Abscisic acid has a key role in modulating diverse plant-pathogen interactions

Exploration of a multifunctional biocontrol agent Streptomyces sp. JCK-8055 for the management of apple fire blight

Apple fire blight, caused by the bacterium Erwinia amylovora, is a devastating disease of apple and pear trees. Biological control methods have attracted much attention from researchers to manage plant diseases as they are eco-friendly and viable alternatives to synthetic pesticides. Herein, we isolated Streptomyces sp. JCK-8055 from the root of pepper and investigated its mechanisms of action against E. amylovora. Streptomyces sp. JCK-8055 produced aureothricin and thiolutin, which antagonistically affect E. amylovora. JCK-8055 and its two active metabolites have a broad-spectrum in vitro activity against various phytopathogenic bacteria and fungi. They also effectively suppressed tomato bacterial wilt and apple fire blight in in vivo experiments. Interestingly, JCK-8055 colonizes roots as a tomato seed coating and induces apple leaf shedding at the abscission zone, ultimately halting the growth of pathogenic bacteria. Additionally, JCK-8055 can produce the plant growth regulation hormone indole-3-acetic acid (IAA) and hydrolytic enzymes, including protease, gelatinase, and cellulase. JCK-8055 treatment also triggered the expression of salicylate (SA) and jasmonate (JA) signaling pathway marker genes, such as PR1, PR2, and PR3. Overall, our findings demonstrate that Streptomyces sp. JCK-8055 can control a wide range of plant diseases, particularly apple fire blight, through a combination of mechanisms such as antibiosis and induced resistance, highlighting its excellent potential as a biocontrol agent. KEY POINTS: • JCK-8055 produces the systemic antimicrobial metabolites, aureothricin, and thiolutin. • JCK-8055 treatment upregulates PR gene expression in apple plants against E. amylovora. • JCK-8055 controls plant diseases with antibiotics and induced resistance.

Exogenous ABA promotes resistance to Sitobion avenae (Fabricius) in rice seedlings

BACKGROUND

Over the course of evolution, plants have developed various sophisticated defense mechanisms to resist pests and diseases. The phytohormone abscisic acid (ABA) has an important role in the growth and development of plants and confers tolerance to selected abiotic stressors, such as drought. Previous studies have shown that ABA promotes the deposit of callose in response to piercing/sucking insect pests. The English grain aphid, Sitobion avenae Fabricius, causes huge losses in rice and is especially harmful to rice seedlings.

RESULTS

Exogenous ABA promoted growth and reduced the feeding behavior of S. avenae nymphs in rice. Our results suggested that enhanced trichome density and increased expression of related genes may be associated with rice resistance to aphids. An analysis of volatiles revealed the production of seven compounds associated with pest resistance.

CONCLUSION

These results indicate that ABA reduces aphid feeding in rice. Our findings provide a basis for understanding ABA-mediated defense responses in rice and provide insights on more environmentally-friendly approaches to control. © 2024 Society of Chemical Industry.

Drought: A context-dependent damper and aggravator of plant diseases

Drought dynamically influences the interactions between plants and pathogens, thereby affecting disease outbreaks. Understanding the intricate mechanistic aspects of the multiscale interactions among plants, pathogens, and the environment-known as the disease triangle-is paramount for enhancing the climate resilience of crop plants. In this review, we systematically compile and comprehensively analyse current knowledge on the influence of drought on the severity of plant diseases. We emphasise that studying these stresses in isolation is not sufficient to predict how plants respond to combined stress from both drought and pathogens. The impact of drought and pathogens on plants is complex and multifaceted, encompassing the activation of antagonistic signalling cascades in response to stress factors. The nature, intensity, and temporality of drought and pathogen stress occurrence significantly influence the outcome of diseases. We delineate the drought-sensitive nodes of plant immunity and highlight the emerging points of crosstalk between drought and defence signalling under combined stress. The limited mechanistic understanding of these interactions is acknowledged as a key research gap in this area. The information synthesised herein will be crucial for crafting strategies for the accurate prediction and mitigation of future crop disease risks, particularly in the context of a changing climate.

Physiochemical interaction between osmotic stress and a bacterial exometabolite promotes plant disease

Various microbes isolated from healthy plants are detrimental under laboratory conditions, indicating the existence of molecular mechanisms preventing disease in nature. Here, we demonstrated that application of sodium chloride (NaCl) in natural and gnotobiotic soil systems is sufficient to induce plant disease caused by an otherwise non-pathogenic root-derived Pseudomonas brassicacearum isolate (R401). Disease caused by combinatorial treatment of NaCl and R401 triggered extensive, root-specific transcriptional reprogramming that did not involve down-regulation of host innate immune genes, nor dampening of ROS-mediated immunity. Instead, we identified and structurally characterized the R401 lipopeptide brassicapeptin A as necessary and sufficient to promote disease on salt-treated plants. Brassicapeptin A production is salt-inducible, promotes root colonization and transitions R401 from being beneficial to being detrimental on salt-treated plants by disturbing host ion homeostasis, thereby bolstering susceptibility to osmolytes. We conclude that the interaction between a global change stressor and a single exometabolite from a member of the root microbiome promotes plant disease in complex soil systems.

Strategies for Enhancing Plant Immunity and Resilience Using Nanomaterials for Sustainable Agriculture

Research on plant-nanomaterial interactions has greatly advanced over the past decade. One particularly fascinating discovery encompasses the immunomodulatory effects in plants. Due to the low doses needed and the comparatively low toxicity of many nanomaterials, nanoenabled immunomodulation is environmentally and economically promising for agriculture. It may reduce environmental costs associated with excessive use of chemical pesticides and fertilizers, which can lead to soil and water pollution. Furthermore, nanoenabled strategies can enhance plant resilience against various biotic and abiotic stresses, contributing to the sustainability of agricultural ecosystems and the reduction of crop losses due to environmental factors. While nanoparticle immunomodulatory effects are relatively well-known in animals, they are still to be understood in plants. Here, we provide our perspective on the general components of the plant's immune system, including the signaling pathways, networks, and molecules of relevance for plant nanomodulation. We discuss the recent scientific progress in nanoenabled immunomodulation and nanopriming and lay out key avenues to use plant immunomodulation for agriculture. Reactive oxygen species (ROS), the mitogen-activated protein kinase (MAPK) cascade, and the calcium-dependent protein kinase (CDPK or CPK) pathway are of particular interest due to their interconnected function and significance in the response to biotic and abiotic stress. Additionally, we underscore that understanding the plant hormone salicylic acid is vital for nanoenabled applications to induce systemic acquired resistance. It is suggested that a multidisciplinary approach, incorporating environmental impact assessments and focusing on scalability, can expedite the realization of enhanced crop yields through nanotechnology while fostering a healthier environment.

NaWRKY70 is a key regulator of Nicotiana attenuata resistance to Alternaria alternata through regulation of phytohormones and phytoalexins biosynthesis

Jasmonate (JA) and abscisic acid (ABA) are two major phytohormones involved in pathogen resistance. However, how their biosynthesis is regulated is not well understood. We silenced NaWRKY70 in wild tobacco Nicotiana attenuata and determined its role in regulating genes involved in the production of JA, ABA and the phytoalexin capsidiol in response to the fungal pathogen Alternaria alternata using techniques including electrophoretic mobility shift, chromatin immunoprecipitation, transient overexpression and virus-induced gene silencing. Silencing NaWRKY70 dramatically reduced both basal and A. alternata-induced jasmonoyl-isoleucine (JA-Ile) and ABA. Further evidence showed that NaWRKY70 directly binds to the W-boxes of the promoters of NaAOS and NaJAR4 (JA biosynthesis), NaNCED1 and NaXD1-like (ABA biosynthesis), and NaMPK4 (ABA signaling) to activate their expression, while binding but repressing the expression of NaCYP707A4-like3 (ABA degradation). Additionally, NaWRKY70 regulates capsidiol production through its key enzyme genes NaEASs and NaEAHs, and interacts with its regulator NaERF2-like to enhance their expression, whereas ABA negatively regulates capsidiol biosynthesis. Our results highlight the key role of NaWRKY70 in controlling both JA-Ile and ABA production, as well as capsidiol production, thus providing new insight into the defense mechanism of plant resistance to A. alternata.

The Role of Cytokinins and Abscisic Acid in the Growth, Development and Virulence of the Pathogenic Fungus Stagonospora nodorum (Berk.)

Cytokinins (CKs) and abscisic acid (ABA) play an important role in the life of both plants and pathogenic fungi. However, the role of CKs and ABA in the regulation of fungal growth, development and virulence has not been sufficiently studied. We compared the ability of two virulent isolates (SnB and Sn9MN-3A) and one avirulent isolate (Sn4VD) of the pathogenic fungus Stagonospora nodorum Berk. to synthesize three groups of hormones (CKs, ABA and auxins) and studied the effect of exogenous ABA and zeatin on the growth, sporulation and gene expression of necrotrophic effectors (NEs) and transcription factors (TFs) in them. Various isolates of S. nodorum synthesized different amounts of CKs, ABA and indoleacetic acid. Using exogenous ABA and zeatin, we proved that the effect of these hormones on the growth and sporulation of S. nodorum isolates can be opposite, depends on both the genotype of the isolate and on the concentration of the hormone and is carried out through the regulation of carbohydrate metabolism. ABA and zeatin regulated the expression of fungal TF and NE genes, but correlation analysis of these parameters showed that this effect depended on the genotype of the isolate. This study will contribute to our understanding of the role of the hormones ABA and CKs in the biology of the fungal pathogen S. nodorum.

A plant NLR receptor employs ABA central regulator PP2C-SnRK2 to activate antiviral immunity

Defence against pathogens relies on intracellular nucleotide-binding, leucine-rich repeat immune receptors (NLRs) in plants. Hormone signaling including abscisic acid (ABA) pathways are activated by NLRs and play pivotal roles in defence against different pathogens. However, little is known about how hormone signaling pathways are activated by plant immune receptors. Here, we report that a plant NLR Sw-5b mimics the behavior of the ABA receptor and directly employs the ABA central regulator PP2C-SnRK2 complex to activate an ABA-dependent defence against viral pathogens. PP2C4 interacts with and constitutively inhibits SnRK2.3/2.4. Behaving in a similar manner as the ABA receptor, pathogen effector ligand recognition triggers the conformational change of Sw-5b NLR that enables binding to PP2C4 via the NB domain. This receptor-PP2C4 binding interferes with the interaction between PP2C4 and SnRK2.3/2.4, thereby releasing SnRK2.3/2.4 from PP2C4 inhibition to activate an ABA-specific antiviral immunity. These findings provide important insights into the activation of hormone signaling pathways by plant immune receptors.

Genome-Wide Identification and Characterization of the Sweet Orange (Citrus sinensis) GATA Family Reveals a Role for CsGATA12 as a Regulator of Citrus Bacterial Canker Resistance

Citrus bacterial canker (CBC) is a severe bacterial infection caused by Xanthomonas citri subsp. citri (Xcc), which continues to adversely impact citrus production worldwide. Members of the GATA family are important regulators of plant development and regulate plant responses to particular stressors. This report aimed to systematically elucidate the Citrus sinensis genome to identify and annotate genes that encode GATAs and evaluate the functional importance of these CsGATAs as regulators of CBC resistance. In total, 24 CsGATAs were identified and classified into four subfamilies. Furthermore, the phylogenetic relationships, chromosomal locations, collinear relationships, gene structures, and conserved domains for each of these GATA family members were also evaluated. It was observed that Xcc infection induced some CsGATAs, among which CsGATA12 was chosen for further functional validation. CsGATA12 was found to be localized in the nucleus and was differentially upregulated in the CBC-resistant and CBC-sensitive Kumquat and Wanjincheng citrus varieties. When transiently overexpressed, CsGATA12 significantly reduced CBC resistance with a corresponding increase in abscisic acid, jasmonic acid, and antioxidant enzyme levels. These alterations were consistent with lower levels of salicylic acid, ethylene, and reactive oxygen species. Moreover, the bacteria-induced CsGATA12 gene silencing yielded the opposite phenotypic outcomes. This investigation highlights the important role of CsGATA12 in regulating CBC resistance, underscoring its potential utility as a target for breeding citrus varieties with superior phytopathogen resistance.

A Comprehensive Analysis of Transcriptomics and Metabolomics Revealed Key Pathways Involved in Saccharum spontaneum Defense against Sporisorium scitamineum

Sugarcane smut, caused by Sporisorium scitamineum, poses a severe threat to sugarcane production. The genetic basis of sugarcane resistance to S. scitamineum remains elusive. A comparative transcriptomic and metabolomic study was conducted on two wild Saccharum species of S. spontaneum with contrast smut resistance. Following infection, the resistant line exhibited greater down-regulation of genes and metabolites compared to the susceptible line, indicating distinct biological processes. Lignan and lignin biosynthesis and SA signal transduction were activated in the resistant line, while flavonoid biosynthesis and auxin signal transduction were enhanced in the susceptible line. TGA2.2 and ARF14 were identified as playing positive and negative roles, respectively, in plant defense. Exogenous auxin application significantly increased the susceptibility of S. spontaneum to S. scitaminum. This study established the significant switching of defense signaling pathways in contrast-resistant S. spontaneum following S. scitamineum infection, offering a hypothetical model and candidate genes for further research into sugarcane smut disease.

Characterization key genes of Arabidopsis seedlings in response to β-caryophyllene, eugenol using combined transcriptome and WGCN analysis

Weeds present a significant challenge to high crop yield and quality. In our study, we investigated the phytotoxic activity of β-caryophyllene (BCP) and eugenol, which are natural allelopathic chemical compounds, on Arabidopsis seedlings. We found that these compounds inhibited the growth of Arabidopsis thaliana plants. When either BCP or eugenol was applied, it led to decrease in the content of cell wall components such as lignin, cellulose, hemicellulose, and pectin; and increase in the levels of endogenous hormones like ETH, ABA, SA, and JA in the seedlings. Through transcriptome profiling, we identified 7181 differentially expressed genes (DEGs) in the roots and shoots that were induced by BCP or eugenol. The genes involved in the synthesis of lignin, cellulose, hemicellulose, and pectin were down-regulated, whereas genes related to synthesis and signal transduction of ABA, ETH, SA, and JA were up-regulated. However, genes related to IAA synthesis and signal transduction were found to be down-regulated. Furthermore, we characterized 24 hub genes using Weighted Correlation Network Analysis (WGCNA). Among them, the identified 16 genes in response to BCP was primarily associated with hypoxia stress, while 8 genes induced by eugenol were linked to inhibition of cell division. Our results suggested that BCP and eugenol had ability to target multiple genes to inhibit growth and development of Arabidopsis plants. Therefore, they can serve as excellent candidates for natural biological herbicides.

Voltage-dependent anion channel 3 (VDAC3) mediates P. syringae induced ABA-SA signaling crosstalk in Arabidopsis thaliana

Pathogen severely affects plant mitochondrial processes including respiration, however, the roles and mechanism of mitochondrial protein during the immune response remain largely unexplored. The interplay of plant hormone signaling during defense is an outcome of plant pathogen interaction. We recently discovered that the Arabidopsis calcineurin B-like interacting protein kinase 9 (AtCIPK9) interacts with the voltage-dependent anion channel 3 (AtVDAC3) and inhibits MV-induced oxidative damage. Here we report the characterization of AtVDAC3 in an antagonistic interaction pathway between abscisic acid (ABA) and salicylic acid (SA) signaling in Pseudomonas syringae -Arabidopsis interaction. In this study, we observed that mutants of AtVDAC3 were highly susceptible to Pseudomonas syringae infection as compared to the wild type (WT) Arabidopsis plants. Transcripts of VDAC3 and CIPK9 were inducible upon ABA application. Following pathogen exposure, expression analyses of ABA and SA biosynthesis genes indicated that the function of VDAC3 is required for isochorisimate synthase 1 (ICS1) expression but not for Nine-cis-epoxycaotenoid dioxygenase 3 (NCED3) expression. Despite the fact that vdac3 mutants had increased NCED3 expression in response to pathogen challenge, transcripts of ABA sensitive genes such as AtRD22 and AtRAB18 were downregulated even after exogenous ABA application. VDAC3 is required for ABA responsive genes expression upon exogenous ABA application. We also found that Pseudomonas syringae-induced SA signaling is downregulated in vdac3 mutants since overexpression of VDAC3 resulted in hyperaccumulation of Pathogenesis related gene1 (PR1) transcript. Interestingly, ABA application prior to P. syringae inoculation resulted in the upregulation of ABA responsive genes like Responsive to ABA18 (RAB18) and Responsive to dehydration 22 (RD22). Intriguingly, in the absence of AtVDAC3, Pst challenge can dramatically increase ABA-induced RD22 and RAB18 expression. Altogether our results reveal a novel Pathogen-SA-ABA interaction pathway in plants. Our findings show that ABA plays a significant role in modifying plant-pathogen interactions, owing to cross-talk with the biotic stress signaling pathways of ABA and SA.

Abscisic acid- and ethylene-induced abscission of yellow lupine flowers is mediated by jasmonates

The appropriate timing of organ abscission determines plant growth, development, reproductive success, and yield in relation to crop species. Among these, yellow lupine is an example of a crop species that loses many fully developed flowers, which limits the formation of pods with high-protein seeds and affects its economic value. Lupine flower abscission, similarly to the separation of other organs, depends on a complex regulatory network functioning in the cells of the abscission zone (AZ). In the present study, genetic, biochemical, and cellular methods were used to highlight the complexity of the interactions among strong hormonal stimulators of abscission, including abscisic acid (ABA), ethylene, and jasmonates (JAs) precisely in the AZ cells, with all results supporting that the JA-related pathway has an important role in the phytohormonal cross-talk leading to flower abscission in yellow lupine. Based on obtained results, we conclude that ABA and ET have positive influence on JAs biosynthesis and signaling pathway in time-dependent manner. Both phytohormones changes lipoxygenase (LOX) gene expression, affects LOX protein abundance, and JA accumulation in AZ cells. We have also shown that the signaling pathway of JA is highly sensitive to ABA and ET, given the accumulation of COI1 receptor and MYC2 transcription factor in response to these phytohormones. The results presented provide novel information about the JAs-dependent separation of organs and provide insight and details about the phytohormone-related mechanisms of lupine flower abscission.

Abscisic Acid Is Involved in the Resistance Response of Arabidopsis thaliana Against Meloidogyne paranaensis

Abscisic acid (ABA) is a classical hormone involved in the plant defense against abiotic stresses, especially drought. However, its role in the defense response against biotic stresses is controversial: it can induce resistance to some pathogens but can also increase the susceptibility to other pathogens. Information regarding the effect of ABA on the relationship between plants and sedentary phytonematodes, such as Meloidogyne paranaensis, is scarce. In this study, we found that ABA changed the susceptibility level of Arabidopsis thaliana against M. paranaensis. The population of M. paranaensis was reduced by 58.3% with the exogenous application of ABA 24 h before the nematode inoculation, which demonstrated that ABA plays an important role in the preinfectional defense of A. thaliana against M. paranaensis. The increase in the nematode population density in the ABA biosynthesis mutant, aba2-1, corroborated the results observed with the exogenous application of ABA. The phytohormone did not show nematicide or nematostatic effects on M. paranaensis juveniles in in vitro tests, indicating that the response is linked to intrinsic plant factors, which was corroborated by the decrease in the number of nematodes in the abi4-1 mutant. This reduction indicates that the gene expression regulation by transcript factors is possibly related to regulatory cascades mediated by ABA in the response of A. thaliana against M. paranaensis.

The RNA-binding protein MdHYL1 modulates cold tolerance and disease resistance in apple

Apple (Malus domestica) trees often experience various abiotic and biotic stresses. However, due to the long juvenile period of apple and its high degree of genetic heterozygosity, only limited progress has been made in developing cold-hardy and disease-resistant cultivars through traditional approaches. Numerous studies reveal that biotechnology is a feasible approach to improve stress tolerance in woody perennial plants. HYPONASTIC LEAVES1 (HYL1), a double-stranded RNA-binding protein, is a key regulator involved in apple drought stress response. However, whether HYL1 participates in apple cold response and pathogen resistance remains unknown. In this study, we revealed that MdHYL1 plays a positive role in cold tolerance and pathogen resistance in apple. MdHYL1 acted upstream to positively regulate freezing tolerance and Alternaria alternata resistance by positively modulating transcripts of MdMYB88 and MdMYB124 in response to cold stress or A. alternata infection. In addition, MdHYL1 regulated the biogenesis of several miRNAs responsive to cold and A. alternata infection in apple. Furthermore, we identified Mdm-miRNA156 (Mdm-miR156) as a negative regulator of cold tolerance and Mdm-miRNA172 (Mdm-miR172) as a positive regulator of cold tolerance, and that Mdm-miRNA160 (Mdm-miR160) decreased plant resistance to infection by A. alternata. In summary, we highlight the molecular role of MdHYL1 regarding cold tolerance and A. alternata infection resistance, thereby providing candidate genes for breeding apple with freezing tolerance and A. alternata resistance using biotechnology.

Integrated transcriptome and metabolome analysis unveil the response mechanism in wild rice (Zizania latifolia griseb.) against sheath rot infection

Sheath rot disease (SRD) is one of the most devastating diseases of Manchurian wild rice (MWR) (Zizania latifolia Griseb). Pilot experiments in our laboratory have shown that an MWR cultivar "Zhejiao NO.7"exhibits signs of SRD tolerance. To explore the responses of Zhejiao No. 7 to SRD infection, we used a combined transcriptome and metabolome analysis approach. A total of 136 differentially accumulated metabolites (DAMs, 114 up- and 22 down-accumulated in FA compared to CK) were detected. These up-accumulated metabolites were enriched in tryptophan metabolism, amino acid biosynthesis, flavonoids, and phytohormone signaling. Transcriptome sequencing results showed the differential expression of 11,280 genes (DEGs, 5,933 up-, and 5,347 downregulated in FA compared to CK). The genes expressed in tryptophan metabolism, amino acid biosynthesis, phytohormone biosynthesis and signaling, and reactive oxygen species homeostasis confirmed the metabolite results. In addition, genes related to the cell wall, carbohydrate metabolism, and plant-pathogen interaction (especially hypersensitive response) showed changes in expression in response to SRD infection. These results provide a basis for understanding the response mechanisms in MWR to FA attack that can be used for breeding SRD-tolerant MWR.

Efficient strategies for controlled release of nanoencapsulated phytohormones to improve plant stress tolerance

Climate change due to different human activities is causing adverse environmental conditions and uncontrolled extreme weather events. These harsh conditions are directly affecting the crop areas, and consequently, their yield (both in quantity and quality) is often impaired. It is essential to seek new advanced technologies to allow plants to tolerate environmental stresses and maintain their normal growth and development. Treatments performed with exogenous phytohormones stand out because they mitigate the negative effects of stress and promote the growth rate of plants. However, the technical limitations in field application, the putative side effects, and the difficulty in determining the correct dose, limit their widespread use. Nanoencapsulated systems have attracted attention because they allow a controlled delivery of active compounds and for their protection with eco-friendly shell biomaterials. Encapsulation is in continuous evolution due to the development and improvement of new techniques economically affordable and environmentally friendly, as well as new biomaterials with high affinity to carry and coat bioactive compounds. Despite their potential as an efficient alternative to phytohormone treatments, encapsulation systems remain relatively unexplored to date. This review aims to emphasize the potential of phytohormone treatments as a means of enhancing plant stress tolerance, with a specific focus on the benefits that can be gained through the improved exogenous application of these treatments using encapsulation techniques. Moreover, the main encapsulation techniques, shell materials and recent work on plants treated with encapsulated phytohormones have been compiled.

Identification and expression of the CCO family during development, ripening and stress response in banana

Plant hormone abscisic acid (ABA) plays an important role in plant growth, development and response to biotic / abiotic stressors. Thus, it is necessary to investigate the crucial genes associated with ABA synthesis. Currently, the carotenoid cleavage oxygenases (CCOs) family that function as the key step for ABA synthesis are not well understood in banana. In this study, 13 MaCCO genes and 12 MbCCO genes, divided into NCED subgroup and CCD subgroup, were identified from the banana genome, and their evolutionary relationship, protein motifs, and gene structures were also determined. Transcriptomic analysis suggested the involvement of CCO genes in banana development, ripening, and response to abiotic and biotic stressors, and homologous gene pairs showed homoeologue expression bias in the A or B subgenome. Our results identified MaNCED3A, MaCCD1, and MbNCED3B as the genes with the highest expression during fruit development and ripening. MaNCED5 / MbNCED5 and MaNCED9A might respond to abiotic stress, and MaNCED3A, 3B, 6 A, 9 A, and MbNCED9A showed transcriptional changes that could be a response to Foc4 infection. These findings may contribute to the characterization of key enzymes involved in ABA biosynthesis, as well as to identify potential targets for the genetic improvement of banana.

The MAX2-KAI2 module promotes salicylic acid-mediated immune responses in Arabidopsis

Arabidopsis MORE AXILLARY GROWTH2 (MAX2) is a key component in the strigolactone (SL) and karrikin (KAR) signaling pathways and regulates the degradation of SUPPRESSOR OF MAX2 1/SMAX1-like (SMAX1/SMXL) proteins, which are transcriptional co-repressors that regulate plant architecture, as well as abiotic and biotic stress responses. The max2 mutation reduces resistance against Pseudomonas syringae pv. tomato (Pst). To uncover the mechanism of MAX2-mediated resistance, we evaluated the resistance of various SL and KAR signaling pathway mutants. The resistance of SL-deficient mutants and of dwarf 14 (d14) was similar to that of the wild-type, whereas the resistance of the karrikin insensitive 2 (kai2) mutant was compromised, demonstrating that the KAR signaling pathway, not the SL signaling pathway, positively regulates the immune response. We measured the resistance of smax1 and smxl mutants, as well as the double, triple, and quadruple mutants with max2, which revealed that both the smax1 mutant and smxl6/7/8 triple mutant rescue the low resistance phenotype of max2 and that SMAX1 accumulation diminishes resistance. The susceptibility of smax1D, containing a degradation-insensitive form of SMAX1, further confirmed the SMAX1 function in the resistance. The relationship between the accumulation of SMAX1/SMXLs and disease resistance suggested that the inhibitory activity of SMAX1 to resistance requires SMXL6/7/8. Moreover, the exogenous application of KAR2 enhanced resistance against Pst, but KAR-induced resistance depended on salicylic acid (SA) signaling. Inhibition of karrikin signaling delayed SA-mediated defense responses and inhibited pathogen-induced protein biosynthesis. Together, we propose that the MAX2-KAI2-SMAX1 complex regulates resistance with the assistance of SMXL6/7/8 and SA signaling and that SMAX1/SMXLs possibly form a multimeric complex with their target transcription factors to fine tune immune responses.

Integrated Transcriptome and Metabolome Analysis Reveals Phenylpropanoid Biosynthesis and Phytohormone Signaling Contribute to "Candidatus Liberibacter asiaticus" Accumulation in Citrus Fruit Piths (Fluffy Albedo)

"Candidatus Liberibacter asiaticus" (CLas) is a phloem-restricted α-proteobacterium that is associated with citrus huanglongbing (HLB), which is the most destructive disease that affects all varieties of citrus. Although midrib is usually used as a material for CLas detection, we recently found that the bacterium was enriched in fruits, especially in the fruit pith. However, no study has revealed the molecular basis of these two parts in responding to CLas infection. Therefore, we performed transcriptome and UHPLC-MS-based targeted and untargeted metabolomics analyses in order to organize the essential genes and metabolites that are involved. Transcriptome and metabolome characterized 4834 differentially expressed genes (DEGs) and 383 differentially accumulated metabolites (DAMs) between the two materials, wherein 179 DEGs and 44 DAMs were affected by HLB in both of the tissues, involving the pathways of phenylpropanoid biosynthesis, phytohormone signaling transduction, starch and sucrose metabolism, and photosynthesis. Notably, we discovered that the gene expression that is related to beta-glucosidase and endoglucanase was up-regulated in fruits. In addition, defense-related gene expression and metabolite accumulation were significantly down-regulated in infected fruits. Taken together, the decreased amount of jasmonic acid, coupled with the reduced accumulation of phenylpropanoid and the increased proliferation of indole-3-acetic acid, salicylic acid, and abscisic acid, compared to leaf midribs, may contribute largely to the enrichment of CLas in fruit piths, resulting in disorders of photosynthesis and starch and sucrose metabolism.

Walking through crossroads-rice responses to heat and biotic stress interactions

Rice, the most important source of calories for humans is prone to severe yield loss due to changing climate including heat stress. Additionally, rice encounters biotic stresses in conjunction with heat stress, which exacerbates the adverse effects, and exponentially increase such losses. Several investigations have identified biotic and heat stress-related quantitative trait loci (QTLs) that may contribute to improved tolerance to these stresses. However, a significant knowledge gap exists in identifying the genomic regions imparting tolerance against combined biotic and heat stress. Hereby, we are presenting a conceptual meta-analysis identifying genomic regions that may be promising candidates for enhancing combined biotic and heat stress tolerance in rice. Fourteen common genomic regions were identified along chromosomes 1, 2, 3, 4, 6, 10 and 12, which harbored 1265 genes related to heat stress and defense responses in rice. Further, the meta expression analysis revealed 24 differentially expressed genes (DEGs) involved in calcium-mediated stress signaling including transcription factors Myb, bHLH, ROS signaling, molecular chaperones HSP110 and pathogenesis related proteins. Additionally, we also proposed a hypothetical model based on GO and MapMan analysis representing the pathways intersecting heat and biotic stresses. These DEGs can be potential candidate genes for improving tolerance to combined biotic and heat stress in rice. We present a framework highlighting plausible connecting links (QTLs/genes) between rice response to heat stress and different biotic factors associated with yield, that can be extended to other crops.

Inducible expression of truncated NAC62 provides tolerance against Alternaria brassicicola and imparts developmental changes in Indian mustard

Indian mustard (Brassica juncea) faces significant yield loss due to the 'Black Spot Disease,' caused by a fungus Alternaria brassicicola. In plants, NAC transcription factors (NAC TFs) are known for their roles in development and stress tolerance. One such NAC TF, NAC 62, was induced during A. brassicicola challenge in Sinapis alba, a non-host resistant plant against this fungus. Sequence analyses of BjuNAC62 from B. juncea showed that it belonged to the membrane-bound class of transcription factors. Gene expression study revealed differential protein processing of NAC62 between B. juncea and S. alba on pathogen challenge. Furthermore, NAC62 processing to 25 kDa protein was found to be unique to the resistant plant during pathogenesis. Conditional expression of BjuNAC62ΔC, which lacks its transmembrane domain, in B. juncea showed improved tolerance to A. brassicicola. BjuNAC62ΔC processing to 25 kDa product was also observed in tolerant transgenic plants. Additionally, transgenic plants showed induced expression of genes associated with defense-related phytohormone signaling pathways on pathogen challenge. Again, altered phenotypes suggest a possible developmental effect of BjuNAC62∆C in transgenic plants. The overall results suggest that the processing of BjuNAC62 might be playing a crucial role in resistance response against Black Spot disease by modulating defense-associated genes.

OsWRKY114 Inhibits ABA-Induced Susceptibility to Xanthomonas oryzae pv. oryzae in Rice

The phytohormone abscisic acid (ABA) regulates various aspects of plant growth, development, and stress responses. ABA suppresses innate immunity to Xanthomonas oryzae pv. oryzae (Xoo) in rice (Oryza sativa), but the identity of the underlying regulator is unknown. In this study, we revealed that OsWRKY114 is involved in the ABA response during Xoo infection. ABA-induced susceptibility to Xoo was reduced in OsWRKY114-overexpressing rice plants. OsWRKY114 attenuated the negative effect of ABA on salicylic acid-dependent immunity. Furthermore, OsWRKY114 decreased the transcript levels of ABA-associated genes involved in ABA response and biosynthesis. Moreover, the endogenous ABA level was lower in OsWRKY114-overexpressing plants than in the wild-type plants after Xoo inoculation. Taken together, our results suggest that OsWRKY114 is a negative regulator of ABA that confers susceptibility to Xoo in rice.

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.

Systematic identification of miRNA-regulatory networks unveils their potential roles in sugarcane response to Sorghum mosaic virus infection

BACKGROUND

Sugarcane mosaic disease (SMD) is a major viral disease of sugarcane (Saccharum spp.) worldwide. Sorghum mosaic virus (SrMV) is the dominant pathogen of SMD in the sugarcane planting areas of China. There is no report on miRNAs and their regulatory networks in sugarcane response to SrMV infection.

RESULTS

In this study, small RNA sequencing (sRNA-seq) of samples from the leaves of SMD-susceptible variety ROC22 and -resistant variety FN39 infected by SrMV was performed. A total of 132 mature miRNAs (55 known miRNAs and 77 novel miRNAs) corresponding to 1,037 target genes were identified. After the SrMV attack, there were 30 differentially expressed miRNAs (17 up-regulated and 13 down-regulated) in FN39 and 19 in ROC22 (16 up-regulated and 3 down-regulated). Besides, there were 18 and 7 variety-specific differentially expressed miRNAs for FN39 and ROC22, respectively. KEGG enrichment analysis showed that the differentially expressed miRNAs targeted genes involved in several disease resistance-related pathways, such as mRNA surveillance, plant pathway interaction, sulfur metabolism, and regulation of autophagy. The reliability of sequencing data, and the expression patterns / regulation relationships between the selected differentially expressed miRNAs and their target genes in ROC22 and FN39 were confirmed by quantitative real-time PCR. A regulatory network diagram of differentially expressed miRNAs and their predicted target genes in sugarcane response to SrMV infection was sketched. In addition, precursor sequences of three candidate differentially expressed novel miRNAs (nov_3741, nov_22650 and nov_40875) were cloned from the ROC22 leaf infected by SrMV. Transient overexpression demonstrated that they could induce the accumulation of hydrogen peroxide and the expression level of hypersensitive response marker genes, salicylic acid-responsive genes and ethylene synthesis-depended genes in Nicotiana benthamiana. It is thus speculated that these three miRNAs may be involved in regulating the early immune response of sugarcane plants following SrMV infection.

CONCLUSIONS

This study lays a foundation for revealing the miRNA regulation mechanism in the interaction of sugarcane and SrMV, and also provides a resource for miRNAs and their predicted target genes for SrMV resistance improvement in sugarcane.

Abscisic Acid-Defensive Player in Flax Response to Fusarium culmorum Infection

Fusarium culmorum is a ubiquitous soil pathogen with a wide host range. In flax (Linum ussitatissimum), it causes foot and root rot and accumulation of mycotoxins in flax products. Fungal infections lead to huge losses in the flax industry. Moreover, due to mycotoxin accumulation, flax products constitute a potential threat to the consumers. We discovered that the defense against this pathogen in flax is based on early oxidative burst among others. In flax plants infected with F. culmorum, the most affected genes are connected with ROS production and processing, callose synthesis and ABA production. We hypothesize that ABA triggers defense mechanism in flax and is a significant player in a successful response to infection.

Evolutionarily conserved bacterial effectors hijack abscisic acid signaling to induce an aqueous environment in the apoplast

High atmospheric humidity levels profoundly impact host-pathogen interactions in plants by enabling the establishment of an aqueous living space that benefits pathogens. The effectors HopM1 and AvrE1 of the bacterial pathogen Pseudomonas syringae have been shown to induce an aqueous apoplast under such conditions. However, the mechanisms by which this happens remain unknown. Here, we show that HopM1 and AvrE1 work redundantly to establish an aqueous living space by inducing a major reprogramming of the Arabidopsis thaliana transcriptome landscape. These effectors induce a strong abscisic acid (ABA) signature, which promotes stomatal closure, resulting in reduced leaf transpiration and water-soaking lesions. Furthermore, these effectors preferentially increase ABA accumulation in guard cells, which control stomatal aperture. Notably, a guard-cell-specific ABA transporter, ABCG40, is necessary for HopM1 induction of water-soaking lesions. This study provides molecular insights into a chain of events of stomatal manipulation that create an ideal microenvironment to facilitate infection.

Comparative Transcriptome Analysis of Onion in Response to Infection by Alternaria porri (Ellis) Cifferi

Purple blotch (PB) is one of the most destructive foliar diseases of onion and other alliums, caused by a necrotrophic fungal pathogen Alternaria porri. There are no reports on the molecular response of onion to PB infection. To elucidate the response of onion to A. porri infection, we consequently carried out an RNAseq analysis of the resistant (Arka Kalyan; AK) and susceptible (Agrifound rose; AFR) genotype after an artificial infection. Through differential expression analyses between control and pathogen-treated plants, we identified 8,064 upregulated and 248 downregulated genes in AFR, while 832 upregulated and 564 downregulated genes were identified in AK. A further significant reprogramming in the gene expression profile was also demonstrated by a functional annotation analysis. Gene ontology (GO) terms, which are particularly involved in defense responses and signaling, are overrepresented in current analyses such as "oxidoreductase activity," "chitin catabolic processes," and "defense response." Several key plant defense genes were differentially expressed on A. porri infection, which includes pathogenesis-related (PR) proteins, receptor-like kinases, phytohormone signaling, cell-wall integrity, cytochrome P450 monooxygenases, and transcription factors. Some of the genes were exclusively overexpressed in resistant genotype, namely, GABA transporter1, ankyrin repeat domain-containing protein, xyloglucan endotransglucosylase/hydrolase, and PR-5 (thaumatin-like). Antioxidant enzyme activities were observed to be increased after infection in both genotypes but higher activity was found in the resistant genotype, AK. This is the first report of transcriptome profiling in onion in response to PB infection and will serve as a resource for future studies to elucidate the molecular mechanism of onion-A. porri interaction and to improve PB resistance in onions.

Drought attenuates plant defence against bacterial pathogens by suppressing the expression of CBP60g/SARD1 during combined stress

In nature, plants are frequently exposed to drought and bacterial pathogens simultaneously. However, information on how the drought and defence pathways interact and orchestrate global transcriptional regulation is limited. Here, we show that moderate drought stress enhances the susceptibility of Arabidopsis thaliana to Pseudomonas syringae pv. tomato DC3000. Using transcriptome meta-analysis, we found that drought and bacterial stress antagonistically modulate a large set of genes predominantly involved in salicylic acid (SA) and abscisic acid (ABA) signalling networks. We identified that the levels of SA and ABA are dynamically regulated during the course of stress. Importantly, under combined stress, drought through the ABA pathway downregulates the induction of Calmodulin-binding Protein 60 g (CBP60g) and Systemic Acquired Resistance Deficient 1 (SARD1), two transcription factors crucial for SA production upon bacterial infection. We also identified an important role of NPR1-LIKE PROTEIN 3 and 4 (NPR3/4) transcriptional repressors in the drought-mediated negative regulation of CBP60g/SARD1 expression. Using a genetic approach, we show that CBP60g/SARD1 expression is the key determinant of plant defence against bacterial pathogens under combined stress. Thus, these transcription factors act as critical nodes for the crosstalk between drought and bacterial stress signalling under combined stress in plants.

Non-Expresser of PR-Genes 1 Positively Regulates Abscisic Acid Signaling in Arabidopsis thaliana

The plant hormone, abscisic acid (ABA), is not only important for promoting abiotic stress responses but also plays a versatile and crucial role in plant immunity. The pathogen infection-induced dynamic accumulation of ABA mediates the degradation of non-expresser of PR genes 1 (NPR1) through the CUL3^NPR3NPR4 proteasome pathway. However, the functional significance of NPR1 degradation by other E3 ligases in response to ABA remains unclear. Here, we report that NPR1 is induced transcriptionally by ABA and that npr1-1 mutation results in ABA insensitivity during seed germination and seedling growth. Mutants lacking NPR1 downregulate the expression of ABA-responsive transcription factors ABA INSENSITIVE4 (ABI4) and ABA INSENSITIVE5 (ABI5), and that of their downstream targets EM6, RAB18, RD26, and RD29B. The npr1-1 mutation also affects the transcriptional activity of WRKY18, which activates WRKY60 in the presence of ABA. Furthermore, NPR1 directly interacts with and is degraded by HOS15, a substrate receptor for the DDB1-CUL4 ubiquitin E3 ligase complex. Collectively, our findings demonstrate that NPR1 acts as a positive regulator of ABA-responsive genes, whereas HOS15 promotes NPR1 degradation in a proteasome-dependent manner.

Comprehensive Analysis of Carotenoid Cleavage Dioxygenases Gene Family and Its Expression in Response to Abiotic Stress in Poplar

Carotenoid cleavage dioxygenases (CCDs) catalyzes the cleavage of various carotenoids into smaller apocarotenoids which are essential for plant growth and development and response to abiotic stresses. CCD family is divided into two subfamilies: 9-cis epoxycarotenoid dioxygenases (NCED) family and CCD family. A better knowledge of carotenoid biosynthesis and degradation could be useful for regulating carotenoid contents. Here, 23 CCD genes were identified from the Populus trichocarpa genome, and their characterizations and expression profiling were validated. The PtCCD members were divided into PtCCD and PtNCED subfamilies. The PtCCD family contained the PtCCD1, 4, 7, and 8 classes. The PtCCDs clustered in the same clade shared similar intron/exon structures and motif compositions and distributions. In addition, the tandem and segmental duplications resulted in the PtCCD gene expansion based on the collinearity analysis. An additional integrated collinearity analysis among poplar, Arabidopsis, rice, and willow revealed the gene pairs between poplar and willow more than that between poplar and rice. Identifying tissue-special expression patterns indicated that PtCCD genes display different expression patterns in leaves, stems, and roots. Abscisic acid (ABA) treatment and abiotic stress suggested that many PtCCD genes are responsive to osmotic stress regarding the comprehensive regulation networks. The genome-wide identification of PtCCD genes may provide the foundation for further exploring the putative regulation mechanism on osmotic stress and benefit poplar molecular breeding.

Evolutionary Origin of the Carotenoid Cleavage Oxygenase Family in Plants and Expression of Pepper Genes in Response to Abiotic Stresses

Plant carotenoid cleavage oxygenase (CCO) is an enzyme that catalyzes the synthesis of carotenoids and participates in many important physiological functions. The plant CCOs exist in two forms, namely carotenoid cleavage dioxygenase (CCD) and nine-cis epoxide carotenoid dioxygenase (NCED). Although studies have shown that this gene family has been identified in many species, such as Arabidopsis, grape, and tomato, the evolutionary origin of the CCO family and the expression pattern of pepper genes in response to H₂O₂ and other abiotic stresses are still unclear. In this study, we used the bioinformatics method to identify and analyze the members of the CCO gene family from pepper and other 13 plants from lower to higher plant species based on the whole genome sequence. A total of 158 CCO genes were identified in different plant species and further divided into two groups (e.g., groups I and II). The former was subdivided into CCD7 and CCD8 and have independent evolutionary origins, respectively, while the latter was subdivided into CCD1, CCD4, CCD-like, and NCED, which may have come from a common ancestor. In addition, the results of RNA-seq showed that the expression patterns of pepper CaCCO genes were different in the tissues tested, and only few genes were expressed at high levels such as CaCCD1a, CaCCD4a, CaNCED3, and CaCCD1b. For hydrogen peroxide (H₂O₂) and other abiotic stresses, such as plant hormones, heat, cold, drought, and NaCl treatments, induction of about half of the CaCCO genes was observed. Moreover, the expression patterns of CaCCOs were further investigated under heat, cold, drought, and NaCl treatments using quantitative real-time PCR (qRT-PCR), and most members were responsive to these stresses, especially some CaCCOs with significant expression changes were identified, such as CaCCD4c, CaCCD-like1, CaCCD8, and CaCCD1b, suggesting the important roles of CaCCOs in abiotic stress responses. All these results will provide a valuable analytical basis for understanding the evolution and functions of the CCO family in plants.

Dual Transcriptional Profiling of Carrot and 'Candidatus Liberibacter solanacearum' at Different Stages of Infection Suggests Complex Host-Pathogen Interaction

The interactions between the phloem-limited pathogen 'Candidatus Liberibacter solanacearum' haplotype C and carrot (Daucus carota subsp. sativus) were studied at 4, 5, and 9 weeks postinoculation (wpi), by combining dual RNA-Seq results with data on bacterial colonization and observations of the plant phenotype. In the infected plants, genes involved in jasmonate biosynthesis, salicylate signaling, pathogen-associated molecular pattern- and effector-triggered immunity, and production of pathogenesis-related proteins were up-regulated. At 4 wpi, terpenoid synthesis-related genes were up-regulated, presumably as a response to the psyllid feeding, whereas at 5 and 9 wpi, genes involved in both the terpenoid and flavonoid production were down-regulated and phenylpropanoid genes were up-regulated. Chloroplast-related gene expression was down-regulated, in concordance with the observed yellowing of the infected plant leaves. Both the RNA-Seq data and electron microscopy suggested callose accumulation in the infected phloem vessels, likely to impair the transport of photosynthates, while phloem regeneration was suggested by the formation of new sieve cells and the upregulation of cell wall-related gene expression. The 'Ca. L. solanacearum' genes involved in replication, transcription, and translation were expressed at high levels at 4 and 5 wpi, whereas, at 9 wpi, the Flp pilus genes were highly expressed, suggesting adherence and reduced mobility of the bacteria. The 'Ca. L. solanacearum' genes encoding ATP and C4-dicarboxylate uptake were differentially expressed between the early and late infection stages, suggesting a change in the dependence on different host-derived energy sources. HPE1 effector and salicylate hydroxylase were expressed, presumably to suppress host cell death and salicylic acid-dependent defenses during the infection.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Non-Targeted Metabolite Profiling Reveals Host Metabolomic Reprogramming during the Interaction of Black Pepper with Phytophthora capsici

Phytophthora capsici is one of the most destructive pathogens causing quick wilt (foot rot) disease in black pepper (Piper nigrum L.) to which no effective resistance has been defined. To better understand the P. nigrum-P. capsici pathosystem, we employed metabolomic approaches based on flow-infusion electrospray-high-resolution mass spectrometry. Changes in the leaf metabolome were assessed in infected and systemic tissues at 24 and 48 hpi. Principal Component Analysis of the derived data indicated that the infected leaves showed a rapid metabolic response by 24 hpi whereas the systemic leaves took 48 hpi to respond to the infection. The major sources of variations between infected leaf and systemic leaf were identified, and enrichment pathway analysis indicated, major shifts in amino acid, tricarboxylic acid cycle, nucleotide and vitamin B6 metabolism upon infection. Moreover, the individual metabolites involved in defensive phytohormone signalling were identified. RT-qPCR analysis of key salicylate and jasmonate biosynthetic genes indicated a transient reduction of expression at 24 hpi but this increased subsequently. Exogenous application of jasmonate and salicylate reduced P. capsici disease symptoms, but this effect was suppressed with the co-application of abscisic acid. The results are consistent with abscisic acid reprogramming, salicylate and jasmonate defences in infected leaves to facilitate the formation of disease. The augmentation of salicylate and jasmonate defences could represent an approach through which quick wilt disease could be controlled in black pepper.

Abscisic acid stimulates wound suberisation in kiwifruit (Actinidia chinensis) by regulating the production of jasmonic acid, cytokinin and auxin

Wounding induces a cascade of correlative physiological responses that lead to the repair of damaged tissue. In this study, the effect of wounding on suberin, endogenous hormones and their metabolic genes expression was observed during the wound healing of kiwifruit (Actinidia chinensis Planch.). In addition, the role of abscisic acid (ABA) in wound suberisation was investigated by analysing the coordinated regulation between ABA and other hormones. The wound healing process in kiwifruit could be divided into two stages including: (1) initial accumulation of suberin polyphenolic (SPP) and long carbon chain suberin polyaliphatic monomers (LSPA) before 24h; and (2) massive synthesis of SPP and very long carbon chain suberin polyaliphatic monomers (VLSPA) after 24h. ABA content rapidly increased and induced the jasmonic acid (JA) biosynthesis at the early stage of wound healing. ABA level gradually decreased with the expression of AchCYP707A genes, while the contents of trans-zeatin (t-ZT) and indole-3-acetic acid (IAA) steadily increased at the late stage of wound healing. Exogenous ABA stimulated JA and suberin monomers accumulation, but suppressed both t-ZT and IAA biosynthesis. The role of ABA in wound healing of kiwifruit might be involved in the coordination of both JA-mediated suberin monomers biosynthesis and t-ZT- and IAA-mediated formation of suberised cells via an interaction mechanism.

Responses of pea plants to multiple antagonists are mediated by order of attack and phytohormone crosstalk

Plants are often attacked by multiple antagonists and traits of the attacking organisms and their order of arrival onto hosts may affect plant defences. However, few studies have assessed how multiple antagonists, and varying attack order, affect plant defence or nutrition. To address this, we assessed defensive and nutritional responses of Pisum sativum plants after attack by a vector herbivore (Acrythosiphon pisum), a nonvector herbivore (Sitona lineatus), and a pathogen (Pea enation mosaic virus, PEMV). We show viruliferous A. pisum induced several antipathogen plant defence signals, but these defences were inhibited by S. lineatus feeding on peas infected with PEMV. In contrast, S. lineatus feeding induced antiherbivore defence signals, and these plant defences were enhanced by PEMV. Sitona lineatus also increased abundance of plant amino acids, but only when they attacked after viruliferous A. pisum. Our results suggest that diverse communities of biotic antagonists alter defence and nutritional traits of plants through complex pathways that depend on the identity of attackers and their order of arrival onto hosts. Moreover, we show interactions among a group of biotic stressors can vary along a spectrum from antagonism to enhancement/synergism based on the identity and order of attackers, and these interactions are mediated by a multitude of phytohormone pathways.

The CrRLK1L subfamily: One of the keys to versatility in plants

Catharanthus roseous kinase 1L receptors (CrRLK1Ls) are a subfamily of membrane receptors unique to plant cells that perceive internal and external signals, integrate metabolic, physiological, and molecular processes, and regulate plant development. Recent genomic studies have suggested that this receptor subfamily arose during the emergence of terrestrial plants and has since diversified, preserving its essential functions. Participation of some of these CrRLK1Ls in different processes is presented and discussed herein, as well as the increasing number of interactors necessary for their function. At least five different responses have been detected after activating these receptors, such as physiological changes, formation or disassembly of protein complexes, metabolic responses, modification of gene expression, and modulation of phytohormone activity. To date, a common response mechanism for all processes involving CrRLK1Ls has not been described. In this review, the information available on the different functions of CrRLK1Ls was compiled. Additionally, the physiological and/or molecular mechanisms involved in the signaling processes triggered by these receptors are also discussed. In this review, we propose a possible common signaling mechanism for all processes regulated by CrRLK1Ls and pose questions to be answered in the future.

Ferredoxin 1 is downregulated by the accumulation of abscisic acid in an ABI5-dependent manner to facilitate rice stripe virus infection in Nicotiana benthamiana and rice

Ferredoxin 1 (FD1) accepts and distributes electrons in the electron transfer chain of plants. Its expression is universally downregulated by viruses and its roles in plant immunity have been brought into focus over the past decade. However, the mechanism by which viruses regulate FD1 remains to be defined. In a previous report, we found that the expression of Nicotiana benthamiana FD1 (NbFD1) was downregulated following infection with potato virus X (PVX) and that NbFD1 regulates callose deposition at plasmodesmata to play a role in defense against PVX infection. We now report that NbFD1 is downregulated by rice stripe virus (RSV) infection and that silencing of NbFD1 also facilitates RSV infection, while viral infection was inhibited in a transgenic line overexpressing NbFD1, indicating that NbFD1 also functions in defense against RSV infection. Next, a RSV-derived small interfering RNA was identified that contributes to the downregulation of FD1 transcripts. Further analysis showed that the abscisic acid (ABA) which accumulates in RSV-infected plants also represses NbFD1 transcription. It does this by stimulating expression of ABA insensitive 5 (ABI5), which binds the ABA response element motifs in the NbFD1 promoter, resulting in negative regulation. Regulation of FD1 by ABA was also confirmed in RSV-infected plants of the natural host rice. The results therefore suggest a mechanism by which virus regulates chloroplast-related genes to suppress their defense roles.

Abscisic acid implicated in differential plant responses of Phaseolus vulgaris during endophytic colonization by Metarhizium and pathogenic colonization by Fusarium

Metarhizium robertsii is an insect pathogen as well as an endophyte, and can antagonize the phytopathogen, Fusarium solani during bean colonization. However, plant immune responses to endophytic colonization by Metarhizium are largely unknown. We applied comprehensive plant hormone analysis, transcriptional expression and stomatal size analysis in order to examine plant immune responses to colonization by Metarhizium and/or Fusarium. The total amount of abscisic acid (ABA) and ABA metabolites decreased significantly in bean leaves by plant roots colonized by M. robertsii and increased significantly with F. solani compared to the un-inoculated control bean plant. Concomitantly, in comparison to the un-inoculated bean, root colonization by Metarhizium resulted in increased stomatal size in leaves and reduced stomatal size with Fusarium. Meanwhile, expression of plant immunity genes was repressed by Metarhizium and, alternately, triggered by Fusarium compared to the un-inoculated plant. Furthermore, exogenous application of ABA resulted in reduction of bean root colonization by Metarhizium but increased colonization by Fusarium compared to the control without ABA application. Our study suggested that ABA plays a central role in differential responses to endophytic colonization by Metarhizium and pathogenic colonization by Fusarium and, we also observed concomitant differences in stomatal size and expression of plant immunity genes.

Extreme Resistance to Viruses in Potato and Soybean

Plant pathogens, including viruses, negatively impact global crop production. Plants have evolved complex immune responses to pathogens. These responses are often controlled by nucleotide-binding leucine-rich repeat proteins (NLRs), which recognize intracellular, pathogen-derived proteins. Genetic resistance to plant viruses is often phenotypically characterized by programmed cell death at or near the infection site; a reaction termed the hypersensitive response. Although visualization of the hypersensitive response is often used as a hallmark of resistance, the molecular mechanisms leading to the hypersensitive response and associated cell death vary. Plants with extreme resistance to viruses rarely exhibit symptoms and have little to no detectable virus replication or spread beyond the infection site. Both extreme resistance and the hypersensitive response can be activated by the same NLR genes. In many cases, genes that normally provide an extreme resistance phenotype can be stimulated to cause a hypersensitive response by experimentally increasing cellular levels of pathogen-derived elicitor protein(s). The molecular mechanisms of extreme resistance and its relationship to the hypersensitive response are largely uncharacterized. Studies on potato and soybean cultivars that are resistant to strains of Potato virus Y (PVY), Potato virus X (PVX), and Soybean mosaic virus (SMV) indicate that abscisic acid (ABA)-mediated signaling and NLR nuclear translocation are important for the extreme resistance response. Recent research also indicates that some of the same proteins are involved in both extreme resistance and the hypersensitive response. Herein, we review and synthesize published studies on extreme resistance in potato and soybean, and describe studies in additional species, including model plant species, to highlight future research avenues that may bridge the gaps in our knowledge of plant antiviral defense mechanisms.

Wheat Varietal Response to Tilletia controversa J. G. Kühn Using qRT-PCR and Laser Confocal Microscopy

Tilletia controversa J. G. Kühn is a causal organism of dwarf bunt in wheat. Understanding the interaction of wheat and T. controversa is of practical and scientific importance for disease control. In this study, the relative expression of TaLHY and TaPR-4 and TaPR-5 genes was higher in a resistant (Yinong 18) and moderately resistant (Pin 9928) cultivars rather than susceptible (Dongxuan 3) cultivar at 72 h post inoculation (hpi) with T. controversa. Similarly, the expression of defensin, TaPR-2 and TaPR-10 genes was observed higher in resistant and moderately resistant cultivars after exogenous application of phytohormones, including methyl jasmonate, salicylic acid, and abscisic acid. Laser confocal microscopy was used to track the fungal hyphae in the roots, leaves, and tapetum cells, which of susceptible cultivar were infected harshly by T. controversa than moderately resistant and resistant cultivars. There were no fungal hyphae in tapetum cells in susceptible cultivar after methyl jasmonate, salicylic acid and abscisic acid treatments. Moreover, after T. controversa infection, the pollen germination was of 80.06, 58.73, and 0.67% in resistant, moderately resistant and susceptible cultivars, respectively. The above results suggested that the use using of resistant cultivar is a good option against the dwarf bunt disease.

A Novel Role of Salt- and Drought-Induced RING 1 Protein in Modulating Plant Defense Against Hemibiotrophic and Necrotrophic Pathogens

Many plant-encoded E3 ligases are known to be involved in plant defense. Here, we report a novel role of E3 ligase SALT- AND DROUGHT-INDUCED RING FINGER1 (SDIR1) in plant immunity. Even though SDIR1 is reasonably well-characterized, its role in biotic stress response is not known. The silencing of SDIR1 in Nicotiana benthamiana reduced the multiplication of the virulent bacterial pathogen Pseudomonas syringae pv. tabaci. The Arabidopsis sdir1 mutant is resistant to virulent pathogens, whereas SDIR1 overexpression lines are susceptible to both host and nonhost hemibiotrophic bacterial pathogens. However, sdir1 mutant and SDIR1 overexpression lines showed hypersusceptibility and resistance, respectively, against the necrotrophic pathogen Erwinia carotovora. The mutant of SDIR1 target protein, i.e., SDIR-interacting protein 1 (SDIR1P1), also showed resistance to host and nonhost pathogens. In SDIR1 overexpression plants, transcripts of NAC transcription factors were less accumulated and the levels of jasmonic acid (JA) and abscisic acid were increased. In the sdir1 mutant, JA signaling genes JAZ7 and JAZ8 were downregulated. These data suggest that SDIR1 is a susceptibility factor and its activation or overexpression enhances disease caused by P. syringae pv. tomato DC3000 in Arabidopsis. Our results show a novel role of SDIR1 in modulating plant defense gene expression and plant immunity.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Whole Genome Analysis of Sugarcane Root-Associated Endophyte Pseudomonas aeruginosa B18-A Plant Growth-Promoting Bacterium With Antagonistic Potential Against Sporisorium scitamineum

Sugarcane smut is a significant fungal disease that causes a major loss in sugar yield and quality. In this study, we isolated an endophytic strain B18 from a sugarcane root, which showed plant growth-promotion, hydrolytic enzyme production, antifungal activity against sugarcane pathogens (Sporisorium scitamineum, Ceratocystis paradoxa, Fusarium verticillioides), and the presence of nifH, acdS, and antibiotic genes (hcn, prn, and phCA) under in vitro conditions. BIOLOG^(R) phenotypic profiling of B18 established its ability to use various carbon and nitrogen sources and tolerate a range of pH and osmotic and temperature stresses. Whole-genome analysis of B18, identified as Pseudomonas aeruginosa, showed that it consists of a single circular chromosome of 6,490,014 bp with 66.33% GC content. Genome annotation has identified 5,919 protein-coding genes, and 65 tRNA, and 12 rRNA genes. The P. aeruginosa B18 genome encodes genes related to ethylene, nitrogen (nifU, norBCDERQ, gltBDPS, and aatJMPQ), and phosphate (pstABCS and phoBDHRU) metabolism and produce indole-3-acetic acid and siderophores. This also includes genes encoding hydrolases and oxidoreductases, those associated with biocontrol mechanisms (hcnABC, phzA_B, phzDEFGMS, and pchA), colonization (minCDE and lysC), and biofilm formation (efp, hfq, flgBCDEFGHI, and motAB), and those associated with metabolism of secondary metabolites. Collectively, these results suggest a role for P. aeruginosa B18 in plant growth enhancement and biocontrol mechanisms. The P. aeruginosa B18 strain was found to be an efficient colonizer in sugarcane; it can improve growth through modulation of plant hormone production and enhanced host-plant resistance to smut pathogen S. scitamineum in a smut-susceptible sugarcane variety (Yacheng71-374). These biocontrol and plant growth promotion properties of P. aeruginosa B18 area are discussed in this report.

A transcriptomic analysis of sugarcane response to Leifsonia xyli subsp. xyli infection

Sugarcane ratoon stunting disease (RSD) caused by Leifsonia xyli subsp. xyli (Lxx) is a common destructive disease that occurs around the world. Lxx is an obligate pathogen of sugarcane, and previous studies have reported some physiological responses of RSD-affected sugarcane. However, the molecular understanding of sugarcane response to Lxx infection remains unclear. In the present study, transcriptomes of healthy and Lxx-infected sugarcane stalks and leaves were studied to gain more insights into the gene activity in sugarcane in response to Lxx infection. RNA-Seq analysis of healthy and diseased plants transcriptomes identified 107,750 unigenes. Analysis of these unigenes showed a large number of differentially expressed genes (DEGs) occurring mostly in leaves of infected plants. Sugarcane responds to Lxx infection mainly via alteration of metabolic pathways such as photosynthesis, phytohormone biosynthesis, phytohormone action-mediated regulation, and plant-pathogen interactions. It was also found that cell wall defense pathways and protein phosphorylation/dephosphorylation pathways may play important roles in Lxx pathogeneis. In Lxx-infected plants, significant inhibition in photosynthetic processes through large number of differentially expressed genes involved in energy capture, energy metabolism and chloroplast structure. Also, Lxx infection caused down-regulation of gibberellin response through an increased activity of DELLA and down-regulation of GID1 proteins. This alteration in gibberellic acid response combined with the inhibition of photosynthetic processes may account for the majority of growth retardation occurring in RSD-affected plants. A number of genes associated with plant-pathogen interactions were also differentially expressed in Lxx-infected plants. These include those involved in secondary metabolite biosynthesis, protein phosphorylation/dephosphorylation, cell wall biosynthesis, and phagosomes, implicating an active defense response to Lxx infection. Considering the fact that RSD occurs worldwide and a significant cause of sugarcane productivity, a better understanding of Lxx resistance-related processes may help develop tools and technologies for producing RSD-resistant sugarcane varieties through conventional and/or molecular breeding.

Abscisic Acid-Induced Stomatal Closure: An Important Component of Plant Defense Against Abiotic and Biotic Stress

Abscisic acid (ABA) is a stress hormone that accumulates under different abiotic and biotic stresses. A typical effect of ABA on leaves is to reduce transpirational water loss by closing stomata and parallelly defend against microbes by restricting their entry through stomatal pores. ABA can also promote the accumulation of polyamines, sphingolipids, and even proline. Stomatal closure by compounds other than ABA also helps plant defense against both abiotic and biotic stress factors. Further, ABA can interact with other hormones, such as methyl jasmonate (MJ) and salicylic acid (SA). Such cross-talk can be an additional factor in plant adaptations against environmental stresses and microbial pathogens. The present review highlights the recent progress in understanding ABA's multifaceted role under stress conditions, particularly stomatal closure. We point out the importance of reactive oxygen species (ROS), reactive carbonyl species (RCS), nitric oxide (NO), and Ca²⁺ in guard cells as key signaling components during the ABA-mediated short-term plant defense reactions. The rise in ROS, RCS, NO, and intracellular Ca²⁺ triggered by ABA can promote additional events involved in long-term adaptive measures, including gene expression, accumulation of compatible solutes to protect the cell, hypersensitive response (HR), and programmed cell death (PCD). Several pathogens can counteract and try to reopen stomata. Similarly, pathogens attempt to trigger PCD of host tissue to their benefit. Yet, ABA-induced effects independent of stomatal closure can delay the pathogen spread and infection within leaves. Stomatal closure and other ABA influences can be among the early steps of defense and a crucial component of plants' innate immunity response. Stomatal guard cells are quite sensitive to environmental stress and are considered good model systems for signal transduction studies. Further research on the ABA-induced stomatal closure mechanism can help us design strategies for plant/crop adaptations to stress.

Multiple levels of crosstalk in hormone networks regulating plant defense

Plant hormones are essential for regulating the interactions between plants and their complex biotic and abiotic environments. Each hormone initiates a specific molecular pathway and these different hormone pathways are integrated in a complex network of synergistic, antagonistic and additive interactions. This inter-pathway communication is called hormone crosstalk. By influencing the immune network topology, hormone crosstalk is essential for tailoring plant responses to diverse microbes and insects in diverse environmental and internal contexts. Crosstalk provides robustness to the immune system but also drives specificity of induced defense responses against the plethora of biotic interactors. Recent advances in dry-lab and wet-lab techniques have greatly enhanced our understanding of the broad-scale effects of hormone crosstalk on immune network functioning and have revealed underlying principles of crosstalk mechanisms. Molecular studies have demonstrated that hormone crosstalk is modulated at multiple levels of regulation, such as by affecting protein stability, gene transcription and hormone homeostasis. These new insights into hormone crosstalk regulation of plant defense are reviewed here, with a focus on crosstalk acting on the jasmonic acid pathway in Arabidopsis thaliana, highlighting the transcription factors MYC2 and ORA59 as major targets for modulation by other hormones.

The role of signal production and transduction in induced resistance of harvested fruits and vegetables

Postharvest diseases are the primary reason causing postharvest loss of fruits and vegetables. Although fungicides show an effective way to control postharvest diseases, the use of fungicides is gradually being restricted due to safety, environmental pollution, and resistance development in the pathogen. Induced resistance is a new strategy to control postharvest diseases by eliciting immune activity in fruits and vegetables with exogenous physical, chemical, and biological elicitors. After being stimulated by elicitors, fruits and vegetables respond immediately against pathogens. This process is actually a continuous signal transduction, including the generation, transduction, and interaction of signal molecules. Each step of response can lead to corresponding physiological functions, and ultimately induce disease resistance by upregulating the expression of disease resistance genes and activating a variety of metabolic pathways. Signal molecules not only mediate defense response alone, but also interact with other signal transduction pathways to regulate the disease resistance response. Among various signal molecules, the second messenger (reactive oxygen species, nitric oxide, calcium ions) and plant hormones (salicylic acid, jasmonic acid, ethylene, and abscisic acid) play an important role in induced resistance. This article summarizes and reviews the research progress of induced resistance in recent years, and expounds the role of the above-mentioned signal molecules in induced resistance of harvested fruits and vegetables, and prospects for future research.

Advances and perspectives in the metabolomics of stomatal movement and the disease triangle

Crops are continuously exposed to microbial pathogens that cause tremendous yield losses worldwide. Stomatal pores formed by pairs of specialized guard cells in the leaf epidermis represent a major route of pathogen entry. Guard cells have an essential role as a first line of defense against pathogens. Metabolomics is an indispensable systems biology tool that has facilitated discovery and functional studies of metabolites that regulate stomatal movement in response to pathogens and other environmental factors. Guard cells, pathogens and environmental factors constitute the "stomatal disease triangle". The aim of this review is to highlight recent advances toward understanding the stomatal disease triangle in the context of newly discovered signaling molecules, hormone crosstalk, and consequent molecular changes that integrate pathogens and environmental sensing into stomatal immune responses. Future perspectives on emerging single-cell studies, multiomics and molecular imaging in the context of stomatal defense are discussed. Advances in this important area of plant biology will inform rational crop engineering and breeding for enhanced stomatal defense without disruption of other pathways that impact crop yield.

A Meta-Analysis Reveals Opposite Effects of Biotic and Abiotic Stresses on Transcript Levels of Arabidopsis Intracellular Immune Receptor Genes

Plant intracellular immune receptor NLR (nucleotide-binding leucine-rich repeat) proteins sense the presence of pathogens and trigger strong and robust immune responses. NLR genes are known to be tightly controlled at the protein level, but little is known about their dynamics at the transcript level. In this study, we presented a meta-analysis of transcript dynamics of all 207 NLR genes in the Col-0 accession of Arabidopsis thaliana under various biotic and abiotic stresses based on 88 publicly available RNA sequencing datasets from 27 independent studies. We find that about two thirds of the NLR genes are generally induced by pathogens, immune elicitors, or salicylic acid (SA), suggesting that transcriptional induction of NLR genes might be an important mechanism in plant immunity regulation. By contrast, NLR genes induced by biotic stresses are often repressed by abscisic acid, high temperature and drought, suggesting that transcriptional regulation of NLR genes might be important for interaction between abiotic and biotic stress responses. In addition, pathogen-induced expression of some NLR genes are dependent on SA induction. Interestingly, a small group of NLR genes are repressed under certain biotic stress treatments, suggesting an unconventional function of this group of NLRs. This meta-analysis thus reveals the transcript dynamics of NLR genes under biotic and abiotic stress conditions and suggests a contribution of NLR transcript regulation to plant immunity as well as interactions between abiotic and biotic stress responses.