Monoterpenes Support Systemic Acquired Resistance within and between Plants

WRKY1-Mediated Interconversion of MeSA and SA in Neighbouring Apple Plants Enhances Defence Against Powdery Mildew

Powdery mildew (PM), caused by the biotrophic fungus Podospharea leucotricha, is a major threat to apple production. Plant-plant communication (PPC) is a crucial strategy for plant communities to enhance their defence against pathogens. The interconversion of methyl salicylate (MeSA) and salicylic acid (SA) is critical for PPC regulation, but the mechanism of MeSA-mediated PPC is not fully understood. This study reveals a significant increase in SA and MeSA levels in neighbouring plants (receivers) following PM attack on emitter plants, activating defence responses in receivers. Notably, the expression of WRKY1, a previously characterized transcription factor, was upregulated in receivers, implicating its role in defence response modulation. WRKY1 was found to promote SA accumulation and enhance PM resistance in receivers. Importantly, WRKY1 positively regulates the expression of SABP2a, which catalysers MeSA to SA conversion, and negatively regulates SAMT1a, which functions in the reverse reaction. Consequently, WRKY1 facilitates the conversion of MeSA to SA in receivers, preventing its reversion and sustaining elevated SA levels. Collectively, our findings clarify the role of WRKY1 in enhancing the defence response to PM in receivers.

Herbivory-induced green leaf volatiles increase plant performance through jasmonate-dependent plant-soil feedbacks

Plants influence each other chemically by releasing leaf volatiles and root exudates, but whether and how these two phenomena interact remains unknown. Here we demonstrate that volatiles that are released by herbivore-attacked leaves trigger plant-soil feedbacks, resulting in increased performance of different plant species. We show that this phenomenon is due to green leaf volatiles that induce jasmonate-dependent systemic defence signalling in receiver plants, which results in the accumulation of beneficial soil bacteria in the rhizosphere. These soil bacteria then increase plant growth and enhance plant defences. In maize, a cysteine-rich receptor-like protein kinase, ZmCRK25, is required for this effect. In four successive year-field experiments, we demonstrate that this phenomenon can suppress leaf herbivore abundance and enhance maize growth and yield. Thus, volatile-mediated plant-plant interactions trigger plant-soil feedbacks that shape plant performance across different plant species through broadly conserved defence signalling mechanisms and changes in soil microbiota. This phenomenon expands the repertoire of biologically relevant plant-plant interactions in space and time and holds promise for the sustainable intensification of agriculture.

Pipecolic acid: A positive regulator of systemic acquired resistance and plant immunity

Pipecolic acid (Pip) is a naturally occurring non-protein amino acid, that builds up in plants in response to pathogen infection. Pip is upregulated in autophagy mutants, indicating its role as a crucial regulator of plant immunity by upregulating systemic acquired resistance (SAR). This broad-spectrum defense mechanism protects uninfected parts of the plant during subsequent pathogen attacks. Pip has been identified as a SAR chemical signal and acts before the NO-ROS-AzA-G3P. The biosynthesis of Pip begins with lysine by the activity of ALD1 and SARD4 in a sequential manner; ALD1, a lysine aminotransferase, catabolizes lysine to Δ 1-piperidine-2-carboxylic acid, which is further modified to Pip by the activity of ornithine cyclodeaminase activity of SARD4. Additionally, FMO 1, a flavin monooxygenase, catalyzes the synthesis of N-hydroxy-pipecolic acid (NHP, the final, SAR-inducing defense hormone) from Pip. Pip and its active form accumulate at the infection site in the phloem and are transported to distal parts of the plant via symplast to trigger SAR. This review focuses on the roles of Pip and NHP in regulating SAR and how they interact with other defense signals like salicylic acid (SA) to modulate plant immunity.

Transcriptome and metabolome analysis of the responses of salt resistance of different Helianthus annuus germplasms to melatonin

Introduction

Salt stress always causes irreversible damages to the growth of Helianthus annuus seedlings in arid and semi-arid areas due to the weakest salt resistance at the seedling stage. Melatonin is a multifunctional molecule that can enhance the salt stress resistance of several crops. However, the effect of melatonin on the salt stress resistance of H. annuus is still unclear.

Methods

In this study, four H. annuus germplasms with different salt resistance (YE988, S2102, Longkuiza 4, and 909S) were selected from a total of 164 germplasms from China, France, Chili, the Unit States, etc. Then, four treatments for the four germplasms were designed, including (1) CK, no salt stress + no melatonin application; (2) MT, no salt stress + melatonin application; (3) K, salt stress + no melatonin application; (4) MK, salt stress + melatonin application. After that, the key genes and metabolic pathways involved in the responses of salt resistance of H. annuus germplasms to melatonin were determined by transcriptome and metabolome analysis.

Results and discussion

The results showed that there were 530 differentially expressed genes (37 upregulated genes and 493 down-regulated genes) in H. annuus leaves in MK vs. K, and these genes were mainly involved in fatty acids, diterpenoid biosynthesis, linolenic acid metabolism, cysteine and methionine metabolism. There were 60 differentially abundant metabolites (17 up-regulated metabolites and 43 downregulated metabolites) in leaves in MK vs. K, mainly concentrating in tryptophan metabolism, biosynthesis of amino acids, biosynthesis of secondary metabolites and metabolic pathways. The integrated transcriptome and metabolome analysis results showed that melatonin regulated the b-alanine metabolism, monoterpene biosynthesis, and glutathione metabolism pathways, and increased the contents of spermine and spermidine in cells by promoting the expression of genes such as HannXRQ_Chr07g0195521 and HannXRQ_Chr03g0093321 in the b-alanine metabolic pathway. In summary, melatonin could enhance salt stress signaling by up-regulating the expression of genes related to the synthesis of spermine and spermidine in H. annuus leaves, to regulate photosynthesis and reactive oxygen species metabolism, ultimately enhancing the salt resistance of H. annuus. This study will advance understanding of mechanism by which melatonin enhances salt resistance of H. annuus, and provide a scientific basis for the breeding of saltresistant H. annuus cultivars.

A Simplified and Integrated View of Disease Control in Varietal Mixtures Using the Phytobiome Framework

Increasing intraspecific diversity within crop systems is a promising strategy to manage aerial diseases, particularly those caused by fungal aerial pathogens. This review examines how cultivar mixtures reduce disease incidence and severity using the phytobiome framework, identifying three major types of processes: (1) physical ones, which alter disease dynamics through dilution effects, barrier effects, and microclimate modifications; (2) processes that are mediated by microbial interactions, which influence disease severity via induced resistance and indirect plant-plant interactions mediated by the microbiome; and (3) processes involving direct plant-plant interactions, where danger signaling and signaling from healthy neighbors modulate plant physiology and immunity through resource management and molecular cues. This review provides a comprehensive understanding of how cultivar mixtures enhance disease resistance and emphasizes that direct plant-plant interactions are likely stronger contributors than so far considered. It highlights the need for further research into the roles of microbiomes and direct plant-plant interactions to optimize mixtures' performance.

Molecular changes in agroinfiltrated leaves of Nicotiana benthamiana expressing suppressor of silencing P19 and coronavirus-like particles

The production of coronavirus disease 2019 vaccines can be achieved by transient expression of the spike (S) protein of severe acute respiratory syndrome coronavirus 2 in agroinfiltrated leaves of Nicotiana benthamiana. Relying on bacterial vector Agrobacterium tumefaciens, this process is favoured by co-expression of viral silencing suppressor P19. Upon expression, the S protein enters the cell secretory pathway, before being trafficked to the plasma membrane where formation of coronavirus-like particles (CoVLPs) occurs. We previously characterized the effects of influenza virus hemagglutinin forming VLPs through similar processes. However, leaf samples were only collected after 6 days of expression, and it is unknown whether influenza VLPs (HA-VLPs) and CoVLPs induce similar responses. Here, time course sampling was used to profile responses of N. benthamiana leaf cells expressing P19 only, or P19 and the S protein. The latter triggered early but transient activation of the unfolded protein response and waves of transcription factor genes involved in immunity. Accordingly, defence genes were induced with different expression kinetics, including those promoting lignification, terpene biosynthesis, and oxidative stress. Cross-talk between stress hormone pathways also occurred, including repression of jasmonic acid biosynthesis genes after agroinfiltration, and dampening of salicylic acid responses upon S protein accumulation. Overall, HA-VLP- and CoVLP-induced responses broadly overlapped, suggesting nanoparticle production to have the most effects on plant immunity, regardless of the virus surface proteins expressed. Taking advantage of RNAseq inferences, we finally show the co-expression of Kunitz trypsin inhibitors to reduce CoVLP-induced defence and leaf symptoms, with no adverse effect on plant productivity.

Biocontrol potential of natamycin-producing Streptomyces lydicus JCK-6019 against soil-borne fungal diseases of cucumber and characterization of its biocontrol mechanism

BACKGROUND

Fusarium oxysporum f. sp. cucumerinum and Rhizoctonia solani AG-4 are the two most important fungal pathogens causing soil-borne fungal diseases of cucumber; they are difficult to control and cause serious economic losses. Given the detrimental effects of the indiscriminate use of chemical fungicides, biocontrol emerges as an efficient and ecofriendly alternative for managing soil-borne fungal diseases.

RESULTS

Streptomyces lydicus JCK-6019 (hereafter, JCK-6019) was isolated from rhizosphere soil. Its fermentation filtrate and volatile organic compounds exhibited broad-spectrum antifungal activity against various phytopathogenic fungi and oomycetes. JCK-6019 produced natamycin as an agar-diffusible antifungal metabolite. It also produced indole-3-acetic acid and various hydrolytic enzymes. In vivo experiments revealed that a ten-fold-diluted optimized JCK-6019 fermentation broth exhibited 100% control efficiency against cucumber damping-off disease and 62.5% control efficiency against cucumber Fusarium wilt disease. Pretreatment of cucumber seedlings with 1000-fold-diluted optimized JCK-6019 fermentation broth resulted in 68.18% and 23.91% disease control values against cucumber damping-off and Fusarium wilt disease, respectively. Moreover, peroxidase activity in cucumbers after 1 day of treatment was 1.5-fold higher than that in the control. Similarly, polyphenol oxidase activity in cucumbers after 3 days of treatment was 2.34-fold higher than that in the control, indicating that JCK-6019 can induce plant resistance.

CONCLUSION

The natamycin-producing strain JCK-6019 could effectively suppress the development of cucumber Fusarium wilt and damping-off disease by inducing plant resistance and producing antifungal metabolites, including natamycin and volatile organic compounds. Thus, JCK-6019 possesses high potential for application in the development of biocontrol agents against soil-borne fungal diseases of cucumber. © 2024 Society of Chemical Industry.

Loss of tomato geranylgeranyl diphosphate synthase 2 increases monoterpenoid levels and enhances immune responses to bacterial infection

Many plastidial isoprenoids, including diterpenes and photosynthesis-related isoprenoids such as carotenoids and chlorophylls, derive from C20 geranylgeranyl diphosphate (GGPP), produced by GGPP synthase (GGPPS) enzymes. Heterodimers of GGPPS and non-catalytic type I small subunit (SSU-I) proteins produce C10 geranyl diphosphate (GPP), the precursor of monoterpenes. Three plastidial GGPPS isoforms, referred to as SlG1-3, are present in tomato (Solanum lycopersicum). Here we explored their contribution to the production of volatile organic compounds (VOCs) of isoprenoid origin under normal conditions and in response to infection with Pseudomonas syringae pathovar tomato (Pst). Edited lines lacking SlG2 showed a distinctive VOC profile compared to unedited (WT) plants and mutants impaired in SlG1 or SlG3. In particular, only slg2 mutants showed constitutively increased levels of GPP-derived hydroxylated monoterpenes (HMTPs). Upon Pst infection, slg2 plants accumulated higher levels of salicylic acid (SA) and exhibited increase resistance compared to WT controls, resulting in reduced levels of VOCs associated to cell death. Our findings suggest that SlG2 regulates GPP synthesis, potentially by specifically competing with other GGPPS isoforms for heterodimerization with SSU-I. Increased GPP production in slg2 plants could lead to higher HMTPs levels, which may result in elevated SA content, and subsequently enhanced protection against bacterial infection.

Assessment of potential candidate genes for partial resistance to Sclerotinia stem rot caused by Sclerotinia sclerotiorum using real-time quantitative PCR

Sclerotinia stem rot (SSR), caused by Sclerotinia sclerotiorum (Lib) de Bary (S. sclerotiorum), is one of the most important diseases that causes significant soybean [Glycine max (L.) Merr.] seed yield and quality losses in Canada and globally. Initiation of plant defense mechanisms is crucial for establishing partial resistance to the pathogenic fungus. To understand plant response to S. sclerotiorum, we conducted a temporal (1, 3, and 5 days post-inoculation [DPI]) assessment of gene expression changes in the stem of soybean genotypes with contrasting phenotypic response. We focused on four genes that have been previously reported as associated with SSR partial resistance and are known to be involved in defense-related functions such as cell wall modification, signaling, response to wounding, and response to fungus. The results showed a higher and earlier expression of the genes in partially resistant cultivars compared to the susceptible. Expression of some genes increased up to 11- (Glyma.02G059700) to 16-fold (Glyma.09G232100) by 3 DPI in the partially resistant cultivar, OAC Drayton, while the genes were generally downregulated in the susceptible cultivar, OAC Shire, at the same DPI. This study improves our understanding of expression patterns of genes involved in plant defense against fungal pathogens in soybean. More importantly, the knowledge of genes that are essential in defense against S. sclerotiorum can be used to fine-map the quantitative trait loci for SSR resistance and facilitate accelerated breeding of SSR-resistant cultivars through gene-based marker-assisted selection.

The Efficacy of Orange Terpene and Bacillus mycoides Strain BM103 on the Control of Periwinkle Leaf Yellowing Phytoplasma

Phytoplasmas are obligate phytopathogenic bacteria belonging to the class Mollicutes. The pathogens, transmitted by insect vectors, are associated with hundreds of plant diseases worldwide. Because of the regulations banning the use of antibiotics and the limited efficacy of traditional disease management manners, an eco-friendly alternative is needed. Given that terpene and probiotics have antibiotic activity and the ability to induce systemic resistance, in this study, the effectiveness of orange terpene and a Bacillus mycoides strain, BM103, was evaluated in periwinkle plants infected with periwinkle leaf yellowing (PLY) phytoplasma derived from a shoot-tip tissue culture system. Weekly drenching of 1,000 ppm diluted orange terpene emulsion or preactivated strain BM103 liquid culture dilution exhibited the ability to inhibit PLY phytoplasma accumulation. The expression of the genes associated with plant defense response and flower development was upregulated after treatment. Moreover, pretreatment of orange terpene or strain BM103 delayed PLY infection via cleft-grafting inoculation. Although orange terpene did not suppress the symptoms, strain BM103 did result in a milder symptom expression that might partially attribute to its plant growth-promoting characteristics. Additionally, the preactivation of strain BM103 may contribute to its efficacy. Taken together, this research indicates that orange terpene and B. mycoides BM103, with the ability to rapidly induce plant defense responses, could potentially be developed into biologic control materials as preventive agents or biofertilizers.

Salicylic Acid Modulates Volatile Organic Compound Profiles During CEVd Infection in Tomato Plants

Background:Citrus Exocortis Viroid (CEVd) is a non-coding RNA pathogen capable of infecting a wide range of plant species, despite its lack of protein-coding ability. Viroid infections induce significant alterations in various physiological and biochemical processes, particularly impacting plant metabolism. This study shows the metabolic changes upon viroid infection in tomato plants (Solanum lycopersicum var. 'MoneyMaker') exhibiting altered levels of salicylic acid (SA), a key signal molecule involved in the plant defence against this pathogen. Methods: Transgenic RNAi_S5H lines, which have the salicylic acid 5-hydroxylase gene silenced to promote SA accumulation, and NahG lines, which overexpress a salicylate hydroxylase to degrade SA into catechol and prevent its accumulation, were used to establish different SA levels in plants, resulting in varying degrees of resistance to viroid infection. The analysis was performed by using gas chromatography-mass spectrometry (GC-MS) to explore the role of volatile organic compounds (VOCs) in plant immunity against this pathogen. Results: Our results revealed distinct volatile profiles associated with plant immunity, where RNAi_S5H-resistant plants showed significantly enhanced production of monoterpenoids upon viroid infection. Moreover, viroid-susceptible NahG plants emitted a broad range of VOCs, whilst viroid-tolerant RNAi_S5H plants exhibited less variation in VOC emission. Conclusions: This study demonstrates that SA levels significantly influence metabolic responses and immunity in tomato plants infected by CEVd. The identification of differential emitted VOCs upon CEVd infection could allow the development of biomarkers for disease or strategies for disease control.

Atmospheric pollution affects the morphoanatomical and physiological responses of plants in urban Atlantic Forest remnants

The Metropolitan Area of São Paulo (MASP) in southern Brazil is impacted by high ozone levels posing significant threats to its urban forests and the Atlantic Forest remnants. These green areas, covering 540 km2 and constituting 30% of MASP's territory, necessitate an urgent assessment of air pollution impacts on their flora. Our study investigates the effects of atmospheric pollution on the morphoanatomical and physiological responses of four native tree species (Alchornea sidifolia, Casearia sylvestris, Guarea macrophylla, and Machaerium nyctitans) across two Atlantic Forest remnants in MASP. We examined visual and morphoanatomical changes in leaves, gas exchange, photosynthetic pigments, and plant volatile organic compounds to identify markers for biomonitoring urban environments. Our results reveal that MASP vegetation is adversely affected by tropospheric ozone. Species with porous mesophyll structures, such as M. nyctitans and G. macrophylla, exhibited greater visual and structural damage. In contrast, species with compact mesophyll, such as A. sidifolia and C. sylvestris, demonstrated higher tolerance. This suggests that anatomical architecture critically influences species' responses to atmospheric pollutants, such as tropospheric ozone. Additionally, we propose that ozone influx occurs through both stomatal pathways and as a result of direct and indirect injuries to the plant tissues. Additionally, our study identifies non-visual markers, including anatomical and physiological parameters and plant volatile organic compounds (e.g., presence of salicylates), as effective tools for monitoring plant species in urban environments. These insights highlight key anatomical and metabolic markers that help distinguish ozone-tolerant species from sensitive species, providing valuable information for monitoring air pollution in urban forests.

Piperideine-6-carboxylic acid regulates vitamin B6 homeostasis and modulates systemic immunity in plants

Dietary consumption of lysine in humans leads to the biosynthesis of Δ1-piperideine-6-carboxylic acid (P6C), with elevated levels linked to the neurological disorder epilepsy. Here we demonstrate that P6C biosynthesis is also a critical component of lysine catabolism in Arabidopsis thaliana. P6C regulates vitamin B6 homeostasis, and increased P6C levels deplete B6 vitamers, resulting in compromised plant immunity. We further establish a key role for pyridoxal and pyridoxal-5-phosphate biosynthesis in plant immunity. Our analysis indicates that P6C metabolism probably evolved through combining select lysine and proline metabolic enzymes horizontally acquired from diverse bacterial sources at different points during evolution. More generally, certain enzymes from the lysine and proline metabolic pathways were probably recruited in evolution as potential guardians of B6 vitamers and for semialdehyde detoxification.

Interspecific allelopathic interaction primes direct and indirect resistance in neighboring plants within agroforestry systems

The agroforestry system with high biodiversity enhances ecosystem stability and reduces vulnerability to environmental disturbances and diseases. Investigating the mechanisms of interspecies allelopathic interactions for disease suppression in agroforestry offers a sustainable strategy for plant disease management. Here, we used Panax ginseng cultivated under Pinus koraiensis forests, which have low occurrences of Alternaria leaf spot, as a model to explore the role of allelochemicals in disease suppression. Our findings demonstrate that foliar application of P. koraiensis needle leachates effectively enhanced the resistance of P. ginseng against Alternaria leaf spot. Using gas chromatography-mass spectrometry, we identified and quantified endo-borneol as a key compound in P. koraiensis leachates and confirmed its ability to prime resistance in neighboring P. ginseng plants. We discovered that endo-borneol not only directly activates defense-related pathways in P. ginseng to confer resistance but also indirectly recruits its beneficial rhizospheric microbiota by promoting the secretion of ginsenosides, thereby triggering induced systemic resistance. Notably, higher concentrations of endo-borneol, ranging from 10 to 100 mg/l, have a greater capacity to induce plant resistance and enhance root secretion, thereby recruiting more microbiota compared to lower concentrations ranging from 0.01 to 1 mg/l. Additionally, endo-borneol exhibits antifungal activities against the growth of the pathogen Alternaria panax when concentrations exceeded 10 mg/l. These results reveal the multifaceted functions of allelochemical endo-borneol in disease suppression within agroforestry systems and highlight its potential as an environmentally friendly agent for sustainable agriculture.

Limonene enhances rice plant resistance to a piercing-sucking herbivore and rice pathogens

Terpene synthases (TPSs) are key enzymes in terpenoids synthesis of plants and play crucial roles in regulating plant defence against pests and diseases. Here, we report the functional characterization of OsTPS19 and OsTPS20, which were upregulated by the attack of brown planthopper (BPH). BPH female adults performed concentration-dependent behavioural responses to (S)-limonene showing preference behaviour at low concentrations and avoidance behaviour at high concentrations. Overexpression lines of OsTPS19 and OsTPS20, which emitted higher amounts of the monoterpene (S)-limonene, decreased the hatching rate of BPH eggs, reduced the lesion length of sheath blight caused by Rhizoctonia solani and bacterial blight caused by Xanthomonas oryzae. While knockout lines of OsTPS19 and OsTPS20, which emitted lower amounts of (S)-limonene, were more susceptible to these pathogens. Overexpression of OsTPS19 and OsTPS20 in rice plants had adverse effects on the incidence of BPH, rice blast, and sheath blight in the field and had no significant impacts on rice yield traits. OsTPS19 and OsTPS20 were found to be involved in fine-tuning the emission of (S)-limonene in rice plants and play an important role in defence against both BPH and rice pathogens.

Cracking the plant VOC sensing code and its practical applications

Volatile organic compounds (VOCs) are essential airborne mediators of interactions between plants. These plant-plant interactions require sophisticated VOC-sensing mechanisms that enable plants to regulate their defenses against pests. However, these interactions are not limited to specific plants or even conspecifics, and can function in very flexible interactions between plants. Sensing and responding to VOCs in plants is finely controlled by their uptake and transport systems as well as by cellular signaling via, for example, chromatin remodeling system-based transcriptional regulation for defense gene activation. Based on the accumulated knowledge about the interactions between plants and their major VOCs, companion plants and biostimulants are being developed for practical applications in agricultural and horticultural pest control, providing a sustainable alternative to harmful chemicals.

Phytol-induced interplant signaling in maize facilitates EXP-A20-driven resistance through ACO31-dependent ethylene accumulation against Ostrinia furnacalis

Plants have evolved sophisticated defense mechanisms against insect herbivores, including cell wall fortification through lignin biosynthesis. Insect attack primes systemic acquired resistance in plants, preparing them to respond more swiftly and vigorously to subsequent insect assaults. Here, we found that Beauveria bassiana-exposed maize plants can emit phytol upon infestation by Spodoptera frugiperda, inducing plant-to-plant (PTP) communication of alert signals for neighboring plants, and revealed the expansin protein EXP-A20 as a pivotal node mediating maize defense responses in neighboring plants against the destructive pest Ostrinia furnacalis via stimulation of ethylene (ET) synthesis and lignin production. Through virus-induced gene silencing, we showed that EXP-A20 is essential for maize resistance, while downregulating ET and lignin pathways. Critically, protein-protein interactions determined via luciferase complementation and yeast two-hybrid assays demonstrated that EXP-A20 binds to and likely activates the ET-forming enzyme gene ACO31 to initiate defense signaling cascades, representing a novel signaling modality for expansins. Treatment with the plant volatile phytol has known insecticidal/priming activity, but we found that its effectiveness requires EXP-A20. This finding highlights the importance of EXP-A20 upstream of hormone-cell wall crosstalk in defense activation by volatiles. Overall, our multifaceted dissection of EXP-A20 revealed key molecular intersections underlying inducible maize immunity against herbivores. Furthermore, we provide functional evidence that extensive cell growth processes directly stimulate defense programs in plants. Our work opens new avenues for enhancing durable, broad-spectrum pest resistance in maize through the use of volatile organic compounds and PTP interactions.

Plant extracellular vesicles contribute to the amplification of immune signals during systemic acquired resistance

KEY MESSAGE

Plant extracellular vesicles play a role in systemic acquired resistance by facilitating the transmission of immune signals between plant cells. Extracellular vesicles (EVs) play a critical role in facilitating the transfer of nucleic acids and proteins between plants and pathogens. However, the involvement of plant EVs in intercellular communication and their contribution to the regulation of physiological and pathological conditions in plants remains unclear. In this study, we isolated EVs from the apoplast of Arabidopsis plants induced by systemic acquired resistance (SAR) and conducted proteomic and physiological analyses to investigate the role of EVs in SAR. The results demonstrated that plant cells are capable of internalizing EVs, and EV secretion was enhanced in SAR-induced plants. EVs isolated from SAR-induced plants effectively inhibited the spore production of Botrytis cinerea, activated the transcription of several SAR marker genes, and improved plant resistance to Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). Several proteins associated with defense responses were enriched in EVs upon SAR induction. Among these, the receptor-like kinase H2O2-Induced Ca2+ Increase 1 (HPCA1) was identified as a crucial component in SAR. In addition, plant EVs contained numerous proteins involved in the transmission of signals related to pathogen-associated molecular patterns-triggered immunity (PTI) and effector-triggered immunity (ETI). Our findings suggest that plant EVs are functionally involved in the propagation of SAR signals and may play diverse roles in plant immune responses.

Phosphoproteomic analysis uncovers phosphorylated proteins in response to salicylic acid and N-hydroxypipecolic acid in Arabidopsis

BACKGROUND

The phytohormone salicylic acid (SA) serves as a crucial signaling molecule within the realm of plant immunity, playing an indispensable role in both local and systemic acquired resistance (SAR). N-hydroxypipecolic acid (NHP), a derivative of L-lysine, is integral to the induction of SAR. Recent investigations have illuminated the intricate manner in which NHP orchestrates the establishment of SAR in conjunction with the immune signal SA.

METHODS AND RESULTS

To further explore the mechanisms governing the synergistic regulation of SAR by SA and NHP, we conducted an extensive phosphoproteomic analysis aimed at identifying the phosphoproteins modulated either commonly or uniquely by SA and NHP, employing a phosphoproteomics platform built upon high-resolution mass spectrometry. Our study revealed a total of 133 phosphopeptides, derived from 115 distinct proteins, exhibiting exclusive responsiveness to NHP treatment. In contrast, 229 phosphopeptides sourced from 204 proteins demonstrated exclusive sensitivity to SA treatment. Additionally, the phosphorylation status of 215 proteins, including numerous kinases, phosphatases, transcription factors, and proteins implicated in membrane trafficking, was commonly modulated by both SA and NHP.

CONCLUSION

This investigation offers detailed insights into the key phosphoproteins influenced either collectively or specifically by SA and NHP, thereby enabling further exploration of the mechanisms underlying the synergistic regulation of immune responses orchestrated by these two potent molecules.

Blumeria hordei affects volatile emission of susceptible and resistant barley plants and modifies the defense response of recipient plants

The barley powdery mildew disease caused by the biotrophic fungus Blumeria hordei (Bh) poses enormous risks to crop production due to yield and quality losses. Plants and fungi can produce and release volatile organic compounds (VOCs) that serve as signals in plant communication and defense response to protect themselves. The present study aims to identify VOCs released by barley (Hordeum vulgare) during Bh-infection and to decipher VOC-induced disease resistance in receiver plants. VOC profiles of susceptible MLO wild type (MLO WT) and a resistant near-isogenic backcross line (mlo5) were characterized over time (one day or three days after Bh inoculation) using TD-GC/MS. Comparative analysis revealed genotype-dependent VOC profiles and significant differences in emission rates for β-caryophyllene, linalool, (Z)-3-hexenol, and methyl salicylate. Furthermore, susceptible barley plants were exposed to the complex VOC bouquet of MLO WT or mlo5 sender plants in plant-to-plant communication. We found that VOC-induced resistance in receiver plants depended on the sender genotype in a Bh susceptibility assay. Additionally, untargeted metabolomics and gene expression studies provide evidence toward an SA-dependent pathway mediating VOC-induced resistance against powdery mildew. The exogenous application of methyl salicylate resulted in the enhanced expression of the BARLEY CHEMICALLY INDUCED-4 marker gene and induced resistance in receiver plants. The findings suggest genotype-dependent alterations in barley VOC profiles during biotrophic plant-fungus interactions and show a VOC-mediated resistance that shares components with salicylic acid-related pathways. The VOC signals identified here could serve as non-invasive markers for disease progression in barley-powdery mildew interactions and as signals for resistance induction in recipient plants.

Impact of plant monoterpenes on insect pest management and insect-associated microbes

The fight against insect pests primarily relies on the utilization of synthetic insecticides. However, improper application of these chemicals can lead to detrimental effects on both the environment and human health, as well as foster the development of insect resistance. Consequently, novel strategies must be implemented to address the challenges stemming from the prolonged use of synthetic insecticides in agricultural and public health environments. Certain strategies involve the combination of crop protectants, which not only enhance insecticidal effectiveness but also reduce application rates. Plant-based natural products emerge as promising alternatives for insect management. Monoterpenes, which are abundant plant compounds produced through the activation of various enzymes, have attracted significant attention for their effectiveness in insect control. Notably, they are prolific in fragrance-producing plants. This review explores the plant defense, insecticidal, and antimicrobial characteristics of monoterpenes against insect pests, shedding light on their potential modes of action and possibilities for commercialization. Emphasizing their role as targeted and environmentally safer, the review highlights the practical viability of monoterpenes within integrated pest management programs.

Herbivore-induced volatiles reduce the susceptibility of neighboring tomato plants to transmission of a whitefly-borne begomovirus

Plant viruses exist in a broader ecological community that includes non-vector herbivores that can impact vector abundance, behavior, and virus transmission within shared host plants. However, little is known about the effects of non-vector herbivore infestation on virus transmission by vector insects on neighboring plants through inter-plant airborne chemicals. In this study, we investigated how volatiles emitted from tomato plants infested with the two-spotted spider mite (Tetranychus urticae) affect the infection of neighboring plants by tomato yellow leaf curl virus (TYLCV) transmitted by whitefly (Bemisia tabaci). Exposure of neighboring tomato plants to volatiles released from T. urticae-infested tomato plants reduced subsequent herbivory as well as TYLCV transmission and infection, and the jasmonic acid signaling pathway was essential for generation of the inter-plant defense signals. We also demonstrated that (E)-β-ocimene and methyl salicylic acid were two volatiles induced by T. urticae that synergistically attenuated TYLCV transmission and infection in tomato. Thus, our findings suggest that plant-plant communication via volatiles likely represents a widespread defensive mechanism that substantially contributes to plant fitness. Understanding such phenomena may help us to predict the occurrence and epidemics of multiple herbivores and viruses in agroecosystems, and ultimately to manage pest and virus outbreaks.

Composing functional food from agro-forest wastes: Selectively extracting bioactive compounds using supercritical fluid extraction

Supercritical fluid extraction (SFE) employing carbon dioxide (SC-CO2) is an efficient method to extract bioactive compounds from agro-forest wastes. These compounds maintain and/or improve food nutrition, safety, freshness, taste, and health and are employed as natural functional food components. To highlight the potential of this technology, we focus on the following current advances: (I) parameters affecting solubility in SFE (pressure, temperature, SC-CO2 flow rate, extraction time, and co-solvents); (II) extraction spectra and yield obtained according to proportion and composition of co-solvents; (III) extract bioactivity for functional food production. Fatty acids, monoterpenes, sesquiterpenes, diterpenoids, and low-polarity phenolic acids and triterpenoids were extracted using SFE without a co-solvent. High-polarity phenolic acids and flavonoids, tannins, carotenoids, and alkaloids were only extracted with the help of co-solvents. Using a co-solvent significantly improved the triterpenoid, flavonoid, and phenolic acid yield with a medium polarity.

Test of Specificity in Signalling between Potato Plants in Response to Infection by Fusarium Solani and Phytophthora Infestans

Plant-plant signalling via volatile organic compounds (VOCs) in response to insect herbivory has been widely studied, but its occurrence and specificity in response to pathogen attack has received much less attention. To fill this gap, we carried out a greenhouse experiment using two fungal pathogens (Fusarium solani and Phytophthora infestans) to test for specificity in VOC induction and signalling between potato plants (Solanum tuberosum). We paired potato plants in plastic cages, one acting as VOC emitter and the other as receiver, and subjected emitters to one of the following treatments: no infection (control), infected by F. solani, or infected by P. infestans. We measured total emission and composition of VOCs released by emitter plants to test for pathogen-specificity in VOC induction, and then conducted a pathogen infection bioassay to assess resistance levels on receiver plants by subjecting half of the receivers of each emitter treatment to F. solani infection and the other half to P. infestans infection. This allowed us to test for specificity in plant VOC signalling by comparing its effects on conspecific and heterospecific sequential infections. Results showed that infection by neither F. solani or P. infestans produced quantitative (total emissions) or qualitative (compositional) changes in VOC emissions. Mirroring these patterns, emitter infection treatment (control vs. pathogen infection) did not produce a significant change in pathogen infection levels on receiver plants in any case (i.e., either for conspecific or heterospecific sequential infections), indicating a lack of signalling effects which precluded pathogen-based specificity in signalling. We discuss possible mechanisms for lack of pathogen effects on VOC emissions and call for future work testing for pathogen specificity in plant-plant signalling and its implications for plant-pathogen interactions under ecologically relevant scenarios involving infections by multiple pathogens.

Where You Protonate Matters: Deciphering the Unimolecular Chemistry of Protonated Myrcene and Linalool

The unimolecular reactions of protonated myrcene and linalool were investigated by collision-induced dissociation and density functional theory calculations. Experiments on a triple quadrupole mass spectrometer showed that protonated myrcene undergoes two major unimolecular reactions losing propene and isobutene, and two minor reactions of ethene and propane loss. In each case, the product ion consists of a substituted five-member ring. Protonation of myrcene was found to form four distinct protomers, three of which can be significantly populated in the ion source. The observed fragmentation reactions were calculated and found to depend on the starting protomer. Each pathway consisted of several hydrogen-migration and ring-forming/opening steps on the way to the observed products. Likewise, protonation of linalool also produces three distinct protomers, with the global minimum being formed by protonation of a central double bond. The major reaction is water loss to form protonated myrcene, but two minor channels were also observed resulting in loss of acetone and isobutene. The calculated minimum energy reaction pathways were found to be consistent with the relative abundances of the ions in the experimental breakdown diagrams.

Overexpression of the Liriodendron tulipifera TPS32 gene in tobacco enhances terpenoid compounds synthesis

Liriodendron, a relic genus from the Magnoliaceae family, comprises two species, L. tulipifera and L. chinense. L. tulipifera is distinguished by its extensive natural distribution in Eastern North America. Conversely, L. chinense is nearing endangerment due to its low regeneration rate. A pivotal aspect in the difference of these species involves terpenoids, which play crucial roles in plant growth and attracting pollinators. However, the complex molecular mechanisms underlying terpenoid roles in Liriodendron are not well understood. Terpene Synthases (TPS) genes are widely reported to play a role in terpenoid biosynthesis, hence, this study centers on TPS genes in Liriodendron spp. Employing multiple bioinformatics methods, a differential expression gene in L. tulipifera, LtuTPS32, was discerned for further functional analysis. Subcellular localization results reveal the involvement of LtuTPS32 in chloroplast-associated processes, hence participate in terpenoid biosynthesis within chloroplasts. Heterologous transformation of the LtuTPS32 gene into tobacco significantly elevates the levels of common terpenoid compounds, including chlorophyll, gibberellin, and carotenoids. Collectively, these findings not only underscore the role of the LtuTPS32 gene in the biosynthesis of terpenoids but also lay a foundation for future research on interspecific differences in Liriodendron.

H2O2 sulfenylates CHE, linking local infection to the establishment of systemic acquired resistance

In plants, a local infection can lead to systemic acquired resistance (SAR) through increased production of salicylic acid (SA). For many years, the identity of the mobile signal and its direct transduction mechanism for systemic SA synthesis in initiating SAR have been debated. We found that in Arabidopsis thaliana, after a local infection, the conserved cysteine residue of the transcription factor CCA1 HIKING EXPEDITION (CHE) undergoes sulfenylation in systemic tissues, which enhances its binding to the promoter of the SA-synthesis gene ISOCHORISMATE SYNTHASE1 (ICS1) and increases SA production. Furthermore, hydrogen peroxide (H2O2) produced through NADPH oxidases is the mobile signal that sulfenylates CHE in a concentration-dependent manner. Accumulation of SA and the previously reported signal molecules, such as N-hydroxypipecolic acid (NHP), then form a signal amplification loop to establish SAR.

Exogenous stimulation of Tanacetum vulgare roots with pipecolic acid leads to tissue-specific responses in terpenoid composition

Tanacetum vulgare L., tansy, is a perennial plant with highly variable terpenoid composition, with mono- and sesquiterpenoids being the most abundant. The high diversity of terpenoids plays an important role in mediating ecological interactions. However, the distribution of terpenoids in different tissues and inducibility of terpenoids in these tissues via biotic stress are poorly understood. We investigated changes in terpenoid profiles and concentrations in different organs following treatment of roots with pipecolic acid (Pip), a non-proteinogenic amino acid that triggers defence responses leading to induce systemic resistance (SAR) in plants. Tansy leaves and midribs contained mainly monoterpenoids, while coarse and fine roots contained mainly sesquiterpenoids. Rhizomes contained terpenoid profiles of both midribs and roots but also unique compounds. Treatment with Pip led to an increase in concentrations of mono- and sesquiterpenoids in all tissues except rhizomes. However, significantly more sesquiterpenoids was formed in root tissues in response to Pip treatment, compared to shoots. The metabolic atlas for terpenoids presented here shows that there is exceptionally strong differentiation of terpenoid patterns and terpenoid content in different tissues of tansy. This, together with differential inducibility by Pip, suggests that the chemical diversity of terpenoids may play an important role in tansy ecological interactions and defence against biotic stressors that feed on below- and aboveground organs.

Redox signaling and oxidative stress in systemic acquired resistance

Plants fully depend on their immune systems to defend against pathogens. Upon pathogen attack, plants not only activate immune responses at the infection site but also trigger a defense mechanism known as systemic acquired resistance (SAR) in distal systemic tissues to prevent subsequent infections by a broad-spectrum of pathogens. SAR is induced by mobile signals produced at the infection site. Accumulating evidence suggests that reactive oxygen species (ROS) play a central role in SAR signaling. ROS burst at the infection site is one of the earliest cellular responses following pathogen infection and can spread to systemic tissues through membrane-associated NADPH oxidase-dependent relay production of ROS. It is well known that ROS ignite redox signaling and, when in excess, cause oxidative stress, damaging cellular components. In this review, we summarize current knowledge on redox regulation of several SAR signaling components. We discuss the ROS amplification loop in systemic tissues involving multiple SAR mobile signals. Moreover, we highlight the essential role of oxidative stress in generating SAR signals including azelaic acid and extracellular NAD(P) [eNAD(P)]. Finally, we propose that eNAD(P) is a damage-associated molecular pattern serving as a converging point of SAR mobile signals in systemic tissues.

A salicylic acid-associated plant-microbe interaction attracts beneficial Flavobacterium sp. to the Arabidopsis thaliana phyllosphere

Both above- and below-ground parts of plants are constantly challenged with microbes and interact closely with them. Many plant-growth-promoting rhizobacteria, mostly interacting with the plant's root system, enhance the immunity of plants in a process described as induced systemic resistance (ISR). Here, we characterized local induced resistance (IR) triggered by the model PGPR Pseudomonas simiae WCS417r (WCS417) in Arabidopsis thaliana. Hydroponic application of WCS417 to Arabidopsis roots resulted in propagation of WCS417 in/on leaves and the establishment of local IR. WCS417-triggered local IR was dependent on salicylic acid (SA) biosynthesis and signalling and on functional biosynthesis of pipecolic acid and monoterpenes, which are classically associated with systemic acquired resistance (SAR). WCS417-triggered local IR was further associated with a priming of gene expression changes related to SA signalling and SAR. A metabarcoding approach applied to the leaf microbiome revealed a significant local IR-associated enrichment of Flavobacterium sp.. Co-inoculation experiments using WCS417 and At-LSPHERE Flavobacterium sp. Leaf82 suggest that the proliferation of these bacteria is influenced by both microbial and immunity-related, plant-derived factors. Furthermore, application of Flavobacterium Leaf82 to Arabidopsis leaves induced SAR in an NPR1-dependent manner, suggesting that recruitment of this bacterium to the phyllosphere resulted in propagation of IR. Together, the data highlight the importance of plant-microbe-microbe interactions in the phyllosphere and reveal Flavobacterium sp. Leaf82 as a new beneficial promoter of plant health.

Plant terpenoid biosynthetic network and its multiple layers of regulation

Terpenoids constitute one of the largest and most chemically diverse classes of primary and secondary metabolites in nature with an exceptional breadth of functional roles in plants. Biosynthesis of all terpenoids begins with the universal five‑carbon building blocks, isopentenyl diphosphate (IPP) and its allylic isomer dimethylallyl diphosphate (DMAPP), which in plants are derived from two compartmentally separated but metabolically crosstalking routes, the mevalonic acid (MVA) and methylerythritol phosphate (MEP) pathways. Here, we review the current knowledge on the terpenoid precursor pathways and highlight the critical hidden constraints as well as multiple regulatory mechanisms that coordinate and homeostatically govern carbon flux through the terpenoid biosynthetic network in plants.

Metabolic crosstalk between hydroxylated monoterpenes and salicylic acid in tomato defense response against bacteria

Hydroxylated monoterpenes (HMTPs) are differentially emitted by tomato (Solanum lycopersicum) plants resisting bacterial infection. We have studied the defensive role of these volatiles in the tomato response to bacteria, whose main entrance is through stomatal apertures. Treatments with some HMTPs resulted in stomatal closure and pathogenesis-related protein 1 (PR1) induction. Particularly, α-terpineol induced stomatal closure in a salicylic acid (SA) and abscisic acid-independent manner and conferred resistance to bacteria. Interestingly, transgenic tomato plants overexpressing or silencing the monoterpene synthase MTS1, which displayed alterations in the emission of HMTPs, exhibited changes in the stomatal aperture but not in plant resistance. Measures of both 2-C-methyl-D-erythritol-2,4-cyclopyrophosphate (MEcPP) and SA levels revealed competition for MEcPP by the methylerythritol phosphate (MEP) pathway and SA biosynthesis activation, thus explaining the absence of resistance in transgenic plants. These results were confirmed by chemical inhibition of the MEP pathway, which alters MEcPP levels. Treatments with benzothiadiazole (BTH), a SA functional analog, conferred enhanced resistance to transgenic tomato plants overexpressing MTS1. Additionally, these MTS1 overexpressors induced PR1 gene expression and stomatal closure in neighboring plants. Our results confirm the role of HMTPs in both intra- and interplant immune signaling and reveal a metabolic crosstalk between the MEP and SA pathways in tomato plants.

Terpenoids are involved in the expression of systemic-induced resistance in Austrian pine

Terpenoids are defense metabolites that are induced upon infection or wounding. However, their role in systemic-induced resistance (SIR) is not known. Here, we explored the role of terpenoids in this phenomenon at a very early stage in the interaction between Austrian pine and the tip blight and canker pathogen Diplodia pinea. We induced Austrian pine saplings by either wounding or inoculating the lower stems with D. pinea. The seedlings were then challenged after 12 h, 72 h, or 10 days with D. pinea on the stem 15 cm above the induction. Lesion lengths and terpenoids were quantified at both induction and challenge locations. Key terpenoids were assayed for antifungal activity in in vitro bioassays. SIR increased with time and was correlated with the inducibility of several compounds. α-Pinene and a cluster of β-pinene, limonene, benzaldehyde, dodecanol, and n-dodecyl acrylate were positively correlated with SIR and were fungistatic in vitro, while other compounds were negatively correlated with SIR and appeared to serve as a carbon source for D. pinea. This study shows that, overall, terpenoids are involved in SIR in this system, but their role is nuanced, depending on the type of induction and time of incubation. We hypothesize that some, such as α-pinene, could serve in SIR signaling.

Molecular responses of agroinfiltrated Nicotiana benthamiana leaves expressing suppressor of silencing P19 and influenza virus-like particles

The production of influenza vaccines in plants is achieved through transient expression of viral hemagglutinins (HAs), a process mediated by the bacterial vector Agrobacterium tumefaciens. HA proteins are then produced and matured through the secretory pathway of plant cells, before being trafficked to the plasma membrane where they induce formation of virus-like particles (VLPs). Production of VLPs unavoidably impacts plant cells, as do viral suppressors of RNA silencing (VSRs) that are co-expressed to increase recombinant protein yields. However, little information is available on host molecular responses to foreign protein expression. This work provides a comprehensive overview of molecular changes occurring in Nicotiana benthamiana leaf cells transiently expressing the VSR P19, or co-expressing P19 and an influenza HA. Our data identifies general responses to Agrobacterium-mediated expression of foreign proteins, including shutdown of chloroplast gene expression, activation of oxidative stress responses and reinforcement of the plant cell wall through lignification. Our results also indicate that P19 expression promotes salicylic acid (SA) signalling, a process dampened by co-expression of the HA protein. While reducing P19 level, HA expression also induces specific signatures, with effects on lipid metabolism, lipid distribution within membranes and oxylipin-related signalling. When producing VLPs, dampening of P19 responses thus likely results from lower expression of the VSR, crosstalk between SA and oxylipin pathways, or a combination of both outcomes. Consistent with the upregulation of oxidative stress responses, we finally show that reduction of oxidative stress damage through exogenous application of ascorbic acid improves plant biomass quality during production of VLPs.

Salicylic acid in plant immunity and beyond

As the most widely used herbal medicine in human history and a major defence hormone in plants against a broad spectrum of pathogens and abiotic stresses, salicylic acid (SA) has attracted major research interest. With applications of modern technologies over the past 30 years, studies of the effects of SA on plant growth, development, and defence have revealed many new research frontiers and continue to deliver surprises. In this review, we provide an update on recent advances in our understanding of SA metabolism, perception, and signal transduction mechanisms in plant immunity. An overarching theme emerges that SA executes its many functions through intricate regulation at multiple steps: SA biosynthesis is regulated both locally and systemically, while its perception occurs through multiple cellular targets, including metabolic enzymes, redox regulators, transcription cofactors, and, most recently, an RNA-binding protein. Moreover, SA orchestrates a complex series of post-translational modifications of downstream signaling components and promotes the formation of biomolecular condensates that function as cellular signalling hubs. SA also impacts wider cellular functions through crosstalk with other plant hormones. Looking into the future, we propose new areas for exploration of SA functions, which will undoubtedly uncover more surprises for many years to come.

Development of Menthyl Esters of Valine for Pest Control in Tomato and Lettuce Crops

Menthyl ester of valine (MV) has been developed as a plant defense potentiator to induce pest resistance in crops. In this study, we attempted to establish MV hydrochloride (MV-HCl) in lettuce and tomato crops. When MV-HCl solutions were used to treat soil or leaves of potted tomato and lettuce plants, 1 µM MV-HCl solution applied to potted plant soil was most effective in increasing the transcript level of defense genes such as pathogenesis-related 1 (PR1). As a result, leaf damage caused by Spodoptera litura and oviposition by Tetranychus urticae were significantly reduced. In addition, MV-HCl-treated plants showed an increased ability to attract Phytoseiulus persimilis, a predatory mite of T. urticae, when they were attacked by T. urticae. Overall, our findings showed that MV-HCl is likely to be effective in promoting not only direct defense by activating defense genes, but also indirect defense mediated by herbivore-induced plant volatiles. Moreover, based on the results of the sustainability of PR1 expression in tomato plants treated with MV-HCl every 3 days, field trials were conducted and showed a 70% reduction in natural leaf damage. Our results suggest a practical approach to promoting organic tomato and lettuce production using this new plant defense potentiator.

A nondestructive asymptomatic early disease prediction method employing ROS-induced differential volatile emissions from dry rot-infected potatoes

Potatoes are a staple crop with many health benefits. Postharvest storage of potatoes takes a considerable amount of time. Potato dry rot is one of the most serious postharvest storage diseases, caused primarily by the fungus Fusarium sambucinum. It is possible to minimize losses if disease is detected early, which allows it to be controlled promptly. A phytopathogen infection can alter the volatile profile of plants. Identifying unique volatile organic compounds (VOCs) as biomarkers for early disease detection is an area of considerable research interest. In this study, we compared the VOC profiles of healthy and dry rot inoculated potatoes (cv. "Kufri Pukhraj") over a time course using gas chromatography-mass spectrometry (GC-MS). There were 29 differentially emitting VOCs between healthy and dry rot inoculated potatoes. Nevertheless, only four of these compounds (linalool tetrahydride, γ-muurolene, alloaromadendrene, and α-isomethyl ionone) were exclusively found in dry rot inoculated potatoes, and hence they were considered biomarkers. Furthermore, reactive oxygen species (ROS) levels were altered in potatoes that were inoculated with dry rot, suggesting a role for ROS signaling in differential VOC emissions. In the early stages of dry rot infection, when symptoms were barely visible, these four biomarker VOCs were robustly useful in distinguishing healthy and dry rot-infected potatoes. These novel biomarkers associated with this disease are promising candidates for non-destructive detection of dry rot in stored potatoes at an early asymptomatic stage. These biomarkers can be used to develop an e-nose sensor to predict dry rot in the future.

Comparative study of two indoor microbial volatile pollutants, 2-Methyl-1-butanol and 3-Methyl-1-butanol, on growth and antioxidant system of rice (Oryza sativa) seedlings

2-Methyl-1-butanol (2MB) and 3-Methyl-1-butanol (3MB) are microbial volatile organic compounds (VOCs) and found in indoor air. Here, we applied rice as a bioindicator to investigate the effects of these indoor microbial volatile pollutants. A remarkable decrease in germination percentage, shoot and root elongation, as well as lateral root numbers were observed in 3MB. Furthermore, ROS production increased by 2MB and 3MB, suggesting that pentanol isomers could induce cytotoxicity in rice seedlings. The enhancement of peroxidase (POD) and catalase (CAT) activity provided evidence that pentanol isomers activated the enzymatic antioxidant scavenging systems, with a more significant effect observed in 3MB. Furthermore, 3MB induced higher activity levels of glutathione (GSH), oxidized glutathione (GSSG), and the GSH/GSSG ratio in rice compared to the levels induced by 2MB. Additionally, qRT-PCR analysis showed more up-regulation in the expression of glutaredoxins (GRXs), peroxiredoxins (PRXs), thioredoxins (TRXs), and glutathione S-transferases (GSTUs) genes in 3MB. Taking the impacts of pentanol isomers together, the present study suggests that 3MB exhibits more cytotoxic than 2MB, as such has critical effects on germination and the early seedling stage of rice. Our results provide molecular insights into how isomeric indoor microbial volatile pollutants affect plant growth through airborne signals.

High-resolution kinetics of herbivore-induced plant volatile transfer reveal clocked response patterns in neighboring plants

Volatiles emitted by herbivore-attacked plants (senders) can enhance defenses in neighboring plants (receivers), however, the temporal dynamics of this phenomenon remain poorly studied. Using a custom-built, high-throughput proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) system, we explored temporal patterns of volatile transfer and responses between herbivore-attacked and undamaged maize plants. We found that continuous exposure to natural blends of herbivore-induced volatiles results in clocked temporal response patterns in neighboring plants, characterized by an induced terpene burst at the onset of the second day of exposure. This delayed burst is not explained by terpene accumulation during the night, but coincides with delayed jasmonate accumulation in receiver plants. The delayed burst occurs independent of day:night light transitions and cannot be fully explained by sender volatile dynamics. Instead, it is the result of a stress memory from volatile exposure during the first day and secondary exposure to bioactive volatiles on the second day. Our study reveals that prolonged exposure to natural blends of stress-induced volatiles results in a response that integrates priming and direct induction into a distinct and predictable temporal response pattern. This provides an answer to the long-standing question of whether stress volatiles predominantly induce or prime plant defenses in neighboring plants, by revealing that they can do both in sequence.

Lipid peroxidation and stress-induced signalling molecules in systemic resistance mediated by azelaic acid/AZELAIC ACID INDUCED1: signal initiation and propagation

Systemic acquired resistance protects plants against a broad spectrum of secondary infections by pathogens. A crucial compound involved in the systemic spread of the threat information after primary pathogen infection is the C9 oxylipin azelaic acid (AZA), a breakdown product of unsaturated C18 fatty acids. AZA is generated during lipid peroxidation in the plastids and accumulates in response to various abiotic and biotic stresses. AZA stimulates the expression of AZELAIC ACID INDUCED1 (AZI1), and a pool of AZI1 accumulates in the plastid envelope in association with AZA. AZA and AZI1 utilize the symplastic pathway to travel through the plasmodesmata to neighbouring cells to induce systemic stress resistance responses in distal tissues. Here, we describe the synthesis, travel and function of AZA and AZI1 and discuss open questions of signal initiation and propagation.

Adjusting function of camphor on primary metabolism in Cinnamomum camphora stressed by high temperature

Cinnamomum camphora has great economic value for its wide utilization in traditional medicine and furniture material, and releases lots of monoterpenes to tolerate high temperature. To uncover the adjusting function of monoterpenes on primary metabolism and promoting their utilization as anti-high temperature agents, the photosynthetic capacities, primary metabolite levels, cell ultrastructure and associated gene expression were surveyed in C. camphora when it was blocked monoterpene biosynthesis with fosmidomycin (Fos) and fumigated with camphor (a typical monoterpene in the plant) under high temperature (Fos+38 °C+camphor). Compared with the control (28 °C), high temperature at 38 °C decreased the starch content and starch grain size, and increased the fructose, glucose, sucrose and soluble sugar content. Meanwhile, high temperature also raised the lipid content, with the increase of lipid droplet size and numbers. These variations were further intensified in Fos+ 38 °C treatment. Compared with Fos+ 38 °C treatment, Fos+ 38 °C+camphor treatment improved the starch accumulation by promoting 4 gene expression in starch biosynthesis, and lowered the sugar content by suppressing 3 gene expression in pentose phosphate pathway and promoting 15 gene expression in glycolysis and tricarboxylic acid cycle. Meanwhile, Fos+ 38 °C+camphor treatment also lowered the lipid content, which may be caused by the down-regulation of 2 genes in fatty acid formation and up-regulation of 4 genes in fatty acid decomposition. Although Fos+ 38 °C+camphor treatment improved the photosynthetic capacities in contrast to Fos+ 38 °C treatment, it cannot explain the variations of these primary metabolite levels. Therefore, camphor should adjust related gene expression to maintain the primary metabolism in C. camphora tolerating high temperature.

Damage-Associated Molecular Patterns and Systemic Signaling

Cellular damage inflicted by wounding, pathogen infection, and herbivory releases a variety of host-derived metabolites, degraded structural components, and peptides into the extracellular space that act as alarm signals when perceived by adjacent cells. These so-called damage-associated molecular patterns (DAMPs) function through plasma membrane localized pattern recognition receptors to regulate wound and immune responses. In plants, DAMPs act as elicitors themselves, often inducing immune outputs such as calcium influx, reactive oxygen species generation, defense gene expression, and phytohormone signaling. Consequently, DAMP perception results in a priming effect that enhances resistance against subsequent pathogen infections. Alongside their established function in local tissues, recent evidence supports a critical role of DAMP signaling in generation and/or amplification of mobile signals that induce systemic immune priming. Here, we summarize the identity, signaling, and synergy of proposed and established plant DAMPs, with a focus on those with published roles in systemic signaling.

(E)-2-hexenal fumigation control the gray mold on fruits via consuming glutathione of Botrytis cinerea

(E)-2-hexenal fumigation inhibits the growth of Botrytis. cinerea, but the direct target and its effect on postharvest strawberry have not yet been discovered. In the present study, we applied increasing level of (E)-2-hexenal fumigation from 0.524 μM to 1048 μM to B. cinerea on strawberry and medium. Results showed that (E)-2-hexenal fumigation inhibited lesion diameter on strawberry by 13.96 %, 20.41 % and 100 % with the dosage increasing. On medium, (E)-2-hexenal fumigation increased both the ROS and mitochondria membrane potential level of B. cinerea. LC-MS/MS and FTIR results demonstrate a 1:1 Michael addition reaction between (E)-2-hexenal and glutathione with the product GSH-H in B. cinerea under (E)-2-hexenal fumigation. Furthermore, the consumption of glutathione and glutathione disulfide along with the production of GSH-H during fumigation in B. cinerea caused by (E)-2-hexenal were both concentration- and time-dependent. This study locates the direct target and discovered the functional model of (E)-2-hexenal to B. cinerea.

Characterization of Larix decidua Mill. (Pinaceae) oleoresin's essential oils composition using GC-MS

Introduction

Larch oleoresin has been described regarding several biological activities and medicinal applications, such as wound healing and treatment of ulcers, but little is known about its chemical composition.

Material and methods

Eight oleoresins from Larix decidua Mill. obtained from four companies and one adulterated control were therefore investigated to determine their content of essential oils and to verify possible differences in their composition in relation to the harvest and manufacturing processes. Essential oils (EOs) were isolated by distillation and the yield was analysed.

Results and discussion

The yield of EO varied among all samples. The yield of the pure larch samples covered a range of 7.8% to 15.5%. A higher yield (19.0%) was observed for adulterated control, which contained oleoresins from different Pinaceae trees. Age of samples had no impact on yield. However, there was a significant statistical variation (p<0.05) in the yields of the mid-summer oleoresins (>10%) compared to early or late summer (<10%), emphasising the importance of the time of collection. Samples were subsequently analysed by GC-MS. EO samples confirmed the presence of various chemical classes, such as monoterpenes, sesquiterpenes, and diterpenes. α-pinene was the compound with the highest concentrations (>50%), followed by β-pinene (>6%), D-limonene (>2.5%), α-terpineol (>0.9%), β-myrcene (>0.2%), and 3-carene (>0.05%). Samples were grouped using multivariate data analysis (MVDA) with respect to the chemical variation between the oleoresins' EOs. The resulting four clusters were named low (low yield obtained for the samples), mixed (mixed oleoresin from different Pinaceae species, adulteration control), old (old oleoresin kept in the institute), and normal (other oleoresins) samples, each presenting distinct chemical biomarkers. There were considerable differences between site and time of collection. Essential oil yield did not always meet requirements as defined by the German Homeopathic Pharmacopoeia. In addition, adulterated or aged samples could be identified as compared to pure and fresh larch oleoresins.

Conclusion

We conclude that larch oleoresin used for pharmaceutical applications has to be carefully analysed and standardised to guarantee reproducible product quality.

Transcriptome Analysis of Sweet Cherry (Prunus avium L.) Cultivar 'Lapins' upon Infection of Pseudomonas syringae pv. syringae

Bacterial canker caused by Pseudomonas syringae pv. syringae (Pss) is responsible for substantial loss to the production of sweet cherry in Chile. To date, the molecular mechanisms of the Pss-sweet cherry interaction and the disease-related genes in the plant are poorly understood. In order to gain insight into these aspects, a transcriptomic analysis of the sweet cherry cultivar 'Lapins' for differentially expressed genes (DEGs) in response to Pss inoculation was conducted. Three Pss strains, A1M3, A1M197, and 11116_b1, were inoculated in young twigs, and RNA was extracted from tissue samples at the inoculation site and distal sections. RNA sequencing and transcriptomic expression analysis revealed that the three strains induced different patterns of responses in local and distal tissues. In the local tissues, A1M3 triggered a much more extensive response than the other two strains, enriching DEGs especially involved in photosynthesis. In the distal tissues, the three strains triggered a comparable extent of responses, among which 11116_b1 induced a group of DEGs involved in defense responses. Furthermore, tissues from various inoculations exhibited an enrichment of DEGs related to carbohydrate metabolism, terpene metabolism, and cell wall biogenesis. This study opened doors to future research on the Pss-sweet cherry interaction, immunity responses, and disease control.

N-hydroxypipecolic acid triggers systemic acquired resistance through extracellular NAD(P)

Systemic acquired resistance (SAR) is a long-lasting broad-spectrum plant defense mechanism induced in distal systemic tissues by mobile signals generated at the primary infection site. Despite the discoveries of multiple potential mobile signals, how these signals cooperate to trigger downstream SAR signaling is unknown. Here, we show that endogenous extracellular nicotinamide adenine dinucleotide (phosphate) [eNAD(P)] accumulates systemically upon pathogen infection and that both eNAD(P) and the lectin receptor kinase (LecRK), LecRK-VI.2, are required in systemic tissues for the establishment of SAR. Moreover, putative mobile signals, e.g., N-hydroxypipecolic acid (NHP), trigger de novo systemic eNAD(P) accumulation largely through the respiratory burst oxidase homolog RBOHF-produced reactive oxygen species (ROS). Importantly, NHP-induced systemic immunity mainly depends on ROS, eNAD(P), LecRK-VI.2, and BAK1, indicating that NHP induces SAR primarily through the ROS-eNAD(P)-LecRK-VI.2/BAK1 signaling pathway. Our results suggest that mobile signals converge on eNAD(P) in systemic tissues to trigger SAR through LecRK-VI.2.

Analysis of the apoplast fluid proteome during the induction of systemic acquired resistance in Arabidopsis thaliana

Background

Plant-pathogen interactions occur in the apoplast comprising the cell wall matrix and the fluid in the extracellular space outside the plasma membrane. However, little is known regarding the contribution of the apoplastic proteome to systemic acquired resistance (SAR).

Methods

Specifically, SAR was induced by inoculating plants with Pst DC3000 avrRps4. The apoplast washing fluid (AWF) was collected from the systemic leaves of the SAR-induced or mock-treated plants. A label free quantitative proteomic analysis was performed to identified the proteins related to SAR in AWF.

Results

A total of 117 proteins were designated as differentially accumulated proteins (DAPs), including numerous pathogenesis-related proteins, kinases, glycosyl hydrolases, and redox-related proteins. Functional enrichment analyses shown that these DAPs were mainly enriched in carbohydrate metabolic process, cell wall organization, hydrogen peroxide catabolic process, and positive regulation of catalytic activity. Comparative analysis of proteome data indicated that these DAPs were selectively enriched in the apoplast during the induction of SAR.

Conclusions

The findings of this study indicate the apoplastic proteome is involved in SAR. The data presented herein may be useful for future investigations on the molecular mechanism mediating the establishment of SAR.

Anti-fungal bioactive terpenoids in the bioenergy crop switchgrass (Panicum virgatum) may contribute to ecotype-specific microbiome composition

Plant derived bioactive small molecules have attracted attention of scientists across fundamental and applied scientific disciplines. We seek to understand the influence of these phytochemicals on rhizosphere and root-associated fungi. We hypothesize that - consistent with accumulating evidence that switchgrass genotype impacts microbiome assembly - differential terpenoid accumulation contributes to switchgrass ecotype-specific microbiome composition. An initial in vitro Petri plate-based disc diffusion screen of 18 switchgrass root derived fungal isolates revealed differential responses to upland- and lowland-isolated metabolites. To identify specific fungal growth-modulating metabolites, we tested fractions from root extracts on three ecologically important fungal isolates - Linnemania elongata, Trichoderma sp. and Fusarium sp. Saponins and diterpenoids were identified as the most prominent antifungal metabolites. Finally, analysis of liquid chromatography-purified terpenoids revealed fungal inhibition structure - activity relationships (SAR). Saponin antifungal activity was primarily determined by the number of sugar moieties - saponins glycosylated at a single core position were inhibitory whereas saponins glycosylated at two core positions were inactive. Saponin core hydroxylation and acetylation were also associated with reduced activity. Diterpenoid activity required the presence of an intact furan ring for strong fungal growth inhibition. These results inform future breeding and biotechnology strategies for crop protection with reduced pesticide application.

Volatile compound-mediated plant-plant interactions under stress with the tea plant as a model

Plants respond to environmental stimuli via the release of volatile organic compounds (VOCs), and neighboring plants constantly monitor and respond to these VOCs with great sensitivity and discrimination. This sensing can trigger increased plant fitness and reduce future plant damage through the priming of their own defenses. The defense mechanism in neighboring plants can either be induced by activation of the regulatory or transcriptional machinery, or it can be delayed by the absorption and storage of VOCs for the generation of an appropriate response later. Despite much research, many key questions remain on the role of VOCs in interplant communication and plant fitness. Here we review recent research on the VOCs induced by biotic (i.e. insects and pathogens) and abiotic (i.e. cold, drought, and salt) stresses, and elucidate the biosynthesis of stress-induced VOCs in tea plants. Our focus is on the role of stress-induced VOCs in complex ecological environments. Particularly, the roles of VOCs under abiotic stress are highlighted. Finally, we discuss pertinent questions and future research directions for advancing our understanding of plant interactions via VOCs.

Mechanisms of systemic resistance to pathogen infection in plants and their potential application in forestry

BACKGROUND

The complex systemic responses of tree species to fight pathogen infection necessitate attention due to the potential for yield protection in forestry.

RESULTS

In this paper, both the localized and systemic responses of model plants, such as Arabidopsis and tobacco, are reviewed. These responses were compared to information available that investigates similar responses in woody plant species and their key differences were highlighted. In addition, tree-specific responses that have been documented were summarised, with the critical responses still relying on certain systemic acquired resistance pathways. Importantly, coniferous species have been shown to utilise phenolic compounds in their immune responses. Here we also highlight the lack of focus on systemic induced susceptibility in trees, which can be important to forest health.

CONCLUSIONS

This review highlights the possible mechanisms of systemic response to infection in woody plant species, their potential applications, and where research may be best focused in future.