The oxylipin signal jasmonic acid is activated by an enzyme that conjugates it to isoleucine in Arabidopsis

Metabolic Control within the Jasmonate Biochemical Pathway

Regulation of defense and developmental responses by jasmonates (JAs) has been intensively investigated at genetic and transcriptional levels. Plasticity in the jasmonic acid (JA) metabolic pathway as a means to control signal output has received less attention. Although the amplitude of JA responses generally follows the accumulation dynamics of the active hormone jasmonoyl-isoleucine (JA-Ile), emerging evidence has identified cases where this relationship is distorted and that we discuss in this review. JA-Ile is turned over in Arabidopsis by two inducible, intertwined catabolic pathways; one is oxidative and mediated by cytochrome P450 enzymes of the subfamily 94 (CYP94), and the other proceeds via deconjugation by amidohydrolases. Their genetic inactivation has profound effects on JAs homeostasis, including strong JA-Ile overaccumulation, but this correlates with enhanced defense and tolerance to microbial or insect attacks only in the absence of overinduction of negative signaling regulators. By contrast, the impairment of JA oxidation in the jasmonic acid oxidase 2 (jao2) mutant turns on constitutive defense responses without elevating JA-Ile levels in naive leaves and enhances resistance to subsequent biotic stress. This latter and other recent cases of JA signaling are associated with JA-Ile catabolites accumulation rather than more abundant hormone, reflecting increased metabolic flux through the pathway. Therefore, manipulating upstream and downstream JA-Ile homeostatic steps reveals distinct metabolic nodes controlling defense signaling output.

Jasmonate Signaling during Arabidopsis Stamen Maturation

The last stages of stamen development, collectively called stamen maturation, encompass pollen viability, filament elongation and anther dehiscence or opening. These processes are essential for male fertility in Arabidopsis and require the function of jasmonate signaling. There is a good understanding of jasmonate synthesis, perception and transcriptional outputs in Arabidopsis stamens. In addition, the spatiotemporal localization of jasmonate signaling components at the tissue and cellular levels has started to emerge in recent years. However, the ultimate cellular functions activated by jasmonate to promote stamen maturation remain unknown. The hormones auxin and gibberellin have been proposed to control the activation of jasmonate synthesis to promote stamen maturation, although we hypothesize that this action is rather indirect. In this review, we examine these different areas, attempt to clarify some confusing aspects found in the literature and raise testable hypothesis that may help to further understand how jasmonate controls male fertility in Arabidopsis.

Use of Yellow Fluorescent Protein Fluorescence to Track OPR3 Expression in Arabidopsis Thaliana Responses to Insect Herbivory

Feeding by chewing insects induces chemical defenses in plants that are regulated by the jasmonic acid (JA) pathway. Jasmonates are usually quantified by liquid chromatography-mass spectrometry (LC-MS) analysis of precursors and products in the biosynthetic pathway or inferred from the extraction and expression of genes known to respond to elevated levels of JA. Both approaches are costly and time consuming. To address these limitations, we developed a rapid reporter for the synthesis of JA based on the OPR3promoter:YFP-PTS1. Yellow fluorescent protein (YFP) fluorescence was increased by mechanical wounding and methyl jasmonate (MeJA) treatment and by caterpillar feeding. To develop an optimal sampling time for a quantitative bioassay, OPR3promoter:YFP-PTS1 plants were sampled at 1, 2, 3, and 24 h after treatment with 115 µM MeJA. The first increase in YFP fluorescence was detected at 2 h and remained elevated 3 and 24 h later; as a result, 3 h was chosen as the sampling time for a quantitative bioassay of jasmonate response to insect attack. Feeding by Pieris rapae caterpillars induced a 1.8-fold increase in YFP fluorescence, consistent with the known induction of JA production by this insect. We also assessed the utility of this reporter in studies of plant responses to caterpillar feeding vibrations, which are known to potentiate the JA-dependent production of chemical defenses. Pretreatment with feeding vibrations increased expression of the OPR3promoter:YFP-PTS1 in response to 14 µM MeJA. Feeding vibrations did not potentiate responses at higher MeJA concentrations, suggesting that potentiating effects of prior treatments can only be detected when plants are below a response threshold to the elicitor. The expression of OPR3 does not indicate levels of specific downstream jasmonates and quantification of specific jasmonates still requires detailed analysis by LC-MS. However, OPR3 expression does provide a rapid and inexpensive way to screen large numbers of plants for the involvement of jasmonate signaling in their response to a wide variety of treatments, and to study the induction and expression of AtOPR3.

Reconstitution of the Jasmonate Signaling Pathway in Plant Protoplasts

The phytohormone jasmonic acid (JA) plays an important role in various plant developmental processes and environmental adaptations. The JA signaling pathway has been well-elucidated in the reference plant Arabidopsis thaliana. It starts with the perception of the active JA derivative, jasmonoyl-isoleucine (JA-Ile), by the F-box protein COI1 which is part of the E3-ligase SCF^COI1. Binding of JA-Ile enables the interaction between COI1 and JAZ repressor proteins. Subsequent degradation of JAZ proteins leads to the activation of transcription factors like e.g., MYC2. Here we demonstrate that the pathway can be reconstituted in transiently transformed protoplasts. Analysis of the stability of a JAZ1-fLuc fusion protein as a function of COI1 transiently expressed in coi1 protoplasts allows structure function analysis of both JAZs and COI1. Using this system, we found that conserved cysteines in COI1 influence steady state COI1 protein levels. Using a luciferase reporter gene under the control of the JAZ1 promoter enable to address those features of JAZ1 that are required for MYC2 repression. Interestingly, the conserved TIFY-motif previously described to interact with NINJA to recruit the corepressor TOPLESS is not necessary for repression. This result is in favor of the alternative repression mode that proposes a direct competition between repressive JAZs and promotive MEDIATOR25 at MYC2. Finally, using protoplasts from the aos coi1 double mutant, which is deficient in JA synthesis and perception, we provide a system that has the potential to study the activity of different COI1 variants in the presence of different ligands.

Mass Spectrometric Approaches to Study the Metabolism of Jasmonates: Biotransformation of Exogenously Supplemented Methyl Jasmonate by Cell Suspension Cultures of Moringa oleifera

Jasmonic acid (JA) and derivatives play a crucial role in plant adaptation to environmental stress. The exogenous application of methyl jasmonate (MeJA) results in activation of stress-related genes and subsequent production of secondary metabolites. This implies the biotransformation of the hormone into a more active form. In this study, Moringa oleifera cell suspension cultures were treated with 100 μM MeJA. Methanolic cell extracts were analyzed with reverse phase ultrahigh performance liquid chromatography (UHPLC) coupled to a quadrupole time-of-flight high definition mass spectrometer (QTOF-HDMS, with MS^E data acquisition). Using a targeted approach for jasmonate profiling, extracted ion chromatograms were generated. MS fragmentation patterns were subsequently derived and molecular formulae calculated from the MS data of each ion. Furthermore, triple quadrupole (QqQ) MS, operated in selected reaction monitoring (SRM) mode, was employed to target masses of interest. In addition to MeJA and JA, three jasmonoyl-amino acids were annotated: jasmonoyl-valine (JA-Val), jasmonoyl-isoleucine/leucine (JA-Ile/Leu), and jasmonoyl-phenylalanine (JA-Phe), based on characteristic precursor and product ions. Furthermore, JA conjugated to a hexose was observed, as well as hydroxylated and carboxylated derivatives of JA-amino acid conjugates. The data point to active metabolism of the externally added MeJA by the M. oleifera cells through biotransformation and bioconversion reactions that can be investigated in depth using advanced mass spectrometric analyses.

A Role for Tocopherol Biosynthesis in Arabidopsis Basal Immunity to Bacterial Infection

Tocopherols are lipid-soluble antioxidants synthesized in plastids of plants and other photosynthetic organisms. The four known tocopherols, α-, β-, γ-, and δ-tocopherol, differ in number and position of methyl groups on their chromanol head group. In unstressed Arabidopsis (Arabidopsis thaliana) leaves, α-tocopherol constitutes the main tocopherol form, whereas seeds predominantly contain γ-tocopherol. Here, we show that inoculation of Arabidopsis leaves with the bacterial pathogen Pseudomonas syringae induces the expression of genes involved in early steps of tocopherol biosynthesis and triggers strong accumulation of γ-tocopherol, moderate production of δ-tocopherol, and generation of the benzoquinol precursors of tocopherols. The pathogen-inducible biosynthesis of tocopherols is promoted by the immune regulators ENHANCED DISEASE SUSCEPTIBILITY1 and PHYTOALEXIN-DEFICIENT4. In addition, tocopherols accumulate in response to bacterial flagellin and reactive oxygen species. By quantifying tocopherol forms in inoculated wild-type plants and biosynthetic pathway mutants, we provide biochemical insights into the pathogen-inducible tocopherol pathway. Notably, vitamin E deficient2 (vte2) mutant plants, which are compromised in both tocopherol and benzoquinol precursor accumulation, exhibit increased susceptibility toward compatible P. syringae and possess heightened levels of markers of lipid peroxidation after bacterial infection. The deficiency of triunsaturated fatty acids in vte2-1 fatty acid desaturase3-2 (fad3-2) fad7-2 fad8 quadruple mutants prevents increased lipid peroxidation in the vte2 background and restores pathogen resistance to wild-type levels. Therefore, the tocopherol biosynthetic pathway positively influences salicylic acid accumulation and guarantees effective basal resistance of Arabidopsis against compatible P. syringae, possibly by protecting leaves from the pathogen-induced oxidation of trienoic fatty acid-containing lipids.

An engineered pathway for N-hydroxy-pipecolic acid synthesis enhances systemic acquired resistance in tomato

Systemic acquired resistance (SAR) is a powerful immune response that triggers broad-spectrum disease resistance throughout a plant. In the model plant Arabidopsis thaliana, long-distance signaling and SAR activation in uninfected tissues occur without circulating immune cells and instead rely on the metabolite N-hydroxy-pipecolic acid (NHP). Engineering SAR in crop plants would enable external control of a plant's ability to mount a global defense response upon sudden changes in the environment. Such a metabolite-engineering approach would require the molecular machinery for producing and responding to NHP in the crop plant. Here, we used heterologous expression in Nicotiana benthamiana leaves to identify a minimal set of Arabidopsis genes necessary for the biosynthesis of NHP. Local expression of these genes in tomato leaves triggered SAR in distal tissues in the absence of a pathogen, suggesting that the SAR trait can be engineered to enhance a plant's endogenous ability to respond to pathogens. We also showed tomato produces endogenous NHP in response to a bacterial pathogen and that NHP is present across the plant kingdom, raising the possibility that an engineering strategy to enhance NHP-induced defenses could be possible in many crop plants.

A metabolomic strategy revealed the role of JA and SA balance in Clematis terniflora DC. Response to UVB radiation and dark

Clematis terniflora DC. is a valuable resource with potential high pharmaceutical value. Proteomic, transcriptomic and metabolomic analyses of C. terniflora that has been exposed to high levels of UVB irradiation and dark conditions (HUVB + D) have revealed the mechanisms underlying its medicinal potential. However, the signal transduction pathways and the mechanisms of regulation for the accumulation of secondary metabolites remain unclear. In this study, we show that the jasmonic acid (JA) and salicylic acid (SA) signals were activated in C. terniflora in response to HUVB + D. Metabolomic analysis demonstrated that the perturbation in JA and SA balance led to additional reallocation of carbon and nitrogen resources. Evaluating the fold change ratios of differentially changed metabolites proved that JA signal enhanced the transformation of nitrogen to carbon through the 4-aminobutyric acid (GABA) shunt pathway, which increased the carbon reserve to be utilized in the production of secondary metabolites. However, SA signal induced the synthesis of proline, while avoiding the accumulation of secondary metabolites. Over all, the results indicate that the co-increase of JA and SA reconstructed the dynamic stability of transformation from nitrogen to carbon, which effectively enhanced the oxidative defense to HUVB + D in C. terniflora by increasing the secondary metabolites.

12-Hydroxy-Jasmonoyl-l-Isoleucine Is an Active Jasmonate That Signals through CORONATINE INSENSITIVE 1 and Contributes to the Wound Response in Arabidopsis

12-hydroxy-jasmonoyl-isoleucine (12OH-JA-Ile) is a metabolite in the catabolic pathway of the plant hormone jasmonate, and is synthesized by the cytochrome P450 subclade 94 enzymes. Contrary to the well-established function of jasmonoyl-isoleucine (JA-Ile) as the endogenous bioactive form of jasmonate, the function of 12OH-JA-Ile is unclear. Here, the potential role of 12OH-JA-Ile in jasmonate signaling and wound response was investigated. Exogenous application of 12OH-JA-Ile mimicked several JA-Ile effects including marker gene expression, anthocyanin accumulation and trichome induction in Arabidopsis thaliana. Genome-wide transcriptomics and untargeted metabolite analyses showed large overlaps between those affected by 12OH-JA-Ile and JA-Ile. 12OH-JA-Ile signaling was blocked by mutation in CORONATINE INSENSITIVE 1. Increased anthocyanin accumulation by 12OH-JA-Ile was additionally observed in tomato and sorghum, and was disrupted by the COI1 defect in tomato jai1 mutant. In silico ligand docking predicted that 12OH-JA-Ile can maintain many of the key interactions with COI1-JAZ1 residues identified earlier by crystal structure studies using JA-Ile as ligand. Genetic alternation of jasmonate metabolic pathways in Arabidopsis to deplete both JA-Ile and 12OH-JA-Ile displayed enhanced jasmonate deficient wound phenotypes and was more susceptible to insect herbivory than that depleted in only JA-Ile. Conversely, mutants overaccumulating 12OH-JA-Ile showed intensified wound responses compared with wild type with similar JA-Ile content. These data are indicative of 12OH-JA-Ile functioning as an active jasmonate signal and contributing to wound and defense response in higher plants.

Calcium-mediation of jasmonate biosynthesis and signaling in plants

Jasmonates (JAs) play vital roles in regulating a range of plant growth and development processes including seed germination, seedling development, reproduction, formation and development of storage organs, and senescence. JAs are also involved in the regulation of plant responses to environmental stimuli. The biosynthesis of JAs takes place in three different subcellular compartments, namely, the chloroplast, peroxisome, and cytoplasm. JAs activate the expression of JA-responsive genes by degrading jasmonate zinc-finger-inflorescence meristem (Zim) domain (JAZ) repressors via the E3 ubiquitin-ligase Skp/Cullin/F-box protein CORONATINE INSENSITIVE1 (COI1) complex (SCF^COI1) by using 26S proteasome. Calcium, reactive oxygen species (ROS), mitogen-activated protein kinase (MAPK), and nitric oxide (NO) are involved in the regulation of the biosynthesis and signaling of JAs in plants. Among these signaling molecules, calcium is one of the most important within plant cells. In plants, intracellular calcium levels change in response to JAs, resulting in calcium signatures with temporal and spatial features. Calcium channels are involved in the generation of calcium signatures. Calcium sensors, including calmodulins (CaMs), CaM-like proteins (CMLs), calcineurin B-like proteins (CBLs), and calcium-dependent protein kinases (CDPKs), can act to regulate the biosynthesis and signaling of JAs.

Jasmonate-Related MYC Transcription Factors Are Functionally Conserved in Marchantia polymorpha

The lipid-derived phytohormone jasmonoyl-isoleucine regulates plant immunity, growth and development in vascular plants by activating genome-wide transcriptional reprogramming. In Arabidopsis (Arabidopsis thaliana), this process is largely orchestrated by the master regulator MYC2 and related transcription factors (TFs). However, the TFs activating this pathway in basal plant lineages are currently unknown. We report the functional conservation of MYC-related TFs between the eudicot Arabidopsis and the liverwort Marchantia polymorpha, a plant belonging to an early diverging lineage of land plants. Phylogenetic analysis suggests that MYC function first appeared in charophycean algae and therefore predates the evolutionary appearance of any other jasmonate pathway component. M. polymorpha possesses two functionally interchangeable MYC genes, one in females and one in males. Similar to AtMYC2, MpMYCs showed nuclear localization, interaction with JASMONATE-ZIM-DOMAIN PROTEIN repressors, and regulation by light. Phenotypic and molecular characterization of loss- and gain-of-function mutants demonstrated that MpMYCs are necessary and sufficient for activating the jasmonate pathway in M. polymorpha, but unlike their Arabidopsis orthologs, do not regulate fertility. Therefore, despite 450 million years of independent evolution, MYCs are functionally conserved between bryophytes and eudicots. Genetic conservation in an early diverging lineage suggests that MYC function existed in the common ancestor of land plants and evolved from a preexisting MYC function in charophycean algae.

LEUNIG_HOMOLOG Mediates MYC2-Dependent Transcriptional Activation in Cooperation with the Coactivators HAC1 and MED25

Groucho/Thymidine uptake 1 (Gro/Tup1) family proteins are evolutionarily conserved transcriptional coregulators in eukaryotic cells. Despite their prominent function in transcriptional repression, little is known about their role in transcriptional activation and the underlying mechanism. Here, we report that the plant Gro/Tup1 family protein LEUNIG_HOMOLOG (LUH) activates MYELOCYTOMATOSIS2 (MYC2)-directed transcription of JAZ2 and LOX2 via the Mediator complex coactivator and the histone acetyltransferase HAC1. We show that the Mediator subunit MED25 physically recruits LUH to MYC2 target promoters that then links MYC2 with HAC1-dependent acetylation of Lys-9 of histone H3 (H3K9ac) to activate JAZ2 and LOX2 Moreover, LUH promotes hormone-dependent enhancement of protein interactions between MYC2 and its coactivators MED25 and HAC1. Our results demonstrate that LUH interacts with MED25 and HAC1 through its distinct domains, thus imposing a selective advantage by acting as a scaffold for MYC2 activation. Therefore, the function of LUH in regulating jasmonate signaling is distinct from the function of TOPLESS, another member of the Gro/Tup1 family that represses MYC2-dependent gene expression in the resting stage.

Omega hydroxylated JA-Ile is an endogenous bioactive jasmonate that signals through the canonical jasmonate signaling pathway

Jasmonates are fatty acid derivatives that control several plant processes including growth, development and defense. Despite the chemical diversity of jasmonates, only jasmonoyl-L-isoleucine (JA-Ile) has been clearly characterized as the endogenous ligand of the jasmonate co-receptors (COI1-JAZs) in higher plants. Currently, it is accepted that ω-hydroxylation of JA-Ile leads to inactivation of the molecule. This study shows that ω-hydroxylated JA-Ile (12-OH-JA-Ile) retains bioactivity and signals through the canonical JA-pathway. The results suggest that 12-OH-JA-Ile differentially activates a subset of JA-Ile co-receptors that may control and/or modulate particular jasmonate dependent responses. It is proposed that after a strong immune response mediated by JA-Ile, the ω-hydroxylated form modulates JA-Ile activated processes thereby improving plant resilience.

Arabidopsis Flowers Unlocked the Mechanism of Jasmonate Signaling

The Arabidopsis male-sterile phenotype has been a wonderful model for jasmonate action in plants. It has allowed us to identify transcription factors that control gene expression during stamen and pollen maturation and provided for the discovery of the JAZ repressor proteins and the mechanism of jasmonate signaling. More recently, it has revealed intriguing details of the spatial localization of jasmonate synthesis and perception in stamen tissues. The extensive and thoughtful application of protein–protein interaction assays to identify JAZ-interacting partners has led to a much richer appreciation of the mechanisms by which jasmonate integrates with the actions of other hormones to regulate plant growth and physiological responses. This integration is strikingly evident in stamen and pollen development in Arabidopsis, which requires the actions of many hormones. Just as importantly, it is now evident that jasmonate has very different actions during flower development and reproduction in other plant species. This integration and diversity of action indicates that many exciting discoveries remain to be made in this area of jasmonate hormone signaling and response.

Genome-wide identification and characterization of Gretchen Hagen3 (GH3) family genes in Brassica napus

The hormone auxin is involved in many biological processes throughout a plant's lifecycle. However, genes in the GH3 (Gretchen Hagen3) family, one of the three major auxin-responsive gene families, have not yet been identified in oilseed rape (Brassica napus). In this study, we identified 63 BnaGH3 genes in oilseed rape using homology searches. We analyzed the chromosome locations, gene structures, and phylogenetic relationships of the BnaGH3 genes, as well as the cis-elements in their promoters. Most BnaGH3 genes are located on chromosomes A03, A09, C02, C03, and C09, each with 4-7 members. In addition, we analyzed the expression patterns of BnaGH3 genes in seven tissues by transcriptome sequencing and quantitative RT-PCR analysis of plants under exogenous IAA treatment. The BnaGH3 genes showed different expression patterns in various tissues. BnaA.GH3.2-1 and BnaC.GH3.2-1 were expressed in the seed and seed coat during development and in response to IAA treatment. These results shed light on the possible roles of the GH3 gene family in oilseed rape.

Jasmonic Acid Modulates Xylem Development by Controlling Expression of PIN-FORMED 7

Jasmonic acid (JA) modulates plant development, growth, and responses to stress. Previously, we showed that in Arabidopsis thaliana, JA promotes the formation of extra xylem in roots, and mutant plants unable to express PIN-FORMED 3 (PIN3) and PIN7 formed extra xylem in the absence of exogenous JA. Those results suggested that JA modulates root xylem development by controlling PIN-mediated polar auxin transport. Consistent with this, treatment with an auxin transport inhibitor induced extra xylem formation. Here, we characterized the expression of PIN3 and PIN7 in JA-treated Arabidopsis plants. PIN3 expression was not altered in response to JA; by contrast, PIN7 expression was reduced by JA, which suggested that PIN7 is involved in JA-mediated xylem development. Indeed, overexpressing PIN7 suppressed the formation of extra xylem in response to JA. Based on these results, we propose that JA mediates xylem development by controlling polar auxin transport with PIN7 critically involved in this process.

Leaf-Derived Jasmonate Mediates Water Uptake from Hydrated Cotton Roots under Partial Root-Zone Irrigation

Partial root-zone irrigation (PRI), a water-saving technique, improves water uptake in hydrated roots by inducing specific responses that are thought to be regulated by signals originating from leaves; however, this signaling is poorly understood. Using a split-root system and polyethylene glycol 6000 to simulate PRI in cotton (Gossypium hirsutum), we showed that increased root hydraulic conductance (L) and water uptake in the hydrated roots may be due to the elevated expression of cotton plasma membrane intrinsic protein (PIP) genes. Jasmonate (jasmonic acid [JA] and jasmonic acid-isoleucine conjugate [JA-Ile]) content and the expression of three JA biosynthesis genes increased in the leaves of the PRI plants compared with those of the polyethylene glycol-free control. JA/JA-Ile content also increased in the hydrated roots, although the expression of the three JA genes was unaltered, compared with the control. The JA/JA-Ile contents in leaves increased after the foliar application of exogenous JA and was followed by an increase in both JA/JA-Ile content and L in the hydrated roots, whereas the silencing of the three JA genes had the opposite effect in the leaves. Ring-barking the hydrated hypocotyls increased the JA/JA-Ile content in the leaves but decreased the JA/JA-Ile content and L in the hydrated roots. These results suggested that the increased JA/JA-Ile in the hydrated roots was mostly transported from the leaves through the phloem, thus increasing L by increasing the expression of GhPIP in the hydrated roots under PRI. We believe that leaf-derived JA/JA-Ile, as a long-distance signal, positively mediates water uptake from the hydrated roots of cotton under PRI.

1-Aminocyclopropane-1-Carboxylic Acid Oxidase (ACO): The Enzyme That Makes the Plant Hormone Ethylene

The volatile plant hormone ethylene regulates many plant developmental processes and stress responses. It is therefore crucial that plants can precisely control their ethylene production levels in space and time. The ethylene biosynthesis pathway consists of two dedicated steps. In a first reaction, S-adenosyl-L-methionine (SAM) is converted into 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC-synthase (ACS). In a second reaction, ACC is converted into ethylene by ACC-oxidase (ACO). Initially, it was postulated that ACS is the rate-limiting enzyme of this pathway, directing many studies to unravel the regulation of ACS protein activity, and stability. However, an increasing amount of evidence has been gathered over the years, which shows that ACO is the rate-limiting step in ethylene production during certain dedicated processes. This implies that also the ACO protein family is subjected to a stringent regulation. In this review, we give an overview about the state-of-the-art regarding ACO evolution, functionality and regulation, with an emphasis on the transcriptional, post-transcriptional, and post-translational control. We also highlight the importance of ACO being a prime target for genetic engineering and precision breeding, in order to control plant ethylene production levels.

Facile preparation of optically active jasmonates and their biological activities in rice

A facile and efficient method has been developed for the optical resolution of racemic jasmonic acid (JA) on a relatively large scale and was successfully utilized for the preparation of optically pure (+)-JA and (−)-JA. We indicated that (+)-JA has lower growth inhibitory activity than (−)-JA in the rice seedling growth test and confirmed in line with an earlier observation that their respective biologically-active forms, (+)-JA-Ile and (−)-JA-Ile, show comparable inhibitory activities. We compared the metabolism of (+)-JA and (−)-JA into (+)-JA-Ile and (−)-JA-Ile, respectively, in the JA-deficient rice cpm2, and found that the exogenously applied (+)-JA was metabolized to the corresponding Ile conjugate less efficiently as compared with (−)-JA. Such metabolic rate difference may cause a discrepancy between biological potencies of (+)-JA and (−)-JA in rice.

Abbreviations: FW: fresh weight; Ile: isoleucine; JA: jasmonic acid; JA-Ile: jasmonoyl-l-isoleucine; LC-ESI-MS/MS: liquid chromatography and electrospray ionization tandem mass spectrometry; MeJA: methyl jasmonate; OPDA: 12-oxophytodienoic acid

Hydrophobin HFBII-4 from Trichoderma asperellum induces antifungal resistance in poplar

Herein, the class II hydrophobin gene HFBII-4 was cloned from the biocontrol agent Trichoderma asperellum ACCC30536 and recombinant rHFBII-4 was expressed in Pichia pastoris GS115. Treatment of Populus davidiana × P. alba var. pyramidalis (PdPap poplar) with rHFBII-4 altered the expression levels of genes in the auxin, salicylic acid (SA), and jasmonic acid (JA) signal transduction pathways. Polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) enzyme activities were induced with rHFBII-4. Evans Blue and nitro blue tetrazolium (NBT) staining indicated that cell membrane permeability and reactive oxygen species were lower in the leaves of plants treated with rHFBII-4. The chlorophyll content was higher than that of control at 2-5 days after treatment. Furthermore, poplar seedlings were inoculated with Alternaria alternata, disease symptoms were observed. The diseased area was smaller in leaves induced with rHFBII-4 compared with control. In summary, rHFBII-4 enhances resistance to A. alternata.

Contribution of cell wall peroxidase- and NADPH oxidase-derived reactive oxygen species to Alternaria brassicicola-induced oxidative burst in Arabidopsis

Cell wall peroxidases and plasma membrane-localized NADPH oxidases are considered to be the main sources of the apoplastic oxidative burst in plants attacked by microbial pathogens. In spite of this established doctrine, approaches attempting a comparative, side-by-side analysis of the functions of extracellular reactive oxygen species (ROS) generated by the two enzymatic sources are scarce. Previously, we have reported the role of Arabidopsis NADPH oxidase RBOHD (respiratory burst oxidase homologue D) in plants challenged with the necrotrophic fungus Alternaria brassicicola. Here, we present results on the activity of apoplastic class III peroxidases PRX33 (At3g49110) and PRX34 (At3g49120) investigated in the same Arabidopsis-Alternaria pathosystem. ROS generated by Arabidopsis peroxidases PRX33 and PRX34 increase the necrotic symptoms and colonization success of A. brassicicola. In addition, the knockdown of PRX33 and PRX34 transcript levels leads to a reduced number of host cells showing an extracellular burst of ROS after inoculation with A. brassicicola. Our results also reveal an age-dependent transcript distribution of ROS-producing peroxidase and NADPH oxidase enzymes, and some potential new components of the RBOHD, PRX33 and PRX34 signalling networks.

BIG regulates stomatal immunity and jasmonate production in Arabidopsis

Plants have evolved an array of responses that provide them with protection from attack by microorganisms and other predators. Many of these mechanisms depend upon interactions between the plant hormones jasmonate (JA) and ethylene (ET). However, the molecular basis of these interactions is insufficiently understood. Gene expression and physiological assays with mutants were performed to investigate the role of Arabidopsis BIG gene in stress responses. BIG transcription is downregulated by methyl JA (MeJA), necrotrophic infection or mechanical injury. BIG deficiency promotes JA-dependent gene induction, increases JA production but restricts the accumulation of both ET and salicylic acid. JA-induced anthocyanin accumulation and chlorophyll degradation are enhanced and stomatal immunity is impaired by BIG disruption. Bacteria- and lipopolysaccaride (LPS)-induced stomatal closure is reduced in BIG gene mutants, which are hyper-susceptible to microbial pathogens with different lifestyles, but these mutants are less attractive to phytophagous insects. Our results indicate that BIG negatively and positively regulate the MYC2 and ERF1 arms of the JA signalling pathway. BIG warrants recognition as a new and distinct regulator that regulates JA responses, the synergistic interactions of JA and ET, and other hormonal interactions that reconcile the growth and defense dilemma in Arabidopsis.

Tasselseed5 overexpresses a wound-inducible enzyme, ZmCYP94B1, that affects jasmonate catabolism, sex determination, and plant architecture in maize

Maize is monecious, with separate male and female inflorescences. Maize flowers are initially bisexual but achieve separate sexual identities through organ arrest. Loss-of-function mutants in the jasmonic acid (JA) pathway have only female flowers due to failure to abort silks in the tassel. Tasselseed5 (Ts5) shares this phenotype but is dominant. Positional cloning and transcriptomics of tassels identified an ectopically expressed gene in the CYP94B subfamily, Ts5 (ZmCYP94B1). CYP94B enzymes are wound inducible and inactivate bioactive jasmonoyl-L-isoleucine (JA-Ile). Consistent with this result, tassels and wounded leaves of Ts5 mutants displayed lower JA and JA-lle precursors and higher 12OH-JA-lle product than the wild type. Furthermore, many wounding and jasmonate pathway genes were differentially expressed in Ts5 tassels. We propose that the Ts5 phenotype results from the interruption of JA signaling during sexual differentiation via the upregulation of ZmCYP94B1 and that its proper expression maintains maize monoecy.

The moss jasmonate ZIM-domain protein PnJAZ1 confers salinity tolerance via crosstalk with the abscisic acid signalling pathway

Abscisic acid (ABA) and jasmonates (JAs) are the primary plant hormones involved in mediating salt tolerance. In addition, these two plant hormones exert a synergistic effect to inhibit seed germination. However, the molecular mechanism of the interaction between ABA signalling and JA signalling is still not well documented. Here, a moss jasmonate ZIM-domain gene (PnJAZ1), which encodes a nucleus-localized protein with conserved ZIM and Jas domains, was cloned from Pohlia nutans. PnJAZ1 expression was rapidly induced by various abiotic stresses. The PnJAZ1 protein physically interacted with MYC2 and was degraded by exogenous 12-oxo-phytodienoic acid (OPDA) treatment, implying that the JAZ protein-mediated signalling pathway is conserved in plants. Transgenic Arabidopsis and Physcomitrella plants overexpressing PnJAZ1 showed increased tolerance to salt stress and decreased ABA sensitivity during seed germination and early development. The overexpression of PnJAZ1 inhibited the expression of ABA pathway genes related to seed germination and seedling growth. Moreover, the transgenic Arabidopsis lines exhibited enhanced tolerance to auxin (IAA) and glucose, mimicking the phenotypes of abi4 or abi5 mutants. These results suggest that PnJAZ1 acts as a repressor, mediates JA-ABA synergistic crosstalk and enhances plant growth under salt stress.

Signaling Crosstalk between Salicylic Acid and Ethylene/Jasmonate in Plant Defense: Do We Understand What They Are Whispering?

During their lifetime, plants encounter numerous biotic and abiotic stresses with diverse modes of attack. Phytohormones, including salicylic acid (SA), ethylene (ET), jasmonate (JA), abscisic acid (ABA), auxin (AUX), brassinosteroid (BR), gibberellic acid (GA), cytokinin (CK) and the recently identified strigolactones (SLs), orchestrate effective defense responses by activating defense gene expression. Genetic analysis of the model plant Arabidopsis thaliana has advanced our understanding of the function of these hormones. The SA- and ET/JA-mediated signaling pathways were thought to be the backbone of plant immune responses against biotic invaders, whereas ABA, auxin, BR, GA, CK and SL were considered to be involved in the plant immune response through modulating the SA-ET/JA signaling pathways. In general, the SA-mediated defense response plays a central role in local and systemic-acquired resistance (SAR) against biotrophic pathogens, such as Pseudomonas syringae, which colonize between the host cells by producing nutrient-absorbing structures while keeping the host alive. The ET/JA-mediated response contributes to the defense against necrotrophic pathogens, such as Botrytis cinerea, which invade and kill hosts to extract their nutrients. Increasing evidence indicates that the SA- and ET/JA-mediated defense response pathways are mutually antagonistic.

The roles of methyl jasmonate to stress in plants

Plants are constantly exposed to various stresses, which can degrade their health. The stresses can be alleviated by the application of methyl jasmonate (MeJA), which is a hormone involved in plant signalling. MeJA induces synthesis of defensive compounds and initiates the expression of pathogenesis-related genes involved in systemic acquired resistance and local resistance. Thus, MeJA may be used against pathogens, salt stress, drought stress, low temperature, heavy metal stress and toxicities of other elements. The application of MeJA improves growth, induces the accumulation of active compounds, and affects endogenous hormones levels, and other physiological and biochemical characteristics in stressed plants. Furthermore, MeJA antagonises the adverse effects of osmotic stress by regulating inorganic penetrating ions or organic penetrants to suppress the absorption of toxic ions. MeJA also mitigates oxidative stress by activating antioxidant systems to scavenge reactive oxygen species (ROS) in stressed plants. For these reasons, we reviewed the use of exogenous MeJA in alleviating biotic (pathogens and insects) and abiotic stresses in plants.

The Gene Structure and Expression Level Changes of the GH3 Gene Family in Brassica napus Relative to Its Diploid Ancestors

The GH3 gene family plays a vital role in the phytohormone-related growth and developmental processes. The effects of allopolyploidization on GH3 gene structures and expression levels have not been reported. In this study, a total of 38, 25, and 66 GH3 genes were identified in Brassica rapa (A_rA_r), Brassica oleracea (C_oC_o), and Brassica napus (A_nAC_nC_n), respectively. BnaGH3 genes were unevenly distributed on chromosomes with 39 on A_n and 27 on C_n, in which six BnaGH3 genes may appear as new genes. The whole genome triplication allowed the GH3 gene family to expand in diploid ancestors, and allopolyploidization made the GH3 gene family re-expand in B. napus. For most BnaGH3 genes, the exon-intron compositions were similar to diploid ancestors, while the cis-element distributions were obviously different from its ancestors. After allopolyploidization, the expression patterns of GH3 genes from ancestor species changed greatly in B. napus, and the orthologous gene pairs between A_n/A_r and C_n/C_o had diverged expression patterns across four tissues. Our study provides a comprehensive analysis of the GH3 gene family in B. napus, and these results could contribute to identifying genes with vital roles in phytohormone-related growth and developmental processes.

Beyond Light: Insights Into the Role of Constitutively Photomorphogenic1 in Plant Hormonal Signaling

Light is an important environmental factor with profound effects in plant growth and development. Constitutively photomorphogenic1 (COP1) is a vital component of the light signaling pathway as a negative regulator of photomorphogenesis. Although the role of COP1 in light signaling has been firmly established for some time, recent studies have proven that COP1 is also a crucial part of multiple plant hormonal regulatory pathways. In this article, we review the available evidence involving COP1 in hormone signaling, its molecular mechanisms, and its contribution to the complicated regulatory network linking light and plant hormone signaling.

BnaMPK3 Is a Key Regulator of Defense Responses to the Devastating Plant Pathogen Sclerotinia sclerotiorum in Oilseed Rape

The disease caused by Sclerotinia sclerotiorum has traditionally been difficult to control, resulting in tremendous economic losses in oilseed rape (Brassica napus). Identification of important genes in the defense responses is critical for molecular breeding, an important strategy for controlling the disease. Here, we report that a B. napus mitogen-activated protein kinase gene, BnaMPK3, plays an important role in the defense against S. sclerotiorum in oilseed rape. BnaMPK3 is highly expressed in the stems, flowers and leaves, and its product is localized in the nucleus. Furthermore, BnaMPK3 is highly responsive to infection by S. sclerotiorum and treatment with jasmonic acid (JA) or the biosynthesis precursor of ethylene (ET), but not to treatment with salicylic acid (SA) or abscisic acid. Moreover, overexpression (OE) of BnaMPK3 in B. napus and Nicotiana benthamiana results in significantly enhanced resistance to S. sclerotiorum, whereas resistance is diminished in RNAi transgenic plants. After S. sclerotiorum infection, defense responses associated with ET, JA, and SA signaling are intensified in the BnaMPK3-OE plants but weakened in the BnaMPK3-RNAi plants when compared to those in the wild type plants; by contrast the level of both H₂O₂ accumulation and cell death exhibits a reverse pattern. The candidate gene association analyses show that the BnaMPK3-encoding BnaA06g18440D locus is a cause of variation in the resistance to S. sclerotiorum in natural B. napus population. These results suggest that BnaMPK3 is a key regulator of multiple defense responses to S. sclerotiorum, which may guide the resistance improvement of oilseed rape and related economic crops.

Hexaacetyl-chitohexaose, a chitin-derived oligosaccharide, transiently activates citrus defenses and alters the feeding behavior of Asian citrus psyllid

Plants have a perception system triggered by pathogen and pest signals to initiate defense. These signals include evolutionarily conserved molecules from microbes and insects termed pathogen/herbivore-associated molecular patterns (PAMPs/HAMPs). Here we showed that hexaacetyl-chitohexaose (HC), an oligosaccharide from chitin, a structural component in insect exoskeletons and fungi cell walls, upregulated defense-associated genes WRKY22, GST1, RAR1, EDS1, PAL1 and NPR2, and downregulated ICS1 at 1 h after HC treatment in Sun Chu Sha mandarin leaves. The effect was transient as defense gene transcriptional changes were not observed at 18 h after the treatment. Electrical penetration graph (EPG) recordings were used to study the feeding behavior of Asian citrus psyllid (ACP) following the HC treatment. ACP is the hemipteran vector of Candidatus Liberibacter asiaticus (CLas), the pathogen associated with huanglongbing (HLB). Adult ACP displayed reduced intercellular probing, reduced xylem feeding count and duration, and increased non-probing activity on HC-treated citrus compared to controls. During an 18-h recording, percentage for total duration of xylem ingestion, phloem ingestion, intercellular probing were lower, and the percentage of non-probing behavior was higher in HC-treated leaves than in controls. In host-selection behavior studies, HC treatment did not alter the attractiveness of citrus leaves under light or dark conditions. In addition, ACP feeding on HC-treated leaves did not show differences in mortality for up to 10 day of exposure. In summary, we report that HC induced a transient defense in citrus and an inhibitory effect on ACP feeding but did not affect host selection or the insect fitness under the tested conditions.

1-Aminocyclopropane 1-Carboxylic Acid and Its Emerging Role as an Ethylene-Independent Growth Regulator

1-Aminocyclopropane 1-carboxylic acid (ACC) is the direct precursor of the plant hormone ethylene. ACC is synthesized from S-adenosyl-L-methionine (SAM) by ACC synthases (ACSs) and subsequently oxidized to ethylene by ACC oxidases (ACOs). Exogenous ACC application has been used as a proxy for ethylene in numerous studies as it is readily converted by nearly all plant tissues to ethylene. However, in recent years, a growing body of evidence suggests that ACC plays a signaling role independent of the biosynthesis. In this review, we briefly summarize our current knowledge of ACC as an ethylene precursor, and present new findings with regards to the post-translational modifications of ACS proteins and to ACC transport. We also summarize the role of ACC in regulating plant development, and its involvement in cell wall signaling, guard mother cell division, and pathogen virulence.

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

Myrosinase enzymes and their substrate glucosinolates provide a specific defensive mechanism against biotic invaders in the Brassicaceae family. In these plants, myrosinase hydrolyzes glucosinolates into diverse products, which can have direct antibiotic activity or function as signaling molecules that initiate a variety of defense reactions. A myrosinase, β-thioglucoside glucohydrolase 1 (TGG1) was previously found to be strikingly abundant in guard cells, and it is required for the abscisic acid (ABA) response of stomata. However, it remains unknown which particular physiological processes actually involve stomatal activity as modulated by TGG1. In this experimental study, a homologous TGG1 gene from broccoli (Brassica oleracea var. italica), BoTGG1, was overexpressed in Arabidopsis. The transgenic plants showed enhanced resistance against the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000 via improved stomatal defense. Upon Pst DC3000 infection, overexpressing BoTGG1 accelerated stomatal closure and inhibited the reopening of stomata. Compared with the wild type, 35S::BoTGG1 was more sensitive to ABA- and salicylic acid (SA)-induced stomatal closure but was less sensitive to indole-3-acetic acid (IAA)-inhibited stomatal closure, thus indicating these hormone signaling pathways were possibly involved in stomatal defense regulated by TGG1. Furthermore, overexpression of BoTGG1 delayed flowering by promoting the expression of FLOWERING LOCUS C (FLC), which encodes a MADS-box transcription factor known as floral repressor. Taken together, our study's results suggest glucosinolate metabolism mediated by TGG1 plays a role in plant stomatal defense against P. syringae and also modulates flowering time by affecting the FLC pathway.

Jasmonate Zim-Domain Protein 9 Interacts With Slender Rice 1 to Mediate the Antagonistic Interaction Between Jasmonic and Gibberellic Acid Signals in Rice

The jasmonic acid (JA) and gibberellic acid (GA) signaling pathways interact to coordinate stress responses and developmental processes. This coordination affects plant growth and yield, and is mediated by interactions between the repressors of each pathway, the JASMONATE ZIM-DOMAIN PROTEIN (JAZ) and DELLA proteins. In this study we attempted to identify rice (Oryza sativa) JAZs that interact with rice DELLAs such as SLENDER RICE 1 (SLR1). Analysis of protein-protein interactions showed that OsJAZ8 and OsJAZ9 interact with SLR1; OsJAZ9 also interacted with the SLR1-LIKE (SLRL) protein SLRL2. Based on this broader interaction, we explored the function of OsJAZ9 in JA and GA responses by analyzing transcript levels of the JA-responsive gene OsbHLH148 and the GA-responsive gene OsPIL14 in OsJAZ9-overexpressing (OsJAZ9-Ox) and osjaz9 mutant plants. OsbHLH148 and OsPIL14 encode key transcription factors controlling JA and GA responses, respectively, and JA and GA antagonistically regulate their expression. In OsJAZ9-Ox, the expression of OsbHLH148 was downregulated and the expression of OsPIL14 was upregulated. By contrast, in osjaz9 mutants, the expression of OsbHLH148 was upregulated and the expression of OsPIL14 was downregulated. These observations indicated that OsJAZ9 regulates both JA and GA responses in rice, and this finding was supported by the opposite expression patterns of OsDREB1s, downstream targets of OsbHLH148 and OsPIL14, in the OsJAZ9-Ox and osjaz9 plants. Together, these findings indicate that OsJAZ9 suppresses JA responses and promotes GA responses in rice, and the protein-protein interaction between OsJAZ9 and SLR1 is involved in the antagonistic interplay between JA and GA.

Genome-wide analysis of cotton GH3 subfamily II reveals functional divergence in fiber development, hormone response and plant architecture

BACKGROUND

Auxin-induced genes regulate many aspects of plant growth and development. The Gretchen Hagen 3 (GH3) gene family, one of three major early auxin-responsive families, is ubiquitous in the plant kingdom and its members function as regulators in modulating hormonal homeostasis, and stress adaptations. Specific Auxin-amido synthetase activity of GH3 subfamily II genes is reported to reversibly inactivate or fully degrade excess auxin through the formation of amino acid conjugates. Despite these crucial roles, to date, genome-wide analysis of the GH3 gene family has not been reported in cotton.

RESULTS

We identified a total of 10 GH3 subfamily II genes in G. arboreum, 10 in G. raimondii, and 20 in G. hirsutum, respectively. Bioinformatic analysis showed that cotton GH3 genes are conserved with the established GH3s in plants. Expression pattern analysis based on RNA-seq data and qRT-PCR revealed that 20 GhGH3 genes were differentially expressed in a temporally and spatially specific manner, indicating their diverse functions in growth and development. We further summarized the organization of promoter regulatory elements and monitored their responsiveness to treatment with IAA (indole-3-acetic acid), SA (salicylic acid), GA (gibberellic acid) and BL (brassinolide) by qRT-PCR in roots and stems. These hormones seemed to regulate the expression of GH3 genes in both a positive and a negative manner while certain members likely have higher sensitivity to all four hormones. Further, we tested the expression of GhGH3 genes in the BR-deficient mutant pag1 and the corresponding wild-type (WT) of CCRI24. The altered expression reflected the true responsiveness to BL and further suggested possible reasons, at least in part, responsible for the dramatic dwarf and shriveled phenotypes of pag1.

CONCLUSION

We comprehensively identified GH3 subfamily II genes in cotton. GhGH3s are differentially expressed in various tissues/organs/stages. Their response to IAA, SA, BL and GA and altered expression in pag1 suggest that some GhGH3 genes might be simultaneously involved in multiple hormone signaling pathways. Taken together, our results suggest that members of the GhGH3 gene family could be possible candidate genes for mechanistic study and applications in cotton fiber development in addition to the reconstruction of plant architecture.

Local and systemic hormonal responses in pepper (Capsicum annuum L.) leaves under green peach aphid (Myzus persicae Sulzer) infestation

This study examined the temporal changes in the leaf content of defence-involved phytohormones in pepper (Capsicum annuum L.) plants responding to the green peach aphid (Myzus persicae Sulzer) infestation, at both local and systemic level. Aphid infestation did not alter the content of cis-12-oxo-phytodienoic acid, the jasmonic acid (JA) precursor, even though endogenous levels of JA and its bioactive isoleucine-conjugated form (JA-Ile) significantly increased from 8 to 96 h in local infested leaves. Systemic effects in jasmonates were only showed at 48 h for JA, and 8 and 48 h in the case of JA-Ile. SA accumulated only in local infested leaves after 96 h of infestation, when the level of JA-Ile decreased in these leaves. This suggests a possible antagonistic interaction between JA and SA pathways, although other pathways may be also involved. Endogenous level of indole-3-acetic acid was higher in systemic relative to local infested leaves at 3 and 24 h, although no significant changes in its content were found compared to control leaves. Abscisic acid content was lower in local infested relative to control leaves at 24 h, but was higher at 48 h when it also increased systemically. The possible roles of the studied phytohormones in plant defence responses against aphids are discussed.

Chitosan Oligosaccharide Induces Resistance to Pseudomonas syringae pv. tomato DC3000 in Arabidopsis thaliana by Activating Both Salicylic Acid- and Jasmonic Acid-Mediated Pathways

Chitosan oligosaccharide (COS) is an effective plant immunity elicitor; however, its induction mechanism in plants is complex and needs further investigation. In this study, the Arabidopsis-Pseudomonas syringae pv. tomato DC3000 (hereafter called DC3000) interaction was used to investigate the induction effect and the underlying mechanisms of COS. COS is effective in inducing resistance to DC3000 in Arabidopsis, and our results demonstrate that treatment with COS 3 days before DC3000 inoculation provided the most effective resistance. Disease severity in jar1 (jasmonic acid [JA]-deficient mutant), NahG, and sid2 (salicylic acid [SA]-deficient mutants) suggest both the SA and JA pathways are required for the Arabidopsis response to DC3000. COS pretreatment induced resistance in wild type (WT), jar1, and also, although to a lesser degree, in NahG and sid2 plants, implying that the SA and JA pathways play redundant roles in COS-induced resistance to DC3000. In COS-pretreated plants, expression of genes related to the SA pathway (PR1, PR2, and PR5) and SA content increased in both WT and jar1. Moreover, expression of genes related to the JA pathway (PDF1.2 and VSP2) and JA content both increased in WT and NahG. In conclusion, COS induces resistance to DC3000 in Arabidopsis by activating both SA- and JA-mediated pathways, although SA and JA pathways play redundant roles in this COS-induced resistance.

Network Analysis Reveals a Role for Salicylic Acid Pathway Components in Shade Avoidance

Plants have sophisticated mechanisms for sensing neighbor shade. To maximize their ability to compete for light, plants respond to shade through enhanced elongation and physiological changes. The shade avoidance response affects many different organs and growth stages, yet the signaling pathways underlying this response have mostly been studied in seedlings. We assayed transcriptome changes in response to shade across a 2-d time course in the wild type and 12 Arabidopsis (Arabidopsis thaliana) mutants. The resulting temporal map of transcriptional responses to shade defines early and late responses in adult plants, enabling us to determine connections between key signaling genes and downstream responses. We found a pervasive and unexpectedly strong connection between shade avoidance and genes related to salicylic acid, suggesting salicylic acid signaling to be an important shade avoidance growth regulator. We tested this connection and found that several mutants disrupting salicylic acid levels or signaling were defective in shade avoidance. The effect of these mutations on shade avoidance was specific to petiole elongation; neither hypocotyl nor flowering time responses were altered, thereby defining important stage-specific differences in the downstream shade avoidance signaling pathway. Shade treatment did not change salicylic acid levels, indicating that the mediation of shade avoidance by salicylic acid is not dependent on the modulation of salicylic acid levels. These results demonstrate that salicylic acid pathway genes also are key components of petiole shade avoidance.

Salt-inducible expression of OsJAZ8 improves resilience against salt-stress

BACKGROUND

Productivity of important crop rice is greatly affected by salinity. The plant hormone jasmonate plays a vital role in salt stress adaptation, but also evokes detrimental side effects if not timely shut down again. As novel strategy to avoid such side effects, OsJAZ8, a negative regulator of jasmonate signalling, is expressed under control of the salt-inducible promoter of the transcription factor ZOS3-11, to obtain a transient jasmonate signature in response to salt stress. To modulate the time course of jasmonate signalling, either a full-length or a dominant negative C-terminally truncated version of OsJAZ8 driven by the ZOS3-11 promoter were expressed in a stable manner either in tobacco BY-2 cells, or in japonica rice.

RESULTS

The transgenic tobacco cells showed reduced mortality and efficient cycling under salt stress adaptation. This was accompanied by reduced sensitivity to Methyl jasmonate and increased responsiveness to auxin. In the case of transgenic rice, the steady-state levels of OsJAZ8 transcripts were more efficiently induced under salt stress compared to the wild type, this induction was more pronounced in the dominant-negative OsJAZ8 variant.

CONCLUSIONS

The result concluded that, more efficient activation of OsJAZ8 was accompanied by improved salt tolerance of the transgenic seedlings and demonstrates the impact of temporal signatures of jasmonate signalling for stress tolerance.

Salt-inducible expression of OsJAZ8 improves resilience against salt-stress

BACKGROUND

Productivity of important crop rice is greatly affected by salinity. The plant hormone jasmonate plays a vital role in salt stress adaptation, but also evokes detrimental side effects if not timely shut down again. As novel strategy to avoid such side effects, OsJAZ8, a negative regulator of jasmonate signalling, is expressed under control of the salt-inducible promoter of the transcription factor ZOS3-11, to obtain a transient jasmonate signature in response to salt stress. To modulate the time course of jasmonate signalling, either a full-length or a dominant negative C-terminally truncated version of OsJAZ8 driven by the ZOS3-11 promoter were expressed in a stable manner either in tobacco BY-2 cells, or in japonica rice.

RESULTS

The transgenic tobacco cells showed reduced mortality and efficient cycling under salt stress adaptation. This was accompanied by reduced sensitivity to Methyl jasmonate and increased responsiveness to auxin. In the case of transgenic rice, the steady-state levels of OsJAZ8 transcripts were more efficiently induced under salt stress compared to the wild type, this induction was more pronounced in the dominant-negative OsJAZ8 variant.

CONCLUSIONS

The result concluded that, more efficient activation of OsJAZ8 was accompanied by improved salt tolerance of the transgenic seedlings and demonstrates the impact of temporal signatures of jasmonate signalling for stress tolerance.

Metabolic Alternations of Amino Acids, γ-Aminobutyric Acid, and Salicylic Acid in Solanum lycopersicum (L.) Following Preplanting Seedling Spray with Salicylic Acid

Preplanting foliar spray of salicylic acid (SA) (0.0, 5.0, and 10.0 μg/mL) to Solanum lycopersicum (L.) altered the metabolite profile of amino acids, γ-aminobutyric acid (GABA), and SA in leaf, root, and fruits. Free amino acid pools increased; bound amino acid pools reduced. In vegetative tissues, amino acid biosyntheses linked to osmo-compatibility (Pro, Leu, Val and GABA); N (Arg, Asn, Asp, Gln, and Glu); C (Pro, Ser, and Tyr); S (Cys) assimilation; stress tolerance (Ala, Gly, Hyp, His, Lys, Met, and Thr); and central metabolism (Phe, Trp, and Tyr) enhanced for 60-120 days. Concentrations of Ala, Arg, Gln, Gly, Leu, and Ser in leaf and of Asp, Cys, Glu, His, Hyp, Lys, Met, Pro, and Val in root predominated. In planta SA and GABA biosynthesis increased concurrently. SA affected GABA biosynthesis via Glu, Pro, and Arg metabolism. SA, GABA, Glu, and Pro were key canonical variables. This study first reported SA-induced metabolites promoting health (SA/GABA; Cys/Met) and palatability (Glu/Asp; Gln) in table tomato.

Jasmonic and salicylic acid response in the fern Azolla filiculoides and its cyanobiont

Plants sense and respond to microbes utilizing a multilayered signalling cascade. In seed plants, the phytohormones jasmonic and salicylic acid (JA and SA) are key denominators of how plants respond to certain microbes. Their interplay is especially well-known for tipping the scales in plants' strategies of dealing with phytopathogens. In non-angiosperm lineages, the interplay is less well understood, but current data indicate that it is intertwined to a lesser extent and the canonical JA/SA antagonism appears to be absent. Here, we used the water fern Azolla filiculoides to gain insights into the fern's JA/SA signalling and the molecular communication with its unique nitrogen fixing cyanobiont Nostoc azollae, which the fern inherits both during sexual and vegetative reproduction. By mining large-scale sequencing data, we demonstrate that Azolla has most of the genetic repertoire to produce and sense JA and SA. Using qRT-PCR on the identified biosynthesis and signalling marker genes, we show that Azolla is responsive to exogenously applied SA. Furthermore, exogenous SA application influenced the abundance and gene expression of Azolla's cyanobiont. Our data provide a framework for JA/SA signalling in ferns and suggest that SA might be involved in Azolla's communication with its vertically inherited cyanobiont.

Members of the GH3 Family of Proteins Conjugate 2,4-D and Dicamba with Aspartate and Glutamate

Auxin homeostasis is a highly regulated process that must be maintained to allow auxin to exert critical growth and developmental controls. Auxin conjugase and hydrolase family proteins play important roles in auxin homeostasis through means of storage, activation, inactivation, response inhibition and degradation of auxins in plants. We systematically evaluated 60 GRETCHEN HAGEN3 (GH3) proteins from diverse plant species for amino acid conjugation activity with the known substrates jasmonic acid (JA), IAA and 4-hydroxybenzoate (4-HBA). While our results largely confirm that Group II conjugases prefer IAA, we observed no clear substrate preference among Group III proteins, and only three of 11 Group I proteins showed the expected preference for JA, indicating that sequence similarity does not always predict substrate specificity. Such a sequence-substrate relationship held true when sequence similarity at the acyl acid-binding site was used for grouping. Several GH3 proteins could catalyze formation of the potentially degradation-destined aspartate (Asp) and glutamate (Glu) conjugates of IAA and the synthetic auxins 2,4-D and dicamba. We found that 2,4-D-Asp/Glu conjugates, but not dicamba and IAA conjugates, were hydrolyzed in Arabidopsis and soybean by AtILL5- and AtIAR3-like amidohydrolases, releasing free 2,4-D in plant cells when conjugates were exogenously applied to seedlings. Dicamba-Asp or dicamba-Glu conjugates were not hydrolyzed in vivo in infiltrated plants nor in vitro with recombinant amidohydrolases. These findings could open the door for exploration of a dicamba herbicide tolerance strategy through conjugation.

From Acetoin to ( Z)-3-Hexen-1-ol: The Diversity of Volatile Organic Compounds that Induce Plant Responses

Evidence that plants can respond to volatile organic compounds (VOCs) was first presented 35 years ago. Since then, over 40 VOCs have been found to induce plant responses. These include VOCs that are produced not only by plants but also by microbes and insects. Here, we summarize what is known about how these VOCs are produced and how plants detect and respond to them. In doing so, we highlight notable observations we believe are worth greater consideration. For example, the VOCs that induce plant responses appear to have little in common. They are derived from many different biosynthetic pathways and have few distinguishing chemical or structural features. Likewise, plants appear to use several mechanisms to detect VOCs rather than a single dedicated "olfactory" system. Considering these observations, we advocate for more discovery-oriented experiments and propose that future research take a fresh look at the ways plants detect and respond to VOCs.

A rationally designed JAZ subtype-selective agonist of jasmonate perception

The phytohormone 7-iso-(+)-jasmonoyl-L-isoleucine (JA-Ile) mediates plant defense responses against herbivore and pathogen attack, and thus increases plant resistance against foreign invaders. However, JA-Ile also causes growth inhibition; and therefore JA-Ile is not a practical chemical regulator of plant defense responses. Here, we describe the rational design and synthesis of a small molecule agonist that can upregulate defense-related gene expression and promote pathogen resistance at concentrations that do not cause growth inhibition in Arabidopsis. By stabilizing interactions between COI1 and JAZ9 and JAZ10 but no other JAZ isoforms, the agonist leads to formation of JA-Ile co-receptors that selectively activate the JAZ9-EIN3/EIL1-ORA59 signaling pathway. The design of a JA-Ile agonist with high selectivity for specific protein subtypes may help promote the development of chemical regulators that do not cause a tradeoff between growth and defense.

Jasmonates: News on Occurrence, Biosynthesis, Metabolism and Action of an Ancient Group of Signaling Compounds

: Jasmonic acid (JA) and its related derivatives are ubiquitously occurring compounds of land plants acting in numerous stress responses and development. Recent studies on evolution of JA and other oxylipins indicated conserved biosynthesis. JA formation is initiated by oxygenation of α-linolenic acid (α-LeA, 18:3) or 16:3 fatty acid of chloroplast membranes leading to 12-oxo-phytodienoic acid (OPDA) as intermediate compound, but in Marchantiapolymorpha and Physcomitrellapatens, OPDA and some of its derivatives are final products active in a conserved signaling pathway. JA formation and its metabolic conversion take place in chloroplasts, peroxisomes and cytosol, respectively. Metabolites of JA are formed in 12 different pathways leading to active, inactive and partially active compounds. The isoleucine conjugate of JA (JA-Ile) is the ligand of the receptor component COI1 in vascular plants, whereas in the bryophyte M. polymorpha COI1 perceives an OPDA derivative indicating its functionally conserved activity. JA-induced gene expressions in the numerous biotic and abiotic stress responses and development are initiated in a well-studied complex regulation by homeostasis of transcription factors functioning as repressors and activators.

Accumulation of 9- and 13-KODEs in response to jasmonic acid treatment and pathogenic infection in rice

The inducible metabolites in rice leaves treated with 1 mM jasmonic acid (JA) were analyzed using HPLC. We detected an increase in the levels of two compounds, 1 and 2. Based on the comparison with mass spectra and chromatographic behavior with authentic compounds, 1 and 2 were identified as 13-oxooctadeca-9,11-dienoic acid (13-KODE) and 9-oxooctadeca-10,12-dienoic acid (9-KODE), respectively, which have not been detected in rice to date. The accumulation of these compounds was also induced by an infection by Bipolaris oryzae. Treatment of rice leaves with KODEs induced the accumulation of defensive secondary metabolites, sakuranetin, naringenin, and serotonin, suggesting that KODEs may play a role in the elicitation of defense responses. The compounds that have an α, β-unsaturated carbonyl group similar to KODEs did not reproduce the response of accumulation of defensive secondary metabolites, suggesting that additional structural factors such as long hydrophobic carbon chain are needed to elicit defense responses.