Ozone-induced programmed cell death in the Arabidopsis radical-induced cell death1 mutant

The role of redox-active small molecules and oxidative protein post-translational modifications in seed aging

Seed vigor is a crucial indicator of seed quality. Variations in seed vigor are closely associated with seed properties and storage conditions. The vigor of mature seeds progressively declines during storage, which is called seed deterioration or aging. Seed aging induces a cascade of cellular damage, including impaired subcellular structures and macromolecules, such as lipids, proteins, and DNA. Reactive oxygen species (ROS) act as signaling molecules during seed aging causing oxidative damage and triggering programmed cell death (PCD). Mitochondria are the main site of ROS production and change morphology and function before other organelles during aging. The roles of other small redox-active molecules in regulating cell and seed vigor, such as nitric oxide (NO) and hydrogen sulfide (H2S), were identified later. ROS, NO, and H2S typically regulate protein function through post-translational modifications (PTMs), including carbonylation, S-glutathionylation, S-nitrosylation, and S-sulfhydration. These signaling molecules as well as the PTMs they induce interact to regulate cell fate and seed vigor. This review was conducted to describe the physiological changes and underlying molecular mechanisms that in seed aging and provides a comprehensive view of how ROS, NO, and H2S affect cell death and seed vigor.

GhRCD1 promotes cotton tolerance to cadmium by regulating the GhbHLH12-GhMYB44-GhHMA1 transcriptional cascade

Heavy metal pollution poses a significant risk to human health and wreaks havoc on agricultural productivity. Phytoremediation, a plant-based, environmentally benign, and cost-effective method, is employed to remove heavy metals from contaminated soil, particularly in agricultural or heavy metal-sensitive lands. However, the phytoremediation capacity of various plant species and germplasm resources display significant genetic diversity, and the mechanisms underlying these differences remain hitherto obscure. Given its potential benefits, genetic improvement of plants is essential for enhancing their uptake of heavy metals, tolerance to harmful levels, as well as overall growth and development in contaminated soil. In this study, we uncover a molecular cascade that regulates cadmium (Cd2+) tolerance in cotton, involving GhRCD1, GhbHLH12, GhMYB44, and GhHMA1. We identified a Cd2+-sensitive cotton T-DNA insertion mutant with disrupted GhRCD1 expression. Genetic knockout of GhRCD1 by CRISPR/Cas9 technology resulted in reduced Cd2+ tolerance in cotton seedlings, while GhRCD1 overexpression enhanced Cd2+ tolerance. Through molecular interaction studies, we demonstrated that, in response to Cd2+ presence, GhRCD1 directly interacts with GhbHLH12. This interaction activates GhMYB44, which subsequently activates a heavy metal transporter, GhHMA1, by directly binding to a G-box cis-element in its promoter. These findings provide critical insights into a novel GhRCD1-GhbHLH12-GhMYB44-GhHMA1 regulatory module responsible for Cd2+ tolerance in cotton. Furthermore, our study paves the way for the development of elite Cd2+-tolerant cultivars by elucidating the molecular mechanisms governing the genetic control of Cd2+ tolerance in cotton.

Unraveling the enigma of root-knot nematodes: from origins to advanced management strategies in agriculture

MAIN CONCLUSION

Integrated management strategies, including novel nematicides and resilient cultivars, offer sustainable solutions to combat root-knot nematodes, crucial for safeguarding global agriculture against persistent threats. Root-knot nematodes (RKN) pose a significant threat to a diverse range of host plants, with their obligatory endoparasitic nature leading to substantial agricultural losses. RKN spend much of their lives inside or in contact by secreting plant cell wall-modifying enzymes resulting in the giant cell development for establishing host-parasite relationships. Additionally, inflicting physical harm to host plants, RKN also contributes to disease complexes creation with fungi and bacteria. This review comprehensively explores the origin, history, distribution, and physiological races of RKN, emphasizing their economic impact on plants through gall formation. Management strategies, ranging from cultural and physical to biological and chemical controls, along with resistance mechanisms and marker-assisted selection, are explored. While recognizing the limitations of traditional nematicides, recent breakthroughs in non-fumigant alternatives like fluensulfone, spirotetramat, and fluopyram offer promising avenues for sustainable RKN management. Despite the success of resistance mechanisms like the Mi gene, challenges persist, prompting the need for integrative approaches to tackle Mi-virulent isolates. In conclusion, the review stresses the importance of innovative and resilient control measures for sustainable agriculture, emphasizing ongoing research to address evolving challenges posed by RKN. The integration of botanicals, resistant cultivars, and biological controls, alongside advancements in non-fumigant nematicides, contributes novel insights to the field, laying the ground work for future research directions to ensure the long-term sustainability of agriculture in the face of persistent RKN threats.

Reactive oxygen species may be involved in the distinctive biological effects of different doses of 12C6+ ion beams on Arabidopsis

Introduction

Heavy ion beam is a novel approach for crop mutagenesis with the advantage of high energy transfer line density and low repair effect after injury, however, little investigation on the biological effect on plant was performed. 50 Gy irradiation significantly stimulated the growth of Arabidopsis seedlings, as indicated by an increase in root and biomass, while 200 Gy irradiation significantly inhibited the growth of seedlings, causing a visible decrease in plant growth.

Methods

The Arabidopsis seeds were irradiated by 12C6+. Monte Carlo simulations were used to calculate the damage to seeds and particle trajectories by ion implantation. The seed epidermis received SEM detection and changes in its organic composition were detected using FTIR. Evidence of ROS and antioxidant systems were analyzed. RNA-seq and qPCR were used to detect changes in seedling transcript levels.

Results and discussion

Monte Carlo simulations revealed that high-dose irradiation causes various damage. Evidence of ROS and antioxidant systems implies that the emergence of phenotypes in plant cells may be associated with oxidative stress. Transcriptomic analysis of the seedlings demonstrated that 170 DEGs were present in the 50 Gy and 200 Gy groups and GO enrichment indicated that they were mainly associated with stress resistance and cell wall homeostasis. Further GO enrichment of DEGs unique to 50 Gy and 200 Gy revealed 58 50Gy-exclusive DEGs were enriched in response to oxidative stress and jasmonic acid entries, while 435 200 Gy-exclusive DEGs were enriched in relation to oxidative stress, organic cyclic compounds, and salicylic acid. This investigation advances our insight into the biological effects of heavy ion irradiation and the underlying mechanisms.

Fusaric acid-evoked oxidative stress affects plant defence system by inducing biochemical changes at subcellular level

Fusaric acid (FA) is one of the most harmful phytotoxins produced in various plant-pathogen interactions. Fusarium species produce FA as a secondary metabolite, which can infect many agronomic crops at all stages of development from seed to fruit, and FA production can further compromise plant survival because of its phytotoxic effects. FA exposure in plant species adversely affects plant growth, development and crop yield. FA exposure in plants leads to the generation of reactive oxygen species (ROS), which cause cellular damage and ultimately cell death. Therefore, FA-induced ROS accumulation in plants has been a topic of interest for many researchers to understand the plant-pathogen interactions and plant defence responses. In this study, we reviewed the FA-mediated oxidative stress and ROS-induced defence responses of antioxidants, as well as hormonal signalling in plants. The effects of FA phytotoxicity on lipid peroxidation, physiological changes and ultrastructural changes at cellular and subcellular levels were reported. Additionally, DNA damage, cell death and adverse effects on photosynthesis have been explained. Some possible approaches to overcome the harmful effects of FA in plants were also discussed. It is concluded that FA-induced ROS affect the enzymatic and non-enzymatic antioxidant system regulated by phytohormones. The effects of FA are also associated with other photosynthetic, ultrastructural and genotoxic modifications in plants.

GhRCD1 regulates cotton somatic embryogenesis by modulating the GhMYC3-GhMYB44-GhLBD18 transcriptional cascade

Plant somatic embryogenesis (SE) is a multifactorial developmental process where embryos that can develop into whole plants are produced from somatic cells rather than through the fusion of gametes. The molecular regulation of plant SE, which involves the fate transition of somatic cells into embryogenic cells, is intriguing yet remains elusive. We deciphered the molecular mechanisms by which GhRCD1 interacts with GhMYC3 to regulate cell fate transitions during SE in cotton. While silencing of GhMYC3 had no discernible effect on SE, its overexpression accelerated callus formation, and proliferation. We identified two of GhMYC3 downstream SE regulators, GhMYB44 and GhLBD18. GhMYB44 overexpression was unconducive to callus growth but bolstered EC differentiation. However, GhLBD18 can be triggered by GhMYC3 but inhibited by GhMYB44, which positively regulates callus growth. On top of the regulatory cascade, GhRCD1 antagonistically interacts with GhMYC3 to inhibit the transcriptional function of GhMYC3 on GhMYB44 and GhLBD18, whereby a CRISPR-mediated rcd1 mutation expedites cell fate transition, resembling the effects of GhMYC3 overexpression. Furthermore, we showed that reactive oxygen species (ROS) are involved in SE regulation. Our findings elucidated that SE homeostasis is maintained by the tetrapartite module, GhRCD1-GhMYC3-GhMYB44-GhLBD18, which acts to modulate intracellular ROS in a temporal manner.

Functions of nitric oxide-mediated post-translational modifications under abiotic stress

Environmental conditions greatly impact plant growth and development. In the current context of both global climate change and land degradation, abiotic stresses usually lead to growth restriction limiting crop production. Plants have evolved to sense and respond to maximize adaptation and survival; therefore, understanding the mechanisms involved in the different converging signaling networks becomes critical for improving plant tolerance. In the last few years, several studies have shown the plant responses against drought and salinity, high and low temperatures, mechanical wounding, heavy metals, hypoxia, UV radiation, or ozone stresses. These threats lead the plant to coordinate a crosstalk among different pathways, highlighting the role of phytohormones and reactive oxygen and nitrogen species (RONS). In particular, plants sense these reactive species through post-translational modification (PTM) of macromolecules such as nucleic acids, proteins, and fatty acids, hence triggering antioxidant responses with molecular implications in the plant welfare. Here, this review compiles the state of the art about how plant systems sense and transduce this crosstalk through PTMs of biological molecules, highlighting the S-nitrosylation of protein targets. These molecular mechanisms finally impact at a physiological level facing the abiotic stressful traits that could lead to establishing molecular patterns underlying stress responses and adaptation strategies.

Ozone and nitrogen dioxide regulate similar gene expression responses in Arabidopsis but natural variation in the extent of cell death is likely controlled by different genetic loci

High doses of ozone (O₃) and nitrogen dioxide (NO₂) cause damage and cell death in plants. These two gases are among the most harmful air pollutants for ecosystems and therefore it is important to understand how plant resistance or sensitivity to these gases work at the molecular level and its genetic control. We compared transcriptome data from O₃ and NO₂ fumigations to other cell death related treatments, as well as individual marker gene transcript level in different Arabidopsis thaliana accessions. Our analysis revealed that O₃ and NO₂ trigger very similar gene expression responses that include genes involved in pathogen resistance, cell death and ethylene signaling. However, we also identified exceptions, for example RBOHF encoding a reactive oxygen species producing RESPIRATORY BURST OXIDASE PROTEIN F. This gene had increased transcript levels by O₃ but decreased transcript levels by NO₂, showing that plants can identify each of the gases separately and activate distinct signaling pathways. To understand the genetics, we conducted a genome wide association study (GWAS) on O₃ and NO₂ tolerance of natural Arabidopsis accessions. Sensitivity to both gases seem to be controlled by several independent small effect loci and we did not find an overlap in the significantly associated regions. Further characterization of the GWAS candidate loci identified new regulators of O₃ and NO₂ induced cell death including ABH1, a protein that functions in abscisic acid signaling, mRNA splicing and miRNA processing. The GWAS results will facilitate further characterization of the control of programmed cell death and differences between oxidative and nitrosative stress in plants.

CYSTEINE-RICH RECEPTOR-LIKE KINASE5 (CRK5) and CRK22 regulate the response to Verticillium dahliae toxins

Cysteine-rich receptor-like kinases (CRKs) play critical roles in responses to biotic and abiotic stresses. However, the molecular mechanisms of CRKs in plant defense responses remain unknown. Here, we demonstrated that two CRKs, CRK5 and CRK22, are involved in regulating defense responses to Verticillium dahliae toxins (Vd-toxins) in Arabidopsis (Arabidopsis thaliana). Biochemical and genetic analyses showed that CRK5 and CRK22 may act upstream of MITOGEN-ACTIVATED PROTEIN KINASE3 (MPK3) and MPK6 to regulate the salicylic acid (SA)-signaling pathway in response to Vd-toxins. In addition, MPK3 and MPK6 interact with the transcription factor WRKY70 to modulate defense responses to Vd-toxins. WRKY70 directly binds the promoter domains of the SA-signaling-related transcription factor genes TGACG SEQUENCE-SPECIFIC BINDING PROTEIN (TGA2) and TGA6 to regulate their expression in response to Vd-toxins. Thus, our study reveals a mechanism by which CRK5 and CRK22 regulate SA signaling through the MPK3/6-WRKY70-TGA2/6 pathway in response to Vd-toxins.

Insights into the Genomic Regions and Candidate Genes of Senescence-Related Traits in Upland Cotton via GWAS

Senescence is the last stage of plant development and is controlled by both internal and external factors. Premature senescence significantly affects the yield and quality of cotton. However, the genetic architecture underlying cotton senescence remains unclear. In this study, genome-wide association studies (GWAS) were performed based on 3,015,002 high-quality SNP markers from the resequencing data of 355 upland cotton accessions to detect genomic regions for cotton senescence. A total of 977 candidate genes within 55 senescence-related genomic regions (SGRs), SGR1-SGR55, were predicted. Gene ontology (GO) analysis of candidate genes revealed that a set of biological processes was enriched, such as salt stress, ethylene processes, and leaf senescence. Furthermore, in the leaf senescence GO term, one candidate gene was focused on: Gohir.A12G270900 (GhMKK9), located in SGR36, which encodes a protein of the MAP kinase kinase family. Quantitative real-time PCR (qRT-PCR) analysis showed that GhMKK9 was up-regulated in old cotton leaves. Overexpression of GhMKK9 in Arabidopsis accelerated natural leaf senescence. Virus-induced gene silencing (VIGS) of GhMKK9 in cotton increased drought tolerance. These results suggest that GhMKK9 is a positive regulator and might be involved in drought-induced senescence in cotton. The results provide new insights into the genetic basis of cotton senescence and will be useful for improving cotton breeding in the future.

In silico characterization, molecular phylogeny, and expression profiling of genes encoding legume lectin-like proteins under various abiotic stresses in Arabidopsis thaliana

Background

Lectin receptor-like kinases (Lec-RLKs), a subfamily of RLKs, have been demonstrated to play an important role in signal transduction from cell wall to the plasma membrane during biotic stresses. Lec-RLKs include legume lectin-like proteins (LLPs), an important group of apoplastic proteins that are expressed in regenerating cell walls and play a role in immune-related responses. However, it is unclear whether LLPs have a function in abiotic stress mitigation and related signaling pathways. Therefore, in this study, we examined the possible role of LLPs in Arabidopsis thaliana (AtLLPs) under various abiotic stresses.

Results

The study was initiated by analyzing the chromosomal localization, gene structure, protein motif, peptide sequence, phylogeny, evolutionary divergence, and sub-cellular localization of AtLLPs. Furthermore, the expression profiling of these AtLLPs was performed using publicly accessible microarray datasets under various abiotic stresses, which indicated that all AtLLPs were differently expressed in both root and shoot tissues in response to abiotic stresses. The cis-regulatory elements (CREs) analysis in 500 bp promoter sequences of AtLLPs suggested the presence of multiple important CREs implicated for regulating abiotic stress responses, which was further supported by expressional correlation analysis between AtLLPs and their CREs cognate transcription factors (TFs). qRT-PCR analysis of these AtLLPs after 2, 6, and 12 h of cold, high light, oxidative (MV), UV-B, wound, and ozone stress revealed that all AtLLPs displayed differential expression patterns in most of the tested stresses, supporting their roles in abiotic stress response and signaling again. Out of these AtLLPs, AT1g53070 and AT5g03350 appeared to be important players. Furthermore, the mutant line of AT5g03350 exhibited higher levels of ROS than wild type plants till 12 h of exposure to high light, MV, UV-B, and wound, whereas its overexpression line exhibited comparatively lower levels of ROS, indicating a positive role of this gene in abiotic stress response in A. thaliana.

Conclusions

This study provides basic insights in the involvement of two important representative AtLLPs, AT1g53070 and AT5g03350, in abiotic stress response. However, further research is needed to determine the specific molecular mechanism of these AtLLPs in abiotic stress mitigation and related signaling pathways in A. thaliana.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08708-0.

Dynamic Expression, Differential Regulation and Functional Diversity of the CNGC Family Genes in Cotton

Cyclic nucleotide-gated channels (CNGCs) constitute a family of non-selective cation channels that are primarily permeable to Ca2+ and activated by the direct binding of cyclic nucleotides (i.e., cAMP and cGMP) to mediate cellular signaling, both in animals and plants. Until now, our understanding of CNGCs in cotton (Gossypium spp.) remains poorly addressed. In the present study, we have identified 40, 41, 20, 20, and 20 CNGC genes in G. hirsutum, G. barbadense, G. herbaceum, G. arboreum, and G. raimondii, respectively, and demonstrated characteristics of the phylogenetic relationships, gene structures, chromosomal localization, gene duplication, and synteny. Further investigation of CNGC genes in G. hirsutum, named GhCNGC1-40, indicated that they are not only extensively expressed in various tissues and at different developmental stages, but also display diverse expression patterns in response to hormones (abscisic acid, salicylic acid, methyl jasmonate, ethylene), abiotic (salt stress) and biotic (Verticillium dahlia infection) stimuli, which conform with a variety of cis-acting regulatory elements residing in the promoter regions; moreover, a set of GhCNGCs are responsive to cAMP signaling during cotton fiber development. Protein–protein interactions supported the functional aspects of GhCNGCs in plant growth, development, and stress responses. Accordingly, the silencing of the homoeologous gene pair GhCNGC1&18 and GhCNGC12&31 impaired plant growth and development; however, GhCNGC1&18-silenced plants enhanced Verticillium wilt resistance and salt tolerance, whereas GhCNGC12&31-silenced plants had opposite effects. Together, these results unveiled the dynamic expression, differential regulation, and functional diversity of the CNGC family genes in cotton. The present work has laid the foundation for further studies and the utilization of CNGCs in cotton genetic improvement.

In silico analysis and expression profiling of S-domain receptor-like kinases (SD-RLKs) under different abiotic stresses in Arabidopsis thaliana

BACKGROUND

S-domain receptor-like kinases (SD-RLKs) are an important and multi-gene subfamily of plant receptor-like/pelle kinases (RLKs), which are known to play a significant role in the development and immune responses of Arabidopsis thaliana. The conserved cysteine residues in the extracellular domain of SD-RLKs make them interesting candidates for sensing reactive oxygen species (ROS), assisting oxidative stress mitigation and associated signaling pathways during abiotic stresses. However, how closely SD-RLKs are interrelated to abiotic stress mitigation and signaling remains unknown in A. thaliana.

RESULTS

This study was initiated by examining the chromosomal localization, phylogeny, sequence and differential expression analyses of 37 SD-RLK genes using publicly accessible microarray datasets under cold, osmotic stress, genotoxic stress, drought, salt, UV-B, heat and wounding. Out of 37 SD-RLKs, 12 genes displayed differential expression patterns in both the root and the shoot tissues. Promoter structure analysis suggested that these 12 SD-RLK genes harbour several potential cis-regulatory elements (CREs), which are involved in regulating multiple abiotic stress responses. Based on these observations, we investigated the expression patterns of 12 selected SD-RLKs under ozone, wounding, oxidative (methyl viologen), UV-B, cold, and light stress at different time points using semi-qRT-PCR. Of these 12 SD-SRKs, the genes At1g61360, At1g61460, At1g61380, and At4g27300 emerged as potential candidates that maintain their expression in most of the stress treatments till exposure for 12 h. Expression patterns of these four genes were further verified under similar stress treatments using qRT-PCR. The expression analysis indicated that the gene At1g61360, At1g61380, and At1g61460 were mostly up-regulated, whereas the expression of At4g27300 either up- or down-regulated in these conditions.

CONCLUSIONS

To summarize, the computational analysis and differential transcript accumulation of SD-RLKs under various abiotic stresses suggested their association with abiotic stress tolerance and related signaling in A. thaliana. We believe that a further detailed study will decipher the specific role of these representative SD-RLKs in abiotic stress mitigation vis-a-vis signaling pathways in A. thaliana.

Dynamics of Foliar Responses to O3 Stress as a Function of Phytotoxic O3 Dose in Hybrid Poplar

With background concentrations having reached phytotoxic levels during the last century, tropospheric ozone (O₃) has become a key climate change agent, counteracting carbon sequestration by forest ecosystems. One of the main knowledge gaps for implementing the recent O₃ flux-based critical levels (CLs) concerns the assessment of effective O₃ dose leading to adverse effects in plants. In this study, we investigate the dynamics of physiological, structural, and morphological responses induced by two levels of O₃ exposure (80 and 100 ppb) in the foliage of hybrid poplar, as a function of phytotoxic O₃ dose (POD₀) and foliar developmental stage. After a latency period driven by foliar ontological development, the gas exchanges and chlorophyll content decreased with higher POD₀ monotonically. Hypersensitive response-like lesions appeared early during exposure and showed sigmoidal-like dynamics, varying according to leaf age. At current POD_{1_SPEC} CL, notwithstanding the aforementioned reactions and initial visible injury to foliage, the treated poplars had still not shown any growth or biomass reduction. Hence, this study demonstrates the development of a complex syndrome of early reactions below the flux-based CL, with response dynamics closely determined by the foliar ontological stage and environmental conditions. General agreement with patterns observed in the field appears indicative of early O₃ impacts on processes relevant, e.g., biodiversity ecosystem services before those of economic significance - i.e., wood production, as targeted by flux-based CL.

Ozone Induced Stomatal Regulations, MAPK and Phytohormone Signaling in Plants

Ozone (O₃) is a gaseous environmental pollutant that can enter leaves through stomatal pores and cause damage to foliage. It can induce oxidative stress through the generation of reactive oxygen species (ROS) like hydrogen peroxide (H₂O₂) that can actively participate in stomatal closing or opening in plants. A number of phytohormones, including abscisic acid (ABA), ethylene (ET), salicylic acid (SA), and jasmonic acid (JA) are involved in stomatal regulation in plants. The effects of ozone on these phytohormones’ ability to regulate the guard cells of stomata have been little studied, however, and the goal of this paper is to explore and understand the effects of ozone on stomatal regulation through guard cell signaling by phytohormones. In this review, we updated the existing knowledge by considering several physiological mechanisms related to stomatal regulation after response to ozone. The collected information should deepen our understanding of the molecular pathways associated with response to ozone stress, in particular, how it influences stomatal regulation, mitogen-activated protein kinase (MAPK) activity, and phytohormone signaling. After summarizing the findings and noting the gaps in the literature, we present some ideas for future research on ozone stress in plants

Ozone responses in Arabidopsis: beyond stomatal conductance

Tropospheric ozone (O3) is a major air pollutant that decreases yield of important crops worldwide. Despite long-lasting research of its negative effects on plants, there are many gaps in our knowledge on how plants respond to O3. In this study, we used natural variation in the model plant Arabidopsis (Arabidopsis thaliana) to characterize molecular and physiological mechanisms underlying O3 sensitivity. A key parameter in models for O3 damage is stomatal uptake. Here we show that the extent of O3 damage in the sensitive Arabidopsis accession Shahdara (Sha) does not correspond with O3 uptake, pointing toward stomata-independent mechanisms for the development of O3 damage. We compared tolerant (Col-0) versus sensitive accessions (Sha, Cvi-0) in assays related to photosynthesis, cell death, antioxidants, and transcriptional regulation. Acute O3 exposure increased cell death, development of lesions in the leaves, and decreased photosynthesis in sensitive accessions. In both Sha and Cvi-0, O3-induced lesions were associated with decreased maximal chlorophyll fluorescence and low quantum yield of electron transfer from Photosystem II to plastoquinone. However, O3-induced repression of photosynthesis in these two O3-sensitive accessions developed in different ways. We demonstrate that O3 sensitivity in Arabidopsis is influenced by genetic diversity given that Sha and Cvi-0 developed accession-specific transcriptional responses to O3. Our findings advance the understanding of plant responses to O3 and set a framework for future studies to characterize molecular and physiological mechanisms allowing plants to respond to high O3 levels in the atmosphere as a result of high air pollution and climate change.

Dissecting Contrasts in Cell Death, Hormone, and Defense Signaling in Response to Botrytis cinerea and Reactive Oxygen Species

Plants require interaction between signaling pathways to differentiate and integrate stress responses and deploy appropriate defenses. The hormones ethylene, salicylic acid (SA), and jasmonic acid (JA) are important regulators of plant defenses. Numerous interactions between these signaling pathways are the cornerstone of robust plant immunity. Additionally, during the early response to pathogens, reactive oxygen species (ROS) act as signaling molecules. Here, we examined the extent of signal interaction in the early stages of Botrytis cinerea infection. To enable a comparison between B. cinerea infection with ROS signaling, we subjected plants to ozone treatment, which stimulates an apoplastic ROS burst. We used a collection of single, double, and triple signaling mutants defective in hormone signaling and biosynthesis and subjected them to B. cinerea infection and ozone treatment at different timepoints. We examined lesion size, cell death, and gene expression (both quantitatively and spatially). The two treatments shared many similarities, especially in JA-insensitive mutants, which were sensitive to both treatments. Unexpectedly, a B. cinerea-susceptible JA-insensitive mutant (coi1), became tolerant when both SA biosynthesis and signaling was impaired (coi1 npr1 sid2), demonstrating that JA responses may be under the control of SA. Extensive marker gene analysis indicated JA as the main regulator of both B. cinerea and ozone defenses. In addition, we identified the transcription factor SR1 as a crucial regulator of PLANT DEFENSIN expression and cell-death regulation, which contributes to resistance to B. cinerea. Overall, our work further defines the context of ROS in plant defense signaling.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A Rapeseed WRKY Transcription Factor Phosphorylated by CPK Modulates Cell Death and Leaf Senescence by Regulating the Expression of ROS and SA-Synthesis-Related Genes

Salicylic acid (SA) and reactive oxygen species (ROS) are two well-defined inducers of leaf senescence. Here, we identified a novel WRKY transcription factor gene WSR1 (WRKY regulating SA and ROS 1) in Brassica napus (rapeseed) in promoting SA and ROS production, which eventually led to leaf senescence thereafter. Its expression increased in senescing leaves. Ca²⁺-dependent protein kinase (CPK) 5 and -6 interacted with and phosphorylated BnaWSR1. Overexpression of phosphomimic BnaWSR1 (BnaWSR1ca) in rapeseed protoplasts elicited ROS production and cell death while its ectopic expression in Arabidopsis enhanced SA and ROS levels and, hence, accelerated leaf senescence. Furthermore, BnaWSR1ca activated the expression of Isochorismate Synthase 1 (ICS1), Respiratory Burst Oxidase Homologue (Rboh) D, and Senescence-Associated Gene 14 (SAG14). Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation-quantitative PCR (ChIP-qPCR) assays demonstrated that BnaWSR1ca directly bound to promoter regions of ICS1, RbohD, and SAG14. These data have identified a CPK-WSR1 module that integrates SA and ROS to control cell death and leaf senescence.

A common wild rice-derived BOC1 allele reduces callus browning in indica rice transformation

Callus browning, a common trait derived from the indica rice cultivar (Oryza sativa L.), is a challenge to transformation regeneration. Here, we report the map-based cloning of BROWNING OF CALLUS1 (BOC1) using a population derived from crossing Teqing, an elite indica subspecies exhibiting callus browning, and Yuanjiang, a common wild rice accession (Oryza rufipogon Griff.) that is less susceptible to callus browning. We show that BOC1 encodes a SIMILAR TO RADICAL-INDUCED CELL DEATH ONE (SRO) protein. Callus browning can be reduced by appropriate upregulation of BOC1, which consequently improves the genetic transformation efficiency. The presence of a Tourist-like miniature inverted-repeat transposable element (Tourist MITE) specific to wild rice in the promoter of BOC1 increases the expression of BOC1 in callus. BOC1 may decrease cell senescence and death caused by oxidative stress. Our study provides a gene target for improving tissue culturability and genetic transformation.

Callus browning heavily affects indica rice transformation regeneration. Here, the authors show transposon insertion in the promoter of BOC1 gene, encoding a SIMILAR TO RADICAL-INDUCED CELL DEATH ONE protein, can upregulate its expression and decrease callus browning in cultivated rice by releasing oxidative stress.

Does the alternative respiratory pathway offer protection against the adverse effects resulting from climate change?

Elevated greenhouse gases (GHGs) induce adverse conditions directly and indirectly, causing decreases in plant productivity. To deal with climate change effects, plants have developed various mechanisms including the fine-tuning of metabolism. Plant respiratory metabolism is highly flexible due to the presence of various alternative pathways. The mitochondrial alternative oxidase (AOX) respiratory pathway is responsive to these changes, and several lines of evidence suggest it plays a role in reducing excesses of reactive oxygen species (ROS) and reactive nitrogen species (RNS) while providing metabolic flexibility under stress. Here we discuss the importance of the AOX pathway in dealing with elevated carbon dioxide (CO2), nitrogen oxides (NOx), ozone (O3), and the main abiotic stresses induced by climate change.

The nuclear transporter SAD2 plays a role in calcium- and H2 O2 -mediated cell death in Arabidopsis

In response to pathogens, plant cells exhibit a rapid increase in the intracellular calcium concentration and a burst of reactive oxygen species (ROS). The cytosolic increase in Ca²⁺ and the accumulation of ROS are critical for inducing programmed cell death (PCD), but the molecular mechanism is not fully understood. We screened an Arabidopsis mutant, sad2-5, which harbours a T-DNA insertion in the 18^th exon of the importin beta-like gene, SAD2. The H₂ O₂ -induced increase in the [Ca²⁺ ]_cyt of the sad2-5 mutant was greater than that of the wild type, and the sad2-5 mutant showed clear cell death phenotypes and abnormal H₂ O₂ accumulation under fumonisin-B1 (FB1) treatment. CaCl₂ could enhance the FB1-induced cell death of the sad2-5 mutant, whereas lanthanum chloride (LaCl₃ ), a broad-spectrum calcium channel blocker, could restore the FB1-induced PCD phenotype of sad2-5. The sad2-5 fbr11-1 double mutant exhibited the same FB1-insensitive phenotype as fbr11-1, which plays a critical role in novo sphingolipid synthesis, indicating that SAD2 works downstream of FBR11. These results suggest the important role of nuclear transporters in calcium- and ROS-mediated PCD response as well as provide an important theoretical basis for further analysis of the molecular mechanism of SAD2 function in PCD and for improvement of the resistance of crops to adverse environments.

Dynamics of plant responses to combinations of air pollutants

The focus of this review is on how plants respond to combinations of multiple air pollutants. Global pollution trends, plant physiological responses and ecological perspectives in natural and agricultural systems are all discussed. In particular, we highlight the importance of studying sequential or simultaneous exposure of plants to pollutants, rather than exposure to individual pollutants in isolation, and explore how these responses may interfere with the way plants interact with their biotic community. Air pollutants can alter the normal physiology and metabolic functioning of plants. Here we describe how the phenotypic and molecular changes in response to multiple pollutants can differ compared to those elicited by single pollutants, and how different responses have been observed between plants in the field and in controlled laboratory conditions and between trees and crop plants. From an ecological perspective, we discuss how air pollution can result in greater susceptibility to biotic stressors and in direct or indirect effects on interactions with organisms that occupy higher trophic levels. Finally, we provide an overview of the potential uses of plants to mitigate air pollution, exploring the feasibility for pollution removal via the processes of bio-accumulation and phytoremediation. We conclude by proposing some new directions for future research in the field.

Arabidopsis PLDs with C2-domain function distinctively in hypoxia

Hypoxia (oxygen deprivation) causes metabolic disturbances at physiological, biochemical and genetic levels and results in decreased plant growth and development. Phospholipase D (PLD)-mediated signaling was reported for abiotic and biotic stress signaling events in plants. To investigate the participatory role of PLDs also in hypoxia signaling, we used wild type of Arabidopsis thaliana and 10 pld isoform mutants containing C2-domain. Hypoxia-induced changes in three major signaling players, namely, cytosolic free calcium (Ca²⁺ _cyt ), reactive oxygen species (ROS) and phosphatidic acid (PA), were determined in mesophyll protoplasts. The Ca²⁺ _cyt and ROS levels were monitored by fluorescence microscopy and confocal imaging, while PA levels were quantified by an enzymatic method. Our findings reveal that the elevations of cytosolic calcium and PA are reduced in all the 10 mutants dysfunctional in PLD isoforms. The hypoxia-related changes in both calcium and ROS show different kinetic patterns depending on the type of PLD studied. Pharmacological experiments confirm that both external and internal sources contribute to calcium and ROS accumulation under hypoxia. PLDα1-3, PLDβ1 and PLDγ1-3 are likely involved in calcium signaling under hypoxia as well as in PA production, while all investigated PLDs, except for PLDγ3, take part in ROS elevation.

Extracellular peptide Kratos restricts cell death during vascular development and stress in Arabidopsis

During plant vascular development, xylem tracheary elements (TEs) form water-conducting, empty pipes by genetically regulated cell death. Cell death is prevented from spreading to non-TEs by unidentified intercellular mechanisms, downstream of METACASPASE9 (MC9)-mediated regulation of autophagy in TEs. Here, we identified differentially abundant extracellular peptides in vascular-differentiating wild-type and MC9-down-regulated Arabidopsis cell suspensions. A peptide named Kratos rescued the abnormally high ectopic non-TE death resulting from either MC9 knockout or TE-specific overexpression of the ATG5 autophagy protein during experimentally induced vascular differentiation in Arabidopsis cotyledons. Kratos also reduced cell death following mechanical damage and extracellular ROS production in Arabidopsis leaves. Stress-induced but not vascular non-TE cell death was enhanced by another identified peptide, named Bia. Bia is therefore reminiscent of several known plant cell death-inducing peptides acting as damage-associated molecular patterns. In contrast, Kratos plays a novel extracellular cell survival role in the context of development and during stress response.

Poly(ADP-Ribose) Polymerases in Plants and Their Human Counterparts: Parallels and Peculiarities

Poly(ADP-ribosyl)ation is a rapid and transient post-translational protein modification that was described first in mammalian cells. Activated by the sensing of DNA strand breaks, poly(ADP-ribose)polymerase1 (PARP1) transfers ADP-ribose units onto itself and other target proteins using NAD⁺ as a substrate. Subsequently, DNA damage responses and other cellular responses are initiated. In plants, poly(ADP-ribose) polymerases (PARPs) have also been implicated in responses to DNA damage. The Arabidopsis genome contains three canonical PARP genes, the nomenclature of which has been uncoordinated in the past. Albeit assumptions concerning the function and roles of PARP proteins in planta have often been inferred from homology and structural conservation between plant PARPs and their mammalian counterparts, plant-specific roles have become apparent. In particular, PARPs have been linked to stress responses of plants. A negative role under abiotic stress has been inferred from studies in which a genetic or, more commonly, pharmacological inhibition of PARP activity improved the performance of stressed plants; in response to pathogen-associated molecular patterns, a positive role has been suggested. However, reports have been inconsistent, and the effects of PARP inhibitors appear to be more robust than the genetic abolition of PARP gene expression, indicating the presence of alternative targets of those drugs. Collectively, recent evidence suggests a conditionality of stress-related phenotypes of parp mutants and calls for a reconsideration of PARP inhibitor studies on plants. This review critically summarizes our current understanding of poly(ADP-ribosylation) and PARP proteins in plants, highlighting similarities and differences to human PARPs, areas of controversy, and requirements for future studies.

Interaction of methyl viologen-induced chloroplast and mitochondrial signalling in Arabidopsis

Reactive oxygen species (ROS) are key signalling intermediates in plant metabolism, defence, and stress adaptation. In plants, both the chloroplast and mitochondria are centres of metabolic control and ROS production, which coordinate stress responses in other cell compartments. The herbicide and experimental tool, methyl viologen (MV) induces ROS generation in the chloroplast under illumination, but is also toxic in non-photosynthetic organisms. We used MV to probe plant ROS signalling in compartments other than the chloroplast. Taking a genetic approach in the model plant Arabidopsis (Arabidopsis thaliana), we used natural variation, QTL mapping, and mutant studies with MV in the light, but also under dark conditions, when the chloroplast electron transport is inactive. These studies revealed a light-independent MV-induced ROS-signalling pathway, suggesting mitochondrial involvement. Mitochondrial Mn SUPEROXIDE DISMUTASE was required for ROS-tolerance and the effect of MV was enhanced by exogenous sugar, providing further evidence for the role of mitochondria. Mutant and hormone feeding assays revealed roles for stress hormones in organellar ROS-responses. The radical-induced cell death1 mutant, which is tolerant to MV-induced ROS and exhibits altered mitochondrial signalling, was used to probe interactions between organelles. Our studies suggest that mitochondria are involved in the response to ROS induced by MV in plants.

When "thirsty" means "less able to activate the signalling wave trigged by a pulse of ozone": A case of study in two Mediterranean deciduous oak species with different drought sensitivity

There is a lack of knowledge about the possibility that plants facing abiotic stressors, such as drought, have an altered perception of a pulse of O₃ and incur in alterations of their signalling network. This poses some concerns as to whether defensive strategy to cope episodic O₃ peaks in healthy plants may fail under stress. In this study, a set of saplings of two Mediterranean deciduous species, Quercus cerris and Q. pubescens, was subjected to water withholding (20% of daily evapotranspiration for 15 days) while another set was kept well-watered. Saplings were then subjected to a pulse of O₃ (200 nl l^-1 for 5 h) or maintained in filtered air. Q. pubescens had a more severe decline of photosynthesis and leaf PDΨ_w (about -65% and 5-fold lower than in well-watered ones) and events of cell death were observed under drought when compared to Q. cerris, which is supportive for a higher sensitivity to drought exhibited by this species. When O₃ was applied after drought, patterns of signalling compounds were altered in both species. Only in Q. pubescens, the typical O₃-induced accumulation of apoplastic reactive oxygen species, which is the first necessary step for the activation of signalling cascade, was completely lost. In Q. cerris the most frequent changes encompassed the weakening of peaks of key signalling molecules (ethylene and salicylic acid), whereas in Q. pubescens both delayed (salicylic and jasmonic acid) or weakened (ethylene and salicylic acid) peaks were observed. This is translated to a higher ability of Q. cerris to maintain a prompt activation of defensive reaction to counteract oxidative damage due to the pollutant. Our results reveal the complexity of the signalling network in plants facing multiple stresses and highlight the need to further investigate possible alteration of defensive mechanism of tree species to predict their behavior.

BAKing up to Survive a Battle: Functional Dynamics of BAK1 in Plant Programmed Cell Death

In plants, programmed cell death (PCD) has diverse, essential roles in vegetative and reproductive development, and in the responses to abiotic and biotic stresses. Despite the rapid progress in understanding the occurrence and functions of the diverse forms of PCD in plants, the signaling components and molecular mechanisms underlying the core PCD machinery remain a mystery. The roles of BAK1 (BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1), an essential co-receptor of multiple receptor complexes, in the regulation of immunity and development- and defense-related PCD have been well characterized. However, the ways in which BAK1 functions in mediating PCD need to be further explored. In this review, different forms of PCD in both plants and mammals are discussed. Moreover, we mainly summarize recent advances in elucidating the functions and possible mechanisms of BAK1 in controlling diverse forms of PCD. We also highlight the involvement of post-translational modifications (PTMs) of multiple signaling component proteins in BAK1-mediated PCD.

Arabidopsis downy mildew effector HaRxL106 suppresses plant immunity by binding to RADICAL-INDUCED CELL DEATH1

The oomycete pathogen Hyaloperonospora arabidopsidis (Hpa) causes downy mildew disease on Arabidopsis. To colonize its host, Hpa translocates effector proteins that suppress plant immunity into infected host cells. Here, we investigate the relevance of the interaction between one of these effectors, HaRxL106, and Arabidopsis RADICAL-INDUCED CELL DEATH1 (RCD1). We use pathogen infection assays as well as molecular and biochemical analyses to test the hypothesis that HaRxL106 manipulates RCD1 to attenuate transcriptional activation of defense genes. We report that HaRxL106 suppresses transcriptional activation of salicylic acid (SA)-induced defense genes and alters plant growth responses to light. HaRxL106-mediated suppression of immunity is abolished in RCD1 loss-of-function mutants. We report that RCD1-type proteins are phosphorylated, and we identified Mut9-like kinases (MLKs), which function as phosphoregulatory nodes at the level of photoreceptors, as RCD1-interacting proteins. An mlk1,3,4 triple mutant exhibits stronger SA-induced defense marker gene expression compared with wild-type plants, suggesting that MLKs also affect transcriptional regulation of SA signaling. Based on the combined evidence, we hypothesize that nuclear RCD1/MLK complexes act as signaling nodes that integrate information from environmental cues and pathogen sensors, and that the Arabidopsis downy mildew pathogen targets RCD1 to prevent activation of plant immunity.

To defend or to grow: lessons from Arabidopsis C24

The emergence of Arabidopsis as a model species and the availability of genetic and genomic resources have resulted in the identification and detailed characterization of abiotic stress signalling pathways. However, this has led only to limited success in engineering abiotic stress tolerance in crops. This is because there needs to be a deeper understanding of how to combine resistances to a range of stresses with growth and productivity. The natural variation and genomic resources of Arabidopsis thaliana (Arabidopsis) are a great asset to understand the mechanisms of multiple stress tolerances. One natural variant in Arabidopsis is the accession C24, and here we provide an overview of the increasing research interest in this accession. C24 is highlighted as a source of tolerance for multiple abiotic and biotic stresses, and a key accession to understand the basis of basal immunity to infection, high water use efficiency, and water productivity. Multiple biochemical, physiological, and phenological mechanisms have been attributed to these traits in C24, and none of them constrains productivity. Based on the uniqueness of C24, we postulate that the use of variation derived from natural selection in undomesticated species provides opportunities to better understand how complex environmental stress tolerances and resource use efficiency are co-ordinated.

Reactive Oxygen Species in Plant Signaling

As fixed organisms, plants are especially affected by changes in their environment and have consequently evolved extensive mechanisms for acclimation and adaptation. Initially considered by-products from aerobic metabolism, reactive oxygen species (ROS) have emerged as major regulatory molecules in plants and their roles in early signaling events initiated by cellular metabolic perturbation and environmental stimuli are now established. Here, we review recent advances in ROS signaling. Compartment-specific and cross-compartmental signaling pathways initiated by the presence of ROS are discussed. Special attention is dedicated to established and hypothetical ROS-sensing events. The roles of ROS in long-distance signaling, immune responses, and plant development are evaluated. Finally, we outline the most challenging contemporary questions in the field of plant ROS biology and the need to further elucidate mechanisms allowing sensing, signaling specificity, and coordination of multiple signals.

Simultaneous Downregulation of MTHFR and COMT in Switchgrass Affects Plant Performance and Induces Lesion-Mimic Cell Death

Switchgrass (Panicum virgatum) has been developed into a model lignocellulosic bioenergy crop. Downregulation of caffeic acid O-methyltransferase (COMT), a key enzyme in lignin biosynthesis, has been shown to alter lignification and increase biofuel yield in switchgrass. Methylenetetrahydrofolate reductase (MTHFR) mediates C1 metabolism and provides methyl units consumed by COMT. It was predicted that co-silencing of MTHFR and COMT would impact lignification even more than either of the single genes. However, our results showed that strong downregulation of MTHFR in a COMT-deficient background led to altered plant growth and development, but no significant change in lignin content or composition was found when compared with COMT plants. Another unexpected finding was that the double MTHFR/COMT downregulated plants showed a novel lesion-mimic leaf phenotype. Molecular analyses revealed that the lesion-mimic phenotype was caused by the synergistic effect of MTHFR and COMT genes, with MTHFR playing a predominant role. Microarray analysis showed significant induction of genes related to oxidative and defense responses. The results demonstrated the lack of additive effects of MTHFR and COMT on lignification. Furthermore, this research revealed an unexpected role of the two genes in the modulation of lesion-mimic cell death as well as their synergistic effects on agronomic performance.

Losing the Warning Signal: Drought Compromises the Cross-Talk of Signaling Molecules in Quercus ilex Exposed to Ozone

Understanding the interactions between drought and acute ozone (O3) stress in terms of signaling molecules and cell death would improve the predictions of plant responses to climate change. The aim was to investigate whether drought stress influences the responses of plants to acute episodes of O3 exposure. In this study, the behavior of 84 Mediterranean evergreen Quercus ilex plants was evaluated in terms of cross-talk responses among signaling molecules. Half of the sample was subjected to drought (20% of the effective daily evapotranspiration, for 15 days) and was later exposed to an acute O3 exposure (200 nL L-1 for 5 h). First, our results indicate that in well-water conditions, O3 induced a signaling pathway specific to O3-sensitive behavior. Second, different trends and consequently different roles of phytohormones and signaling molecules (ethylene, ET; abscisic acid, ABA; salycilic acid, SA and jasmonic acid, JA) were observed in relation to water stress and O3. A spatial and functional correlation between these signaling molecules was observed in modulating O3-induced responses in well-watered plants. In contrast, in drought-stressed plants, these compounds were not involved either in O3-induced signaling mechanisms or in leaf senescence (a response observed in water-stressed plants before the O3-exposure). Third, these differences were ascribable to the fact that in drought conditions, most defense processes induced by O3 were compromised and/or altered. Our results highlight how Q. ilex plants suffering from water deprivation respond differently to an acute O3 episode compared to well-watered plants, and suggest new effect to be considered in plant responses to environmental changes. This poses the serious question as to whether or not multiple high-magnitude O3 events (as predicted) can change these cross-talk responses, thus opening it up possible further investigations.

The IDA-LIKE peptides IDL6 and IDL7 are negative modulators of stress responses in Arabidopsis thaliana

Small signalling peptides have emerged as important cell to cell messengers in plant development and stress responses. However, only a few of the predicted peptides have been functionally characterized. Here, we present functional characterization of two members of the IDA-LIKE (IDL) peptide family in Arabidopsis thaliana, IDL6 and IDL7. Localization studies suggest that the peptides require a signal peptide and C-terminal processing to be correctly transported out of the cell. Both IDL6 and IDL7 appear to be unstable transcripts under post-transcriptional regulation. Treatment of plants with synthetic IDL6 and IDL7 peptides resulted in down-regulation of a broad range of stress-responsive genes, including early stress-responsive transcripts, dominated by a large group of ZINC FINGER PROTEIN (ZFP) genes, WRKY genes, and genes encoding calcium-dependent proteins. IDL7 expression was rapidly induced by hydrogen peroxide, and idl7 and idl6 idl7 double mutants displayed reduced cell death upon exposure to extracellular reactive oxygen species (ROS). Co-treatment of the bacterial elicitor flg22 with IDL7 peptide attenuated the rapid ROS burst induced by treatment with flg22 alone. Taken together, our results suggest that IDL7, and possibly IDL6, act as negative modulators of stress-induced ROS signalling in Arabidopsis.

Ozonated water reduces susceptibility in tomato plants to Meloidogyne incognita by the modulation of the antioxidant system

Few studies have been carried out on the effect of ozonated water (O₃ wat) on the oxidative stress of root systems and, in particular, in combination with biotic stress. The aim of this study was to determine whether aqueous ozone is effective in the control of root-knot nematode (RKN) infection and to investigate the concomitant changes in the basal defence system. A tomato cultivar susceptible to Meloidogyne incognita was treated with O₃ wat as a soil drench. No negative effects were seen following ozone application in comparison with the control under the exposure conditions used. The treatment reduced significantly the nematode infection rate and induced changes in the morphology of nematode feeding sites, some of which were characterized by visible symptoms of senescence. The antioxidant response, as well as parameters of oxidative damage, were examined in untreated and O₃ wat-treated galls at 2, 4 and 7 days after inoculation and compared with uninfected roots. High levels of reactive oxygen species (ROS), H₂ O₂ and malondialdehyde were generated in galls in response to combined abiotic and biotic stresses. Throughout the experimental period, the activities and relative transcript levels of the antioxidant enzymes catalase, superoxide dismutase and ascorbate peroxidase produced different responses when exposed to ozone treatment and/or infection. The results demonstrate how O₃ wat protects tomato against the RKN M. incognita through the modulation of basal defence mechanisms.

No Silver Bullet - Canonical Poly(ADP-Ribose) Polymerases (PARPs) Are No Universal Factors of Abiotic and Biotic Stress Resistance of Arabidopsis thaliana

Abiotic and biotic stress can have a detrimental impact on plant growth and productivity. Hence, there is a substantial demand for key factors of stress responses to improve yield stability of crops. Members of the poly(ADP-ribose)polymerase (PARP) protein family, which post-translationally modify (PARylate) nuclear proteins, have been suggested as such universal determinants of plant stress responses. A role under abiotic stress has been inferred from studies in which a genetic or, more commonly, pharmacological inhibition of PARP activity improved the performance of stressed plants. To further elucidate the role of PARP proteins under stress, T-DNA knockout mutants for the three Arabidopsis thaliana PARP genes were subjected to drought, osmotic, salt, and oxidative stress. To exclude a functional redundancy, which was indicated by a transcriptional upregulation of the remaining parp genes, a parp triple mutant was generated. Surprisingly, parp mutant plants did not differ from wild type plants in any of these stress experiments, independent from the number of PARP genes mutated. The parp triple mutant was also analyzed for callose formation in response to the pathogenassociated molecular pattern flg22. Unexpectedly, callose formation was unaltered in the mutant, albeit pharmacological PARP inhibition robustly blocked this immune response, confirming previous reports. Evidently, pharmacological inhibition appears to be more robust than the abolition of all PARP genes, indicating the presence of so-far undescribed proteins with PARP activity. This was supported by the finding that protein PARylation was not absent, but even increased in the parp triple mutant. Candidates for novel PARP-inhibitor targets may be found in the SRO protein family. These proteins harbor a catalytic PARP-like domain and are centrally involved in stress responses. Molecular modeling analyses, employing animal PARPs as templates, indeed indicated a capability of the SRO proteins RCD1 and SRO1 to bind nicotinamide-derived inhibitors. Collectively, the results of our study suggest that the stress-related phenotypes of parp mutants are highly conditional, and they call for a reconsideration of PARP inhibitor studies. In the context of this study, we also propose a unifying nomenclature of PARP genes and parp mutants, which is currently highly inconsistent and redundant.

A review of redox signaling and the control of MAP kinase pathway in plants

Mitogen-activated protein kinase (MAPK) cascades are evolutionarily conserved modules among eukaryotic species that range from yeast, plants, flies to mammals. In eukaryotic cells, reactive oxygen species (ROS) has both physiological and toxic effects. Both MAPK cascades and ROS signaling are involved in plant response to various biotic and abiotic stresses. It has been observed that not only can ROS induce MAPK activation, but also that disturbing MAPK cascades can modulate ROS production and responses. This review will discuss the potential mechanisms by which ROS may activate and/or regulate MAPK cascades in plants. The role of MAPK cascades and ROS signaling in regulating gene expression, stomatal function, and programmed cell death (PCD) is also discussed. In addition, the relationship between Rboh-dependent ROS production and MAPK activation in PAMP-triggered immunity will be reviewed.

Serotonin modulates Arabidopsis root growth via changes in reactive oxygen species and jasmonic acid-ethylene signaling

Serotonin (5-hydroxytryptamine) is a bioactive indoleamine with neurotransmitter function in vertebrates, which represents an emerging signaling molecule in plants, playing key roles in the development and defense. In this study, the role of reactive oxygen species (ROS) and jasmonic acid (JA)-ethylene (Et) signaling in root developmental alterations induced by serotonin was investigated. An Arabidopsis thaliana mutant defective at the RADICAL-INDUCED CELL DEATH1 (RCD1) locus was resistant to paraquat-induced ROS accumulation in primary roots and showed decreased inhibition or root growth in response to serotonin. A suite of JA- and Et-related mutants including coronatine insensitive1, jasmonic acid resistant1 (jar1), etr1, ein2 and ein3 showed tolerance to serotonin in the inhibition of primary root growth and ROS redistribution within the root tip when compared with wild-type (WT) seedlings. Competence assays between serotonin and AgNO3 , a well-known blocker of Et action, showed that primary root growth in medium supplemented with serotonin was normalized by AgNO3 , whereas roots of eto3, an Et overproducer mutant, were oversensitive to serotonin. Comparison of ROS levels in WT, etr1, jar1 and rcd1 primary root tips using the ROS-specific probe 2',7'-dichlorofluorescein diacetate and confocal imaging showed that serotonin inhibition of primary root growth likely occurs independently of its conversion into melatonin. Our results provide compelling evidence that serotonin affects ROS distribution in roots, involving RCD1 and components of the JA-Et signaling pathways.

Dissecting Abscisic Acid Signaling Pathways Involved in Cuticle Formation

The cuticle is the outer physical barrier of aerial plant surfaces and an important interaction point between plants and the environment. Many environmental stresses affect cuticle formation, yet the regulatory pathways involved remain undefined. We used a genetics and gene expression analysis in Arabidopsis thaliana to define an abscisic acid (ABA) signaling loop that positively regulates cuticle formation via the core ABA signaling pathway, including the PYR/PYL receptors, PP2C phosphatase, and SNF1-Related Protein Kinase (SnRK) 2.2/SnRK2.3/SnRK2.6. Downstream of the SnRK2 kinases, cuticle formation was not regulated by the ABA-responsive element-binding transcription factors but rather by DEWAX, MYB16, MYB94, and MYB96. Additionally, low air humidity increased cuticle formation independent of the core ABA pathway and cell death/reactive oxygen species signaling attenuated expression of cuticle-biosynthesis genes. In Physcomitrella patens, exogenous ABA suppressed expression of cuticle-related genes, whose Arabidopsis orthologs were ABA-induced. Hence, the mechanisms regulating cuticle formation are conserved but sophisticated in land plants. Signaling specifically related to cuticle deficiency was identified to play a major role in the adaptation of ABA signaling pathway mutants to increased humidity and in modulating their immunity to Botrytis cinerea in Arabidopsis. These results define a cuticle-specific downstream branch in the ABA signaling pathway that regulates responses to the external environment.

Suppression of PCD-related genes affects salt tolerance in Arabidopsis

This work aims at examining a natural exciting phenomenon suggesting that suppression of genes inducing programmed cell death (PCD) might confer tolerance against abiotic stresses in plants. PCD-related genes were induced in tobacco under oxalic acid (OA) treatment (20 mM), and plant cells were characterized to confirm the incidence of PCD. The results indicated that PCD was triggered 24 h after the exposure to OA. Then, RNAs were extracted from tobacco cells 0, 2, 6, 12 and 24 h after treatment for deep sequencing. RNA-Seq analyses were done with a special emphasis to clusters whose PCD-related genes were upregulated after 2 h of OA exposure. Accordingly, 23 tobacco PCD-related genes were knocked down via virus-induced gene silencing (VIGS), whereas our results indicated the influence of five of them on inducing or suppressing PCD. Knockout T-DNA insertion mutants of these five genes in Arabidopsis were tested under salt stress (0, 100, 150, and 200 mM NaCl), and the results indicated that a mutant of an antiapoptotic gene, namely Bax Inhibitor-1 (BI-1), whose VIGS induced PCD in tobacco, was salt sensitive, while a mutant of an apoptotic gene, namely mildew resistance locus O (Mlo), whose VIGS suppressed PCD, was salt tolerant as compared to the WT (Col) control. These data support our hypothesis that retarding PCD-inducing genes can result in higher levels of salt tolerance, while retarding PCD-suppressing genes can result in lower levels of salt tolerance in plants.

Roles of Defense Hormones in the Regulation of Ozone-Induced Changes in Gene Expression and Cell Death

Apoplast, the diffusional space between plant cell plasma membranes, is an important medium for signaling within and between the cells. Apoplastic reactive oxygen species (ROS) are crucial signaling molecules in various biological processes. ROS signaling is interconnected with the response to several hormones, including jasmonic acid (JA), salicylic acid (SA) and ethylene. Using ozone (O3) to activate apoplastic ROS signaling, we performed global and targeted analysis of transcriptional changes and cell death assays to dissect the contribution of hormone signaling and various transcription factors (TFs) in the regulation of gene expression and cell death. The contributions of SA, JA, and ethylene were assessed through analysis of single, double, and triple mutants deficient in biosynthesis or signaling for all three hormones. Even in the triple mutant, the global gene expression responses to O3 were mostly similar to the wild-type. Cell death in the JA receptor mutant coi1-16 was suppressed by impairment of the NADPH oxidase RBOHF, suggesting a role for a ROS signal in limiting the spread of cell death. In response to apoplastic ROS, there is not a single signaling pathway that regulates gene expression or cell death. Instead, several pathways regulate the apoplastic ROS response via combinatorial or overlapping mechanisms.

An Arabidopsis WDR protein coordinates cellular networks involved in light, stress response and hormone signals

The WD-40 repeat (WDR) protein acts as a scaffold for protein interactions in various cellular events. An Arabidopsis WDR protein exhibited sequence similarity with human WDR26, a scaffolding protein implicated in H2O2-induced cell death in neural cells. The AtWDR26 transcript was induced by auxin, abscisic acid (ABA), ethylene (ET), osmostic stress and salinity. The expression of AtWDR26 was regulated by light, and seed germination of the AtWDR26 overexpression (OE) and seedling growth of the T-DNA knock-out (KO) exhibited altered sensitivity to light. Root growth of the OE seedlings increased tolerance to ZnSO4 and NaCl stresses and were hypersensitive to inhibition of osmotic stress. Seedlings of OE and KO altered sensitivities to multiple hormones. Transcriptome analysis of the transgenic plants overexpressing AtWDR26 showed that genes involved in the chloroplast-related metabolism constituted the largest group of the up-regulated genes. AtWDR26 overexpression up-regulated a large number of genes related to defense cellular events including biotic and abiotic stress response. Furthermore, several members of genes functioning in the regulation of Zn homeostasis, and hormone synthesis and perception of auxin and JA were strongly up-regulated in the transgenic plants. Our data provide physiological and transcriptional evidence for AtWDR26 role in hormone, light and abiotic stress cellular events.

A genetic framework for H2O2 induced cell death in Arabidopsis thaliana

BACKGROUND

To survive in a changing environment plants constantly monitor their surroundings. In response to several stresses and during photorespiration plants use reactive oxygen species as signaling molecules. The Arabidopsis thaliana catalase2 (cat2) mutant lacks a peroxisomal catalase and under photorespiratory conditions accumulates H2O2, which leads to activation of cell death.

METHODS

A cat2 double mutant collection was generated through crossing and scored for cell death in different assays. Selected double mutants were further analyzed for photosynthetic performance and H2O2 accumulation.

RESULTS

We used a targeted mutant analysis with more than 50 cat2 double mutants to investigate the role of stress hormones and other defense regulators in H2O2-mediated cell death. Several transcription factors (AS1, MYB30, MYC2, WRKY70), cell death regulators (RCD1, DND1) and hormone regulators (AXR1, ERA1, SID2, EDS1, SGT1b) were essential for execution of cell death in cat2. Genetic loci required for cell death in cat2 was compared with regulators of cell death in spontaneous lesion mimic mutants and led to the identification of a core set of plant cell death regulators. Analysis of gene expression data from cat2 and plants undergoing cell death revealed similar gene expression profiles, further supporting the existence of a common program for regulation of plant cell death.

CONCLUSIONS

Our results provide a genetic framework for further study on the role of H2O2 in regulation of cell death. The hormones salicylic acid, jasmonic acid and auxin, as well as their interaction, are crucial determinants of cell death regulation.

Effect of nitrogen dioxide and sulfur dioxide on viability and morphology of oak pollen

BACKGROUND

Nitrogen dioxide (NO2) and sulfur dioxide (SO2) generated by excessive coal combustion and motor vehicle emissions are major air pollutants in the large cities of China. The objective of our study was to determine the effects of the exposure of oak pollens (Quercusmongolica) to several concentrations of NO2 or SO2.

METHODS

Pollen grains were exposed to 0.5 ppm to 5.0 ppm NO2 or SO2 for 4 hours and assessed for morphological damage by field emission scanning electron microscopy and for viability using the trypan blue stain. Morphological changes in pollen grains were also examined after contact with acid solutions at pH 4.0 to pH 7.0.

RESULTS

Exposure to NO2 or SO2 significantly damaged pollen grains at all concentrations investigated, compared to exposure to air; with exposure to concentrations of 0.5 ppm to 2 ppm resulting in fissures or complete breaks in the exine and a concentration of 5 ppm resulting in complete breakdown and release of pollen cytoplasmic granules. Significantly greater amounts of pollen grain were damaged after exposure to SO2 (15.5-20.4%) than after exposure to NO2 (7.1-14.7%). Similarly, exposure to NO2 or SO2 significantly decreased the viability of pollen grains, compared with exposure to air; with SO2 being slightly more detrimental than NO2. Exposure to acid solutions also induced pollen damage, which appeared to be pH-dependent (from 24.6% at pH 6.0 to 55.8% at pH 4.0; compared to 3.8% at pH 7.0).

CONCLUSION

Short-term exposure of oak pollen to high concentrations of SO2 or NO2 significantly increases their fragility and disruption, leading to subsequent release of pollen cytoplasmic granules into the atmosphere. These results suggest that heightened air pollution during the oak pollen season may possibly increase the incidence of allergic airway disease in sensitized individuals by facilitating the bioavailability of airborne pollen allergens.

Quantitative trait loci mapping and transcriptome analysis reveal candidate genes regulating the response to ozone in Arabidopsis thaliana

As multifaceted molecules, reactive oxygen species (ROS) are known to accumulate in response to various stresses. Ozone (O3 ) is an air pollutant with detrimental effect on plants and O3 can also be used as a tool to study the role of ROS in signalling. Genetic variation of O3 sensitivity in different Arabidopsis accessions highlights the complex genetic architecture of plant responses to ROS. To investigate the genetic basis of O3 sensitivity, a recombinant inbred line (RIL) population between two Arabidopsis accessions with distinct O3 sensitivity, C24 (O3 tolerant) and Te (O3 sensitive) was used for quantitative trait loci (QTL) mapping. Through analysis of QTL mapping combined with transcriptome changes in response to O3 , we identified three causal QTLs and several potential candidate genes regulating the response to O3 . Based on gene expression data, water loss and stomatal conductance measurement, we found that a combination of relatively low stomatal conductance and constitutive activation of salicylic acid (SA)-mediated defence signalling were responsible for the O3 tolerance in C24. Application of exogenous SA prior to O3 exposure can mimic the constitutive SA signalling in C24 and could attenuate O3 -induced leaf damage in the sensitive Arabidopsis accessions Te and Cvi-0.

Internucleosomal DNA fragmentation in wild emmer wheat is catalyzed by S1-type endonucleases translocated to the nucleus upon induction of cell death

Leaves of cereal plants display nucleosomal fragmentation of DNA attributed to the action of nucleases induced during program cell death (PCD). Yet, the specific nuclease activity responsible for generating double strand DNA breaks (DSBs) that lead to DNA fragmentation has not been fully described. Here, we characterized a Ca2+/Mg2+-dependent S1-type endonuclease activity in leaves of wild emmer wheat (Triticum dicoccoides Köern.) capable of introducing DSBs as demonstrated by the conversion of supercoiled plasmid DNA into a linear duplex DNA. In-gel nuclease assay revealed a nuclease of about 35 kDa capable of degrading both single stranded DNA and RNA. We further showed that the endonuclease activity can be purified on Concanavalin A and treatment with peptide-N-glycosidase F (PNGase F) did not abolish its activity. Furthermore, ConA-associated endonuclease was capable of generating nucleosomal DNA fragmentation in tobacco nuclei. Since S1-type endonucleases lack canonical nuclear localization signal it was necessary to determine their subcellular localization. To this end, a cDNA encoding for a putative 34 kDa S1-type nuclease, designated TaS1-like (TaS1L) was synthesized based on available sequence data of Triticum aestivum and fused with RFP. Introduction into protoplasts showed that TaS1L-RFP is cytoplasmic 24h post transformation but gradually turn nuclear at 48 h concomitantly with induction of cell death. Our results suggest that DNA fragmentation occurring in leaves of wild emmer wheat may be attributed to S1-type endonuclease(s) that reside in the cytoplasm but translocate to the nucleus upon induction of cell death.

Reactive oxygen species-provoked mitochondria-dependent cell death during ageing of elm (Ulmus pumila L.) seeds

Previous studies have shown that controlled deterioration treatment (CDT) induces programmed cell death in elm (Ulmus pumila L.) seeds, which undergo certain fundamental processes that are comparable to apoptosis in animals. In this study, the essential characteristics of mitochondrial physiology in elm seeds during CDT were identified by cellular ultrastructural analysis, whole-body optical imaging, Western blotting and semi-quantitative RT-PCR. The alteration in mitochondrial morphology was an early event during CDT, as indicated by progressive dynamic mitochondrial changes and rupture of the mitochondrial outer membrane; loss of mitochondrial transmembrane potential (Δψ(m)) ensued, and mitochondrial ATP levels decreased. The mitochondrial permeability transition pore inhibitor cyclosporine A effectively suppressed these changes during ageing. The in situ localization of production of reactive oxygen species (ROS), and evaluation of the expression of voltage-dependent anion-selective channel and cyclophilin D indicated that the levels of mitochondrial permeability transition pore components were positively correlated with ROS production, leading to an imbalance of the cellular redox potential and ultimately to programmed cell death. Pre-incubation with ascorbic acid slowed loss of mitochondrial Δψ(m), and decreased the effect of CDT on seed viability. However, there were no significant changes in multiple antioxidant elements or chaperones in the mitochondria during early stages of ageing. Our results indicate that CDT induces dynamic changes in mitochondrial physiology via increased ROS production, ultimately resulting in an irreversible loss of seed viability.

Plant signalling in acute ozone exposure

Exposure of plants to high ozone concentrations causes lesion formation in sensitive plants. Plant responses to ozone involve fast and massive changes in protein activities, gene expression and metabolism even before any tissue damage can be detected. Degradation of ozone and subsequent accumulation of reactive oxygen species (ROS) in the extracellular space activates several signalling cascades, which are integrated inside the cell into a fine-balanced network of ROS signalling. Reversible protein phosphorylation and degradation plays an important role in the regulation of signalling mechanisms in a complex crosstalk with plant hormones and calcium, an essential second messenger. In this review, we discuss the recent advances in understanding the molecular mechanisms of ozone uptake, perception and signalling pathways activated during the early steps of ozone response, and discuss the use of ozone as a tool to study the function of apoplastic ROS in signalling.