The cytokinin trans-zeatine riboside increased resistance to heavy metals in the halophyte plant species Kosteletzkya pentacarpos in the absence but not in the presence of NaCl

Mitigating Effect of Trans-Zeatin on Cadmium Toxicity in Desmodesmus armatus

Phytohormones, particularly cytokinin trans-zeatin (tZ), were studied for their impact on the green alga Desmodesmus armatus under cadmium (Cd) stress, focusing on growth, metal accumulation, and stress response mechanisms. Using atomic absorption spectroscopy for the Cd level and high-performance liquid chromatography for photosynthetic pigments and phytochelatins, along with spectrophotometry for antioxidants and liquid chromatography-mass spectrometry for phytohormones, we found that tZ enhances Cd uptake in D. armatus, potentially improving phycoremediation of aquatic environments. Cytokinin mitigates Cd toxicity by regulating internal phytohormone levels and activating metal tolerance pathways, increasing phytochelatin synthase activity and phytochelatin accumulation essential for Cd sequestration. Treatment with tZ and Cd also resulted in increased cell proliferation, photosynthetic pigment and antioxidant levels, and antioxidant enzyme activities, reducing oxidative stress. This suggests that cytokinin-mediated mechanisms in D. armatus enhance its capacity for Cd uptake and tolerance, offering promising avenues for more effective aquatic phycoremediation techniques.

Phytohormones-mediated strategies for mitigation of heavy metals toxicity in plants focused on sustainable production

KEY MESSAGE

In this manuscript, authors reviewed and explore the information on beneficial role of phytohormones to mitigate adverse effects of heavy metals toxicity in plants. Global farming systems are seriously threatened by heavy metals (HMs) toxicity, which can result in decreased crop yields, impaired food safety, and negative environmental effects. A rise in curiosity has been shown recently in creating sustainable methods to reduce HMs toxicity in plants and improve agricultural productivity. To accomplish this, phytohormones, which play a crucial role in controlling plant development and adaptations to stress, have emerged as intriguing possibilities. With a particular focus on environmentally friendly farming methods, the current review provides an overview of phytohormone-mediated strategies for reducing HMs toxicity in plants. Several physiological and biochemical activities, including metal uptake, translocation, detoxification, and stress tolerance, are mediated by phytohormones, such as melatonin, auxin, gibberellin, cytokinin, ethylene, abscisic acid, salicylic acid, and jasmonates. The current review offers thorough explanations of the ways in which phytohormones respond to HMs to help plants detoxify and strengthen their resilience to metal stress. It is crucial to explore the potential uses of phytohormones as long-term solutions for reducing the harmful effects of HMs in plants. These include accelerating phytoextraction, decreasing metal redistribution to edible plant portions, increasing plant tolerance to HMs by hormonal manipulation, and boosting metal sequestration in roots. These methods seek to increase plant resistance to HMs stress while supporting environmentally friendly agricultural output. In conclusion, phytohormones present potential ways to reduce the toxicity of HMs in plants, thus promoting sustainable agriculture.

Futuristic advancements in phytoremediation of endocrine disruptor Bisphenol A: A step towards sustainable pollutant degradation for rehabilitated environment

Bisphenol A (BPA) accumulates in the environment at lethal concentrations because of its high production rate and utilization. BPA, originating from industrial effluent, plastic production, and consumer products, poses serious risks to both the environment and human health. The widespread aggregation of BPA leads to endocrine disruption, reactive oxygen species-mediated DNA damage, epigenetic modifications and carcinogenicity, which can disturb the normal homeostasis of the body. The living being in a population is subjected to BPA exposure via air, water and food. Globally, urinary analysis reports have shown higher BPA concentrations in all age groups, with children being particularly susceptible due to its occurrence in items such as milk bottles. The conventional methods are costly with a low removal rate. Since there is no proper eco-friendly and cost-effective degradation of BPA reported so far. The phytoremediation, green-biotechnology based method which is a cost-effective and renewable resource can be used to sequestrate BPA. Phytoremediation is observed in numerous plant species with different mechanisms to remove harmful contaminants. Plants normally undergo several improvements in genetic and molecular levels to withstand stress and lower levels of toxicants. But such natural adaptation requires more time and also higher concentration of contaminants may disrupt the normal growth, survival and yield of the plants. Therefore, natural or synthetic amendments and genetic modifications can improve the xenobiotics removal rate by the plants. Also, constructed wetlands technique utilizes the plant's phytoremediation mechanisms to remove industrial effluents and medical residues. In this review, we have discussed the limitations and futuristic advancement strategies for degrading BPA using phytoremediation-associated mechanisms.

Unveiling Innovative Approaches to Mitigate Metals/Metalloids Toxicity for Sustainable Agriculture

Due to anthropogenic activities, environmental pollution of heavy metals/metalloids (HMs) has increased and received growing attention in recent decades. Plants growing in HM-contaminated soils have slower growth and development, resulting in lower agricultural yield. Exposure to HMs leads to the generation of free radicals (oxidative stress), which alters plant morpho-physiological and biochemical pathways at the cellular and tissue levels. Plants have evolved complex defense mechanisms to avoid or tolerate the toxic effects of HMs, including HMs absorption and accumulation in cell organelles, immobilization by forming complexes with organic chelates, extraction via numerous transporters, ion channels, signaling cascades, and transcription elements, among others. Nonetheless, these internal defensive mechanisms are insufficient to overcome HMs toxicity. Therefore, unveiling HMs adaptation and tolerance mechanisms is necessary for sustainable agriculture. Recent breakthroughs in cutting-edge approaches such as phytohormone and gasotransmitters application, nanotechnology, omics, and genetic engineering tools have identified molecular regulators linked to HMs tolerance, which may be applied to generate HMs-tolerant future plants. This review summarizes numerous systems that plants have adapted to resist HMs toxicity, such as physiological, biochemical, and molecular responses. Diverse adaptation strategies have also been comprehensively presented to advance plant resilience to HMs toxicity that could enable sustainable agricultural production.

Modulation of plant photosynthetic processes during metal and metalloid stress, and strategies for manipulating photosynthesis-related traits

Metals constitute vital elements for plant metabolism and survival, acting as essential co-factors in cellular processes which are indispensable for plant growth and survival. Excess or deficient provision of metal/metalloids puts plant's life and survival at risk, thus considered a potent stress for plants. Chloroplasts as an organelle with a high metal demand form a pivotal site within the metal homeostasis network. Therefore, the metal-mediated electron transport chain (ETC) in chloroplasts is a primary target site of metal/metalloid-induced stresses. Both excess and deficient availability of metal/metalloids threatens plant's photosynthesis in several ways. Energy demands from the photosynthetic carbon reactions should be in balance with energy output of ETC. Malfunctioning of ETC components as a result of metal/metalloid stress initiates photoinhiition. A feedback inhibition from carbon fixation process also impedes the ETC. Metal stress impairs antioxidant enzyme activity, pigment biosynthesis, and stomatal function. However, genetic manipulations, nutrient management, keeping photostasis, and application of phytohormones are among strategies for coping with metal stress. Consequently, a comprehensive understanding of the underlying mechanisms of metal/metalloid stress, as well as the exploration of potential strategies to mitigate its impact on plants are imperative. This review offers a mechanistic insight into the disruption of photosynthesis regulation by metal/metalloids and highlights adaptive approaches to ameliorate their effects on plants. Focus was made on photostasis, nutrient interactions, phytohormones, and genetic interventions for mitigating metal/metalloid stresses.

Maintenance of stem cell activity in plant development and stress responses

Stem cells residing in plant apical meristems play an important role during postembryonic development. These stem cells are the wellspring from which tissues and organs of the plant emerge. The shoot apical meristem (SAM) governs the aboveground portions of a plant, while the root apical meristem (RAM) orchestrates the subterranean root system. In their sessile existence, plants are inextricably bound to their environment and must adapt to various abiotic stresses, including osmotic stress, drought, temperature fluctuations, salinity, ultraviolet radiation, and exposure to heavy metal ions. These environmental challenges exert profound effects on stem cells, potentially causing severe DNA damage and disrupting the equilibrium of reactive oxygen species (ROS) and Ca2+ signaling in these vital cells, jeopardizing their integrity and survival. In response to these challenges, plants have evolved mechanisms to ensure the preservation, restoration, and adaptation of the meristematic stem cell niche. This enduring response allows plants to thrive in their habitats over extended periods. Here, we presented a comprehensive overview of the cellular and molecular intricacies surrounding the initiation and maintenance of the meristematic stem cell niche. We also delved into the mechanisms employed by stem cells to withstand and respond to abiotic stressors.

Evaluation of the potential of two halophytes to extract Cd and Zn from contaminated saltwater

The contamination of saltwater by toxic heavy metals has become a worldwide problem. The application of phytoextraction to remove these pollutants seems to be more efficient and cheaper compared to physicochemical methods. In this work, we evaluated the potential of two halophyte species to accumulate cadmium and zinc from contaminated water. Seedlings of Carpobrotus edulis L. and Sesuvium portulacastrum L. were cultivated during 1 month on pots filled with saltwater (200 mM NaCl) containing different concentrations of Cd2+ (0, 50, 100 μM) and of Zn2+ (0, 200, and 400 μM) applied separately. Results showed that both halophytes were more resistant to Zn2+ than Cd2+ and that Sesuvium better tolerates the two metals. Zn2+ and Cd2+ concentrations in the shoot as well as the values of translocation factors suggest that these species are able to absorb and to concentrate Cd2+ and Zn2+ in their roots and shoots. Hence, after 1 month of culture on 50 μM Cd2+, plants were able to extract 31% and 21% of Cd, respectively, in S. portulacastrum and C. edulis. The Zn-extraction efficiency was less important and reached 18 and 19%, respectively, in S. portulacastrum and C. edulis cultivated under 200 μM Zn2+. Given together, data demonstrate the efficiency of the use of halophytes, especially S. portulacastrum, to extract Zn2+ and Cd2+ from salt wastewater.

NaCl effect on Cd accumulation and cell compartmentalization in barley

The effect of sodium chloride (NaCl) on cadmium (Cd) tolerance, uptake, translocation, and compartmentation was investigated in 3 barley genotypes. Seedlings were cultivated hydroponically in the absence of NaCl and Cd (control), in the presence of 50 mM NaCl alone, in the presence of 10 µM Cd alone, and in the combined addition of NaCl (50 mM) and Cd (10 µM). Plants were cultivated during one month under 16 h light period at a minimal light intensity of 250 µmol m-2 s-1, a temperature of 25 ± 3 °C, and 70-80% of relative humidity. Results showed that NaCl alone did not significantly affect plant development and biomass production; however, Cd alone reduced plant development rate leading to a decline in biomass production in Raihane and Giza 127 but did not affect that in Amalou. NaCl addition in Cd-treated plants accentuated the Cd effect on plant growth. NaCl limited Cd accumulation in the roots and in the shoots in all tested barley varieties by reducing Cd-absorption efficiency and the translocation of Cd from the root to the shoot. In all Cd-treated plants, cell Cd compartmentalization showed the following gradient: organelles < cell wall < vacuole. NaCl in the medium increased Cd accumulation in the soluble fraction and reduced that in organelle and cell wall fractions. Globally our results showed that, although NaCl reduces Cd accumulation in barley, it accentuates the Cd toxic effects, hence limiting the plant yield. We advise farmers to avoid barley cultivation near mine sites and its irrigation with moderately salty water, although this plant is considered as salt tolerant.

Molecular Regulation and Evolution of Cytokinin Signaling in Plant Abiotic Stresses

The sustainable production of crops faces increasing challenges from global climate change and human activities, which leads to increasing instances of many abiotic stressors to plants. Among the abiotic stressors, drought, salinity and excessive levels of toxic metals cause reductions in global agricultural productivity and serious health risks for humans. Cytokinins (CKs) are key phytohormones functioning in both normal development and stress responses in plants. Here, we summarize the molecular mechanisms on the biosynthesis, metabolism, transport and signaling transduction pathways of CKs. CKs act as negative regulators of both root system architecture plasticity and root sodium exclusion in response to salt stress. The functions of CKs in mineral-toxicity tolerance and their detoxification in plants are reviewed. Comparative genomic analyses were performed to trace the origin, evolution and diversification of the critical regulatory networks linking CK signaling and abiotic stress. We found that the production of CKs and their derivatives, pathways of signal transduction and drought-response root growth regulation are evolutionarily conserved in land plants. In addition, the mechanisms of CK-mediated sodium exclusion under salt stress are suggested for further investigations. In summary, we propose that the manipulation of CK levels and their signaling pathways is important for plant abiotic stress and is, therefore, a potential strategy for meeting the increasing demand for global food production under changing climatic conditions.

microRNAs: Key Players in Plant Response to Metal Toxicity

Environmental metal pollution is a common problem threatening sustainable and safe crop production. Heavy metals (HMs) cause toxicity by targeting key molecules and life processes in plant cells. Plants counteract excess metals in the environment by enhancing defense responses, such as metal chelation, isolation to vacuoles, regulating metal intake through transporters, and strengthening antioxidant mechanisms. In recent years, microRNAs (miRNAs), as a small non-coding RNA, have become the central regulator of a variety of abiotic stresses, including HMs. With the introduction of the latest technologies such as next-generation sequencing (NGS), more and more miRNAs have been widely recognized in several plants due to their diverse roles. Metal-regulated miRNAs and their target genes are part of a complex regulatory network. Known miRNAs coordinate plant responses to metal stress through antioxidant functions, root growth, hormone signals, transcription factors (TF), and metal transporters. This article reviews the research progress of miRNAs in the stress response of plants to the accumulation of HMs, such as Cu, Cd, Hg, Cr, and Al, and the toxicity of heavy metal ions.

Plant hormone profile and control over isoprene biosynthesis in a tropical tree Ficus septica

Plant hormone signalling and the circadian clock have been implicated in the transcriptional control of isoprene biosynthesis. To gain more insight into the hormonal control of isoprene biosynthesis, the present study measured plant hormone concentrations in jasmonic acid (JA)-treated leaves of our previous model study, examined their relationship with gene expression of isoprene synthase (IspS) and hormone signalling transcription factors. Of the plant hormones, IAA and JA-Ile and their related transcription factors (MYC2 and SAUR21) were significantly correlated with IspS gene expression. Concentrations of cytokinins, isopentenyladenine (iP), trans-zeatin riboside (tZR) and cis-zeatin riboside (cZR), were similarly significantly correlated with IspS expression. However, there was no significant correlation between their related transcription factor (ARR-B) and IspS expression. The circadian clock-related gene PRR7, but not the transcription factor LHY, was highly correlated with IspS expression. These results suggest that the hormonal balance between JA-Ile and IAA plays a central role in transcriptional regulation of IspS through the transcription factors MYC2 and SAUR21, the early auxin responsive genes. The putative cis-acting elements for SAUR on the IspS promoter (TGTCNN and CATATG), in addition to the G-box for MYC2, support the above proposal. These results provide insightful information on the core components of plant hormone-related regulation of IspS under coordination with the circadian clock genes.

Silver Nanoparticle Production by the Cyanobacterium Cyanothece sp.: De Novo Manipulation of Nano-Biosynthesis by Phytohormones

Background: Numerous cyanobacteria have the potential to reduce metallic ions to form pure metal nanoparticles in a green biosynthesis process. Aim: To investigate the production capacity of silver nanoparticles by the cyanobacterium Cyanothece sp. and to examine the effect of five different phytohormones, indole acetic acid, kinetin; gibberellic acid; abscisic acid; and methyl jasmonate, on this capacity. Methods: The cyanobacterial strain was grown for 60 days and the harvested cyanobacterium biomass was incubated with 0.1 mM of AgNO₃. Percentage conversion of Ag⁺ to Ag⁰ was calculated to indicate the AgNPs’ production capacity. Different concentrations of the five phytohormones were added to cultures and the AgNP production was monitored throughout different time intervals. Results: Cyanothece sp. biosynthesized spherical AgNPs (diameter range 70 to 140 nm, average diameter 84.37 nm). The addition of indole acetic acid and kinetin provoked the maximum conversion (87.29% and 55.16%, respectively) of Ag⁺ to Ag⁰, exceeding or slightly below that of the control (56%). Gibberellic and abscisic acids failed to elevate the Ag⁺ to Ag⁰ conversion rate (45.23% and 47.95%, respectively) above that of the control. Methyl jasmonate increased the Ag⁺ to Ag⁰ conversion rate to 90.29%, although nearly all the cyanobacterial cultures died at the end. Conclusion: Phytohormones could be used to induce or inhibit the green production of AgNPs with the cyanobacterium Cyanothece sp. This novel manipulation technique may have several applications in agriculture or biomedicine.

Kosteletzkya pentacarpos: A Potential Halophyte Candidate for Phytoremediation in the Meta(loid)s Polluted Saline Soils

Kosteletzkya pentacarpos (L.) Ledebour is a perennial facultative halophyte species from the Malvacea family that grows in coastal areas with high amounts of salt. The tolerance of K. pentacarpos to the high concentration of salt (0.5–1.5% salinity range of coastal saline land) has been widely studied for decades. Nowadays, with the dramatic development of the economy and urbanization, in addition to the salt, the accumulation of mate(loid)s in coastal soil is increasing, which is threatening the survival of halophyte species as well as the balance of wetland ecosystems. Recently, the capacity of K. pentacarpos to cope with either single heavy metal stress or a combination of multiple meta(loid) toxicities was studied. Hence, this review focused on summarizing the physiological and biochemical behaviors of K. pentacarpos that has been simultaneously exposed to the combination of several meta(loid) toxicities. How the salt accumulated by K. pentacarpos impacts the response to meta(loid) stress was discussed. We conclude that as a potential candidate for phytoremediation, K. pentacarpos was able to cope with various environmental constrains such as multiple meta(loid) stresses due to its relative tolerance to meta(loid) toxicity.

Phytohormones: Key players in the modulation of heavy metal stress tolerance in plants

Heavy metal (HM) stress in plants has received considerable global attention as it threatens sustainable growth in agriculture worldwide. Hence, desperate efforts have been undertaken for combating the effects of this stress in plants. Interestingly, the use of phytohormones in reducing the impact of HM toxicity has gained much momentum in the recent past. Phytohormones act as chemical messengers that improve the HM stress resistance in plants, thus allowing them to retain their growth and developmental plasticity. Their exogenous application as well as manipulation of endogenous levels through precise targeting of their biosynthesis/signaling components is a promising approach for providing a protective shield against HM stress in plants. However, for the successful use of phytohormones for field plants exposed to HM toxicity, in-depth knowledge of the key pathways regulated by them is of prime importance. Hence, the present review mainly summarizes the key conceptual developments on the involvement of phytohormones in the mitigation of HM stress in plants. The role of various genes, proteins, and signaling components involved in phytohormones associated HM stress tolerance and their modulation has also been discussed. Thus, this update will pave the way for improving HM stress tolerance in plants with the advent of phytohormones for sustainable agriculture growth in the future.

Application of sulphate and cytokinin in assisted arsenic phytoextraction by industrial Cannabis sativa L

Phytoextraction is currently investigated to effectively remediate soil contaminated by metals and provide highly competitive biomass for energy production. This research aimed to increase arsenic (As) removal from contaminated soil using industrial Cannabis sativa L., a suitable energy crop for biofuel production. Assisted phytoextraction experiments were conducted on a microcosm scale to explore the ability of two friendly treatments, sodium sulphate (SO₄) and exogenous cytokinin (CK), in increasing As phytoextraction efficiency. The results showed that the treatments significantly increased As phytoextraction. Cytokinin was the most effective agent for effectively increasing translocation and the amount of As in aerial parts of C. sativa. In fact, the concentration of As in the shoots of CK-treated plants increased by 172% and 44% compared to untreated and SO₄-treated plants, respectively. However, the increased As amount accumulated in C. sativa tissues due to the two treatments negatively affected plant growth. Arsenic toxicity caused a significant decrease in aerial C. sativa biomass treated with CK and SO₄ of about 32.7% and 29.8% compared to untreated plants, respectively. However, for our research purposes, biomass reduction has been counterbalanced by an increase in As phytoextraction, such as to consider C. sativa and CK an effective combination for the remediation of As-contaminated soils. Considering that C. sativa has the suitable characteristics to provide valuable resources for bioenergy production, our work can help improve the implementation of a sustainable management model for As contaminated areas, such as phytoremediation coupled with bioenergy generation.

Beyond transport: cytokinin ribosides are translocated and active in regulating the development and environmental responses of plants

Riboside type cytokinins are key components in cytokinin metabolism, transport, and sensitivity, making them important functional signals in plant growth and development and environmental stress responses. Cytokinin (CKs) are phytohormones that regulate multiple processes in plants and are critical for agronomy, as they are involved in seed filling and plant responses to biotic and abiotic stress. Among the over 30 identified CKs, there is uncertainty about the roles of many of the individual CK structural forms. Cytokinin free bases (CKFBs), have been studied in great detail, but, by comparison, roles of riboside-type CKs (CKRs) in CK metabolism and associated signaling pathways and their distal impacts on plant physiology remain largely unknown. Here, recent findings on CKR abundance, transport and localization, are summarized, and their importance in planta is discussed. The history of CKR analyses is reviewed, in the context of the determination of CK metabolic pathways, and research on CKR affinity for CK receptors, all of which yield essential insights into their functions. Recent studies suggest that CKR forms are a lot more than a group of transport CKs and, beyond this, they play important roles in plant development and responses to environmental stress. In this context, this review discusses the involvement of CKRs in plant development, and highlight the less anticipated functions of CKRs in abiotic stress tolerance. Based on this, possible mechanisms for CKR modes of action are proposed and experimental approaches to further uncover their roles and future biotechnological applications are suggested.

Regulatory hubs and strategies for improving heavy metal tolerance in plants: Chemical messengers, omics and genetic engineering

Heavy metal (HM) accumulation in the agricultural soil and its toxicity is a major threat for plant growth and development. HMs disrupt functional integrity of the plants, induces altered phenological and physiological responses and slashes down qualitative crop yield. Chemical messengers such as phytohormones, plant growth regulators and gasotransmitters play a crucial role in regulating plant growth and development under metal toxicity in plants. Understanding the intricate network of these chemical messengers as well as interactions of genes/metabolites/proteins associated with HM toxicity in plants is necessary for deciphering insights into the regulatory circuit involved in HM tolerance. The present review describes (a) the role of chemical messengers in HM-induced toxicity mitigation, (b) possible crosstalk between phytohormones and other signaling cascades involved in plants HM tolerance and (c) the recent advancements in biotechnological interventions including genetic engineering, genome editing and omics approaches to provide a step ahead in making of improved plant against HM toxicities.

Kinetin mitigates Cd-induced damagesto growth, photosynthesis and PS II photochemistry of Trigonella seedlings by up-regulating ascorbate-glutathione cycle

Cytokinins (CKs) plays a key role in plant adaptation over a range of different stress conditions. Here, we analyze the effects of a cytokinin (i.e., kinetin, KN) on the growth, photosynthesis (rate of O2 evolution), PS II photochemistry and AsA-GSH cycle in Trigonella seedlings grown under cadmium (Cd) stress. Trigonella seeds were sown in soil amended with 0, 3 and 9 mg Cd kg-1 soil, and after 15 days resultant seedlings were sprayed with three doses of KN, i.e.,10 μM (low, KNL), 50 μM (medium, KNM) and 100 μM (high, KNH); subsequent experiments were performed after 15 days of KN application, i.e., 30 days after sowing. Cadmium toxicity induced oxidative damage as shown by decreased seedling growth and photosynthetic pigment production (Chl a, Chl b and Car), rates of O2-evolution, and photochemistry of PS II of Trigonella seedlings, all accompanied by an increase in H2O2 accumulation. Supplementation with doses of KN at KNL and KNM significantly improved the growth and photosynthetic activity by reducing H2O2 accumulation through the up-regulation AsA-GSH cycle. Notably, KNL and KNM doses stimulated the rate of enzyme activities of APX, GR and DHAR, involved in the AsA-GSH cycle thereby efficiently regulates the level of AsA and GSH in Trigonella grown under Cd stress. The study concludes that KN can mitigate the damaging effects of Cd stress on plant growth by maintaining the redox status (>ratios: AsA/DHA and GSH/GSSG) of cells through the regulation of AsA-GSH cycle at 10 and 50 μM KN under Cd stress conditions. At 100 μM KN, the down-regulation of AsA-GSH cycle did not support the growth and PS II activity of the test seedlings.

Cytokinins as central regulators during plant growth and stress response

Cytokinins are a class of phytohormone that participate in the regulation of the plant growth, development, and stress response. In this review, the potential regulating mechanism during plant growth and stress response are discussed. Cytokinins are a class of phytohormone that participate in the regulation of plant growth, physiological activities, and yield. Cytokinins also play a key role in response to abiotic stresses, such as drought, salt and high or low temperature. Through the signal transduction pathway, cytokinins interact with various transcription factors via a series of phosphorylation cascades to regulate cytokinin-target gene expression. In this review, we systematically summarize the biosynthesis and metabolism of cytokinins, cytokinin signaling, and associated gene regulation, and highlight the function of cytokinins during plant development and resistance to abiotic stress. We also focus on the importance of crosstalk between cytokinins and other classes of phytohormones, including auxin, ethylene, strigolactone, and gibberellin. Our aim is to provide a comprehensive overview of recent findings on the mechanisms by which cytokinins act as central regulators of plant development and stress reactions, and highlight topics for future research.

Soilless revegetation: An efficient means of improving physicochemical properties and reshaping microbial communities of high-salty gold mine tailings

Soilless revegetation is a cost-effective and eco-friendly method for the ecological restoration of gold mine tailings. However, due to gold mine tailings are high-salty, alkaline and low-nutrient, little research has been done on soilless revegetation of gold mine tailings. The aim of study was to apply soilless revegetation to gold mine tailings, and investigate the changes of physicochemical properties and microbial communities of tailings after soilless revegetation. Six selected herbaceous plants (Melilotus officinalis, Xanthium sibiricum, Festuca elata, Zoysia japonica, Amaranthus tricolor L., Artemisia desertorum) grew well on the bare tailings, and their heights reached as high as 16.28 cm after 90 days. After soilless revegetation, tailings salinity dramatically dropped from 547.15 to 129.24 μS cm^-1, and pH went down from 8.68 to 7.59 at most. The content of available phosphorus (AP), available nitrogen (AN) and organic matter (OM) in tailings gradually improved, especially the content of AP and OM increased 53.36% and 52.58%, respectively. Furthermore, microbial metabolic activity and diversity in tailings obviously increased 70.33-264.70% and 1.64-13.97% respectively. The relative abundance of potential plant growth-promoting bacteria increased 1.40-3.05%, while the relative abundance of opportunistic pathogens and halophilic bacteria decreased 10.58-17.03% and 2.98-6.52% respectively. Such variations of microbial communities were beneficial for tailings restoration. This study provided insight into soilless revegetation and its impact on tailings microorganisms, which could be a new strategy for ecological restoration of gold mine tailings.

Spatio-temporal dynamics of arbuscular mycorrhizal fungi and soil organic carbon in coastal saline soil of China

A comprehensive understanding of the relationship between arbuscular mycorrhizal (AM) fungi and coastal saline soil organic carbon (SOC) is crucial for analysis of the function of coastal wetlands in soil carbon sequestration. In a field experiment, the temporal and spatial dynamics of AM fungi, glomalin-related soil protein (GRSP) – which is described as a N-linked glycoprotein and the putative gene product of AM fungi, SOC, and soil aggregates were investigated in halophyte Kosteletzkya virginica rhizosphere soil of coastal saline areas of North Jiangsu, China. Soil samples were collected from a depth of up to 30 cm in two plantation regions from August 2012 to May 2013. Results showed K. virginica formed a strong symbiotic relationship to AM fungi. AM colonization and spore density were the highest in the 10–20 cm soil layer of Jinhai farm in August 2012, because of the presence of numerous fibrous roots in this soil layer. The total GRSP and SOC were the highest in the 0–10 cm soil layer in May 2013 and November 2012, respectively. Correlation coefficient analysis revealed that AM colonization and spore density were positively correlated with total GRSP. Meanwhile, total GRSP was significantly positively correlated with large macroaggregates (>3 mm), SOC, total P, Olsen P, and soil microbial biomass carbon (SMBC), but negatively correlated with microaggregates (<0.25 mm), soil EC, total N, and pH. SOC was positively correlated with spore density, large macroaggregates, small macroaggregates (2–0.25 mm), alkaline N, and SMBC and negatively correlated with microaggregates, EC, pH, and total K. Although it may be a statistical artifact, we found an interesting phenomenon that there was no significant correlation between soil aggregates and AM colonization or spore density. Hence, total GRSP is a vital source of saline soil C pool and an important biological indicator for evaluating coastal saline SOC pool and soil fertility, while AM colonization or spore density may not be.