with 2007 mg Cu kg^-1 and 585 mg Pb kg^-1 was increased by 28% and 20%, respectively. At 585 mg Pb kg^-1, the bioinoculant also increased the kernel attributes. At 2007 mg Cu kg^-1 strain CAZ3 enhanced the number, yield and protein of kernels by 10%, 45% and 6%, respectively. Interestingly, strain CAZ3 significantly reduced the levels of proline, malondialdehyde and antioxidant enzymes in foliage. The roots of inoculated plants accumulated greatest amounts of metals compared to other organs. In kernels, the concentration of Pb was more as compared to Cu. The metal concentrations in roots, shoots and kernels, however, declined following CAZ3 inoculation. Copper and lead had substantial distortive impact on root and leaf morphology while cell death were visible under CLSM and SEM. Conclusively, A. chroococcum CAZ3 could be a most suitable and promising option to increase maize production in metal polluted soils despite the soils being contaminated with heavy metals.

Application of flow cytometry with a fluorescent dye to measurement of intracellular nitric oxide in plant cells

A simple and rapid method involving flow cytometry and NO-specific probe (DAF-FM DA) proved useful for detection and determination of intracellular NO production in Medicago truncatula suspension cells and leaves as well as in cells of Avena fatua, Amaranthus retroflexus embryos and leaves. The measurement of nitric oxide (NO) in plant material is important for examining the regulatory roles of endogenous NO in various physiological processes. The possibility of detecting and determining intracellular NO production by flow cytometry (FCM) with 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM DA), an NO-specific probe in Medicago truncatula cells in suspension and leaves as well as in cells of embryos and leaves of Avena fatua L. or Amaranthus retroflexus L. was explored. To detect and measure NO production by cell suspension or embryos and leaves, the recommended DAF-FM DA concentration is 5 or 10 µM, respectively, applied for 30 min. Exogenous NO increased the intensity of the fluorescent signal in embryos and leaves of both plants, while carboxy-PTIO (cPTIO), an NO scavenger, decreased it. Thus, these results demonstrate that NO can be detected and an increase and a decrease of its intracellular level can be estimated. Wounding was observed to increase the fluorescence signal, indicating an increase in the intracellular NO level. In addition, the levels of exogenous and endogenous ascorbic acid were demonstrated to have no effect on the NO-related fluorescence signal, indicating the signal's specificity only in relation with NO. The applicability of the proposed method for detection and determination of NO was confirmed (1) by in situ NO imaging in cell suspensions and (2) by determining the NO concentration in embryos and leaves using the Griess reagent. In view of the data obtained, FCM is recommended as a rapid and simple method with which to detect and determine intracellular NO production in plant cells.

Trace metal metabolism in plants

Many trace metals are essential micronutrients, but also potent toxins. Due to natural and anthropogenic causes, vastly different trace metal concentrations occur in various habitats, ranging from deficient to toxic levels. Therefore, one focus of plant research is on the response to trace metals in terms of uptake, transport, sequestration, speciation, physiological use, deficiency, toxicity, and detoxification. In this review, we cover most of these aspects for the essential micronutrients copper, iron, manganese, molybdenum, nickel, and zinc to provide a broader overview than found in other recent reviews, to cross-link aspects of knowledge in this very active research field that are often seen in a separated way. For example, individual processes of metal usage, deficiency, or toxicity often were not mechanistically interconnected. Therefore, this review also aims to stimulate the communication of researchers following different approaches, such as gene expression analysis, biochemistry, or biophysics of metalloproteins. Furthermore, we highlight recent insights, emphasizing data obtained under physiologically and environmentally relevant conditions.

Responses of contrasting rice genotypes to excess manganese and their implications for lignin synthesis

Manganese (Mn) toxicity is frequently encountered in crops grown on soils with low pH or low redox potential, and harmful to plant development and growth. This study aimed at exploring adaptive mechanisms to Mn toxicity in rice, and investigated the effects of Mn toxicity on shoot lignification. Sixteen rice genotypes were grown in hydroponic solutions and exposed to normal (0.5 mg dm^-3) or toxic (5 mg dm^-3) Mn concentrations for three weeks. Morphological responses to Mn toxicity included a significant reduction in shoot length and the formation of visible symptoms scored as leaf damage index (LDI). Based on shoot Mn concentrations in the Mn toxic treatment, genotypes were classified as Mn includers and excluders. Across different genotypes, shoot Mn concentrations were significantly negatively correlated with relative shoot length and positively correlated with LDI. Consequently, the most tolerant genotypes in terms of morphology were all excluders, while the most sensitive genotypes were includers. The sensitive genotypes were also more responsive to manganese in terms of lipid peroxidation than tolerant genotypes. Shoots of rice plants grown in the high Mn treatment showed a higher level of lignification measured as thioglycolic acid lignin (TGAL), especially among Mn includers. TGAL was positively correlated with shoot Mn concentration and the levels of phenolics. In contrast, peroxidase activity was not responsive to the Mn treatment and was not significantly correlated with shoot lignification. In conclusion, exclusion is a dominant tolerance mechanism to Mn toxicity in rice. Further, Mn stimulated lignin biosynthesis in rice, especially in genotypes that were unable to exclude Mn.

Control of Endogenous Auxin Levels in Plant Root Development

In this review, we summarize the different biosynthesis-related pathways that contribute to the regulation of endogenous auxin in plants. We demonstrate that all known genes involved in auxin biosynthesis also have a role in root formation, from the initiation of a root meristem during embryogenesis to the generation of a functional root system with a primary root, secondary lateral root branches and adventitious roots. Furthermore, the versatile adaptation of root development in response to environmental challenges is mediated by both local and distant control of auxin biosynthesis. In conclusion, auxin homeostasis mediated by spatial and temporal regulation of auxin biosynthesis plays a central role in determining root architecture.

Lanthanum Inhibits Primary Root Growth by Repressing Auxin Carrier Abundances in Arabidopsis

Lanthanum (La) is one of rare earth elements that was used as a crop growth stimulants; however, high concentration of La markedly inhibited plant growth. Our previous study indicated that, although La induced the expression of auxin biosynthesis-related genes, it markedly repressed primary root (PR) elongation by reducing auxin accumulation in PR tips. In this study, we exhibited that La reduces the abundances of auxin carriers. Treatment with La markedly inhibited the auxins IAA-, 2,4-D-, and NAA-induced elevation of DR5:GUS activity in the roots, suggesting that La inhibited auxin transport through both the influx and efflux transporters. Supplementation with auxin transport inhibitor naphthylphthalamic acid in La-treated seedlings did not further reduce PR growth compared with that of the La treatment alone, further confirmed that auxin transport is involved in La-induced inhibition of PR growth. Analysis of the protein abundances using the transgenic AUX1-YFP and PIN1/2/4/7-GFP marker lines indicated that La treatment reduced the abundances of all these auxin carriers in the PR tips. La also increased the stabilization of Aux/IAA protein AXR3. Taken together, these results indicated that La treatment inhibits PIN-mediated auxin transport and subsequently impairs auxin distribution and PR growth via reducing auxin carrier abundances.