Plant perception of β-aminobutyric acid is mediated by an aspartyl-tRNA synthetase

Phytohormones in a universe of regulatory metabolites: lessons from jasmonate

Small-molecule phytohormones exert control over plant growth, development, and stress responses by coordinating the patterns of gene expression within and between cells. Increasing evidence indicates that currently recognized plant hormones are part of a larger group of regulatory metabolites that have acquired signaling properties during the evolution of land plants. This rich assortment of chemical signals reflects the tremendous diversity of plant secondary metabolism, which offers evolutionary solutions to the daunting challenges of sessility and other unique aspects of plant biology. A major gap in our current understanding of plant regulatory metabolites is the lack of insight into the direct targets of these compounds. Here, we illustrate the blurred distinction between classical phytohormones and other bioactive metabolites by highlighting the major scientific advances that transformed the view of jasmonate from an interesting floral scent to a potent transcriptional regulator. Lessons from jasmonate research generally apply to other phytohormones and thus may help provide a broad understanding of regulatory metabolite-protein interactions. In providing a framework that links small-molecule diversity to transcriptional plasticity, we hope to stimulate future research to explore the evolution, functions, and mechanisms of perception of a broad range of plant regulatory metabolites.

Translation machinery: the basis of translational control

Messenger RNA (mRNA) translation consists of initiation, elongation, termination, and ribosome recycling, carried out by the translation machinery, primarily including tRNAs, ribosomes, and translation factors (TrFs). Translational regulators transduce signals of growth and development, as well as biotic and abiotic stresses, to the translation machinery, where global or selective translational control occurs to modulate mRNA translation efficiency (TrE). As the basis of translational control, the translation machinery directly determines the quality and quantity of newly synthesized peptides and, ultimately, the cellular adaption. Thus, regulating the availability of diverse machinery components is reviewed as the central strategy of translational control. We provide classical signaling pathways (e.g., integrated stress responses) and cellular behaviors (e.g., liquid-liquid phase separation) to exemplify this strategy within different physiological contexts, particularly during host-microbe interactions. With new technologies developed, further understanding this strategy will speed up translational medicine and translational agriculture.

Join the green team: Inducers of plant immunity in the plant disease sustainable control toolbox

BACKGROUND

Crops are constantly attacked by various pathogens. These pathogenic microorganisms, such as fungi, oomycetes, bacteria, viruses, and nematodes, threaten global food security by causing detrimental crop diseases that generate tremendous quality and yield losses worldwide. Chemical pesticides have undoubtedly reduced crop damage; however, in addition to increasing the cost of agricultural production, the extensive use of chemical pesticides comes with environmental and social costs. Therefore, it is necessary to vigorously develop sustainable disease prevention and control strategies to promote the transition from traditional chemical control to modern green technologies. Plants possess sophisticated and efficient defense mechanisms against a wide range of pathogens naturally. Immune induction technology based on plant immunity inducers can prime plant defense mechanisms and greatly decrease the occurrence and severity of plant diseases. Reducing the use of agrochemicals is an effective way to minimize environmental pollution and promote agricultural safety.

AIM OF REVIEW

The purpose of this workis to offer valuable insights into the current understanding and future research perspectives of plant immunity inducers and their uses in plant disease control, ecological and environmental protection, and sustainable development of agriculture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

In this work, we have introduced the concepts of sustainable and environment-friendly concepts of green disease prevention and control technologies based on plant immunity inducers. This article comprehensively summarizes these recent advances, emphasizes the importance of sustainable disease prevention and control technologies for food security, and highlights the diverse functions of plant immunity inducers-mediated disease resistance. The challenges encountered in the potential applications of plant immunity inducers and future research orientation are also discussed.

Glutamine Metabolism, Sensing and Signaling in Plants

Glutamine (Gln) is the first amino acid synthesized in nitrogen (N) assimilation in plants. Gln synthetase (GS), converting glutamate (Glu) and NH4+ into Gln at the expense of ATP, is one of the oldest enzymes in all life domains. Plants have multiple GS isoenzymes that work individually or cooperatively to ensure that the Gln supply is sufficient for plant growth and development under various conditions. Gln is a building block for protein synthesis and an N-donor for the biosynthesis of amino acids, nucleic acids, amino sugars and vitamin B coenzymes. Most reactions using Gln as an N-donor are catalyzed by Gln amidotransferase (GAT) that hydrolyzes Gln to Glu and transfers the amido group of Gln to an acceptor substrate. Several GAT domain-containing proteins of unknown function in the reference plant Arabidopsis thaliana suggest that some metabolic fates of Gln have yet to be identified in plants. In addition to metabolism, Gln signaling has emerged in recent years. The N regulatory protein PII senses Gln to regulate arginine biosynthesis in plants. Gln promotes somatic embryogenesis and shoot organogenesis with unknown mechanisms. Exogenous Gln has been implicated in activating stress and defense responses in plants. Likely, Gln signaling is responsible for some of the new Gln functions in plants.

Elevated CO2 alters photosynthesis, growth and susceptibility to powdery mildew of oak seedlings

Elevated CO2 (eCO2) is a determinant factor of climate change and is known to alter plant processes such as physiology, growth and resistance to pathogens. Quercus robur, a tree species integrated in most forest regeneration strategies, shows high vulnerability to powdery mildew (PM) disease at the seedling stage. PM is present in most oak forests and it is considered a bottleneck for oak woodland regeneration. Our study aims to decipher the effect of eCO2 on plant responses to PM. Oak seedlings were grown in controlled environment at ambient (aCO2, ∼400 ppm) and eCO2 (∼1000 ppm), and infected with Erysiphe alphitoides, the causal agent of oak PM. Plant growth, physiological parameters and disease progression were monitored. In addition, to evaluate the effect of eCO2 on induced resistance (IR), these parameters were assessed after treatments with IR elicitor β-aminobutyric acid (BABA). Our results show that eCO2 increases photosynthetic rates and aerial growth but in contrast, reduces root length. Importantly, under eCO2 seedlings were more susceptible to PM. Treatments with BABA protected seedlings against PM and this protection was maintained under eCO2. Moreover, irrespectively of the concentration of CO2, BABA did not significantly change aerial growth but resulted in longer radicular systems, thus mitigating the effect of eCO2 in root shortening. Our results demonstrate the impact of eCO2 in plant physiology, growth and defence, and warrant further biomolecular studies to unravel the mechanisms by which eCO2 increases oak seedling susceptibility to PM.

Aspartyl tRNA-synthetase (AspRS) gene family enhances drought tolerance in poplar through BABA-PtrIBIs-PtrVOZ signaling module

BACKGROUND

Drought stress is a prevalent abiotic stress that significantly hinders the growth and development of plants. According to studies, β-aminobutyric acid (BABA) can influence the ABA pathway through the AtIBI1 receptor gene to enhance cold resistance in Arabidopsis. However, the Aspartate tRNA-synthetase (AspRS) gene family, which acts as the receptor for BABA, has not yet been investigated in poplar. Particularly, it is uncertain how the AspRS gene family (PtrIBIs)r can resist drought stress after administering various concentrations of BABA to poplar.

RESULTS

In this study, we have identified 12 AspRS family genes and noted that poplar acquired four PtrIBI pairs through whole genome duplication (WGD). We conducted cis-action element analysis and found a significant number of stress-related action elements on different PtrIBI genes promoters. The expression of most PtrIBI genes was up-regulated under beetle and mechanical damage stresses, indicating their potential role in responding to leaf damage stress. Our results suggest that a 50 mM BABA treatment can alleviate the damage caused by drought stress in plants. Additionally, via transcriptome sequencing, we observed that the partial up-regulation of BABA receptor genes, PtrIBI2/4/6/8/11, in poplars after drought treatment. We hypothesize that poplar responds to drought stress through the BABA-PtrIBIs-PtrVOZ coordinated ABA signaling pathway. Our research provides molecular evidence for understanding how plants respond to drought stress through external application of BABA.

CONCLUSIONS

In summary, our study conducted genome-wide analysis of the AspRS family of P. trichocarpa and identified 12 PtrIBI genes. We utilized genomics and bioinformatics to determine various characteristics of PtrIBIs such as chromosomal localization, evolutionary tree, gene structure, gene doubling, promoter cis-elements, and expression profiles. Our study found that certain PtrIBI genes are regulated by drought, beetle, and mechanical damage implying their crucial role in enhancing poplar stress tolerance. Additionally, we observed that external application of low concentrations of BABA increased plant drought resistance under drought stress. Through the BABA-PtrIBIs-PtrVOZ signaling module, poplar plants were able to transduce ABA signaling and regulate their response to drought stress. These results suggest that the PtrIBI genes in poplar have the potential to improve drought tolerance in plants through the topical application of low concentrations of BABA.

Chemical priming of plant defense responses to pathogen attacks

Plants can acquire an improved resistance against pathogen attacks by exogenous application of natural or artificial compounds. In a process called chemical priming, application of these compounds causes earlier, faster and/or stronger responses to pathogen attacks. The primed defense may persist over a stress-free time (lag phase) and may be expressed also in plant organs that have not been directly treated with the compound. This review summarizes the current knowledge on the signaling pathways involved in chemical priming of plant defense responses to pathogen attacks. Chemical priming in induced systemic resistance (ISR) and systemic acquired resistance (SAR) is highlighted. The roles of the transcriptional coactivator NONEXPRESSOR OF PR1 (NPR1), a key regulator of plant immunity, induced resistance (IR) and salicylic acid signaling during chemical priming are underlined. Finally, we consider the potential usage of chemical priming to enhance plant resistance to pathogens in agriculture.

Simultaneous determination of all aminobutyric acids by chiral derivatization and liquid chromatography-tandem mass spectrometry

Aminobutyric acids include eight structural or stereoisomers that exhibit a wide range of biological activities. Recent evidence on some low abundant isomers have increased the demand for highly selective analysis of all the isomers; however, simultaneous separation of all the aminobutyric acid isomers has not been successful yet, except for a specialized method that uses multiple separation columns and a split of samples. In this study, we developed a new analytical method using chiral derivatization and liquid chromatography-tandem mass spectrometry to separate all the aminobutyric acid isomers in a single separation column. All the diastereomeric derivatives were resolved in a C18 column, and the derivatives showed characteristic fragmentation patterns in tandem mass spectrometry. By using the method, we analyzed the isomers in the Arabidopsis thaliana seeds and revealed the existence of three low abundant isomers, i.e., D-, L-β-aminoisobutyric acid, and D-β-aminobutyric acid. The proposed method uses a commercially available chiral derivatizing reagent and a broadly used column; therefore, it can be widely used in biological and food analyses.

Plant HEM1 specifies a condensation domain to control immune gene translation

Translational reprogramming is a fundamental layer of immune regulation, but how such a global regulatory mechanism operates remains largely unknown. Here we perform a genetic screen and identify Arabidopsis HEM1 as a global translational regulator of plant immunity. The loss of HEM1 causes exaggerated cell death to restrict bacterial growth during effector-triggered immunity (ETI). By improving ribosome footprinting, we reveal that the hem1 mutant increases the translation efficiency of pro-death immune genes. We show that HEM1 contains a plant-specific low-complexity domain (LCD) absent from animal homologues. This LCD endows HEM1 with the capability of phase separation in vitro and in vivo. During ETI, HEM1 interacts and condensates with the translation machinery; this activity is promoted by the LCD. CRISPR removal of this LCD causes more ETI cell death. Our results suggest that HEM1 condensation constitutes a brake mechanism of immune activation by controlling the tissue health and disease resistance trade-off during ETI.

β-Aminobutyric acid induced phytotoxicity and effectiveness against nematode is stereomer-specific and dose-dependent in tomato

β-Aminobutyric acid (BABA) induces resistance to a/biotic stress but is associated with phytotoxicity in some plant species. There are two enantiomers of BABA, the R and S enantiomers. We evaluated the phytotoxicity caused by the RS BABA (racemic mixture of R and S BABA), evaluating the dose-response effect and different modes of application on tomato. Results show that RS BABA-induced phytotoxicity in tomato is dose-dependent and stronger with foliar applications than with soil drench. We further evaluated the phytotoxicity of the two enantiomers separately and observed that BABA-induced phytotoxicity is stereomer-specific. In comparison with less phytotoxic effects induced by S BABA, R BABA induces dose-dependent and systemic phytotoxic symptoms. To investigate the possible physiological causes of this phytotoxicity, we measured levels of oxidative stress and anthocyanins and validated the findings with gene expression analyses. Our results show that high doses of RS and R BABA induce hydrogen peroxide, lipid peroxidation, and anthocyanin accumulation in tomato leaves, while this response is milder and more transient upon S BABA application. Next, we evaluated BABA induced resistance against root-knot nematode Meloidogyne incognita in tomato. BABA-induced resistance was found to be stereomer-specific and dependent on dose and mode of application. R or RS BABA multiple soil drench application at low doses induces resistance to nematodes with less phytotoxic effects. Taken together, our data provide useful knowledge on how BABA can be applied in crop production by enhancing stress tolerance and limiting phytotoxicity.

Distinct chemical resistance-inducing stimuli result in common transcriptional, metabolic, and nematode community signatures in rice root and rhizosphere

Induced resistance (IR), a phenotypic state induced by an exogenous stimulus and characterized by enhanced resistance to future (a)biotic challenge, is an important component of plant immunity. Numerous IR-inducing stimuli have been described in various plant species, but relatively little is known about 'core' systemic responses shared by these distinct IR stimuli and the effects of IR on plant-associated microbiota. In this study, rice (Oryza sativa) leaves were treated with four distinct IR stimuli (β-aminobutyric acid, acibenzolar-S-methyl, dehydroascorbic acid, and piperonylic acid) capable of inducing systemic IR against the root-knot nematode Meloidogyne graminicola and evaluated their effect on the root transcriptome and exudome, and root-associated nematode communities. Our results reveal shared transcriptional responses-notably induction of jasmonic acid and phenylpropanoid metabolism-and shared alterations to the exudome that include increased amino acid, benzoate, and fatty acid exudation. In rice plants grown in soil from a rice field, IR stimuli significantly affected the composition of rhizosphere nematode communities 3 d after treatment, but by 14 d after treatment these changes had largely reverted. Notably, IR stimuli did not reduce nematode diversity, which suggests that IR might offer a sustainable option for managing plant-parasitic nematodes.

A single amino acid transporter controls the uptake of priming-inducing beta-amino acids and the associated tradeoff between induced resistance and plant growth

Selected β-amino acids, such as β-aminobutyric acid (BABA) and R-β-homoserine (RBH), can prime plants for resistance against a broad spectrum of diseases. Here, we describe a genome-wide screen of fully annotated Arabidopsis thaliana T-DNA insertion lines for impaired in RBH-induced immunity (iri) mutants against the downy mildew pathogen Hyaloperonospora arabidopsidis, yielding 104 lines that were partially affected and four lines that were completely impaired in RBH-induced resistance (IR). We confirmed the iri1-1 mutant phenotype with an independent T-DNA insertion line in the same gene, encoding the high-affinity amino acid transporter LYSINE HISTIDINE TRANSPORTER 1 (LHT1). Uptake experiments with yeast cells expressing LHT1 and mass spectrometry-based quantification of RBH and BABA in leaves of lht1 mutant and LHT1 overexpression lines revealed that LHT1 acts as the main transporter for cellular uptake and systemic distribution of RBH and BABA. Subsequent characterization of lht1 mutant and LHT1 overexpression lines for IR and growth responses revealed that the levels of LHT1-mediated uptake determine the tradeoff between IR and plant growth by RBH and BABA.

The Shared Proteome of the Apomictic Fern Dryopteris affinis ssp. affinis and Its Sexual Relative Dryopteris oreades

Ferns are a diverse evolutionary lineage, sister to the seed plants, which is of great ecological importance and has a high biotechnological potential. Fern gametophytes represent one of the simplest autotrophic, multicellular plant forms and show several experimental advantages, including a simple and space-efficient in vitro culture system. However, the molecular basis of fern growth and development has hardly been studied. Here, we report on a proteomic study that identified 417 proteins shared by gametophytes of the apogamous fern Dryopteris affinis ssp. affinis and its sexual relative Dryopteris oreades. Most proteins are predicted to localize to the cytoplasm, the chloroplast, or the nucleus, and are linked to enzymatic, binding, and structural activities. A subset of 145 proteins are involved in growth, reproduction, phytohormone signaling and biosynthesis, and gene expression, including homologs of SHEPHERD (SHD), HEAT SHOCK PROTEIN 90-5 (CR88), TRP4, BOBBER 1 (BOB1), FLAVONE 3'-O-METHYLTRANSFERASE 1 (OMT1), ZEAXANTHIN EPOXIDASE (ABA1), GLUTAMATE DESCARBOXYLASE 1 (GAD), and dsRNA-BINDING DOMAIN-LIKE SUPERFAMILY PROTEIN (HLY1). Nearly 25% of the annotated proteins are associated with responses to biotic and abiotic stimuli. As for biotic stress, the proteins PROTEIN SGT1 HOMOLOG B (SGT1B), SUPPRESSOR OF SA INSENSITIVE2 (SSI2), PHOSPHOLIPASE D ALPHA 1 (PLDALPHA1), SERINE/THREONINE-PROTEIN KINASE SRK2E (OST1), ACYL CARRIER PROTEIN 4 (ACP4), and NONHOST RESISTANCE TO P. S. PHASEOLICOLA1 (GLPK) are worth mentioning. Regarding abiotic stimuli, we found proteins associated with oxidative stress: SUPEROXIDE DISMUTASE[CU-ZN] 1 (CSD1), and GLUTATHIONE S-TRANSFERASE U19 (GSTU19), light intensity SERINE HYDROXYMETHYLTRANSFERASE 1 (SHM1) and UBIQUITIN-CONJUGATING ENZYME E2 35 (UBC35), salt and heavy metal stress included MITOCHONDRIAL PHOSPHATE CARRIER PROTEIN 3 (PHT3;1), as well as drought and thermotolerance: LEA7, DEAD-BOX ATP-DEPENDENT RNA HELICASE 38 (LOS4), and abundant heat-shock proteins and other chaperones. In addition, we identified interactomes using the STRING platform, revealing protein-protein associations obtained from co-expression, co-occurrence, text mining, homology, databases, and experimental datasets. By focusing on ferns, this proteomic study increases our knowledge on plant development and evolution, and may inspire future applications in crop species.

L-Aspartate and L-Glutamine Inhibit Beta-Aminobutyric Acid-Induced Resistance in Tomatoes

Plant diseases caused by pathogens lead to economic and agricultural losses, while plant resistance is defined by robustness and timing of defence response. Exposure to microbial-associated molecular patterns or specific chemical compounds can promote plants into a primed state with more robust defence responses. β-aminobutyric acid (BABA) is an endogenous stress metabolite that induces resistance, thereby protecting various plants' diverse stresses by induction of non-canonical activity after binding into aspartyl-tRNA synthetase (AspRS). In this study, by integrating BABA-induced changes in selected metabolites and transcript data, we describe the molecular processes involved in BABA-induced resistance (BABA-IR) in tomatoes. BABA significantly restricted the growth of the pathogens P. syringae pv. tomato DC3000 and was related to the accumulation of transcripts for pathogenesis-related proteins and jasmonic acid signalling but not salicylic acid signalling in Arabidopsis. The resistance was considerably reduced by applying amino acids L-Asp and L-Gln when L-Gln prevents general amino acid inhibition in plants. Analysis of amino acid changes suggests that BABA-IR inhibition by L-Asp is due to its rapid metabolisation to L-Gln and not its competition with BABA for the aspartyl-tRNA synthetase (AspRS) binding site. Our results showed differences between the effect of BABA on tomatoes and other model plants. They highlighted the importance of comparative studies between plants of agronomic interest subjected to treatment with BABA.

Amino acids and their derivatives mediating defense priming and growth tradeoff

Plant response to pathogens attacks generally comes at the expense of growth. Defense priming is widely accepted as an efficient strategy used for augmenting resistance with reduced fitness in terms of growth and yield. Plant-derived small molecules, both primary as well as secondary metabolites, can function as activators to prime plant defense. Amino acids and their derivatives regulate numerous aspects of plant growth and development, and biotic and abiotic stress responses. In this review, we discuss the recent progress in understanding the roles of amino acids and related molecules in defense priming and their link with plant growth. We also highlight some of the outstanding questions and provide an outlook on the prospects of 'engineering' the tradeoff between defense and growth in plants.

Immune priming in plants: from the onset to transgenerational maintenance

Enhancing plant resistance against pests and diseases by priming plant immunity is an attractive concept for crop protection because it provides long-lasting broad-spectrum protection against pests and diseases. This review provides a selected overview of the latest advances in research on the molecular, biochemical and epigenetic drivers of plant immune priming. We review recent findings about the perception and signalling mechanisms controlling the onset of priming by the plant stress metabolite β-aminobutyric acid. In addition, we review the evidence for epigenetic regulation of long-term maintenance of priming and discuss how stress-induced reductions in DNA hypomethylation at transposable elements can prime defence genes. Finally, we examine how priming can be exploited in crop protection and articulate the opportunities and challenges of translating research results from the Arabidopsis model system to crops.

Review: Emerging roles of the signaling network of the protein kinase GCN2 in the plant stress response

The pan-eukaryotic protein kinase GCN2 (General Control Nonderepressible2) regulates the translation of mRNAs in response to external and metabolic conditions. Although GCN2 and its substrate, translation initiation factor 2 (eIF2) α, and several partner proteins are substantially conserved in plants, this kinase has assumed novel functions in plants, including in innate immunity and retrograde signaling between the chloroplast and cytosol. How exactly some of the biochemical paradigms of the GCN2 system have diverged in the green plant lineage is only partially resolved. Specifically, conflicting data underscore and cast doubt on whether GCN2 regulates amino acid biosynthesis; also whether phosphorylation of eIF2α can in fact repress global translation or activate mRNA specific translation via upstream open reading frames; and whether GCN2 is controlled in vivo by the level of uncharged tRNA. This review examines the status of research on the eIF2α kinase, GCN2, its function in the response to xenobiotics, pathogens, and abiotic stress conditions, and its rather tenuous role in the translational control of mRNAs.

The Rice Aspartyl-tRNA Synthetase YLC3 Regulates Amino Acid Homeostasis and Chloroplast Development Under Low Temperature

Aminoacyl tRNA synthetases primarily function to attach specific amino acids to the corresponding tRNAs during protein translation. However, their roles in regulating plant growth and development still remain elusive. Here we reported a rice thermo-sensitive mutant yellow leaf chlorosis3 (ylc3) with reduced chlorophyll content, altered thylakoid structure, and substantially elevated levels of free aspartate, asparagine and glutamine in leaves under low temperature condition. Map-based cloning identified that YLC3 encodes an aspartyl-tRNA synthetase which is localized in cytosol and mitochondria. In addition, quantitative proteomics analysis revealed that both nuclear and chloroplast-encoded thylakoid proteins were significantly down-regulated in the mutant. On the other hand, proteins involved in amino acid metabolism and the process of protein synthesis were up-regulated in ylc3, particularly for key enzymes that convert aspartate to asparagine. Moreover, uncharged tRNA-Asp accumulation and phosphorylation of the translation initiation factor eIF2α was detected in the mutant, suggesting that YLC3 regulates the homeostasis of amino acid metabolism and chloroplast thylakoid development through modulation of processes during protein synthesis.

Extracellular DNA as an elicitor of broad-spectrum resistance in Arabidopsis thaliana

Like in mammals, the plant immune system has evolved to perceive damage. Damaged-associated molecular patterns (DAMPs) are endogenous signals generated in wounded or infected tissue after pathogen or insect attack. Although extracellular DNA (eDNA) is a DAMP signal that induces immune responses, plant responses after eDNA perception remain largely unknown. Here, we report that signaling defenses but not direct defense responses are induced after eDNA applications enhancing broad-range plant protection. A screening of defense signaling and hormone biosynthesis marker genes revealed that OXI1, CML37 and MPK3 are relevant eDNA-Induced Resistance markers (eDNA-IR). Additionally, we observed that eDNA from several Arabidopsis ecotypes and other phylogenetically distant plants such as citrus, bean and, more surprisingly, a monocotyledonous plant such as maize upregulates eDNA-IR marker genes. Using 3,3'-Diaminobenzidine (DAB) and aniline blue staining methods, we observed that H₂O₂ but not callose was strongly accumulated following self-eDNA treatments. Finally, eDNA resulted in effective induced resistance in Arabidopsis against the pathogens Hyaloperonospora arabidopsidis, Pseudomonas syringae, and Botrytis cinerea and against aphid infestation, reducing the number of nymphs and moving forms. Hence, the unspecificity of DNA origin and the wide range of insects to which eDNA can protect opens many questions about the mechanisms behind eDNA-IR.

Disclosure of salicylic acid and jasmonic acid-responsive genes provides a molecular tool for deciphering stress responses in soybean

Hormones orchestrate the physiology of organisms. Measuring the activity of defense hormone-responsive genes can help understanding immune signaling and facilitate breeding for plant health. However, different from model species like Arabidopsis, genes that respond to defense hormones salicylic acid (SA) and jasmonic acid (JA) have not been disclosed in the soybean crop. We performed global transcriptome analyses to fill this knowledge gap. Upon exogenous application, endogenous levels of SA and JA increased in leaves. SA predominantly activated genes linked to systemic acquired resistance and defense signaling whereas JA mainly activated wound response-associated genes. In general, SA-responsive genes were activated earlier than those responding to JA. Consistent with the paradigm of biotrophic pathogens predominantly activating SA responses, free SA and here identified most robust SA marker genes GmNIMIN1, GmNIMIN1.2 and GmWRK40 were induced upon inoculation with Phakopsora pachyrhizi, whereas JA marker genes did not respond to infection with the biotrophic fungus. Spodoptera exigua larvae caused a strong accumulation of JA-Ile and JA-specific mRNA transcripts of GmBPI1, GmKTI1 and GmAAT whereas neither free SA nor SA-marker gene transcripts accumulated upon insect feeding. Our study provides molecular tools for monitoring the dynamic accumulation of SA and JA, e.g. in a given stress condition.

The rise, fall and resurrection of chemical-induced resistance agents

Since the discovery that the plant immune system could be augmented for improved deployment against biotic stressors through the exogenous application of chemicals that lead to induced resistance (IR), many such IR-eliciting agents have been identified. Initially it was hoped that these chemical IR agents would be a benign alternative to traditional chemical biocides. However, owing to low efficacy and/or a realization that their benefits sometimes come at the cost of growth and yield penalties, chemical IR agents fell out of favour and were seldom used as crop protection products. Despite the lack of interest in agricultural use, researchers have continued to explore the efficacy and mechanisms of chemical IR. Moreover, as we move away from the approach of 'zero tolerance' toward plant pests and pathogens toward integrated pest management, chemical IR agents could have a place in the plant protection product list. In this review, we chart the rise and fall of chemical IR agents, and then explore a variety of strategies used to improve their efficacy and remediate their negative adverse effects. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Induced Resistance by Ascorbate Oxidation Involves Potentiating of the Phenylpropanoid Pathway and Improved Rice Tolerance to Parasitic Nematodes

Anticipating an increased ecological awareness, scientists have been exploring new strategies to reduce the use of chemical pesticides to control pests and diseases. Triggering the intrinsic plant defense system is one of the promising strategies to reduce yield loss by pathogenic organisms, such as nematodes. Ascorbate oxidase (AO) enzyme plays an important role in plant defense by regulating the apoplastic ascorbate/dehydroascorbate (DHA) ratio via the ascorbate oxidation process. Ascorbate oxidation is known to induce systemic resistance in rice against parasitic root-knot nematodes (RKN). Here, we sought to evaluate if AO- or DHA-induced resistance (IR) against RKN M. graminicola involves activation of the phenylpropanoid pathway and whether this IR phenotype has potential effects on growth of rice seedlings under stressed and unstressed conditions. Our results show that AO/DHA-IR against these parasitic nematodes is dependent on activation of phenylalanine ammonia lyase (PAL). However, application of reduced ascorbic acid (AA) did not induce this response. Gene expression analysis via qRT-PCR showed that OsPAL2 and OsPAL4 are highly expressed in AO/DHA-sprayed nematode-infected roots and PAL-activity measurements confirmed that AO/DHA spraying triggers the plants for primed activation of this enzyme upon nematode infection. AO/DHA-IR is not effective in plants sprayed with a chemical PAL inhibitor confirming that AO/DHA-induced resistance is dependent on PAL activity. Improved plant growth and low nematode infection in AO/DHA-sprayed plants was found to be correlated with an increase in shoot chlorophyll fluorescence (Fv/Fm), chlorophyll index (ChlIdx), and modified anthocyanin reflection index which were proven to be good above-ground parameters for nematode infestation. A detailed growth analysis confirmed the improved growth of AO/DHA-treated plants under nematode-infected conditions. Taken together, our results indicate that ascorbate oxidation enhances the phenylpropanoid-based response to nematode infection and leads to a tolerance phenotype in treated rice plants.

Understanding and Exploiting Post-Translational Modifications for Plant Disease Resistance

Plants are constantly threatened by pathogens, so have evolved complex defence signalling networks to overcome pathogen attacks. Post-translational modifications (PTMs) are fundamental to plant immunity, allowing rapid and dynamic responses at the appropriate time. PTM regulation is essential; pathogen effectors often disrupt PTMs in an attempt to evade immune responses. Here, we cover the mechanisms of disease resistance to pathogens, and how growth is balanced with defence, with a focus on the essential roles of PTMs. Alteration of defence-related PTMs has the potential to fine-tune molecular interactions to produce disease-resistant crops, without trade-offs in growth and fitness.

β-Aminobutyric Acid Induced Resistance against Alternaria Fruit Rot in Apple Fruits

Fruit body rot and calyx rot caused by Alternaria alternata f. sp. mali is an important disease of apple worldwide. The disease has recently become severe in cv. Pink Lady apple in Israel to an extent that has never been reported elsewhere in the world. No alternative control measures of the disease except fungicides are known. Here, we show for the first time that dl-β-aminobutyric acid (BABA) induces resistance against Alternaria fruit rot (AFR) in apple fruits in the laboratory and in the orchard. AFR was inhibited in fruits treated with BABA of 1000 μg/mL. BABA did not inhibit spore germination or mycelial growth of the pathogen in vitro (up to 2000 μg/mL). It was most inhibitory when applied 4 days prior to inoculation of detached fruits. BABA inhibited AFR also curatively when applied at 24 h post inoculation. Five other isomers of aminobutyric acid failed to protect the fruits from rot formation. Three field trials in commercial apple orchards proved that BABA was as protective against AFR as the commercial standard fungicidal mixture of azoxystrobin and difenoconazole. This research suggests that BABA may serve as a resistance inducer in apple against AFR. It can be used as an adequate alternative to the currently used fungicides or integrated in disease management programs to reduce fungicide load and buildup of fungicide resistance.

C4 Bacterial Volatiles Improve Plant Health

Plant growth-promoting rhizobacteria (PGPR) associated with plant roots can trigger plant growth promotion and induced systemic resistance. Several bacterial determinants including cell-wall components and secreted compounds have been identified to date. Here, we review a group of low-molecular-weight volatile compounds released by PGPR, which improve plant health, mostly by protecting plants against pathogen attack under greenhouse and field conditions. We particularly focus on C4 bacterial volatile compounds (BVCs), such as 2,3-butanediol and acetoin, which have been shown to activate the plant immune response and to promote plant growth at the molecular level as well as in large-scale field applications. We also disc/ uss the potential applications, metabolic engineering, and large-scale fermentation of C4 BVCs. The C4 bacterial volatiles act as airborne signals and therefore represent a new type of biocontrol agent. Further advances in the encapsulation procedure, together with the development of standards and guidelines, will promote the application of C4 volatiles in the field.

Increased Expression of UMAMIT Amino Acid Transporters Results in Activation of Salicylic Acid Dependent Stress Response

In addition to their role in the biosynthesis of important molecules such as proteins and specialized metabolites, amino acids are known to function as signaling molecules through various pathways to report nitrogen status and trigger appropriate metabolic and cellular responses. Moreover, changes in amino acid levels through altered amino acid transporter activities trigger plant immune responses. Specifically, loss of function of major amino acid transporter, over-expression of cationic amino acid transporter, or over-expression of the positive regulators of membrane amino acid export all lead to dwarfed phenotypes and upregulated salicylic acid (SA)-induced stress marker genes. However, whether increasing amino acid exporter protein levels lead to similar stress phenotypes has not been investigated so far. Recently, a family of transporters, namely USUALLY MULTIPLE ACIDS MOVE IN AND OUT TRANSPORTERS (UMAMITs), were identified as amino acid exporters. The goal of this study was to investigate the effects of increased amino acid export on plant development, growth, and reproduction to further examine the link between amino acid transport and stress responses. The results presented here show strong evidence that an increased expression of UMAMIT transporters induces stress phenotypes and pathogen resistance, likely due to the establishment of a constitutive stress response via a SA-dependent pathway.

Chitosan primes plant defence mechanisms against Botrytis cinerea, including expression of Avr9/Cf-9 rapidly elicited genes

Current crop protection strategies against the fungal pathogen Botrytis cinerea rely on a combination of conventional fungicides and host genetic resistance. However, due to pathogen evolution and legislation in the use of fungicides, these strategies are not sufficient to protect plants against this pathogen. Defence elicitors can stimulate plant defence mechanisms through a phenomenon known as defence priming. Priming results in a faster and/or stronger expression of resistance upon pathogen recognition by the host. This work aims to study defence priming by a commercial formulation of the elicitor chitosan. Treatments with chitosan result in induced resistance (IR) in solanaceous and brassicaceous plants. In tomato plants, enhanced resistance has been linked with priming of callose deposition and accumulation of the plant hormone jasmonic acid (JA). Large-scale transcriptomic analysis revealed that chitosan primes gene expression at early time-points after infection. In addition, two novel tomato genes with a characteristic priming profile were identified, Avr9/Cf-9 rapidly elicited protein 75 (ACRE75) and 180 (ACRE180). Transient and stable over-expression of ACRE75, ACRE180 and their Nicotiana benthamiana homologs, revealed that they are positive regulators of plant resistance against B. cinerea. This provides valuable information in the search for strategies to protect Solanaceae plants against B. cinerea.

Translational gene regulation in plants: A green new deal

The molecular machinery for protein synthesis is profoundly similar between plants and other eukaryotes. Mechanisms of translational gene regulation are embedded into the broader network of RNA-level processes including RNA quality control and RNA turnover. However, over eons of their separate history, plants acquired new components, dropped others, and generally evolved an alternate way of making the parts list of protein synthesis work. Research over the past 5 years has unveiled how plants utilize translational control to defend themselves against viruses, regulate translation in response to metabolites, and reversibly adjust translation to a wide variety of environmental parameters. Moreover, during seed and pollen development plants make use of RNA granules and other translational controls to underpin developmental transitions between quiescent and metabolically active stages. The economics of resource allocation over the daily light-dark cycle also include controls over cellular protein synthesis. Important new insights into translational control on cytosolic ribosomes continue to emerge from studies of translational control mechanisms in viruses. Finally, sketches of coherent signaling pathways that connect external stimuli with a translational response are emerging, anchored in part around TOR and GCN2 kinase signaling networks. These again reveal some mechanisms that are familiar and others that are different from other eukaryotes, motivating deeper studies on translational control in plants. This article is categorized under: Translation > Translation Regulation RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

The IBI1 Receptor of β-Aminobutyric Acid Interacts with VOZ Transcription Factors to Regulate Abscisic Acid Signaling and Callose-Associated Defense

External and internal signals can prime the plant immune system for a faster and/or stronger response to pathogen attack. β-aminobutyric acid (BABA) is an endogenous stress metabolite that induces broad-spectrum disease resistance in plants. BABA perception in Arabidopsis is mediated by the aspartyl tRNA synthetase IBI1, which activates priming of multiple immune responses, including callose-associated cell wall defenses that are under control by abscisic acid (ABA). However, the immediate signaling components after BABA perception by IBI1, as well as the regulatory role of ABA therein, remain unknown. Here, we have studied the early signaling events controlling IBI1-dependent BABA-induced resistance (BABA-IR), using untargeted transcriptome and protein interaction analyses. Transcriptome analysis revealed that IBI1-dependent expression of BABA-IR against the biotrophic oomycete Hyaloperonospora arabidopsidis is associated with suppression of ABA-inducible abiotic stress genes. Protein interaction studies identified the VOZ1 and VOZ2 transcription factors (TFs) as IBI1-interacting partners, which are transcriptionally induced by ABA but suppress pathogen-induced expression of ABA-dependent genes. Furthermore, we show that VOZ TFs require nuclear localization for their contribution to BABA-IR by mediating augmented expression of callose-associated defense. Collectively, our study indicates that the IBI1-VOZ signaling module channels pathogen-induced ABA signaling toward cell wall defense while simultaneously suppressing abiotic stress-responsive genes.

Plant Immune Mechanisms: From Reductionistic to Holistic Points of View

After three decades of the amazing progress made on molecular studies of plant-microbe interactions (MPMI), we have begun to ask ourselves "what are the major questions still remaining?" as if the puzzle has only a few pieces missing. Such an exercise has ultimately led to the realization that we still have many more questions than answers. Therefore, it would be an impossible task for us to project a coherent "big picture" of the MPMI field in a single review. Instead, we provide our opinions on where we would like to go in our research as an invitation to the community to join us in this exploration of new MPMI frontiers.

Plant-Specific Domains and Fragmented Sequences Imply Non-Canonical Functions in Plant Aminoacyl-tRNA Synthetases

Aminoacyl-tRNA synthetases (aaRSs) play essential roles in protein translation. In addition, numerous aaRSs (mostly in vertebrates) have also been discovered to possess a range of non-canonical functions. Very few studies have been conducted to elucidate or characterize non-canonical functions of plant aaRSs. A genome-wide search for aaRS genes in Arabidopsis thaliana revealed a total of 59 aaRS genes. Among them, asparaginyl-tRNA synthetase (AsnRS) was found to possess a WHEP domain inserted into the catalytic domain in a plant-specific manner. This insertion was observed only in the cytosolic isoform. In addition, a long stretch of sequence that exhibited weak homology with histidine ammonia lyase (HAL) was found at the N-terminus of histidyl-tRNA synthetase (HisRS). This HAL-like domain has only been seen in plant HisRS, and only in cytosolic isoforms. Additionally, a number of genes lacking minor or major portions of the full-length aaRS sequence were found. These genes encode 14 aaRS fragments that lack key active site sequences and are likely catalytically null. These identified genes that encode plant-specific additional domains or aaRS fragment sequences are candidates for aaRSs possessing non-canonical functions.

Redox status regulates subcelluar localization of PpTGA1 associated with a BABA-induced priming defence against Rhizopus rot in peach fruit

This study attempted to characterize the involvement of a change in the redox status and subcellular localization in the BABA-induced priming resistance of peach fruit against Rhizopus rot. Specifically, 50 mM BABA primed the peaches for the enhanced disease resistance against R. stolonifer, as demonstrated by suppression of the disease development upon pathogen challenge accompanied by the clearly elevated level of TGA transcription factor (PpTGA1) and NPR1 gene (PpNPR1). In addition, the BABA elicitation enhanced the activities of a series of critical enzymes in the PPP and AsA-GSH cycle, and eventually promoted the NADPH and GSH pools, which altered the intracellular redox state towards a highly reductive condition. Additionally, PpTGA1-GFP was localized in the cytoplasm in the absence of BABA treatment or R. stolonifer inoculation, while BABA elicitation plus R. stolonifer inoculation caused PpTGA1-GFP to specifically translocate to the nucleus, where it interacted with PpNPR1 and regulated the positive expression of PR genes. Therefore, the observations implied that BABA could promote the reduction of the redox state, resulting in the translocation of PpTGA1 to the nucleus, which was a prerequisite for the induction of a priming defence against Rhizopus rot in peach.

γ-Aminobutyric acid and related amino acids in plant immune responses: Emerging mechanisms of action

The entanglement between primary metabolism regulation and stress responses is a puzzling and fascinating theme in plant sciences. Among the major metabolites found in plants, γ-aminobutyric acid (GABA) fulfils important roles in connecting C and N metabolic fluxes through the GABA shunt. Activation of GABA metabolism is known since long to occur in plant tissues following biotic stresses, where GABA appears to have substantially different modes of action towards different categories of pathogens and pests. While it can harm insects thanks to its inhibitory effect on the neuronal transmission, its capacity to modulate the hypersensitive response in attacked host cells was proven to be crucial for host defences in several pathosystems. In this review, we discuss how plants can employ GABA's versatility to effectively deal with all the major biotic stressors, and how GABA can shape plant immune responses against pathogens by modulating reactive oxygen species balance in invaded plant tissues. Finally, we discuss the connections between GABA and other stress-related amino acids such as BABA (β-aminobutyric acid), glutamate and proline.

Aminoacyl-tRNA synthetase inhibition activates a pathway that branches from the canonical amino acid response in mammalian cells

Signaling pathways that sense amino acid abundance are integral to tissue homeostasis and cellular defense. Our laboratory has previously shown that halofuginone (HF) inhibits the prolyl-tRNA synthetase catalytic activity of glutamyl-prolyl-tRNA synthetase (EPRS), thereby activating the amino acid response (AAR). We now show that HF treatment selectively inhibits inflammatory responses in diverse cell types and that these therapeutic benefits occur in cells that lack GCN2, the signature effector of the AAR. Depletion of arginine, histidine, or lysine from cultured fibroblast-like synoviocytes recapitulates key aspects of HF treatment, without utilizing GCN2 or mammalian target of rapamycin complex 1 pathway signaling. Like HF, the threonyl-tRNA synthetase inhibitor borrelidin suppresses the induction of tissue remodeling and inflammatory mediators in cytokine-stimulated fibroblast-like synoviocytes without GCN2, but both aminoacyl-tRNA synthetase (aaRS) inhibitors are sensitive to the removal of GCN1. GCN1, an upstream component of the AAR pathway, binds to ribosomes and is required for GCN2 activation. These observations indicate that aaRS inhibitors, like HF, can modulate inflammatory response without the AAR/GCN2 signaling cassette, and that GCN1 has a role that is distinct from its activation of GCN2. We propose that GCN1 participates in a previously unrecognized amino acid sensor pathway that branches from the canonical AAR.

Metabolomics to Exploit the Primed Immune System of Tomato Fruit

Tomato is a major crop suffering substantial yield losses from diseases, as fruit decay at a postharvest level can claim up to 50% of the total production worldwide. Due to the environmental risks of fungicides, there is an increasing interest in exploiting plant immunity through priming, which is an adaptive strategy that improves plant defensive capacity by stimulating induced mechanisms. Broad-spectrum defence priming can be triggered by the compound ß-aminobutyric acid (BABA). In tomato plants, BABA induces resistance against various fungal and bacterial pathogens and different methods of application result in durable protection. Here, we demonstrate that the treatment of tomato plants with BABA resulted in a durable induced resistance in tomato fruit against Botrytis cinerea, Phytophthora infestans and Pseudomonas syringae. Targeted and untargeted metabolomics were used to investigate the metabolic regulations that underpin the priming of tomato fruit against pathogenic microbes that present different infection strategies. Metabolomic analyses revealed major changes after BABA treatment and after inoculation. Remarkably, primed responses seemed specific to the type of infection, rather than showing a common fingerprint of BABA-induced priming. Furthermore, top-down modelling from the detected metabolic markers allowed for the accurate prediction of the measured resistance to fruit pathogens and demonstrated that soluble sugars are essential to predict resistance to fruit pathogens. Altogether, our results demonstrate that metabolomics is particularly insightful for a better understanding of defence priming in fruit. Further experiments are underway in order to identify key metabolites that mediate broad-spectrum BABA-induced priming in tomato fruit.

Exogenous Treatment with Glutamate Induces Immune Responses in Arabidopsis

Plant resistance inducers (PRIs) are compounds that protect plants from diseases by activating immunity responses. Exogenous treatment with glutamate (Glu), an important amino acid for all living organisms, induces resistance against fungal pathogens in rice and tomato. To understand the molecular mechanisms of Glu-induced immunity, we used the Arabidopsis model system. We found that exogenous treatment with Glu induces resistance against pathogens in Arabidopsis. Consistent with this, transcriptome analyses of Arabidopsis seedlings showed that Glu significantly induces the expression of wound-, defense-, and stress-related genes. Interestingly, Glu activates the expression of genes induced by pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns at much later time points than the flg22 peptide, which is a bacterial-derived PAMP. The Glu receptor-like (GLR) proteins GLR3.3 and GLR3.6 are involved in the early expression of Glu-inducible genes; however, the sustained expression of these genes does not require the GLR proteins. Glu-inducible gene expression is also not affected by mutations in genes that encode PAMP receptors (EFR, FLS2, and CERK1), regulators of pattern-triggered immunity (BAK1, BKK1, BIK1, and PBL1), or a salicylic acid biosynthesis enzyme (SID2). The treatment of roots with Glu activates the expression of PAMP-, salicylic acid-, and jasmonic acid-inducible genes in leaves. Moreover, the treatment of roots with Glu primes chitin-induced responses in leaves, possibly through transcriptional activation of LYSIN-MOTIF RECEPTOR-LIKE KINASE 5 (LYK5), which encodes a chitin receptor. Because Glu treatment does not cause discernible growth retardation, Glu can be used as an effective PRI.

Comparative Transcriptome Profiling Reveals Compatible and Incompatible Patterns of Potato Toward Phytophthora infestans

Late blight, caused by Phytophthora infestans (P. infestans), is a devastating disease in potato worldwide. Our previous study revealed that the Solanum andigena genotype 03112-233 is resistant to P. infestans isolate 90128, but susceptible to the super race isolate, CN152. In this study, we confirmed by diagnostic resistance gene enrichment sequencing (dRenSeq) that the resistance of 03112-233 toward 90128 is most likely based on a distinct new R gene(s). To gain an insight into the mechanism that governs resistance or susceptibility in 03112-223, comparative transcriptomic profiling analysis based on RNAseq was initiated. Changes in transcription at two time points (24 h and 72 h) after inoculation with isolates 90128 or CN152 were analyzed. A total of 8,881 and 7,209 genes were differentially expressed in response to 90128 and CN152, respectively, and 1,083 differentially expressed genes (DEGs) were common to both time points and isolates. A substantial number of genes were differentially expressed in an isolate-specific manner with 3,837 genes showing induction or suppression following infection with 90128 and 2,165 genes induced or suppressed after colonization by CN152. Hierarchical clustering analysis suggested that isolates with different virulence profiles can induce different defense responses at different time points. Further analysis revealed that the compatible interaction caused higher induction of susceptibility genes such as SWEET compared with the incompatible interaction. The salicylic acid, jasmonic acid, and abscisic acid mediated signaling pathways were involved in the response against both isolates, while ethylene and brassinosteroids mediated defense pathways were suppressed. Our results provide a valuable resource for understanding the interactions between P. infestans and potato.

An Aminoacyl tRNA Synthetase, OKI1, Is Required for Proper Shoot Meristem Size in Arabidopsis

In plants, the stem cells that form the shoot system reside within the shoot apical meristem (SAM), which is regulated by feedback signaling between the WUSCHEL (WUS) homeobox protein and CLAVATA (CLV) peptides and receptors. WUS-CLV feedback signaling can be modulated by various endogenous or exogenous factors, such as chromatin state, hormone signaling, reactive oxygen species (ROS) signaling and nutrition, leading to a dynamic control of SAM size corresponding to meristem activity. Despite these insights, however, the knowledge of genes that control SAM size is still limited, and in particular, the regulation by ROS signaling is only beginning to be comprehended. In this study, we report a new function in maintenance of SAM size, encoded by the OKINA KUKI1 (OKI1) gene. OKI1 is expressed in the SAM and encodes a mitochondrial aspartyl tRNA synthetase (AspRS). oki1 mutants display enlarged SAMs with abnormal expression of WUS and CLV3 and overaccumulation of ROS in the meristem. Our findings support the importance of normal AspRS function in the maintenance of the WUS-CLV3 feedback loop and SAM size.

Perception of double-stranded RNA in plant antiviral immunity

RNA silencing and antiviral pattern-triggered immunity (PTI) both rely on recognition of double-stranded (ds)RNAs as defence-inducing signals. While dsRNA recognition by dicer-like proteins during antiviral RNA silencing is thoroughly investigated, the molecular mechanisms involved in dsRNA perception leading to antiviral PTI are just about to be untangled. Parallels to antimicrobial PTI thereby only partially facilitate our view on antiviral PTI. PTI against microbial pathogens involves plasma membrane bound receptors; however, dsRNAs produced during virus infection occur intracellularly. Hence, how dsRNA may be perceived during this immune response is still an open question. In this short review, we describe recent discoveries in PTI signalling upon sensing of microbial patterns and endogenous 'danger' molecules with emphasis on immune signalling-associated subcellular trafficking processes in plants. Based on these studies, we develop different scenarios how dsRNAs could be sensed during antiviral PTI.

Arabidopsis GCN2 kinase contributes to ABA homeostasis and stomatal immunity

General Control Non-derepressible 2 (GCN2) is an evolutionarily conserved serine/threonine kinase that modulates amino acid homeostasis in response to nutrient deprivation in yeast, human and other eukaryotes. However, the GCN2 signaling pathway in plants remains largely unknown. Here, we demonstrate that in Arabidopsis, bacterial infection activates AtGCN2-mediated phosphorylation of eIF2α and promotes TBF1 translational derepression. Consequently, TBF1 regulates a subset of abscisic acid signaling components to modulate pre-invasive immunity. We show that GCN2 fine-tunes abscisic acid accumulation and signaling during both pre-invasive and post-invasive stages of an infection event. Finally, we also demonstrate that AtGCN2 participates in signaling triggered by phytotoxin coronatine secreted by P. syringae. During the preinvasive phase, AtGCN2 regulates stomatal immunity by affecting pathogen-triggered stomatal closure and coronatine-mediated stomatal reopening. Our conclusions support a conserved role of GCN2 in various forms of immune responses across kingdoms, highlighting GCN2's importance in studies on both plant and mammalian immunology.

Rice plants overexpressing OsEPF1 show reduced stomatal density and increased root cortical aerenchyma formation

Stomata are adjustable pores in the aerial epidermis of plants. The role of stomata is usually described in terms of the trade-off between CO₂ uptake and water loss. Little consideration has been given to their interaction with below-ground development or diffusion of other gases. We overexpressed the rice EPIDERMAL PATTERNING FACTOR1 (OsEPF1) to produce rice plants with reduced stomatal densities, resulting in lowered leaf stomatal conductance and enhanced water use efficiency. Surprisingly, we found that root cortical aerenchyma (RCA) is formed constitutively in OsEPF1OE lines regardless of tissue age and position. Aerenchyma is tissue containing air-spaces that can develop in the plant root during stressful conditions, e.g. oxygen deficiency when it functions to increase O₂ diffusion from shoot to root. The relationship with stomata is unknown. We conclude that RCA development and stomatal development are linked by two possible mechanisms: first that reduced stomatal conductance inhibits the diffusion of oxygen to the root, creating an oxygen deficit and stimulating the formation of RCA, second that an unknown EPF signalling pathway may be involved. Our observations have fundamental implications for the understanding of whole plant gas diffusion and root-to-shoot signalling events.

Molecular Dissection of Early Defense Signaling Underlying Volatile-Mediated Defense Regulation and Herbivore Resistance in Rice

Herbivore-induced plant volatiles prime plant defenses and resistance, but how they are integrated into early defense signaling and whether a causal relationship exists between volatile defense priming and herbivore resistance is unclear. Here, we investigated the impact of indole, a common herbivore-induced plant volatile and modulator of many physiological processes in plants, bacteria, and animals, on early defense signaling and herbivore resistance in rice (Oryza sativa). Rice plants infested by fall armyworm (Spodoptera frugiperda) caterpillars release indole at a rate of up to 25 ng*h^-1 Exposure to equal doses of exogenous indole enhances rice resistance to S. frugiperda Screening of early signaling components revealed that indole pre-exposure directly enhances the expression of the leucine-rich repeat-receptor-like kinase OsLRR-RLK1 Pre-exposure to indole followed by simulated herbivory increases (i.e. primes) the transcription, accumulation, and activation of the mitogen-activated protein kinase OsMPK3 and the expression of the downstream WRKY transcription factor gene OsWRKY70 as well as several jasmonate biosynthesis genes, resulting in higher jasmonic acid (JA) accumulation. Analysis of transgenic plants defective in early signaling showed that OsMPK3 is required and that OsMPK6 and OsWRKY70 contribute to indole-mediated defense priming of JA-dependent herbivore resistance. Therefore, herbivore-induced plant volatiles increase plant resistance to herbivores by positively regulating early defense signaling components.

Accumulation patterns of endogenous β-aminobutyric acid during plant development and defence in Arabidopsis thaliana

We recently discovered that β-aminobutyric acid (BABA), a molecule known for its ability to prime defences in plants, is a natural plant metabolite. However, the role played by endogenous BABA in plants is currently unknown. In this study we investigated the systemic accumulation of BABA during pathogen infection, levels of BABA during plant growth and development and analysed mutants possibly involved in BABA transport or regulation. BABA was quantified by LC-MS using an improved method adapted from a previously published protocol. Systemic accumulation of BABA was determined by analysing non-infected leaves and roots after localised infections with Plectosphaerella cucumerina or Pseudomonas syringae pv. tomato (Pst) DC3000 avrRpt2. The levels of BABA were also quantified in different plant tissues and organs during normal plant growth, and in leaves during senescence. Mutants affecting amino acid transport (aap6, aap3, prot1 and gat1), γ-aminobutyric acid levels (pop2) and senescence/defence (cpr5-2) were analysed. BABA was found to accumulate only locally after bacterial or fungal infection, with no detectable increase in non-infected systemic plant parts. In leaves, BABA content increased during natural and induced senescence. Reproductive organs had the highest levels of BABA, and the mutant cpr5-2 produced constitutively high levels of BABA. Synthetic BABA is highly mobile in the receiving plant, whereas endogenous BABA appears to be produced and accumulated locally in a tissue-specific way. We discuss a possible role for BABA in age-related resistance and propose a comprehensive model for endogenous and synthetic BABA.

Molecular Dissection of Early Defense Signaling Underlying Volatile-Mediated Defense Regulation and Herbivore Resistance in Rice

Herbivore-induced plant volatiles prime plant defenses and resistance, but how they are integrated into early defense signaling and whether a causal relationship exists between volatile defense priming and herbivore resistance is unclear. Here, we investigated the impact of indole, a common herbivore-induced plant volatile and modulator of many physiological processes in plants, bacteria, and animals, on early defense signaling and herbivore resistance in rice (Oryza sativa). Rice plants infested by fall armyworm (Spodoptera frugiperda) caterpillars release indole at a rate of up to 25 ng*h^-1 Exposure to equal doses of exogenous indole enhances rice resistance to S. frugiperda Screening of early signaling components revealed that indole pre-exposure directly enhances the expression of the leucine-rich repeat-receptor-like kinase OsLRR-RLK1 Pre-exposure to indole followed by simulated herbivory increases (i.e. primes) the transcription, accumulation, and activation of the mitogen-activated protein kinase OsMPK3 and the expression of the downstream WRKY transcription factor gene OsWRKY70 as well as several jasmonate biosynthesis genes, resulting in higher jasmonic acid (JA) accumulation. Analysis of transgenic plants defective in early signaling showed that OsMPK3 is required and that OsMPK6 and OsWRKY70 contribute to indole-mediated defense priming of JA-dependent herbivore resistance. Therefore, herbivore-induced plant volatiles increase plant resistance to herbivores by positively regulating early defense signaling components.

Single-dose β-aminobutyric acid treatment modifies tobacco (Nicotiana tabacum L.) leaf acclimation to consecutive UV-B treatment

β-Aminobutyric acid (BABA) pre-treatment has been shown to alter both biotic and abiotic stress responses. The present study extends this observation to acclimative UV-B-response, which has not been explored in this context so far. A single soil application of 300 ppm BABA modified the non-enzymatic antioxidant capacities and the leaf hydrogen peroxide levels in tobacco (Nicotiana tabacum L.) leaves in response to a 9-day treatment with 5.4 kJ m-2 d-1 biologically effective supplementary UV-B radiation in a model experiment that was performed in a growth chamber. BABA decreased leaf hydrogen peroxide levels both as a single factor and in combination with UV-B, but neither BABA nor UV-B affected leaf photochemistry significantly. The total antioxidant capacities were increased by either BABA or UV-B, and this response was additive in BABA pre-treated leaves. These results together with the observed changes in hydroxyl radical neutralising ability and non-enzymatic hydrogen peroxide antioxidant capacities show that BABA pre-treatment (i) has a long-term effect on leaf antioxidants even in the absence of other factors and (ii) modifies acclimative readjustment of prooxidant-antioxidant balance in response to UV-B. BABA-inducible antioxidants do not include phenolic compounds as a UV-B-induced increase in the adaxial leaf flavonoid index and total leaf extract UV absorption were unaffected by BABA.

Identification of Lysine Histidine Transporter 2 as an 1-Aminocyclopropane Carboxylic Acid Transporter in Arabidopsis thaliana by Transgenic Complementation Approach

1-Aminocyclopropane-1-carboxylic acid (ACC), a biosynthetic precursor of ethylene, has long been proposed to act as a mobile messenger in higher plants. However, little is known about the transport system of ACC. Recently, our genetic characterization of an ACC-resistant mutant with normal ethylene sensitivity revealed that lysine histidine transporter 1 (LHT1) functions as a transporter of ACC. As amino acid transporters might have broad substrate specificity, we hypothesized that other amino acid transporters including LHT1 paralogs might have the ACC-transporter activity. Here, we took a gain-of-function approach by transgenic complementation of lht1 mutant with a selected set of amino acid transporters. When we introduced transgene into the lht1 mutant, the transgenic expression of LHT2, but not of LHT3 or amino acid permease 5 (AAP5), restored the ACC resistance phenotype of the lht1 mutant. The result provides genetic evidence that some, if not all, amino acid transporters in Arabidopsis can function as ACC transporters. In support, when expressed in Xenopus laevis oocytes, both LHT1 and LHT2 exhibited ACC-transporting activity, inducing inward current upon addition of ACC. Interestingly, the transgenic expression of LHT2, but not of LHT3 or AAP5, could also suppress the early senescence phenotypes of the lht1 mutant. Taking together, we propose that plants have evolved a multitude of ACC transporters based on amino acid transporters, which would contribute to the differential distribution of ACC under various spatiotemporal contexts.

Recent Advances in Synthetic Chemical Inducers of Plant Immunity

Different from the conventional biocidal agrochemicals, synthetic chemical inducers of plant immunity activate, bolster, or prime plant defense machineries rather than directly acting on the pathogens. Advances in combinatorial synthesis and high-throughput screening methods have led to the discovery of various synthetic plant immune activators as well as priming agents. The availability of their structures and recent progress in the mechanistic understanding of plant immune responses have opened up the possibility of identifying new or more potent chemical inducers through rational design. In this review, we first summarize the chemical inducers identified through large-scale screening and then discuss the emerging trends in the identification and development of novel plant immune inducers including natural elicitor based chemical derivation, bifunctional combination, and computer-aided design.

Fruit Decay to Diseases: Can Induced Resistance and Priming Help?

Humanity faces the challenge of having to increase food production to feed an exponentially growing world population, while crop diseases reduce yields to levels that we can no longer afford. Besides, a significant amount of waste is produced after fruit harvest. Fruit decay due to diseases at a post-harvest level can claim up to 50% of the total production worldwide. Currently, the most effective means of disease control is the use of pesticides. However, their use post-harvest is extremely limited due to toxicity. The last few decades have witnessed the development of safer methods of disease control post-harvest. They have all been included in programs with the aim of achieving integrated pest (and disease) management (IPM) to reduce pesticide use to a minimum. Unfortunately, these approaches have failed to provide robust solutions. Therefore, it is necessary to develop alternative strategies that would result in effective control. Exploiting the immune capacity of plants has been described as a plausible route to prevent diseases post-harvest. Post-harvest-induced resistance (IR) through the use of safer chemicals from biological origin, biocontrol, and physical means has also been reported. In this review, we summarize the successful activity of these different strategies and explore the mechanisms behind. We further explore the concept of priming, and how its long-lasting and broad-spectrum nature could contribute to fruit resistance.