MAPKs regulate root growth by influencing auxin signaling and cell cycle-related gene expression in cadmium-stressed rice

Physiological response and molecular mechanism of Quercus variabilis under cadmium stress

Heavy metal pollution is a global environmental problem, and Quercus variabilis has a stronger tolerance to Cd stress than do other species. We aimed to explore the physiological response and molecular mechanisms of Q. variabilis to Cd stress. In this study, the antioxidant enzyme activities of leaves were determined, while the photosynthetic parameters of leaves were measured using Handy PEA, and ion fluxes and DEGs in the roots were investigated using noninvasive microtest technology (NMT) and RNA sequencing techniques, respectively. Cd stress at different concentrations and for different durations affected the uptake patterns of Cd2+ and H+ by Q. variabilis and affected the photosynthetic efficiency of leaves. Moreover, there was a positive relationship between antioxidant enzyme (CAT and POD) activity and Cd concentration. Transcriptome analysis revealed that many genes, including genes related to the cell wall, glutathione metabolism, ion uptake and transport, were significantly upregulated in response to cadmium stress in Q. variabilis roots. WGCNA showed that these DEGs could be divided into eight modules. The turquoise and blue modules exhibited the strongest correlations, and the most significantly enriched pathways were the phytohormone signaling pathway and the phenylpropanoid biosynthesis pathway, respectively. These findings suggest that Q. variabilis can bolster plant tolerance by modulating signal transduction and increasing the synthesis of compounds, such as lignin, under Cd stress. In summary, Q. variabilis can adapt to Cd stress by increasing the activity of antioxidant enzymes, and regulating the fluxes of Cd2+ and H+ ions and the expression of Cd stress-related genes.

Heavy metal stress and mitogen activated kinase transcription factors in plants: Exploring heavy metal-ROS influences on plant signalling pathways

Due to their stationary nature, plants are exposed to a diverse range of biotic and abiotic stresses, of which heavy metal (HM) stress poses one of the most detrimental abiotic stresses, targeting diverse plant processes. HMs instigate the overproduction of reactive oxygen species (ROS), and to mitigate the adverse effects of ROS, plants induce multiple defence mechanisms. Besides the negative implications of overproduction of ROS, these molecules play a multitude of signalling roles in plants, acting as a central player in the complex signalling network of cells. One of the ROS-associated signalling mechanisms is the mitogen-activated protein kinase (MAPK) cascade, a signalling pathway which transduces extracellular stimuli into intracellular responses. Plant MAPKs have been implicated in signalling involved in stress response, phytohormone regulation, and cell cycle cues. However, the influence of various HMs on MAPK activation has not been well documented. In this review, we address and summarise several aspects related to various HM-induced ROS signalling. Additionally, we touch on how these signals activate the MAPK cascade and the downstream transcription factors that influence plant responses to HMs. Moreover, we propose a workflow that could characterise genes associated with MAPKs and their roles during plant HM stress responses.

Protein post-translational modifications in auxin signaling

Protein post-translational modifications (PTMs), such as ubiquitination, phosphorylation, and small ubiquitin-like modifier (SUMO)ylation, are crucial for regulating protein stability, activity, subcellular localization, and binding with cofactors. Such modifications remarkably increase the variety and complexity of proteomes, which are essential for regulating numerous cellular and physiological processes. The regulation of auxin signaling is finely tuned in time and space to guide various plant growth and development. Accumulating evidence indicates that PTMs play critical roles in auxin signaling regulations. Thus, a thorough and systematic review of the functions of PTMs in auxin signal transduction will improve our profound comprehension of the regulation mechanism of auxin signaling and auxin-mediated various processes. This review discusses the progress of protein ubiquitination, phosphorylation, histone acetylation and methylation, SUMOylation, and S-nitrosylation in the regulation of auxin signaling.

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.

Mitigation of heavy metal stress in the soil through optimized interaction between plants and microbes

Agricultural as well as industrial processes, such as mining and textile activities, are just a few examples of anthropogenic activities that have a long-term negative impact on the environment. Each of the aforementioned factors increases the concentration of heavy metals in soil. Heavy metal contamination in soil causes a wide range of environmental issues and is harmful to microbes, plants, and animals. Because of their non-biodegradability and toxic effects, preventing additional metal contamination and remediating the vast majority of contaminated sites around the world is critical. Hence, this review focuses on the effects of metal contamination on soil microbes, as well as plant-microbe interactions. Plant-associated probiotics reduce metal accumulation; the introduction of beneficial microbes is regarded as one of the most promising approaches to improving metal stress tolerance; thus, the study focuses on plant-microbe interactions as well as their actual implications via phytoremediation. Plant-microbe interaction can play an important role in acclimating vegetation (plants) to metalliferous conditions and should thus be studied to improve microbe-aided metal tolerance in plants. Plant-interacted microbes reduce metal accumulation in plant cells and metal bioaccumulation in the soil through a variety of processes. A novel phytobacterial approach, such as genetically modified microbes, is now being used to improve heavy metal cleanup as well as stress tolerance among plants. This review examines our current understanding of such negative consequences of heavy metal stresses, signaling responses, and the role of plant-associated microbiota in heavy metal stress tolerance and interaction.

Effects of PmaIAA27 and PmaARF15 genes on drought stress tolerance in pinus massoniana

BACKGROUND

Auxin plays an important role in plant resistance to abiotic stress. The modulation of gene expression by Auxin response factors (ARFs) and the inhibition of auxin/indole-3-acetic acid (Aux/IAA) proteins play crucial regulatory roles in plant auxin signal transduction. However, whether the stress resistance of Masson pine (Pinus massoniana), as a representative pioneer species, is related to Aux/IAA and ARF genes has not been thoroughly studied and explored.

RESULTS

The present study provides preliminary evidence for the regulatory role of the PmaIAA27 gene in abiotic stress response in Masson pine. We investigated the effects of drought and hormone treatments on Masson pine by examining the expression patterns of PmaIAA27 and PmaARF15 genes. Subsequently, we conducted gene cloning, functional testing using transgenic tobacco, and explored gene interactions. Exogenous auxin irrigation significantly downregulated the expression of PmaIAA27 while upregulating PmaARF15 in Masson pine seedlings. Moreover, transgenic tobacco with the PmaIAA27 gene exhibited a significant decrease in auxin content compared to control plants, accompanied by an increase in proline content - a known indicator of plant drought resistance. These findings suggest that overexpression of the PmaIAA27 gene may enhance drought resistance in Masson pine. To further investigate the interaction between PmaIAA27 and PmaARF15 genes, we performed bioinformatics analysis and yeast two-hybrid experiments which revealed interactions between PB1 structural region of PmaARF15 and PmaIAA27.

CONCLUSION

The present study provides new insights into the regulatory functions of Aux/IAA and ARF genes in Masson pine. Overexpression of PmaIAA gene may have negative effects on the growth of Masson pine, but may improve the drought resistance. Therefore, this study has great application prospects.

Novel Insights into Exogenous Phytohormones: Central Regulators in the Modulation of Physiological, Biochemical, and Molecular Responses in Rice under Metal(loid) Stress

Rice (Oryza sativa) is a research model for monocotyledonous plants. Rice is also one of the major staple foods and the primary crop for more than half of the world's population. Increasing industrial activities and the use of different fertilizers and pesticides containing heavy metals (HMs) contribute to the contamination of agriculture fields. HM contamination is among the leading causes that affect the health of rice plants by limiting their growth and causing plant death. Phytohormones have a crucial role in stress-coping mechanisms and in determining a range of plant development and growth aspects during heavy metal stress. This review summarizes the role of different exogenous applications of phytohormones including auxin, cytokinin, gibberellins, ethylene, abscisic acid, strigolactones, jasmonates, brassinosteroids, and salicylic acids in rice plants for mitigating heavy metal stress via manipulation of their stress-related physiological and biochemical processes, and alterations of signaling and biosynthesis of genes. Exogenous administration of phytohormones and regulation of endogenous levels by targeting their biosynthesis/signaling machineries is a potential strategy for protecting rice from HM stress. The current review primarily emphasizes the key mechanistic phytohormonal-mediated strategies for reducing the adverse effects of HM toxicity in rice. Herein, we have provided comprehensive evidence for the effective role of exogenous phytohormones in employing defense responses and tolerance in rice to the phytotoxic effects of HM toxicity along with endogenous hormonal crosstalk for modulation of subcellular mechanisms and modification of stress-related signaling pathways, and uptake and translocation of metals. Altogether, this information offers a systematic understanding of how phytohormones modulate a plant's tolerance to heavy metals and may assist in directing the development of new approaches to strengthen rice plant resistance to HM toxicity.

Unraveling the mutualistic interaction between endophytic Curvularia lunata CSL1 and tomato to mitigate cadmium (Cd) toxicity via transcriptomic insights

In this study, endophytic fungus Curvularia lunata strain SL1 was used to explore its bioremediation potential and growth restoration of tomato (Solanum lycopersicum) under cadmium (Cd) stress. Our findings demonstrate that SL1 establishes a symbiotic relationship with tomato plants, which modulates the antioxidant system, secondary metabolites, and gene expression in tomato plants exposed to Cd stress. Under Cd stress, tomato seedling growth was significantly reduced by up to 42.8 %, although this reduction was mitigated by up to 25 % after SL1 inoculation. Similar to this, SLI inoculation inhibits Cd absorption and translocation to the upper parts of the plant. Additionally, during Cd stress, phytohormones related to stress, including jasmonic acid (JA), abscisic acid (ABA), and ethylene (ET), were elevated; however, SL1 inoculation lowered their level. RNA-Seq data revealed that the highest number of differentially expressed genes (DEGs) was detected in the comparison between control and 1 mM Cd, followed by 2 mM Cd stress. These DEGs were mostly related to oxidoreductase activity, catalytic activity, plant hormones transduction, and photosynthesis. The findings also suggested that SL1 could improve tomato tolerance to Cd stress by modulating Ca²⁺ signaling, phytohormone biosynthesis, MAPK signaling pathway, and some transcription factors.

Genome-wide identification and transcriptome analysis of the heavy metal-associated (HMA) gene family in Tartary buckwheat and their regulatory roles under cadmium stress

Heavy metal-associated (HMA) genes are those related to heavy metal transport and detoxification in plants. HMA genes have not been reported in Tartary buckwheat so far. In this study, we accessed the HMA genes of Tartary buckwheat by genome-wide identification for the first time. A total of 56 HMA genes were identified, including 36 ATX1 (antioxidant protein1) genes, 13 HIPP (heavy metal-associated isoprenylated plant protein) genes, and 7 P_1B-ATPase (P_1B-type adenosine triphosphatase) genes. These gene structures, motif compositions, chromosomal distribution, phylogenetic relationship, duplication events, interaction networks, cis-acting elements, and transcriptional expression under cadmium (Cd) stress were investigated. Among them, genes in HIPP and ATX1 subfamilies were more closely related. The 56 HMA genes were involved in the regulation of metal ion transport and homeostasis by binding metal ions, likely triggered by signals transducted by plant hormones. Fifteen of these HMA genes played regulatory roles under Cd stress. FtP1bA1 was identified to be a core gene involved in the defense regulation of Cd stress. Our results provide not only the first overview and characteristics of HMA genes in the whole genome of Tartary buckwheat but also a valuable reference for the functional analysis of HMA genes under Cd stress. Understanding changes in gene regulation induced by Cd stress lays the foundation for breeding resistant varieties.

The MKK2a Gene Involved in the MAPK Signaling Cascades Enhances Populus Salt Tolerance

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction modules, which transmit environmental signals in plant cells through stepwise phosphorylation and play indispensable roles in a wide range of physiological and biochemical processes. Here, we isolated and characterized a gene encoding MKK2 protein from poplar through the rapid amplification of cDNA ends (RACE). The full-length PeMKK2a gene was 1571 bp, including a 1068 bp open reading frame (ORF) encoding 355 amino acids, and the putative PeMKK2a protein belongs to the PKc_like (protein kinase domain) family (70–336 amino acids) in the PKc_MAPKK_plant subfamily and contains 62 sites of possible phosphorylation and two conserved domains, DLK and S/T-xxxxx-S/T. Detailed information about its gene structure, sequence similarities, subcellular localization, and transcript profiles under salt-stress conditions was revealed. Transgenic poplar lines overexpressing PeMKK2a exhibited higher activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) than non-transgenic poplar under salt stress conditions. These results will provide insight into the roles of MAPK signaling cascades in poplar response to salt stress.

A Preliminary Study on the Mechanisms of Growth and Physiological Changes in Response to Different Temperatures in Neopyropia yezoensis (Rhodophyta)

As an economically valuable red seaweed, Neopyropia yezoensis (Rhodophyta) is cultivated in intertidal areas, and its growth and development are greatly influenced by environmental factors such as temperature. Although much effort has been devoted to delineating the influence, the underlying cellular and molecular mechanisms remain elusive. In this study, the gametophyte blades and protoplasts were cultured at different temperatures (13 °C, 17 °C, 21 °C, 25 °C). Only blades cultured at 13 °C maintained a normal growth state (the relative growth rate of thalli was positive, and the content of phycobiliprotein and pigments changed little); the survival and division rates of protoplasts were high at 13 °C, but greatly decreased with the increase in temperature, suggesting that 13 °C is suitable for the growth of N. yezoensis. In our efforts to delineate the underlying mechanism, a partial coding sequence (CDS) of Cyclin B and the complete CDS of cyclin-dependent-kinase B (CDKB) in N. yezoensis were cloned. Since Cyclin B controls G2/M phase transition by activating CDK and regulates the progression of cell division, we then analyzed how Cyclin B expression in the gametophyte blades might change with temperatures by qPCR and Western blotting. The results showed that the expression of Cyclin B first increased and then decreased after transfer from 13 °C to higher temperatures, and the downregulation of Cyclin B was more obvious with the increase in temperature. The phosphorylation of extracellular signal-regulated kinase (ERK) decreased with the increase in temperature, suggesting inactivation of ERK at higher temperatures; inhibition of ERK by FR180204 significantly decreased the survival and division rates of protoplasts cultured at 13 °C. These results suggest that downregulation of Cyclin B and inactivation of ERK might be involved in negatively regulating the survival and division of protoplasts and the growth of gametophyte blades of N. yezoensis at high temperatures.

The Impact of Bio-Stimulants on Cd-Stressed Wheat (Triticum aestivum L.): Insights Into Growth, Chlorophyll Fluorescence, Cd Accumulation, and Osmolyte Regulation

It has been established that wheat (Triticum aestivum L.) has a higher Cd absorption capacity than other cereal crops causing an excess daily Cd intake and a huge threat for public health. Therefore, the reduction of Cd accumulation in wheat from the soil is a crucial food-security issue. A pot trial was performed on Cd-stressed wheat seedlings to evaluate the morphological and physio-biochemical responses via foliage spray of two different bio-stimulants, i.e., ascorbic acid (AsA) and moringa leaf extract (MLE). Two wheat cultivars (Fsd-08 and Glxy-13) were exposed to cadmium (CdCl₂.5H₂O) stress (0, 500, and 1,000 μM), along with foliar spray of AsA (0 and 50 mM) and MLE (0 and 3%). The most observable growth reduction was documented in plants that are exposed to a higher Cd concentration (1,000 μM), followed by the lower Cd level (500 μM). The wheat growth attributes, such as number of leaves per plant, number of tillers per plant, biomass yield, shoot/root length, and leaf area, were greatly depressed under the Cd stress, irrespective of the cultivar. Under the increasing Cd stress, a significant diminution was observed in maximum photochemical efficiency (Fv/Fm), photochemical quenching (qP), and electron transport rate (ETR) accompanied with reduced gas exchange attributes. However, Cd-induced phytotoxicity enhanced the non-photochemical quenching (NPQ) and internal carbon dioxide concentration (Ci), which was confirmed by their significant positive correlation with Cd contents in shoot and root tissues of both cultivars. The contents of proline, AsA, glycine betaine (GB), tocopherol, total free amino acid (TFAA), and total soluble sugar (TSS) were greatly decreased with Cd stress (1,000 μM), while MLE and AsA significantly enhanced the osmolytes accumulation under both Cd levels (especially 500 μM level). The Cd accumulation was predominantly found in the root as compared to shoots in both cultivars, which has declined after the application of MLE and AsA. Conclusively, MLE was found to be more effective to mitigate Cd-induced phytotoxicity up to 500 μM Cd concentration, compared with the AsA amendment.

Transcriptome-wide m6A methylation profile reveals regulatory networks in roots of barley under cadmium stress

Cadmium (Cd) pollutants restrict crop yield and food security in long-term agricultural activities. Crops have evolved adaptive strategies under Cd condition, however, the transcriptional regulatory mechanism of Cd-tolerant genes remains to be largely illustrated. In this study, barley roots were exposed to 5 µM CdCl₂ for physiological response and transcriptome-wide m6A methylation profile. Cd stress inhibited root growth after 7 d Cd treatment, which is mainly associated with inhibited absorption of Mn. After Cd treatment, 8151 significantly modified m6A sites and 3920 differentially expressed genes were identified. Transcriptome-wide m6A hypermethylation was widely induced by Cd stress and enriched near the stop codon and 3' UTR regions. Among 435 m6A modified DEGs, 319 hypermethylated genes were up-regulated and 84 hypomethylated genes were down-regulated, respectively, indicating a positive correlation of m6A methylation and expression. But well-known Cd transporter genes (HvNramp5, HvIRT1, HvHMA3, etc.) were not modified by m6A methylation, except for ABC transporters. We further found key Cd-responding regulatory genes were positively modulated with m6A methylation, including MAPK, WRKY and MYB members. This study proposed a transcriptional regulatory network of Cd stress response in barley roots, which may provide new insight into gene manipulation of controlling low Cd accumulation for crops.

Recent advances in physiological and molecular mechanisms of heavy metal accumulation in plants

Among abiotic stress, the toxicity of metals impacts negatively on plants' growth and productivity. This toxicity promotes various perturbations in plants at different levels. To withstand stress, plants involve efficient mechanisms through the implication of various signaling pathways. These pathways enhance the expression of many target genes among them gene coding for metal transporters. Various metal transporters which are localized at the plasma membrane and/or at the tonoplast are crucial in metal stress response. Furthermore, metal detoxification is provided by metal-binding proteins like phytochelatins and metallothioneins. The understanding of the molecular basis of metal toxicities signaling pathways and tolerance mechanisms is crucial for genetic engineering to produce transgenic plants that enhance phytoremediation. This review presents an overview of the recent advances in our understanding of metal stress response. Firstly, we described the effect of metal stress on plants. Then, we highlight the mechanisms involved in metal detoxification and the importance of the regulation in the response to heavy metal stress. Finally, we mentioned the importance of genetic engineering for enhancing the phytoremediation technique. In the end, the response to heavy metal stress is complex and implicates various components. Thus, further studies are needed to better understand the mechanisms involved in response to this abiotic stress.

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.

The CaM1-associated CCaMK-MKK1/6 cascade positively affects lateral root growth via auxin signaling under salt stress in rice

Ca2+/calmodulin (CaM)-dependent protein kinases (CCaMKs) and mitogen-activated protein kinase kinases (MAPKKs) are two types of kinases that regulate salt stress response in plants. It remains unclear, however, how they cooperatively affect lateral root growth under salt stress. Here, two conserved phosphorylation sites (S102 and T118) of OsCaM1 were identified, and found to affect the ability to bind to Ca2+in vitro and the kinase activity of OsCCaMK in vivo. OsCCaMK specifically interacted with OsMKK1/6 in a Ca2+/CaM-dependent manner. In vitro kinase and in vivo dual-luciferase assays revealed that OsCCaMK phosphorylated OsMKK6 while OsMKK1 phosphorylated OsCCaMK. Overexpression and antisense-RNA repression expression of OsCaM1-1, and CRISPR/Cas9-mediated gene editing mutations of OsMKK1, OsMKK6, and OsMKK1/6 proved that OsCaM1-1, OsMKK1, and OsMKK6 enhanced the auxin content in roots and lateral root growth under salt stress. Consistently, OsCaM1-1, OsMKK1, and OsMKK6 regulated the transcript levels of the genes of this cascade, and salt stress-related and lateral root growth-related auxin signaling under salt stress in rice roots. These findings demonstrate that the OsCaM1-associated OsCCaMK-OsMKK1/6 cascade plays a critical role in recruiting auxin signaling in rice roots. These results also provide new insight into the regulatory mechanism of the CaM-mediated phosphorylation relay cascade to auxin signaling in lateral root growth under salt stress in plants.

Physiological, anatomical, and transcriptional responses of mulberry (Morus alba L.) to Cd stress in contaminated soil

Mulberry has been widely studied for its capacity to tolerate heavy metals. However, the anatomical and molecular response mechanisms of Cd detoxification and transportation in mulberry have not been fully elucidated. In this study, the anatomical characteristics, Cd and mineral element uptake and transport, and transcriptome profiling of mulberry were studied under Cd stress. The results showed that mulberry possessed strong detoxification and self-protection abilities against Cd stress. The growth and photosynthetic pigment contents of mulberry were only slightly affected when the soil Cd content was less than 37.0 mg/kg, while the Ca and Mg contents in the mulberry roots were clearly (p < 0.05) increased by 37.85%-40.87% and 36.63%-53.06% in 37.0-55.4 mg/kg Cd-contaminated soil. Meanwhile, the relationships between antioxidant enzyme activities, such as peroxidase, catalase, and ascorbate peroxidase, and Cd content in plants were positive. Furthermore, the structures of leaf cells, root and stem tissues were largely intact; simultaneously, the increase in osmiophilic particles and the dissolution of starch granules in mulberry leaves significantly responded to Cd stress. Clusters of Orthologous Groups of proteins (COG) and Gene Ontology (GO) classification analysis indicated that mulberry can enhance the catalytic activity, regulate the transport and metabolism of inorganic ions, and strengthen its antioxidant enzyme activity and defense mechanism to decrease Cd intoxication. Large numbers of differentially expressed genes associated with cell wall biosynthesis, antioxidant enzyme activities, glutathione metabolism, chelation, plant hormone signal transduction, and the mitogen-activated protein kinase (MAPK) signaling pathway were upregulated under Cd stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis indicated that plant hormone signal transduction was significantly (p < 0.05) enriched in roots, stems, and leaves of mulberry, and abscisic acid and ethylene can mediate MAPK signaling pathways to increase plant tolerance to Cd stress. The results suggested that the physiological, cellular and tissue, and transcriptional regulation of mulberry can facilitate its stress adaptation in Cd-contaminated soil.

Mitogen-Activated Protein Kinase Expression Profiling Revealed Its Role in Regulating Stress Responses in Potato (Solanum tuberosum)

Mitogen-activated protein kinase (MAPK) cascades are the universal signal transduction networks that regulate cell growth and development, hormone signaling, and other environmental stresses. However, their essential contribution to plant tolerance is very little known in the potato (Solanum tuberosum) plant. The current study carried out a genome-wide study of StMAPK and provided a deep insight using bioinformatics tools. In addition, the relative expression of StMAPKs was also assessed in different plant tissues. The similarity search results identified a total of 22 StMAPK genes in the potato genome. The sequence alignment also showed conserved motif TEY/TDY in most StMAPKs with conserved docking LHDXXEP sites. The phylogenetic analysis divided all 22 StMAPK genes into five groups, i.e., A, B, C, D, and E, showing some common structural motifs. In addition, most of the StMAPKs were found in a cluster form at the terminal of chromosomes. The promoter analysis predicted several stress-responsive Cis-acting regulatory elements in StMAPK genes. Gene duplication under selection pressure also indicated several purifying and positive selections in StMAPK genes. In potato, StMAPK2, StMAPK6, and StMAPK19 showed a high expression in response to heat stress. Under ABA and IAA treatment, the expression of the total 20 StMAPK genes revealed that ABA and IAA played an essential role in this defense process. The expression profiling and real-time qPCR (RT-qPCR) exhibited their high expression in roots and stems compared to leaves. These results deliver primary data for functional analysis and provide reference data for other important crops.

OsIAA20, an Aux/IAA protein, mediates abiotic stress tolerance in rice through an ABA pathway

In plants, auxin and ABA play significant roles in conferring tolerance to environmental abiotic stresses. Earlier studies have been shown that some Aux/IAA genes, with important signaling factors in the auxin pathway, were induced in response to drought and other abiotic stresses. However, the mechanistic links between Aux/IAA expression and general drought response remain largely unknown. In this study, OsIAA20, a rice Aux/IAA protein, shown with important roles in abiotic stress. Phenotypic analyses revealed that OsIAA20 RNAi transgenic rice reduced drought and salt tolerance; whereas, OsIAA20 overexpression plants displayed the opposite phenotype. Physiological analyses of OsIAA20 RNAi rice grown under drought or salt stress showed that proline and chlorophyll content significantly decreased, while malondialdehyde content and the ratio of Na⁺/ K⁺ significantly increased. In addition, OsIAA20down-regulation reduced stomatal closure and increased the rate of water loss, while transgenic plants overexpressing OsIAA20 exhibited the opposite physiological responses. Furthermore, an ABA-responsive gene, OsRab21, was down-regulated in OsIAA20 RNAi rice lines and upregulated in OsIAA20 overexpression plants. Those results means OsIAA20 played an important role in plant drought and salt stress responses, by an ABA dependent mechanism, and it will be a candidate target gene used to breed abiotic stress tolerance.

Cadmium Accumulation Involves Synthesis of Glutathione and Phytochelatins, and Activation of CDPK, CaMK, CBLPK, and MAPK Signaling Pathways in Ulva compressa

In order to analyze the effect of cadmium in Ulva compressa (Chlorophyta), the alga was cultivated with 10, 25, and 50 μM of cadmium for 7 days, and the level of intracellular cadmium was determined. Intracellular cadmium showed an increase on day 1, no change until day 5, and an increase on day 7. Then, the alga was cultivated with 10 μM for 7 days, and the level of hydrogen peroxide, superoxide anions, and lipoperoxides; activities of antioxidant enzymes ascorbate peroxidase (AP), dehydroascorbate reductase (DHAR), and glutathione reductase (GR); the level of glutathione (GSH) and ascorbate (ASC); and the level of phytochelatins (PCs) and transcripts encoding metallothioneins (UcMTs) levels were determined. The level of hydrogen peroxide increased at 2 and 12 h, superoxide anions on day 1, and lipoperoxides on days 3 to 5. The activities of AP and GR were increased, but not the DHAR activity. The level of GSH increased on day 1, decreased on day 3, and increased again on day 5, whereas ASC slightly increased on days 3 and 7, and activities of enzymes involved in GSH and ASC synthesis were increased on days 3 to 7. The level of PC2 and PC4 decreased on day 3 but increased again on day 5. The level of transcripts encoding UcMT1 and UcMT2 increased on days 3 to 5, mainly that of UcMT2. Thus, cadmium accumulation induced an oxidative stress condition that was mitigated by the activation of antioxidant enzymes and synthesis of GSH and ASC. Then, the alga cultivated with inhibitors of calcium-dependent protein kinases (CDPKs), calmodulin-dependent protein kinases (CaMKs), calcineurin B-like protein kinases (CBLPKs), and MAPKs and 10 μM of cadmium for 5 days showed a decrease in intracellular cadmium and in the level of GSH and PCs, with the four inhibitors, and in the level of transcripts encoding UcMTs, with two inhibitors. Thus, CDPKs, CaMK, CBLPKS, and MAPKs are involved in cadmium accumulation and GSH and PC synthesis, and GSH and PCs and/or UcMTs may participate in cadmium accumulation.

Jasmonates, Ethylene and Brassinosteroids Control Adventitious and Lateral Rooting as Stress Avoidance Responses to Heavy Metals and Metalloids

Developmental and environmental signaling networks often converge during plant growth in response to changing conditions. Stress-induced hormones, such as jasmonates (JAs), can influence growth by crosstalk with other signals like brassinosteroids (BRs) and ethylene (ET). Nevertheless, it is unclear how avoidance of an abiotic stress triggers local changes in development as a response. It is known that stress hormones like JAs/ET and BRs can regulate the division rate of cells from the first asymmetric cell divisions (ACDs) in meristems, suggesting that stem cell activation may take part in developmental changes as a stress-avoidance-induced response. The root system is a prime responder to stress conditions in soil. Together with the primary root and lateral roots (LRs), adventitious roots (ARs) are necessary for survival in numerous plant species. AR and LR formation is affected by soil pollution, causing substantial root architecture changes by either depressing or enhancing rooting as a stress avoidance/survival response. Here, a detailed overview of the crosstalk between JAs, ET, BRs, and the stress mediator nitric oxide (NO) in auxin-induced AR and LR formation, with/without cadmium and arsenic, is presented. Interactions essential in achieving a balance between growth and adaptation to Cd and As soil pollution to ensure survival are reviewed here in the model species Arabidopsis and rice.

Decoding Heavy Metal Stress Signalling in Plants: Towards Improved Food Security and Safety

The mining of heavy metals from the environment leads to an increase in soil pollution, leading to the uptake of heavy metals into plant tissue. The build-up of toxic metals in plant cells often leads to cellular damage and senescence. Therefore, it is of utmost importance to produce plants with improved tolerance to heavy metals for food security, as well as to limit heavy metal uptake for improved food safety purposes. To achieve this goal, our understanding of the signaling mechanisms which regulate toxic heavy metal uptake and tolerance in plants requires extensive improvement. In this review, we summarize recent literature and data on heavy metal toxicity (oral reference doses) and the impact of the metals on food safety and food security. Furthermore, we discuss some of the key events (reception, transduction, and response) in the heavy metal signaling cascades in the cell wall, plasma membrane, and cytoplasm. Our future perspectives provide an outlook of the exciting advances that will shape the plant heavy metal signaling field in the near future.

Cadmium Inhibits Lateral Root Emergence in Rice by Disrupting OsPIN-Mediated Auxin Distribution and the Protective Effect of OsHMA3

Cadmium (Cd) strongly inhibits root growth, especially the formation of lateral roots (LRs). The mechanism of Cd inhibition on LR formation in rice (Oryza sativa) remains unclear. In this study, we found that LR emergence in rice was inhibited significantly by 1 �M Cd and almost completely arrested by 5 �M Cd. Cd suppressed both the formation and subsequent development of the lateral root primordium (LRP). By using transgenic rice expressing the auxin response reporters DR5::GUS and DR5rev::VENUS, we found that Cd markedly reduced the auxin levels in the stele and LRP. Cd rapidly downregulated the expression of the auxin efflux transporter genes OsPIN1b, OsPIN1c and OsPIN9 in the stele and LRP. The emergence of LRs in a rice cultivar with a null allele of OsHMA3 (Heavy Metal ATPase 3) was more sensitive to Cd than cultivars with functional alleles. Overexpression of functional OsHMA3 in rice greatly alleviated the inhibitory effect of Cd, but the protective effect of OsHMA3 was abolished by the auxin polar transport inhibitor 1-N-naphthylphthalamic acid. The results suggest that Cd inhibits LR development in rice by disrupting OsPIN-mediated auxin distribution to LRP and OsHMA3 protects against Cd toxicity by sequestering Cd into the vacuoles.

Cadmium inhibits cell cycle progression and specifically accumulates in the maize leaf meristem

It is well known that cadmium (Cd) pollution inhibits plant growth, but how this metal impacts leaf growth processes at the cellular and molecular level is still largely unknown. In the current study, we show that Cd specifically accumulates in the meristematic tissue of the growing maize leaf, while Cd concentration in the elongation zone rapidly declines as the deposition rates diminish and cell volumes increase due to cell expansion. A kinematic analysis shows that, at the cellular level, a lower number of meristematic cells together with a significantly longer cell cycle duration explain the inhibition of leaf growth by Cd. Flow cytometry analysis suggests an inhibition of the G1/S transition, resulting in a lower proportion of cells in the S phase and reduced endoreduplication in expanding cells under Cd stress. Lower cell cycle activity is also reflected by lower expression levels of key cell cycle genes (putative wee1, cyclin-B2-4, and minichromosome maintenance4). Cell elongation rates are also inhibited by Cd, which is possibly linked to the inhibited endoreduplication. Taken together, our results complement studies on Cd-induced growth inhibition in roots and link inhibited cell cycle progression to Cd deposition in the leaf meristem.

Genome-wide identification of the mitogen-activated protein kinase (MAPK) family in cotton (Gossypium hirsutum) reveals GhMPK6 involved in fiber elongation

Mitogen-activated protein kinases (MAPKs) are important in regulating plant development as well as stress response. In this study, we genome-widely identified 56 MAPK genes in upland cotton. These MAPK genes unequally distribute on 22 chromosomes of cotton genome, but no MAPK gene is located on At_Chr6, Dt_Chr6, At_Chr13 and Dt_Chr13. The exons and introns in GhMAPK gene family vary widely at the position, number and length. Furthermore, GhMAPK family can be divided into 4 groups (A, B, C and D), and the TEY type of T-loop exists in three groups (A, B and C), but the TDY type of T-loop is only in group D. Further study revealed that some GhMAPK genes (including GhMPK6) are preferentially expressed in elongating fibers. GhMPK6 maintains a high phosphorylation level in elongating fibers, and its phosphorylation was enhanced in fibers by phytohormones brassinosteroid (BR), ethylene and indole-3-acetic acid (IAA). Additionally, GhMPK6 could interact with GhMKK2-2 and GhMKK4, suggesting that GhMKK2-2/4-GhMPK6 module may be involved in phosphorylation of its downstream proteins for regulating fiber elongation of cotton.

Genome-wide identification of mitogen-activated protein kinase (MAPK) cascade and expression profiling of CmMAPKs in melon (Cucumis melo L.)

Mitogen-activated protein kinase (MAPK) is a form of serine/threonine protein kinase that activated by extracellular stimulation acting through the MAPK cascade (MAPKKK-MAPKK-MAPK). The MAPK cascade gene family, an important family of protein kinases, plays a vital role in responding to various stresses and hormone signal transduction processes in plants. In this study, we identified 14 CmMAPKs, 6 CmMAPKKs and 64 CmMAPKKKs in melon genome. Based on structural characteristics and a comparison of phylogenetic relationships of MAPK gene families from Arabidopsis, cucumber and watermelon, CmMAPKs and CmMAPKKs were categorized into 4 groups, and CmMAPKKKs were categorized into 3 groups. Furthermore, chromosome location revealed an unevenly distribution on chromosomes of MAPK cascade genes in melon, respectively. Eventually, qRT-PCR analysis showed that all 14 CmMAPKs had different expression patterns under drought, salt, salicylic acid (SA), methyl jasmonate (MeJA), red light (RL), and Podosphaera xanthii (P. xanthii) treatments. Overall, the expression levels of CmMAPK3 and CmMAPK7 under different treatments were higher than those in control. Our study provides an important basis for future functional verification of MAPK genes in regulating responses to stress and signal substance in melon.

Expression analysis of the BpARF genes in Betula platyphylla under drought stress

Auxin response factors (ARFs) play an important role in modulating plant growth and development processes by regulating the expression of auxin-responsive genes. However, the modes of action of ARFs in birch (Betula platyphylla) remain largely unknown. In this study, fifteen ARF genes were identified in the birch (B. platyphylla) genome. Bioinformatics analysis revealed that the 15 BpARF genes were unevenly distributed on 7 chromosomes. The 15 BpARF proteins clustered into 6 groups, and all of them contained ARF and B3 motifs. The cis-acting elements present within the promoters of the BpARF genes were mostly related to stress resistance. Expression analysis revealed that most of the BpARF genes were significantly upregulated or downregulated in response to drought treatment in at least one organ. In particular, the expression of BpARF1 was significantly induced by drought stress. The function of BpARF1 was further studied via a transient transformation system. Under drought stress conditions, compared with vector control plants, BpARF1 RNA interference (RNAi)-inhibited plants presented reduced reactive oxygen species (ROS) accumulation, enhanced peroxide (POD) and superoxide dismutase (SOD) activities, increased ascorbic acid (AsA) and proline contents, and reduced electrolyte leakage and water loss rates. Conversely, BpARF1 overexpression plants displayed the opposite physiological changes. These results suggest that the silencing of BpARF1 can improve the drought tolerance of B. platyphylla.

Non-canonical AUX/IAA protein IAA33 competes with canonical AUX/IAA repressor IAA5 to negatively regulate auxin signaling

The phytohormone auxin controls plant growth and development via TIR1-dependent protein degradation of canonical AUX/IAA proteins, which normally repress the activity of auxin response transcription factors (ARFs). IAA33 is a non-canonical AUX/IAA protein lacking a TIR1-binding domain, and its role in auxin signaling and plant development is not well understood. Here, we show that IAA33 maintains root distal stem cell identity and negatively regulates auxin signaling by interacting with ARF10 and ARF16. IAA33 competes with the canonical AUX/IAA repressor IAA5 for binding to ARF10/16 to protect them from IAA5-mediated inhibition. In contrast to auxin-dependent degradation of canonical AUX/IAA proteins, auxin stabilizes IAA33 protein via MITOGEN-ACTIVATED PROTEIN KINASE 14 (MPK14) and does not affect IAA33 gene expression. Taken together, this study provides insight into the molecular functions of non-canonical AUX/IAA proteins in auxin signaling transduction.

Biopriming with Piriformospora indica ameliorates cadmium stress in rice by lowering oxidative stress and cell death in root cells

Piriformospora indica is known for plant growth promotion and abiotic stress alleviation potential in several agricultural crops. However, a systemic analysis is warranted to explore potential application of this important fungus to augment heavy metal tolerance in rice. The present study explores potential of P. indica in ameliorating the effect of cadmium (Cd) stress in rice cultivars N22 and IR64. Seedlings inoculated with P. indica recorded significantly higher root-shoot length and biomass as compared to non-inoculated plants under control and Cd stress, respectively. Moreover, P. indica inoculated stressed roots accumulated more Cd as compared to non-inoculated stressed roots in both the varieties. Interestingly, cell death and reactive oxygen species (ROS) accumulation were significantly lower in the inoculated plant roots as compare with non-inoculated roots under Cd stress. The results emphasized significantly higher accumulation of Cd in fungal spores could reduce ROS accumulation in root cells resulting in lower cell death.

The Complex Genetic Architecture of Early Root and Shoot Traits in Flax Revealed by Genome-Wide Association Analyses

Roots are fundamental organs for water and nutrient uptake as well as for signal transduction in response to biotic and abiotic stresses. Flax has a shallow tap root system that relies mostly on top soil nutrient and moisture resources. The crop can easily be outcompeted by weeds or other crops in intercropping systems, especially in moisture deficit conditions. However, there is a wide range of variation among genotypes in terms of performance under scarce resources such as moisture limitation. Here we phenotyped 15 root, two shoot traits and shoot to root dry weight ratio on 115 flax accessions grown in a hydroponic pouch system and performed a genome-wide association study (GWAS) based on seven different models to identify quantitative trait loci underlying these traits. Significant variation among genotypes was observed for the two shoot and 12 of the 14 root traits. Shoot dry weight was correlated with root network volume, length, surface area, and root dry weight (r > 0.5, P < 0.001) but not significantly correlated with root depth (r = 0.033, P > 0.05). The seven GWAS models detected a total of 228 quantitative trait nucleotides (QTNs) for 16 traits. Most loci, defined by an interval of 100 kb up and downstream of the QTNs, harbored genes known to play role(s) in root and shoot development, suggesting them as candidates. Examples of candidate genes linked to root network QTNs included genes encoding GRAS transcription factors, mitogen-activated protein kinases, and auxin related lateral organ boundary proteins while QTN loci for shoot dry weight harbored genes involved in photomorphogenesis and plant immunity. These results provide insights into the genetic bases of early shoot and root development traits in flax that could be capitalized upon to improve its root architecture, particularly in view of better withstanding water limiting conditions during the cropping season.

Reactive oxygen species, auxin and nitric oxide in metal-stressed roots: toxicity or defence

The presented review is a summary on the current knowledge about metal induced stress response in plants, focusing on the roles of reactive oxygen species, auxin and nitric oxide in roots. The article focuses mainly on the difference between defence and toxicity symptoms of roots during metal-induced stress. Nowadays, pollution of soils by heavy metals is a rapidly growing issue, which affects agriculture and human health. In order to deal with these problems, we must first understand the basic mechanisms and responses to environmental conditions in plants growing under such conditions. Studies so far show somewhat conflicting data, interpreting the same stress responses as both symptoms of defence and toxicity. Therefore, the aim of this review is to give a report about current knowledge of heavy metal-induced stress research, and also to differentiate between toxicity and defence, and outline the challenges of research, focusing on reactive oxygen and nitrogen species, auxin, and the interplay among them. There are still remaining questions on how reactive oxygen and nitrogen species, as well as auxin, can activate either symptoms of toxicity or defence, and adaptation responses.

Advances in the Uptake and Transport Mechanisms and QTLs Mapping of Cadmium in Rice

Cadmium (Cd), as a heavy metal, presents substantial biological toxicity and has harmful effects on human health. To lower the ingress levels of human Cd, it is necessary for Cd content in food crops to be reduced, which is of considerable significance for ensuring food safety. This review will summarize the genetic traits of Cd accumulation in rice and examine the mechanism of Cd uptake and translocation in rice. The status of genes related to Cd stress and Cd accumulation in rice in recent years will be summarized, and the genes related to Cd accumulation in rice will be classified according to their functions. In addition, an overview of quantitative trait loci (QTLs) mapping populations in rice will be introduced, aiming to provide a theoretical reference for the breeding of rice varieties with low Cd accumulation. Finally, existing problems and prospects will be put forward.

Genome-wide characterization and expression profiling of the MAPKKK genes in Gossypium arboreum L

Mitogen-activated protein kinase kinase kinases (MAPKKKs) are important components of MAPK cascades, which have different functions during developmental processes and stress responses. To date, there has been no systematic investigation of this gene family in the diploid cotton Gossypium arboreum L. In this study, a genome-wide survey was performed that identified 78 MAPKKK genes in G. arboreum. Phylogenetic analysis classified these genes into three subgroups: 14 belonged to ZIK, 20 to MEKK, and 44 to Raf. Chromosome location, phylogeny, and the conserved protein motifs of the MAPKKK gene family in G. arboreum were analyzed. The MAPKKK genes had a scattered genomic distribution across 13 chromosomes. The members in the same subfamily shared similar conserved motifs. The MAPKKK expression patterns were analyzed in mature leaves, stems, roots, and at different ovule developmental stages, as well as under salt and drought stresses. Transcriptome analysis showed that 76 MAPKKK genes had different transcript accumulation patterns in the tested tissues and 38 MAPKKK genes were differentially expressed in response to salt and drought stresses. These results lay the foundation for understanding the complex mechanisms behind MAPKKK-mediated developmental processes and abiotic stress-signaling transduction pathways in cotton.

A transcriptomic (RNA-seq) analysis of genes responsive to both cadmium and arsenic stress in rice root

Cadmium (Cd) and arsenic (As) are nonessential and toxic elements in rice that often occur together in contaminated paddy field soils. To understand whether rice has a common molecular response mechanism against Cd and As toxicity, 30-day seedlings (Oryza sativa L. indica) were exposed separately to Cd and As³⁺ in hydroponic cultures for up to 7 days. Root transcriptomic analysis of plants exposed to Cd and As for 3 days revealed that a total of 2224 genes in rice roots responded to Cd stress, while 1503 genes responded to As stress. Of these, 841 genes responded to both stressors. The genes in common to Cd and As stress were associated with redox control, stress response, transcriptional regulation, transmembrane transport, signal transduction, as well as biosynthesis and metabolism of macromolecules and sulfur compounds. In plants exposed to Cd and As separately or in combination for 3 and 7 days, qRT-PCR verification revealed that the glutathione metabolism associated gene Os09g0367700 was significantly up-regulated with respect to unexposed controls and had a positive synergistic effect under combined Cd and As stress. In addition, the redox control related genes Os06g0216000, Os07g0638300 and Os01g0294500, the glutathione metabolism related gene Os01g0530900, the cell wall biogenesis related genes Os05g0247800, Os11g0592000 and Os03g0416200, the expression regulation related genes Os07g0597200 and Os02g0168200, and the transmembrane transport related genes Os04g0524500, also varied significantly with respect to an unexposed control and displayed synergistic effects after 7 days of simultaneous exposure to Cd and As. Our identification of a novel set of genes in rice which responded to both Cd and As³⁺ stress may be of value in mitigating the toxicity of co-contaminated soils. These results also provide a deeper understanding of the molecular mechanisms involved in response to multi-metal/loids stress, and may be used in the genetic improvement of rice varieties.

New insight into the molecular basis of cadmium stress responses of wild paper mulberry plant by transcriptome analysis

BACKGROUND

Heavy metal contamination is becoming a limitation to the utilization of soil and the distribution of vegetation. In particular, cadmium (Cd) pollution has had a serious impact on the food chain. Broussonetia papyrifera is a widely distributed pioneer tree species of heavy metal contaminated areas with important economic value. However, little is known about the genomic background of the Cd-tolerance mechanism in B. papyrifera.

RESULTS

The CdCl₂ responsive physiology was evaluated and proved to be involved in antioxidase activity and active oxygen species (ROS) accumulation. The leaf and root transcriptomes derived from B. papyrifera grown under normal and CdCl₂ stress conditions were systematically investigated using the Illumina HiSeq method. A total of 180,678,660 bp (27.1 GB) clean reads were assembled into 589,487 high-quality unigenes, of which 256,025 (43.43% of the total) and 250,251 (42.45% of the total) were aligned in Gene Ontology (GO) and Protein family (Pfam), respectively. A total of 24,414 differentially expressed genes (DEGs) were GO-annotated into 53, 23, 55, and 60 terms from the transcriptomes of root and leaf tissues under Cd stress and control conditions. A total of 117,547 Kyoto Encyclopedia of Genes and Genomes (KEGG) Orthology (KO)-annotated DEGs were enriched in at least 47 KEGG pathway terms among the four comparisons. Many genes encoding important transcription factors (e.g., auxin/indole-3-acetic acid (AUX/IAA), basic helix-loop-helix (bHLH), DNA-binding one zinc finger (Dof), and MYB) and proteins involved in plant-pathogen interactions, phenylpropanoid biosynthesis, plant hormone signal transduction, oxidative phosphorylation, carbon fixation, peroxisomes, flavonoid biosynthesis, and glutathione metabolism, among others, were substantially upregulated under CdCl₂ stress.

CONCLUSIONS

These genes represent important candidates for studying Cd-response mechanisms and molecular biology of B. papyrifera and related species. Our findings provide a genomic sequence resource for functional genetic assignments in B. papyrifera, which will help elucidate the molecular mechanisms of its Cd-stress responses and facilitate the bioremediation of heavy metal contaminated areas via breeding of new stress-tolerant cultivars.

Translating auxin responses into ovules, seeds and yield: Insight from Arabidopsis and the cereals

Grain production in cereal crops depends on the stable formation of male and female gametes in the flower. In most angiosperms, the female gamete is produced from a germline located deep within the ovary, protected by several layers of maternal tissue, including the ovary wall, ovule integuments and nucellus. In the field, germline formation and floret fertility are major determinants of yield potential, contributing to traits such as seed number, weight and size. As such, stimuli affecting the timing and duration of reproductive phases, as well as the viability, size and number of cells within reproductive organs can significantly impact yield. One key stimulant is the phytohormone auxin, which influences growth and morphogenesis of female tissues during gynoecium development, gametophyte formation, and endosperm cellularization. In this review we consider the role of the auxin signaling pathway during ovule and seed development, first in the context of Arabidopsis and then in the cereals. We summarize the gene families involved and highlight distinct expression patterns that suggest a range of roles in reproductive cell specification and fate. This is discussed in terms of seed production and how targeted modification of different tissues might facilitate improvements.

MhMAPK4 from Malus hupehensis Rehd. decreases cell death in tobacco roots by controlling Cd2+ uptake

Cadmium (Cd) induces cell death in plant roots. Mitogen-activated protein kinase (MAPK) plays a role in the regulation of cell death induced by Cd in plant roots. In this study, MhMAPK4 was isolated from the roots of Malus hupehensis. Subcellular localization showed that the MhMAPK4 protein was located in the cell membrane and cytoplasm and is a transmembrane protein that is characterized by hydrophily. The expression of MhMAPK4 in the roots of M. hupehensis was up-regulated by Cd sulfate and Cd chloride. Phenotypic comparison under Cd stress showed that the growth of wild-type (WT) tobacco was lower than the transgenic lines overexpressing MhMAPK4. The fresh weight and the root length of WT also was lower than that of the transgenic tobacco. The net Cd2+ influx in the tobacco roots was decreased by the overexpression of MhMAPK4, as was root Cd accumulation. The recovery time of the Cd2+ influx to stable state in the transgenic tobacco was also shorter than the WT. The expression of iron-regulated transporter 1 (NtIRT1) and natural resistance associated macrophage protein 5 (NtNRAMP5) was relatively low in the transgenic lines under Cd stress. Cell death and apoptosis in the tobacco roots was reduced following the overexpression of MhMAPK4. The activity of vacuolar processing enzyme (VPE) and the transcript level of VPE in the transgenic tobacco was lower than that of WT under Cd stress. In addition, the electrolyte leakage and malondialdehyde and hydrogen peroxide contents in the transgenic tobacco were lower than those of WT, whereas the antioxidant enzyme activity and expression were higher. These results suggest that MhMAPK4 regulates Cd accumulation by mediating Cd2+ uptake by the roots, and controls Cd-caused cell death by adjusting VPE activity.

Mitogen-Activated Protein Kinase Cascades in Plant Hormone Signaling

Mitogen-activated protein kinase (MAPK) modules play key roles in the transduction of environmental and developmental signals through phosphorylation of downstream signaling targets, including other kinases, enzymes, cytoskeletal proteins or transcription factors, in all eukaryotic cells. A typical MAPK cascade consists of at least three sequentially acting serine/threonine kinases, a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK) and finally, the MAP kinase (MAPK) itself, with each phosphorylating, and hence activating, the next kinase in the cascade. Recent advances in our understanding of hormone signaling pathways have led to the discovery of new regulatory systems. In particular, this research has revealed the emerging role of crosstalk between the protein components of various signaling pathways and the involvement of this crosstalk in multiple cellular processes. Here we provide an overview of current models and mechanisms of hormone signaling with a special emphasis on the role of MAPKs in cell signaling networks. One-sentence summary: In this review we highlight the mechanisms of crosstalk between MAPK cascades and plant hormone signaling pathways and summarize recent findings on MAPK regulation and function in various cellular processes.

Genome-Wide Identification of Mitogen-activated Protein Kinase Cascade Genes and Transcriptional Profiling Analysis during Organ Development in Eucommia ulmoides

The mitogen-activated protein kinase (MAPK) cascades, which play crucial roles in plant development processes, are universal modules of signal transduction in eukaryotes and consist of a core module of three sequentially phosphorylated kinases: MAPK, MAPK kinase (MAPKK), and MAPKK kinase (MAPKKK). This is the first report on the identification and analysis of MAPK cascades in Eucommia ulmoides. We conducted a genome-wide screening and identified 13 EuMAPKs, five EuMAPKKs, and 57 EuMAPKKKs. The construction of phylogenetic trees revealed that EuMAPKs and EuMAPKKs were divided into four groups (A, B, C, and D), and EuMAPKKKs were divided into three subfamilies (MEKK, RAF, and ZIK). These subfamilies were further confirmed by conserved domain/motif analysis and gene structure analysis. Based on the expression profiles of all identified EuMAPK cascades in various organs at different developmental stages, three genes (EuRAF22-2, EuRAF34-1, and EuRAF33-2) with stable expression patterns at all stages of fruit or leaf development, three genes (EuRAF2-3, EuMPK11, and EuMEKK21) with differential expression patterns, and two highly expressed genes (EuZIK1 and EuMKK2) were screened and validated by qRT-PCR. Overall, our results could be used for further research on the precise role of MAPK cascades during organ development in E. ulmoides.

The MAPKKK gene family in cassava: Genome-wide identification and expression analysis against drought stress

Mitogen-activated protein kinase kinase kinases (MAPKKKs), an important unit of MAPK cascade, play crucial roles in plant development and response to various stresses. However, little is known concerning the MAPKKK family in the important subtropical and tropical crop cassava. In this study, 62 MAPKKK genes were identified in the cassava genome, and were classified into 3 subfamilies based on phylogenetic analysis. Most of MAPKKKs in the same subfamily shared similar gene structures and conserved motifs. The comprehensive transcriptome analysis showed that MAPKKK genes participated in tissue development and response to drought stress. Comparative expression profiles revealed that many MAPKKK genes were activated in cultivated varieties SC124 and Arg7 and the function of MeMAPKKKs in drought resistance may be different between SC124/Arg7 and W14. Expression analyses of the 7 selected MeMAPKKK genes showed that most of them were significantly upregulated by osmotic, salt and ABA treatments, whereas slightly induced by H₂O₂ and cold stresses. Taken together, this study identified candidate MeMAPKKK genes for genetic improvement of abiotic stress resistance and provided new insights into MAPKKK -mediated cassava resistance to drought stress.

An Arabidopsis kinase cascade influences auxin-responsive cell expansion

Mitogen-activated protein kinase (MPK) cascades are conserved mechanisms of signal transduction across eukaryotes. Despite the importance of MPK proteins in signaling events, specific roles for many Arabidopsis MPK proteins remain unknown. Multiple studies have suggested roles for MPK signaling in a variety of auxin-related processes. To identify MPK proteins with roles in auxin response, we screened mpk insertional alleles and identified mpk1-1 as a mutant that displays hypersensitivity in auxin-responsive cell expansion assays. Further, mutants defective in the upstream MAP kinase kinase MKK3 also display hypersensitivity in auxin-responsive cell expansion assays, suggesting that this MPK cascade affects auxin-influenced cell expansion. We found that MPK1 interacts with and phosphorylates ROP BINDING PROTEIN KINASE 1 (RBK1), a protein kinase that interacts with members of the Rho-like GTPases from Plants (ROP) small GTPase family. Similar to mpk1-1 and mkk3-1 mutants, rbk1 insertional mutants display auxin hypersensitivity, consistent with a possible role for RBK1 downstream of MPK1 in influencing auxin-responsive cell expansion. We found that RBK1 directly phosphorylates ROP4 and ROP6, supporting the possibility that RBK1 effects on auxin-responsive cell expansion are mediated through phosphorylation-dependent modulation of ROP activity. Our data suggest a MKK3 • MPK1 • RBK1 phosphorylation cascade that may provide a dynamic module for altering cell expansion.

Cadmium stress in rice: toxic effects, tolerance mechanisms, and management: a critical review

Cadmium (Cd) is one of the main pollutants in paddy fields, and its accumulation in rice (Oryza sativa L.) and subsequent transfer to food chain is a global environmental issue. This paper reviews the toxic effects, tolerance mechanisms, and management of Cd in a rice paddy. Cadmium toxicity decreases seed germination, growth, mineral nutrients, photosynthesis, and grain yield. It also causes oxidative stress and genotoxicity in rice. Plant response to Cd toxicity varies with cultivars, growth condition, and duration of Cd exposure. Under Cd stress, stimulation of antioxidant defense system, osmoregulation, ion homeostasis, and over production of signaling molecules are important tolerance mechanisms in rice. Several strategies have been proposed for the management of Cd-contaminated paddy soils. One such approach is the exogenous application of hormones, osmolytes, and signaling molecules. Moreover, Cd uptake and toxicity in rice can be decreased by proper application of essential nutrients such as nitrogen, zinc, iron, and selenium in Cd-contaminated soils. In addition, several inorganic (liming and silicon) and organic (compost and biochar) amendments have been applied in the soils to reduce Cd stress in rice. Selection of low Cd-accumulating rice cultivars, crop rotation, water management, and exogenous application of microbes could be a reasonable approach to alleviate Cd toxicity in rice. To draw a sound conclusion, long-term field trials are still required, including risks and benefit analysis for various management strategies.

Genome-wide identification, phylogeny and expressional profiles of mitogen activated protein kinase kinase kinase (MAPKKK) gene family in bread wheat (Triticum aestivum L.)

BACKGROUND

Mitogen-activated protein kinase kinase kinases (MAPKKKs) are the important components of MAPK cascades, which play the crucial role in plant growth and development as well as in response to diverse stresses. Although this family has been systematically studied in many plant species, little is known about MAPKKK genes in wheat (Triticum aestivum L.), especially those involved in the regulatory network of stress processes.

RESULTS

In this study, we identified 155 wheat MAPKKK genes through a genome-wide search method based on the latest available wheat genome information, of which 29 belonged to MEKK, 11 to ZIK and 115 to Raf subfamily, respectively. Then, chromosome localization, gene structure and conserved protein motifs and phylogenetic relationship as well as regulatory network of these TaMAPKKKs were systematically investigated and results supported the prediction. Furthermore, a total of 11 homologous groups between A, B and D sub-genome and 24 duplication pairs among them were detected, which contributed to the expansion of wheat MAPKKK gene family. Finally, the expression profiles of these MAPKKKs during development and under different abiotic stresses were investigated using the RNA-seq data. Additionally, 10 tissue-specific and 4 salt-responsive TaMAPKKK genes were selected to validate their expression level through qRT-PCR analysis.

CONCLUSIONS

This study for the first time reported the genome organization, evolutionary features and expression profiles of the wheat MAPKKK gene family, which laid the foundation for further functional analysis of wheat MAPKKK genes, and contributed to better understanding the roles and regulatory mechanism of MAPKKKs in wheat.

Calcium alleviates cadmium-induced inhibition on root growth by maintaining auxin homeostasis in Arabidopsis seedlings

Cadmium (Cd) toxicity has been widely studied in different plant species. However, the mechanism involved in its toxicity and the cell response to Cd has not been well established. In the present study, we investigated the possible mechanism of calcium (Ca) in protecting Arabidopsis from Cd toxicity. The results showed that 50 μM Cd significantly inhibited the seedling growth and decreased the chlorophyll content in Arabidopsis. Specifically, the primary root (PR) length was decreased but the lateral root (LR) number was increased under Cd stress. Furthermore, Cd enhanced the hydrogen peroxide (H2O2) content and lipid peroxidation as indicated by malondialdehyde (MDA) accumulation. Cd also altered the level and the distribution of auxin in PR tips (as evidenced by DR5::GUS and PIN:GFP reporter expression) and the expression of several putative auxin biosynthetic, catabolic, and transport pathway-related genes. Application of 3 mM Ca alleviated the inhibition of Cd on the root growth. Ca application not only led to reducing oxidative injuries but also restoring the normal auxin transport and distribution in Arabidopsis root under Cd stress. Taken together, these results suggest that Ca alleviates the root growth inhibition caused by Cd through maintaining auxin homeostasis in Arabidopsis seedlings.

Genome-Wide Identification, Evolution, and Co-expression Network Analysis of Mitogen-Activated Protein Kinase Kinase Kinases in Brachypodium distachyon

Mitogen-activated protein kinase (MAPK) cascades are the conserved and universal signal transduction modules in all eukaryotes, which play the vital roles in plant growth, development, and in response to multiple stresses. In this study, we used bioinformatics methods to identify 86 MAPKKK protein encoded by 73 MAPKKK genes in Brachypodium. Phylogenetic analysis of MAPKKK family from Arabidopsis, rice, and Brachypodium has classified them into three subfamilies, of which 28 belonged to MEKK, 52 to Raf, and 6 to ZIK subfamily, respectively. Conserved protein motif, exon-intron organization, and splicing intron phase in kinase domains supported the evolutionary relationships inferred from the phylogenetic analysis. And gene duplication analysis suggested the chromosomal segment duplication happened before the divergence of the rice and Brachypodium, while all of three tandem duplicated gene pairs happened after their divergence. We further demonstrated that the MAPKKKs have evolved under strong purifying selection, implying the conservation of them. The splicing transcripts expression analysis showed that the splicesome translating longest protein tended to be adopted. Furthermore, the expression analysis of BdMAPKKKs in different organs and development stages as well as heat, virus and drought stresses revealed that the MAPKKK genes were involved in various signaling pathways. And the circadian analysis suggested there were 41 MAPKKK genes in Brachypodium showing cycled expression in at least one condition, of which seven MAPKKK genes expressed in all conditions and the promoter analysis indicated these genes possessed many cis-acting regulatory elements involved in circadian and light response. Finally, the co-expression network of MAPK, MAPKK, and MAPKKK in Brachypodium was constructed using 144 microarray and RNA-seq datasets, and ten potential MAPK cascades pathway were predicted. To conclude, our study provided the important information for evolutionary and functional characterization of MAPKKK family in Brachypodium, which will facilitate the functional analysis of BdMAPKKK genes, and also will facilitate better understanding the MAPK signal pathway in Brachypodium and beyond.

Minimising toxicity of cadmium in plants--role of plant growth regulators

A range of man-made activities promote the enrichment of world-wide agricultural soils with a myriad of chemical pollutants including cadmium (Cd). Owing to its significant toxic consequences in plants, Cd has been one of extensively studied metals. However, sustainable strategies for minimising Cd impacts in plants have been little explored. Plant growth regulators (PGRs) are known for their role in the regulation of numerous developmental processes. Among major PGRs, plant hormones (such as auxins, gibberellins, cytokinins, abscisic acid, jasmonic acid, ethylene and salicylic acid), nitric oxide (a gaseous signalling molecule), brassinosteroids (steroidal phytohormones) and polyamines (group of phytohormone-like aliphatic amine natural compounds with aliphatic nitrogen structure) have gained attention by agronomist and physiologist as a sustainable media to induce tolerance in abiotic-stressed plants. Considering recent literature, this paper: (a) overviews Cd status in soil and its toxicity in plants, (b) introduces major PGRs and overviews their signalling in Cd-exposed plants, (c) appraises mechanisms potentially involved in PGR-mediated enhanced plant tolerance to Cd and (d) highlights key aspects so far unexplored in the subject area.

Salicylic acid alleviates cadmium-induced stress responses through the inhibition of Cd-induced auxin-mediated reactive oxygen species production in barley root tips

Auxin is a master regulator of root growth by modulating its development under the constantly changing environment. Recently, an antagonistic interaction was suggested between SA and IAA signaling. Therefore, the purpose of this work was to analyze and compare the effect of the indole-3-acetic acid (IAA) signaling inhibitor p-chlorophenoxyisobutyric acid (PCIB) and salicylic acid (SA) as a potential IAA signaling inhibitor on the root growth, enzyme activity and reactive oxygen species (ROS) production in Cd- and IAA-treated barley root tips. Exposure of plants to Cd resulted in a more than threefold increase of IAA content in the root apex even 3h after the treatment. In addition, exogenously applied IAA evoked root responses such as root growth inhibition and swelling, ROS generation and activation of lipoxygenase or glutathione peroxidase identical to those induced by Cd. Furthermore, both Cd- and IAA-induced stress responses were markedly reversed by PCIB or SA post-treatment. Similarly to PCIB, SA did not affect the IAA content of root tips, suggesting the action of SA on the IAA signaling pathway in barley roots. SA probably does not alleviate the Cd toxicity in roots, but rather prevents or partially inhibits the root defense response to the presence of Cd through the inhibition of Cd-induced IAA-mediated ROS generation in roots.