Protein phosphatase 2A controls ethylene biosynthesis by differentially regulating the turnover of ACC synthase isoforms

Molecular mechanisms underlying the toxicity and detoxification of trace metals and metalloids in plants

Plants take up a wide range of trace metals/metalloids (hereinafter referred to as trace metals) from the soil, some of which are essential but become toxic at high concentrations (e.g., Cu, Zn, Ni, Co), while others are non-essential and toxic even at relatively low concentrations (e.g., As, Cd, Cr, Pb, and Hg). Soil contamination of trace metals is an increasing problem worldwide due to intensifying human activities. Trace metal contamination can cause toxicity and growth inhibition in plants, as well as accumulation in the edible parts to levels that threatens food safety and human health. Understanding the mechanisms of trace metal toxicity and how plants respond to trace metal stress is important for improving plant growth and food safety in contaminated soils. The accumulation of excess trace metals in plants can cause oxidative stress, genotoxicity, programmed cell death, and disturbance in multiple physiological processes. Plants have evolved various strategies to detoxify trace metals through cell-wall binding, complexation, vacuolar sequestration, efflux, and translocation. Multiple signal transduction pathways and regulatory responses are involved in plants challenged with trace metal stresses. In this review, we discuss the recent progress in understanding the molecular mechanisms involved in trace metal toxicity, detoxification, and regulation, as well as strategies to enhance plant resistance to trace metal stresses and reduce toxic metal accumulation in food crops.

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.

Phosphorylation status of Bβ subunit acts as a switch to regulate the function of phosphatase PP2A in ethylene-mediated root growth inhibition

The various combinations and regulations of different subunits of phosphatase PP2A holoenzymes underlie their functional complexity and importance. However, molecular mechanisms governing the assembly of PP2A complex in response to external or internal signals remain largely unknown, especially in Arabidopsis thaliana. We found that the phosphorylation status of Bβ of PP2A acts as a switch to regulate the activity of PP2A. In the absence of ethylene, phosphorylated Bβ leads to an inactivation of PP2A; the substrate EIR1 remains to be phosphorylated, preventing the EIR1-mediated auxin transport in epidermis, leading to normal root growth. Upon ethylene treatment, the dephosphorylated Bβ mediates the formation of the A2-C4-Bβ protein complex to activate PP2A, resulting in the dephosphorylation of EIR1 to promote auxin transport in epidermis of elongation zone, leading to root growth inhibition. Altogether, our research revealed a novel molecular mechanism by which the dephosphorylation of Bβ subunit switches on PP2A activity to dephosphorylate EIR1 to establish EIR1-mediated auxin transport in the epidermis in elongation zone for root growth inhibition in response to ethylene.

The B'ζ subunit of protein phosphatase 2A negatively regulates ethylene signaling in Arabidopsis

Ethylene is a major plant hormone that regulates plant growth, development, and defense responses to biotic and abiotic stresses. The major pieces of the ethylene signaling pathway have been put together, although several details still need to be elucidated. For instance, the phosphorylation and dephosphorylation processes controlling the ethylene responses are poorly understood and need to be further explored. The type 2A protein phosphatase (PP2A) was suggested to play an important role in the regulation of ethylene biosynthesis, where the A1 subunit of PP2A was shown to be involved in the regulation of the rate-limiting enzyme of the ethylene biosynthetic pathway. However, whether other subunits of PP2A play roles in the ethylene signal transduction pathway is yet to be answered. In this study, we demonstrate that a B subunit, PP2A-B'ζ, positively regulates plant germination and seedling development, as a pp2a-b'ζ mutant is very sensitive to ethylene treatment. Furthermore, PP2A-B'ζ interacts with and stabilizes the kinase CTR1 (Constitutive Triple Response 1), a key enzyme in the ethylene signal transduction pathway, and like CTR1, PP2A-B'ζ negatively regulates ethylene signaling in plants.

Essential Roles of the Linker Sequence Between Tetratricopeptide Repeat Motifs of Ethylene Overproduction 1 in Ethylene Biosynthesis

Ethylene Overproduction 1 (ETO1) is a negative regulator of ethylene biosynthesis. However, the regulation mechanism of ETO1 remains largely unclear. Here, a novel eto1 allele (eto1-16) was isolated with typical triple phenotypes due to an amino acid substitution of G480C in the uncharacterized linker sequence between the TPR1 and TPR2 motifs. Further genetic and biochemical experiments confirmed the eto1-16 mutation site. Sequence analysis revealed that G480 is conserved not only in two paralogs, EOL1 and EOL2, in Arabidopsis, but also in the homologous protein in other species. The glycine mutations (eto1-11, eto1-12, and eto1-16) do not influence the mRNA abundance of ETO1, which is reflected by the mRNA secondary structure similar to that of WT. According to the protein-protein interaction analysis, the abnormal root phenotype of eto1-16 might be caused by the disruption of the interaction with type 2 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACSs) proteins. Overall, these data suggest that the linker sequence between tetratricopeptide repeat (TPR) motifs and the glycine in TPR motifs or the linker region are essential for ETO1 to bind with downstream mediators, which strengthens our knowledge of ETO1 regulation in balancing ACSs.

The regulation of ethylene biosynthesis: a complex multilevel control circuitry

The gaseous plant hormone ethylene is produced by a fairly simple two-step biosynthesis route. Despite this pathway's simplicity, recent molecular and genetic studies have revealed that the regulation of ethylene biosynthesis is far more complex and occurs at different layers. Ethylene production is intimately linked with the homeostasis of its general precursor S-adenosyl-l-methionine (SAM), which experiences transcriptional and posttranslational control of its synthesising enzymes (SAM synthetase), as well as the metabolic flux through the adjacent Yang cycle. Ethylene biosynthesis continues from SAM by two dedicated enzymes: 1-aminocyclopropane-1-carboxylic (ACC) synthase (ACS) and ACC oxidase (ACO). Although the transcriptional dynamics of ACS and ACO have been well documented, the first transcription factors that control ACS and ACO expression have only recently been discovered. Both ACS and ACO display a type-specific posttranslational regulation that controls protein stability and activity. The nonproteinogenic amino acid ACC also shows a tight level of control through conjugation and translocation. Different players in ACC conjugation and transport have been identified over the years, however their molecular regulation and biological significance is unclear, yet relevant, as ACC can also signal independently of ethylene. In this review, we bring together historical reports and the latest findings on the complex regulation of the ethylene biosynthesis pathway in plants.

Strigolactone elevates ethylene biosynthesis in etiolated Arabidopsis seedlings

The gaseous phytohormone ethylene influences many aspects of plant life, including germination, fruit ripening, senescence, and stress responses. These diverse roles of ethylene occur in part through crosstalk with other phytohormones, which affects ethylene biosynthesis and signaling pathways. We have recently shown that the phytohormones, including gibberellic acid, abscisic acid, auxin, methyl jasmonate, and salicylic acid, regulate the stability of ACC synthases (ACSs), the rate-limiting enzymes in ethylene biosynthesis. Here, we report that treatment of etiolated Arabidopsis seedlings with strigolactone (SL) increases ethylene biosynthesis. SL does not influence ACS stability or ACS gene expression, but it increases the transcript levels of a subset of ACC oxidase (ACO) genes, thereby enhancing ethylene biosynthesis. Taken together with the results of our previous study, these findings demonstrate that most phytohormones differentially regulate ethylene biosynthesis in dark-grown Arabidopsis seedlings by affecting ACS stability and/or the transcript levels of ethylene biosynthesis genes.

Engineering Smut Resistance in Maize by Site-Directed Mutagenesis of LIPOXYGENASE 3

Biotic stresses caused by microbial pathogens impair crop yield and quality if not restricted by expensive and often ecologically problematic pesticides. For a sustainable agriculture of tomorrow, breeding or engineering of pathogen-resistant crop varieties is therefore a major cornerstone. Maize is one of the four most important cereal crops in the world. The biotrophic fungal pathogen Ustilago maydis causes galls on all aerial parts of the maize plant. Biotrophic pathogens like U. maydis co-evolved with their host plant and depend during their life cycle on successful manipulation of the host's cellular machinery. Therefore, removing or altering plant susceptibility genes is an effective and usually durable way to obtain resistance in plants. Transcriptional time course experiments in U. maydis-infected maize revealed numerous maize genes being upregulated upon establishment of biotrophy. Among these genes is the maize LIPOXYGENASE 3 (LOX3) previously shown to be a susceptibility factor for other fungal genera as well. Aiming to engineer durable resistance in maize against U. maydis and possibly other pathogens, we took a Cas endonuclease technology approach to generate loss of function mutations in LOX3. lox3 maize mutant plants react with an enhanced PAMP-triggered ROS burst implicating an enhanced defense response. Based on visual assessment of disease symptoms and quantification of relative fungal biomass, homozygous lox3 mutant plants exposed to U. maydis show significantly decreased susceptibility. U. maydis infection assays using a transposon mutant lox3 maize line further substantiated that LOX3 is a susceptibility factor for this important maize pathogen.

Identifying the Pressure Points of Acute Cadmium Stress Prior to Acclimation in Arabidopsis thaliana

The toxic metal cadmium (Cd) is a major soil pollutant. Knowledge on the acute Cd-induced stress response is required to better understand the triggers and sequence of events that precede plant acclimation. Therefore, we aimed to identify the pressure points of Cd stress using a short-term exposure set-up ranging from 0 h to 24 h. Acute responses related to glutathione (GSH), hydrogen peroxide (H₂O₂), 1-aminocyclopropane-1-carboxylic acid (ACC), ethylene and the oxidative challenge were studied at metabolite and/or transcript level in roots and leaves of Arabidopsis thaliana either exposed or not to 5 µM Cd. Cadmium rapidly induced root GSH depletion, which might serve as an alert response and modulator of H₂O₂ signalling. Concomitantly, a stimulation of root ACC levels was observed. Leaf responses were delayed and did not involve GSH depletion. After 24 h, a defined oxidative challenge became apparent, which was most pronounced in the leaves and concerted with a strong induction of leaf ACC synthesis. We suggest that root GSH depletion is required for a proper alert response rather than being a merely adverse effect. Furthermore, we propose that roots serve as command centre via a.o. root-derived ACC/ethylene to engage the leaves in a proper stress response.

Maize Plant Architecture Is Regulated by the Ethylene Biosynthetic Gene ZmACS7

Plant height and leaf angle are two crucial determinants of plant architecture in maize (Zea mays) and are closely related to lodging resistance and canopy photosynthesis at high planting density. These two traits are primarily regulated by several phytohormones. However, the mechanism of ethylene in regulating plant architecture in maize, especially plant height and leaf angle, is unclear. Here, we characterized a maize mutant, Semidwarf3 (Sdw3), which exhibits shorter stature and larger leaf angle than the wild type. Histological analysis showed that inhibition of longitudinal cell elongation in the internode and promotion in the auricle were mainly responsible for reduced plant height and enlarged leaf angle in the Sdw3 mutant. Through positional cloning, we identified a transposon insertion in the candidate gene ZmACS7, encoding 1-aminocyclopropane-1-carboxylic acid (ACC) Synthase 7 in ethylene biosynthesis of maize. The transposon alters the C terminus of ZmACS7. Transgenic analysis confirmed that the mutant ZmACS7 gene confers the phenotypes of the Sdw3 mutant. Enzyme activity and protein degradation assays indicated that the altered C terminus of ZmACS7 in the Sdw3 mutant increases this protein's stability but does not affect its catalytic activity. The ACC and ethylene contents are dramatically elevated in the Sdw3 mutant, leading to reduced plant height and increased leaf angle. In addition, we demonstrated that ZmACS7 plays crucial roles in root development, flowering time, and leaf number, indicating that ZmACS7 is an important gene with pleiotropic effects during maize growth and development.

Protein phosphatase 2A promotes stomatal development by stabilizing SPEECHLESS in Arabidopsis

Stomatal guard cells control gas exchange that allows plant photosynthesis but limits water loss from plants to the environment. In Arabidopsis, stomatal development is mainly controlled by a signaling pathway comprising peptide ligands, membrane receptors, a mitogen-activated protein kinase (MAPK) cascade, and a set of transcription factors. The initiation of the stomatal lineage requires the activity of the bHLH transcription factor SPEECHLESS (SPCH) with its partners. Multiple kinases were found to regulate SPCH protein stability and function through phosphorylation, yet no antagonistic protein phosphatase activities have been identified. Here, we identify the conserved PP2A phosphatases as positive regulators of Arabidopsis stomatal development. We show that mutations in genes encoding PP2A subunits result in lowered stomatal production in Arabidopsis Genetic analyses place the PP2A function upstream of SPCH. Pharmacological treatments support a role for PP2A in promoting SPCH protein stability. We further find that SPCH directly binds to the PP2A-A subunits in vitro. In plants, nonphosphorylatable SPCH proteins are less affected by PP2A activity levels. Thus, our research suggests that PP2A may function to regulate the phosphorylation status of the master transcription factor SPCH in stomatal development.

Protein Phosphatases Type 2C Group A Interact with and Regulate the Stability of ACC Synthase 7 in Arabidopsis

Ethylene is an important plant hormone that controls growth, development, aging and stress responses. The rate-limiting enzymes in ethylene biosynthesis, the 1-aminocyclopropane-1-carboxylate synthases (ACSs), are strictly regulated at many levels, including posttranslational control of protein half-life. Reversible phosphorylation/dephosphorylation events play a pivotal role as signals for ubiquitin-dependent degradation. We showed previously that ABI1, a group A protein phosphatase type 2C (PP2C) and a key negative regulator of abscisic acid signaling regulates type I ACS stability. Here we provide evidence that ABI1 also contributes to the regulation of ethylene biosynthesis via ACS7, a type III ACS without known regulatory domains. Using various approaches, we show that ACS7 interacts with ABI1, ABI2 and HAB1. We use molecular modeling to predict the amino acid residues involved in ABI1/ACS7 complex formation and confirm these predictions by mcBiFC–FRET–FLIM analysis. Using a cell-free degradation assay, we show that proteasomal degradation of ACS7 is delayed in protein extracts prepared from PP2C type A knockout plants, compared to a wild-type extract. This study therefore shows that ACS7 undergoes complex regulation governed by ABI1, ABI2 and HAB1. Furthermore, this suggests that ACS7, together with PP2Cs, plays an essential role in maintaining appropriate levels of ethylene in Arabidopsis.

Target of Rapamycin (TOR) Negatively Regulates Ethylene Signals in Arabidopsis

Target of rapamycin (TOR) acts as a master regulator in coordination of cell growth with energy and nutrient availability. Despite the increased appreciation of the essential role of the TOR complex in interaction with phytohormone signaling, little is known about its function on ethylene signaling. Here, through expression analysis, genetic and biochemical approaches, we reveal that TOR functions in the regulation of ethylene signals. Transcriptional analysis indicates that TOR inhibition by AZD8055 upregulated senescence- and ethylene-related genes expression. Furthermore, ethylene insensitive mutants like etr1-1, ein2-5 and ein3 eil1, showed more hyposensitivity to AZD8055 than that of WT in hypocotyl growth inhibition. Similarly, blocking ethylene signals by ethylene action inhibitor Ag+ or biosynthesis inhibitor aminoethoxyvinylglycine (AVG) largely rescued hypocotyl growth even in presence of AZD8055. In addition, we also demonstrated that Type 2A phosphatase-associated protein of 46 kDa (TAP46), a downstream component of TOR signaling, physically interacts with 1-aminocy-clopropane-1-carboxylate (ACC) synthase ACS2 and ACS6. Arabidopsis overexpressing ACS2 or ACS6 showed more hypersensitivity to AZD8055 than WT in hypocotyl growth inhibition. Moreover, ACS2/ACS6 protein was accumulated under TOR suppression, implying TOR modulates ACC synthase protein levels. Taken together, our results indicate that TOR participates in negatively modulating ethylene signals and the molecular mechanism is likely involved in the regulation of ethylene biosynthesis by affecting ACSs in transcription and protein levels.

Protein phosphatase 2A alleviates cadmium toxicity by modulating ethylene production in Arabidopsis thaliana

Cadmium (Cd) is phytotoxic and detoxified primarily via phytochelatin (PC) complexation in Arabidopsis. Here, we explore Cd toxicity responses and defence mechanisms beyond the PC pathway using forward genetics approach. We isolated an Arabidopsis thaliana Cd-hypersensitive mutant, Cd-induced short root 1 (cdsr1) in the PC synthase mutant (cad1-3) background. Using genomic resequencing and complementation, we identified PP2A-4C as the causal gene for the mutant phenotype, which encodes a catalytic subunit of protein phosphatase 2A (PP2A). Root and shoot growth of cdsr1 cad1-3 and cdsr1 were more sensitive to Cd than their respective wild-type cad1-3 and Col-0. A mutant of the PP2A scaffolding subunit 1A was also more sensitive to Cd. PP2A-4C was localized in the cytoplasm and nucleus and PP2A-4C expression was downregulated by Cd in cad1-3. PP2A enzyme activity was decreased in cdsr1 and cdsr1 cad1-3 under Cd stress. The expression of 1-aminocyclopropane-1-carboxylic acid synthase genes ACS2 and ACS6 was upregulated by Cd more in cad1-3 and cdsr1 cad1-3 than in Col-0 and the double mutant had a higher ACS activity. cdsr1 cad1-3 and cdsr1 overproduced ethylene under Cd stress. The results suggest that PP2A containing 1A and 4C subunits alleviates Cd-induced growth inhibition by modulating ethylene production.

PP2A and microtubules function in 5-aminolevulinic acid-mediated H2 O2 signaling in Arabidopsis guard cells

5-aminolevulinic acid (ALA), a plant growth regulator with great application potential in agriculture and horticulture, induces stomatal opening and inhibits stomatal closure by decreasing guard cell H₂ O₂ . However, the mechanisms behind ALA-decreased H₂ O₂ in guard cells are not fully understood. Here, using type 2A protein phosphatase (PP2A) inhibitors, microtubule-stabilizing/disrupting drugs and green fluorescent protein-tagged α-tubulin 6 transgenic Arabidopsis (GFP-TUA6), we find that PP2A and cortical microtubules (MTs) are involved in ALA-regulated stomatal movement. Then, we analyze stomatal responses of Arabidopsis overexpressing C2 catalytic subunit of PP2A (PP2A-C2) and pp2a-c2 mutant to ALA and abscisic acid (ABA) under both light and dark conditions, and show that PP2A-C2 participates in ALA-induced stomatal movement. Furthermore, using pharmacological methods and confocal studies, we reveal that PP2A and MTs function upstream and downstream, respectively, of H₂ O₂ in guard cell signaling. Finally, we demonstrate the role of H₂ O₂ -mediated microtubule arrangement in ALA inhibiting ABA-induced stomatal closure. Our findings indicate that MTs regulated by PP2A-mediated H₂ O₂ decreasing play an important role in ALA guard cell signaling, revealing new insights into stomatal movement regulation.

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.

BES1 directly binds to the promoter of the ACC oxidase 1 gene to regulate gravitropic response in the roots of Arabidopsis thaliana

Brassinosteroids (BRs) are known to be endogenous regulators of ethylene production, suggesting that some BR activity in plant growth and development is associated with ethylene. Here, we demonstrated that ethylene production in Arabidopsis thaliana roots is increased by BR signaling via the ethylene biosynthetic gene for ACC oxidase 1 (ACO1). Electrophoretic mobility shift and chromatin immune-precipitation assays showed that the BR transcription factor BES1 directly binds to two E-box sequences located in the intergenic region of ACO1. GUS expression using site mutations of the E-box sequences verified that ACO1 is normally expressed only when BES1 binds to the E-boxes in the putative promoter of ACO1, indicating that this binding is essential for ACO1 expression and the subsequent production of ethylene in A. thaliana roots. BR exogenously applied to A. thaliana roots enhanced the gravitropic response. Additionally, bes1-D exhibited a greater gravitropic response than did the wild-type specimens, proving that BR is a positive regulator of the gravitropic response in A. thaliana roots. The knock-down mutant aco1-1 showed a slightly lower gravitropic response than did the wild-type specimens, while bes1-D X aco1-1 exhibited a lower gravitropic response than did bes1-D. Therefore, ACO1 is a direct downstream target for BR transcription factor BES1, which controls ethylene production for gravitropism in A. thaliana roots.

The Role of Serine-Threonine Protein Phosphatase PP2A in Plant Oxidative Stress Signaling-Facts and Hypotheses

Abiotic and biotic factors induce oxidative stress involving the production and scavenging of reactive oxygen species (ROS). This review is a survey of well-known and possible roles of serine-threonine protein phosphatases in plant oxidative stress signaling, with special emphasis on PP2A. ROS mediated signaling involves three interrelated pathways: (i) perception of extracellular ROS triggers signal transduction pathways, leading to DNA damage and/or the production of antioxidants; (ii) external signals induce intracellular ROS generation that triggers the relevant signaling pathways and (iii) external signals mediate protein phosphorylation dependent signaling pathway(s), leading to the expression of ROS producing enzymes like NADPH oxidases. All pathways involve inactivation of serine-threonine protein phosphatases. The metal dependent phosphatase PP2C has a negative regulatory function during ABA mediated ROS signaling. PP2A is the most abundant protein phosphatase in eukaryotic cells. Inhibitors of PP2A exert a ROS inducing activity as well and we suggest that there is a direct relationship between these two effects of drugs. We present current findings and hypotheses regarding PP2A-ROS signaling connections related to all three ROS signaling pathways and anticipate future research directions for this field. These mechanisms have implications in the understanding of stress tolerance of vascular plants, having applications regarding crop improvement.

Light-induced stabilization of ACS contributes to hypocotyl elongation during the dark-to-light transition in Arabidopsis seedlings

Hypocotyl growth during seedling emergence is a crucial developmental transition influenced by light and phytohormones such as ethylene. Ethylene and light antagonistically control hypocotyl growth in either continuous light or darkness. However, how ethylene and light regulate hypocotyl growth, including seedling emergence, during the dark-to-light transition remains elusive. Here, we show that ethylene and light cooperatively stimulate a transient increase in hypocotyl growth during the dark-to-light transition via the light-mediated stabilization of 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACSs), the rate-limiting enzymes in ethylene biosynthesis. We found that, in contrast to the known inhibitory role of light in hypocotyl growth, light treatment transiently increases hypocotyl growth in wild-type etiolated seedlings. Moreover, ACC, the direct precursor of ethylene, accentuates the effects of light on hypocotyl elongation during the dark-to-light transition. We determined that light leads to the transient elongation of hypocotyls by stabilizing the ACS5 protein during the dark-to-light transition. Furthermore, biochemical analysis of an ACS5 mutant protein bearing an alteration in the C-terminus indicated that light stabilizes ACS5 by inhibiting the degradation mechanism that acts through the C-terminus of ACS5. Our study reveals that plants regulate hypocotyl elongation during seedling establishment by coordinating light-induced ethylene biosynthesis at the post-translational level. Moreover, the stimulatory role of light on hypocotyl growth during the dark-to-light transition provides additional insights into the known inhibitory role of light in hypocotyl development.

The regulatory module MdPUB29-MdbHLH3 connects ethylene biosynthesis with fruit quality in apple

The plant hormone ethylene is critical for climacteric fruit ripening, while glucose and anthocyanins determine the fruit quality of climacteric fruits such as apple. Understanding the exact molecular mechanism for this process is important for elucidating the interconnection of ethylene and fruit quality. Overexpression of apple MdbHLH3 gene, an anthocyanin-related basic helix-loop-helix transcription factor (bHLH TF) gene, promotes ethylene production, and transgenic apple plantlets and trees exhibit ethylene-related root developmental abnormalities, premature leaf senescence, and fruit ripening. Biochemical analyses demonstrate that MdbHLH3 binds to the promoters of three genes that are involved in ethylene biosynthesis, including MdACO1, MdACS1, and MdACS5A, activating their transcriptional expression, thereby promoting ethylene biosynthesis. High glucose-inhibited U-box-type E3 ubiquitin ligase MdPUB29, the ortholog of Arabidopsis AtPUB29 in apple, influences the expression of ethylene biosynthetic genes and ethylene production by direct ubiquitination of the MdbHLH3 protein. Our findings provide new insights into the ubiquitination of MdbHLH3 by glucose-inhibited ubiquitin E3 ligase MdPUB29 in the regulation of ethylene biosynthesis as well as indicate that the regulatory module MdPUB29-MdbHLH3 connects ethylene biosynthesis with fruit quality in apple.

Light Modulates Ethylene Synthesis, Signaling, and Downstream Transcriptional Networks to Control Plant Development

The inhibition of hypocotyl elongation by ethylene in dark-grown seedlings was the basis of elegant screens that identified ethylene-insensitive Arabidopsis mutants, which remained tall even when treated with high concentrations of ethylene. This simple approach proved invaluable for identification and molecular characterization of major players in the ethylene signaling and response pathway, including receptors and downstream signaling proteins, as well as transcription factors that mediate the extensive transcriptional remodeling observed in response to elevated ethylene. However, the dark-adapted early developmental stage used in these experiments represents only a small segment of a plant's life cycle. After a seedling's emergence from the soil, light signaling pathways elicit a switch in developmental programming and the hormonal circuitry that controls it. Accordingly, ethylene levels and responses diverge under these different environmental conditions. In this review, we compare and contrast ethylene synthesis, perception, and response in light and dark contexts, including the molecular mechanisms linking light responses to ethylene biology. One powerful method to identify similarities and differences in these important regulatory processes is through comparison of transcriptomic datasets resulting from manipulation of ethylene levels or signaling under varying light conditions. We performed a meta-analysis of multiple transcriptomic datasets to uncover transcriptional responses to ethylene that are both light-dependent and light-independent. We identified a core set of 139 transcripts with robust and consistent responses to elevated ethylene across three root-specific datasets. This "gold standard" group of ethylene-regulated transcripts includes mRNAs encoding numerous proteins that function in ethylene signaling and synthesis, but also reveals a number of previously uncharacterized gene products that may contribute to ethylene response phenotypes. Understanding these light-dependent differences in ethylene signaling and synthesis will provide greater insight into the roles of ethylene in growth and development across the entire plant life cycle.

Manipulation of Phytohormone Pathways by Effectors of Filamentous Plant Pathogens

Phytohormones regulate a large variety of physiological processes in plants. In addition, salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are responsible for primary defense responses against abiotic and biotic stresses, while plant growth regulators, such as auxins, brassinosteroids (BRs), cytokinins (CKs), abscisic acid (ABA), and gibberellins (GAs), also contribute to plant immunity. To successfully colonize plants, filamentous pathogens like fungi and oomycetes have evolved diverse strategies to interfere with phytohormone pathways with the help of secreted effectors. These include proteins, toxins, polysaccharides as well as phytohormones or phytohormone mimics. Such pathogen effectors manipulate phytohormone pathways by directly altering hormone levels, by interfering with phytohormone biosynthesis, or by altering or blocking important components of phytohormone signaling pathways. In this review, we outline the various strategies used by filamentous phytopathogens to manipulate phytohormone pathways to cause disease.

GhL1L1 affects cell fate specification by regulating GhPIN1-mediated auxin distribution

Auxin is as an efficient initiator and regulator of cell fate during somatic embryogenesis (SE), but the molecular mechanisms and regulating networks of this process are not well understood. In this report, we analysed SE process induced by Leafy cotyledon1-like 1 (GhL1L1), a NF-YB subfamily gene specifically expressed in embryonic tissues in cotton. We also identified the target gene of GhL1L1, and its role in auxin distribution and cell fate specification during embryonic development was analysed. Overexpression of GhL1L1 accelerated embryonic cell formation, associated with an increased concentration of IAA in embryogenic calluses (ECs) and in the shoot apical meristem, corresponding to altered expression of the auxin transport gene GhPIN1. By contrast, GhL1L1-deficient explants showed retarded embryonic cell formation, and the concentration of IAA was decreased in GhL1L1-deficient ECs. Disruption of auxin distribution accelerated the specification of embryonic cell fate together with regulation of GhPIN1. Furthermore, we showed that PHOSPHATASE 2AA2 (GhPP2AA2) was activated by GhL1L1 through targeting the G-box of its promoter, hence regulating the activity of GhPIN1 protein. Our results indicate that GhL1L1 functions as a key regulator in auxin distribution to regulate cell fate specification in cotton and contribute to the understanding of the complex process of SE in plant species.

Arabidopsis ACC Oxidase 1 Coordinated by Multiple Signals Mediates Ethylene Biosynthesis and Is Involved in Root Development

Ethylene regulates numerous aspects of plant growth and development. Multiple external and internal factors coordinate ethylene production in plant tissues. Transcriptional and post-translational regulations of ACC synthases (ACSs), which are key enzymes mediating a rate-limiting step in ethylene biosynthesis have been well characterized. However, the regulation and physiological roles of ACC oxidases (ACOs) that catalyze the final step of ethylene biosynthesis are largely unknown in Arabidopsis. Here, we show that Arabidopsis ACO1 exhibits a tissue-specific expression pattern that is regulated by multiple signals, and plays roles in the lateral root development in Arabidopsis. Histochemical analysis of the ACO1 promoter indicated that ACO1 expression was largely modulated by light and plant hormones in a tissue-specific manner. We demonstrated that point mutations in two E-box motifs on the ACO1 promoter reduce the light-regulated expression patterns of ACO1. The aco1-1 mutant showed reduced ethylene production in root tips compared to wild-type. In addition, aco1-1 displayed altered lateral root formation. Our results suggest that Arabidopsis ACO1 integrates various signals into the ethylene biosynthesis that is required for ACO1's intrinsic roles in root physiology.

EIN3-LIKE1, MYB1, and ETHYLENE RESPONSE FACTOR3 Act in a Regulatory Loop That Synergistically Modulates Ethylene Biosynthesis and Anthocyanin Accumulation

Ethylene regulates climacteric fruit ripening, and EIN3-LIKE1 (EIL1) plays an important role in this process. In apple (Malus domestica), fruit coloration is accompanied by ethylene release during fruit ripening, but the molecular mechanism that underlies these two physiological processes is unknown. In this study, we found that ethylene treatment markedly induced fruit coloration as well as the expression of MdMYB1, a positive regulator of anthocyanin biosynthesis and fruit coloration. In addition, we found that MdEIL1 directly bound to the promoter of MdMYB1 and transcriptionally activated its expression, which resulted in anthocyanin biosynthesis and fruit coloration. Furthermore, MdMYB1 interacted with the promoter of ETHYLENE RESPONSE FACTOR3, a key regulator of ethylene biosynthesis, thereby providing a positive feedback for ethylene biosynthesis regulation. Overall, our findings provide insight into a mechanism involving the synergistic interaction of the ethylene signal with the MdMYB1 transcription factor to regulate ethylene biosynthesis and fruit coloration in apple.

Co-overexpression of AVP1 and PP2A-C5 in Arabidopsis makes plants tolerant to multiple abiotic stresses

Abiotic stresses are major threats to agricultural production. Drought and salinity as two of the major abiotic stresses cause billions of losses in agricultural productivity worldwide each year. Thus, it is imperative to make crops more tolerant. Overexpression of AVP1 or PP2A-C5 was previously shown to increase drought and salt stress tolerance, respectively, in transgenic plants. In this study, the hypothesis that co-overexpression of AVP1 and PP2A-C5 would combine their respective benefits and further improve salt tolerance was tested. The two genes were inserted into the same T-DNA region of the binary vector and then introduced into the Arabidopsis genome through Agrobacterium-mediated transformation. Transgenic Arabidopsis plants expressing both AVP1 and PP2A-C5 at relatively high levels were identified and analyzed. These plants displayed enhanced tolerance to NaCl compared to either AVP1 or PP2A-C5 overexpressing plants. They also showed tolerance to other stresses such as KNO₃ and LiCl at harmful concentrations, drought, and phosphorus deficiency at comparable levels with either AVP1 or PP2A-C5 overexpressing plants. This study demonstrates that introducing multiple genes in single T-DNA region is an effective approach to create transgenic plants with enhanced tolerance to multiple stresses.

The PP2A-interactor TIP41 modulates ABA responses in Arabidopsis thaliana

Modulation of growth in response to environmental cues is a fundamental aspect of plant adaptation to abiotic stresses. TIP41 (TAP42 INTERACTING PROTEIN OF 41 kDa) is the Arabidopsis thaliana orthologue of proteins isolated in mammals and yeast that participate in the Target-of-Rapamycin (TOR) pathway, which modifies cell growth in response to nutrient status and environmental conditions. Here, we characterized the function of TIP41 in Arabidopsis. Expression analyses showed that TIP41 is constitutively expressed in vascular tissues, and is induced following long-term exposure to NaCl, polyethylene glycol and abscisic acid (ABA), suggesting a role of TIP41 in adaptation to abiotic stress. Visualization of a fusion protein with yellow fluorescent protein indicated that TIP41 is localized in the cytoplasm and the nucleus. Abolished expression of TIP41 results in smaller plants with a lower number of rosette leaves and lateral roots, and an increased sensitivity to treatments with chemical TOR inhibitors, indicating that TOR signalling is affected in these mutants. In addition, tip41 mutants are hypersensitive to ABA at germination and seedling stage, whereas over-expressing plants show higher tolerance. Several TOR- and ABA-responsive genes are differentially expressed in tip41, including iron homeostasis, senescence and ethylene-associated genes. In yeast and mammals, TIP41 provides a link between the TOR pathway and the protein phosphatase 2A (PP2A), which in plants participates in several ABA-mediated mechanisms. Here, we showed an interaction of TIP41 with the catalytic subunit of PP2A. Taken together, these results offer important insights into the function of Arabidopsis TIP41 in the modulation of plant growth and ABA responses.

Petal senescence: a hormone view

Flowers are highly complex organs that have evolved to enhance the reproductive success of angiosperms. As a key component of flowers, petals play a vital role in attracting pollinators and ensuring successful pollination. Having fulfilled this function, petals senesce through a process that involves many physiological and biochemical changes that also occur during leaf senescence. However, petal senescence is distinct, due to the abundance of secondary metabolites in petals and the fact that petal senescence is irreversible. Various phytohormones are involved in regulating petal senescence, and are thought to act both synergistically and antagonistically. In this regard, there appears to be developmental point during which such regulatory signals are sensed and senescence is initiated. Here, we review current understanding of petal senescence, and discuss associated regulatory mechanisms involving hormone interactions and epigenetic regulation.

Mitogen-Activated Protein Kinase Cascades in Plant Hormone Signaling

Mitogen-activated protein kinase (MAPK) modules play key roles in the transduction of environmental and developmental signals through phosphorylation of downstream signaling targets, including other kinases, enzymes, cytoskeletal proteins or transcription factors, in all eukaryotic cells. A typical MAPK cascade consists of at least three sequentially acting serine/threonine kinases, a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK) and finally, the MAP kinase (MAPK) itself, with each phosphorylating, and hence activating, the next kinase in the cascade. Recent advances in our understanding of hormone signaling pathways have led to the discovery of new regulatory systems. In particular, this research has revealed the emerging role of crosstalk between the protein components of various signaling pathways and the involvement of this crosstalk in multiple cellular processes. Here we provide an overview of current models and mechanisms of hormone signaling with a special emphasis on the role of MAPKs in cell signaling networks. One-sentence summary: In this review we highlight the mechanisms of crosstalk between MAPK cascades and plant hormone signaling pathways and summarize recent findings on MAPK regulation and function in various cellular processes.

Regulation of the turnover of ACC synthases by phytohormones and heterodimerization in Arabidopsis

Ethylene influences many aspects of plant growth and development. The biosynthesis of ethylene is highly regulated by a variety of internal and external cues. A key target of this regulation is 1-aminocyclopropane-1-carboxylic acid (ACC) synthases (ACS), generally the rate-limiting step in ethylene biosynthesis, which is regulated both transcriptionally and post-transcriptionally. Prior studies have demonstrated that cytokinin and brassinosteroid (BR) act as regulatory inputs to elevate ethylene biosynthesis by increasing the stability of ACS proteins. Here, we demonstrate that several additional phytohormones also regulate ACS protein turnover. Abscisic acid, auxin, gibberellic acid, methyl jasmonic acid, and salicylic acid differentially regulate the stability of ACS proteins, with distinct effects on various isoforms. In addition, we demonstrate that heterodimerization influences the stability of ACS proteins. Heterodimerization between ACS isoforms from distinct subclades results in increased stability of the shorter-lived partner. Together, our study provides a comprehensive understanding of the roles of various phytohormones on ACS protein stability, which brings new insights into crosstalk between ethylene and other phytohormones, and a novel regulatory mechanism that controls ACS protein stability through a heterodimerization of ACS isoforms.

'Bending' models of halotropism: incorporating protein phosphatase 2A, ABCB transporters, and auxin metabolism

Salt stress causes worldwide reductions in agricultural yields, a problem that is exacerbated by the depletion of global freshwater reserves and the use of contaminated or recycled water (i.e. effluent water). Additionally, salt stress can occur as cultivated areas are subjected to frequent rounds of irrigation followed by periods of moderate to severe evapotranspiration, which can result in the heterogeneous aggregation of salts in agricultural soils. Our understanding of the later stages of salt stress and the mechanisms by which salt is transported out of cells and roots has greatly improved over the last decade. The precise mechanisms by which plant roots perceive salt stress and translate this perception into adaptive, directional growth away from increased salt concentrations (i.e. halotropism), however, are not well understood. Here, we provide a review of the current knowledge surrounding the early responses to salt stress and the initiation of halotropism, including lipid signaling, protein phosphorylation cascades, and changes in auxin metabolism and/or transport. Current models of halotropism have focused on the role of PIN2- and PIN1-mediated auxin efflux in initiating and controlling halotropism. Recent studies, however, suggest that additional factors such as ABCB transporters, protein phosphatase 2A activity, and auxin metabolism should be included in the model of halotropic growth.

Effect of brain-derived neurotrophic factor (BDNF) on hepatocyte metabolism

Brain-derived neurotrophic factor (BDNF) plays crucial roles in the development, maintenance, plasticity and homeostasis of the central and peripheral nervous systems. Perturbing BDNF signaling in mouse brain results in hyperphagia, obesity, hyperinsulinemia and hyperglycemia. Currently, little is known whether BDNF affects liver tissue directly. Our aim was to determine the metabolic signaling pathways activated after BDNF treatment in hepatocytes. Unlike its effect in the brain, BDNF did not lead to activation of the liver AKT pathway. However, AMP protein activated kinase (AMPK) was ∼3 times more active and fatty acid synthase (FAS) ∼2-fold less active, suggesting increased fatty acid oxidation and reduced fatty acid synthesis. In addition, cAMP response element binding protein (CREB) was ∼3.5-fold less active together with its output the gluconeogenic transcript phosphoenolpyruvate carboxykinase (Pepck), suggesting reduced gluconeogenesis. The levels of glycogen synthase kinase 3b (GSK3b) was ∼3-fold higher suggesting increased glycogen synthesis. In parallel, the expression levels of the clock genes Bmal1 and Cry1, whose protein products play also a metabolic role, were ∼2-fold increased and decreased, respectively. In conclusion, BDNF binding to hepatocytes leads to activation of catabolic pathways, such as fatty acid oxidation. In parallel gluconeogenesis is inhibited, while glycogen storage is triggered. This metabolic state mimics that of after breakfast, in which the liver continues to oxidize fat, stops gluconeogenesis and replenishes glycogen stores.

Interactions between plant hormones and heavy metals responses

Heavy metals are natural non-biodegradable constituents of the Earth's crust that accumulate and persist indefinitely in the ecosystem as a result of human activities. Since the industrial revolution, the concentration of cadmium, arsenic, lead, mercury and zinc, amongst others, have increasingly contaminated soil and water resources, leading to significant yield losses in plants. These issues have become an important concern of scientific interest. Understanding the molecular and physiological responses of plants to heavy metal stress is critical in order to maximize their productivity. Recent research has extended our view of how plant hormones can regulate and integrate growth responses to various environmental cues in order to sustain life. In the present review we discuss current knowledge about the role of the plant growth hormones abscisic acid, auxin, brassinosteroid and ethylene in signaling pathways, defense mechanisms and alleviation of heavy metal toxicity.

Regulation of seedling growth by ethylene and the ethylene-auxin crosstalk

This review highlights that the auxin gradient, established by local auxin biosynthesis and transport, can be controlled by ethylene, and steers seedling growth. A better understanding of the mechanisms in Arabidopsis will increase potential applications in crop species. In dark-grown Arabidopsis seedlings, exogenous ethylene treatment triggers an exaggeration of the apical hook, the inhibition of both hypocotyl and root elongation, and radial swelling of the hypocotyl. These features are predominantly based on the differential cell elongation in different cells/tissues mediated by an auxin gradient. Interestingly, the physiological responses regulated by ethylene and auxin crosstalk can be either additive or synergistic, as in primary root and root hair elongation, or antagonistic, as in hypocotyl elongation. This review focuses on the crosstalk of these two hormones at the seedling stage. Before illustrating the crosstalk, ethylene and auxin biosynthesis, metabolism, transport and signaling are briefly discussed.

Diverse responses of wild and cultivated tomato to BABA, oligandrin and Oidium neolycopersici infection

Background and Aims

Current strategies for increased crop protection of susceptible tomato plants against pathogen infections include treatment with synthetic chemicals, application of natural pathogen-derived compounds or transfer of resistance genes from wild tomato species within breeding programmes. In this study, a series of 45 genes potentially involved in defence mechanisms was retrieved from the genome sequence of inbred reference tomato cultivar Solanum lycopersicum 'Heinz 1706'. The aim of the study was to analyse expression of these selected genes in wild and cultivated tomato plants contrasting in resistance to the biotrophic pathogen Oidium neolycopersici , the causative agent of powdery mildew. Plants were treated either solely with potential resistance inducers or by inducers together with the pathogen.

Methods

The resistance against O. neolycopersici infection as well as RT-PCR-based analysis of gene expression in response to the oomycete elicitor oligandrin and chemical agent β-aminobutyric acid (BABA) were investigated in the highly susceptible domesticated inbred genotype Solanum lycopersicum 'Amateur' and resistant wild genotype Solanum habrochaites .

Key Results

Differences in basal expression levels of defensins, germins, β-1,3-glucanases, heveins, chitinases, osmotins and PR1 proteins in non-infected and non-elicited plants were observed between the highly resistant and susceptible genotypes. Moreover, these defence genes showed an extensive up-regulation following O. neolycopersici infection in both genotypes. Application of BABA and elicitin induced expression of multiple defence-related transcripts and, through different mechanisms, enhanced resistance against powdery mildew in the susceptible tomato genotype.

Conclusions

The results indicate that non-specific resistance in the resistant genotype S. habrochaites resulted from high basal levels of transcripts with proven roles in defence processes. In the susceptible genotype S. lycopersicum 'Amateur', oligandrin- and BABA-induced resistance involved different signalling pathways, with BABA-treated leaves displaying direct activation of the ethylene-dependent signalling pathway, in contrast to previously reported jasmonic acid-mediated signalling for elicitins.

Atypical Protein Phosphatase 2A Gene Families Do Not Expand via Paleopolyploidization

Protein phosphatase 2A (PP2A) presents unique opportunities for analyzing molecular mechanisms of functional divergence between gene family members. The canonical PP2A holoenzyme regulates multiple eukaryotic signaling pathways by dephosphorylating target proteins and contains a catalytic (C) subunit, a structural/scaffolding (A) subunit, and a regulatory (B) subunit. Genes encoding PP2A subunits have expanded into multigene families in both flowering plants and mammals, and the extent to which different isoform functions may overlap is not clearly understood. To gain insight into the diversification of PP2A subunits, we used phylogenetic analyses to reconstruct the evolutionary histories of PP2A gene families in Arabidopsis (Arabidopsis thaliana). Genes encoding PP2A subunits in mammals represent ancient lineages that expanded early in vertebrate evolution, while flowering plant PP2A subunit lineages evolved much more recently. Despite this temporal difference, our data indicate that the expansion of PP2A subunit gene families in both flowering plants and animals was driven by whole-genome duplications followed by nonrandom gene loss. Selection analysis suggests that the expansion of one B subunit gene family (B56/PPP2R5) was driven by functional diversification rather than by the maintenance of gene dosage. We also observed reduced expansion rates in three distinct B subunit subclades. One of these subclades plays a highly conserved role in cell division, while the distribution of a second subclade suggests a specialized function in supporting beneficial microbial associations. Thus, while whole-genome duplications have driven the expansion and diversification of most PP2A gene families, members of functionally specialized subclades quickly revert to singleton status after duplication events.

Accumulation and Transport of 1-Aminocyclopropane-1-Carboxylic Acid (ACC) in Plants: Current Status, Considerations for Future Research and Agronomic Applications

1-aminocyclopropane-1-carboxylic acid (ACC) is a non-protein amino acid acting as the direct precursor of ethylene, a plant hormone regulating a wide variety of vegetative and developmental processes. ACC is the central molecule of ethylene biosynthesis. The rate of ACC formation differs in response to developmental, hormonal and environmental cues. ACC can be conjugated to three derivatives, metabolized in planta or by rhizobacteria using ACC deaminase, and is transported throughout the plant over short and long distances, remotely leading to ethylene responses. This review highlights some recent advances related to ACC. These include the regulation of ACC synthesis, conjugation and deamination, evidence for a role of ACC as an ethylene-independent signal, short and long range ACC transport, and the identification of a first ACC transporter. Although unraveling the complex mechanism of ACC transport is in its infancy, new questions emerge together with the identification of a first transporter. In the light of the future quest for additional ACC transporters, this review presents perspectives of the novel findings and includes considerations for future research toward applications in agronomy.

Know where your clients are: subcellular localization and targets of calcium-dependent protein kinases

Calcium-dependent protein kinases (CDPKs) are at the forefront of decoding transient Ca(2+) signals into physiological responses. They play a pivotal role in many aspects of plant life starting from pollen tube growth, during plant development, and in stress response to senescence and cell death. At the cellular level, Ca(2+) signals have a distinct, narrow distribution, thus requiring a conjoined localization of the decoders. Accordingly, most CDPKs have a distinct subcellular distribution which enables them to 'sense' the local Ca(2+) concentration and to interact specifically with their targets. Here we present a comprehensive overview of identified CDPK targets and discuss them in the context of kinase-substrate specificity and subcellular distribution of the CDPKs. This is particularly relevant for calcium-mediated phosphorylation where different CDPKs, as well as other kinases, were frequently reported to be involved in the regulation of the same target. However, often these studies were not performed in an in situ context. Thus, considering the specific expression patterns, distinct subcellular distribution, and different Ca(2+) affinities of CDPKs will narrow down the number of potential CDPKs for one given target. A number of aspects still remain unresolved, giving rise to pending questions for future research.

Direct and Indirect Targeting of PP2A by Conserved Bacterial Type-III Effector Proteins

Bacterial AvrE-family Type-III effector proteins (T3Es) contribute significantly to the virulence of plant-pathogenic species of Pseudomonas, Pantoea, Ralstonia, Erwinia, Dickeya and Pectobacterium, with hosts ranging from monocots to dicots. However, the mode of action of AvrE-family T3Es remains enigmatic, due in large part to their toxicity when expressed in plant or yeast cells. To search for targets of WtsE, an AvrE-family T3E from the maize pathogen Pantoea stewartii subsp. stewartii, we employed a yeast-two-hybrid screen with non-lethal fragments of WtsE and a synthetic genetic array with full-length WtsE. Together these screens indicate that WtsE targets maize protein phosphatase 2A (PP2A) heterotrimeric enzyme complexes via direct interaction with B' regulatory subunits. AvrE1, another AvrE-family T3E from Pseudomonas syringae pv. tomato strain DC3000 (Pto DC3000), associates with specific PP2A B' subunit proteins from its susceptible host Arabidopsis that are homologous to the maize B' subunits shown to interact with WtsE. Additionally, AvrE1 was observed to associate with the WtsE-interacting maize proteins, indicating that PP2A B' subunits are likely conserved targets of AvrE-family T3Es. Notably, the ability of AvrE1 to promote bacterial growth and/or suppress callose deposition was compromised in Arabidopsis plants with mutations of PP2A genes. Also, chemical inhibition of PP2A activity blocked the virulence activity of both WtsE and AvrE1 in planta. The function of HopM1, a Pto DC3000 T3E that is functionally redundant to AvrE1, was also impaired in specific PP2A mutant lines, although no direct interaction with B' subunits was observed. These results indicate that sub-component specific PP2A complexes are targeted by bacterial T3Es, including direct targeting by members of the widely conserved AvrE-family.

Protein Phosphatase 2A in the Regulatory Network Underlying Biotic Stress Resistance in Plants

Biotic stress factors pose a major threat to plant health and can significantly deteriorate plant productivity by impairing the physiological functions of the plant. To combat the wide range of pathogens and insect herbivores, plants deploy converging signaling pathways, where counteracting activities of protein kinases and phosphatases form a basic mechanism for determining appropriate defensive measures. Recent studies have identified Protein Phosphatase 2A (PP2A) as a crucial component that controls pathogenesis responses in various plant species. Genetic, proteomic and metabolomic approaches have underscored the versatile nature of PP2A, which contributes to the regulation of receptor signaling, organellar signaling, gene expression, metabolic pathways, and cell death, all of which essentially impact plant immunity. Associated with this, various PP2A subunits mediate post-translational regulation of metabolic enzymes and signaling components. Here we provide an overview of protein kinase/phosphatase functions in plant immunity signaling, and position the multifaceted functions of PP2A in the tightly inter-connected regulatory network that controls the perception, signaling and responding to biotic stress agents in plants.

Ethylene and Metal Stress: Small Molecule, Big Impact

The phytohormone ethylene is known to mediate a diverse array of signaling processes during abiotic stress in plants. Whereas many reports have demonstrated enhanced ethylene production in metal-exposed plants, the underlying molecular mechanisms are only recently investigated. Increasing evidence supports a role for ethylene in the regulation of plant metal stress responses. Moreover, crosstalk appears to exist between ethylene and the cellular redox balance, nutrients and other phytohormones. This review highlights our current understanding of the key role ethylene plays during responses to metal exposure. Moreover, particular attention is paid to the integration of ethylene within the broad network of plant responses to metal stress.

Protein Phosphatase 2A in the Regulatory Network Underlying Biotic Stress Resistance in Plants

Biotic stress factors pose a major threat to plant health and can significantly deteriorate plant productivity by impairing the physiological functions of the plant. To combat the wide range of pathogens and insect herbivores, plants deploy converging signaling pathways, where counteracting activities of protein kinases and phosphatases form a basic mechanism for determining appropriate defensive measures. Recent studies have identified Protein Phosphatase 2A (PP2A) as a crucial component that controls pathogenesis responses in various plant species. Genetic, proteomic and metabolomic approaches have underscored the versatile nature of PP2A, which contributes to the regulation of receptor signaling, organellar signaling, gene expression, metabolic pathways, and cell death, all of which essentially impact plant immunity. Associated with this, various PP2A subunits mediate post-translational regulation of metabolic enzymes and signaling components. Here we provide an overview of protein kinase/phosphatase functions in plant immunity signaling, and position the multifaceted functions of PP2A in the tightly inter-connected regulatory network that controls the perception, signaling and responding to biotic stress agents in plants.

Effects of volatile organic compound ether on cell responses and gene expressions in Arabidopsis

BACKGROUND

The volatile organic compound ether is widely used as an industrial solvent and easily released to the environment. Our previous research indicated that ether triggers reactive oxygen species (ROS) production and activates ethylene biosynthetic genes and defense gene expressions in tomato. In the present study, we investigated the effect of ether on cell responses and gene expressions in Arabidopsis and compared the ROS and phytohormones produced in Arabidopsis and tomato plants in response to different air pollutants (O₃ vs. ether).

RESULTS

Ether induced the sequential production of superoxide anion and hydrogen peroxide in Arabidopsis. Ether also triggered expressions of ethylene, salicylic acid and jasmonic acid biosynthetic genes. The temporal expression patterns of MAP kinase and protein phosphatase genes are in good accordance with those of the ethylene and salicylic acid biosynthetic genes, suggesting that induction of these phytohormone biosynthesis were through signaling pathways including both phosphorylation and/or dephosphorylation. By contrast, expression pattern of protein phosphatase PP2A3&4 coincided well with the expression of jasmonic acid biosynthetic gene LOX4, suggesting that induction of jasmonic acid biosynthesis is through PP2A3&4. However, the production of ROS and temporal expression patterns of phytohormone biosynthetic genes in Arabidopsis in response to ether were different from those to O₃ and were different from those in tomato as well.

CONCLUSIONS

Different plants have different strategies to respond to the same abiotic stress, and each plant species possesses its own unique signaling pathways that regulate the responding process.

Identification of miRNAs involved in fruit ripening in Cavendish bananas by deep sequencing

BACKGROUND

MicroRNAs (miRNAs) are a family of non-coding small RNAs that play an important regulatory role in various biological processes. Previous studies have reported that miRNAs are closely related to the ripening process in model plants. However, the miRNAs that are closely involved in the banana fruit ripening process remain unknown.

METHODS

Here, we investigated the miRNA populations from banana fruits in response to ethylene or 1-MCP treatment using a deep sequencing approach and bioinformatics analysis combined with quantitative RT-PCR validation.

RESULTS

A total of 125 known miRNAs and 26 novel miRNAs were identified from three libraries. MiRNA profiling of bananas in response to ethylene treatment compared with 1-MCP treatment showed differential expression of 82 miRNAs. Furthermore, the differentially expressed miRNAs were predicted to target a total of 815 target genes. Interestingly, some targets were annotated as transcription factors and other functional proteins closely involved in the development and the ripening process in other plant species. Analysis by qRT-PCR validated the contrasting expression patterns between several miRNAs and their target genes.

CONCLUSIONS

The miRNAome of the banana fruit in response to ethylene or 1-MCP treatment were identified by high-throughput sequencing. A total of 82 differentially expressed miRNAs were found to be closely associated with the ripening process. The miRNA target genes encode transcription factors and other functional proteins, including SPL, APETALA2, EIN3, E3 ubiquitin ligase, β-galactosidase, and β-glucosidase. These findings provide valuable information for further functional research of the miRNAs involved in banana fruit ripening.

Ethylene signalling is mediating the early cadmium-induced oxidative challenge in Arabidopsis thaliana

Cadmium (Cd) induces the generation of reactive oxygen species (ROS) and stimulates ethylene biosynthesis. The phytohormone ethylene is a regulator of many developmental and physiological plant processes as well as stress responses. Previous research indicated various links between ethylene signalling and oxidative stress. Our results support a correlation between the Cd-induced oxidative challenge and ethylene signalling in Arabidopsis thaliana leaves. The effects of 24 or 72 h exposure to 5 μM Cd on plant growth and several oxidative stress-related parameters were compared between wild-type (WT) and ethylene insensitive mutants (etr1-1, ein2-1, ein3-1). Cadmium-induced responses observed in WT plants were mainly affected in etr1-1 and ein2-1 mutants, of which the growth was less inhibited by Cd exposure as compared to WT and ein3-1 mutants. Both etr1-1 and ein2-1 showed a delayed response in the glutathione (GSH) metabolism, including GSH levels and transcript levels of GSH synthesising and recycling enzymes. Furthermore, the expression of different oxidative stress marker genes was significantly lower in Cd-exposed ein2-1 mutants, evidencing that ethylene signalling is involved in early responses to Cd stress. A model for the cross-talk between ethylene signalling and oxidative stress is proposed.

Producing the Ethylene Signal: Regulation and Diversification of Ethylene Biosynthetic Enzymes

Strictly controlled production of ethylene gas lies upstream of the signaling activities of this crucial regulator throughout the plant life cycle. Although the biosynthetic pathway is enzymatically simple, the regulatory circuits that modulate signal production are fine tuned to allow integration of responses to environmental and intrinsic cues. Recently identified posttranslational mechanisms that control ethylene production converge on one family of biosynthetic enzymes and overlay several independent reversible phosphorylation events and distinct mediators of ubiquitin-dependent protein degradation. Although the core pathway is conserved throughout seed plants, these posttranslational regulatory mechanisms may represent evolutionarily recent innovations. The evolutionary origins of the pathway and its regulators are not yet clear; outside the seed plants, numerous biochemical and phylogenetic questions remain to be addressed.

Identification of Open Stomata1-Interacting Proteins Reveals Interactions with Sucrose Non-fermenting1-Related Protein Kinases2 and with Type 2A Protein Phosphatases That Function in Abscisic Acid Responses

The plant hormone abscisic acid (ABA) controls growth and development and regulates plant water status through an established signaling pathway. In the presence of ABA, pyrabactin resistance/regulatory component of ABA receptor proteins inhibit type 2C protein phosphatases (PP2Cs). This, in turn, enables the activation of Sucrose Nonfermenting1-Related Protein Kinases2 (SnRK2). Open Stomata1 (OST1)/SnRK2.6/SRK2E is a major SnRK2-type protein kinase responsible for mediating ABA responses. Arabidopsis (Arabidopsis thaliana) expressing an epitope-tagged OST1 in the recessive ost1-3 mutant background was used for the copurification and identification of OST1-interacting proteins after osmotic stress and ABA treatments. These analyses, which were confirmed using bimolecular fluorescence complementation and coimmunoprecipitation, unexpectedly revealed homo- and heteromerization of OST1 with SnRK2.2, SnRK2.3, OST1, and SnRK2.8. Furthermore, several OST1-complexed proteins were identified as type 2A protein phosphatase (PP2A) subunits and as proteins involved in lipid and galactolipid metabolism. More detailed analyses suggested an interaction network between ABA-activated SnRK2-type protein kinases and several PP2A-type protein phosphatase regulatory subunits. pp2a double mutants exhibited a reduced sensitivity to ABA during seed germination and stomatal closure and an enhanced ABA sensitivity in root growth regulation. These analyses add PP2A-type protein phosphatases as another class of protein phosphatases to the interaction network of SnRK2-type protein kinases.

New Insights into the Protein Turnover Regulation in Ethylene Biosynthesis

Biosynthesis of the phytohormone ethylene is under tight regulation to satisfy the need for appropriate levels of ethylene in plants in response to exogenous and endogenous stimuli. The enzyme 1-aminocyclopropane-1-carboxylic acid synthase (ACS), which catalyzes the rate-limiting step of ethylene biosynthesis, plays a central role to regulate ethylene production through changes in ACS gene expression levels and the activity of the enzyme. Together with molecular genetic studies suggesting the roles of post-translational modification of the ACS, newly emerging evidence strongly suggests that the regulation of ACS protein stability is an alternative mechanism that controls ethylene production, in addition to the transcriptional regulation of ACS genes. In this review, recent new insight into the regulation of ACS protein turnover is highlighted, with a special focus on the roles of phosphorylation, ubiquitination, and novel components that regulate the turnover of ACS proteins. The prospect of cross-talk between ethylene biosynthesis and other signaling pathways to control turnover of the ACS protein is also considered.

Plant signalling in acute ozone exposure

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

Cantharidin, a protein phosphatase inhibitor, strongly upregulates detoxification enzymes in the Arabidopsis proteome

Cantharidin, a potent inhibitor of plant serine/threonine protein phosphatases (PPPs), is highly phytotoxic and dramatically affects the transcriptome in Arabidopsis. To investigate the effect of cantharidin on the Arabidopsis proteome, a combination of two-dimensional difference gel electrophoresis (2-D DIGE) and matrix-assisted laser desorption ionization time-of-flight (MALDI/TOF) mass spectrometry was employed for protein profiling. Multivariate statistical analysis identified 75 significant differential spots corresponding to 59 distinct cantharidin-responsive proteins, which were representative of different biological processes, cellular components, and molecular functions categories. The majority of identified proteins localized in the chloroplast had a significantly decreased presence, especially proteins involved in photosynthesis. Detoxification enzymes, especially glutathione-S-transferases (GSTs), were the most upregulated group (ca. 1.5- to 3.3-fold). Given that the primary role of GSTs is involved in the process of detoxification of both xenobiotic and endobiotic compounds, the induction of GSTs suggests that cantharidin promoted inhibition of PPPs may lead to defense-like responses through regulation of GST enzymes as well as other metabolic pathways.