PP2A-B'γ modulates foliar trans-methylation capacity and the formation of 4-methoxy-indol-3-yl-methyl glucosinolate in Arabidopsis leaves

Protein-Protein Interactions and Quantitative Phosphoproteomic Analysis Reveal Potential Mitochondrial Substrates of Protein Phosphatase 2A-B'ζ Holoenzyme

Protein phosphatase 2A (PP2A) is a heterotrimeric conserved serine/threonine phosphatase complex that includes catalytic, scaffolding, and regulatory subunits. The 3 A subunits, 17 B subunits, and 5 C subunits that are encoded by the Arabidopsis genome allow 255 possible PP2A holoenzyme combinations. The regulatory subunits are crucial for substrate specificity and PP2A complex localization and are classified into the B, B', and B" non-related families in land plants. In Arabidopsis, the close homologs B'η, B'θ, B'γ, and B'ζ are further classified into a subfamily of B' called B'η. Previous studies have suggested that mitochondrial targeted PP2A subunits (B'ζ) play a role in energy metabolism and plant innate immunity. Potentially, the PP2A-B'ζ holoenzyme is involved in the regulation of the mitochondrial succinate/fumarate translocator, and it may affect the enzymes involved in energy metabolism. To investigate this hypothesis, the interactions between PP2A-B'ζ and the enzymes involved in the mitochondrial energy flow were investigated using bimolecular fluorescence complementation in tobacco and onion cells. Interactions were confirmed between the B'ζ subunit and the Krebs cycle proteins succinate/fumarate translocator (mSFC1), malate dehydrogenase (mMDH2), and aconitase (ACO3). Additional putative interacting candidates were deduced by comparing the enriched phosphoproteomes of wild type and B'ζ mutants: the mitochondrial regulator Arabidopsis pentatricopeptide repeat 6 (PPR6) and the two metabolic enzymes phosphoenolpyruvate carboxylase (PPC3) and phosphoenolpyruvate carboxykinase (PCK1). Overall, this study identifies potential PP2A substrates and highlights the role of PP2A in regulating energy metabolism in mitochondria.

"B" Regulatory Subunits of PP2A: Their Roles in Plant Development and Stress Reactions

Protein phosphatase PP2A is an enzyme complex consisting of C (catalytic), A (scaffold) and B (regulatory) subunits. B subunits are a large family of proteins that regulate activity, substrate specificity and subcellular localization of the holoenzyme. Knowledge on the molecular functions of PP2A in plants is less than for protein kinases, but it is rapidly increasing. B subunits are responsible for the large diversity of PP2A functioning. This paper intends to give a survey on their multiple regulatory mechanisms. Firstly, we give a short description on our current knowledge in terms of "B"-mediated regulation of metabolic pathways. Next, we present their subcellular localizations, which extend from the nucleus to the cytosol and membrane compartments. The next sections show how B subunits regulate cellular processes from mitotic division to signal transduction pathways, including hormone signaling, and then the emerging evidence for their regulatory (mostly modulatory) roles in both abiotic and biotic stress responses in plants. Knowledge on these issues should be increased in the near future, since it contributes to a better understanding of how plant cells work, it may have agricultural applications, and it may have new insights into how vascular plants including crops face diverse environmental challenges.

Protein phosphatases and their targets: Comprehending the interactions in plant signaling pathways

Protein phosphorylation is a vital reversible post-translational modification. This process is established by two classes of enzymes: protein kinases and protein phosphatases. Protein kinases phosphorylate proteins while protein phosphatases dephosphorylate phosphorylated proteins, thus, functioning as 'critical regulators' in signaling pathways. The eukaryotic protein phosphatases are classified as phosphoprotein phosphatases (PPP), metallo-dependent protein phosphatases (PPM), protein tyrosine (Tyr) phosphatases (PTP), and aspartate (Asp)-dependent phosphatases. The PPP and PPM families are serine (Ser)/threonine (Thr) specific phosphatases (STPs) that dephosphorylate Ser and Thr residues. The PTP family dephosphorylates Tyr residues while dual-specificity phosphatases (DsPTPs/DSPs) dephosphorylate Ser, Thr, and Tyr residues. The composition of these enzymes as well as their substrate specificity are important determinants of their functional significance in a number of cellular processes and stress responses. Their role in animal systems is well-understood and characterized. The functional characterization of protein phosphatases has been extensively covered in plants, although the comprehension of their mechanistic basis is an ongoing pursuit. The nature of their interactions with other key players in the signaling process is vital to our understanding. The substrates or targets determine their potential as well as magnitude of the impact they have on signaling pathways. In this article, we exclusively overview the various substrates of protein phosphatases in plant signaling pathways, which are a critical determinant of the outcome of various developmental and stress stimuli.

Quantitative trait loci analysis of glucosinolate, sugar, and organic acid concentrations in Eruca vesicaria subsp. sativa

Eruca vesicaria subsp. sativa is a leafy vegetable of the Brassicaceae family known for its pungency. Variation in growing conditions, leaf age, agronomic practices, and variety choice lead to inconsistent quality, especially in content of isothiocyanates (ITCs) and their precursor glucosinolates (GSLs). We present the first linkage and Quantitative Trait Loci (QTL) map for Eruca, generated using a population of 139 F4 lines. A significant environmental effect on the abundance of primary and secondary metabolites was observed, with UK-grown plants containing significantly higher concentrations of glucoraphanin, malic acid, and total sugars. Italian-grown plants were characterized by higher concentrations of glucoerucin, indolic GSLs, and low monosaccharides. 20 QTL were identified and associated with robust SNP markers. Five genes putatively associated with the synthesis of the GSL 4-methoxyglucobrassicin (4MGB) were identified as candidate regulators underlying QTL. Analysis revealed that orthologs of MYB51, IGMT1 and IGMT4 present on LG1 are associated with 4MGB concentrations in Eruca. This research illustrates the utility of the map for identifying genes associated with nutritional composition in Eruca and its value as a genetic resource to assist breeding programs for this leafy vegetable crop.

The MIK2/SCOOP Signaling System Contributes to Arabidopsis Resistance Against Herbivory by Modulating Jasmonate and Indole Glucosinolate Biosynthesis

Initiation of plant immune signaling requires recognition of conserved molecular patterns from microbes and herbivores by plasma membrane-localized pattern recognition receptors. Additionally, plants produce and secrete numerous small peptide hormones, termed phytocytokines, which act as secondary danger signals to modulate immunity. In Arabidopsis, the Brassicae-specific SERINE RICH ENDOGENOUS PEPTIDE (SCOOP) family consists of 14 members that are perceived by the leucine-rich repeat receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR LIKE KINASE 2 (MIK2). Recognition of SCOOP peptides elicits generic early signaling responses but knowledge on how and if SCOOPs modulate specific downstream immune defenses is limited. We report here that depletion of MIK2 or the single PROSCOOP12 precursor results in decreased Arabidopsis resistance against the generalist herbivore Spodoptera littoralis but not the specialist Pieris brassicae. Increased performance of S. littoralis on mik2-1 and proscoop12 is accompanied by a diminished accumulation of jasmonic acid, jasmonate-isoleucine and indolic glucosinolates. Additionally, we show transcriptional activation of the PROSCOOP gene family in response to insect herbivory. Our data therefore indicate that perception of endogenous SCOOP peptides by MIK2 modulates the jasmonate pathway and thereby contributes to enhanced defense against a generalist herbivore.

ACONITASE 3 is part of theANAC017 transcription factor-dependent mitochondrial dysfunction response

Mitochondria are tightly embedded within metabolic and regulatory networks that optimize plant performance in response to environmental challenges. The best-known mitochondrial retrograde signaling pathway involves stress-induced activation of the transcription factor NAC DOMAIN CONTAINING PROTEIN 17 (ANAC017), which initiates protective responses to stress-induced mitochondrial dysfunction in Arabidopsis (Arabidopsis thaliana). Posttranslational control of the elicited responses, however, remains poorly understood. Previous studies linked protein phosphatase 2A subunit PP2A-B'γ, a key negative regulator of stress responses, with reversible phosphorylation of ACONITASE 3 (ACO3). Here we report on ACO3 and its phosphorylation at Ser91 as key components of stress regulation that are induced by mitochondrial dysfunction. Targeted mass spectrometry-based proteomics revealed that the abundance and phosphorylation of ACO3 increased under stress, which required signaling through ANAC017. Phosphomimetic mutation at ACO3-Ser91 and accumulation of ACO3S91D-YFP promoted the expression of genes related to mitochondrial dysfunction. Furthermore, ACO3 contributed to plant tolerance against ultraviolet B (UV-B) or antimycin A-induced mitochondrial dysfunction. These findings demonstrate that ACO3 is both a target and mediator of mitochondrial dysfunction signaling, and critical for achieving stress tolerance in Arabidopsis leaves.

Characterization of the Free and Membrane-Associated Fractions of the Thylakoid Lumen Proteome in Arabidopsis thaliana

The thylakoid lumen houses proteins that are vital for photosynthetic electron transport, including water-splitting at photosystem (PS) II and shuttling of electrons from cytochrome b₆f to PSI. Other lumen proteins maintain photosynthetic activity through biogenesis and turnover of PSII complexes. Although all lumen proteins are soluble, these known details have highlighted interactions of some lumen proteins with thylakoid membranes or thylakoid-intrinsic proteins. Meanwhile, the functional details of most lumen proteins, as well as their distribution between the soluble and membrane-associated lumen fractions, remain unknown. The current study isolated the soluble free lumen (FL) and membrane-associated lumen (MAL) fractions from Arabidopsis thaliana, and used gel- and mass spectrometry-based proteomics methods to analyze the contents of each proteome. These results identified 60 lumenal proteins, and clearly distinguished the difference between the FL and MAL proteomes. The most abundant proteins in the FL fraction were involved in PSII assembly and repair, while the MAL proteome was enriched in proteins that support the oxygen-evolving complex (OEC). Novel proteins, including a new PsbP domain-containing isoform, as well as several novel post-translational modifications and N-termini, are reported, and bi-dimensional separation of the lumen proteome identified several protein oligomers in the thylakoid lumen.

Regulation of glucosinolate biosynthesis

Glucosinolates are secondary defense metabolites produced by plants of the order Brassicales, which includes the model species Arabidopsis and many crop species. In the past 13 years, the regulation of glucosinolate synthesis in plants has been intensively studied, with recent research revealing complex molecular mechanisms that connect glucosinolate production with responses to other central pathways. In this review, we discuss how the regulation of glucosinolate biosynthesis is ecologically relevant for plants, how it is controlled by transcription factors, and how this transcriptional machinery interacts with hormonal, environmental, and epigenetic mechanisms. We present the central players in glucosinolate regulation, MYB and basic helix-loop-helix transcription factors, as well as the plant hormone jasmonate, which together with other hormones and environmental signals allow the coordinated and rapid regulation of glucosinolate genes. Furthermore, we highlight the regulatory connections between glucosinolates, auxin, and sulfur metabolism and discuss emerging insights and open questions on the regulation of glucosinolate biosynthesis.

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.

Evolutionary conservation and post-translational control of S-adenosyl-L-homocysteine hydrolase in land plants

Trans-methylation reactions are intrinsic to cellular metabolism in all living organisms. In land plants, a range of substrate-specific methyltransferases catalyze the methylation of DNA, RNA, proteins, cell wall components and numerous species-specific metabolites, thereby providing means for growth and acclimation in various terrestrial habitats. Trans-methylation reactions consume vast amounts of S-adenosyl-L-methionine (SAM) as a methyl donor in several cellular compartments. The inhibitory reaction by-product, S-adenosyl-L-homocysteine (SAH), is continuously removed by SAH hydrolase (SAHH), which essentially maintains trans-methylation reactions in all living cells. Here we report on the evolutionary conservation and post-translational control of SAHH in land plants. We provide evidence suggesting that SAHH forms oligomeric protein complexes in phylogenetically divergent land plants and that the predominant protein complex is composed by a tetramer of the enzyme. Analysis of light-stress-induced adjustments of SAHH in Arabidopsis thaliana and Physcomitrella patens further suggests that regulatory actions may take place on the levels of protein complex formation and phosphorylation of this metabolically central enzyme. Collectively, these data suggest that plant adaptation to terrestrial environments involved evolution of regulatory mechanisms that adjust the trans-methylation machinery in response to environmental cues.

Genome-wide identification and expression analysis of the phosphatase 2A family in rubber tree (Hevea brasiliensis)

The protein phosphatase 2As (PP2As) play a key role in manipulating protein phosphorylation. Although a number of proteins in the latex of laticifers are phosphorylated during latex regeneration in rubber tree, information about the PP2A family is limited. In the present study, 36 members of the HbPP2A family were genome-wide identified. They were clustered into five subgroups: the subgroup HbPP2AA (4), HbPP2AB' (14), HbPP2AB'' (6), HbPP2AB55 (4), and HbPP2AC (8). The members within the same subgroup shared highly conserved gene structures and protein motifs. Most of HbPP2As possessed ethylene- and wounding-responsive cis-acting elements. The transcripts of 29 genes could be detected in latex by using published high-throughput sequencing data. Of the 29 genes, seventeen genes were significantly down-regulated while HbPP2AA1-1 and HbPP2AB55α/Bα-1were up-regulated by tapping. Of the 17 genes, 14 genes were further significantly down-regulated by ethrel application. The down-regulated expression of a large number of HbPP2As may attribute to the enhanced phosphorylation of the proteins in latex from the tapped trees and the trees treated with ethrel application.

The significance of methionine cycle enzymes in plant virus infections

Both biotic and abiotic stresses cause changes in the activities of plant methionine cycle (MTC) enzymes. These changes contribute to the ability of the plant to manage stress. On the other hand, viruses utilize MTC enzymes to promote infection. Here, we review the growing but still limited knowledge of the interactions between plant viral proteins and MTC enzymes. Virus-induced changes in S-adenosyl methionine synthetase and S-adenosyl homocysteine hydrolase activities debilitate transcriptional and post-transcriptional RNA silencing and affect antiviral defense reactions connected to ethylene and polyamine biosynthesis pathways. Viral perturbations of host methionine homeostasis couple trans-sulfuration and gluthathione biosynthesis pathways to MTC functions. Large multiprotein complexes, which contain viral proteins and MTC enzymes, may represent metabolons assembled for specific viral functions or host defense responses. Proper understanding of the MTC-associated metabolic and regulatory interactions will reveal those with potential to create virus resistance in plants.

A novel Ca2+-binding protein influences photosynthetic electron transport in Anabaena sp. PCC 7120

Ca²⁺ is a potent signalling molecule that regulates many cellular processes. In cyanobacteria, Ca²⁺ has been linked to cell growth, stress response and photosynthesis, and to the development of specialist heterocyst cells in certain nitrogen-fixing species. Despite this, the pathways of Ca²⁺ signal transduction in cyanobacteria are poorly understood, and very few protein components are known. The current study describes a previously unreported Ca²⁺-binding protein which was called the Ca²⁺ Sensor EF-hand (CSE), which is conserved in filamentous, nitrogen-fixing cyanobacteria. CSE is shown to bind Ca²⁺, which induces a conformational change in the protein structure. Poor growth of a strain of Anabaena sp. PCC 7120 overexpressing CSE was attributed to diminished photosynthetic performance. Transcriptomics, biophysics and proteomics analyses revealed modifications in the light-harvesting phycobilisome and photosynthetic reaction centre protein complexes.

Trans-methylation reactions in plants: focus on the activated methyl cycle

Trans-methylation reactions are vital in basic metabolism, epigenetic regulation, RNA metabolism, and posttranslational control of protein function and therefore fundamental in determining the physiological processes in all living organisms. The plant kingdom is additionally characterized by the production of secondary metabolites that undergo specific hydroxylation, oxidation and methylation reactions to obtain a wide array of different chemical structures. Increasing research efforts have started to reveal the enzymatic pathways underlying the biosynthesis of complex metabolites in plants. Further engineering of these enzymatic machineries offers significant possibilities in the development of bio-based technologies, but necessitates deep understanding of their potential metabolic and regulatory interactions. Trans-methylation reactions are tightly coupled with the so-called activated methyl cycle (AMC), an essential metabolic circuit that maintains the trans-methylation capacity in all living cells. Tight regulation of the AMC is crucial in ensuring accurate trans-methylation reactions in different subcellular compartments, cell types, developmental stages and environmental conditions. This review addresses the organization and posttranslational regulation of the AMC and elaborates its critical role in determining metabolic regulation through modulation of methyl utilization in stress-exposed plants.

Bioactive Compounds in Brassicaceae Vegetables with a Role in the Prevention of Chronic Diseases

The beneficial role of the Mediterranean diet in the prevention of chronic diseases, including cardiovascular diseases, diabetes, and obesity, is well-recognized. In this context, Brassicaceae are considered important vegetables due to several evidences of their health promoting effects that are associated to bioactive compounds present in the edible parts of the plants. In this review, the mechanisms of action and the factors regulating the levels of the bioactive compounds in Brassicaceae have been discussed. In addition, the impact of industrial and domestic processing on the amount of these compounds have been considered, in order to identify the best conditions that are able to preserve the functional properties of the Brassicaceae products before consumption. Finally, the main strategies used to increase the content of health-promoting metabolites in Brassica plants through biofortification have been analyzed.

PSB33 sustains photosystem II D1 protein under fluctuating light conditions

On Earth, solar irradiance varies as the sun rises and sets over the horizon, and sunlight is thus in constant fluctuation, following a slow dark-low-high-low-dark curve. Optimal plant growth and development are dependent on the capacity of plants to acclimate and regulate photosynthesis in response to these changes of light. Little is known of regulative processes for photosynthesis during nocturnal events. The nucleus-encoded plant lineage-specific protein PSB33 has been described as stabilizing the photosystem II complex, especially under light stress conditions, and plants lacking PSB33 have a dysfunctional state transition. To clarify the localization and function of this protein, we used phenomic, biochemical and proteomics approaches in the model plant Arabidopsis. We report that PSB33 is predominantly located in non-appressed thylakoid regions and dynamically associates with a thylakoid protein complex in a light-dependent manner. Moreover, plants lacking PSB33 show an accelerated D1 protein degradation in nocturnal periods, and show severely stunted growth when challenged with fluctuating light. We further show that the function of PSB33 precedes the STN7 kinase to regulate or balance the excitation energy of photosystems I and II in fluctuating light conditions.