Overexpressing the Myrosinase Gene TGG1 Enhances Stomatal Defense Against Pseudomonas syringae and Delays Flowering in Arabidopsis

Multifaceted roles of plant glycosyl hydrolases during pathogen infections: more to discover

MAIN CONCLUSION

Carbohydrates are hydrolyzed by a family of carbohydrate-active enzymes (CAZymes) called glycosidases or glycosyl hydrolases. Here, we have summarized the roles of various plant defense glycosidases that possess different substrate specificities. We have also highlighted the open questions in this research field. Glycosidases or glycosyl hydrolases (GHs) are a family of carbohydrate-active enzymes (CAZymes) that hydrolyze glycosidic bonds in carbohydrates and glycoconjugates. Compared to those of all other sequenced organisms, plant genomes contain a remarkable diversity of glycosidases. Plant glycosidases exhibit activities on various substrates and have been shown to play important roles during pathogen infections. Plant glycosidases from different GH families have been shown to act upon pathogen components, host cell walls, host apoplastic sugars, host secondary metabolites, and host N-glycans to mediate immunity against invading pathogens. We could classify the activities of these plant defense GHs under eleven different mechanisms through which they operate during pathogen infections. Here, we have provided comprehensive information on the catalytic activities, GH family classification, subcellular localization, domain structure, functional roles, and microbial strategies to regulate the activities of defense-related plant GHs. We have also emphasized the research gaps and potential investigations needed to advance this topic of research.

Flavin-containing monooxygenases FMOGS-OXs integrate flowering transition and salt tolerance in Arabidopsis thaliana

Salt stress substantially leads to flowering delay. The regulation of salt-induced late flowering has been studied at the transcriptional and protein levels; however, the involvement of secondary metabolites has rarely been investigated. Here, we report that FMOGS-OXs (EC 1.14.13.237), the enzymes that catalyze the biosynthesis of glucosinolates (GSLs), promote flowering transition in Arabidopsis thaliana. It has been reported that WRKY75 is a positive regulator, and MAF4 is a negative regulator of flowering transition. The products of FMOGS-OXs, methylsulfinylalkyl GSLs (MS GSLs), facilitate flowering by inducing WRKY75 and repressing the MAS-MAF4 module. We further show that the degradation of MS GSLs is involved in salt-induced late flowering and salt tolerance. Salt stress induces the expression of myrosinase genes, resulting in the degradation of MS GSLs, thereby relieving the promotion of WRKY75 and inhibition of MAF4, leading to delayed flowering. In addition, the degradation products derived from MS GSLs enhance salt tolerance. Previous studies have revealed that FMOGS-OXs exhibit alternative catalytic activity to form trimethylamine N-oxide (TMAO) under salt stress, which activates multiple stress-related genes to promote salt tolerance. Therefore, FMOGS-OXs integrate flowering transition and salt tolerance in various ways. Our study shed light on the functional diversity of GSLs and established a connection between flowering transition, salt resistance, and GSL metabolism.

Identification of nitric oxide regulated low abundant myrosinases from seeds and seedlings of Brassica juncea

Myrosinases constitute an important component of the glucosinolate-myrosinase system responsible for interaction of plants with microorganisms, insects, pest, and herbivores. It is a distinctive feature of Brassicales. Multiple isozymes of myrosinases are present in the vacuoles. Active myrosinases are also present in the apoplast and the nucleus however, the similarity or difference in the biochemical properties with the vacuolar myrosinases are not known. Here, we have attempted to isolate, characterize, and identify myrosinases from seeds, seedlings, apoplast, and nucleus to understand these forms. 2D-CN/SDS-PAGE coupled with western blotting and MS have shown low abundant myrosinases (65/70/72/75 kDa) in seeds and seedlings and apoplast & nucleus of seedlings to exist as dimers, oligomers, and as protein complex. Nuclear membrane associated form of myrosinase was also identified. The present study for the first time has shown enzymatically active myrosinase-alpha-mannosidase complex in seedlings. Both 65 and 70 kDa myrosinase in seedlings were S-nitrosated. Nitric oxide donor treatment (GSNO) led to 25% reduction in myrosinase activity which was reversed by DTT suggesting redox regulation of myrosinase. These S-nitrosated myrosinases might be a component of NO signalling in B. juncea.

Glucosinolate Catabolism Maintains Glucosinolate Profiles and Transport in Sulfur-Starved Arabidopsis

Glucosinolates (GSLs) are sulfur (S)-rich specialized metabolites present in Brassicales order plants. Our previous study found that GSL can function as a S source in Arabidopsis seedlings via its catabolism catalyzed by two β-glucosidases (BGLUs), BGLU28 and BGLU30. However, as GSL profiles in plants vary among growth stages and organs, the potential contribution of BGLU28/30-dependent GSL catabolism at the reproductive growth stage needs verification. Thus, in this study, we assessed growth, metabolic and transcriptional phenotypes of mature bglu28/30 double mutants grown under different S conditions. Our results showed that compared to wild-type plants grown under -S, mature bglu28/30 mutants displayed impaired growth and accumulated increased levels of GSL in their reproductive organs and rosette leaves of before-bolting plants. In contrast, the levels of primary S-containing metabolites, glutathione and cysteine decreased in their mature seeds. Furthermore, the transport of GSL from rosette leaves to the reproductive organs was stimulated in the bglu28/30 mutants under -S. Transcriptome analysis revealed that genes related to other biological processes, such as ethylene response, defense response and plant response to heat, responded differentially to -S in the bglu28/30 mutants. Altogether, these findings broadened our understanding of the roles of BGLU28/30-dependent GSL catabolism in plant adaptation to nutrient stress.

Nitric oxide (NO) modulates low temperature-stress signaling via S-nitrosation, a NO PTM, inducing ethylene biosynthesis inhibition leading to enhanced post-harvest shelf-life of agricultural produce

Low temperature (cold) stress is one of the major abiotic stress conditions affecting crop productivity worldwide. Nitric oxide (NO) is a dynamic signaling molecule that interacts with various stress regulators and provides abiotic stress tolerance. Stress enhanced NO contributes to S-nitrosothiol accumulation which causes oxidation of the -SH group in proteins leading to S-nitrosation, a post-translational modification. Cold stress induced in vivo S-nitrosation of > 240 proteins majorly belonging to stress/signaling/redox (myrosinase, SOD, GST, CS, DHAR), photosynthesis (RuBisCO, PRK), metabolism (FBA, GAPDH, TPI, SBPase), and cell wall modification (Beta-xylosidases, alpha-l-arabinogalactan) in different crop plants indicated role of NO in these important cellular and metabolic pathways. NO mediated regulation of a transcription factor CBF (C-repeat Binding Factor, a transcription factor) at transcriptional and post-translational level was shown in Solanum lycopersicum seedlings. NO donor priming enhances seed germination, breaks dormancy and provides tolerance to stress in crops. Its role in averting stress, promoting seed germination, and delaying senescence paved the way for use of NO and NO releasing compounds to prevent crop loss and increase the shelf-life of fruits and vegetables. An alternative to energy consuming and expensive cold storage led to development of a storage device called "shelf-life enhancer" that delays senescence and increases shelf-life at ambient temperature (25-27 °C) using NO donor. The present review summarizes NO research in plants and exploration of NO for its translational potential to improve agricultural yield and post-harvest crop loss.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01371-z.

Cell specialization and coordination in Arabidopsis leaves upon pathogenic attack revealed by scRNA-seq

Plant defense responses involve several biological processes that allow plants to fight against pathogenic attacks. How these different processes are orchestrated within organs and depend on specific cell types is poorly known. Here, using single-cell RNA sequencing (scRNA-seq) technology on three independent biological replicates, we identified several cell populations representing the core transcriptional responses of wild-type Arabidopsis leaves inoculated with the bacterial pathogen Pseudomonas syringae DC3000. Among these populations, we retrieved major cell types of the leaves (mesophyll, guard, epidermal, companion, and vascular S cells) with which we could associate characteristic transcriptional reprogramming and regulators, thereby specifying different cell-type responses to the pathogen. Further analyses of transcriptional dynamics, on the basis of inference of cell trajectories, indicated that the different cell types, in addition to their characteristic defense responses, can also share similar modules of gene reprogramming, uncovering a ubiquitous antagonism between immune and susceptible processes. Moreover, it appears that the defense responses of vascular S cells, epidermal cells, and mesophyll cells can evolve along two separate paths, one converging toward an identical cell fate, characterized mostly by lignification and detoxification functions. As this divergence does not correspond to the differentiation between immune and susceptible cells, we speculate that this might reflect the discrimination between cell-autonomous and non-cell-autonomous responses. Altogether our data provide an upgraded framework to describe, explore, and explain the specialization and the coordination of plant cell responses upon pathogenic challenge.

Genome-Wide Identification and Expression Analysis of ESPs and NSPs Involved in Glucosinolate Hydrolysis and Insect Attack Defense in Chinese Cabbage (Brassica rapa subsp. pekinensis)

Glucosinolates are secondary plant metabolites that are part of the plant's defense system against pathogens and pests and are activated via enzymatic degradation by thioglucoside glucohydrolases (myrosinases). Epithiospecifier proteins (ESPs) and nitrile-specifier proteins (NSPs) divert the myrosinase-catalyzed hydrolysis of a given glucosinolate to form epithionitrile and nitrile rather than isothiocyanate. However, the associated gene families have not been explored in Chinese cabbage. We identified three ESP and fifteen NSP genes randomly distributed on six chromosomes in Chinese cabbage. Based on a phylogenetic tree, the ESP and NSP gene family members were divided into four clades and had similar gene structure and motif composition of Brassica rapa epithiospecifier proteins (BrESPs) and B. rapa nitrile-specifier proteins (BrNSPs) in the same clade. We identified seven tandem duplicated events and eight pairs of segmentally duplicated genes. Synteny analysis showed that Chinese cabbage and Arabidopsis thaliana are closely related. We detected the proportion of various glucosinolate hydrolysates in Chinese cabbage and verified the function of BrESPs and BrNSPs in glucosinolate hydrolysis. Furthermore, we used quantitative RT-PCR to analyze the expression of BrESPs and BrNSPs and demonstrated that these genes responded to insect attack. Our findings provide novel insights into BrESPs and BrNSPs that can help further promote the regulation of glucosinolate hydrolysates by ESP and NSP to resist insect attack in Chinese cabbage.

Protein glycosylation changes during systemic acquired resistance in Arabidopsis thaliana

N-glycosylation, an important post-translational modification of proteins in all eukaryotes, has been clearly shown to be involved in numerous diseases in mammalian systems. In contrast, little is known regarding the role of protein N-glycosylation in plant defensive responses to pathogen infection. We identified, for the first time, glycoproteins related to systemic acquired resistance (SAR) in an Arabidopsis thaliana model, using a glycoproteomics platform based on high-resolution mass spectrometry. 407 glycosylation sites corresponding to 378 glycopeptides and 273 unique glycoproteins were identified. 65 significantly changed glycoproteins with 80 N-glycosylation sites were detected in systemic leaves of SAR-induced plants, including numerous GDSL-like lipases, thioglucoside glucohydrolases, kinases, and glycosidases. Functional enrichment analysis revealed that significantly changed glycoproteins were involved mainly in N-glycan biosynthesis and degradation, phenylpropanoid biosynthesis, cutin and wax biosynthesis, and plant-pathogen interactions. Comparative analysis of glycoproteomics and proteomics data indicated that glycoproteomics analysis is an efficient method for screening proteins associated with SAR. The present findings clarify glycosylation status and sites of A. thaliana proteins, and will facilitate further research on roles of glycoproteins in SAR induction.

Mechanism of Pb accumulation in Chinese cabbage leaves: Stomata and trichomes regulate foliar uptake of Pb in atmospheric PM2.5

Chinese cabbage (Brassica rapa ssp. pekinensis) is one of the most popular and frequently consumed leafy vegetables. It was found that atmospheric PM_2.5-Pb contributes to Pb accumulation in the edible leaves of Chinese cabbage via stomata in North China during haze seasons with high concentrations of fine particulate matter in autumn and winter. However, it is unclear whether both stomata and trichomes co-regulate foliar transfer of PM_2.5-Pb from atmospheric deposition to the leaf of Chinese cabbage genotypes with trichomes. Field and hydroponic experiments were conducted to investigate the effects of foliar uptake of PM_2.5-Pb on Pb accumulation in leaves using two genotypes of Chinese cabbage, one without trichomes and one with trichomes. It was verified that open stoma is a prominent pathway of foliar PM_2.5-Pb transfer in the short-term exposure for 6 h, contributing 74.5% of Pb accumulation in leaves, whereas Pb concentrations in the leaves of with-trichome genotype in the rosette stage were 6.52- and 1.04-fold higher than that of without-trichome genotype in greenhouse and open field, respectively, which suggests that stomata and trichomes co-regulate foliar Pb uptake of from atmospheric PM_2.5. Moreover, subcellular Pb in the leaves was distributed in the following order of cytoplasm (53.8%) > cell wall (38.5%)> organelle (7.8%), as confirmed through high-resolution secondary ion mass spectrometry (NanoSIMS). The Leadmium™ Green AM dye manifested that Pb in PM_2.5 entered cellular space of trichomes and accumulated in the basal compartment, enhancing foliar Pb uptake in the edible leaves of cabbage. The results of these experiments are evidence that both stomata and trichomes are important pathways in the regulation of foliar Pb uptake and translocation in Chinese cabbage.

Quantitative Proteome Profiling of a S-Nitrosoglutathione Reductase (GSNOR) Null Mutant Reveals a New Class of Enzymes Involved in Nitric Oxide Homeostasis in Plants

Nitric oxide (NO) is a short-lived radical gas that acts as a signaling molecule in all higher organisms, and that is involved in multiple plant processes, including germination, root growth, and fertility. Regulation of NO-levels is predominantly achieved by reaction of oxidation products of NO with glutathione to form S-nitrosoglutathione (GSNO), the principal bioactive form of NO. The enzyme S-nitrosoglutathione reductase (GSNOR) is a major route of NADH-dependent GSNO catabolism and is critical to NO homeostasis. Here, we performed a proteomic analysis examining changes in the total leaf proteome of an Arabidopsis thaliana GSNOR null mutant (hot5-2/gsnor1-3). Significant increases or decreases in proteins associated with chlorophyll metabolism and with redox and stress metabolism provide insight into phenotypes observed in hot5-2/gsnor1-3 plants. Importantly, we identified a significant increase in proteins that belong to the aldo-keto reductase (AKR) protein superfamily, AKR4C8 and 9. Because specific AKRs have been linked to NO metabolism in mammals, we expressed and purified A. thaliana AKR4C8 and 9 and close homologs AKR4C10 and 11 and determined that they have NADPH-dependent activity in GSNO and S-nitroso-coenzyme A (SNO-CoA) reduction. Further, we found an increase of NADPH-dependent GSNO reduction activity in hot5-2/gsnor1-3 mutant plants. These data uncover a new, NADPH-dependent component of NO metabolism that may be integrated with NADH-dependent GSNOR activity to control NO homeostasis in plants.

Improvement of glucosinolates by metabolic engineering in Brassica crops

Glucosinolates (GSLs) are a class of sulfur- and nitrogen-containing, and amino acid-derived important secondary metabolites, which mainly present in plants of Brassicaceae family, including Brassica crops, such as broccoli, cabbage, and oilseed rape. The bioactive GSL metabolites confer benefits to plant defense, human health, and the unique flavor of some Brassica crops. However, certain GSL profiles have adverse effects and are known as anti-nutritional factors. This has attracted mounting attempts to increase beneficial GSLs and reduce detrimental ones in the most commonly consumed Brassica crops. We provide a comprehensive overview of metabolic engineering applied in Brassica crops to achieve this purpose, including modulation of GSL biosynthesis, ablation of GSL hydrolysis, inhibition of GSL transport processes, and redirection of metabolic flux to GSL. Moreover, advances in omics approaches, i.e., genomics, transcriptome, and metabolome, applied in the elucidation of GSL metabolism in Brassica crops, as well as promising and potential genome-editing technologies are also discussed.

The Imaging of Guard Cells of thioglucosidase (tgg) Mutants of Arabidopsis Further Links Plant Chemical Defence Systems with Physical Defence Barriers

The glucosinolate-myrosinase system is a well-known plant chemical defence system. Two functional myrosinase-encoding genes, THIOGLUCOSIDASE 1 (TGG1) and THIOGLUCOSIDASE 2 (TGG2), express in aerial tissues of Arabidopsis. TGG1 expresses in guard cells (GCs) and is also a highly abundant protein in GCs. Recently, by studying wild type (WT), tgg single, and double mutants, we showed a novel association between the glucosinolate-myrosinase system defence system, and a physical barrier, the cuticle. In the current study, using imaging techniques, we further analysed stomata and ultrastructure of GCs of WT, tgg1, tgg2 single, and tgg1 tgg2 double mutants. The tgg mutants showed distinctive features of GCs. The GCs of tgg1 and tgg1 tgg2 mutants showed vacuoles that had less electron-dense granular material. Both tgg single mutants had bigger stomata complexes. The WT and tgg mutants also showed variations for cell wall, chloroplasts, and starch grains of GCs. Abscisic acid (ABA)-treated stomata showed that the stomatal aperture was reduced in tgg1 single and tgg1 tgg2 double mutants. The data provides a basis to perform comprehensive further studies to find physiological and molecular mechanisms associated with ultrastructure differences in tgg mutants. We speculate that the absence of myrosinase alters the endogenous chemical composition, hence affecting the physical structure of plants and the plants' physical defence barriers.

Advances and perspectives in the metabolomics of stomatal movement and the disease triangle

Crops are continuously exposed to microbial pathogens that cause tremendous yield losses worldwide. Stomatal pores formed by pairs of specialized guard cells in the leaf epidermis represent a major route of pathogen entry. Guard cells have an essential role as a first line of defense against pathogens. Metabolomics is an indispensable systems biology tool that has facilitated discovery and functional studies of metabolites that regulate stomatal movement in response to pathogens and other environmental factors. Guard cells, pathogens and environmental factors constitute the "stomatal disease triangle". The aim of this review is to highlight recent advances toward understanding the stomatal disease triangle in the context of newly discovered signaling molecules, hormone crosstalk, and consequent molecular changes that integrate pathogens and environmental sensing into stomatal immune responses. Future perspectives on emerging single-cell studies, multiomics and molecular imaging in the context of stomatal defense are discussed. Advances in this important area of plant biology will inform rational crop engineering and breeding for enhanced stomatal defense without disruption of other pathways that impact crop yield.

The Versatile Roles of Sulfur-Containing Biomolecules in Plant Defense-A Road to Disease Resistance

Sulfur (S) is an essential plant macronutrient and the pivotal role of sulfur compounds in plant disease resistance has become obvious in recent decades. This review attempts to recapitulate results on the various functions of sulfur-containing defense compounds (SDCs) in plant defense responses to pathogens. These compounds include sulfur containing amino acids such as cysteine and methionine, the tripeptide glutathione, thionins and defensins, glucosinolates and phytoalexins and, last but not least, reactive sulfur species and hydrogen sulfide. SDCs play versatile roles both in pathogen perception and initiating signal transduction pathways that are interconnected with various defense processes regulated by plant hormones (salicylic acid, jasmonic acid and ethylene) and reactive oxygen species (ROS). Importantly, ROS-mediated reversible oxidation of cysteine residues on plant proteins have profound effects on protein functions like signal transduction of plant defense responses during pathogen infections. Indeed, the multifaceted plant defense responses initiated by SDCs should provide novel tools for plant breeding to endow crops with efficient defense responses to invading pathogens.

Glucosinolate Biosynthesis and the Glucosinolate–Myrosinase System in Plant Defense

Insect pests represent a major global challenge to important agricultural crops. Insecticides are often applied to combat such pests, but their use has caused additional challenges such as environmental contamination and human health issues. Over millions of years, plants have evolved natural defense mechanisms to overcome insect pests and pathogens. One such mechanism is the production of natural repellents or specialized metabolites like glucosinolates. There are three types of glucosinolates produced in the order Brassicales: aliphatic, indole, and benzenic glucosinolates. Upon insect herbivory, a “mustard oil bomb” consisting of glucosinolates and their hydrolyzing enzymes (myrosinases) is triggered to release toxic degradation products that act as insect deterrents. This review aims to provide a comprehensive summary of glucosinolate biosynthesis, the “mustard oil bomb”, and how these metabolites function in plant defense against pathogens and insects. Understanding these defense mechanisms will not only allow us to harness the benefits of this group of natural metabolites for enhancing pest control in Brassicales crops but also to transfer the “mustard oil bomb” to non-glucosinolate producing crops to boost their defense and thereby reduce the use of chemical pesticides.

Human, Animal and Plant Health Benefits of Glucosinolates and Strategies for Enhanced Bioactivity: A Systematic Review

Glucosinolates (GSs) are common anionic plant secondary metabolites in the order Brassicales. Together with glucosinolate hydrolysis products (GSHPs), they have recently gained much attention due to their biological activities and mechanisms of action. We review herein the health benefits of GSs/GSHPs, approaches to improve the plant contents, their bioavailability and bioactivity. In this review, only literature published between 2010 and March 2020 was retrieved from various scientific databases. Findings indicate that these compounds (natural, pure, synthetic, and derivatives) play an important role in human/animal health (disease therapy and prevention), plant health (defense chemicals, biofumigants/biocides), and food industries (preservatives). Overall, much interest is focused on in vitro studies as anti-cancer and antimicrobial agents. GS/GSHP levels improvement in plants utilizes mostly biotic/abiotic stresses and short periods of phytohormone application. Their availability and bioactivity are directly proportional to their contents at the source, which is affected by methods of food preparation, processing, and extraction. This review concludes that, to a greater extent, there is a need to explore and improve GS-rich sources, which should be emphasized to obtain natural bioactive compounds/active ingredients that can be included among synthetic and commercial products for use in maintaining and promoting health. Furthermore, the development of advanced research on compounds pharmacokinetics, their molecular mode of action, genetics based on biosynthesis, their uses in promoting the health of living organisms is highlighted.

Fasciclin-like arabinogalactan gene family in Nicotiana benthamiana: genome-wide identification, classification and expression in response to pathogens

BACKGROUND

Nicotiana benthamiana is widely used as a model plant to study plant-pathogen interactions. Fasciclin-like arabinogalactan proteins (FLAs), a subclass of arabinogalactan proteins (AGPs), participate in mediating plant growth, development and response to abiotic stress. However, the members of FLAs in N. benthamiana and their response to plant pathogens are unknown.

RESULTS

38 NbFLAs were identified from a genome-wide study. NbFLAs could be divided into four subclasses, and their gene structure and motif composition were conserved in each subclass. NbFLAs may be regulated by cis-acting elements such as STRE and MBS, and may be the targets of transcription factors like C2H2. Quantitative real time polymerase chain reaction (RT-qPCR) results showed that selected NbFLAs were differentially expressed in different tissues. All of the selected NbFLAs were significantly downregulated following infection by turnip mosaic virus (TuMV) and most of them also by Pseudomonas syringae pv tomato strain DC3000 (Pst DC3000), suggesting possible roles in response to pathogenic infection.

CONCLUSIONS

This study systematically identified FLAs in N. benthamiana, and indicates their potential roles in response to biotic stress. The identification of NbFLAs will facilitate further studies of their role in plant immunity in N. benthamiana.