Cyclophilin 20-3 relays a 12-oxo-phytodienoic acid signal during stress responsive regulation of cellular redox homeostasis

The centrality of redox regulation and sensing of reactive oxygen species in abiotic and biotic stress acclimatization

During land plant evolution, the number of genes encoding for components of the thiol redox regulatory network and the generator systems of reactive oxygen species (ROS) expanded, tentatively indicating that they have a role in tailored environmental acclimatization. This hypothesis has been validated both experimentally and theoretically during the last few decades. Recent developments of dynamic redox-sensitive GFP (roGFP)-based in vivo sensors for H2O2 and the redox potential of the glutathione pool have paved the way for dissecting the kinetics changes that occur in these crucial parameters in response to environmental stressors. The versatile cellular redox sensory and response regulatory system monitors alterations in redox metabolism and controls the activity of redox target proteins, and thereby affects most, if not all, cellular processes ranging from transcription to translation and metabolism. This review uses examples to describe the role of the redox- and ROS-dependent regulatory network in realising the appropriate responses to diverse environmental stresses. The selected case studies concern different environmental challenges, namely excess excitation energy, the heavy metal cadmium and the metalloid arsenic, nitrogen or phosphate shortages as examples for nutrient deficiency, wounding, and nematode infestation. Each challenge affects the redox-regulatory and ROS network, but our present state of knowledge also points toward pressing questions that remain open in relation to the translation of redox regulation to environmental acclimatization.

Glutathione: a key modulator of plant defence and metabolism through multiple mechanisms

Redox reactions are fundamental to energy conversion in living cells, and also determine and tune responses to the environment. Within this context, the tripeptide glutathione plays numerous roles. As an important antioxidant, glutathione confers redox stability on the cell and also acts as an interface between signalling pathways and metabolic reactions that fuel growth and development. It also contributes to the assembly of cell components, biosynthesis of sulfur-containing metabolites, inactivation of potentially deleterious compounds, and control of hormonal signalling intensity. The multiplicity of these roles probably explains why glutathione status has been implicated in influencing plant responses to many different conditions. In particular, there is now a considerable body of evidence showing that glutathione is a crucial player in governing the outcome of biotic stresses. This review provides an overview of glutathione synthesis, transport, degradation, and redox turnover in plants. It examines the expression of genes associated with these processes during pathogen challenge and related conditions, and considers the diversity of mechanisms by which glutathione can influence protein function and gene expression.

OPDA/dn-OPDA actions: biosynthesis, metabolism, and signaling

Plants cannot move, so they have evolved sophisticated strategies that integrate the external environmental cues and internal signaling networks for adaptation to dynamic circumstances. Cis-(+)-12-oxo-phytodienoic acid (OPDA) and 2,3-dinor-OPDA (dn-OPDA), the cyclopentenone-containing oxylipins, ubiquitously occur in the green lineage to orchestrate a series of growth and developmental processes as well as various stress and defense responses. OPDA/dn-OPDA are precursors of jasmonate (JA) biosynthesis in vascular plants. Dn-OPDA and its isomer also serve as bioactive JAs perceived by the coronatine insensitive 1/jasmonate ZIM-domain (COI1/JAZ) co-receptor complex in bryophytes and lycophytes. In addition, OPDA/dn-OPDA display signaling activities independent of (+)-7-iso-jasmonoyl-L-isoleucine (JA-Ile) and COI1 in both vascular and non-vascular plants. In this review, we discuss recent advances in the biosynthesis, metabolism, and signaling of OPDA/dn-OPDA, and provide an overview of the evolution of OPDA/dn-OPDA actions to obtain a deeper understanding of the pervasive role of OPDA/dn-OPDA in the plant life cycle.

Metabolomics comparison of four varieties apple with different browning characters in response to pretreatment during pulp processing

Browning commonly appeared in apple processing, which varied in different apple varieties. Present work investigated the metabolomics of four varieties apple of Yataka, Gala, Sansa, and Fuji, which possessed different browning characteristics and related enzymes. Sansa as browning insensitive apple variety, exhibited the least chroma change with the lowest PPO activity and the highest SOD activity among the four apple varieties. Browning inhibition pretreatment increased the activity of SOD and PAL and decreased PPO and POD activity. In addition, metabolomic variances among the four apple varieties (FC), their browning pulp (BR) and browning inhibition pulp (CM) were compared. And the key metabolites were in-depth analyzed to match the relevant KEGG pathways and speculated metabolic networks. There were 487, 644, and 494 significant differential metabolites detected in FC, BR and CM, which were consisted of lipids, benzenoids, phenylpropanoids, organheterocyclic compounds, organic acids, nucleosides, accounting for 23 %, 11 %, 15 %, 16 %, 11 % of the total metabolites. The differential metabolites were matched with 39, 49, and 36 KEGG pathways in FC, BR, and CM, respectively, in which other secondary metabolites biosynthesis metabolism was the most significant in FC, lipid metabolism was the most significant in BR and CM, and energy metabolism was markedly annotated in CM. Notably, Sansa displayed the highest number of differential metabolites in both its BR (484) and CM (342). The BR of Sansa was characterized by flavonoid biosynthesis, while the other three apple varieties were associated with α-linolenic acid metabolism. Furthermore, in browning sensitive apple varieties, the flavonoid and phenylpropanoid biosynthesis pathway was significantly activated by browning inhibition pretreatment. Phenolic compounds, lipids, sugars, organic acids, nucleotides, and adenosine were regulated differently in the four apple varieties, potentially serving as key regulatory sites. Overall, this work provides novel insight for browning prevention in different apple varieties.

The ascorbate-glutathione cycle coming of age

Concepts regarding the operation of the ascorbate-glutathione cycle and the associated water/water cycle in the processing of metabolically generated hydrogen peroxide and other forms of reactive oxygen species (ROS) are well established in the literature. However, our knowledge of the functions of these cycles and their component enzymes continues to grow and evolve. Recent insights include participation in the intrinsic environmental and developmental signalling pathways that regulate plant growth, development, and defence. In addition to ROS processing, the enzymes of the two cycles not only support the functions of ascorbate and glutathione, they also have 'moonlighting' functions. They are subject to post-translational modifications and have an extensive interactome, particularly with other signalling proteins. In this assessment of current knowledge, we highlight the central position of the ascorbate-glutathione cycle in the network of cellular redox systems that underpin the energy-sensitive communication within the different cellular compartments and integrate plant signalling pathways.

Sulfate Availability and Hormonal Signaling in the Coordination of Plant Growth and Development

Sulfur (S), one of the crucial macronutrients, plays a pivotal role in fundamental plant processes and the regulation of diverse metabolic pathways. Additionally, it has a major function in plant protection against adverse conditions by enhancing tolerance, often interacting with other molecules to counteract stresses. Despite its significance, a thorough comprehension of how plants regulate S nutrition and particularly the involvement of phytohormones in this process remains elusive. Phytohormone signaling pathways crosstalk to modulate growth and developmental programs in a multifactorial manner. Additionally, S availability regulates the growth and development of plants through molecular mechanisms intertwined with phytohormone signaling pathways. Conversely, many phytohormones influence or alter S metabolism within interconnected pathways. S metabolism is closely associated with phytohormones such as abscisic acid (ABA), auxin (AUX), brassinosteroids (BR), cytokinins (CK), ethylene (ET), gibberellic acid (GA), jasmonic acid (JA), salicylic acid (SA), and strigolactones (SL). This review provides a summary of the research concerning the impact of phytohormones on S metabolism and, conversely, how S availability affects hormonal signaling. Although numerous molecular details are yet to be fully understood, several core signaling components have been identified at the crossroads of S and major phytohormonal pathways.

Jasmonates and salicylic acid: Evolution of defense hormones in land plants

The emergence of plant hormone signaling pathways is deeply intertwined with land plant evolution. In angiosperms, two plant hormones, salicylic Acid (SA) and Jasmonates (JAs), play a key role in plant defense, where JAs-mediated defenses are typically activated in response to herbivores and necrotrophic pathogens, whereas SA is prioritized against hemi/biotrophic pathogens. Thus, studying the evolution of SA and JAs and their crosstalk is essential to understand the evolution of molecular plant-microbe interactions (EvoMPMI) in land plants. Recent advances in the evolution of SA and JAs biosynthesis, signaling, and crosstalk in land plants illustrated that the insight gained in angiosperms does not necessarily apply to non-seed plant lineages, where the receptors perceive different ligands and the hormones activate pathways independently on the canonical receptors. In this review, recent findings on the two main defense hormones (JAs and SA) in non-seed plants, including functional studies in the bryophyte model Marchantia polymorpha, will be discussed.

Interplay between MYZUS PERSICAE-INDUCED LIPASE 1 and OPDA signaling in limiting green peach aphid infestation on Arabidopsis thaliana

MYZUS PERSICAE-INDUCED LIPASE1 (MPL1) encodes a lipase in Arabidopsis thaliana that is required for limiting infestation by the green peach aphid (GPA; Myzus persicae), an important phloem sap-consuming insect pest. Previously, we demonstrated that MPL1 expression was up-regulated in response to GPA infestation, and GPA fecundity was higher on the mpl1 mutant, compared with the wild-type (WT), and lower on 35S:MPL1 plants that constitutively expressed MPL1 from the 35S promoter. Here, we show that the MPL1 promoter is active in the phloem and expression of the MPL1 coding sequence from the phloem-specific SUC2 promoter in mpl1 is sufficient to restore resistance to GPA. The GPA infestation-associated up-regulation of MPL1 requires CYCLOPHILIN 20-3 (CYP20-3), which encodes a 12-oxo-phytodienoic acid (OPDA)-binding protein that is involved in OPDA signaling, and is required for limiting GPA infestation. OPDA promotes MPL1 expression to limit GPA fecundity, a process that requires CYP20-3 function. These results along with our observation that constitutive expression of MPL1 from the 35S promoter restores resistance to GPA in the cyp20-3 mutant, and MPL1 acts in a feedback loop to limit OPDA levels in GPA-infested plants, suggest that an interplay between MPL1, OPDA, and CYP20-3 contributes to resistance to GPA.

Synthetic and analytical routes to the L-amino acid conjugates of cis-OPDA and their identification and quantification in plants

Cis-(+)-12-oxophytodienoic acid (cis-(+)-OPDA) is a bioactive jasmonate, a precursor of jasmonic acid, which also displays signaling activity on its own. Modulation of cis-(+)-OPDA actions may be carried out via biotransformation leading to metabolites of various functions. This work introduces a methodology for the synthesis of racemic cis-OPDA conjugates with amino acids (OPDA-aa) and their deuterium-labeled analogs, which enables the unambiguous identification and accurate quantification of these compounds in plants. We have developed a highly sensitive liquid chromatography-tandem mass spectrometry-based method for the reliable determination of seven OPDA-aa (OPDA-Alanine, OPDA-Aspartate, OPDA-Glutamate, OPDA-Glycine, OPDA-Isoleucine, OPDA-Phenylalanine, and OPDA-Valine) from minute amount of plant material. The extraction from 10 mg of fresh plant tissue by 10% aqueous methanol followed by single-step sample clean-up on hydrophilic-lipophilic balanced columns prior to final analysis was optimized. The method was validated in terms of accuracy and precision, and the method parameters such as process efficiency, recovery and matrix effects were evaluated. In mechanically wounded 30-day-old Arabidopsis thaliana leaves, five endogenous (+)-OPDA-aa were identified and their endogenous levels were estimated. The time-course accumulation revealed a peak 60 min after the wounding, roughly corresponding to the accumulation of cis-(+)-OPDA. Our synthetic and analytical methodologies will support studies on cis-(+)-OPDA conjugation with amino acids and research into the biological significance of these metabolites in plants.

Reduced GSH Acts as a Metabolic Cue of OPDA Signaling in Coregulating Photosynthesis and Defense Activation under Stress

12-oxo-phytodienoic acid (OPDA) is a primary precursor of jasmonates, able to trigger autonomous signaling cascades that activate and fine-tune plant defense responses, as well as growth and development. However, its mechanism of actions remains largely elusive. Here we describe a dual-function messenger of OPDA signaling, reduced glutathione (GSH), that cross-regulates photosynthesis machinery and stress protection/adaptation in concert, optimizing plant plasticity and survival potential. Under stress conditions, the rapid induction of OPDA production stimulates GSH accumulation in the chloroplasts, and in turn leads to protein S-glutathionylation in modulating the structure and function of redox-sensitive enzymes such as 2-cysteine (Cys) peroxiredoxin A (2CPA), a recycler in the water-water cycle. GSH exchanges thiol-disulfides with the resolving CysR175, while donating an electron (e-, H+) to the peroxidatic CysP53, of 2CPA, which revives its reductase activity and fosters peroxide detoxification in photosynthesis. The electron flow protects photosynthetic processes (decreased total non-photochemical quenching, NPQ(T)) and maintains its efficiency (increased photosystem II quantum yield, ΦII). On the other hand, GSH also prompts retrograde signaling from the chloroplasts to the nucleus in adjusting OPDA-responsive gene expressions such as Glutathione S-Transferase 6 (GST6) and GST8, and actuating defense responses against various ecological constraints such as salinity, excess oxidants and light, as well as mechanical wounding. We thus propose that OPDA regulates a unique metabolic switch that interfaces light and defense signaling, where it links cellular and environmental cues to a multitude of plant physiological, e.g., growth, development, recovery, and acclimation, processes.

PSI Photoinhibition and Changing CO2 Levels Initiate Retrograde Signals to Modify Nuclear Gene Expression

Photosystem I (PSI) is a critical component of the photosynthetic machinery in plants. Under conditions of environmental stress, PSI becomes photoinhibited, leading to a redox imbalance in the chloroplast. PSI photoinhibition is caused by an increase in electron pressure within PSI, which damages the iron-sulfur clusters. In this study, we investigated the susceptibility of PSI to photoinhibition in plants at different concentrations of CO2, followed by global gene expression analyses of the differentially treated plants. PSI photoinhibition was induced using a specific illumination protocol that inhibited PSI with minimal effects on PSII. Unexpectedly, the varying CO2 levels combined with the PSI-PI treatment neither increased nor decreased the likelihood of PSI photodamage. All PSI photoinhibition treatments, independent of CO2 levels, upregulated genes generally involved in plant responses to excess iron and downregulated genes involved in iron deficiency. PSI photoinhibition also induced genes encoding photosynthetic proteins that act as electron acceptors from PSI. We propose that PSI photoinhibition causes a release of iron from damaged iron-sulfur clusters, which initiates a retrograde signal from the chloroplast to the nucleus to modify gene expression. In addition, the deprivation of CO2 from the air initiated a signal that induced flavonoid biosynthesis genes, probably via jasmonate production.

Organelle-targeted biosensors reveal distinct oxidative events during pattern-triggered immune responses

Reactive oxygen species are produced in response to pathogens and pathogen-associated molecular patterns, as exemplified by the rapid extracellular oxidative burst dependent on the NADPH oxidase isoform RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD) in Arabidopsis (Arabidopsis thaliana). We used the H2O2 biosensor roGFP2-Orp1 and the glutathione redox state biosensor GRX1-roGFP2 targeted to various organelles to reveal unsuspected oxidative events during the pattern-triggered immune response to flagellin (flg22) and after inoculation with Pseudomonas syringae. roGFP2-Orp1 was oxidized in a biphasic manner 1 and 6 h after treatment, with a more intense and faster response in the cytosol compared to chloroplasts, mitochondria, and peroxisomes. Peroxisomal and cytosolic GRX1-roGFP2 were also oxidized in a biphasic manner. Interestingly, our results suggested that bacterial effectors partially suppress the second phase of roGFP2-Orp1 oxidation in the cytosol. Pharmacological and genetic analyses indicated that the pathogen-associated molecular pattern-induced cytosolic oxidation required the BRI1-ASSOCIATED RECEPTOR KINASE (BAK1) and BOTRYTIS-INDUCED KINASE 1 (BIK1) signaling components involved in the immune response but was largely independent of NADPH oxidases RBOHD and RESPIRATORY BURST OXIDASE HOMOLOG F (RBOHF) and apoplastic peroxidases peroxidase 33 (PRX33) and peroxidase 34 (PRX34). The initial apoplastic oxidative burst measured with luminol was followed by a second oxidation burst, both of which preceded the two waves of cytosolic oxidation. In contrast to the cytosolic oxidation, these bursts were RBOHD-dependent. Our results reveal complex oxidative sources and dynamics during the pattern-triggered immune response, including that cytosolic oxidation is largely independent of the preceding extracellular oxidation events.

Oxylipins and Reactive Carbonyls as Regulators of the Plant Redox and Reactive Oxygen Species Network under Stress

Reactive oxygen species (ROS), and in particular H2O2, serve as essential second messengers at low concentrations. However, excessive ROS accumulation leads to severe and irreversible cell damage. Hence, control of ROS levels is needed, especially under non-optimal growth conditions caused by abiotic or biotic stresses, which at least initially stimulate ROS synthesis. A complex network of thiol-sensitive proteins is instrumental in realizing tight ROS control; this is called the redox regulatory network. It consists of sensors, input elements, transmitters, and targets. Recent evidence revealed that the interplay of the redox network and oxylipins–molecules derived from oxygenation of polyunsaturated fatty acids, especially under high ROS levels–plays a decisive role in coupling ROS generation and subsequent stress defense signaling pathways in plants. This review aims to provide a broad overview of the current knowledge on the interaction of distinct oxylipins generated enzymatically (12-OPDA, 4-HNE, phytoprostanes) or non-enzymatically (MDA, acrolein) and components of the redox network. Further, recent findings on the contribution of oxylipins to environmental acclimatization will be discussed using flooding, herbivory, and establishment of thermotolerance as prime examples of relevant biotic and abiotic stresses.

Untargeted profiling of secondary metabolites and phytotoxins associated with stemphylium blight of lentil

Stemphylium botryosum alters lentil secondary metabolism and differentially affects resistant and susceptible genotypes. Untargeted metabolomics identifies metabolites and their potential biosynthetic pathways that play a crucial role in resistance to S. botryosum. The molecular and metabolic processes that mediate resistance to stemphylium blight caused by Stemphylium botryosum Wallr. in lentil are largely unknown. Identifying metabolites and pathways associated with Stemphylium infection may provide valuable insights and novel targets to breed for enhanced resistance. The metabolic changes following infection of four lentil genotypes by S. botryosum were investigated by comprehensive untargeted metabolic profiling employing reversed-phase or hydrophilic interaction liquid chromatography (HILIC) coupled to a Q-Exactive mass spectrometer. At the pre-flowering stage, plants were inoculated with S. botryosum isolate SB19 spore suspension and leaf samples were collected at 24, 96 and 144 h post-inoculation (hpi). Mock-inoculated plants were used as negative controls. After analyte separation, high-resolution mass spectrometry data was acquired in positive and negative ionization modes. Multivariate modeling revealed significant treatment, genotype and hpi effects on metabolic profile changes that reflect lentil response to Stemphylium infection. In addition, univariate analyses highlighted numerous differentially accumulated metabolites. By contrasting the metabolic profiles of SB19-inoculated and mock-inoculated plants and among lentil genotypes, 840 pathogenesis-related metabolites were detected including seven S. botryosum phytotoxins. These metabolites included amino acids, sugars, fatty acids and flavonoids in primary and secondary metabolism. Metabolic pathway analysis revealed 11 significant pathways including flavonoid and phenylpropanoid biosynthesis, which were affected upon S. botryosum infection. This research contributes to ongoing efforts toward a comprehensive understanding of the regulation and reprogramming of lentil metabolism under biotic stress, which will provide targets for potential applications in breeding for enhanced disease resistance.

The root-knot nematode effector MiMSP32 targets host 12-oxophytodienoate reductase 2 to regulate plant susceptibility

To establish persistent infections in host plants, herbivorous invaders, such as root-knot nematodes, must rely on effectors for suppressing damage-induced jasmonate-dependent host defenses. However, at present, the effector mechanisms targeting the biosynthesis of biologically active jasmonates to avoid adverse host responses are unknown. Using yeast two-hybrid, in planta co-immunoprecipitation, and mutant analyses, we identified 12-oxophytodienoate reductase 2 (OPR2) as an important host target of the stylet-secreted effector MiMSP32 of the root-knot nematode Meloidogyne incognita. MiMSP32 has no informative sequence similarities with other functionally annotated genes but was selected for the discovery of novel effector mechanisms based on evidence of positive, diversifying selection. OPR2 catalyzes the conversion of a derivative of 12-oxophytodienoate to jasmonic acid (JA) and operates parallel to 12-oxophytodienoate reductase 3 (OPR3), which controls the main pathway in the biosynthesis of jasmonates. We show that MiMSP32 targets OPR2 to promote parasitism of M. incognita in host plants independent of OPR3-mediated JA biosynthesis. Artificially manipulating the conversion of the 12-oxophytodienoate by OPRs increases susceptibility to multiple unrelated plant invaders. Our study is the first to shed light on a novel effector mechanism targeting this process to regulate the susceptibility of host plants.

Plant biology: Putting a break on stomatal opening

Blue light triggers stomatal opening through the phototropin-mediated pathway. A new study shows that light-induced stomatal opening is negatively regulated by three closely related plastidial phospholipases and their downstream oxylipin product.

Light-induced stomatal opening in Arabidopsis is negatively regulated by chloroplast-originated OPDA signaling

Stomatal movement is orchestrated by diverse signaling cascades and metabolic activities in guard cells. Light triggers the opening of the pores through the phototropin-mediated pathway, which leads to the activation of plasma membrane H⁺-ATPase and thereby facilitates potassium accumulation through K_in⁺ channels. However, it remains poorly understood how phototropin signaling is fine-tuned to prevent excessive stomatal opening and consequent water loss. Here, we show that the stomatal response to light is negatively regulated by 12-oxo-phytodienoic acid (OPDA), an oxylipin metabolite produced through enzymatic oxygenation of polyunsaturated fatty acids (PUFAs). We identify a set of phospholipase-encoding genes, phospholipase (PLIP)1/2/3, which are transactivated rapidly in guard cells upon illumination in a phototropin-dependent manner. These phospholipases release PUFAs from the chloroplast membrane, which is oxidized by guard-cell lipoxygenases and further metabolized to OPDA. The OPDA-deficient mutants had wider stomatal pores, whereas mutants containing elevated levels of OPDA showed the opposite effect on stomatal aperture. Transmembrane solute fluxes that drive stomatal aperture were enhanced in lox6-1 guard cells, indicating that OPDA signaling ultimately impacts on activities of proton pumps and K_in⁺ channels. Interestingly, the accelerated stomatal kinetics in lox6-1 leads to increased plant growth without cost in water or macronutrient use. Together, our results reveal a new role for chloroplast membrane oxylipin metabolism in stomatal regulation. Moreover, the accelerated stomatal opening kinetics in OPDA-deficient mutants benefits plant growth and water use efficiency.

Jasmonate perception: Ligand-receptor interaction, regulation, and evolution

Phytohormones integrate external environmental and developmental signals with internal cellular responses for plant survival and multiplication in changing surroundings. Jasmonate (JA), which might originate from prokaryotes and benefit plant terrestrial adaptation, is a vital phytohormone that regulates diverse developmental processes and defense responses against various environmental stresses. In this review, we first provide an overview of ligand-receptor binding techniques used for the characterization of phytohormone-receptor interactions, then introduce the identification of the receptor COI1 and active JA molecules, and finally summarize recent advances on the regulation of JA perception and its evolution.

Plant thiol peroxidases as redox sensors and signal transducers in abiotic stress acclimation

The temporal and spatial patterns of reactive oxygen species (ROS) in cells and tissues decisively determine the plant acclimation response to diverse abiotic and biotic stresses. Recent progress in developing dynamic cell imaging probes provides kinetic information on changes in parameters like H₂O₂, glutathione (GSH/GSSG) and NAD(P)H/NAD(P)⁺, that play a crucial role in tuning the cellular redox state. Central to redox-based regulation is the thiol-redox regulatory network of the cell that integrates reductive information from metabolism and oxidative ROS signals. Sensitive proteomics allow for monitoring changes in redox-related posttranslational modifications. Thiol peroxidases act as sensitive peroxide and redox sensors and play a central role in this signal transduction process. Peroxiredoxins (PRX) and glutathione peroxidases (GPX) are the two main thiol peroxidases and their function in ROS sensing and redox signaling in plants is emerging at present and summarized in this review. Depending on their redox state, PRXs and GPXs act as redox-dependent binding partners, direct oxidants of target proteins and oxidants of thiol redox transmitters that in turn oxidize target proteins. With their versatile functions, the multiple isoforms of plant thiol peroxidases play a central role in plant stress acclimation, e.g. to high light or osmotic stress, but also in ROS-mediated immunity and development.

Iron redistribution induces oxidative burst and resistance in maize against Curvularia lunata

ΔClnps6 induced iron redistribution in maize B73 leaf cells and resulted in reactive oxygen species (ROS) burst to enhance plant resistance against Curvularia lunata. Iron is an indispensable co-factor of various crucial enzymes that are involved in cellular metabolic processes and energy metabolism in eukaryotes. For this reason, plants and pathogens compete for iron to maintain their iron homeostasis, respectively. In our previous study, ΔClnps6, the extracellular siderophore biosynthesis deletion mutant of Curvularia lunata, was sensitive to exogenous hydrogen peroxide and virulence reduction. However, the mechanism was not studied. Here, we report that maize B73 displayed highly resistance to ΔClnps6. The plants recruited more iron at cell wall appositions (CWAs) to cause ROS bursts. Intracellular iron deficiency induced by iron redistribution originated form up-regulated expression of genes involved in intracellular iron consumption in leaves and absorption in roots. The RNA-sequencing data also showed that the expression of respiratory burst oxidase homologue (ZmRBOH4) and NADP-dependent malic enzyme 4 (ZmNADP-ME4) involved in ROS production was up-regulated in maize B73 after ΔClnps6 infection. Simultaneously, jasmonic acid (JA) biosynthesis genes lipoxygenase (ZmLOX), allene oxide synthase (ZmAOS), GA degradation gene gibberellin 2-beta-dioxygenase (ZmGA2OX6) and ABA degradation genes abscisic acid hydroxylase (ZmABH1, ZmABH2) involved in iron homeostasis were up-regulated expression. Ferritin1 (ZmFER1) positive regulated maize resistance against C. lunata via ROS burst under Fe-limiting conditions. Overall, our results showed that iron played vital roles in activating maize resistance in B73-C. lunata interaction.

Chitooligosaccharide elicitor and oxylipins synergistically elevate phytoalexin production in rice

We show that in rice, the amino acid-conjugates of JA precursor, OPDA, may function as a non-canonical signal for the production of phytoalexins in coordination with the innate chitin signaling. The core oxylipins, jasmonic acid (JA) and JA-Ile, are well-known as potent regulators of plant defense against necrotrophic pathogens and/or herbivores. However, recent studies also suggest that other oxylipins, including 12-oxo-phytodienoic acid (OPDA), may contribute to plant defense. Here, we used a previously characterized metabolic defense marker, p-coumaroylputrescine (CoP), and fungal elicitor, chitooligosaccharide, to specifically test defense role of various oxylipins in rice (Oryza sativa). While fungal elicitor triggered a rapid production of JA, JA-Ile, and their precursor OPDA, rice cells exogenously treated with the compounds revealed that OPDA, rather than JA-Ile, can stimulate the CoP production. Next, reverse genetic approach and oxylipin-deficient rice mutant (hebiba) were used to uncouple oxylipins from other elicitor-triggered signals. It appeared that, without oxylipins, residual elicitor signaling had only a minimal effect but, in synergy with OPDA, exerted a strong stimulatory activity towards CoP production. Furthermore, as CoP levels were compromised in the OPDA-treated Osjar1 mutant cells impaired in the oxylipin-amino acid conjugation, putative OPDA-amino acid conjugates emerged as hypothetical regulators of CoP biosynthesis. Accordingly, we found several OPDA-amino acid conjugates in rice cells treated with exogenous OPDA, and OPDA-Asp was detected, although in small amounts, in the chitooligosaccharide-treated rice. However, as synthetic OPDA-Asp and OPDA-Ile, so far, failed to induce CoP in cells, it suggests that yet another presumed OPDA-amino acid form(s) could be acting as novel regulator(s) of phytoalexins in rice.

Leaf fitness and stress response after the application of contaminated soil dust particulate matter

In this study, we observed the effect of the application of soil dust enriched with risk elements (Cd, Pb, As and Zn) to leaf surfaces of lettuce (Lactuca sativa var. capitata) while it was grown under hydroponic conditions. This study aimed to determine how low soil dust particulate matter (PM) doses affected the activity of or damaged the photosynthetic apparatus and how the uptake of risk elements was associated with both epigenetic changes (5-methylcytosine content, i.e., 5mC) and stress metabolism. During the study, we obtained many results pertaining to risk element contents and biochemical (total phenolic content (TPC), malondialdehyde (MDA) content and the amount of free amino acids (AAs)) and physiological (photosynthesis parameters: net photosynthetic rate, transpiration rate, intercellular CO2 concentration, stomatal conductance, instantaneous water-use efficiency, maximum quantum yield of PSII, chlorophyll and carotenoid contents, and leaf water potential (WP)) plant features. The results showed an increase in MDA and 5mC. However, the transpiration rate, WP and free AAs decreased. In conclusion, contamination by very low doses of soil dust PM had no direct or significant effect on plant fitness, as shown by the TPC and 5mC content, which indicates that plants can overcome the oxidative stress caused by the accumulation of risk elements. From the above, we propose the use of epigenetic changes as biomarkers of potential changes in the activation of plant metabolism under stress caused by environmental pollution.

OPDAylation of Thiols of the Redox Regulatory Network In Vitro

cis-(+)-12-Oxophytodienoic acid (OPDA) is a reactive oxylipin produced by catalytic oxygenation of polyunsaturated α-linolenic acid (18:3 (ω − 3)) in the chloroplast. Apart from its function as precursor for jasmonic acid synthesis, OPDA serves as a signaling molecule and regulator on its own, namely by tuning enzyme activities and altering expression of OPDA-responsive genes. A possible reaction mechanism is the covalent binding of OPDA to thiols via the addition to the C=C double bond of its α,β-unsaturated carbonyl group in the cyclopentenone ring. The reactivity allows for covalent modification of accessible cysteinyl thiols in proteins. This work investigated the reaction of OPDA with selected chloroplast and cytosolic thioredoxins (TRX) and glutaredoxins (GRX) of Arabidopsis thaliana. OPDA reacted with TRX and GRX as detected by decreased m-PEG maleimide binding, consumption of OPDA, reduced ability for insulin reduction and inability to activate glyceraldehyde-3-phosphate dehydrogenase and regenerate glutathione peroxidase (GPXL8), and with lower efficiency, peroxiredoxin IIB (PRXIIB). OPDAylation of certain protein thiols occurs quickly and efficiently in vitro and is a potent post-translational modification in a stressful environment.

The Phytotoxin Myrigalone A Triggers a Phased Detoxification Programme and Inhibits Lepidium sativum Seed Germination via Multiple Mechanisms including Interference with Auxin Homeostasis

Molecular responses of plants to natural phytotoxins comprise more general and compound-specific mechanisms. How phytotoxic chalcones and other flavonoids inhibit seedling growth was widely studied, but how they interfere with seed germination is largely unknown. The dihydrochalcone and putative allelochemical myrigalone A (MyA) inhibits seed germination and seedling growth. Transcriptome (RNAseq) and hormone analyses of Lepidium sativum seed responses to MyA were compared to other bioactive and inactive compounds. MyA treatment of imbibed seeds triggered the phased induction of a detoxification programme, altered gibberellin, cis-(+)-12-oxophytodienoic acid and jasmonate metabolism, and affected the expression of hormone transporter genes. The MyA-mediated inhibition involved interference with the antioxidant system, oxidative signalling, aquaporins and water uptake, but not uncoupling of oxidative phosphorylation or p-hydroxyphenylpyruvate dioxygenase expression/activity. MyA specifically affected the expression of auxin-related signalling genes, and various transporter genes, including for auxin transport (PIN7, ABCG37, ABCG4, WAT1). Responses to auxin-specific inhibitors further supported the conclusion that MyA interferes with auxin homeostasis during seed germination. Comparative analysis of MyA and other phytotoxins revealed differences in the specific regulatory mechanisms and auxin transporter genes targeted to interfere with auxin homestasis. We conclude that MyA exerts its phytotoxic activity by multiple auxin-dependent and independent molecular mechanisms.

The jasmonate biosynthesis Gene OsOPR7 can mitigate salinity induced mitochondrial oxidative stress

Salinity poses a serious threat to global agriculture and human food security. A better understanding of plant adaptation to salt stress is, therefore, mandatory. In the non-photosynthetic cells of the root, salinity perturbs oxidative balance in mitochondria, leading to cell death. In parallel, plastids accumulate the jasmonate precursor cis (+)12-Oxo-Phyto-Dienoic Acid (OPDA) that is then translocated to peroxisomes and has been identified as promoting factor for salt-induced cell death as well. In the current study, we probed for a potential interaction between these three organelles that are primarily dealing with oxidative metabolism. We made use of two tools: (i) Rice OPDA Reductase 7 (OsOPR7), an enzyme localised in peroxisomes converting OPDA into the precursors of the stress hormone JA-Ile. (ii) A Trojan Peptoid, Plant PeptoQ, which can specifically target to mitochondria and scavenge excessive superoxide accumulating in response to salt stress. We show that overexpression of OsOPR7 as GFP fusion in tobacco (Nicotiana tabacum L. cv. Bright Yellow 2, BY-2) cells, as well as a pretreatment with Plant PeptoQ can mitigate salt stress with respect to numerous aspects including proliferation, expansion, ionic balance, redox homeostasis, and mortality. This mitigation correlates with a more robust oxidative balance, evident from a higher activity of superoxide dismutase (SOD), lower levels of superoxide and lipid peroxidation damage, and a conspicuous and specific upregulation of mitochondrial SOD transcripts. Although both, Plant PeptoQ and ectopic OsOPR7, were acting in parallel and mostly additive, there are two specific differences: (i) OsOPR7 is strictly localised to the peroxisomes, while Plant PeptoQ found in mitochondria. (ii) Plant PeptoQ activates transcripts of NAC, a factor involved in retrograde signalling from mitochondria to the nucleus, while these transcripts are suppressed significantly in the cells overexpressing OsOPR7. The fact that overexpression of a peroxisomal enzyme shifting the jasmonate pathway from the cell-death signal OPDA towards JA-Ile, a hormone linked with salt adaptation, is accompanied by more robust redox homeostasis in a different organelle, the mitochondrion, indicates that cross-talk between peroxisome and mitochondrion is a crucial factor for efficient adaptation to salt stress.

Light Deficiency Inhibits Growth by Affecting Photosynthesis Efficiency as well as JA and Ethylene Signaling in Endangered Plant Magnolia sinostellata

The endangered plant Magnolia sinostellata largely grows in the understory of forest and suffers light deficiency stress. It is generally recognized that the interaction between plant development and growth environment is intricate; however, the underlying molecular regulatory pathways by which light deficiency induced growth inhibition remain obscure. To understand the physiological and molecular mechanisms of plant response to shading caused light deficiency, we performed photosynthesis efficiency analysis and comparative transcriptome analysis in M. sinostellata leaves, which were subjected to shading treatments of different durations. Most of the parameters relevant to the photosynthesis systems were altered as the result of light deficiency treatment, which was also confirmed by the transcriptome analysis. Gene Ontology and KEGG pathway enrichment analyses illustrated that most of differential expression genes (DEGs) were enriched in photosynthesis-related pathways. Light deficiency may have accelerated leaf abscission by impacting the photosynthesis efficiency and hormone signaling. Further, shading could repress the expression of stress responsive transcription factors and R-genes, which confer disease resistance. This study provides valuable insight into light deficiency-induced molecular regulatory pathways in M. sinostellata and offers a theoretical basis for conservation and cultivation improvements of Magnolia and other endangered woody plants.

The Plant-Stress Metabolites, Hexanoic Aacid and Melatonin, Are Potential "Vaccines" for Plant Health Promotion

A plethora of compounds stimulate protective mechanisms in plants against microbial pathogens and abiotic stresses. Some defense activators are synthetic compounds and trigger responses only in certain protective pathways, such as activation of defenses under regulation by the plant regulator, salicylic acid (SA). This review discusses the potential of naturally occurring plant metabolites as primers for defense responses in the plant. The production of the metabolites, hexanoic acid and melatonin, in plants means they are consumed when plants are eaten as foods. Both metabolites prime stronger and more rapid activation of plant defense upon subsequent stress. Because these metabolites trigger protective measures in the plant they can be considered as "vaccines" to promote plant vigor. Hexanoic acid and melatonin instigate systemic changes in plant metabolism associated with both of the major defense pathways, those regulated by SA- and jasmonic acid (JA). These two pathways are well studied because of their induction by different microbial triggers: necrosis-causing microbial pathogens induce the SA pathway whereas colonization by beneficial microbes stimulates the JA pathway. The plant's responses to the two metabolites, however, are not identical with a major difference being a characterized growth response with melatonin but not hexanoic acid. As primers for plant defense, hexanoic acid and melatonin have the potential to be successfully integrated into vaccination-like strategies to protect plants against diseases and abiotic stresses that do not involve man-made chemicals.

Cyclophilins and Their Functions in Abiotic Stress and Plant-Microbe Interactions

Plants have developed a variety of mechanisms and regulatory pathways to change their gene expression profiles in response to abiotic stress conditions and plant–microbe interactions. The plant–microbe interaction can be pathogenic or beneficial. Stress conditions, both abiotic and pathogenic, negatively affect the growth, development, yield and quality of plants, which is very important for crops. In contrast, the plant–microbe interaction could be growth-promoting. One of the proteins involved in plant response to stress conditions and plant–microbe interactions is cyclophilin. Cyclophilins (CyPs), together with FK506-binding proteins (FKBPs) and parvulins, belong to a big family of proteins with peptidyl-prolyl cis-trans isomerase activity (Enzyme Commission (EC) number 5.2.1.8). Genes coding for proteins with the CyP domain are widely expressed in all organisms examined, including bacteria, fungi, animals, and plants. Their different forms can be found in the cytoplasm, endoplasmic reticulum, nucleus, chloroplast, mitochondrion and in the phloem space. They are involved in numerous processes, such as protein folding, cellular signaling, mRNA processing, protein degradation and apoptosis. In the past few years, many new functions, and molecular mechanisms for cyclophilins have been discovered. In this review, we aim to summarize recent advances in cyclophilin research to improve our understanding of their biological functions in plant defense and symbiotic plant–microbe interactions.

Metabolism, signaling, and transport of jasmonates

Biosynthesis/metabolism, perception/signaling, and transport are three essential aspects of the actions of phytohormones. Jasmonates (JAs), including jasmonic acid (JA) and related oxylipins, are implicated in the regulation of a range of ecological interactions, as well as developmental programs to integrate these interactions. Jasmonoyl-isoleucine (JA-Ile) is the most bioactive JAs, and perception of JA-Ile by its coreceptor, the Skp1-Cullin1-F-box-type (SCF) protein ubiquitin ligase complex SCF^COI1-JAZ, in the nucleus derepresses the transcriptional repression of target genes. The biosynthesis and metabolism of JAs occur in the plastid, peroxisome, cytosol, endoplasmic reticulum, and vacuole, whereas sensing of JA-Ile levels occurs in the nucleus. It is increasingly apparent that a number of transporters, particularly members of the jasmonates transporter (JAT) family, located at endomembranes as well as the plasma membrane, constitute a network for modulating and coordinating the metabolic flux and signaling of JAs. In this review, we discuss recent advances in the metabolism, signaling, and especially the transport of JAs, focusing on intracellular compartmentation of these processes. The roles of transporter-mediated cell-cell transport in driving long-distance transport and signaling of JAs are also discussed.

Maize biochemistry in response to root herbivory was mediated by domestication, spread, and breeding

With domestication, northward spread, and breeding, maize defence against root-herbivores relied on induced defences, decreasing levels of phytohormones involved in resistance, and increasing levels of a phytohormone involved in tolerance. We addressed whether a suite of maize (Zea mays mays) phytohormones and metabolites involved in herbivore defence were mediated by three successive processes: domestication, spread to North America, and modern breeding. With those processes, and following theoretical predictions, we expected to find: a change in defence strategy from reliance on induced defences to reliance on constitutive defences; decreasing levels of phytohormones involved in herbivore resistance, and; increasing levels of a phytohormone involved in herbivore tolerance. We tested those predictions by comparing phytohormone levels in seedlings exposed to root herbivory by Diabrotica virgifera virgifera among four plant types encompassing those processes: the maize ancestor Balsas teosinte (Zea mays parviglumis), Mexican maize landraces, USA maize landraces, and USA inbred maize cultivars. With domestication, maize transitioned from reliance on induced defences in teosinte to reliance on constitutive defences in maize, as predicted. One subset of metabolites putatively involved in herbivory defence (13-oxylipins) was suppressed with domestication, as predicted, though another was enhanced (9-oxylipins), and both were variably affected by spread and breeding. A phytohormone (indole-3-acetic acid) involved in tolerance was enhanced with domestication, and with spread and breeding, as predicted. These changes are consistent with documented changes in herbivory resistance and tolerance, and occurred coincidentally with cultivation in increasingly resource-rich environments, i.e., from wild to highly enriched agricultural environments. We concluded that herbivore defence evolution in crops may be mediated by processes spanning thousands of generations, e.g., domestication and spread, as well as by processes spanning tens of generations, e.g., breeding and agricultural intensification.

Resolving diurnal dynamics of the chloroplastic glutathione redox state in Arabidopsis reveals its photosynthetically derived oxidation

Plants are subjected to fluctuations in light intensity, and this might cause unbalanced photosynthetic electron fluxes and overproduction of reactive oxygen species (ROS). Electrons needed for ROS detoxification are drawn, at least partially, from the cellular glutathione (GSH) pool via the ascorbate-glutathione cycle. Here, we explore the dynamics of the chloroplastic glutathione redox potential (chl-EGSH) using high-temporal-resolution monitoring of Arabidopsis (Arabidopsis thaliana) lines expressing the reduction-oxidation sensitive green fluorescent protein 2 (roGFP2) in chloroplasts. This was carried out over several days under dynamic environmental conditions and in correlation with PSII operating efficiency. Peaks in chl-EGSH oxidation during dark-to-light and light-to-dark transitions were observed. Increasing light intensities triggered a binary oxidation response, with a threshold around the light saturating point, suggesting two regulated oxidative states of the chl-EGSH. These patterns were not affected in npq1 plants, which are impaired in non-photochemical quenching. Oscillations between the two oxidation states were observed under fluctuating light in WT and npq1 plants, but not in pgr5 plants, suggesting a role for PSI photoinhibition in regulating the chl-EGSH dynamics. Remarkably, pgr5 plants showed an increase in chl-EGSH oxidation during the nights following light stresses, linking daytime photoinhibition and nighttime GSH metabolism. This work provides a systematic view of the dynamics of the in vivo chloroplastic glutathione redox state during varying light conditions.

Chloroplast-derived photo-oxidative stress causes changes in H2O2 and EGSH in other subcellular compartments

Metabolic fluctuations in chloroplasts and mitochondria can trigger retrograde signals to modify nuclear gene expression. Mobile signals likely to be involved are reactive oxygen species (ROS), which can operate protein redox switches by oxidation of specific cysteine residues. Redox buffers, such as the highly reduced glutathione pool, serve as reservoirs of reducing power for several ROS-scavenging and ROS-induced damage repair pathways. Formation of glutathione disulfide and a shift of the glutathione redox potential (EGSH) toward less negative values is considered as hallmark of several stress conditions. Here we used the herbicide methyl viologen (MV) to generate ROS locally in chloroplasts of intact Arabidopsis (Arabidopsis thaliana) seedlings and recorded dynamic changes in EGSH and H2O2 levels with the genetically encoded biosensors Grx1-roGFP2 (for EGSH) and roGFP2-Orp1 (for H2O2) targeted to chloroplasts, the cytosol, or mitochondria. Treatment of seedlings with MV caused rapid oxidation in chloroplasts and, subsequently, in the cytosol and mitochondria. MV-induced oxidation was significantly boosted by illumination with actinic light, and largely abolished by inhibitors of photosynthetic electron transport. MV also induced autonomous oxidation in the mitochondrial matrix in an electron transport chain activity-dependent manner that was milder than the oxidation triggered in chloroplasts by the combination of MV and light. In vivo redox biosensing resolves the spatiotemporal dynamics of compartmental responses to local ROS generation and provides a basis for understanding how compartment-specific redox dynamics might operate in retrograde signaling and stress acclimation in plants.

Why and How to Dig into Plant Metabolite-Protein Interactions

Interaction between metabolites and proteins drives cellular regulatory processes within and between organisms. Recent reports highlight that numerous plant metabolites embrace multiple biological activities, beyond a sole role as substrates, products, or cofactors of enzymes, or as defense or growth-regulatory compounds. Though several technologies have been developed to identify and characterize metabolite-protein interactions, the systematic implementation of such methods in the plant field remains limited. Here, we discuss the plant metabolic space, with a specific focus on specialized metabolites and their roles, and review the technologies to study their interaction with proteins. We approach it both from a plant's perspective, to increase our understanding of plant metabolite-dependent regulatory networks, and from a human perspective, to empower agrochemical and drug discoveries.

The In Vitro Interaction of 12-Oxophytodienoic Acid and Related Conjugated Carbonyl Compounds with Thiol Antioxidants

α,β-unsaturated carbonyls interfere with numerous plant physiological processes. One mechanism of action is their reactivity toward thiols of metabolites like cysteine and glutathione (GSH). This work aimed at better understanding these interactions. Both 12-oxophytodienoic acid (12-OPDA) and abscisic acid (ABA) conjugated with cysteine. It was found that the reactivity of α,β-unsaturated carbonyls with GSH followed the sequence trans-2-hexenal < 12-OPDA ≈ 12-OPDA-ethylester < 2-cyclopentenone << methyl vinylketone (MVK). Interestingly, GSH, but not ascorbate (vitamin C), supplementation ameliorated the phytotoxic potential of MVK. In addition, 12-OPDA and 12-OPDA-related conjugated carbonyl compounds interacted with proteins, e.g., with members of the thioredoxin (TRX)-fold family. 12-OPDA modified two cysteinyl residues of chloroplast TRX-f1. The OPDAylated TRX-f1 lost its activity to activate the Calvin-Benson-cycle enzyme fructose-1,6-bisphosphatase (FBPase). Finally, we show that 12-OPDA interacts with cyclophilin 20-3 (Cyp20-3) non-covalently and affects its peptidyl-prolyl-cis/trans isomerase activity. The results demonstrate the high potential of 12-OPDA as a diverse interactor and cellular regulator and suggest that OPDAylation may occur in plant cells and should be investigated as novel regulatory mechanism.

Characterization of CYCLOPHILLIN38 shows that a photosynthesis-derived systemic signal controls lateral root emergence

Photosynthesis in leaves generates fixed-carbon resources and essential metabolites that support sink tissues, such as roots. Two of these metabolites, sucrose and auxin, promote growth in root systems, but the explicit connection between photosynthetic activity and control of root architecture has not been explored. Through a mutant screen to identify pathways regulating root system architecture, we identified a mutation in the Arabidopsis thaliana CYCLOPHILIN 38 (CYP38) gene, which causes accumulation of pre-emergent stage lateral roots. CYP38 was previously reported to stabilize photosystem II (PSII) in chloroplasts. CYP38 expression is enriched in shoots, and grafting experiments show that the gene acts non-cell-autonomously to promote lateral root emergence. Growth of wild-type plants under low-light conditions phenocopies the cyp38 lateral root emergence defect, as does the inhibition of PSII-dependent electron transport or Nicotinamide adenine dinucleotide phosphate (NADPH) production. Importantly, these perturbations to photosynthetic activity rapidly suppress lateral root emergence, which is separate from their effects on shoot size. Supplementary exogenous sucrose largely rescued primary root (PR) growth in cyp38, but not lateral root growth. Auxin (indole-3-acetic acid (IAA)) biosynthesis from tryptophan is dependent on reductant generated during photosynthesis. Consistently, we found that wild-type seedlings grown under low light and cyp38 mutants have highly diminished levels of IAA in root tissues. IAA treatment rescued the cyp38 lateral root defect, revealing that photosynthesis promotes lateral root emergence partly through IAA biosynthesis. These data directly confirm the importance of CYP38-dependent photosynthetic activity in supporting root growth, and define the specific contributions of two metabolites in refining root architecture under light-limited conditions.

A molecular switch in sulfur metabolism to reduce arsenic and enrich selenium in rice grain

Rice grains typically contain high levels of toxic arsenic but low levels of the essential micronutrient selenium. Anthropogenic arsenic contamination of paddy soils exacerbates arsenic toxicity in rice crops resulting in substantial yield losses. Here, we report the identification of the gain-of-function arsenite tolerant 1 (astol1) mutant of rice that benefits from enhanced sulfur and selenium assimilation, arsenic tolerance, and decreased arsenic accumulation in grains. The astol1 mutation promotes the physical interaction of the chloroplast-localized O-acetylserine (thiol) lyase protein with its interaction partner serine-acetyltransferase in the cysteine synthase complex. Activation of the serine-acetyltransferase in this complex promotes the uptake of sulfate and selenium and enhances the production of cysteine, glutathione, and phytochelatins, resulting in increased tolerance and decreased translocation of arsenic to grains. Our findings uncover the pivotal sensing-function of the cysteine synthase complex in plastids for optimizing stress resilience and grain quality by regulating a fundamental macronutrient assimilation pathway.

Contamination of paddy soils can lead to toxic arsenic accumulation in rice grains and low levels of the micronutrient selenium. Here the authors show that a gain of function mutant affecting an O-acetylserine (thiol) lyase enhances sulfur and selenium assimilation while reducing arsenic accumulation in grains.

Conserved Residues in the C-Terminal Domain Affect the Structure and Function of CYP38 in Arabidopsis

Arabidopsis cyclophilin38 (CYP38) is a thylakoid lumen protein critial for PSII assembly and maintenance, and its C-terminal region serves as the target binding domain. We hypothesized that four conserved residues (R290, F294, Q372, and F374) in the C-terminal domain are critical for the structure and function of CYP38. In yeast two-hybrid and protein pull-down assays, CYP38s with single-sited mutations (R290A, F294A, Q372A, or F374A) did not interact with the CP47 E-loop as the wild-type CYP38. In contrast, CYP38 with the R290A/F294A/Q372A/F374A quadruple mutation could bind the CP47 E-loop. Gene transformation analysis showed that the quadruple mutation prevented CYP38 to efficiently complement the mutant phenotype of cyp38. The C-terminal domain half protein with the quadruple mutation, like the wild-type one, could interact with the N-terminal domain or the CP47 E-loop in vitro. The cyp38 plants expressing CYP38 with the quadruple mutation showed a similar BN-PAGE profile as cyp38, but distinct from the wild type. The CYP38 protein with the quadruple mutation associated with the thylakoid membrane less efficiently than the wild-type CYP38. We concluded that these four conserved residues are indispensable as changes of all these residues together resulted in a subtle conformational change of CYP38 and reduced its intramolecular N-C interaction and the ability to associate with the thylakoid membrane, thus impairing its function in chloroplast.

Function and Regulation of Chloroplast Peroxiredoxin IIE

Peroxiredoxins (PRX) are thiol peroxidases that are highly conserved throughout all biological kingdoms. Increasing evidence suggests that their high reactivity toward peroxides has a function not only in antioxidant defense but in particular in redox regulation of the cell. Peroxiredoxin IIE (PRX-IIE) is one of three PRX types found in plastids and has previously been linked to pathogen defense and protection from protein nitration. However, its posttranslational regulation and its function in the chloroplast protein network remained to be explored. Using recombinant protein, it was shown that the peroxidatic Cys121 is subjected to multiple posttranslational modifications, namely disulfide formation, S-nitrosation, S-glutathionylation, and hyperoxidation. Slightly oxidized glutathione fostered S-glutathionylation and inhibited activity in vitro. Immobilized recombinant PRX-IIE allowed trapping and subsequent identification of interaction partners by mass spectrometry. Interaction with the 14-3-3 υ protein was confirmed in vitro and was shown to be stimulated under oxidizing conditions. Interactions did not depend on phosphorylation as revealed by testing phospho-mimicry variants of PRX-IIE. Based on these data it is proposed that 14-3-3υ guides PRX‑IIE to certain target proteins, possibly for redox regulation. These findings together with the other identified potential interaction partners of type II PRXs localized to plastids, mitochondria, and cytosol provide a new perspective on the redox regulatory network of the cell.

Cysteine modifications (oxPTM) and protein sulphenylation-mediated sulfenome expression in plants: evolutionary conserved signaling networks?

Plant resilience to oxidative stress possibly operates through the restoration of intracellular redox milieu and the activity of various posttranslationally modified proteins. Among various modes of redox regulation operative in plants cys oxPTMs are brought about by the activity of reactive oxygen species (ROS), reactive nitrogen species (RNS), and hydrogen peroxide. Cysteine oxPTMs are capable of transducing ROS-mediated long-distance hormone signaling (ABA, JA, SA) in plants. S-sulphenylation is an intermediary modification en route to other oxidative states of cysteine. In silico analysis have revealed evolutionary conservation of certain S-sulphenylated proteins across human and plants. Further analysis of protein sulphenylation in plants should be extended to the functional follow-up studies followed by site-specific characterization and case-by-case validation of protein activity. The repertoire of physiological methods (fluorescent conjugates (dimedone) and yeast AP-1 (YAP1)-based genetic probes) in the recent past has been successful in the detection of sulphenylated proteins and other cysteine-based modifications in plants. In view of a better understanding of the sulfur-based redoxome it is necessary to update our timely progress on the methodological advancements for the detection of cysteine-based oxPTM. This substantiative information can extend our investigations on plant-environment interaction thus improving crop manipulation strategies. The simulation-based computational approach has emerged as a new method to determine the directive mechanism of cysteine oxidation in plants. Thus, sulfenome analysis in various plant systems might reflect as a pinnacle of plant redox biology in the future.

Lipid Peroxide-Derived Reactive Carbonyl Species as Mediators of Oxidative Stress and Signaling

Oxidation of membrane lipids by reactive oxygen species (ROS) or O₂/lipoxygenase leads to the formation of various bioactive compounds collectively called oxylipins. Reactive carbonyl species (RCS) are a group of oxylipins that have the α,β-unsaturated carbonyl structure, including acrolein and 4-hydroxy-(E)-2-nonenal. RCS provides a missing link between ROS stimuli and cellular responses in plants via their electrophilic modification of proteins. The physiological significance of RCS in plants has been established based on the observations that the RCS-scavenging enzymes that are overexpressed in plants or the RCS-scavenging chemicals added to plants suppress the plants' responses to ROS, i.e., photoinhibition, aluminum-induced root damage, programmed cell death (PCD), senescence, abscisic acid-induced stomata closure, and auxin-induced lateral root formation. The functions of RCS are thus a key to ROS- and redox-signaling in plants. The chemical species involved in distinct RCS signaling/damaging phenomena were recently revealed, based on comprehensive carbonyl determinations. This review presents an overview of the current status of research regarding RCS signaling functions in plants and discusses present challenges for gaining a more complete understanding of the signaling mechanisms.

12-oxo-Phytodienoic Acid: A Fuse and/or Switch of Plant Growth and Defense Responses?

12-oxo-Phytodienoic acid (OPDA) is a primary precursor of (-)-jasmonic acid (JA), able to trigger autonomous signaling pathways that regulate a unique subset of jasmonate-responsive genes, activating and fine-tuning defense responses, as well as growth processes in plants. Recently, a number of studies have illuminated the physiol-molecular activities of OPDA signaling in plants, which interconnect the regulatory loop of photosynthesis, cellular redox homeostasis, and transcriptional regulatory networks, together shedding new light on (i) the underlying modes of cellular interfaces between growth and defense responses (e.g., fitness trade-offs or balances) and (ii) vital information in genetic engineering or molecular breeding approaches to upgrade own survival capacities of plants. However, our current knowledge regarding its mode of actions is still far from complete. This review will briefly revisit recent progresses on the roles and mechanisms of OPDA and information gaps within, which help in understanding the phenotypic and environmental plasticity of plants.

Grapevine-Downy Mildew Rendezvous: Proteome Analysis of the First Hours of an Incompatible Interaction

Grapevine is one of the most relevant crops in the world being used for economically important products such as wine. However, relevant grapevine cultivars are heavily affected by diseases such as the downy mildew disease caused by Plasmopara viticola. Improvements on grapevine resistance are made mainly by breeding techniques where resistance traits are introgressed into cultivars with desired grape characteristics. However, there is still a lack of knowledge on how resistant or tolerant cultivars tackle the P. viticola pathogen. In this study, using a shotgun proteomics LC-MS/MS approach, we unravel the protein modulation of a highly tolerant grapevine cultivar, Vitis vinifera “Regent”, in the first hours post inoculation (hpi) with P. viticola. At 6 hpi, proteins related to defense and to response to stimuli are negatively modulated while at 12 hpi there is an accumulation of proteins belonging to both categories. The co-occurrence of indicators of effector-triggered susceptibility (ETS) and effector-triggered immunity (ETI) is detected at both time-points, showing that these defense processes present high plasticity. The results obtained in this study unravel the tolerant grapevine defense strategy towards P. viticola and may provide valuable insights on resistance associated candidates and mechanisms, which may play an important role in the definition of new strategies for breeding approaches.

Redox Modification of the Iron-Sulfur Glutaredoxin GRXS17 Activates Holdase Activity and Protects Plants from Heat Stress

Heat stress induces misfolding and aggregation of proteins unless they are guarded by chaperone systems. Here, we examined the function of the glutaredoxin GRXS17, a member of thiol reductase families in the model plant Arabidopsis (Arabidopsis thaliana). GRXS17 is a nucleocytosolic monothiol glutaredoxin consisting of an N-terminal thioredoxin domain and three CGFS active-site motif-containing GRX domains that coordinate three iron-sulfur (Fe-S) clusters in a glutathione-dependent manner. As an Fe-S cluster-charged holoenzyme, GRXS17 is likely involved in the maturation of cytosolic and nuclear Fe-S proteins. In addition to its role in cluster biogenesis, GRXS17 presented both foldase and redox-dependent holdase activities. Oxidative stress in combination with heat stress induced loss of its Fe-S clusters followed by subsequent formation of disulfide bonds between conserved active-site cysteines in the corresponding thioredoxin domains. This oxidation led to a shift of GRXS17 to a high-molecular-weight complex and thus activated its holdase activity in vitro. Moreover, GRXS17 was specifically involved in plant tolerance to moderate high temperature and protected root meristematic cells from heat-induced cell death. Finally, GRXS17 interacted with a different set of proteins upon heat stress, possibly protecting them from heat injuries. Therefore, we propose that the Fe-S cluster enzyme GRXS17 is an essential guard that protects proteins against moderate heat stress, likely through a redox-dependent chaperone activity. We reveal the mechanism of an Fe-S cluster-dependent activity shift that converts the holoenzyme GRXS17 into a holdase, thereby preventing damage caused by heat stress.

A Cyclophilin OsCYP20-2 Interacts with OsSYF2 to Regulate Grain Length by Pre-mRNA Splicing

BACKGROUND

Grain size is one of the key agronomic traits that impact grain yield. Several regulatory pathways had been reported to participate in grain size determination via cell expansion or proliferation in rice. However, little is known about cyclophilin and spliceosome participation in grain shape regulation.

RESULTS

Here, we identified OsCYP20-2, a cyclophilin that influences spliceosome assembly to determine grain length. oscyp20-2 t1, a knock out mutant of OsCYP20-2 caused by T-DNA insertion, produced shorter grains with deficient cell elongation. Through yeast two-hybrid screening and pull-down assays, OsSYF2, a pre-mRNA splicing factor, was identified as an interacting protein of OsCYP20-2. The phenotypes of transgenic lines indicated that OsSYF2 positively regulates grain length via its influence on cell expansion. Transcriptomic analysis showed that OsSYF2 controls the expression and pre-mRNA alternative splicing of genes involved in sugar metabolism. In addition, these two genes have similar effects on panicle architecture.

CONCLUSIONS

Taken together, OsSYF2, an interacting protein of OsCYP20-2, controls grain length and panicle architecture by regulating the alternative splicing of pre-mRNA involved in cell elongation and sugar metabolism.

Chemical Genetics Approach Identifies Abnormal Inflorescence Meristem 1 as a Putative Target of a Novel Sulfonamide That Protects Catalase2-Deficient Arabidopsis against Photorespiratory Stress

Alterations of hydrogen peroxide (H₂O₂) levels have a profound impact on numerous signaling cascades orchestrating plant growth, development, and stress signaling, including programmed cell death. To expand the repertoire of known molecular mechanisms implicated in H₂O₂ signaling, we performed a forward chemical screen to identify small molecules that could alleviate the photorespiratory-induced cell death phenotype of Arabidopsisthaliana mutants lacking H₂O₂-scavenging capacity by peroxisomal catalase2. Here, we report the characterization of pakerine, an m-sulfamoyl benzamide from the sulfonamide family. Pakerine alleviates the cell death phenotype of cat2 mutants exposed to photorespiration-promoting conditions and delays dark-induced senescence in wild-type Arabidopsis leaves. By using a combination of transcriptomics, metabolomics, and affinity purification, we identified abnormal inflorescence meristem 1 (AIM1) as a putative protein target of pakerine. AIM1 is a 3-hydroxyacyl-CoA dehydrogenase involved in fatty acid β-oxidation that contributes to jasmonic acid (JA) and salicylic acid (SA) biosynthesis. Whereas intact JA biosynthesis was not required for pakerine bioactivity, our results point toward a role for β-oxidation-dependent SA production in the execution of H₂O₂-mediated cell death.

CYP20-3 deglutathionylates 2-CysPRX A and suppresses peroxide detoxification during heat stress

In plants, growth-defense trade-offs occur because of limited resources, which demand prioritization towards either of them depending on various external and internal factors. However, very little is known about molecular mechanisms underlying their occurrence. Here, we describe that cyclophilin 20-3 (CYP20-3), a 12-oxo-phytodienoic acid (OPDA)-binding protein, crisscrosses stress responses with light-dependent electron reactions, which fine-tunes activities of key enzymes in plastid sulfur assimilations and photosynthesis. Under stressed states, OPDA, accumulates in the chloroplasts, binds and stimulates CYP20-3 to convey electrons towards serine acetyltransferase 1 (SAT1) and 2-Cys peroxiredoxin A (2CPA). The latter is a thiol-based peroxidase, protecting and optimizing photosynthesis by reducing its toxic byproducts (e.g., H2O2). Reduction of 2CPA then inactivates its peroxidase activity, suppressing the peroxide detoxification machinery, whereas the activation of SAT1 promotes thiol synthesis and builds up reduction capacity, which in turn triggers the retrograde regulation of defense gene expressions against abiotic stress. Thus, we conclude that CYP20-3 is a unique metabolic hub conveying resource allocations between plant growth and defense responses (trade-offs), ultimately balancing optimal growth phonotype.

Infection of human lymphomononuclear cells by SARS-CoV-2

Although SARS-CoV-2 severe infection is associated with a hyperinflammatory state, lymphopenia is an immunological hallmark, and correlates with poor prognosis in COVID-19. However, it remains unknown if circulating human lymphocytes and monocytes are susceptible to SARS-CoV-2 infection. In this study, SARS-CoV-2 infection of human peripheral blood mononuclear cells (PBMCs) was investigated both in vitro and in vivo . We found that in vitro infection of whole PBMCs from healthy donors was productive of virus progeny. Results revealed that monocytes, as well as B and T lymphocytes, are susceptible to SARS-CoV-2 active infection and viral replication was indicated by detection of double-stranded RNA. Moreover, flow cytometry and immunofluorescence analysis revealed that SARS-CoV-2 was frequently detected in monocytes and B lymphocytes from COVID-19 patients, and less frequently in CD4 ⁺ T lymphocytes. The rates of SARS-CoV-2-infected monocytes in PBMCs from COVID-19 patients increased over time from symptom onset. Additionally, SARS-CoV-2-positive monocytes and B and CD4+T lymphocytes were detected by immunohistochemistry in post mortem lung tissue. SARS-CoV-2 infection of blood circulating leukocytes in COVID-19 patients may have important implications for disease pathogenesis, immune dysfunction, and virus spread within the host.

A mitochondria-targeted coenzyme Q peptoid induces superoxide dismutase and alleviates salinity stress in plant cells

Salinity is a serious challenge to global agriculture and threatens human food security. Plant cells can respond to salt stress either by activation of adaptive responses, or by programmed cell death. The mechanisms deciding the respective response are far from understood, but seem to depend on the degree, to which mitochondria can maintain oxidative homeostasis. Using plant PeptoQ, a Trojan Peptoid, as vehicle, it is possible to transport a coenzyme Q10 (CoQ10) derivative into plant mitochondria. We show that salinity stress in tobacco BY-2 cells (Nicotiana tabacum L. cv Bright Yellow-2) can be mitigated by pretreatment with plant PeptoQ with respect to numerous aspects including proliferation, expansion, redox homeostasis, and programmed cell death. We tested the salinity response for transcripts from nine salt-stress related-genes representing different adaptive responses. While most did not show any significant response, the salt response of the transcription factor NtNAC, probably involved in mitochondrial retrograde signaling, was significantly modulated by the plant PeptoQ. Most strikingly, transcripts for the mitochondrial, Mn-dependent Superoxide Dismutase were rapidly and drastically upregulated in presence of the peptoid, and this response was disappearing in presence of salt. The same pattern, albeit at lower amplitude, was seen for the sodium exporter SOS1. The findings are discussed by a model, where plant PeptoQ modulates retrograde signalling to the nucleus leading to a strong expression of mitochondrial SOD, what renders mitochondria more resilient to perturbations of oxidative balance, such that cells escape salt induced cell death and remain viable.

Plant Chloroplast Stress Response: Insights from Thiol Redox Proteomics

Significance: Plant chloroplasts generate reactive oxygen species (ROS) during photosynthesis, especially under stresses. The sulfhydryl groups of protein cysteine residues are susceptible to redox modifications, which regulate protein structure and function, and thus different signaling and metabolic processes. The ROS-governed protein thiol redox switches play important roles in chloroplasts. Recent Advances: Various high-throughput thiol redox proteomic approaches have been developed, and they have enabled the improved understanding of redox regulatory mechanisms in chloroplasts. For example, the thioredoxin-modulated antioxidant enzymes help to maintain cellular ROS homeostasis. The light- and dark-dependent redox regulation of photosynthetic electron transport, the Calvin/Benson cycle, and starch biosynthesis ensures metabolic coordination and efficient energy utilization. In addition, redox cascades link the light with the dynamic changes of metabolites in nitrate and sulfur assimilation, shikimate pathway, and biosynthesis of fatty acid hormone as well as purine, pyrimidine, and thiamine. Importantly, redox regulation of tetrapyrrole and chlorophyll biosynthesis is critical to balance the photodynamic tetrapyrrole intermediates and prevent oxidative damage. Moreover, redox regulation of diverse elongation factors, chaperones, and kinases plays an important role in the modulation of gene expression, protein conformation, and posttranslational modification that contribute to photosystem II (PSII) repair, state transition, and signaling in chloroplasts. Critical Issues: This review focuses on recent advances in plant thiol redox proteomics and redox protein networks toward understanding plant chloroplast signaling, metabolism, and stress responses. Future Directions: Using redox proteomics integrated with biochemical and molecular genetic approaches, detailed studies of cysteine residues, their redox states, cross talk with other modifications, and the functional implications will yield a holistic understanding of chloroplast stress responses.