A novel transcription factor, ERD15 (Early Responsive to Dehydration 15), connects endoplasmic reticulum stress with an osmotic stress-induced cell death signal

Expansion and diversification of the Glycine max (Gm) ERD15-like subfamily of the PAM2-like superfamily

MAIN CONCLUSION

Despite modulating senescence and drought responses, the GmERD15-like subfamily members are differentially induced by multiple stresses and diverge partially in stress signaling functions. The PAM2 motif represents a binding site for poly (A)-binding proteins (PABPs), often associated with RNA metabolism regulation. The PAM2-containing protein ERD15 stands out as a critical regulator of diverse stress responses in plants. Despite the relevance of the PAM2 motif, a comprehensive analysis of the PAM2 superfamily and ERD15-like subfamily in the plant kingdom is lacking. Here, we provide an extensive in silico analysis of the PAM2 superfamily and the ERD15-like subfamily in soybean, using Arabidopsis and rice sequences as prototypes. The Glycine max ERD15-like subfamily members were clustered in pairs, likely originating from DNA-based gene duplication, as the paralogs display high sequence conservation, similar exon/intron genome organization, and are undergoing purifying selection. Complementation analyses of an aterd15 mutant demonstrated that the plant ERD15-like subfamily members are functionally redundant in response to drought, osmotic stress, and dark-induced senescence. Nevertheless, the soybean members displayed differential expression profiles, biochemical activity, and subcellular localization, consistent with functional diversification. The expression profiles of Glyma04G138600 under salicylic acid (SA) and abscisic acid (ABA) treatments differed oppositely from those of the other GmERD15-like genes. Abiotic stress-induced coexpression analysis with soybean PABPs showed that Glyma04G138600 was clustered separately from other GmERD15s. In contrast to the AtERD15 stress-induced nuclear redistribution, Glyma04G138600 and Glyma02G260800 localized to the cytoplasm, while Glyma03G131900 fractionated between the cytoplasm and nucleus under normal and stress conditions. These data collectively indicate that despite modulating senescence and drought responses, the GmERD15-like subfamily members are differentially induced by multiple stresses and may diverge partially in stress signaling functions.

Integrating GWAS with a gene co-expression network better prioritizes candidate genes associated with root metaxylem phenes in maize

Root metaxylems are phenotypically diverse structures whose function is particularly important under drought stress. Significant research has dissected the genetic machinery underlying metaxylem phenotypes in dicots, but that of monocots are relatively underexplored. In maize (Zea mays), a robust pipeline integrated a genome-wide association study (GWAS) of root metaxylem phenes under well-watered and water-stress conditions with a gene co-expression network to prioritize the strongest gene candidates. We identified 244 candidate genes by GWAS, of which 103 reside in gene co-expression modules most relevant to xylem development. Several candidate genes may be involved in biosynthetic processes related to the cell wall, hormone signaling, oxidative stress responses, and drought responses. Of those, six gene candidates were detected in multiple root metaxylem phenes in both well-watered and water-stress conditions. We posit that candidate genes that are more essential to network function based on gene co-expression (i.e., hubs or bottlenecks) should be prioritized and classify 33 essential genes for further investigation. Our study demonstrates a new strategy for identifying promising gene candidates and presents several gene candidates that may enhance our understanding of vascular development and responses to drought in cereals.

Unveiling the role of the ERD15 gene in wheat's tolerance to combined drought and salinity stress: a meta-analysis of QTL and RNA-Seq data

The coexistence of drought and salinity stresses in field conditions significantly hinders wheat (Triticum aestivum L.) productivity. Understanding the molecular mechanisms governing response and tolerance to these stresses is crucial for developing resilient wheat varieties. Our research, employing a combination of meta-QTL and meta-RNA-Seq transcriptome analyses, has uncovered the genome functional landscape of wheat in response to drought and salinity. We identified 118 meta-QTLs (MQTLs) distributed across all 21 wheat chromosomes, with ten designated as the most promising. Additionally, we found 690 meta-differentially expressed genes (mDEGs) shared between drought and salinity stress. Notably, our findings highlight the Early Responsive to Dehydration 15 (ERD15) gene, located in one of the most promising MQTLs, as a key gene in the shared gene network of drought and salinity stress. ERD15, differentially expressed between contrasting wheat genotypes under combined stress conditions, significantly regulates water relations, photosynthetic activity, antioxidant activity, and ion homeostasis. These findings not only provide valuable insights into the molecular genetic mechanisms underlying combined stress tolerance in wheat but also hold the potential to contribute significantly to the development of stress-resilient wheat varieties.

A conserved oomycete effector RxLR23 triggers plant defense responses by targeting ERD15La to release NbNAC68

Oomycete pathogens deliver many effectors to enhance virulence or suppress plant immunity. Plant immune networks are interconnected, in which a few effectors can trigger a strong defense response when recognized by immunity-related proteins. How effectors activate plant defense response remains poorly understood. Here we report Phytophthora capsici effector RxLR23KM can induce plant cell death and plant immunity. RxLR23KM specifically binds to ERD15La, a regulator of abscisic acid and salicylic acid pathway, and the binding intensity depends on the amino acid residues (K93 and M320). NbNAC68, a downstream protein of ERD15La, can stimulate plant immunity that is compromised after binding with ERD15La. Silencing of NbNAC68 substantially prevents the activation of plant defense response. RxLR23KM binds to ERD15La, releasing NbNAC68 to activate plant immunity. These findings highlight a strategy of plant defense response that ERD15La as a central regulator coordinates RxLR23KM to regulate NbNAC68-triggered plant immunity.

Characteristics analysis of Early Responsive to Dehydration genes in Arabidopsis thaliana (AtERD)

Early Responsive to Dehydration (ERD) genes are rapidly induced in response to various biotic and abiotic stresses, such as bacteria, drought, light, temperature and high salt in Arabidopsis thaliana. Sixteen ERD of Arabidopsis thaliana (AtERD) genes have been previously identified. The lengths of the coding region of the genes are 504-2838 bp. They encode 137-745 amino acids. In this study, the AtERD genes structure and promoter are analyzed through bioinformatics, and a overall function is summarized and a systematic signal pathway involving AtERD genes is mapped. AtERD9, AtERD11 and AtERD13 have the GST domain. AtERD10 and AtERD14 have the Dehyd domain. The promoters regions contain 32 light responsive elements, 23 ABA responsive elements, 5 drought responsive elements, 5 meristem expression related elements and 132 core promoter elements. The study provides a theoretical guidance for subsequent studies of AtERD genes.

Plasma Membrane Channel TRPM4 Mediates Immunogenic Therapy-Induced Necrosis

Several emerging therapies kill cancer cells primarily by inducing necrosis. As necrosis activates immune cells, potentially, uncovering the molecular drivers of anticancer therapy-induced necrosis could reveal approaches for enhancing immunotherapy efficacy. To identify necrosis-associated genes, we performed a genome-wide CRISPR-Cas9 screen with negative selection against necrosis-inducing preclinical agents BHPI and conducted follow-on experiments with ErSO. The screen identified transient receptor potential melastatin member 4 (TRPM4), a calcium-activated, ATP-inhibited, sodium-selective plasma membrane channel. Cancer cells selected for resistance to BHPI and ErSO exhibited robust TRPM4 downregulation, and TRPM4 reexpression restored sensitivity to ErSO. Notably, TRPM4 knockout (TKO) abolished ErSO-induced regression of breast tumors in mice. Supporting a broad role for TRPM4 in necrosis, knockout of TRPM4 reversed cell death induced by four additional diverse necrosis-inducing cancer therapies. ErSO induced anticipatory unfolded protein response (a-UPR) hyperactivation, long-term necrotic cell death, and release of damage-associated molecular patterns that activated macrophages and increased monocyte migration, all of which was abolished by TKO. Furthermore, loss of TRPM4 suppressed the ErSO-induced increase in cell volume and depletion of ATP. These data suggest that ErSO triggers initial activation of the a-UPR but that it is TRPM4-mediated sodium influx and cell swelling, resulting in osmotic stress, which sustains and propagates lethal a-UPR hyperactivation. Thus, TRPM4 plays a pivotal role in sustaining lethal a-UPR hyperactivation that mediates the anticancer activity of diverse necrosis-inducing therapies.

SIGNIFICANCE

A genome-wide CRISPR screen reveals a pivotal role for TRPM4 in cell death and immune activation following treatment with diverse necrosis-inducing anticancer therapies, which could facilitate development of necrosis-based cancer immunotherapies.

Comparative Analysis of Antioxidant System and Salt-Stress Tolerance in Two Hibiscus Cultivars Exposed to NaCl Toxicity

Hibiscus (Hibiscus syriacus L.) is known as a horticultural plant of great ornamental and medicinal value. However, the effect of NaCl stress on hibiscus seedlings is unclear. Little is known about H. syriacus 'Duede Brabaul' (DB) and H. syriacus 'Blueberry Smoothie' (BS). Here, the effects of solutions with different concentrations of NaCl on the organic osmolytes, ion accumulation, and antioxidant enzyme activity of hibiscus seedling leaves were determined. The results showed that the Na⁺/K⁺ ratio was imbalanced with increasing NaCl concentration, especially in BS (range 34% to 121%), which was more sensitive than DB (range 32% to 187%) under NaCl concentrations of 50 to 200 mM. To cope with the osmotic stress, the content of organic osmolytes increased significantly. Additionally, NaCl stress caused a large increase in O₂·^- and H₂O₂, and other reactive oxygen species (ROS), and antioxidant enzyme activity was significantly increased to remove excess ROS. The expression level of genes related to salt tolerance was significantly higher in DB than that in BS under different NaCl concentrations. Taken together, DB possessed a stronger tolerance to salt stress and the results suggest membrane stability, Na⁺/K⁺, H₂O₂, catalase and ascorbate peroxidase as salt tolerance biomarkers that can be used for gene transformation and breeding in future hibiscus research.

Genome-Wide Characterization and Function Analysis of ZmERD15 Genes' Response to Saline Stress in Zea mays L

Early responsive dehydration (ERD) genes can be rapidly induced by dehydration. ERD15 genes have been confirmed to regulate various stress responses in plants. However, the maize ERD15 members have not been characterized. In the present study, a total of five ZmERD15 genes were identified from the maize genome and named ZmERD15a, ZmERD15b, ZmERD15c, ZmERD15d, and ZmERD15e. Subsequently, their protein properties, gene structure and duplication, chromosomal location, cis-acting elements, subcellular localization, expression pattern, and over-expression in yeast were analyzed. The results showed that the ZmERD15 proteins were characterized by a similar size (113-159 aa) and contained a common domain structure, with PAM2 and adjacent PAE1 motifs followed by an acidic region. The ZmERD15 proteins exhibited a close phylogenetic relationship with OsERD15s from rice. Five ZmERD15 genes were distributed on maize chromosomes 2, 6, 7, and 9 and showed a different exon-intron organization and were expanded by duplication. Besides, the promoter region of the ZmERD15s contained abundant cis-acting elements that are known to be responsive to stress and hormones. Subcellular localization showed that ZmERD15b and ZmERD15c were localized in the nucleus. ZmERD15a and ZmERD15e were localized in the nucleus and cytoplasm. ZmERD15d was localized in the nucleus and cell membrane. The results of the quantitative real-time PCR (qRT-PCR) showed that the expression of the ZmERD15 genes was regulated by PEG, salinity, and ABA. The heterologous expression of ZmERD15a, ZmERD15b, ZmERD15c, and ZmERD15d significantly enhanced salt tolerance in yeast. In summary, a comprehensive analysis of ZmERD15s was conducted in the study. The results will provide insights into further dissecting the biological function and molecular mechanism of ZmERD15s regulating of the stress response in maize.

Genome-Wide Prediction and Analysis of Oryza Species NRP Genes in Rice Blast Resistance

Members of the N-rich proteins (NRPs) gene family play important roles in the plant endoplasmic reticulum stress in response, which can be triggered by plant pathogens’ infection. Previous studies of the NRP gene family have been limited to only a few plants, such as soybean and Arabidopsis thaliana. Thus, their evolutionary characteristics in the Oryza species and biological functions in rice defense against the pathogenic fungus Magnaporthe oryzae have remained unexplored. In the present study, we demonstrated that the NRP genes family may have originated in the early stages of plant evolution, and that they have been strongly conserved during the evolution of the Oryza species. Domain organization of NRPs was found to be highly conserved within but not between subgroups. OsNRP1, an NRP gene in the Oryza sativa japonica group, was specifically up-regulated during the early stages of rice-M. oryzae interactions-inhibited M. oryzae infection. Predicted protein-protein interaction networks and transcription-factor binding sites revealed a candidate interactor, bZIP50, which may be involved in OsNRP1-mediated rice resistance against M. oryzae infection. Taken together, our results established a basis for future studies of the NRP gene family and provided molecular insights into rice immune responses to M. oryzae.

A Phakopsora pachyrhizi Effector Suppresses PAMP-Triggered Immunity and Interacts with a Soybean Glucan Endo-1,3-β-Glucosidase to Promote Virulence

Asian soybean rust, caused by the fungus Phakopsora pachyrhizi, is one of the most important diseases affecting soybean production in tropical areas. During infection, P. pachyrhizi secretes proteins from haustoria that are transferred into plant cells to promote virulence. To date, only one candidate P. pachyrhizi effector protein has been characterized in detail to understand the mechanism by which it suppresses plant defenses to enhance infection. Here, we aimed to extend understanding of the pathogenic mechanisms of P. pachyrhizi based on the discovery of host proteins that interact with the effector candidate Phapa-7431740. We demonstrated that Phapa-7431740 suppresses pathogen-associated molecular pattern-triggered immunity (PTI) and that it interacts with a soybean glucan endo-1,3-β-glucosidase (GmβGLU), a pathogenesis-related (PR) protein belonging to the PR-2 family. Structural and phylogenetic characterization of the PR-2 protein family predicted in the soybean genome and comparison to PR-2 family members in Arabidopsis thaliana and cotton, demonstrated that GmβGLU is a type IV β-1,3-glucanase. Transcriptional profiling during an infection time course showed that the GmβGLU mRNA is highly induced during the initial hours after infection, coinciding with peak of expression of Phapa-7431740. The effector was able to interfere with the activity of GmβGLU in vitro, with a dose-dependent inhibition. Our results suggest that Phapa-7431740 may suppress PTI by interfering with glucan endo-1,3-β-glucosidase activity. [Formula: see text] The author(s) have dedicated the work to the public domain under the Creative Commons CC0 "No Rights Reserved" license by waiving all of his or her rights to the work worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law, 2022.

Abiotic Stresses in Plants and Their Markers: A Practice View of Plant Stress Responses and Programmed Cell Death Mechanisms

Understanding how plants cope with stress and the intricate mechanisms thereby used to adapt and survive environmental imbalances comprise one of the most powerful tools for modern agriculture. Interdisciplinary studies suggest that knowledge in how plants perceive, transduce and respond to abiotic stresses are a meaningful way to design engineered crops since the manipulation of basic characteristics leads to physiological remodeling for plant adaption to different environments. Herein, we discussed the main pathways involved in stress-sensing, signal transduction and plant adaption, highlighting biochemical, physiological and genetic events involved in abiotic stress responses. Finally, we have proposed a list of practice markers for studying plant responses to multiple stresses, highlighting how plant molecular biology, phenotyping and genetic engineering interconnect for creating superior crops.

Cadmium-mediated toxicity in plant cells is associated with the DCD/NRP-mediated cell death response

Cadmium (Cd2+ ) is highly harmful to plant growth. Although Cd2+ induces programmed cell death (PCD) in plant cells, Cd2+ stress in whole plants during later developmental stages and the mechanism underlying Cd2+ -mediated toxicity are poorly understood. Here, we showed that Cd2+ limits plant growth, causes intense redness in leaf vein, leaf yellowing, and chlorosis during the R1 reproductive stage of soybean (Glycine max). These symptoms were associated with Cd2+ -induced PCD, as Cd2+ -stressed soybean leaves displayed decreased number of nuclei, enhanced cell death, DNA damage, and caspase 1 activity compared to unstressed leaves. Accordingly, Cd2+ -induced NRPs, GmNAC81, GmNAC30 and VPE, the DCD/NRP-mediated cell death signalling components, which execute PCD via caspase 1-like VPE activity. Furthermore, overexpression of the positive regulator of this cell death signalling GmNAC81 enhanced sensitivity to Cd2+ stress and intensified the hallmarks of Cd2+ -mediated PCD. GmNAC81 overexpression enhanced Cd2+ -induced H2 O2 production, cell death, DNA damage, and caspase-1-like VPE expression. Conversely, BiP overexpression negatively regulated the NRPs/GmNACs/VPE signalling module, conferred tolerance to Cd2+ stress and reduced Cd2+ -mediated cell death. Collectively, our data indicate that Cd2+ induces PCD in plants via activation of the NRP/GmNAC/VPE regulatory circuit that links developmentally and stress-induced cell death.

Abscisic acid employs NRP-dependent PIN2 vacuolar degradation to suppress auxin-mediated primary root elongation in Arabidopsis

How plants balance growth and stress adaptation is a long-standing topic in plant biology. Abscisic acid (ABA) induces the expression of the stress-responsive Asparagine Rich Protein (NRP), which promotes the vacuolar degradation of PP6 phosphatase FyPP3, releasing ABI5 transcription factor to initiate transcription. Whether NRP is required for growth remains unknown. We generated an nrp1 nrp2 double mutant, which had a dwarf phenotype that can be rescued by inhibiting auxin transport. Insufficient auxin in the transition zone and over-accumulation of auxin at the root tip was responsible for the short elongation zone and short-root phenotype of nrp1 nrp2. The auxin efflux carrier PIN2 over-accumulated in nrp1 nrp2 and became de-polarized at the plasma membrane, leading to slower root basipetal auxin transport. Knock-out of PIN2 suppressed the dwarf phenotype of nrp1 nrp2. Furthermore, ABA can induce NRP-dependent vacuolar degradation of PIN2 to inhibit primary root elongation. FyPP3 also is required for NRP-mediated PIN2 turnover. In summary, in growth condition, NRP promotes PIN2 vacuolar degradation to help maintain PIN2 protein concentration and polarity, facilitating the establishment of the elongation zone and primary root elongation. When stressed, ABA employs this pathway to inhibit root elongation for stress adaptation.

Cell Death Signaling From Endoplasmic Reticulum Stress: Plant-Specific and Conserved Features

The endoplasmic reticulum (ER) stress response is triggered by any condition that disrupts protein folding and promotes the accumulation of unfolded proteins in the lumen of the organelle. In eukaryotic cells, the evolutionarily conserved unfolded protein response is activated to clear unfolded proteins and restore ER homeostasis. The recovery from ER stress is accomplished by decreasing protein translation and loading into the organelle, increasing the ER protein processing capacity and ER-associated protein degradation activity. However, if the ER stress persists and cannot be reversed, the chronically prolonged stress leads to cellular dysfunction that activates cell death signaling as an ultimate attempt to survive. Accumulating evidence implicates ER stress-induced cell death signaling pathways as significant contributors for stress adaptation in plants, making modulators of ER stress pathways potentially attractive targets for stress tolerance engineering. Here, we summarize recent advances in understanding plant-specific molecular mechanisms that elicit cell death signaling from ER stress. We also highlight the conserved features of ER stress-induced cell death signaling in plants shared by eukaryotic cells.

Stress response proteins NRP1 and NRP2 are pro-survival factors that inhibit cell death during ER stress

Environmental stresses cause an increased number of unfolded or misfolded proteins to accumulate in the endoplasmic reticulum (ER), resulting in ER stress. To restore ER homeostasis and survive, plants initiate an orchestrated signaling pathway known as the unfolded protein response (UPR). Asparagine-rich protein (NRP) 1 and NRP2, two homologous proteins harboring a Development and Cell Death domain, are associated with various stress responses in Arabidopsis (Arabidopsis thaliana), but the relevant molecular mechanism remains obscure. Here, we show that NRP1 and NRP2 act as key pro-survival factors during the ER stress response and that they inhibit cell death. Loss-of-function of NRP1 and NRP2 results in decreased tolerance to the ER stress inducer tunicamycin (TM), accelerating cell death. NRP2 is constitutively expressed while NRP1 is induced in plants under ER stress. In Arabidopsis, basic leucine zipper protein (bZIP) 28 and bZIP60 are important transcription factors in the UPR that activates the expression of many ER stress-related genes. Notably, under ER stress, bZIP60 activates NRP1 by directly binding to the UPRE-I element in the NRP1 promoter. These findings reveal a pro-survival strategy in plants wherein the bZIP60-NRPs cascade suppresses cell death signal transmission, improving survival under adverse conditions.

Senescence-Associated Glycine max (Gm)NAC Genes: Integration of Natural and Stress-Induced Leaf Senescence

Leaf senescence is a genetically regulated developmental process that can be triggered by a variety of internal and external signals, including hormones and environmental stimuli. Among the senescence-associated genes controlling leaf senescence, the transcriptional factors (TFs) comprise a functional class that is highly active at the onset and during the progression of leaf senescence. The plant-specific NAC (NAM, ATAF, and CUC) TFs are essential for controlling leaf senescence. Several members of Arabidopsis AtNAC-SAGs are well characterized as players in elucidated regulatory networks. However, only a few soybean members of this class display well-known functions; knowledge about their regulatory circuits is still rudimentary. Here, we describe the expression profile of soybean GmNAC-SAGs upregulated by natural senescence and their functional correlation with putative AtNAC-SAGs orthologs. The mechanisms and the regulatory gene networks underlying GmNAC081- and GmNAC030-positive regulation in leaf senescence are discussed. Furthermore, new insights into the role of GmNAC065 as a negative senescence regulator are presented, demonstrating extraordinary functional conservation with the Arabidopsis counterpart. Finally, we describe a regulatory circuit which integrates a stress-induced cell death program with developmental leaf senescence via the NRP-NAC-VPE signaling module.

Natural variation in the promoter of GsERD15B affects salt tolerance in soybean

Salt stress has detrimental effects on crop growth and yield, and the area of salt-affected land is increasing. Soybean is a major source of vegetable protein, oil and feed, but considered as a salt-sensitive crop. Cultivated soybean (Glycine max) is domesticated from wild soybean (G. soja) but lost considerable amount of genetic diversity during the artificial selection. Therefore, it is important to exploit the gene pool of wild soybean. In this study, we identified 34 salt-tolerant accessions from wild soybean germplasm and found that a 7-bp insertion/deletion (InDel) in the promoter of GsERD15B (early responsive to dehydration 15B) significantly affects the salt tolerance of soybean. GsERD15B encodes a protein with transcriptional activation function and contains a PAM2 domain to mediate its interaction with poly(A)-binding (PAB) proteins. The 7-bp deletion in GsERD15B promoter enhanced the salt tolerance of soybean, with increased up-regulation of GsERD15B, two GmPAB genes, the known stress-related genes including GmABI1, GmABI2, GmbZIP1, GmP5CS, GmCAT4, GmPIP1:6, GmMYB84 and GmSOS1 in response to salt stress. We propose that natural variation in GsERD15B promoter affects soybean salt tolerance, and overexpression of GsERD15B enhanced salt tolerance probably by increasing the expression levels of genes related to ABA-signalling, proline content, catalase peroxidase, dehydration response and cation transport.

GmNAC81 Inversely Modulates Leaf Senescence and Drought Tolerance

Glycine max NAC81 (GmNAC81) is a downstream effector of the DCD/NRP-mediated cell death signaling, which interacts with GmNAC30 to fully induce the caspase 1-like vacuolar processing enzyme (VPE) expression, the executioner of the cell death program. GmNAC81 has been previously shown to positively modulate leaf senescence via the NRP/GmNAC81/VPE signaling module. Here, we examined the transcriptome induced by GmNAC81 overexpression and leaf senescence and showed that GmNAC81 further modulates leaf senescence by regulating an extensive repertoire of functionally characterized senescence-associated genes (SAGs). Because the NRP/GmNAC81/VPE signaling circuit also relays stress-induced cell death signals, we examined the effect of GmNAC81 overexpression in drought responses. Enhanced GmNAC81 expression in the transgenic lines increased sensitivity to water deprivation. Under progressive drought, the GmNAC81-overexpressing lines displayed severe leaf wilting, a larger and faster decline in leaf Ψw, relative water content (RWC), photosynthesis rate, stomatal conductance, and transpiration rate, in addition to higher Ci/Ca and lower Fm/Fv ratios compared to the BR16 control line. Collectively, these results indicate that the photosynthetic activity and apparatus were more affected by drought in the transgenic lines. Consistent with hypersensitivity to drought, chlorophyll loss, and lipid peroxidation were higher in the GmNAC81-overexpressing lines than in BR16 under dehydration. In addition to inducing VPE expression, GmNAC81 overexpression uncovered the regulation of typical drought-responsive genes. In particular, key regulators and effectors of ABA signaling were suppressed by GmNAC81 overexpression. These results suggest that GmNAC81 may negatively control drought tolerance not only via VPE activation but also via suppression of ABA signaling.

ABAS1 from soybean is a 1R-subtype MYB transcriptional repressor that enhances ABA sensitivity

Transcription factors (TFs) help plants respond to environmental stresses by regulating gene expression. Up till now, studies on the MYB family of TFs have mainly focused on the highly abundant R2R3-subtype. While the less well-known 1R-subtype has been generally shown to enhance abscisic acid (ABA) sensitivity by acting as transcriptional activators, the mechanisms of their functions are unclear. Here we identified an ABA sensitivity-associated gene from soybean, ABA-Sensitive 1 (GmABAS1), of the 1R-subtype of MYB. Using the GFP-GmABAS1 fusion protein, we demonstrated that GmABAS1 is localized in the nucleus, and with yeast reporter systems, we showed that it is a transcriptional repressor. We then identified the target gene of GmABAS1 to be Glyma.01G060300, an annotated ABI five-binding protein 3 and showed that GmABAS1 binds to the promoter of Glyma.01G060300 both in vitro and in vivo. Furthermore, Glyma.01G060300 and GmABAS1 exhibited reciprocal expression patterns under osmotic stress, inferring that GmABAS1 is a transcriptional repressor of Glyma.01G060300. As a further confirmation, AtAFP2, an orthologue of Glyma.01G060300, was down-regulated in GmABAS1-transgenic Arabidopsis thaliana, enhancing the plant's sensitivity to ABA. This is the first time a 1R-subtype of MYB from soybean has been reported to enhance ABA sensitivity by acting as a transcriptional repressor.

OXI1 and DAD Regulate Light-Induced Cell Death Antagonistically through Jasmonate and Salicylate Levels

Singlet oxygen produced from triplet excited chlorophylls in photosynthesis is a signal molecule that can induce programmed cell death (PCD) through the action of the OXIDATIVE STRESS INDUCIBLE 1 (OXI1) kinase. Here, we identify two negative regulators of light-induced PCD that modulate OXI1 expression: DAD1 and DAD2, homologs of the human antiapoptotic protein DEFENDER AGAINST CELL DEATH. Overexpressing OXI1 in Arabidopsis (Arabidopsis thaliana) increased plant sensitivity to high light and induced early senescence of mature leaves. Both phenomena rely on a marked accumulation of jasmonate and salicylate. DAD1 or DAD2 overexpression decreased OXI1 expression, jasmonate levels, and sensitivity to photooxidative stress. Knock-out mutants of DAD1 or DAD2 exhibited the opposite responses. Exogenous applications of jasmonate upregulated salicylate biosynthesis genes and caused leaf damage in wild-type plants but not in the salicylate biosynthesis mutant Salicylic acid induction-deficient2, indicating that salicylate plays a crucial role in PCD downstream of jasmonate. Treating plants with salicylate upregulated the DAD genes and downregulated OXI1 We conclude that OXI1 and DAD are antagonistic regulators of cell death through modulating jasmonate and salicylate levels. High light-induced PCD thus results from a tight control of the relative activities of these regulating proteins, with DAD exerting a negative feedback control on OXI1 expression.

RAMOSA1 ENHANCER LOCUS2-Mediated Transcriptional Repression Regulates Vegetative and Reproductive Architecture

Transcriptional repression in multicellular organisms orchestrates dynamic and precise gene expression changes that enable complex developmental patterns. Here, we present phenotypic and molecular characterization of the maize (Zea mays) transcriptional corepressor RAMOSA1 ENHANCER LOCUS2 (REL2), a unique member of the highly conserved TOPLESS (TPL) family. Analysis of single recessive mutations in rel2 revealed an array of vegetative and reproductive phenotypes, many related to defects in meristem initiation and maintenance. To better understand how REL2-mediated transcriptional complexes relate to rel2 phenotypes, we performed protein interaction assays and transcriptional profiling of mutant inflorescences, leading to the identification of different maize transcription factors and regulatory pathways that employ REL2 repression to control traits directly impacting maize yields. In addition, we used our REL2 interaction data to catalog conserved repression motifs present on REL2 interactors and showed that two of these, RLFGV- and DLN-type motifs, interact with the C-terminal WD40 domain of REL2 rather than the N terminus, which is known to bind LxLxL EAR motifs. These findings establish that the WD40 domain of TPL family proteins is an independent protein interaction surface that may work together with the N-terminal domain to allow the formation of large macromolecular complexes of functionally related transcription factors.

Cloning and Expression Analysis of ZmERD3 Gene From Zea mays

Background

Stresses (such as drought, salt, viruses, and others) seriously affect plant productivity. To cope with these threats, plants express a large number of genes, including several members of ERD (early responsive to dehydration) genes to synthesize and assemble adaptive molecules. But, the function of ERD3 gene hasn’t been known so far.

Objectives

The purpose of the present study was to clone the stress-resistance gene: ZmERD3, and to analyze its expression pattern in the maize plant organs at different stages and under various stress treatments.

Materials and Methods

MaizeGDB database search together with the bioinformatics analysis led to the identification of ZmERD3 gene in Zea mays. The cDNA sequence and promoter of ZmERD3 gene were obtained through PCR. Bioinformatics analysis was performed through online tools. The tissue-specific expression profile of the ZmERD3 gene in maize plant was carried out using the quantitative real time PCR (qRT-PCR) technique and its expression pattern in response to stress treatments (such as PEG, NaCl, ABA, and low temperature) was also analyzed through qRT-PCR method.

Results

Based on the homology alignment with AtERD3 (XP_002867953) in MaizeGDB (http://www. maizegdb.org/), the cDNA sequence and promoter region of the ZmERD3 gene were obtained. The bioinformatic analysis showed that ZmERD3 protein has one specific hit of methyltransferase and a high probability of location in the cytoplasm, and there are many cis-regulatory elements responsive to light, heat, cold, dehydration, as well as other stresses in its promoter sequence. Expression analysis revealed that the amount of ZmERD3 mRNA is different in all indicated organs of the maize plant. In addition, the ZmERD3 expression could be induced by abiotic stress treatments. Compared to the control, treatment with NaCl or PEG-6000 could significantly enhance the expression ability of ZmERD3 gene. As well, its expression level was increased about 20 times above the control after exposure to NaCl and PEG-6000 treatments for 3-6 h.

Conclusions

One putative methyltransferase gene, ZmERD3 was cloned. ZmERD3 expression exhibited an obvious tissue-specificity, and its expression could make a significant response to NaCl and PEG-6000 treatments.

Age-related cataracts: Role of unfolded protein response, Ca2+ mobilization, epigenetic DNA modifications, and loss of Nrf2/Keap1 dependent cytoprotection

Age-related cataracts are closely associated with lens chronological aging, oxidation, calcium imbalance, hydration and crystallin modifications. Accumulating evidence indicates that misfolded proteins are generated in the endoplasmic reticulum (ER) by most cataractogenic stresses. To eliminate misfolded proteins from cells before they can induce senescence, the cells activate a clean-up machinery called the ER stress/unfolded protein response (UPR). The UPR also activates the nuclear factor-erythroid-2-related factor 2 (Nrf2), a central transcriptional factor for cytoprotection against stress. Nrf2 activates nearly 600 cytoprotective target genes. However, if ER stress reaches critically high levels, the UPR activates destructive outputs to trigger programmed cell death. The UPR activates mobilization of ER-Ca2+ to the cytoplasm and results in activation of Ca2+-dependent proteases to cleave various enzymes and proteins which cause the loss of normal lens function. The UPR also enhances the overproduction of reactive oxygen species (ROS), which damage lens constituents and induce failure of the Nrf2 dependent cytoprotection. Kelch-like ECH-associated protein 1 (Keap1) is an oxygen sensor protein and regulates the levels of Nrf2 by the proteasomal degradation. A significant loss of DNA methylation in diabetic cataracts was found in the Keap1 promoter, which overexpresses the Keap1 protein. Overexpressed Keap1 significantly decreases the levels of Nrf2. Lower levels of Nrf2 induces loss of the redox balance toward to oxidative stress thereby leading to failure of lens cytoprotection. Here, this review summarizes the overall view of ER stress, increases in Ca2+ levels, protein cleavage, and loss of the well-established stress protection in somatic lens cells.

VaERD15, a Transcription Factor Gene Associated with Cold-Tolerance in Chinese Wild Vitis amurensis

Early responsive to dehydration (ERD) genes can be rapidly induced to counteract abiotic stresses, such as drought, low temperatures or high salinities. Here, we report on an ERD gene (VaERD15) related to cold tolerance from Chinese wild Vitis amurensis accession 'Heilongjiang seedling'. The full-length VaERD15 cDNA is 685 bp, including a 66 bp 5'-untranslated region (UTR), a 196 bp 3'-UTR region and a 423 bp open reading frame encoding 140 amino acids. The VaERD15 protein shares a high amino acid sequence similarity with ERD15 of Arabidopsis thaliana. In our study, VaERD15 was shown to have a nucleic localization function and a transcriptional activation function. Semi-quantitative PCR and Western blot analyses showed that VaERD15 was constitutively expressed in young leaves, stems and roots of V. amurensis accession 'Heilongjiang seedling' plants, and expression levels increased after low-temperature treatment. We also generated a transgenic Arabidopsis Col-0 line that over-expressed VaERD15 and carried out a cold-treatment assay. Real-time quantitative PCR (qRT-PCR) and Western blot analyses showed that as the duration of cold treatment increased, the expression of both gene and protein levels increased continuously in the transgenic plants, while almost no expression was detected in the wild type Arabidopsis. Moreover, the plants that over-expressed VaERD15 showed higher cold tolerance and accumulation of proline, soluble sugars, proteins, malondialdehyde and three antioxidases (superoxide dismutase, peroxidase, and catalase). Lower levels of relative ion leakage also occurred under cold stress. Taken together, our results indicate that the transcription factor VaERD15 was induced by cold stress and was able to enhance cold tolerance.

Transcription activation activity of ERD15 protein from Morus indica

Early Responsive to Dehydration (ERD) genes are described as rapid response mediators of dehydration stress. Recently, ERD15 has emerged as a novel stress induced transcription factor which might be involved in mediating distinct stress responses in plants. In order to determine whether mulberry ERD15 can act as functional transcription factor, yeast-based assays were performed. Mulberry ERD15 was found to drive high level reporter gene expression in yeast which suggests it may function as a transcription factor. However, due to lack of an identifiable DNA binding domain, deletion analysis was carried out to determine the putative region of the protein involved in mediating protein-DNA interaction. Our results indicate that the region between 70 and 100 amino acids is critical in conferring transcription activation activity and might harbor the DNA binding region of ERD15.

Functional and regulatory conservation of the soybean ER stress-induced DCD/NRP-mediated cell death signaling in plants

BACKGROUND

The developmental and cell death domain (DCD)-containing asparagine-rich proteins (NRPs) were first identified in soybean (Glycine max) as transducers of a cell death signal derived from prolonged endoplasmic reticulum (ER) stress, osmotic stress, drought or developmentally-programmed leaf senescence via the GmNAC81/GmNAC30/GmVPE signaling module. In spite of the relevance of the DCD/NRP-mediated signaling as a versatile adaptive response to multiple stresses, mechanistic knowledge of the pathway is lacking and the extent to which this pathway may operate in the plant kingdom has not been investigated.

RESULTS

Here, we demonstrated that the DCD/NRP-mediated signaling also propagates a stress-induced cell death signal in other plant species with features of a programmed cell death (PCD) response. In silico analysis revealed that several plant genomes harbor conserved sequences of the pathway components, which share functional analogy with their soybean counterparts. We showed that GmNRPs, GmNAC81and VPE orthologs from Arabidopsis, designated as AtNRP-1, AtNRP-2, ANAC036 and gVPE, respectively, induced cell death when transiently expressed in N. benthamiana leaves. In addition, loss of AtNRP1 and AtNRP2 function attenuated ER stress-induced cell death in Arabidopsis, which was in marked contrast with the enhanced cell death phenotype displayed by overexpressing lines as compared to Col-0. Furthermore, atnrp-1 knockout mutants displayed enhanced sensitivity to PEG-induced osmotic stress, a phenotype that could be complemented with ectopic expression of either GmNRP-A or GmNRP-B. In addition, AtNRPs, ANAC036 and gVPE were induced by osmotic and ER stress to an extent that was modulated by the ER-resident molecular chaperone binding protein (BiP) similarly as in soybean. Finally, as putative downstream components of the NRP-mediated cell death signaling, the stress induction of AtNRP2, ANAC036 and gVPE was dependent on the AtNRP1 function. BiP overexpression also conferred tolerance to water stress in Arabidopsis, most likely due to modulation of the drought-induced NRP-mediated cell death response.

CONCLUSION

Our results indicated that the NRP-mediated cell death signaling operates in the plant kingdom with conserved regulatory mechanisms and hence may be target for engineering stress tolerance and adaptation in crops.

The Stress-Induced Soybean NAC Transcription Factor GmNAC81 Plays a Positive Role in Developmentally Programmed Leaf Senescence

The onset of leaf senescence is a highly regulated developmental change that is controlled by both genetics and the environment. Senescence is triggered by massive transcriptional reprogramming, but functional information about its underlying regulatory mechanisms is limited. In the current investigation, we performed a functional analysis of the soybean (Glycine max) osmotic stress- and endoplasmic reticulum (ER) stress-induced NAC transcription factor GmNAC81 during natural leaf senescence using overexpression studies and reverse genetics. GmNAC81-overexpressing lines displayed accelerated flowering and leaf senescence but otherwise developed normally. The precocious leaf senescence of GmNAC81-overexpressing lines was associated with greater Chl loss, faster photosynthetic decay and higher expression of hydrolytic enzyme-encoding GmNAC81 target genes, including the vacuolar processing enzyme (VPE), an executioner of vacuole-triggered programmed cell death (PCD). Conversely, virus-induced gene silencing-mediated silencing of GmNAC81 delayed leaf senescence and was associated with reductions in Chl loss, lipid peroxidation and the expression of GmNAC81 direct targets. Promoter-reporter studies revealed that the expression pattern of GmNAC81 was associated with senescence in soybean leaves. Our data indicate that GmNAC81 is a positive regulator of age-dependent senescence and may integrate osmotic stress- and ER stress-induced PCD responses with natural leaf senescence through the GmNAC81/VPE regulatory circuit.

Comprehensive analysis of the endoplasmic reticulum stress response in the soybean genome: conserved and plant-specific features

BACKGROUND

Despite the relevance of the eukaryotic endoplasmic reticulum (ER)-stress response as an integrator of multiple stress signals into an adaptive response, knowledge about these ER-mediated cytoprotective pathways in soybean (Glycine max) is lacking. Here, we searched for genes involved in the highly conserved unfolded protein response (UPR) and ER stress-induced plant-specific cell death signaling pathways in the soybean genome.

METHODS

Previously characterized Arabidopsis UPR genes were used as prototypes for the identification of the soybean orthologs and the in silico assembly of the UPR in soybean, using eggNOG v4.0 software. Functional studies were also conducted by analyzing the transcriptional activity of soybean UPR transducers.

RESULTS

As a result of this search, we have provided a complete profile of soybean UPR genes with significant predicted protein similarities to A. thaliana UPR-associated proteins. Both arms of the plant UPR were further examined functionally, and evidence is presented that the soybean counterparts are true orthologs of previously characterized UPR transducers in Arabidopsis. The bZIP17/bZI28 orthologs (GmbZIP37 and GmbZIP38) and ZIP60 ortholog (GmbZIP68) from soybean have similar structural organizations as their Arabidopsis counterparts, were induced by ER stress and activated an ERSE- and UPRE-containing BiP promoter. Furthermore, the transcript of the putative substrate of GmIREs, GmbZIP68, harbors a canonical site for IRE1 endonuclease activity and was efficiently spliced under ER stress conditions. In a reverse approach, we also examined the Arabidopsis genome for components of a previously characterized ER stress-induced cell death signaling response in soybean. With the exception of GmERD15, which apparently does not possess an Arabidopsis ortholog, the Arabidopsis genome harbors conserved GmNRP, GmNAC81, GmNAC30 and GmVPE sequences that share significant structural and sequence similarities with their soybean counterparts. These results suggest that the NRP/GmNAC81 + GmNAC30/VPE regulatory circuit may transduce cell death signals in plant species other than soybean.

CONCLUSIONS

Our in silico analyses, along with current and previous functional data, permitted generation of a comprehensive overview of the ER stress response in soybean as a framework for functional prediction of ER stress signaling components and their possible connections with multiple stress responses.

Evolutionary history exposes radical diversification among classes of interaction partners of the MLLE domain of plant poly(A)-binding proteins

Background

Poly(A)-binding proteins (PABPs) are evolutionarily conserved proteins that have important functions in the regulation of translation and the control of mRNA stability in eukaryotes. Most PABPs encode a C-terminal domain known as the MLLE domain (previously PABC or CTC), which can mediate protein interactions. In earlier work we identified and predicted that four classes of MLLE-interacting proteins were present in Arabidopsis thaliana, which we named CID A, B, C, and D. These proteins encode transcription-activating domains (CID A), the Lsm and LsmAD domains of ataxin-2 (CID B), the CUE and small MutS-related domains (CID C), and two RNA recognition domains (CID D). We recently found that a novel class that lacks the LsmAD domain is present in CID B proteins.

Results

We extended our analysis to other classes of CIDs present in the viridiplantae. We found that novel variants also evolved in classes CID A and CID C. A specific transcription factor domain is present in a distinct lineage in class A, and a variant that lacks at least two distinct domains was also identified in a divergent lineage in class C. We did not detect any variants in Class D CIDs. This class often consists of four to six highly conserved RNA-binding proteins, which suggests that major redundancy is present in this class.

Conclusions

CIDs are likely to operate as components of posttranscriptional regulatory assemblies. The evident diversification of CIDs may be neutral or may be important for plant adaptation to the environment and for acquisition of specific traits during evolution. The fact that CIDs subclasses are maintained in early lineages suggest that a presumed interference between duplicates was resolved, and a defined function for each subclass was achieved.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0475-1) contains supplementary material, which is available to authorized users.

Activation of salt shock response leads to solubilisation of mutant huntingtin in Saccharomyces cerevisiae

Formation of cytoplasmic and nuclear aggregates is a hallmark of Huntington's disease (HD). Inhibition of aggregation of mutant huntingtin has been suggested to be a feasible approach to slow down the progress of this neurodegenerative disorder. Exposure to environmental stimuli leads to the activation of the stress response machinery of the cell. In this work, we have investigated the effect of salt shock on the aggregation of mutant huntingtin (103Q-htt) in a yeast model of HD. We found that at an optimum concentration of NaCl, the protein no longer formed aggregates and existed in the soluble form. This led to lower oxidative stress in the cell. Salt shock resulted in the synthesis of the osmolyte glycerol, which was partially responsible for the beneficial effect of stress. Surprisingly, we also found increase in the synthesis of another osmolyte, trehalose. Using deletion strains, we were able to show that the effect on solubilisation of mutant huntingtin is due to the synthesis of optimum amounts of both osmolytes. Stress-induced effect was monitored on gene expression. Genes related to proteins of the osmosensory pathway were upregulated on exposure to salt while those coding for stress response proteins were downregulated when solubilisation of mutant huntingtin occurred. Our study shows that activation of stress response elements can have beneficial effect in the solubilisation of huntingtin in a yeast model of HD.

The membrane-associated transcription factor NAC089 controls ER-stress-induced programmed cell death in plants

The unfolded protein response (UPR) is activated to sustain cell survival by reducing misfolded protein accumulation in the endoplasmic reticulum (ER). The UPR also promotes programmed cell death (PCD) when the ER stress is severe; however, the underlying molecular mechanisms are less understood, especially in plants. Previously, two membrane-associated transcriptions factors (MTFs), bZIP28 and bZIP60, were identified as the key regulators for cell survival in the plant ER stress response. Here, we report the identification of another MTF, NAC089, as an important PCD regulator in Arabidopsis (Arabidopsis thaliana) plants. NAC089 relocates from the ER membrane to the nucleus under ER stress conditions. Inducible expression of a truncated form of NAC089, in which the transmembrane domain is deleted, induces PCD with increased caspase 3/7-like activity and DNA fragmentation. Knock-down NAC089 in Arabidopsis confers ER stress tolerance and impairs ER-stress-induced caspase-like activity. Transcriptional regulation analysis and ChIP-qPCR reveal that NAC089 plays important role in regulating downstream genes involved in PCD, such as NAC094, MC5 and BAG6. Furthermore, NAC089 is up-regulated by ER stress, which is directly controlled by bZIP28 and bZIP60. These results show that nuclear relocation of NAC089 promotes ER-stress-induced PCD, and both pro-survival and pro-death signals are elicited by bZIP28 and bZIP60 during plant ER stress response.

Protein folding is fundamentally important for development and responses to environmental stresses in eukaryotes. When excess misfolded proteins are accumulated in the endoplasmic reticulum (ER), the unfolded protein response (UPR) is triggered to promote cell survival through optimizing protein folding, and also promote programmed cell death (PCD) when the stress is severe. However, the link from ER-stress-sensing to PCD is largely unknown. Here, we report the identification of one membrane-associated transcription factor NAC089 as an important regulator of ER stress-induced PCD in plants. We have established a previously unrecognized molecular connection between ER stress sensors and PCD regulators. We have shown that organelle-to-organelle translocation of a transcription factor is important for its function in transcriptional regulation. Our results have provided novel insights into the molecular mechanisms of PCD in plants, especially under ER stress conditions.

Drying without senescence in resurrection plants

Research into extreme drought tolerance in resurrection plants using species such as Craterostigma plantagineum, C. wilmsii, Xerophyta humilis, Tortula ruralis, and Sporobolus stapfianus has provided some insight into the desiccation tolerance mechanisms utilized by these plants to allow them to persist under extremely adverse environmental conditions. Some of the mechanisms used to ensure cellular preservation during severe dehydration appear to be peculiar to resurrection plants. Apart from the ability to preserve vital cellular components during drying and rehydration, such mechanisms include the ability to down-regulate growth-related metabolism rapidly in response to changes in water availability, and the ability to inhibit dehydration-induced senescence programs enabling reconstitution of photosynthetic capacity quickly following a rainfall event. Extensive research on the molecular mechanism of leaf senescence in non-resurrection plants has revealed a multi-layered regulatory network operates to control programed cell death pathways. However, very little is known about the molecular mechanisms that resurrection plants employ to avoid undergoing drought-related senescence during the desiccation process. To survive desiccation, dehydration in the perennial resurrection grass S. stapfianus must proceed slowly over a period of 7 days or more. Leaves detached from the plant before 60% relative water content (RWC) is attained are desiccation-sensitive indicating that desiccation tolerance is conferred in vegetative tissue of S. stapfianus when the leaf RWC has declined to 60%. Whilst some older leaves remaining attached to the plant during dehydration will senesce, suggesting dehydration-induced senescence may be influenced by leaf age or the rate of dehydration in individual leaves, the majority of leaves do not senesce. Rather these leaves dehydrate to air-dryness and revive fully following rehydration. Hence it seems likely that there are genes expressed in younger leaf tissues of resurrection plants that enable suppression of drought-related senescence pathways. As very few studies have directly addressed this phenomenon, this review aims to discuss current literature surrounding the activation and suppression of senescence pathways and how these pathways may differ in resurrection plants.

The endoplasmic reticulum binding protein BiP displays dual function in modulating cell death events

The binding protein (BiP) has been demonstrated to participate in innate immunity and attenuate endoplasmic reticulum- and osmotic stress-induced cell death. Here, we employed transgenic plants with manipulated levels of BiP to assess whether BiP also controlled developmental and hypersensitive programmed cell death (PCD). Under normal conditions, the BiP-induced transcriptome revealed a robust down-regulation of developmental PCD genes and an up-regulation of the genes involved in hypersensitive PCD triggered by nonhost-pathogen interactions. Accordingly, the BiP-overexpressing line displayed delayed leaf senescence under normal conditions and accelerated hypersensitive response triggered by Pseudomonas syringae pv tomato in soybean (Glycine max) and tobacco (Nicotiana tabacum), as monitored by measuring hallmarks of PCD in plants. The BiP-mediated delay of leaf senescence correlated with the attenuation of N-rich protein (NRP)-mediated cell death signaling and the inhibition of the senescence-associated activation of the unfolded protein response (UPR). By contrast, under biological activation of salicylic acid (SA) signaling and hypersensitive PCD, BiP overexpression further induced NRP-mediated cell death signaling and antagonistically inhibited the UPR. Thus, the SA-mediated induction of NRP cell death signaling occurs via a pathway distinct from UPR. Our data indicate that during the hypersensitive PCD, BiP positively regulates the NRP cell death signaling through a yet undefined mechanism that is activated by SA signaling and related to ER functioning. By contrast, BiP's negative regulation of leaf senescence may be linked to its capacity to attenuate the UPR activation and NRP cell death signaling. Therefore, BiP can function either as a negative or positive modulator of PCD events.

The molecular chaperone binding protein BiP prevents leaf dehydration-induced cellular homeostasis disruption

BiP overexpression improves leaf water relations during droughts and delays drought-induced leaf senescence. However, whether BiP controls cellular homeostasis under drought conditions or simply delays dehydration-induced leaf senescence as the primary cause for water stress tolerance remains to be determined. To address this issue, we examined the drought-induced transcriptomes of BiP-overexpressing lines and wild-type (WT) lines under similar leaf water potential (ψw) values. In the WT leaves, a ψw reduction of -1.0 resulted in 1339 up-regulated and 2710 down-regulated genes; in the BiP-overexpressing line 35S::BiP-4, only 334 and 420 genes were induced and repressed, respectively, at a similar leaf ψw = -1.0 MPa. This level of leaf dehydration was low enough to induce a repertory of typical drought-responsive genes in WT leaves but not in 35S::BiP-4 dehydrated leaves. The responders included hormone-related genes, functional and regulatory genes involved in drought protection and senescence-associated genes. The number of differentially expressed genes in the 35S::BiP-4 line approached the wild type number at a leaf ψw = -1.6 MPa. However, N-rich protein (NRP)- mediated cell death signaling genes and unfolded protein response (UPR) genes were induced to a much lower extent in the 35S::BiP-4 line than in the WT even at ψw = -1.6 MPa. The heatmaps for UPR, ERAD (ER-associated degradation protein system), drought-responsive and cell death-associated genes revealed that the leaf transcriptome of 35S::BiP-4 at ψw = -1.0 MPa clustered together with the transcriptome of well-watered leaves and they diverged considerably from the drought-induced transcriptome of the WT (ψw = -1.0, -1.7 and -2.0 MPa) and 35S::BiP-4 leaves at ψw = -1.6 MPa. Taken together, our data revealed that BiP-overexpressing lines requires a much higher level of stress (ψw = -1.6 MPa) to respond to drought than that of WT (ψw = -1.0). Therefore, BiP overexpression maintains cellular homeostasis under water stress conditions and thus ameliorates endogenous osmotic stress.

Nodulin 22, a novel small heat-shock protein of the endoplasmic reticulum, is linked to the unfolded protein response in common bean

The importance of plant small heat shock proteins (sHsp) in multiple cellular processes has been evidenced by their unusual abundance and diversity; however, little is known about their biological role. Here, we characterized the in vitro chaperone activity and subcellular localization of nodulin 22 of Phaseolus vulgaris (PvNod22; common bean) and explored its cellular function through a virus-induced gene silencing-based reverse genetics approach. We established that PvNod22 facilitated the refolding of a model substrate in vitro, suggesting that it acts as a molecular chaperone in the cell. Through microscopy analyses of PvNod22, we determined its localization in the endoplasmic reticulum (ER). Furthermore, we found that silencing of PvNod22 resulted in necrotic lesions in the aerial organs of P. vulgaris plants cultivated under optimal conditions and that downregulation of PvNod22 activated the ER-unfolded protein response (UPR) and cell death. We also established that PvNod22 expression in wild-type bean plants was modulated by abiotic stress but not by chemicals that trigger the UPR, indicating PvNod22 is not under UPR control. Our results suggest that the ability of PvNod22 to suppress protein aggregation contributes to the maintenance of ER homeostasis, thus preventing the induction of cell death via UPR in response to oxidative stress during plant-microbe interactions.

GmNAC30 and GmNAC81 integrate the endoplasmic reticulum stress- and osmotic stress-induced cell death responses through a vacuolar processing enzyme

Prolonged endoplasmic reticulum and osmotic stress synergistically activate the stress-induced N-rich protein-mediated signaling that transduces a cell death signal by inducing GmNAC81 (GmNAC6) in soybean. To identify novel regulators of the stress-induced programmed cell death (PCD) response, we screened a two-hybrid library for partners of GmNAC81. We discovered another member of the NAC (NAM-ATAF1,2-CUC2) family, GmNAC30, which binds to GmNAC81 in the nucleus of plant cells to coordinately regulate common target promoters that harbor the core cis-regulatory element TGTG[TGC]. We found that GmNAC81 and GmNAC30 can function either as transcriptional repressors or activators and cooperate to enhance the transcriptional regulation of common target promoters, suggesting that heterodimerization may be required for the full regulation of gene expression. Accordingly, GmNAC81 and GmNAC30 display overlapping expression profiles in response to multiple environmental and developmental stimuli. Consistent with a role in PCD, GmNAC81 and GmNAC30 bind in vivo to and transactivate hydrolytic enzyme promoters in soybean protoplasts. A GmNAC81/GmNAC30 binding site is located in the promoter of the caspase-1-like vacuolar processing enzyme (VPE) gene, which is involved in PCD in plants. We demonstrated that the expression of GmNAC81 and GmNAC30 fully transactivates the VPE gene in soybean protoplasts and that this transactivation was associated with an increase in caspase-1-like activity. Collectively, our results indicate that the stress-induced GmNAC30 cooperates with GmNAC81 to activate PCD through the induction of the cell death executioner VPE.

Development of a new wheat microarray from a durum wheat totipotent cDNA library used for a powdery mildew resistance study

Totipotent cDNA libraries representative of all the potentially expressed sequences in a genome would be of great benefit to gene expression studies. Here, we report on an innovative method for creating such a library for durum wheat (Triticum turgidum L. var. durum) and its application for gene discovery. The use of suitable quantities of 5-azacytidine during the germination phase induced the demethylation of total DNA, and the resulting seedlings potentially express all of the genes present in the genome. A new wheat microarray consisting of 4925 unigenes was developed from the totipotent cDNA library and used to screen for genes that may contribute to differences in the disease resistance of two near-isogenic lines, the durum wheat cultivar Latino and the line 5BIL-42, which are respectively susceptible and resistant to powdery mildew. Fluorescently labeled cDNA was prepared from the RNA of seedlings of the two near-isogenic wheat lines after infection with a single powdery mildew isolate under controlled conditions in the greenhouse. Hybridization to the microarray identified six genes that were differently expressed in the two lines. Four of the sequences could be assigned putative functions based on their similarity to known genes in public databases. Physical mapping of the six genes localized them to two regions of the genome: the centromeric region of chromosome 5B, where the Pm36 resistance gene was previously localized, and chromosome 6B.

The unfolded protein response in plants: a fundamental adaptive cellular response to internal and external stresses

In eukaryotic cells, proteins that enter the secretory pathway are translated on membrane-bound ribosomes and translocated into the endoplasmic reticulum (ER), where they are subjected to chaperone-assisted folding, post-translational modification and assembly. During the evolution of the eukaryotic cell, a homeostatic mechanism was developed to maintain the functions of the ER in the face of various internal and external stresses. The most severe stresses imposed on eukaryotic cells can induce ER stress that can overwhelm the processing capacity of the ER, leading to the accumulation of unfolded proteins in the ER lumen. To cope with this accumulation of unfolded proteins, the unfolded protein response (UPR) is activated to alter transcriptional programs through inositol-requiring enzyme 1 (IRE1) and bZIP17/28 in plants. In addition to transcriptional induction of UPR genes, quality control (QC), translational attenuation, ER-associated degradation (ERAD) and ER stress-induced apoptosis are also conserved as fundamental adaptive cellular responses to ER stress in plants. This article is part of a Special Issue entitled: Translational Plant Proteomics.

N-rich protein (NRP)-mediated cell death signaling: a new branch of the ER stress response with implications for plant biotechnology

Upon disruption of ER homeostasis, plant cells activate at least two branches of the unfolded protein response (UPR) through IRE1-like and ATAF6-like transducers, resulting in the upregulation of ER-resident molecular chaperones and the activation of the ER-associated degradation protein system. Here, we discuss a new ER stress response pathway in plants that is associated with an osmotic stress response in transducing a cell death signal. Both ER and osmotic stress induce the expression of the novel transcription factor GmERD15, which binds and activates N-rich protein (NRP) promoters to induce NRP expression and cause the upregulation of GmNAC6, an effector of the cell death response. In contrast to this activation mechanism, the ER-resident molecular chaperone binding protein (BiP) attenuates the propagation of the cell death signal by modulating the expression and activity of components of the ER and osmotic stress-induced NRP-mediated cell death signaling. This interaction attenuates dehydration-induced cell death and promotes a better adaptation of BiP-overexpressing transgenic lines to drought.

The binding protein BiP attenuates stress-induced cell death in soybean via modulation of the N-rich protein-mediated signaling pathway

The molecular chaperone binding protein (BiP) participates in the constitutive function of the endoplasmic reticulum (ER) and protects the cell against stresses. In this study, we investigated the underlying mechanism by which BiP protects plant cells from stress-induced cell death. We found that enhanced expression of BiP in soybean (Glycine max) attenuated ER stress- and osmotic stress-mediated cell death. Ectopic expression of BiP in transgenic lines attenuated the leaf necrotic lesions that are caused by the ER stress inducer tunicamycin and also maintained shoot turgidity upon polyethylene glycol-induced dehydration. BiP-mediated attenuation of stress-induced cell death was confirmed by the decreased percentage of dead cell, the reduced induction of the senescence-associated marker gene GmCystP, and reduced DNA fragmentation in BiP-overexpressing lines. These phenotypes were accompanied by a delay in the induction of the cell death marker genes N-RICH PROTEIN-A (NRP-A), NRP-B, and GmNAC6, which are involved in transducing a cell death signal generated by ER stress and osmotic stress through the NRP-mediated signaling pathway. The prosurvival effect of BiP was associated with modulation of the ER stress- and osmotic stress-induced NRP-mediated cell death signaling, as determined in transgenic tobacco (Nicotiana tabacum) lines with enhanced (sense) and suppressed (antisense) BiP levels. Enhanced expression of BiP prevented NRP- and NAC6-mediated chlorosis and the appearance of senescence-associated markers, whereas silencing of endogenous BiP accelerated the onset of leaf senescence mediated by NRPs and GmNAC6. Collectively, these results implicate BiP as a negative regulator of the stress-induced NRP-mediated cell death response.

EARLY RESPONSIVE to DEHYDRATION 15, a new transcription factor that integrates stress signaling pathways

The Early Responsive to Dehydration (ERD) genes are defined as those genes that are rapidly activated during drought stress. The encoded proteins show a great structural and functional diversity, with a particular class of proteins acting as connectors of stress response pathways. Recent studies have shown that ERD15 proteins from different species of plants operate in cross-talk among different response pathways. In this mini-review, we show the recent progress on the functional role of this diverse family of proteins and demonstrate that a soybean ERD15 homolog can act as a connector in stress response pathways that trigger a programmed cell death signal.

The NAC domain-containing protein, GmNAC6, is a downstream component of the ER stress- and osmotic stress-induced NRP-mediated cell-death signaling pathway

BACKGROUND

The endoplasmic reticulum (ER) is a major signaling organelle, which integrates a variety of responses against physiological stresses. In plants, one such stress-integrating response is the N-rich protein (NRP)-mediated cell death signaling pathway, which is synergistically activated by combined ER stress and osmotic stress signals. Despite the potential of this integrated signaling to protect plant cells against different stress conditions, mechanistic knowledge of the pathway is lacking, and downstream components have yet to be identified.

RESULTS

In the present investigation, we discovered an NAC domain-containing protein from soybean, GmNAC6 (Glycine max NAC6), to be a downstream component of the integrated pathway. Similar to NRP-A and NRP-B, GmNAC6 is induced by ER stress and osmotic stress individually, but requires both signals for full activation. Transient expression of GmNAC6 promoted cell death and hypersensitive-like responses in planta. GmNAC6 and NRPs also share overlapping responses to biotic signals, but the induction of NRPs peaked before the increased accumulation of GmNAC6 transcripts. Consistent with the delayed kinetics of GmNAC6 induction, increased levels of NRP-A and NRP-B transcripts induced promoter activation and the expression of the GmNAC6 gene.

CONCLUSIONS

Collectively, our results biochemically link GmNAC6 to the ER stress- and osmotic stress-integrating cell death response and show that GmNAC6 may act downstream of the NRPs.