ABA3 is a molybdenum cofactor sulfurase required for activation of aldehyde oxidase and xanthine dehydrogenase in Arabidopsis thaliana

Hormonal regulation of plant primary metabolism under drought

Phytohormones are essential signalling molecules globally regulating many processes of plants, including their growth, development, and stress responses. The promotion of growth and the enhancement of stress resistance have to be balanced, especially under adverse conditions such as drought stress, because of limited resources. Plants cope with drought stress via various strategies, including the transcriptional regulation of stress-responsive genes and the adjustment of metabolism, and phytohormones play roles in these processes. Although abscisic acid (ABA) is an important signal under drought, less attention has been paid to other phytohormones. In this review, we summarize progress in the understanding of phytohormone-regulated primary metabolism under water-limited conditions, especially in Arabidopsis thaliana, and highlight recent findings concerning the amino acids associated with ABA metabolism and signalling. We also discuss how phytohormones function antagonistically and synergistically in order to balance growth and stress responses.

Advance Research on the Pre-Harvest Sprouting Trait in Vegetable Crop Seeds

Pre-harvest sprouting (PHS), the germination of seeds on the plant prior to harvest, poses significant challenges to agriculture. It not only reduces seed and grain yield, but also impairs the commodity quality of the fruit, ultimately affecting the success of the subsequent crop cycle. A deeper understanding of PHS is essential for guiding future breeding strategies, mitigating its impact on seed production rates and the commercial quality of fruits. PHS is a complex phenomenon influenced by genetic, physiological, and environmental factors. Many of these factors exert their influence on PHS through the intricate regulation of plant hormones responsible for seed germination. While numerous genes related to PHS have been identified in food crops, the study of PHS in vegetable crops is still in its early stages. This review delves into the regulatory elements, functional genes, and recent research developments related to PHS in vegetable crops. Meanwhile, this paper presents a novel understanding of PHS, aiming to serve as a reference for the study of this trait in vegetable crops.

The mitochondrial amidoxime reducing component-from prodrug-activation mechanism to drug-metabolizing enzyme and onward to drug target

The mitochondrial amidoxime-reducing component (mARC) is one of five known molybdenum enzymes in eukaryotes. mARC belongs to the MOSC domain superfamily, a large group of so far poorly studied molybdoenzymes. mARC was initially discovered as the enzyme activating N-hydroxylated prodrugs of basic amidines but has since been shown to also reduce a variety of other N-oxygenated compounds, for example, toxic nucleobase analogs. Under certain circumstances, mARC might also be involved in reductive nitric oxide synthesis through reduction of nitrite. Recently, mARC enzymes have received a lot of attention due to their apparent involvement in lipid metabolism and, in particular, because many genome-wide association studies have shown a common variant of human mARC1 to have a protective effect against liver disease. The mechanism linking mARC enzymes with lipid metabolism remains unknown. Here, we give a comprehensive overview of what is currently known about mARC enzymes, their substrates, structure, and apparent involvement in human disease.

History of Maturation of Prokaryotic Molybdoenzymes-A Personal View

In prokaryotes, the role of Mo/W enzymes in physiology and bioenergetics is widely recognized. It is worth noting that the most diverse family of Mo/W enzymes is exclusive to prokaryotes, with the probable existence of several of them from the earliest forms of life on Earth. The structural organization of these enzymes, which often include additional redox centers, is as diverse as ever, as is their cellular localization. The most notable observation is the involvement of dedicated chaperones assisting with the assembly and acquisition of the metal centers, including Mo/W-bisPGD, one of the largest organic cofactors in nature. This review seeks to provide a new understanding and a unified model of Mo/W enzyme maturation.

Balanophora genomes display massively convergent evolution with other extreme holoparasites and provide novel insights into parasite-host interactions

Parasitic plants have evolved to be subtly or severely dependent on host plants to complete their life cycle. To provide new insights into the biology of parasitic plants in general, we assembled genomes for members of the sandalwood order Santalales, including a stem hemiparasite (Scurrula) and two highly modified root holoparasites (Balanophora) that possess chimaeric host-parasite tubers. Comprehensive genome comparisons reveal that hemiparasitic Scurrula has experienced a relatively minor degree of gene loss compared with autotrophic plants, consistent with its moderate degree of parasitism. Nonetheless, patterns of gene loss appear to be substantially divergent across distantly related lineages of hemiparasites. In contrast, Balanophora has experienced substantial gene loss for the same sets of genes as an independently evolved holoparasite lineage, the endoparasitic Sapria (Malpighiales), and the two holoparasite lineages experienced convergent contraction of large gene families through loss of paralogues. This unprecedented convergence supports the idea that despite their extreme and strikingly divergent life histories and morphology, the evolution of these and other holoparasitic lineages can be shaped by highly predictable modes of genome reduction. We observe substantial evidence of relaxed selection in retained genes for both hemi- and holoparasitic species. Transcriptome data also document unusual and novel interactions between Balanophora and host plants at the host-parasite tuber interface tissues, with evidence of mRNA exchange, substantial and active hormone exchange and immune responses in parasite and host.

Biosynthetic Pathways of Hormones in Plants

Phytohormones exhibit a wide range of chemical structures, though they primarily originate from three key metabolic precursors: amino acids, isoprenoids, and lipids. Specific amino acids, such as tryptophan, methionine, phenylalanine, and arginine, contribute to the production of various phytohormones, including auxins, melatonin, ethylene, salicylic acid, and polyamines. Isoprenoids are the foundation of five phytohormone categories: cytokinins, brassinosteroids, gibberellins, abscisic acid, and strigolactones. Furthermore, lipids, i.e., α-linolenic acid, function as a precursor for jasmonic acid. The biosynthesis routes of these different plant hormones are intricately complex. Understanding of these processes can greatly enhance our knowledge of how these hormones regulate plant growth, development, and physiology. This review focuses on detailing the biosynthetic pathways of phytohormones.

Mutations in Selected ABA-Related Genes Reduce Level of Arabidopsis thaliana Susceptibility to the Beet Cyst Nematode Heterodera schachtii

Heterodera schachtii is a common parasite of many important crops such as beets and Brassicaceae (oilseed rape, cabbage or mustard). Arabidopsis thaliana is a model plant also used for studying defence responses to pathogens or pest infections. Defence responses of plants are often regulated and fine-tuned by stress phytohormones: salicylic acid (SA), jasmonic acid (JA), ethylene (Et) and abscisic acid (ABA), of which the role of ABA in these responses is the least examined. The aim of this study was to show, if and which genes related to ABA turnover can be modulated during the development of nematode-induced feeding sites in A. thaliana roots. To answer the question, we performed infection tests on wild type and ABA mutant roots and analysed the expression levels of selected ABA-related genes (ABI1, ABI2, ABI5, PYL5, PYL6, CYP707A1 and CYP707A4) at the early stage of root infection. Our results show that the expression of ABI2, ABI5 (ABA signalling) and CYP707A4 (ABA metabolism) genes was upregulated in feeding sites at 4 dpi, whereas the level of expression of PYL5 and PYL6 (ABA receptors) genes was decreased. Mutations in ABI1, ABI2, ABI5, CYP707A1 or CYP707A4 genes led to a decrease of A. thaliana susceptibility verbalised as the number of fully developed females, whereas mutations in PYL5 or PYL6 genes did not influence the number of females of the nematode. Based on the results, it can be concluded that the modifications of analysed ABA-related gene expression are required for the proper development of nematodes; however, further in-depth analyses are required.

Diversity and roles of cysteine desulfurases in photosynthetic organisms

As sulfur is part of many essential protein cofactors such as iron-sulfur clusters, molybdenum cofactors, or lipoic acid, its mobilization from cysteine represents a fundamental process. The abstraction of the sulfur atom from cysteine is catalysed by highly conserved pyridoxal 5'-phosphate-dependent enzymes called cysteine desulfurases. The desulfuration of cysteine leads to the formation of a persulfide group on a conserved catalytic cysteine and the concomitant release of alanine. Sulfur is then transferred from cysteine desulfurases to different targets. Numerous studies have focused on cysteine desulfurases as sulfur-extracting enzymes for iron-sulfur cluster synthesis in mitochondria and chloroplasts but also for molybdenum cofactor sulfuration in the cytosol. Despite this, knowledge about the involvement of cysteine desulfurases in other pathways is quite rudimentary, particularly in photosynthetic organisms. In this review, we summarize current understanding of the different groups of cysteine desulfurases and their characteristics in terms of primary sequence, protein domain architecture, and subcellular localization. In addition, we review the roles of cysteine desulfurases in different fundamental pathways and highlight the gaps in our knowledge to encourage future work on unresolved issues especially in photosynthetic organisms.

Overexpression of NtGCN2 improves drought tolerance in tobacco by regulating proline accumulation, ROS scavenging ability, and stomatal closure

Drought stress is a severe threat to plants. Genes that respond to drought stress are essential for plant growth and development. General control nonderepressible 2 (GCN2) encodes a protein kinase that responds to various biotic and abiotic stresses. However, the mechanism of GCN2 in plant drought tolerance remains unclear. In the present study, the promoters of NtGCN2 from Nicotiana tabacum K326, which contained a drought-responsive Cis-acting element MYB that can be activated by drought stress, were cloned. Furthermore, the drought tolerance function of NtGCN2 was investigated using NtGCN2-overexpressed transgenic tobacco plants. NtGCN2-overexpressed transgenic plants were more tolerant to drought stress than wild-type (WT) plants. The transgenic tobacco plants exhibited higher proline and abscisic acid (ABA) contents, antioxidant enzyme activities, leaf relative water content, and expression levels of genes encoding key antioxidant enzymes and proline synthase, but lower levels of malondialdehyde and reactive oxygen species, and reduced stomatal apertures, stomatal densities, and stomatal opening rates compared to WT plants under drought stress. These results indicated that overexpression of NtGCN2 conferred drought tolerance in transgenic tobacco plants. RNA-seq analysis showed that overexpression of NtGCN2 responded to drought stress by regulating the expression of genes related to proline synthesis and catabolism, abscisic acid synthesis and catabolism, antioxidant enzymes, and ion channels in guard cells. These results showed that NtGCN2 might regulate drought tolerance by regulating proline accumulation, reactive oxygen species (ROS) scavenging, and stomatal closure in tobacco and may have the potential for application in the genetic modification of crop drought tolerance.

PRpnp, a novel dual activity PNP family protein improves plant vigour and confers multiple stress tolerance in Citrus aurantifolia

Under field conditions, plants are often simultaneously exposed to several abiotic and biotic stresses resulting in significant reductions in growth and yield; thus, developing a multi-stress tolerant variety is imperative. Previously, we reported the neofunctionalization of a novel PNP family protein, Putranjiva roxburghii purine nucleoside phosphorylase (PRpnp) to trypsin inhibitor to cater to the needs of plant defence. However, to date, no study has revealed the potential role and mechanism of either member of this protein group in plant defence. Here, we overexpressed PRpnp in Citrus aurantifolia which showed nuclear-cytoplasmic localization, where it functions in maintaining the intracellular purine reservoir. Overexpression of PRpnp significantly enhanced tolerance to salt, oxidative stress, alkaline pH, drought and two pests, Papilio demoleus and Scirtothrips citri in transgenic plants. Global gene expression studies revealed that PRpnp overexpression up-regulated differentially expressed genes (DEGs) related to ABA- and JA-biosynthesis and signalling, plant defence, growth and development. LC-MS/MS analysis validated higher endogenous ABA and JA accumulation in transgenic plants. Taken together, our results suggest that PRpnp functions by enhancing the endogenous ABA and JA, which interact synergistically and it also inhibits trypsin proteases in the insect gut. Also, like other purine salvage genes, PRpnp also regulates CK metabolism and increases the levels of CK-free bases in transgenic Mexican lime. We also suggest that PRpnp can be used as a potential candidate to develop new varieties with improved plant vigour and enhanced multiple stress resistance.

Genome-Wide Characterization and Expression Profiling of ABA Biosynthesis Genes in a Desert Moss Syntrichia caninervis

Syntrichia caninervis can survive under 80-90% protoplasmic water losses, and it is a model plant in desiccation tolerance research. A previous study has revealed that S. caninervis would accumulate ABA under dehydration stress, while the ABA biosynthesis genes in S. caninervis are still unknown. This study identified one ScABA1, two ScABA4s, five ScNCEDs, twenty-nine ScABA2s, one ScABA3, and four ScAAOs genes, indicating that the ABA biosynthesis genes were complete in S. caninervis. Gene location analysis showed that the ABA biosynthesis genes were evenly distributed in chromosomes but were not allocated to sex chromosomes. Collinear analysis revealed that ScABA1, ScNCED, and ScABA2 had homologous genes in Physcomitrella patens. RT-qPCR detection found that all of the ABA biosynthesis genes responded to abiotic stress; it further indicated that ABA plays an important role in S. caninervis. Moreover, the ABA biosynthesis genes in 19 representative plants were compared to study their phylogenetic and conserved motifs; the results suggested that the ABA biosynthesis genes were closely associated with plant taxa, but these genes had the same conserved domain in each plant. In contrast, there is a huge variation in the exon number between different plant taxa; it revealed that ABA biosynthesis gene structures are closely related to plant taxa. Above all, this study provides strong evidence demonstrating that ABA biosynthesis genes were conserved in the plant kingdom and deepens our understanding of the evolution of the phytohormone ABA.

Do Opposites Attract? Auxin-Abscisic Acid Crosstalk: New Perspectives

Plants are constantly exposed to a variety of different environmental stresses, including drought, salinity, and elevated temperatures. These stress cues are assumed to intensify in the future driven by the global climate change scenario which we are currently experiencing. These stressors have largely detrimental effects on plant growth and development and, therefore, put global food security in jeopardy. For this reason, it is necessary to expand our understanding of the underlying mechanisms by which plants respond to abiotic stresses. Especially boosting our insight into the ways by which plants balance their growth and their defense programs appear to be of paramount importance, as this may lead to novel perspectives that can pave the way to increase agricultural productivity in a sustainable manner. In this review, our aim was to present a detailed overview of different facets of the crosstalk between the antagonistic plant hormones abscisic acid (ABA) and auxin, two phytohormones that are the main drivers of plant stress responses, on the one hand, and plant growth, on the other.

Multiple forms of vitamin B6 regulate salt tolerance by balancing ROS and abscisic acid levels in maize root

Salt stress causes osmotic stress, ion toxicity and oxidative stress, inducing the accumulation of abscisic acid (ABA) and excessive reactive oxygen species (ROS) production, which further damage cell structure and inhibit the development of roots in plants. Previous study showed that vitamin B6 (VB6) plays a role in plant responses to salt stress, however, the regulatory relationship between ROS, VB6 and ABA under salt stress remains unclear yet in plants. In our study, we found that salt stress-induced ABA accumulation requires ROS production, in addition, salt stress also promoted VB6 (including pyridoxamine (PM), pyridoxal (PL), pyridoxine (PN), and pyridoxal 5'-phosphate (PLP)) accumulation, which involved in ROS scavenging and ABA biosynthesis. Furthermore, VB6-deficient maize mutant small kernel2 (smk2) heterozygous is more susceptible to salt stress, and which failed to scavenge excessive ROS effectively or induce ABA accumulation in maize root under salt stress, interestingly, which can be restored by exogenous PN and PLP, respectively. According to these results, we proposed that PN and PLP play an essential role in balancing ROS and ABA levels under salt stress, respectively, it laid a foundation for VB6 to be better applied in crop salt resistance than ABA.

The regulatory role of abscisic acid on cadmium uptake, accumulation and translocation in plants

To date, Cd contamination of cropland and crops is receiving more and more attention around the world. As a plant hormone, abscisic acid (ABA) plays an important role in Cd stress response, but its effect on plant Cd uptake and translocation varies among plant species. In some species, such as Arabidopsis thaliana, Oryza sativa, Brassica chinensis, Populus euphratica, Lactuca sativa, and Solanum lycopersicum, ABA inhibits Cd uptake and translocation, while in other species, such as Solanum photeinocarpum and Boehmeria nivea, ABA severs the opposite effect. Interestingly, differences in the methods and concentrations of ABA addition also triggered the opposite result of Cd uptake and translocation in Sedum alfredii. The regulatory mechanism of ABA involved in Cd uptake and accumulation in plants is still not well-established. Therefore, we summarized the latest studies on the ABA synthesis pathway and comparatively analyzed the physiological and molecular mechanisms related to ABA uptake, translocation, and detoxification of Cd in plants at different ABA concentrations or among different species. We believe that the control of Cd uptake and accumulation in plant tissues can be achieved by the appropriate ABA application methods and concentrations in plants.

The History of the Molybdenum Cofactor—A Personal View

The transition element molybdenum (Mo) is an essential micronutrient for plants, animals, and microorganisms, where it forms part of the active center of Mo enzymes. To gain biological activity in the cell, Mo has to be complexed by a pterin scaffold to form the molybdenum cofactor (Moco). Mo enzymes and Moco are found in all kingdoms of life, where they perform vital transformations in the metabolism of nitrogen, sulfur, and carbon compounds. In this review, I recall the history of Moco in a personal view, starting with the genetics of Moco in the 1960s and 1970s, followed by Moco biochemistry and the description of its chemical structure in the 1980s. When I review the elucidation of Moco biosynthesis in the 1990s and the early 2000s, I do it mainly for eukaryotes, as I worked with plants, human cells, and filamentous fungi. Finally, I briefly touch upon human Moco deficiency and whether there is life without Moco.

A briefly overview of the research progress for the abscisic acid analogues

Abscisic acid (ABA) is an important plant endogenous hormone that participates in the regulation of various physiological processes in plants, including the occurrence and development of somatic embryos, seeddevelopment and dormancy. ABA is called “plant stress resistance factor”, while with the limitation of the rapid metabolic inactivation and photoisomerization inactivation of ABA for its large-scale use. Understanding the function and role of ABA in plants is of great significance to promote its application. For decades, scientists have conducted in-depth research on its mechanism of action and signaling pathways, a series of progress were achieved, and hundreds of ABA analogues (similar in structure or function) have been synthesized to develop highly active plant growth regulators and tools to elucidate ABA perception. In this review, we summarize a variety of ABA analogues, especially the ABA receptor analogues, and explore the mechanisms of ABA action and catabolism, which will facilitate the development of novel ABA analogues with high biological activities.

The bacterial effector AvrRxo1 inhibits vitamin B6 biosynthesis to promote infection in rice

Xanthomonas oryzae pv. oryzicola (Xoc), which causes rice bacterial leaf streak, invades leaves mainly through stomata, which are often closed as a plant immune response against pathogen invasion. How Xoc overcomes stomatal immunity is unclear. Here, we show that the effector protein AvrRxo1, an ATP-dependent protease, enhances Xoc virulence and inhibits stomatal immunity by targeting and degrading rice OsPDX1 (pyridoxal phosphate synthase), thereby reducing vitamin B6 (VB6) levels in rice. VB6 is required for the activity of aldehyde oxidase, which catalyzes the last step of abscisic acid (ABA) biosynthesis, and ABA positively regulates rice stomatal immunity against Xoc. Thus, we provide evidence supporting a model in which a major bacterial pathogen inhibits plant stomatal immunity by directly targeting VB6 biosynthesis and consequently inhibiting the biosynthesis of ABA in guard cells to open stomata. Moreover, AvrRxo1-mediated VB6 targeting also explains the poor nutritional quality, including low VB6 levels, of Xoc-infected rice grains.

The Interplay between Hydrogen Sulfide and Phytohormone Signaling Pathways under Challenging Environments

Hydrogen sulfide (H₂S) serves as an important gaseous signaling molecule that is involved in intra- and intercellular signal transduction in plant–environment interactions. In plants, H₂S is formed in sulfate/cysteine reduction pathways. The activation of endogenous H₂S and its exogenous application has been found to be highly effective in ameliorating a wide variety of stress conditions in plants. The H₂S interferes with the cellular redox regulatory network and prevents the degradation of proteins from oxidative stress via post-translational modifications (PTMs). H₂S-mediated persulfidation allows the rapid response of proteins in signaling networks to environmental stimuli. In addition, regulatory crosstalk of H₂S with other gaseous signals and plant growth regulators enable the activation of multiple signaling cascades that drive cellular adaptation. In this review, we summarize and discuss the current understanding of the molecular mechanisms of H₂S-induced cellular adjustments and the interactions between H₂S and various signaling pathways in plants, emphasizing the recent progress in our understanding of the effects of H₂S on the PTMs of proteins. We also discuss future directions that would advance our understanding of H₂S interactions to ultimately mitigate the impacts of environmental stresses in the plants.

Research progress of aldehyde oxidases in plants

Plant aldehyde oxidases (AOs) are multi-functional enzymes, and they could oxidize abscisic aldehyde into ABA (abscisic acid) or indole acetaldehyde into IAA (indoleacetic acid) as the last step, respectively. AOs can be divided into four groups based on their biochemical and physiological functions. In this review, we summarized the recent studies about AOs in plants including the motif information, biochemical, and physiological functions. Besides their role in phytohormones biosynthesis and stress response, AOs could also involve in reactive oxygen species homeostasis, aldehyde detoxification and stress tolerance.

Transcriptomic and proteomic analyses uncover the drought adaption landscape of Phoebe zhennan

BACKGROUND

Phoebe zhennan S.Lee (nanmu) is listed as a threatened tree species in China, whose growth and development, especially during the seedling stage, can be severely limited by drought. Previous studies on nanmu responses to drought stress involved physiological and biochemical analyses, while the molecular mechanisms remained unclear. Therefore, it is of great significance to carry out molecular biology research on the drought resistance of nanmu and reveal the genetic background and molecular regulation mechanism of nanmu drought resistance.

RESULTS

Drought stress enhanced the soluble sugar (SS), free proline(PRO), superoxide anion (O2·-), and hydrogen peroxide (H₂O₂) contents as well as the peroxidase (POD) and monodehydroascorbate reductase (MDHAR) activities of nanmu. However, glutathione S-transferase (GST) activity was sensitive to drought stress. Further transcriptomic and proteomic analyses revealed the abundant members of the differentially expressed genes(DEGs) and differentially expressed proteins(DEPs) that were related to phenylpropanoid and flavonoid biosynthesis, hormone biosynthesis and signal transduction, chlorophyll metabolism, photosynthesis, and oxidation-reduction reaction, which suggested their involvement in the drought response of nanmu. These enhanced the osmotic regulation, detoxification, and enzyme-induced and non-enzyme-induced antioxidant ability of nanmu. Moreover, 52% (447/867) of proteins that were up-regulated and 34% (307/892) down-regulated ones were attributed to the increase and decrease of transcription abundance. Transcript up (T^U) and protein up (P^U) groups had 447 overlaps, while transcript down (T^D) and protein down (P^D) groups had 307 overlaps, accounting for 54% of up and 35% of down-regulated proteins. The lack of overlap between DEGs and DEPs also suggested that post-transcriptional regulation has a critical role in nanmu response to drought.

CONCLUSIONS

Our research results provide significant insights into the regulatory mechanisms of drought stress in nanmu.

Electrochemically driven catalysis of the bacterial molybdenum enzyme YiiM

The Mo-dependent enzyme YiiM enzyme from Escherichia coli is a member of the sulfite oxidase family and shares many similarities with the well-studied human mitochondrial amidoxime reducing component (mARC). We have investigated YiiM catalysis using electrochemical and spectroscopic methods. EPR monitored redox potentiometry found the active site redox potentials to be Mo^VI/V -0.02 V and Mo^V/IV -0.12 V vs NHE at pH 7.2. In the presence of methyl viologen as an electrochemically reduced electron donor, YiiM catalysis was studied with a range of potential substrates. YiiM preferentially reduces N-hydroxylated compounds such as hydroxylamines, amidoximes, N-hydroxypurines and N-hydroxyureas but shows little or no activity against amine-oxides or sulfoxides. The pH optimum for catalysis was 7.1 and a bell-shaped pH profile was found with pK_a values of 6.2 and 8.1 either side of this optimum that are associated with protonation/deprotonations that modulate activity. Simulation of the experimental voltammetry elucidated kinetic parameters associated with YiiM catalysis with the substrates 6-hydroxyaminopurine and benzamidoxime.

Pulmonary inflammation and cellular responses following exposure to benzalkonium chloride: Potential impact of disrupted pulmonary surfactant homeostasis

Benzalkonium chloride (BKC) is a prototypical quaternary ammonium disinfectant. Previously, we suggested a no lethal dose level (0.005%) and an LD50 range (0.5-0.05%) of BKC following a single pharyngeal aspiration. Herein, we exposed BKC repeatedly by pharyngeal aspiration for 14 days (0.005 and 0.01%, female mice, total five times with interval of two days, 5 mice/group) and 28 days (0, 0.001, 0.005, and 0.01%, male and female mice, weekly, 16 mice/sex/group). Death following 14 days-repeated exposure did not occur. Meanwhile, chronic pathological lesions were observed in the lung tissues of mice exposed to BKC for 28 days. The total number of bronchial alveolar lavage cells increased, and pulmonary homeostasis of immunologic messenger molecules was disturbed. Following, we investigated BKC-induced cellular responses using human bronchial epithelial cells. The cytotoxicity increased rapidly with concentration. Lysosomal volume, NO production, and lipid peroxidation increased in BKC-treated cells, whereas intracellular ROS level decreased accompanying structural and functional damage of mitochondria. We also found that BKC affected the expression level of immune response, DNA damage, and amino acid biosynthesis-related molecules. More interestingly, lamellar body- and autophagosome-like structures were notably observed in cells exposed to BKC, and necrotic and apoptotic cell death were identified accompanying cell accumulation in the G2/M phase. Therefore, we suggest that repeated respiratory exposure of BKC causes pulmonary inflammation and lung tissue damage and that dead and damaged cells may contribute to the inflammatory response. In addition, the formation process of lamellar body-like structures may function as a key toxicity mechanism.

The cytosolic Arabidopsis thaliana cysteine desulfurase ABA3 delivers sulfur to the sulfurtransferase STR18

The biosynthesis of many sulfur-containing molecules depends on cysteine as a sulfur source. Both the cysteine desulfurase (CD) and rhodanese (Rhd) domain–containing protein families participate in the trafficking of sulfur for various metabolic pathways in bacteria and human, but their connection is not yet described in plants. The existence of natural chimeric proteins containing both CD and Rhd domains in specific bacterial genera, however, suggests a general interaction between these proteins. We report here the biochemical relationships between two cytosolic proteins from Arabidopsis thaliana, a Rhd domain–containing protein, the sulfurtransferase 18 (STR18), and a CD isoform referred to as ABA3, and compare these biochemical features to those of a natural CD–Rhd fusion protein from the bacterium Pseudorhodoferax sp. We observed that the bacterial enzyme is bifunctional exhibiting both CD and STR activities using l-cysteine and thiosulfate as sulfur donors but preferentially using l-cysteine to catalyze transpersulfidation reactions. In vitro activity assays and mass spectrometry analyses revealed that STR18 stimulates the CD activity of ABA3 by reducing the intermediate persulfide on its catalytic cysteine, thereby accelerating the overall transfer reaction. We also show that both proteins interact in planta and form an efficient sulfur relay system, whereby STR18 catalyzes transpersulfidation reactions from ABA3 to the model acceptor protein roGFP2. In conclusion, the ABA3–STR18 couple likely represents an uncharacterized pathway of sulfur trafficking in the cytosol of plant cells, independent of ABA3 function in molybdenum cofactor maturation.

New insights into flowering date in Prunus: fine mapping of a major QTL in sweet cherry

Flowering date is an important trait in Prunus fruit species, especially for their adaptation in a global warming context. Numerous quantitative trait loci (QTLs) have been identified and a major one was previously located on LG4. The objectives of this study were to fine-map this QTL in sweet cherry, to identify robust candidate genes by using the new sweet cherry genome sequence of the cultivar 'Regina' and to define markers usable in marker-assisted selection (MAS). We performed QTL analyses on two populations derived from crosses using cultivars 'Regina' and 'Garnet' as parents. The first one (n = 117) was phenotyped over ten years, while the second one (n = 1386) was evaluated during three years. Kompetitive allele specific PCR (KASP) markers located within the QTL region on LG4 were developed and mapped within this region, consisting in the first fine mapping in sweet cherry. The QTL interval was narrowed from 380 kb to 68 kb and candidate genes were identified by using the genome sequence of 'Regina'. Their expression was analyzed from bud dormancy period to flowering in cultivars 'Regina' and 'Garnet'. Several genes, such as PavBOI-E3, PavSR45a and PavSAUR71, were differentially expressed in these two cultivars and could be then considered as promising candidate genes. Two KASP markers were validated using a population derived from a cross between cultivars 'Regina' and 'Lapins' and two collections, including landraces and modern cultivars. Thanks to the high synteny within the Prunus genus, these results give new insights into the control of flowering date in Prunus species and pave the way for the development of molecular breeding strategies.

Sulfide and transition metals - A partnership for life

Sulfide and transition metals often came together in Biology. The variety of possible structural combinations enabled living organisms to evolve an array of highly versatile metal-sulfide centers to fulfill different physiological roles. The ubiquitous iron‑sulfur centers, with their structural, redox, and functional diversity, are certainly the best-known partners, but other metal-sulfide centers, involving copper, nickel, molybdenum or tungsten, are equally crucial for Life. This review provides a concise overview of the exclusive sulfide properties as a metal ligand, with emphasis on the structural aspects and biosynthesis. Sulfide as catalyst and as a substrate is discussed. Different enzymes are considered, including xanthine oxidase, formate dehydrogenases, nitrogenases and carbon monoxide dehydrogenases. The sulfide effect on the activity and function of iron‑sulfur, heme and zinc proteins is also addressed.

Arabidopsis aldehyde oxidase 3, known to oxidize abscisic aldehyde to abscisic acid, protects leaves from aldehyde toxicity

The Arabidopsis thaliana aldehyde oxidase 3 (AAO3) catalyzes the oxidation of abscisic aldehyde (ABal) to abscisic acid (ABA). Besides ABal, plants generate other aldehydes that can be toxic above a certain threshold. AAO3 knockout mutants (aao3) exhibited earlier senescence but equivalent relative water content compared with wild-type (WT) during normal growth or upon application of UV-C irradiation. Aldehyde profiling in leaves of 24-day-old plants revealed higher accumulation of acrolein, crotonaldehyde, 3Z-hexenal, hexanal and acetaldehyde in aao3 mutants compared with WT leaves. Similarly, higher levels of acrolein, benzaldehyde, crotonaldehyde, propionaldehyde, trans-2-hexenal and acetaldehyde were accumulated in aao3 mutants upon UV-C irradiation. Aldehydes application to plants hastened profuse senescence symptoms and higher accumulation of aldehydes, such as acrolein, benzaldehyde and 4-hydroxy-2-nonenal, in aao3 mutant leaves as compared with WT. The senescence symptoms included greater decrease in chlorophyll content and increase in transcript expression of the early senescence marker genes, Senescence-Related-Gene1, Stay-Green-Protein2 as well as NAC-LIKE, ACTIVATED-BY AP3/P1. Notably, although aao3 had lower ABA content than WT, members of the ABA-responding genes SnRKs were expressed at similar levels in aao3 and WT. Moreover, the other ABA-deficient mutants [aba2 and 9-cis-poxycarotenoid dioxygenase3-2 (nced3-2), that has functional AAO3] exhibited similar aldehydes accumulation and chlorophyll content like WT under normal growth conditions or UV-C irradiation. These results indicate that the absence of AAO3 oxidation activity and not the lower ABA and its associated function is responsible for the earlier senescence symptoms in aao3 mutant.

A Soft Evaporation and Ionization Technique for Mass Spectrometric Analysis and Bio-Imaging of Metal Ions in Plants Based on Metal-Iodide Cluster Ionization

Protonation/deprotonation is the well-recognized mass spectrometric mechanism in matrix-assisted laser desorption ionization of organic molecules but not for metal ions with different oxidation states. We describe herein a soft evaporation and ionization technique for metal ions based on iodination/de-iodination in metal-iodide cluster ionization (MICI). It is not only able to determine identities and oxidation states of metal ions but also reveal spatial distributions and isotope ratios in response to physiological or environmental changes. A long chain alcohol 1-tetradecanol with no functional groups that can absorb laser irradiation was used to cover and prevent samples from direct laser ablation. Upon the irradiation of the third harmonic Nd³⁺:YAG (355 nm, 3 ns), iohexol containing three covalently bonded iodine atoms instantly generates negative iodide ions that can quantitatively form clusters with at least 14 essential metal ions present in plants. The detection limits vary with different metal ions down to low fmol. MICI eliminates the atomization process that obscures metal charges in inductively coupled plasma mass spectrometry. Because only metal ions can be iodinated with iohexol, interferences from the abundant organic molecules of plants that are confronted by secondary ion mass spectrometry (SIMS) are also greatly decreased.

Transcriptome analysis and identification of genes associated with floral transition and fruit development in rabbiteye blueberry (Vaccinium ashei)

Flowering and fruit set are important traits affecting fruit quality and yield in rabbiteye blueberry (Vaccinium ashei). Intense efforts have been made to elucidate the influence of vernalization and phytohormones on flowering, but the molecular mechanisms of flowering and fruit set remain unclear. To unravel these mechanisms, we performed transcriptome analysis to explore blueberry transcripts from flowering to early fruit stage. We divided flowering and fruit set into flower bud (S2), initial flower (S3), bloom flower (S4), pad fruit (S5), and cup fruit (S6) based on phenotype and identified 1,344, 69, 658, and 189 unique differentially expressed genes (DEGs) in comparisons of S3/S2, S4/S3, S5/S4, and S6/S5, respectively. There were obviously more DEGs in S3/S2 and S5/S4 than in S4/S3, and S6/S5, suggesting that S3/S2 and S5/S4 represent major transitions from buds to fruit in blueberry. GO and KEGG enrichment analysis indicated these DEGs were mostly enriched in phytohormone biosynthesis and signaling, transporter proteins, photosynthesis, anthocyanins biosynthesis, disease resistance protein and transcription factor categories, in addition, transcript levels of phytohormones and transporters changed greatly throughout the flowering and fruit set process. Gibberellic acid and jasmonic acid mainly acted on the early stage of flowering development like expression of the florigen gene FT, while the expression of auxin response factor genes increased almost throughout the process from bud to fruit development. Transporter proteins were mainly associated with minerals during the early flowering development stage and sugars during the early fruit stage. At the early fruit stage, anthocyanins started to accumulate, and the fruit was susceptible to diseases such as fungal infection. Expression of the transcription factor MYB86 was up-regulated during initial fruit development, which may promote anthocyanin accumulation. These results will aid future studies exploring the molecular mechanism underlying flowering and fruit set of rabbiteye blueberry.

Roles of Glutathione in Mediating Abscisic Acid Signaling and Its Regulation of Seed Dormancy and Drought Tolerance

Plant growth and development and interactions with the environment are regulated by phytohormones and other signaling molecules. During their evolution, plants have developed strategies for efficient signal perception and for the activation of signal transduction cascades to maintain proper growth and development, in particular under adverse environmental conditions. Abscisic acid (ABA) is one of the phytohormones known to regulate plant developmental events and tolerance to environmental stresses. The role of ABA is mediated by both its accumulated level, which is regulated by its biosynthesis and catabolism, and signaling, all of which are influenced by complex regulatory mechanisms. Under stress conditions, plants employ enzymatic and non-enzymatic antioxidant strategies to scavenge excess reactive oxygen species (ROS) and mitigate the negative effects of oxidative stress. Glutathione (GSH) is one of the main antioxidant molecules playing a critical role in plant survival under stress conditions through the detoxification of excess ROS, maintaining cellular redox homeostasis and regulating protein functions. GSH has recently emerged as an important signaling molecule regulating ABA signal transduction and associated developmental events, and response to stressors. This review highlights the current knowledge on the interplay between ABA and GSH in regulating seed dormancy, germination, stomatal closure and tolerance to drought.

Crosstalk between abscisic acid and nitric oxide under heat stress: exploring new vantage points

Heat stress adversely affects plants growth potential. Global warming is reported to increase in the intensity, frequency, and duration of heatwaves, eventually affecting ecology, agriculture and economy. With an expected increase in average temperature by 2-3 °C over the next 30-50 years, crop production is facing a severe threat to sub-optimum growth conditions. Abscisic acid (ABA) and nitric oxide (NO) are growth regulators that are involved in the adaptation to heat stress by affecting each other and changing the adaptation process. The interaction between these molecules has been discussed in various studies in general or under stress conditions; however, regarding high temperature, their interaction has little been worked out. In the present review, the focus is shifted on the role of these molecules under heat stress emphasizing the different possible interactions between ABA and NO as both regulate stomatal closure and other molecules including hydrogen peroxide (H₂O₂), hydrogen sulfide (H₂S), antioxidants, proline, glycine betaine, calcium (Ca²⁺) and heat shock protein (HSP). Exploring the crosstalk between ABA and NO with other molecules under heat stress will provide us with a comprehensive knowledge of plants mechanism of heat tolerance which could be useful to develop heat stress-resistant varieties.

Synthesis and regulation of auxin and abscisic acid in maize

Indole-3-acetic acid (IAA), the primary auxin in higher plants, and abscisic acid (ABA) play crucial roles in the ability of maize (Zea mays L.) to acclimatize to various environments by mediating growth, development, defense and nutrient allocation. Although understanding the biochemical reactions for IAA and ABA biosynthesis and signal transduction has progressed, the mechanisms by which auxin and ABA are synthesized and transduced in maize have not been fully elucidated to date. The synthesis and signal transduction pathway of IAA and ABA in maize can be analyzed using an existing model. This article focuses on the research progress toward understanding the synthesis and signaling pathways of IAA and ABA, as well as IAA and ABA regulation of maize growth, providing insight for future development and the significance of IAA and ABA for maize improvement.

ABA Metabolism and Homeostasis in Seed Dormancy and Germination

Abscisic acid (ABA) is a key hormone that promotes dormancy during seed development on the mother plant and after seed dispersal participates in the control of dormancy release and germination in response to environmental signals. The modulation of ABA endogenous levels is largely achieved by fine-tuning, in the different seed tissues, hormone synthesis by cleavage of carotenoid precursors and inactivation by 8'-hydroxylation. In this review, we provide an overview of the current knowledge on ABA metabolism in developing and germinating seeds; notably, how environmental signals such as light, temperature and nitrate control seed dormancy through the adjustment of hormone levels. A number of regulatory factors have been recently identified which functional relationships with major transcription factors, such as ABA INSENSITIVE3 (ABI3), ABI4 and ABI5, have an essential role in the control of seed ABA levels. The increasing importance of epigenetic mechanisms in the regulation of ABA metabolism gene expression is also described. In the last section, we give an overview of natural variations of ABA metabolism genes and their effects on seed germination, which could be useful both in future studies to better understand the regulation of ABA metabolism and to identify candidates as breeding materials for improving germination properties.

Interplay between hydrogen sulfide and other signaling molecules in the regulation of guard cell signaling and abiotic/biotic stress response

Stomatal aperture controls the balance between transpirational water loss and photosynthetic carbon dioxide (CO2) uptake. Stomata are surrounded by pairs of guard cells that sense and transduce environmental or stress signals to induce diverse endogenous responses for adaptation to environmental changes. In a recent decade, hydrogen sulfide (H2S) has been recognized as a signaling molecule that regulates stomatal movement. In this review, we summarize recent progress in research on the regulatory role of H2S in stomatal movement, including the dynamic regulation of phytohormones, ion homeostasis, and cell structural components. We focus especially on the cross talk among H2S, nitric oxide (NO), and hydrogen peroxide (H2O2) in guard cells, as well as on H2S-mediated post-translational protein modification (cysteine thiol persulfidation). Finally, we summarize the mechanisms by which H2S interacts with other signaling molecules in plants under abiotic or biotic stress. Based on evidence and clues from existing research, we propose some issues that need to be addressed in the future.

Gene Expression of Putative Pathogenicity-Related Genes in Verticillium dahliae in Response to Elicitation with Potato Extracts and during Infection Using Quantitative Real-Time PCR

Quantitative real-time PCR was used to monitor the expression of 15 Verticillium dahliae's genes, putatively involved in pathogenicity, highly (HAV) and weakly aggressive (WAV) V. dahliae isolates after either (i) elicitation with potato leaf, stem, or root extracts, or (ii) inoculation of potato detached petioles. These genes, i.e., coding for Ras-GAP-like protein, serine/threonine protein kinase, Ubiquitin-conjugating enzyme variant-MMS2, NADH-ubiquinone oxidoreductase, Thioredoxin, Pyruvate dehydrogenase E1 VdPDHB, myo-inositol 2-dehydrogenase, and HAD-superfamily hydrolase, showed differential upregulation in the HAV versus WAV isolate in response to plant extracts or after inoculation of potato leaf petioles. This suggests their potential involvement in the observed differential aggressiveness between isolates. However, other genes like glucan endo-1,3-alpha-glucosidase and nuc-1 negative regulatory protein VdPREG showed higher activity in the WAV than in the HAV in response to potato extracts and/or during infection. This, in contrast, may suggest a role in their lower aggressiveness. These findings, along with future functional analysis of selected genes, will contribute to improving our understanding of V. dahliae's pathogenesis. For example, expression of VdPREG negatively regulates phosphorus-acquisition enzymes, which may indicate a lower phosphorus acquisition activity in the WAV. Therefore, integrating the knowledge about the activity of both genes enhancing pathogenicity and those restraining it will provide a guild line for further functional characterization of the most critical genes, thus driving new ideas towards better Verticillium wilt management.

Protein-bound molybdenum cofactor is bioavailable and rescues molybdenum cofactor-deficient C. elegans

In this paper, Warnoff et al. investigated the mechanism by which C. elegans stably acquires molybdenum cofactor (Moco), which is essential in animals and causes lethal neurological and developmental defects in humans with mutations in genes that encode Moco biosynthetic enzymes. The authors show that protein-bound Moco is the stable, bioavailable species of Moco taken up by C. elegans from its diet and is an effective dietary supplement in a C. elegans model of Moco deficiency, and that these Moco:protein complexes are very stable, suggesting they may provide a strategy for the production and delivery of therapeutically active Moco to treat human Moco deficiency.

The molybdenum cofactor (Moco) is a 520-Da prosthetic group that is synthesized in all domains of life. In animals, four oxidases (among them sulfite oxidase) use Moco as a prosthetic group. Moco is essential in animals; humans with mutations in genes that encode Moco biosynthetic enzymes display lethal neurological and developmental defects. Moco supplementation seems a logical therapy; however, the instability of Moco has precluded biochemical and cell biological studies of Moco transport and bioavailability. The nematode Caenorhabditis elegans can take up Moco from its bacterial diet and transport it to cells and tissues that express Moco-requiring enzymes, suggesting a system for Moco uptake and distribution. Here we show that protein-bound Moco is the stable, bioavailable species of Moco taken up by C. elegans from its diet and is an effective dietary supplement, rescuing a C. elegans model of Moco deficiency. We demonstrate that diverse Moco:protein complexes are stable and bioavailable, suggesting a new strategy for the production and delivery of therapeutically active Moco to treat human Moco deficiency.

Fosdenopterin: a First-in-class Synthetic Cyclic Pyranopterin Monophosphate for the Treatment of Molybdenum Cofactor Deficiency Type A

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Abscisic Acid and Flowering Regulation: Many Targets, Different Places

Plants can react to drought stress by anticipating flowering, an adaptive strategy for plant survival in dry climates known as drought escape (DE). In Arabidopsis, the study of DE brought to surface the involvement of abscisic acid (ABA) in controlling the floral transition. A central question concerns how and in what spatial context can ABA signals affect the floral network. In the leaf, ABA signaling affects flowering genes responsible for the production of the main florigen FLOWERING LOCUS T (FT). At the shoot apex, FD and FD-like transcription factors interact with FT and FT-like proteins to regulate ABA responses. This knowledge will help separate general and specific roles of ABA signaling with potential benefits to both biology and agriculture.

Molybdenum cofactor biology, evolution and deficiency

The molybdenum cofactor (Moco) represents an ancient metal‑sulfur cofactor, which participates as catalyst in carbon, nitrogen and sulfur cycles, both on individual and global scale. Given the diversity of biological processes dependent on Moco and their evolutionary age, Moco is traced back to the last universal common ancestor (LUCA), while Moco biosynthetic genes underwent significant changes through evolution and acquired additional functions. In this review, focused on eukaryotic Moco biology, we elucidate the benefits of gene fusions on Moco biosynthesis and beyond. While originally the gene fusions were driven by biosynthetic advantages such as coordinated expression of functionally related proteins and product/substrate channeling, they also served as origin for the development of novel functions. Today, Moco biosynthetic genes are involved in a multitude of cellular processes and loss of the according gene products result in severe disorders, both related to Moco biosynthesis and secondary enzyme functions.

The coordination of guard-cell autonomous ABA synthesis and DES1 function in situ regulates plant water deficit responses

Introduction

Drought stress triggers the synthesis and accumulation of the phytohormone abscisic acid (ABA), which regulates stomatal aperture and hence reducing plant water loss. Hydrogen sulfide (H₂S), which is produced by the enzyme L-cysteine desulfhydrase 1 (DES1) that catalyzes the desulfuration of L-cysteine in Arabidopsis, also plays a critical role in the regulation of drought-induced stomatal closure. However, little is known about the regulation of DES1 or the crosstalk between H₂S and ABA signaling in response to dehydration.

Objectives

To demonstrate the potential crosstalk between DES1-dependent H₂S and ABA signaling in response to dehydration and its regulation mechanism.

Methods

Firstly, by introducing guard cell-specific MYB60 promoter, to produce complementary lines of DES1 or ABA3 into guard cell of des1 or aba3 mutant. And the related genes expression and water loss under ABA, NaHS, or dehydration treatment in these mutant or transgenics lines were determinate.

Results

We found that dehydration-induced expression of DES1 is abolished in the abscisic acid deficient 3 (aba3) mutants that are deficient in ABA synthesis. Both the complementation of ABA3 expression in guard cells of the aba3 mutants and ABA treatment rescue the dehydration-induced expression of DES1, as well as the wilting phenotype observed in these mutants. Moreover, the drought-induced expression of ABA synthesis genes was suppressed in des1 mutants. While the addition of ABA or the expression of either ABA3 or DES1 in the guard cells of the aba3/des1 double mutant did not alter the wilting phenotype of these mutants, the wild type phenotype was fully restored by the expression of both ABA3 and DES1, or by the application of NaHS.

Conclusion

These results demonstrate that the coordinated synthesis of ABA and DES1 expression is required for drought-induced stomatal closure in Arabidopsis.

The Requirement of Inorganic Fe-S Clusters for the Biosynthesis of the Organometallic Molybdenum Cofactor

Iron-sulfur (Fe-S) clusters are essential protein cofactors. In enzymes, they are present either in the rhombic [2Fe-2S] or the cubic [4Fe-4S] form, where they are involved in catalysis and electron transfer and in the biosynthesis of metal-containing prosthetic groups like the molybdenum cofactor (Moco). Here, we give an overview of the assembly of Fe-S clusters in bacteria and humans and present their connection to the Moco biosynthesis pathway. In all organisms, Fe-S cluster assembly starts with the abstraction of sulfur from l-cysteine and its transfer to a scaffold protein. After formation, Fe-S clusters are transferred to carrier proteins that insert them into recipient apo-proteins. In eukaryotes like humans and plants, Fe-S cluster assembly takes place both in mitochondria and in the cytosol. Both Moco biosynthesis and Fe-S cluster assembly are highly conserved among all kingdoms of life. Moco is a tricyclic pterin compound with molybdenum coordinated through its unique dithiolene group. Moco biosynthesis begins in the mitochondria in a Fe-S cluster dependent step involving radical/S-adenosylmethionine (SAM) chemistry. An intermediate is transferred to the cytosol where the dithiolene group is formed, to which molybdenum is finally added. Further connections between Fe-S cluster assembly and Moco biosynthesis are discussed in detail.

Transcription factor CDF4 promotes leaf senescence and floral organ abscission by regulating abscisic acid and reactive oxygen species pathways in Arabidopsis

Leaf senescence is a highly complex developmental process that is tightly controlled by multiple layers of regulation. Abscisic acid (ABA) and reactive oxygen species (ROS) are two well-known factors that promote leaf senescence. We show here that the transcription factor CDF4 positively regulates leaf senescence. Constitutive and inducible overexpression of CDF4 accelerates leaf senescence, while knockdown of CDF4 delays it. CDF4 increases endogenous ABA levels by upregulating the transcription of the ABA biosynthesis genes 9-cis-epoxycarotenoid dioxygenase 2, 3 (NCED2, 3) and suppresses H₂ O₂ scavenging by repressing expression of the catalase2 (CAT2) gene. NCED2, 3 knockout and CAT2 overexpression partially rescue premature leaf senescence caused by CDF4 overexpression. We also show that CDF4 promotes floral organ abscission by activating the polygalacturonase PGAZAT gene. Based on these results, we propose that the levels of CDF4, ABA, and ROS undergo a gradual increase driven by their interlinking positive feedback loops during the leaf senescence and floral organ abscission processes.

Functional mononuclear molybdenum enzymes: challenges and triumphs in molecular cloning, expression, and isolation

Mononuclear molybdenum enzymes catalyze a variety of reactions that are essential in the cycling of nitrogen, carbon, arsenic, and sulfur. For decades, the structure and function of these crucial enzymes have been investigated to develop a fundamental knowledge for this vast family of enzymes and the chemistries they carry out. Therefore, obtaining abundant quantities of active enzyme is necessary for exploring this family's biochemical capability. This mini-review summarizes the methods for overexpressing mononuclear molybdenum enzymes in the context of the challenges encountered in the process. Effective methods for molybdenum cofactor synthesis and incorporation, optimization of expression conditions, improving isolation of active vs. inactive enzyme, incorporation of additional prosthetic groups, and inclusion of redox enzyme maturation protein chaperones are discussed in relation to the current molybdenum enzyme literature. This article summarizes the heterologous and homologous expression studies providing underlying patterns and potential future directions.

The Apocarotenoid Zaxinone Is a Positive Regulator of Strigolactone and Abscisic Acid Biosynthesis in Arabidopsis Roots

Carotenoids are ubiquitous precursors of important metabolites including hormones, such as strigolactones (SLs) and abscisic acid (ABA), and signaling and regulatory molecules, such as the recently discovered zaxinone. Strigolactones and ABA are key regulators of plant growth and development, adaptation to environmental changes and response to biotic and abiotic stress. Previously, we have shown that zaxinone, an apocarotenoid produced in rice by the enzyme zaxinone synthase (ZAS) that is common in mycorrhizal plants, is required for normal rice growth and development, and a negative regulator of SL biosynthesis. Zaxinone is also formed in Arabidopsis, which lacks ZAS, via an unknown route. In the present study, we investigated the biological activity of zaxinone in Arabidopsis, focusing on its effect on SL and ABA biosynthesis. For this purpose, we quantified the content of both hormones and determined the levels of related transcripts in Arabidopsis (Arabidopsis thaliana), roots upon zaxinone treatment. For SL quantification, we also employed Striga seed germination bioassay. Results obtained show that zaxinone application to hydroponically grown Arabidopsis seedlings enhanced transcript levels of key biosynthetic genes of both hormones, led to higher root ABA and SL (methyl carlactonoate, MeCLA) content, and increased SL release, even under sufficient phosphate supply. Using the SL insensitive (max2-1) and the ABA deficient (aba1-6, aba2-1, and nced3) mutants, we also show that zaxinone application reduced hypocotyl growth and that this effect is caused by increasing ABA content. Our results suggest that zaxinone is a regulatory metabolite also in Arabidopsis, which triggers the biosynthesis of both carotenoid-derived hormones, SLs and ABA, in roots. In the non-mycotrophic plant Arabidopsis, zaxinone does not increase growth and may be perceived as a stress signal, while it acts as a growth-promoting metabolite and suppressor of SL biosynthesis in rice.

Abscisic acid dynamics, signaling, and functions in plants

Abscisic acid (ABA) is an important phytohormone regulating plant growth, development, and stress responses. It has an essential role in multiple physiological processes of plants, such as stomatal closure, cuticular wax accumulation, leaf senescence, bud dormancy, seed germination, osmotic regulation, and growth inhibition among many others. Abscisic acid controls downstream responses to abiotic and biotic environmental changes through both transcriptional and posttranscriptional mechanisms. During the past 20 years, ABA biosynthesis and many of its signaling pathways have been well characterized. Here we review the dynamics of ABA metabolic pools and signaling that affects many of its physiological functions.

Inhibition of vertebrate aldehyde oxidase as a therapeutic treatment for cancer, obesity, aging and amyotrophic lateral sclerosis

The aldehyde oxidases (AOXs) are a small sub-family of cytosolic molybdo-flavoenzymes, which are structurally conserved proteins and broadly distributed from plants to animals. AOXs play multiple roles in both physiological and pathological processes and AOX inhibition is of increasing significance in the development of novel drugs and therapeutic strategies. This review provides an overview of the evolution and the action mechanism of AOX and the role of each domain. The review provides an update of the polymorphisms in the human AOX. This review also summarises the physiology of AOX in different organs and its role in drug metabolism. The inhibition of AOX is a promising therapeutic treatment for cancer, obesity, aging and amyotrophic lateral sclerosis.

Genome-Scale Metabolic Networks Shed Light on the Carotenoid Biosynthesis Pathway in the Brown Algae Saccharina japonica and Cladosiphon okamuranus

Understanding growth mechanisms in brown algae is a current scientific and economic challenge that can benefit from the modeling of their metabolic networks. The sequencing of the genomes of Saccharina japonica and Cladosiphon okamuranus has provided the necessary data for the reconstruction of Genome-Scale Metabolic Networks (GSMNs). The same in silico method deployed for the GSMN reconstruction of Ectocarpus siliculosus to investigate the metabolic capabilities of these two algae, was used. Integrating metabolic profiling data from the literature, we provided functional GSMNs composed of an average of 2230 metabolites and 3370 reactions. Based on these GSMNs and previously published work, we propose a model for the biosynthetic pathways of the main carotenoids in these two algae. We highlight, on the one hand, the reactions and enzymes that have been preserved through evolution and, on the other hand, the specificities related to brown algae. Our data further indicate that, if abscisic acid is produced by Saccharina japonica, its biosynthesis pathway seems to be different in its final steps from that described in land plants. Thus, our work illustrates the potential of GSMNs reconstructions for formalizing hypotheses that can be further tested using targeted biochemical approaches.

Molecular and biochemical analysis of XDH from Phaseolus vulgaris suggest that uric acid protects the enzyme against the inhibitory effects of nitric oxide in nodules

Xanthine dehydrogenase (XDH) is essential for the assimilation of symbiotically fixed nitrogen in ureidic legumes. Uric acid, produced in the reaction catalyzed by XDH, is the precursor of the ureides, allantoin and allantoate, which are the main N-transporting molecules in these plants. XDH and uric acid have been reported to be involved in the response to stress, both in plants and animals. However, the physiological role of XDH under stressful conditions in ureidic legumes remains largely unexplored. In vitro assays showed that Phaseolus vulgaris XDH (PvXDH) can behave as a dehydrogenase or as an oxidase. Therefore, it could potentially protect against oxidative radicals or, in contrast, it could increase their production. In silico analysis of the upstream genomic region of XDH coding gene from P. vulgaris revealed the presence of several stress-related cis-regulatory elements. PvXDH mRNA and enzymatic activity in plants treated with stress-related phytohormones or subjected to dehydration and stressful temperatures showed several fold induction. However, PvXDH activity was in vivo and in vitro inhibited by nitric oxide in leaves but not in nodules. In extracts from RNAi PvXDH silenced nodules, with lower levels of uric acid, XDH activity was inhibited by SNP which indicates that uric acid produced by XDH in the nodules of this ureidic legume could help to protect XDH against the inhibitory effects of nitric oxide.

Rhodanese domain-containing sulfurtransferases: multifaceted proteins involved in sulfur trafficking in plants

Sulfur is an essential element for the growth and development of plants, which synthesize cysteine and methionine from the reductive assimilation of sulfate. Besides its incorporation into proteins, cysteine is the building block for the biosynthesis of numerous sulfur-containing molecules and cofactors. The required sulfur atoms are extracted either directly from cysteine by cysteine desulfurases or indirectly after its catabolic transformation to 3-mercaptopyruvate, a substrate for sulfurtransferases (STRs). Both enzymes are transiently persulfidated in their reaction cycle, i.e. the abstracted sulfur atom is bound to a reactive cysteine residue in the form of a persulfide group. Trans-persulfidation reactions occur when sulfur atoms are transferred to nucleophilic acceptors such as glutathione, proteins, or small metabolites. STRs form a ubiquitous, multigenic protein family. They are characterized by the presence of at least one rhodanese homology domain (Rhd), which usually contains the catalytic, persulfidated cysteine. In this review, we focus on Arabidopsis STRs, presenting the sequence characteristics of all family members as well as their biochemical and structural features. The physiological functions of particular STRs in the biosynthesis of molybdenum cofactor, thio-modification of cytosolic tRNAs, arsenate tolerance, cysteine catabolism, and hydrogen sulfide formation are also discussed.

Differential Function of Endogenous and Exogenous Abscisic Acid during Bacterial Pattern-Induced Production of Reactive Oxygen Species in Arabidopsis

Abscisic acid (ABA) plays important roles in positively or negatively regulating plant disease resistance to pathogens. Here, we reassess the role of endogenous and exogenous ABA by using: 35S::ABA2, a previously reported transgenic Arabidopsis line with increased endogenous ABA levels; aba2-1, a previously reported ABA2 mutant with reduced endogenous ABA levels; and exogenous application of ABA. We found that bacterial susceptibility promoted by exogenous ABA was suppressed in 35S::ABA2 plants. The 35S::ABA2 and aba2-1 plants displayed elevated and reduced levels, respectively, of bacterial flagellin peptide (flg22)-induced H₂O₂. Surprisingly, ABA pre-treatment reduced flg22-induced H₂O₂ generation. Exogenous, but not endogenous ABA, increased catalase activity. Loss of nicotinamide adenine dinucleotide phosphate oxidase genes, RBOHD and RBOHF, restored exogenous ABA-promoted bacterial susceptibility of 35S::ABA2 transgenic plants. In addition, endogenous and exogenous ABA had similar effects on callose deposition and salicylic acid (SA) signaling. These results reveal an underlying difference between endogenous and exogenous ABA in regulating plant defense responses. Given that some plant pathogens are able to synthesize ABA and affect endogenous ABA levels in plants, our results highlight the importance of reactive oxygen species in the dual function of ABA during plant-pathogen interactions.