Identification of a C2H2-type zinc finger transcription factor (ZAT10) from Arabidopsis as a substrate of MAP kinase

The transcription factor MhZAT10 enhances antioxidant capacity by directly activating the antioxidant genes MhMSD1, MhAPX3a and MhCAT1 in apple rootstock SH6 (Malus honanensis × M. domestica)

Stress tolerance in apple (Malus domestica) can be improved by grafting to a stress-tolerant rootstock, such as 'SH6' (Malus honanensis × M. domestica 'Ralls Genet'). However, the mechanisms of stress tolerance in this rootstock are unclear. In Arabidopsis (Arabidopsis thaliana), the transcription factor ZINC FINGER OF ARABIDOPSIS THALIANA 10 is a key component of plant tolerance to multiple abiotic stresses and positively regulates antioxidant enzymes. However, how reactive oxygen species are eliminated upon activation of ZINC FINGER OF ARABIDOPSIS THALIANA 10 in response to abiotic stress remains elusive. Here, we report that MhZAT10 in the rootstock SH6 directly activates the transcription of three genes encoding the antioxidant enzymes MANGANESE SUPEROXIDE DISMUTASE 1 (MhMSD1), ASCORBATE PEROXIDASE 3A (MhAPX3a) and CATALASE 1 (MhCAT1) by binding to their promoters. Heterologous expression in Arabidopsis protoplasts showed that MhMSD1, MhAPX3a and MhCAT1 localize in multiple subcellular compartments. Overexpressing MhMSD1, MhAPX3a or MhCAT1 in SH6 fruit calli resulted in higher superoxide dismutase, ascorbate peroxidase and catalase enzyme activities in their respective overexpressing calli than in those overexpressing MhZAT10. Notably, the calli overexpressing MhZAT10 exhibited better growth and lower reactive oxygen species levels under simulated osmotic stress. Apple SH6 plants overexpressing MhZAT10 in their roots via Agrobacterium rhizogenes-mediated transformation also showed enhanced tolerance to osmotic stress, with higher leaf photosynthetic capacity, relative water content in roots and antioxidant enzyme activity, as well as less reactive oxygen species accumulation. Overall, our study demonstrates that the transcription factor MhZAT10 synergistically regulates the transcription of multiple antioxidant-related genes and elevates reactive oxygen species detoxification.

A pervasive phosphorylation cascade modulation of plant transcription factors in response to abiotic stress

MAIN CONCLUSION

Transcriptional regulation of stress-responsive genes is a crucial step in establishing the mechanisms behind plant abiotic stress tolerance. A sensitive method of regulating transcription factors activity, stability, protein interaction, and subcellular localization is through phosphorylation. This review highlights a widespread regulation mechanism that involves phosphorylation of plant TFs in response to abiotic stress. Abiotic stress is one of the main components limiting crop yield and sustainability on a global scale. It greatly reduces the land area that is planted and lowers crop production globally. In all living organisms, transcription factors (TFs) play a crucial role in regulating gene expression. They participate in cell signaling, cell cycle, development, and plant stress response. Plant resilience to diverse abiotic stressors is largely influenced by TFs. Transcription factors modulate gene expression by binding to their target gene's cis-elements, which are impacted by genomic characteristics, DNA structure, and TF interconnections. In this review, we focus on the six major TFs implicated in abiotic stress tolerance, namely, DREB, bZIP, WRKY, ABF, MYB, and NAC, and the cruciality of phosphorylation of these transcription factors in abiotic stress signaling, as protein phosphorylation has emerged as one of the key post-translational modifications, playing a critical role in cell signaling, DNA amplification, gene expression and differentiation, and modification of other biological configurations. These TFs have been discovered after extensive study as stress-responsive transcription factors which may be major targets for crop development and important contributors to stress tolerance and crop production.

Transcription factors MhDREB2A/MhZAT10 Play a Role in Drought and Cold Stress Response Crosstalk in Apple

Drought and cold stresses seriously affect tree growth and fruit yield during apple (Malus domestica) production, with combined stress causing injury such as shoot shriveling. However, the molecular mechanism underlying crosstalk between responses to drought and cold stress remains to be clarified. In this study, we characterized the zinc finger transcription factor ZINC FINGER OF ARABIDOPSIS THALIANA 10 (ZAT10) through comparative analysis of shoot-shriveling tolerance between tolerant and sensitive apple rootstocks. MhZAT10 responded to both drought and cold stress. Heterologous expression of MhZAT10 in the sensitive rootstock 'G935' from domesticated apple (Malus domestica) promoted shoot-shriveling tolerance, while silencing of MhZAT10 expression in the tolerant rootstock 'SH6' of Malus honanensis reduced stress tolerance. We determined that the apple transcription factor DEHYDRATION RESPONSE ELEMENT-BINDING PROTEIN 2A (DREB2A) is a direct regulator activating the expression of MhZAT10 in response to drought stress. Apple plants overexpressing both MhDREB2A and MhZAT10 genes exhibited enhanced tolerance to drought and cold stress, while plants overexpressing MhDREB2A but with silenced expression of MhZAT10 showed reduced tolerance, suggesting a critical role of MhDREB2A-MhZAT10 in the crosstalk between drought and cold stress responses. We further identified drought-tolerant MhWRKY31 and cold-tolerant MhMYB88 and MhMYB124 as downstream regulatory target genes of MhZAT10. Our findings reveal a MhDREB2A-MhZAT10 module involved in crosstalk between drought and cold stress responses, which may have applications in apple rootstock breeding programs aimed at developing shoot-shriveling tolerance.

Redox Signaling in Plant Heat Stress Response

The increase in environmental temperature due to global warming is a critical threat to plant growth and productivity. Heat stress can cause impairment in several biochemical and physiological processes. Plants sense and respond to this adverse environmental condition by activating a plethora of defense systems. Among them, the heat stress response (HSR) involves an intricate network of heat shock factors (HSFs) and heat shock proteins (HSPs). However, a growing amount of evidence suggests that reactive oxygen species (ROS), besides potentially being responsible for cellular oxidative damage, can act as signal molecules in HSR, leading to adaptative responses. The role of ROS as toxic or signal molecules depends on the fine balance between their production and scavenging. Enzymatic and non-enzymatic antioxidants represent the first line of defense against oxidative damage and their activity is critical to maintaining an optimal redox environment. However, the HS-dependent ROS burst temporarily oxidizes the cellular environment, triggering redox-dependent signaling cascades. This review provides an overview of the redox-activated mechanisms that participate in the HSR.

Arabidopsis Cys2/His2 Zinc Finger Transcription Factor ZAT18 Modulates the Plant Growth-Defense Tradeoff

Plant defense responses under unfavorable conditions are often associated with reduced growth. However, the mechanisms underlying the growth-defense tradeoff remain to be fully elucidated, especially at the transcriptional level. Here, we revealed a Cys2/His2-type zinc finger transcription factor, namely, ZAT18, which played dual roles in plant immunity and growth by oppositely regulating the signaling of defense- and growth-related hormones. ZAT18 was first identified as a salicylic acid (SA)-inducible gene and was required for plant responses to SA in this study. In addition, we observed that ZAT18 enhanced the plant immunity with growth penalties that may have been achieved by activating SA signaling and repressing auxin signaling. Further transcriptome analysis of the zat18 mutant showed that the biological pathways of defense-related hormones, including SA, ethylene and abscisic acid, were repressed and that the biological pathways of auxin and cytokinin, which are growth-related hormones, were activated by abolishing the function of ZAT18. The ZAT18-mediated regulation of hormone signaling was further confirmed using qRT-PCR. Our results explored a mechanism by which plants handle defense and growth at the transcriptional level under stress conditions.

The C2H2-type zinc finger protein PhZFP1 regulates cold stress tolerance by modulating galactinol synthesis in Petunia hybrida

The C2H2 zinc finger proteins (ZFPs) play essential roles in regulating cold stress responses. Similarly, raffinose accumulation contributes to freezing stress tolerance. However, the relationship between C2H2 functions and raffinose synthesis in cold tolerance remains uncertain. Here, we report the characterization of the cold-induced C2H2-type zinc finger protein PhZFP1 in Petunia hybrida. PhZFP1 was found to be predominantly localized in the nucleus. Overexpression of PhZFP1 conferred enhanced cold tolerance in transgenic petunia lines. In contrast, RNAi mediated suppression of PhZFP1 led to increased cold susceptibility. PhZFP1 regulated the expression of a range of abiotic stress responsive-genes including genes encoding proteins involved in reactive oxygen species (ROS) scavenging and raffinose metabolism. The accumulation of galactinol and raffinose, and the levels of PhGolS1-1 transcripts, were significantly increased in PhZFP1-overexpressing plants and decreased in PhZFP1-RNAi plants under cold stress. Moreover, the galactinol synthase (GolS)-encoding gene PhGolS1-1 was identified as a direct target of PhZFP1. Taken together, these results demonstrate that PhZFP1 functions in cold stress tolerance by modulation of galactinol synthesis via regulation of PhGolS1-1. This study also provides new insights into the mechanisms underlying C2H2 zinc finger protein-mediated cold stress tolerance, and has identified a candidate gene for improving cold stress tolerance.

Genome-Wide Identification of C2H2 ZFPs and Functional Analysis of BRZAT12 under Low-Temperature Stress in Winter Rapeseed (Brassica rapa)

Zinc-finger protein (ZFP) transcription factors are among the largest families of transcription factors in plants. They participate in various biological processes such as apoptosis, autophagy, and stemness maintenance and play important roles in regulating plant growth and development and the response to stress. To elucidate the functions of ZFP genes in the low-temperature response of winter (Brassica rapa L.) B. rapa, this study identified 141 members of the C2H2 ZFP gene family from B. rapa, which are heterogeneously distributed on 10 chromosomes and have multiple cis-acting elements related to hormone regulation and abiotic stress of adversity. Most of the genes in this family contain only one CDS, and genes distributed in the same evolutionary branch share mostly the same motifs and are highly conserved in the evolution of cruciferous species. The genes were significantly upregulated in the roots and growth cones of ‘Longyou-7’, indicating that they play a role in the stress-response process of winter B. rapa. The expression level of the Bra002528 gene was higher in the strongly cold-resistant varieties than in the weakly cold-resistant varieties after low-temperature stress. The survival rate and BrZAT12 gene expression of trans-BrZAT12 Arabidopsis thaliana (Arabidopsis) were significantly higher than those of the wild-type plants at low temperature, and the enzyme activities in vivo were higher than those of the wild-type plants, indicating that the BrZAT12 gene could improve the cold resistance of winter B. rapa. BrZAT12 expression and superoxide dismutase and ascorbate peroxidase enzyme activities were upregulated in winter B. rapa after exogenous ABA treatment. BrZAT12 expression and enzyme activities decreased after the PD98059 treatment, and BrZAT12 expression and enzyme activities were higher than in the PD98059 treatment but lower than in the control after both treatments together. It is speculated that BrZAT12 plays a role in the ABA signaling process in which MAPKK is involved. This study provides a theoretical basis for the resolution of cold-resistance mechanisms in strong winter B. rapa.

C2H2 Zinc Finger Proteins Response to Abiotic Stress in Plants

Abiotic stresses have already exhibited the negative effects on crop growth and development, thereby influencing crop quality and yield. Therefore, plants have developed regulatory mechanisms to adopt against such harsh changing environmental conditions. Recent studies have shown that zinc finger protein transcription factors play a crucial role in plant growth and development as well as in stress response. C2H2 zinc finger proteins are one of the best-studied types and have been shown to play diverse roles in the plant abiotic stress responses. However, the C2H2 zinc finger network in plants is complex and needs to be further studied in abiotic stress responses. Here in this review, we mainly focus on recent findings on the regulatory mechanisms, summarize the structural and functional characterization of C2H2 zinc finger proteins, and discuss the C2H2 zinc finger proteins involved in the different signal pathways in plant responses to abiotic stress.

ZmMPK5 phosphorylates ZmNAC49 to enhance oxidative stress tolerance in maize

Mitogen-activated protein kinase (MPK) is a critical regulator of the antioxidant defence system in response to various stimuli. However, how MPK directly and exactly regulates antioxidant enzyme activities is still unclear. Here, we demonstrated that a NAC transcription factor ZmNAC49 mediated the regulation of antioxidant enzyme activities by ZmMPK5. ZmNAC49 expression is induced by oxidative stress. ZmNAC49 enhances oxidative stress tolerance in maize, and it also reduces superoxide anion generation and increases superoxide dismutase (SOD) activity. A detailed study showed that ZmMPK5 directly interacts with and phosphorylates ZmNAC49 in vitro and in vivo. ZmMPK5 directly phosphorylates Thr-26 in NAC subdomain A of ZmNAC49. Mutation at Thr-26 of ZmNAC49 does not affect the interaction with ZmMPK5 and its subcellular localisation. Further analysis found that ZmNAC49 activates the ZmSOD3 expression by directly binding to its promoter. ZmMPK5-mediated ZmNAC49 phosphorylation improves its ability to bind to the ZmSOD3 promoter. Thr-26 of ZmNAC49 is essential for its transcriptional activity. In addition, ZmSOD3 enhances oxidative stress tolerance in maize. Our results show that phosphorylation of Thr-26 in ZmNAC49 by ZmMPK5 increased its DNA-binding activity to the ZmSOD3 promoter, enhanced SOD activity and thereby improved oxidative stress tolerance in maize.

Genome-wide analysis of ZAT gene family revealed GhZAT6 regulates salt stress tolerance in G. hirsutum

High salt environments can induce stress in different plants. The genes containing the ZAT domain constitute a family that belongs to a branch of the C₂H₂ family, which plays a vital role in responding to abiotic stresses. In this study, we identified 169 ZAT genes from seven plant species, including 44 ZAT genes from G. hirsutum. Phylogenetic tree analysis divided ZAT genes in six groups with conserved gene structure, protein motifs. Two C₂H₂ domains and an EAR domain and even chromosomal distribution on At and Dt sub-genome chromosomes of G. hirsutum was observed. GhZAT6 was primarily expressed in the root tissue and responded to NaCl and ABA treatments. Subcellular localization found that GhZAT6 was located in the nucleus and demonstrated transactivation activity during a transactivation activity assay. Arabidopsis transgenic lines overexpressing the GhZAT6 gene showed salt tolerance and grew more vigorously than WT on MS medium supplemented with 100 mmol NaCl. Additionally, the silencing of the GhZAT6 gene in cotton plants showed more obvious leaf wilting than the control plants, which were subjected to 400 mmol NaCl treatment. Next, the expressions of GhAPX1, GhFSD1, GhFSD2, and GhSOS3 were significantly lower in the GhZAT6-silenced plants treated with NaCl than the control. Based on these findings, GhZAT6 may be involved in the ABA pathway and mediate salt stress tolerance by regulating ROS-related gene expression.

The C2H2-type zinc finger transcription factor MdZAT10 negatively regulates drought tolerance in apple

Various abiotic stressors, particularly drought stress, affect plant growth and yield. Zinc finger proteins play an important role in plant abiotic stress tolerance. Here, we isolated the apple MdZAT10 gene, a C2H2-type zinc finger protein, which is a homolog of Arabidopsis STZ/ZAT10. MdZAT10 was localized to the nucleus and highly expressed in leaves and fruit. Promoter analysis showed that MdZAT10 contained several response elements and the transcription level of MdZAT10 was induced by abiotic stress and hormone treatments. MdZAT10 was responsive to drought treatment both at the transcriptional and post-translational levels. MdZAT10-overexpressing apple calli decreased the expression level of MdAPX2 and increased sensitivity to PEG 6000 treatment. Moreover, ectopically expressed MdZAT10 in Arabidopsis reduced the tolerance to drought stress, and exhibited higher water loss, higher malondialdehyde (MDA) content and higher reactive oxygen species (ROS) accumulation under drought stress. In addition, MdZAT10 reduced the sensitivity to abscisic acid in apple. Ectopically expressed MdZAT10 in Arabidopsis promoted seed germination and seedling growth. These results indicate that MdZAT10 plays a negative regulator in the drought resistance, which can provide theoretical basis for further molecular mechanism research.

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.

Heat-Induced Oxidation of the Nuclei and Cytosol

The concept that heat stress (HS) causes a large accumulation of reactive oxygen species (ROS) is widely accepted. However, the intracellular compartmentation of ROS accumulation has been poorly characterized. We therefore used redox-sensitive green fluorescent protein (roGFP2) to provide compartment-specific information on heat-induced redox changes of the nuclei and cytosol of Arabidopsis leaf epidermal and stomatal guard cells. We show that HS causes a large increase in the degree of oxidation of both compartments, causing large shifts in the glutathione redox potentials of the cells. Heat-induced increases in the levels of the marker transcripts, heat shock protein (HSP)101, and ascorbate peroxidase (APX)2 were maximal after 15 min of the onset of the heat treatment. RNAseq analysis of the transcript profiles of the control and heat-treated seedlings revealed large changes in transcripts encoding HSPs, mitochondrial proteins, transcription factors, and other nuclear localized components. We conclude that HS causes extensive oxidation of the nucleus as well as the cytosol. We propose that the heat-induced changes in the nuclear redox state are central to both genetic and epigenetic control of plant responses to HS.

Protein Phosphorylation in Plant Cell Signaling

Owing to their sessile nature, plants have evolved sophisticated sensory mechanisms to respond quickly and precisely to the changing environment. The extracellular stimuli are perceived and integrated by diverse receptors, such as receptor-like protein kinases (RLKs) and receptor-like proteins (RLPs), and then transmitted to the nucleus by complex cellular signaling networks, which play vital roles in biological processes including plant growth, development, reproduction, and stress responses. The posttranslational modifications (PTMs) are important regulators for the diversification of protein functions in plant cell signaling. Protein phosphorylation is an important and well-characterized form of the PTMs, which influences the functions of many receptors and key components in cellular signaling. Protein phosphorylation in plants predominantly occurs on serine (Ser) and threonine (Thr) residues, which is dynamically and reversibly catalyzed by protein kinases and protein phosphatases, respectively. In this review, we focus on the function of protein phosphorylation in plant cell signaling, especially plant hormone signaling, and highlight the roles of protein phosphorylation in plant abiotic stress responses.

AtMPK6-induced phosphorylation of AtERF72 enhances its DNA binding activity and interaction with TGA4/OBF4 in Arabidopsis

The ethylene-responsive element binding factor (ERF) family is a large family of transcription factors involved in plant development and environmental stress responses. We previously reported the identification of 29 putative substrates of Mitogen-activated Protein Kinase3 (AtMPK3), AtMPK4 and AtMPK6, based on a solid-phase phosphorylation screening using a lambda phage expression library in Arabidopsis thaliana. In this study, a putative MPK substrate, AtERF72 (At3g16770), was strongly phosphorylated by AtMPK6 on the serine residue at position 151 (Ser151). AtERF72 binds to the GCC box (AGCCGCC) in the promoters of several pathogenesis-related (PR) genes and activates their transcription. We also show that the DNA-binding activity of AtERF72 is enhanced upon phosphorylation by AtMPK6 in vitro. In addition, transient co-expression experiments in Arabidopsis protoplasts revealed that effector constructs expressing a mutant variant of AtERF72, AtERF72^S151D (carrying a Ser to aspartic acid [Asp] substitution at amino acid position 151) showed higher expression of the β-glucuronidase (GUS) reporter gene driven by the GCC box element than effector constructs expressing the wild-type AtERF72. Furthermore, yeast two-hybrid assays revealed that the interaction between AtERF72^S151D and TGA4/OBF4 was stronger than that between wild-type AtERF72 and TGA4/OBF4. Since AtERF72^S151D is equivalent to AtERF72 phosphorylated by AtMPK6 at Ser151, these results suggest that the phosphorylation of AtERF72 by AtMPK6 triggers an event of transcriptional regulation from defence signalling in Arabidopsis.

Comparative transcriptomic analysis of rhizomes, stems, and leaves of Polygonatum odoratum (Mill.) Druce reveals candidate genes associated with polysaccharide synthesis

Polygonatum odoratum (Mill.) Druce is a well-known traditional Chinese herb. Polysaccharides are major bioactive components of Polygonatum odoratum, which can improve immunity, and are used to treat rheumatic heart disease, cardiovascular disease, and diabetes. This study identified potential genes and transcription factors (TFs) that regulate polysaccharide synthesis in Polygonatum odoratum (Mill.) Druce using RNA sequencing data from leaf, stem, and rhizome tissues. 76,714 unigenes were annotated in public databases. Analysis of KEGG annotations identified 18 key enzymes responsible for polysaccharide biosynthesis and the most of the upregulated expressed unigenes were enriched in rhizome tissue compared with leaf or stem tissue. 73 TFs involved in polysaccharide synthesis were predicted. In addition, key enzyme genes were verified by quantitative real-time PCR. This study substantially enlarged the public transcriptome datasets of this species, and provided insight into detection of novel genes involved in synthesis of polysaccharides and other secondary metabolites.

Prediction of condition-specific regulatory genes using machine learning

Recent advances in genomic technologies have generated data on large-scale protein-DNA interactions and open chromatin regions for many eukaryotic species. How to identify condition-specific functions of transcription factors using these data has become a major challenge in genomic research. To solve this problem, we have developed a method called ConSReg, which provides a novel approach to integrate regulatory genomic data into predictive machine learning models of key regulatory genes. Using Arabidopsis as a model system, we tested our approach to identify regulatory genes in data sets from single cell gene expression and from abiotic stress treatments. Our results showed that ConSReg accurately predicted transcription factors that regulate differentially expressed genes with an average auROC of 0.84, which is 23.5-25% better than enrichment-based approaches. To further validate the performance of ConSReg, we analyzed an independent data set related to plant nitrogen responses. ConSReg provided better rankings of the correct transcription factors in 61.7% of cases, which is three times better than other plant tools. We applied ConSReg to Arabidopsis single cell RNA-seq data, successfully identifying candidate regulatory genes that control cell wall formation. Our methods provide a new approach to define candidate regulatory genes using integrated genomic data in plants.

Cold priming uncouples light- and cold-regulation of gene expression in Arabidopsis thaliana

BACKGROUND

The majority of stress-sensitive genes responds to cold and high light in the same direction, if plants face the stresses for the first time. As shown recently for a small selection of genes of the core environmental stress response cluster, pre-treatment of Arabidopsis thaliana with a 24 h long 4 °C cold stimulus modifies cold regulation of gene expression for up to a week at 20 °C, although the primary cold effects are reverted within the first 24 h. Such memory-based regulation is called priming. Here, we analyse the effect of 24 h cold priming on cold regulation of gene expression on a transcriptome-wide scale and investigate if and how cold priming affects light regulation of gene expression.

RESULTS

Cold-priming affected cold and excess light regulation of a small subset of genes. In contrast to the strong gene co-regulation observed upon cold and light stress in non-primed plants, most priming-sensitive genes were regulated in a stressor-specific manner in cold-primed plant. Furthermore, almost as much genes were inversely regulated as co-regulated by a 24 h long 4 °C cold treatment and exposure to heat-filtered high light (800 μmol quanta m- 2 s- 1). Gene ontology enrichment analysis revealed that cold priming preferentially supports expression of genes involved in the defence against plant pathogens upon cold triggering. The regulation took place on the cost of the expression of genes involved in growth regulation and transport. On the contrary, cold priming resulted in stronger expression of genes regulating metabolism and development and weaker expression of defence genes in response to high light triggering. qPCR with independently cultivated and treated replicates confirmed the trends observed in the RNASeq guide experiment.

CONCLUSION

A 24 h long priming cold stimulus activates a several days lasting stress memory that controls cold and light regulation of gene expression and adjusts growth and defence regulation in a stressor-specific manner.

The barley stripe mosaic virus expression system reveals the wheat C2H2 zinc finger protein TaZFP1B as a key regulator of drought tolerance

BACKGROUND

Drought stress is one of the major factors limiting wheat production globally. Improving drought tolerance is important for agriculture sustainability. Although various morphological, physiological and biochemical responses associated with drought tolerance have been documented, the molecular mechanisms and regulatory genes that are needed to improve drought tolerance in crops require further investigation. We have used a novel 4-component version (for overexpression) and a 3-component version (for underexpression) of a barley stripe mosaic virus-based (BSMV) system for functional characterization of the C2H2-type zinc finger protein TaZFP1B in wheat. These expression systems avoid the need to produce transgenic plant lines and greatly speed up functional gene characterization.

RESULTS

We show that overexpression of TaZFP1B stimulates plant growth and up-regulates different oxidative stress-responsive genes under well-watered conditions. Plants that overexpress TaZFP1B are more drought tolerant at critical periods of the plant's life cycle. Furthermore, RNA-Seq analysis revealed that plants overexpressing TaZFP1B reprogram their transcriptome, resulting in physiological and physical modifications that help wheat to grow and survive under drought stress. In contrast, plants transformed to underexpress TaZFP1B are significantly less tolerant to drought and growth is negatively affected.

CONCLUSIONS

This study clearly shows that the two versions of the BSMV system can be used for fast and efficient functional characterization of genes in crops. The extent of transcriptome reprogramming in plants that overexpress TaZFP1B indicates that the encoded transcription factor is a key regulator of drought tolerance in wheat.

C2H2 Zinc Finger Proteins: Master Regulators of Abiotic Stress Responses in Plants

Abiotic stresses such as drought and salinity are major environmental factors that limit crop yields. Unraveling the molecular mechanisms underlying abiotic stress resistance is crucial for improving crop performance and increasing productivity under adverse environmental conditions. Zinc finger proteins, comprising one of the largest transcription factor families, are known for their finger-like structure and their ability to bind Zn2+. Zinc finger proteins are categorized into nine subfamilies based on their conserved Cys and His motifs, including the Cys2/His2-type (C2H2), C3H, C3HC4, C2HC5, C4HC3, C2HC, C4, C6, and C8 subfamilies. Over the past two decades, much progress has been made in understanding the roles of C2H2 zinc finger proteins in plant growth, development, and stress signal transduction. In this review, we focus on recent progress in elucidating the structures, functions, and classifications of plant C2H2 zinc finger proteins and their roles in abiotic stress responses.

PeSTZ1, a C2H2-type zinc finger transcription factor from Populus euphratica, enhances freezing tolerance through modulation of ROS scavenging by directly regulating PeAPX2

In the present study, PeSTZ1, a cysteine-2/histidine-2-type zinc finger transcription factor, was isolated from the desert poplar, Populus euphratica, which serves as a model stress adaptation system for trees. PeSTZ1 was preferentially expressed in the young stems and was significantly up-regulated during chilling and freezing treatments. PeSTZ1 was localized to the nucleus and bound specifically to the PeAPX2 promoter. To examine the potential functions of PeSTZ1, we overexpressed it in poplar 84K hybrids (Populus alba × Populus glandulosa), which are known to be stress-sensitive. Upon exposure to freezing stress, transgenic poplars maintained higher photosynthetic activity and dissipated more excess light energy (in the form of heat) than wild-type poplars. Thus, PeSTZ1 functions as a transcription activator to enhance freezing tolerance without sacrificing growth. Under freezing stress, PeSTZ1 acts upstream of ASCORBATE PEROXIDASE2 (PeAPX2) and directly regulates its expression by binding to its promoter. Activated PeAPX2 promotes cytosolic APX that scavenges reactive oxygen species (ROS) under cold stress. PeSTZ1 may operate in parallel with C-REPEAT-BINDING FACTORS to regulate COLD-REGULATED gene expression. Moreover, PeSTZ1 up-regulation reduces malondialdehyde and ROS accumulation by activating the antioxidant system. Taken together, these results suggested that overexpressing PeSTZ1 in 84K poplar enhances freezing tolerance through the modulation of ROS scavenging via the direct regulation of PeAPX2 expression.

The mitogen-activated protein kinase 4-phosphorylated heat shock factor A4A regulates responses to combined salt and heat stresses

Heat shock factors regulate responses to high temperature, salinity, water deprivation, or heavy metals. Their function in combinations of stresses is, however, not known. Arabidopsis HEAT SHOCK FACTOR A4A (HSFA4A) was previously reported to regulate responses to salt and oxidative stresses. Here we show, that the HSFA4A gene is induced by salt, elevated temperature, and a combination of these conditions. Fast translocation of HSFA4A tagged with yellow fluorescent protein from cytosol to nuclei takes place in salt-treated cells. HSFA4A can be phosphorylated not only by mitogen-activated protein (MAP) kinases MPK3 and MPK6 but also by MPK4, and Ser309 is the dominant MAP kinase phosphorylation site. In vivo data suggest that HSFA4A can be the substrate of other kinases as well. Changing Ser309 to Asp or Ala alters intramolecular multimerization. Chromatin immunoprecipitation assays confirmed binding of HSFA4A to promoters of target genes encoding the small heat shock protein HSP17.6A and transcription factors WRKY30 and ZAT12. HSFA4A overexpression enhanced tolerance to individually and simultaneously applied heat and salt stresses through reduction of oxidative damage. Our results suggest that this heat shock factor is a component of a complex stress regulatory pathway, connecting upstream signals mediated by MAP kinases MPK3/6 and MPK4 with transcription regulation of a set of stress-induced target genes.

The Roles of Arabidopsis C1-2i Subclass of C2H2-type Zinc-Finger Transcription Factors

The Cys2His2 (C2H2)-type zinc-finger protein (ZFP) family, which includes 176 members in Arabidopsis thaliana, is one of the largest families of putative transcription factors in plants. Of the Arabidopsis ZFP members, only 33 members are conserved in other eukaryotes, with 143 considered to be plant specific. C2H2-type ZFPs have been extensively studied and have been shown to play important roles in plant development and environmental stress responses by transcriptional regulation. The ethylene-responsive element binding-factor-associated amphiphilic repression (EAR) domain (GCC box) has been found to have a critical role in the tolerance response to abiotic stress. Many of the plant ZFPs containing the EAR domain, such as AZF1/2/3, ZAT7, ZAT10, and ZAT12, have been shown to function as transcriptional repressors. In this review, we mainly focus on the C1-2i subclass of C2H2 ZFPs and summarize the latest research into their roles in various stress responses. The role of C2H2-type ZFPs in response to the abiotic and biotic stress signaling network is not well explained, and amongst them, C1-2i is one of the better-characterized classifications in response to environmental stresses. These studies of the C1-2i subclass ought to furnish the basis for future studies to discover the pathways and receptors concerned in stress defense. Research has implied possible protein-protein interactions between members of C1-2i under various stresses, for which we have proposed a hypothetical model.

A C2H2 zinc-finger protein OsZFP213 interacts with OsMAPK3 to enhance salt tolerance in rice

Improvement of salt tolerance is one of the major targets in rice breeding. Here, we report that the zinc-finger protein (ZFP) OsZFP213 functions in enhancing salt tolerance in rice. OsZFP213 is localized in the nucleus and has transactivation activity. Transgenic rice overexpressing OsZFP213 showed enhanced salt tolerance compared with wild type and OsZFP213 RNAi plants. Furthermore, OsZFP213 overexpression plants showed higher transcription levels of antioxidant system genes and higher catalytic activity of scavenging enzymes of reactive oxygen, such as superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), and glutathione reductase (GR), and a lower level of ROS accumulation than that in wild type and OsZFP213 RNAi plants under salt treatment. Yeast two-hybrid, pull-down, and BiFC analysis showed that OsMAPK3 is a direct partner of OsZFP213, and this interaction enhanced the transactivation activity of OsZFP213. Taken together, these results suggest that OsZFP213 cooperates with OsMAPK3 in the regulation of rice salt stress tolerance by enhancing the ability of scavenging reactive oxygen.

The Quest for MAP Kinase Substrates: Gaining Momentum

Mitogen-activated protein kinase (MAPK) pathways are versatile signaling mechanisms in all eukaryotes. Their signaling outputs are defined by the protein substrates phosphorylated by MAPKs. An expanding list of substrates has been identified by high-throughput screens and targeted approaches in plants. The majority of these are phosphorylated by MPK3/6, and a few by MPK4, which are the best-characterized plant MAPKs, participating in the regulation of numerous biological processes. The identified substrates clearly represent the functional diversity of MAPKs: they are associated with pathogen defense, abiotic stress responses, ethylene signaling, and various developmental functions. Understanding their outputs is integral to unraveling the complex regulatory mechanisms of MAPK cascades. We review here methodological approaches and provide an overview of known MAPK substrates.

GROWTH-REGULATING FACTOR 9 negatively regulates arabidopsis leaf growth by controlling ORG3 and restricting cell proliferation in leaf primordia

Leaf growth is a complex process that involves the action of diverse transcription factors (TFs) and their downstream gene regulatory networks. In this study, we focus on the functional characterization of the Arabidopsis thaliana TF GROWTH-REGULATING FACTOR9 (GRF9) and demonstrate that it exerts its negative effect on leaf growth by activating expression of the bZIP TF OBP3-RESPONSIVE GENE 3 (ORG3). While grf9 knockout mutants produce bigger incipient leaf primordia at the shoot apex, rosette leaves and petals than the wild type, the sizes of those organs are reduced in plants overexpressing GRF9 (GRF9ox). Cell measurements demonstrate that changes in leaf size result from alterations in cell numbers rather than cell sizes. Kinematic analysis and 5-ethynyl-2'-deoxyuridine (EdU) incorporation assay revealed that GRF9 restricts cell proliferation in the early developing leaf. Performing in vitro binding site selection, we identified the 6-base motif 5'-CTGACA-3' as the core binding site of GRF9. By global transcriptome profiling, electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) we identified ORG3 as a direct downstream, and positively regulated target of GRF9. Genetic analysis of grf9 org3 and GRF9ox org3 double mutants reveals that both transcription factors act in a regulatory cascade to control the final leaf dimensions by restricting cell number in the developing leaf.

Photosynthetic response to increased irradiance correlates to variation in transcriptional response of lipid-remodeling and heat-shock genes

Plants have evolved several mechanisms for sensing increased irradiance, involving signal perception by photoreceptors (cryptochromes), and subsequent biochemical (reactive oxygen species, ROS) and metabolic clues to transmit the signals. This results in the increased expression of heat-shock response genes and of the transcription factor LONG HYPOCOTYL 5 (HY5, mediated by the cryptochrome photoreceptor 1, CRY1). Here, we show the existence of another response pathway in Arabidopsis. This pathway evokes the SPX1-mediated expression activation of the transcription factor PHR1 and leads to the expression of several galactolipid biosynthesis genes. Gene expression analysis of accessions Col-0, Ga-0, and Ts-1, showed activated expression of the SPX1/PHR1-mediated gene expression activation pathway acting on galactolipids biosynthesis genes in both Ga-0 and Col-0, but not in Ts-1. The activation of the SPX1/PHR1-mediated response pathway can be associated with lower photosynthesis efficiency in Ts-1, compared to Col-0 and Ga-0. Besides the accession-associated activation of the SPX1/PHR1-mediated response pathway, comparing gene expression in the accessions showed stronger activation of several heat responsive genes in Ga-0, and the opposite in Ts-1, when compared to Col-0, in line with the differences in their efficiency of photosynthesis. We conclude that natural variation in activation of both heat responsive genes and of galactolipids biosynthesis genes contribute to the variation in photosynthesis efficiency in response to irradiance increase.

Traversing the Links between Heavy Metal Stress and Plant Signaling

Plants confront multifarious environmental stresses widely divided into abiotic and biotic stresses, of which heavy metal stress represents one of the most damaging abiotic stresses. Heavy metals cause toxicity by targeting crucial molecules and vital processes in the plant cell. One of the approaches by which heavy metals act in plants is by over production of reactive oxygen species (ROS) either directly or indirectly. Plants act against such overdose of metal in the environment by boosting the defense responses like metal chelation, sequestration into vacuole, regulation of metal intake by transporters, and intensification of antioxidative mechanisms. This response shown by plants is the result of intricate signaling networks functioning in the cell in order to transmit the extracellular stimuli into an intracellular response. The crucial signaling components involved are calcium signaling, hormone signaling, and mitogen activated protein kinase (MAPK) signaling that are discussed in this review. Apart from signaling components other regulators like microRNAs and transcription factors also have a major contribution in regulating heavy metal stress. This review demonstrates the key role of MAPKs in synchronously controlling the other signaling components and regulators in metal stress. Further, attempts have been made to focus on metal transporters and chelators that are regulated by MAPK signaling.

Phosphorylation of the transcriptional repressor MYB15 by mitogen-activated protein kinase 6 is required for freezing tolerance in Arabidopsis

The expression of CBF (C-repeat-binding factor) genes is required for freezing tolerance in Arabidopsis thaliana. CBFs are positively regulated by INDUCER OF CBF EXPRESSION1 (ICE1) and negatively regulated by MYB15. These transcription factors directly interact with specific elements in the CBF promoters. Mitogen-activated protein kinase (MAPK/MPK) cascades function upstream to regulate CBFs. However, the mechanism by which MPKs control CBF expression during cold stress signaling remains unknown. This study showed that the activity of MYB15, a transcriptional repressor of cold signaling, is regulated by MPK6-mediated phosphorylation. MYB15 specifically interacts with MPK6, and MPK6 phosphorylates MYB15 on Ser168. MPK6-induced phosphorylation reduced the affinity of MYB15 binding to the CBF3 promoter and mutation of its phosphorylation site (MYB15S168A) enhanced the transcriptional repression of CBF3 by MYB15. Furthermore, transgenic plants overexpressing MYB15S168A showed significantly reduced CBF transcript levels in response to cold stress, compared with plants overexpressing MYB15. The MYB15S168A-overexpressing plants were also more sensitive to freezing than MYB15-overexpressing plants. These results suggest that MPK6-mediated regulation of MYB15 plays an important role in cold stress signaling in Arabidopsis.

Arg156 in the AP2-Domain Exhibits the Highest Binding Activity among the 20 Individuals to the GCC Box in BnaERF-B3-hy15, a Mutant ERF Transcription Factor from Brassica napus

To develop mutants of the ERF factor with more binding activities to the GCC box, we performed in vitro directed evolution by using DNA shuffling and screened mutants through yeast one-hybrid assay. Here, a series of mutants were obtained and used to reveal key amino acids that induce changes in the DNA binding activity of the BnaERF-B3 protein. With the BnaERF-B3-hy15 as the template, we produced 12 mutants which host individual mutation of potential key residues. We found that amino acid 156 is the key site, and the other 18 mutants host the 18 corresponding individual amino acid residues at site 156. Among the 20 individuals comprising WT (Gly156), Mu3 (Arg156), and 18 mutants with other 18 amino acid residues, Arg156 in the AP2-domain is the amino acid residue with the highest binding activity to the GCC box. The structure of the α-helix in the AP2-domain affects the binding activity. Other residues within AP2-domain modulated binding activity of ERF protein, suggesting that these positions are important for binding activity. Comparison of the mutant and wild-type transcription factors revealed the relationship of protein function and sequence modification. Our result provides a potential useful resource for understanding the trans-activation of ERF proteins.

Novel connections in plant organellar signalling link different stress responses and signalling pathways

To coordinate growth, development and responses to environmental stimuli, plant cells need to communicate the metabolic state between different sub-compartments of the cell. This requires signalling pathways, including protein kinases, secondary messengers such as Ca(2+) ions or reactive oxygen species (ROS) as well as metabolites and plant hormones. The signalling networks involved have been intensively studied over recent decades and have been elaborated more or less in detail. However, it has become evident that these signalling networks are also tightly interconnected and often merge at common targets such as a distinct group of transcription factors, most prominently ABI4, which are amenable to regulation by phosphorylation, potentially also in a Ca(2+)- or ROS-dependent fashion. Moreover, the signalling pathways connect several organelles or subcellular compartments, not only in functional but also in physical terms, linking for example chloroplasts to the nucleus or peroxisomes to chloroplasts thereby enabling physical routes for signalling by metabolite exchange or even protein translocation. Here we briefly discuss these novel findings and try to connect them in order to point out the remaining questions and emerging developments in plant organellar signalling.

Phosphoproteomics Profiling of Tobacco Mature Pollen and Pollen Activated in vitro

Tobacco mature pollen has extremely desiccated cytoplasm, and is metabolically quiescent. Upon re-hydration it becomes metabolically active and that results in later emergence of rapidly growing pollen tube. These changes in cytoplasm hydration and metabolic activity are accompanied by protein phosphorylation. In this study, we subjected mature pollen, 5-min-activated pollen, and 30-min-activated pollen to TCA/acetone protein extraction, trypsin digestion and phosphopeptide enrichment by titanium dioxide. The enriched fraction was subjected to nLC-MS/MS. We identified 471 phosphopeptides that carried 432 phosphorylation sites, position of which was exactly matched by mass spectrometry. These 471 phosphopeptides were assigned to 301 phosphoproteins, because some proteins carried more phosphorylation sites. Of the 13 functional groups, the majority of proteins were put into these categories: transcription, protein synthesis, protein destination and storage, and signal transduction. Many proteins were of unknown function, reflecting the fact that male gametophyte contains many specific proteins that have not been fully functionally annotated. The quantitative data highlighted the dynamics of protein phosphorylation during pollen activation; the identified phosphopeptides were divided into seven groups based on the regulatory trends. The major group comprised mature pollen-specific phosphopeptides that were dephosphorylated during pollen activation. Several phosphopeptides representing the same phosphoprotein had different regulation, which pinpointed the complexity of protein phosphorylation and its clear functional context. Collectively, we showed the first phosphoproteomics data on activated pollen where the position of phosphorylation sites was clearly demonstrated and regulatory kinetics was resolved.

A rice LSD1-like-type ZFP gene OsLOL5 enhances saline-alkaline tolerance in transgenic Arabidopsis thaliana, yeast and rice

Background

Zinc finger proteins (ZFPs) play an important role in regulating plant responses to abiotic stress. However, little is known about the function of LSD1-like-type ZFP in saline-alkaline (SA) stress resistance of rice. In this study, OsLOL5 (GenBank No. AJ620677), containing two LSD1-like-type C2C2 domains, was isolated and analyzed its protection roles in transgenic plants and yeast. OsLOL5 was located in the nucleus as evidenced by the bombardment of onion epidermal cells.

Results

OsLOL5 expression significantly increased in rice leaves and roots under 150 mmol L-1 NaCl, 30 mM NaHCO₃, and 10 mmol L-1 H₂O₂ treatment, respectively. Overexpression of OsLOL5 in yeast resulted in SA tolerance at significant level. Transgenic Arabidopsis plants overexpressing OsLOL5 grew well in the presence ofboth NaCl and NaHCO₃ treatments, whereas wild-type plants exhibited chlorosis, stunted growth phenotype, and even death. SA stress caused significant changes in the malondialdehyde (MDA) contents in non-transgenic plants compared with those in transgenic lines. Transgenic rice overexpressing OsLOL5 exhibited stronger resistance than NT under NaHCO₃ treatment, as demonstrated by its greater shoot length, and fresh weight. The genes associated with oxidative stress, such as OsAPX2, OsCAT, OsCu/Zn-SOD, and OsRGRC2, were significantly upregulated in OsLOL5-overexpressing rice. The results suggested that OsLOL5 improved SA tolerance in plants, and regulated oxidative and salinity stress retardation via the active oxygen detoxification pathway.

Conclusions

The yeast INVScI bacterium grew significantly better than the control strain under NaCl, NaHCO₃, and H₂O₂ treatments. These findings illustrated that OsLOL5 overexpression enhanced yeast resistance for SA stress through active oxygen species. The present study showed that the OsLOL5 genes involved in the ROS signaling pathways may combine with the model plant Arabidopsis and rice in LDS1-type ZFP by ROS signaling pathways that regulate cell necrosis. We speculated that the OsLOL5 active oxygen scavenging system may have coordinating roles. The present study further revealed that OsLOL5 ZFP could regulate oxidative stress function, but could also provide a basis for salt-resistant rice strains.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2460-5) contains supplementary material, which is available to authorized users.

ZmABA2, an interacting protein of ZmMPK5, is involved in abscisic acid biosynthesis and functions

In maize (Zea mays), the mitogen-activated protein kinase ZmMPK5 has been shown to be involved in abscisic acid (ABA)-induced antioxidant defence and to enhance the tolerance of plants to drought, salt stress and oxidative stress. However, the underlying molecular mechanisms are poorly understood. Here, using ZmMPK5 as bait in yeast two-hybrid screening, a protein interacting with ZmMPK5 named ZmABA2, which belongs to a member of the short-chain dehydrogenase/reductase family, was identified. Pull-down assay and bimolecular fluorescence complementation analysis and co-immunoprecipitation test confirmed that ZmMPK5 interacts with ZmABA2 in vitro and in vivo. Phosphorylation of Ser173 in ZmABA2 by ZmMPK5 was shown to increase the activity of ZmABA2 and the protein stability. Various abiotic stimuli induced the expression of ZmABA2 in leaves of maize plants. Pharmacological, biochemical and molecular biology and genetic analyses showed that both ZmMPK5 and ZmABA2 coordinately regulate the content of ABA. Overexpression of ZmABA2 in tobacco plants was found to elevate the content of ABA, regulate seed germination and root growth under drought and salt stress and enhance the tolerance of tobacco plants to drought and salt stress. These results suggest that ZmABA2 is a direct target of ZmMPK5 and is involved in ABA biosynthesis and functions.

Signaling mechanisms in pattern-triggered immunity (PTI)

In nature, plants constantly have to face pathogen attacks. However, plant disease rarely occurs due to efficient immune systems possessed by the host plants. Pathogens are perceived by two different recognition systems that initiate the so-called pattern-triggered immunity (PTI) and effector-triggered immunity (ETI), both of which are accompanied by a set of induced defenses that usually repel pathogen attacks. Here we discuss the complex network of signaling pathways occurring during PTI, focusing on the involvement of mitogen-activated protein kinases.

Modes of MAPK substrate recognition and control

Mitogen-activated protein kinase (MAPK) cascades are universal, evolutionary conserved signalling modules, which translate environmental information into appropriate responses via phosphorylation of their substrate proteins. In Arabidopsis, the MAPK MPK3 regulates numerous cellular processes, including the adaptation to abiotic and biotic stresses. The molecular steps immediately downstream of MPK3 induction have, therefore, received abundant attention, and a respectable number of MPK3 targets are known by now. These proteins illustrate the substrate promiscuity of MPK3. They also are evidence for how manifold phosphorylation-regulated functions can be. This review presents the current knowledge about the function and regulation of MPK3-targeted proteins, takes a close look at their primary protein sequences, and proposes a model of how MPK3 recognises, binds, and phosphorylates its targets.

A mitogen-activated protein kinase cascade module, MKK3-MPK6 and MYC2, is involved in blue light-mediated seedling development in Arabidopsis

Mitogen-activated protein kinase (MAPK) pathways are involved in several signal transduction processes in eukaryotes. Light signal transduction pathways have been extensively studied in plants; however, the connection between MAPK and light signaling pathways is currently unknown. Here, we show that MKK3-MPK6 is activated by blue light in a MYC2-dependent manner. MPK6 physically interacts with and phosphorylates a basic helix-loop-helix transcription factor, MYC2, and is phosphorylated by a MAPK kinase, MKK3. Furthermore, MYC2 binds to the MPK6 promoter and regulates its expression in a feedback regulatory mechanism in blue light signaling. We present mutational and physiological studies that illustrate the function of the MKK3-MPK6-MYC2 module in Arabidopsis thaliana seedling development and provide a revised mechanistic view of photomorphogenesis.

Positive and negative roles for soybean MPK6 in regulating defense responses

It has been well established that MPK6 is a positive regulator of defense responses in model plants such as Arabidopsis and tobacco. However, the functional importance of soybean MPK6 in disease resistance has not been investigated. Here, we showed that silencing of GmMPK6 in soybean using virus-induced gene silencing mediated by Bean pod mottle virus (BPMV) caused stunted growth and spontaneous cell death on the leaves, a typical phenotype of activated defense responses. Consistent with this phenotype, expression of pathogenesis-related (PR) genes and the conjugated form of salicylic acid were significantly increased in GmMPK6-silenced plants. As expected, GmMPK6-silenced plants were more resistant to downy mildew and Soybean mosaic virus compared with vector control plants, indicating a negative role of GmMPK6 in disease resistance. Interestingly, overexpression of GmMPK6, either transiently in Nicotiana benthamiana or stably in Arabidopsis, resulted in hypersensitive response (HR)-like cell death. The HR-like cell death was accompanied by increased PR gene expression, suggesting that GmMPK6, like its counterpart in other plant species, also plays a positive role in cell death induction and defense response. Using bimolecular fluorescence complementation analysis, we determined that GmMKK4 might function upstream of GmMPK6 and GmMKK4 could interact with GmMPK6 independent of its phosphorylation status. Taken together, our results indicate that GmMPK6 functions as both repressor and activator in defense responses of soybean.

The Cysteine2/Histidine2-Type Transcription Factor ZINC FINGER OF ARABIDOPSIS THALIANA6 Modulates Biotic and Abiotic Stress Responses by Activating Salicylic Acid-Related Genes and C-REPEAT-BINDING FACTOR Genes in Arabidopsis

The cysteine2/histidine2-type zinc finger proteins are a large family of transcription regulators, and some of them play essential roles in plant responses to biotic and abiotic stress. In this study, we found that expression of C₂H₂-type ZINC FINGER OF ARABIDOPSIS THALIANA6 (AtZAT6) was transcriptionally induced by salt, dehydration, cold stress treatments, and pathogen infection, and AtZAT6 was predominantly located in the nucleus. AtZAT6-overexpressing plants exhibited improved resistance to pathogen infection, salt, drought, and freezing stresses, while AtZAT6 knockdown plants showed decreased stress resistance. AtZAT6 positively modulates expression levels of stress-related genes by directly binding to the TACAAT motifs in the promoter region of pathogen-related genes (ENHANCED DISEASE SUSCEPTIBILITY1, PHYTOALEXIN DEFICIENT4, PATHOGENESIS-RELATED GENE1 [PR1], PR2, and PR5) and abiotic stress-responsive genes (C-REPEAT-BINDING FACTOR1 [CBF1], CBF2, and CBF3). Moreover, overexpression of AtZAT6 exhibited pleiotrophic phenotypes with curly leaves and small-sized plant at vegetative stage and reduced size of floral organs and siliques at the reproductive stage. Modulation of AtZAT6 also positively regulates the accumulation of salicylic acid and reactive oxygen species (hydrogen peroxide and superoxide radical). Taken together, our findings indicated that AtZAT6 plays important roles in plant development and positively modulates biotic and abiotic stress resistance by activating the expression levels of salicylic acid-related genes and CBF genes.

The heat shock factor A4A confers salt tolerance and is regulated by oxidative stress and the mitogen-activated protein kinases MPK3 and MPK6

Heat shock factors (HSFs) are principal regulators of plant responses to several abiotic stresses. Here, we show that estradiol-dependent induction of HSFA4A confers enhanced tolerance to salt and oxidative agents, whereas inactivation of HSFA4A results in hypersensitivity to salt stress in Arabidopsis (Arabidopsis thaliana). Estradiol induction of HSFA4A in transgenic plants decreases, while the knockout hsfa4a mutation elevates hydrogen peroxide accumulation and lipid peroxidation. Overexpression of HSFA4A alters the transcription of a large set of genes regulated by oxidative stress. In yeast (Saccharomyces cerevisiae) two-hybrid and bimolecular fluorescence complementation assays, HSFA4A shows homomeric interaction, which is reduced by alanine replacement of three conserved cysteine residues. HSFA4A interacts with mitogen-activated protein kinases MPK3 and MPK6 in yeast and plant cells. MPK3 and MPK6 phosphorylate HSFA4A in vitro on three distinct sites, serine-309 being the major phosphorylation site. Activation of the MPK3 and MPK6 mitogen-activated protein kinase pathway led to the transcriptional activation of the HEAT SHOCK PROTEIN17.6A gene. In agreement that mutation of serine-309 to alanine strongly diminished phosphorylation of HSFA4A, it also strongly reduced the transcriptional activation of HEAT SHOCK PROTEIN17.6A. These data suggest that HSFA4A is a substrate of the MPK3/MPK6 signaling and that it regulates stress responses in Arabidopsis.

Dominant repression by Arabidopsis transcription factor MYB44 causes oxidative damage and hypersensitivity to abiotic stress

In any living species, stress adaptation is closely linked with major changes of the gene expression profile. As a substrate protein of the rapidly stress-induced mitogen-activated protein kinase MPK3, Arabidopsis transcription factor MYB44 likely acts at the front line of stress-induced re-programming. We recently characterized MYB44 as phosphorylation-dependent positive regulator of salt stress signaling. Molecular events downstream of MYB44 are largely unknown. Although MYB44 binds to the MBSII element in vitro, it has no discernible effect on MBSII-driven reporter gene expression in plant co-transfection assays. This may suggest limited abundance of a synergistic co-regulator. MYB44 carries a putative transcriptional repression (Ethylene responsive element binding factor-associated Amphiphilic Repression, EAR) motif. We employed a dominant repressor strategy to gain insights into MYB44-conferred stress resistance. Overexpression of a MYB44-REP fusion markedly compromised salt and drought stress tolerance—the opposite was seen in MYB44 overexpression lines. MYB44-mediated resistance likely results from induction of tolerance-enhancing, rather than from repression of tolerance-diminishing factors. Salt stress-induced accumulation of destructive reactive oxygen species is efficiently prevented in transgenic MYB44, but accelerated in MYB44-REP lines. Furthermore, heterologous overexpression of MYB44-REP caused tissue collapse in Nicotiana. A mechanistic model of MAPK-MYB-mediated enhancement in the antioxidative capacity and stress tolerance is proposed. Genetic engineering of MYB44 variants with higher trans-activating capacity may be a means to further raise stress resistance in crops.

Post-translational regulation of cold acclimation response

Cold acclimation is an adaptive response whereby plants from temperate regions increase their capacity to tolerate freezing in response to low-nonfreezing temperatures. Numerous studies have unveiled the large transcriptome re-programming that takes place during cold acclimation in diverse species, and a number of proteins have been identified as important regulators of this adaptive response. Post-translational mechanisms regulating the function of proteins involved in cold acclimation have been, however, much less studied. Several components of the signal transduction pathways mediating cold response have been described to be post-translationally modified. These post-translational modifications, including protein phosphorylation and dephosphorylation, ubiquitination, SUMOylation, N-glycosylation and lipid modification, determine key aspects of protein function such as sub-cellular localization, stability, activity or ability to interact with other proteins. Integrating these post-translational mechanisms within the appropriate spatio-temporal context of cold acclimation is essential to develop new crops with improved cold tolerance. Here, we review available evidence regarding the post-translational regulation of cold acclimation, discuss its relevance for the accurate development of this response, and highlight significant missing data.

The C2H2-type zinc finger protein ZFP182 is involved in abscisic acid-induced antioxidant defense in rice

C(2) H(2) -type zinc finger proteins (ZFPs) are thought to play important roles in modulating the responses of plants to drought, salinity and oxidative stress. However, direct evidence is lacking for the involvement of these ZFPs in abscisic acid (ABA)-induced antioxidant defense in plants. In this study, the role of the rice (Oryza sativa L. sub. japonica cv. Nipponbare) C(2) H(2) -type ZFP ZFP182 in ABA-induced antioxidant defense and the relationship between ZFP182 and two rice MAPKs, OsMPK1 and OsMPK5 in ABA signaling were investigated. ABA treatment induced the increases in the expression of ZFP182, OsMPK1 and OsMPK5, and the activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) in rice leaves. The transient gene expression analysis and the transient RNA interference (RNAi) analysis in protoplasts showed that ZFP182, OsMPK1 and OsMPK5 are involved in ABA-induced up-regulation in the activities of SOD and APX. Besides, OsMPK1 and OsMPK5 were shown to be required for the up-regulation in the expression of ZFP182 in ABA signaling, but ZFP182 did not mediate the ABA-induced up-regulation in the expression of OsMPK1 and OsMPK5. These results indicate that ZFP182 is required for ABA-induced antioxidant defense and the expression of ZFP182 is regulated by rice MAPKs in ABA signaling.