Signal regulators of systemic acquired resistance

Post-translational modifications control the signal at the crossroads of plant-pathogen interactions

The co-evolution of plants and pathogens has enabled them to 'outsmart' each other by promoting their own defence responses and suppressing those of the other. While plants are reliant on their sophisticated immune signalling pathways, pathogens make use of effector proteins to achieve the objective. This entails rapid regulation of underlying molecular mechanisms for prompt induction of associated signalling events in both plants as well as pathogens. The past decade has witnessed the emergence of post-translational modification (PTM) of proteins as a key a factor in modulating cellular responses. The ability of PTMs to expand the functional diversity of the proteome and induce rapid changes at the appropriate time enables them to play crucial roles in the regulation of plant-pathogen interactions. Therefore, this review will delve into the intricate interplay of five major PTMs involved in plant defence and pathogen countermeasures. We discuss how plants employ PTMs to fortify their immune networks, and how pathogen effectors utilize/target host modification systems to gain entry into plants and cause disease. We also emphasize the need for identification of novel PTMs and propose the use of PTM pathways as potential targets for genome editing approaches.

Genetic and physiological characteristics of CsNPR3 edited citrus and their impact on HLB tolerance

Huanglongbing (HLB) disease, caused by Candidatus Liberibacter asiaticus (CaLas), severely impacts citrus production, and currently, there is no cure. Developing HLB-resistant or tolerant cultivars is crucial, with modifying defense-related genes being a promising approach to managing HLB. NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) is a positive regulator of systemic acquired resistance (SAR), which enhances resistance to pathogens, whereas NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 3 (NPR3) is a negative regulator of SAR. To unambiguously address the role of CsNPR3 in HLB, we introduced mutations into the CsNPR3 gene in sweet orange (Citrus sinensis L. Osbeck) through genome editing and assessed their effects on morphology, physiology, and resistance/tolerance to HLB. Several genome-edited 'Hamlin' sweet orange trees harboring frameshift-inducing insertions or deletions were identified. After confirming the genome editing using Sanger sequencing, selected lines were grafted onto C-146 trifoliate hybrid rootstocks for clonal propagation. The progenies were then infected with CaLas using a no-choice Asian Citrus Psyllid (ACP) feeding assay. Evaluation of the genetic and physiological characteristics of CsNPR3-edited citrus trees under greenhouse conditions revealed that the edited trees exhibited greater vigor than the wild-type trees, despite the lack of significant differences in CaLas titers. Although further field evaluation is needed, our findings indicate that CsNPR3 contributes to HLB-caused tree deterioration and demonstrate that editing CsNPR3 can enhance tolerance to HLB.

Augmenting Rice Defenses: Exogenous Calcium Elevates GABA Levels Against WBPH Infestation

This study investigates the impact of exogenous calcium and gamma-aminobutyric acid (GABA) supplementation on rice growth and stress tolerance under white-backed planthopper (WBPH) infestation. We evaluated several phenotypic traits, including shoot/root length, leaf width, tiller number, panicle length, and relative water content, alongside physiological markers such as oxidative stress indicators, antioxidant enzymes activities, hormonal levels, and amino acids biosynthesis. Our results indicate that WBPH stress significantly reduces growth parameters but calcium and GABA supplementation markedly enhance shoot length (by 26% and 36%) and root length (by 38% and 64%), respectively, compared to WBPH-infested plants. Both supplementations also reduced oxidative stress, as evidenced by decreased H2O2 and O2•- levels and a lower electrolyte leakage. Notably, calcium and GABA treatments increased antioxidant enzyme activities, with GABA boosting catalase (CAT) activity by 800%, peroxidase (POD) by 144%, and superoxide dismutase (SOD) by 62% under WBPH stress. Additionally, calcium and GABA enhanced the accumulation of stress hormones (abscisic acid ABA) and salicylic acid (SA) and promoted stomatal closure, contributing to improved water conservation. This study reveals that calcium regulates the GABA shunt pathway, significantly increasing GABA and succinate levels in both root and shoot. Furthermore, calcium and GABA supplementation enhance the biosynthesis of key amino acids and improve ion homeostasis, particularly elevating calcium (Ca), iron (Fe), and magnesium (Mg) levels under WBPH stress. Overall, this study highlights the potential of exogenous calcium and GABA as effective strategies for enhancing rice plant tolerance to WBPH infestation by modulating various physiological and biochemical mechanisms. Further research is warranted to fully elucidate the underlying mechanisms of action.

Genome-Wide Transcriptional Response of Avocado to Fusarium sp. Infection

The avocado crop is relevant for its economic importance and because of its unique evolutionary history. However, there is a lack of information regarding the molecular processes during the defense response against fungal pathogens. Therefore, using a genome-wide approach in this work, we investigated the transcriptional response of the Mexican horticultural race of avocado (Persea americana var. drymifolia), including miRNAs profile and their possible targets. For that, we established an avocado-Fusarium hydroponic pathosystem and studied the response for 21 days. To guarantee robustness in the analysis, first, we improved the avocado genome assembly available for this variety, resulting in 822.49 Mbp in length with 36,200 gene models. Then, using an RNA-seq approach, we identified 13,778 genes differentially expressed in response to the Fusarium infection. According to their expression profile across time, these genes can be clustered into six groups, each associated with specific biological processes. Regarding non-coding RNAs, 8 of the 57 mature miRNAs identified in the avocado genome are responsive to infection caused by Fusarium, and the analysis revealed a total of 569 target genes whose transcript could be post-transcriptionally regulated. This study represents the first research in avocados to comprehensively explore the role of miRNAs in orchestrating defense responses against Fusarium spp. Also, this work provides valuable data about the genes involved in the intricate response of the avocado during fungal infection.

Navigational Signals for Insect and Slug Parasitic Nematodes: The Role of Ascorbate-Glutathione System and Volatiles Released by Insect-Damaged Sweet Pepper Roots

This study of underground multitrophic communication, involving plant roots, insects, and parasitic nematodes, is an emerging field with significant implications for understanding plant-insect-nematode interactions. Our research investigated the impact of wireworm (Agriotes lineatus L. [Coleoptera: Elateridae]) infestations on the ascorbate-glutathione system in sweet pepper (Capsicum annuum L.) plants in order to study the potential role in root-exudate-mediated nematode chemotaxis. We observed that an A. lineatus infestation led to a decrease in leaf ascorbate levels and an increase in root ascorbate, with corresponding increases in the glutathione content in both roots and leaves. Additionally, a pigment analysis revealed increased carotenoid and chlorophyll levels and a shift towards a de-epoxidized state in the xanthophyll cycle. These changes suggest an individual and integrated regulatory function of photosynthetic pigments accompanied with redox modifications of the ascorbate-glutathione system that enhance plant defense. We also noted changes in the root volatile organic compound (VOC). Limonene, methyl salicylate, and benzyl salicylate decreased, whereas hexanal, neoisopulegol, nonanal, phenylethyl alcohol, m-di-tert-butylbenzene, and trans-β-ionone increased in the roots of attacked plants compared to the control group. Most notably, the VOC hexanal and amino acid exudate cysteine were tested for the chemotaxis assay. Nematode responses to chemoattractants were found to be species-specific, influenced by environmental conditions such as temperature. This study highlights the complexity of nematode chemotaxis and suggests that VOC-based biological control strategies must consider nematode foraging strategies and environmental factors. Future research should further explore these dynamics to optimize nematode management in agricultural systems.

The miR7125-MdARF1 module enhances the resistance of apple to Colletotrichum gloeosporioides by promoting lignin synthesis in response to salicylic acid signalling

Apple is an important cash crop in China, and it is susceptible to fungal infections that have deleterious effects on its yield. Apple bitter rot caused by Colletorichum gloeosporioides is one of the most severe fungal diseases of apple. Salicylic acid (SA) is a key signalling molecule in the plant disease resistance signalling pathways. Lignin synthesis also plays a key role in conferring disease resistance. However, few studies have clarified the relationship between the SA disease resistance signalling pathway and the lignin disease resistance pathway in apple. MdMYB46 has previously been shown to promote lignin accumulation in apple and enhance salt and osmotic stress tolerance. Here, we investigated the relationship between MdMYB46 and biological stress; we found that MdMYB46 overexpression enhances the resistance of apple to C. gloeosporioides. We also identified MdARF1, a transcription factor upstream of MdMYB46, via yeast library screening and determined that MdARF1 was regulated by miR7125 through psRNATarget prediction. This regulatory relationship was confirmed through LUC and qRT-PCR experiments, demonstrating that miR7125 negatively regulates MdARF1. Analysis of the miR7125 promoter revealed that miR7125 responds to SA signals. The accumulation of SA level will result in the decrease of miR7125 expression level. In sum, the results of our study provide novel insights into the molecular mechanisms underlying the resistance of apple to C. gloeosporioides and reveal a new pathway that enhances lignin accumulation in apple in response to SA signals. These findings provide valuable information for future studies aimed at breeding apple for disease resistance.

Secret Weapon of Insects: The Oral Secretion Cocktail and Its Modulation of Host Immunity

Plants and insects have co-existed for almost 400 million years and their interactions can be beneficial or harmful, thus reflecting their intricate co-evolutionary dynamics. Many herbivorous arthropods cause tremendous crop loss, impacting the agro-economy worldwide. Plants possess an arsenal of chemical defenses that comprise diverse secondary metabolites that help protect against harmful herbivorous arthropods. In response, the strategies that herbivores use to cope with plant defenses can be behavioral, or molecular and/or biochemical of which salivary secretions are a key determinant. Insect salivary secretions/oral secretions (OSs) play a crucial role in plant immunity as they contain several biologically active elicitors and effector proteins that modulate plants' defense responses. Using this oral secretion cocktail, insects overcome plant natural defenses to allow successful feeding. However, a lack of knowledge of the nature of the signals present in oral secretion cocktails has resulted in reduced mechanistic knowledge of their cellular perception. In this review, we discuss the latest knowledge on herbivore oral secretion derived elicitors and effectors and various mechanisms involved in plant defense modulation. Identification of novel herbivore-released molecules and their plant targets should pave the way for understanding the intricate strategies employed by both herbivorous arthropods and plants in their interactions.

Transcriptional and hormonal profiling uncovers the interactions between plant developmental stages and RNA virus infection

Arabidopsis thaliana is more susceptible to certain viruses during its later developmental stages. The differential responses and the mechanisms behind this development-dependent susceptibility to infection are still not fully understood. Here we explored the outcome of a viral infection at different host developmental stages by studying the response of A. thaliana to infection with turnip mosaic virus at three developmental stages: juvenile vegetative, bolting, and mature flowering plants. We found that infected plants at later stages downregulate cell wall biosynthetic genes and that this downregulation may be one factor facilitating viral spread and systemic infection. We also found that, despite being more susceptible to infection, infected mature flowering plants were more fertile (i.e. produce more viable seeds) than juvenile vegetative and bolting infected plants; that is, plants infected at the reproductive stage have greater fitness than plants infected at earlier developmental stages. Moreover, treatment of mature plants with salicylic acid increased resistance to infection at the cost of significantly reducing fertility. Together, these observations support a negative trade-off between viral susceptibility and plant fertility. Our findings point towards a development-dependent tolerance to infection.

Fluorinated plant activators induced dual-pathway signal transduction and long-lasting ROS burst in chloroplast

Synthetic plant activators represent a promising novel class of green pesticides that can triggering endogenous plant immunity against pathogen invasion. In our previous study, we developed a series of fluorinated compounds capable of eliciting disease resistance in plants; however, the underlying regulatory mechanisms remained unclear. In this study, we systematically investigated the mechanism of plant immune activation using four synthetic plant activators in Arabidopsis thaliana (A. thaliana), including two fluorine-substituted and two non‑fluorine-substituted molecules. Our findings revealed that the fluorinated compounds exhibited superior disease resistance activity compared to the non-fluorinated molecules. Gene expression analysis in systemic acquired resistance (SAR)- and induced systemic resistance (ISR)-related pathways demonstrated that fluorine substitution effectively regulated both SAR- and ISR-pathway activation, highlighting the distinct roles of fluorine in modulating the plant immune system. Notably, the prolonged ROS burst was observed in chloroplasts following treatment with all four plant activators, contrasting with the transient ROS burst induced by natural elicitors. These results provide insights into the unique mechanisms underlying synthetic plant activator-induced plant immunity. Furthermore, comprehensive proteomic analysis revealed a robust immune response mediated by fluorine-substituted plant activators. These findings offer novel insights into the role of fluorine substitution in SAR- and ISR-associated immune signaling pathways and their distinct impact on ROS production within chloroplasts.

How Tea Plant Defends Against Blister Blight Disease: Facts Revealed and Unexplored Horizons

Tea (Camellia sinensis [L.] O. Kuntze) is cultivated as a beverage crop. Despite being a hardy perennial, the tea plant is susceptible to various biotic stresses. Among them, the foliar disease blister blight (BB) is considered the most serious threat to the tea industry, particularly in Asia. BB caused by Exobasidium vexans (Basidiomycetes) was first reported from Northern India in 1868 and gradually established in other tea-growing countries. The fungus E. vexans attacks young harvestable shoots and causes 20 to 50% crop loss. Over the past 150 years, scientific research has delved into various aspects of BB disease, including pathogen biology, disease cycle, epidemiology, disease forecasting, crop loss assessment, and disease management strategies. In a recent shift in research focus, scientists have begun to investigate the resistance mechanisms of tea plants against BB and apply this knowledge to commercial tea cultivation. Although progress has been significant in understanding the fundamental aspects of BB resistance, the detailed molecular mechanisms driving this resistance remain under investigation. This paper focuses on the current understanding of defense mechanisms employed by tea plants against E. vexans and, conversely, how E. vexans overcomes these defenses. Furthermore, we discuss the application of plant resistance strategies in commercial tea cultivation. Lastly, we identify existing research gaps and propose future research directions in the field.

Thermopriming Induces Time-Limited Tolerance to Salt Stress

Implementing sustainable crop protection practices is crucial to protect global harvests and ensure high-quality food supplies. While priming is an established method in seed production for the fortification of plants against various stresses, it is not yet a standard practice in transplant cultivation. Thus, we evaluated the long-term effects of thermopriming-a heat-based priming technique-on the growth, development, and fruit yield of tomato plants. Following a recovery period of about six weeks for thermoprimed plants without stress inducers, we subjected them to subsequent salt stress to ascertain the persistence of the priming effects. Additionally, we compared the efficacy of thermopriming with benzothiadiazole (BTH), a chemical elicitor, in enhancing plant resilience to abiotic stress. While BTH application negatively impacted both plant growth and fruit health, thermopriming showed no such adverse effects on these parameters. Instead, thermopriming initially enhanced the plant defense mechanisms by increasing the accumulation of protective phenols and flavonoids in the leaves. Interestingly, while thermopriming did not alter the response to salt stress, it notably strengthened the overall resilience of the plants. Our findings underscore both the potential and temporal constraints of thermopriming memory. Nonetheless, primed plants exhibited temporarily increased stress tolerance, offering a means to safeguard the offspring.

Endogenous Hormone Levels and Transcriptomic Analysis Reveal the Mechanisms of Bulbil Initiation in Pinellia ternata

Pinellia ternata is a medicinal plant that has important pharmacological value, and the bulbils serve as the primary reproductive organ; however, the mechanisms underlying bulbil initiation remain unclear. Here, we characterized bulbil development via histological, transcriptomic, and targeted metabolomic analyses to unearth the intricate relationship between hormones, genes, and bulbil development. The results show that the bulbils initiate growth from the leaf axillary meristem (AM). In this stage, jasmonic acid (JA), abscisic acid (ABA), isopentenyl adenosine (IPA), and salicylic acid (SA) were highly enriched, while indole-3-acetic acid (IAA), zeatin, methyl jasmonate (MeJA), and 5-dexoxystrigol (5-DS) were notably decreased. Through OPLS-DA analysis, SA has emerged as the most crucial factor in initiating and positively regulating bulbil formation. Furthermore, a strong association between IPA and SA was observed during bulbil initiation. The transcriptional changes in IPT (Isopentenyltransferase), CRE1 (Cytokinin Response 1), A-ARR (Type-A Arabidopsis Response Regulator), B-ARR (Type-B Arabidopsis Response Regulator), AUX1 (Auxin Resistant 1), ARF (Auxin Response Factor), AUX/IAA (Auxin/Indole-3-acetic acid), GH3 (Gretchen Hagen 3), SAUR (Small Auxin Up RNA), GA2ox (Gibberellin 2-oxidase), GA20ox (Gibberellin 20-oxidase), AOS (Allene oxide synthase), AOC (Allene oxide cyclase), OPR (Oxophytodienoate Reductase), JMT (JA carboxy l Methyltransferase), COI1 (Coronatine Insensitive 1), JAZ (Jasmonate ZIM-domain), MYC2 (Myelocytomatosis 2), D27 (DWARF27), SMAX (Suppressor of MAX2), PAL (Phenylalanine Ammonia-Lyase), ICS (Isochorismate Synthase), NPR1 (Non-expressor of Pathogenesis-related Genes1), TGA (TGACG Sequence-specific Binding), PR-1 (Pathogenesis-related), MCSU (Molybdenium Cofactor Sulfurase), PP2C (Protein Phosphatase 2C), and SnRK (Sucrose Non-fermenting-related Protein Kinase 2) were highly correlated with hormone concentrations, indicating that bulbil initiation is coordinately controlled by multiple phytohormones. Notably, eight TFs (transcription factors) that regulate AM initiation have been identified as pivotal regulators of bulbil formation. Among these, WUS (WUSCHEL), CLV (CLAVATA), ATH1 (Arabidopsis Thaliana Homeobox Gene 1), and RAX (Regulator of Axillary meristems) have been observed to exhibit elevated expression levels. Conversely, LEAFY demonstrated contrasting expression patterns. The intricate expression profiles of these TFs are closely associated with the upregulated expression of KNOX(KNOTTED-like homeobox), suggesting a intricate regulatory network underlying the complex process of bulbil initiation. This study offers a profound understanding of the bulbil initiation process and could potentially aid in refining molecular breeding techniques specific to P. ternata.

Decoding early stress signaling waves in living plants using nanosensor multiplexing

Increased exposure to environmental stresses due to climate change have adversely affected plant growth and productivity. Upon stress, plants activate a signaling cascade, involving multiple molecules like H2O2, and plant hormones such as salicylic acid (SA) leading to resistance or stress adaptation. However, the temporal ordering and composition of the resulting cascade remains largely unknown. In this study we developed a nanosensor for SA and multiplexed it with H2O2 nanosensor for simultaneous monitoring of stress-induced H2O2 and SA signals when Brassica rapa subsp. Chinensis (Pak choi) plants were subjected to distinct stress treatments, namely light, heat, pathogen stress and mechanical wounding. Nanosensors reported distinct dynamics and temporal wave characteristics of H2O2 and SA generation for each stress. Based on these temporal insights, we have formulated a biochemical kinetic model that suggests the early H2O2 waveform encodes information specific to each stress type. These results demonstrate that sensor multiplexing can reveal stress signaling mechanisms in plants, aiding in developing climate-resilient crops and pre-symptomatic stress diagnoses.

Comparative transcriptional analysis of Persea americana MYB, WRKY and AP2/ERF transcription factors following Phytophthora cinnamomi infection

Plant cells undergo extensive transcriptional reprogramming following pathogen infection, with these reprogramming patterns becoming more complex when pathogens, such as hemibiotrophs, exhibit different lifestyles. These transcriptional changes are often orchestrated by MYB, WRKY and AP2/ERF transcription factors (TFs), which modulate both growth and defence-related gene expression. Transcriptional analysis of defence-related genes in avocado (Persea americana) infected with Phytophthora cinnamomi indicated differential immune response activation when comparing a partially resistant and susceptible rootstock. This study identified 226 MYB, 82 WRKY, and 174 AP2/ERF TF-encoding genes in avocado, using a genome-wide approach. Phylogenetic analysis revealed substantial sequence conservation within TF groups underscoring their functional significance. RNA-sequencing analysis in a partially resistant and susceptible avocado rootstock infected with P. cinnamomi was indicative of an immune response switch occurring in either rootstock after 24 and 6 h post-inoculation, respectively. Different clusters of co-expressed TF genes were observed at these times, suggesting the activation of necrotroph-related immune responses at varying intervals between the two rootstocks. This study aids our understanding of avocado immune response activation following P. cinnamomi infection, and the role of the TFs therein, elucidating the transcriptional reprogramming disparities between partially resistant and susceptible rootstocks.

Plant Growth Promotion and Plant Disease Suppression Induced by Bacillus amyloliquefaciens Strain GD4a

Botrytis cinerea, the causative agent of gray mold disease (GMD), invades plants to obtain nutrients and disseminates through airborne conidia in nature. Bacillus amyloliquefaciens strain GD4a, a beneficial bacterium isolated from switchgrass, shows great potential in managing GMD in plants. However, the precise mechanism by which GD4a confers benefits to plants remains elusive. In this study, an A. thaliana-B. cinerea-B. amyloliquefaciens multiple-scale interaction model was used to explore how beneficial bacteria play essential roles in plant growth promotion, plant pathogen suppression, and plant immunity boosting. Arabidopsis Col-0 wild-type plants served as the testing ground to assess GD4a's efficacy. Additionally, bacterial enzyme activity and targeted metabolite tests were conducted to validate GD4a's potential for enhancing plant growth and suppressing plant pathogens and diseases. GD4a was subjected to co-incubation with various bacterial, fungal, and oomycete pathogens to evaluate its antagonistic effectiveness in vitro. In vivo pathogen inoculation assays were also carried out to investigate GD4a's role in regulating host plant immunity. Bacterial extracellular exudate (BEE) was extracted, purified, and subjected to untargeted metabolomics analysis. Benzocaine (BEN) from the untargeted metabolomics analysis was selected for further study of its function and related mechanisms in enhancing plant immunity through plant mutant analysis and qRT-PCR analysis. Finally, a comprehensive model was formulated to summarize the potential benefits of applying GD4a in agricultural systems. Our study demonstrates the efficacy of GD4a, isolated from switchgrass, in enhancing plant growth, suppressing plant pathogens and diseases, and bolstering host plant immunity. Importantly, GD4a produces a functional bacterial extracellular exudate (BEE) that significantly disrupts the pathogenicity of B. cinerea by inhibiting fungal conidium germination and hypha formation. Additionally, our study identifies benzocaine (BEN) as a novel small molecule that triggers basal defense, ISR, and SAR responses in Arabidopsis plants. Bacillus amyloliquefaciens strain GD4a can effectively promote plant growth, suppress plant disease, and boost plant immunity through functional BEE production and diverse gene expression.

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.

Comparative transcriptome profiling reveals differential defense responses among Alternaria brassicicola resistant Sinapis alba and susceptible Brassica rapa

Alternaria blight is a devastating disease that causes significant crop losses in oilseed Brassicas every year. Adoption of conventional breeding to generate disease-resistant varieties has so far been unsuccessful due to the lack of suitable resistant source germplasms of cultivated Brassica spp. A thorough understanding of the molecular basis of resistance, as well as the identification of defense-related genes involved in resistance responses in closely related wild germplasms, would substantially aid in disease management. In the current study, a comparative transcriptome profiling was performed using Illumina based RNA-seq to detect differentially expressed genes (DEGs) specifically modulated in response to Alternaria brassicicola infection in resistant Sinapis alba, a close relative of Brassicas, and the highly susceptible Brassica rapa. The analysis revealed that, at 48 hpi (hours post inoculation), 3396 genes were upregulated and 23239 were downregulated, whereas at 72 hpi, 4023 genes were upregulated and 21116 were downregulated. Furthermore, a large number of defense response genes were detected to be specifically regulated as a result of Alternaria infection. The transcriptome data was validated using qPCR-based expression profiling for selected defense-related DEGs, that revealed significantly higher fold change in gene expression in S. alba when compared to B. rapa. Expression of most of the selected genes was elevated across all the time points under study with significantly higher expression towards the later time point of 72 hpi in the resistant germplasm. S. alba activates a stronger defense response reaction against the disease by deploying an array of genes and transcription factors involved in a wide range of biological processes such as pathogen recognition, signal transduction, cell wall modification, antioxidation, transcription regulation, etc. Overall, the study provides new insights on resistance of S. alba against A. brassicicola, which will aid in devising strategies for breeding resistant varieties of oilseed Brassica.

Investigating the ecological implications of nanomaterials: Unveiling plants' notable responses to nano-pollution

The rapid advancement of nanotechnology has led to unprecedented innovations; however, it is crucial to analyze its environmental impacts carefully. This review thoroughly examines the complex relationship between plants and nanomaterials, highlighting their significant impact on ecological sustainability and ecosystem well-being. This study investigated the response of plants to nano-pollution stress, revealing the complex regulation of defense-related genes and proteins, and highlighting the sophisticated defense mechanisms in nature. Phytohormones play a crucial role in the complex molecular communication network that regulates plant responses to exposure to nanomaterials. The interaction between plants and nano-pollution influences plants' complex defense strategies. This reveals the interconnectedness of systems of nature. Nevertheless, these findings have implications beyond the plant domain. The incorporation of hyperaccumulator plants into pollution mitigation strategies has the potential to create more environmentally sustainable urban landscapes and improve overall environmental resilience. By utilizing these exceptional plants, we can create a future in which cities serve as centers of both innovation and ecological balance. Further investigation is necessary to explore the long-term presence of nanoparticles in the environment, their ability to induce genetic changes in plants over multiple generations, and their overall impact on ecosystems. In conclusion, this review summarizes significant scientific discoveries with broad implications beyond the confines of laboratories. This highlights the importance of understanding the interactions between plants and nanomaterials within the wider scope of environmental health. By considering these insights, we initiated a path towards the responsible utilization of nanomaterials, environmentally friendly management of pollution, and interdisciplinary exploration. We have the responsibility to balance scientific advancement and environmental preservation to create a sustainable future that combines nature's wisdom with human innovation.

β-glucan-induced disease resistance in plants: A review

Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are caused by various factors, including both pathogenic and non-pathogenic ones. β-glucan primarily originates from bacteria and fungi, some species of these organisms work as biological agents in causing diseases. When β-glucan enters plants, it triggers the defense system, leading to various reactions such as the production of proteins related to pathogenicity and defense enzymes. By extracting β-glucan from disturbed microorganisms and using it as an inducing agent, plant diseases can be effectively controlled by activating the plant's defense system. β-glucan plays a crucial role during the interaction between plants and pathogens. Therefore, modeling the plant-pathogen relationship and using the molecules involved in this interaction can help in controlling plant diseases, as pathogens have genes related to resistance against pathogenicity. Thus, it is reasonable to identify and use biological induction agents at a large scale by extracting these compounds.

Coordinated recruitment of conserved defense-signaling pathways in PVYO-Infected Nicotiana benthamiana

Potato virus Y (PVY) is an aphid-transmitted potyvirus that affects economically important solanaceous species. In this study, the phenomena and mechanisms following infection with PVY were investigated in tobacco (Nicotiana benthamiana). In tobacco plants, infection with a mild strain of PVY (PVYO) induced stunted growth in the first two leaves at the shoot apex starting 7 days post-infection (dpi), and mosaic symptoms began to appear on newly developing young leaves at 14 dpi. Using enzyme-linked immunosorbent assay and ultrastructure analysis, we confirmed that viral particles accumulated only in the upper developing leaves of infected plants. We analyzed reactive oxygen species (ROS) generation in leaves from the bottom to the top of the plants to investigate whether delayed symptom development in leaves was associated with a defense response to the virus. In addition, the ultrastructural analysis confirmed the increase of ATG4 and ATG8, which are autophagy markers by endoplasmic reticulum (ER) stress, and the expression of genes involved in viral RNA suppression. Overall, our results suggested that viral RNA silencing and induced autophagy may play a role in the inhibition of viral symptom development in host plants in response to PVYO infection.

A 4D Proteome Investigation of the Potential Mechanisms of SA in Triggering Resistance in Kiwifruit to Pseudomonas syringae pv. actinidiae

Kiwifruit bacterial cankers caused by Pseudomonas syringae pv. actinidiae (Psa) are a serious threat to the kiwifruit industry. Salicylic acid (SA) regulates plant defense responses and was previously found to enhance kiwifruit's resistance to Psa. However, the underlying mechanisms of this process remain unclear. In this study, we used 4D proteomics to investigate how SA enhances kiwifruit's resistance to Psa and found that both SA treatment and Psa infection induced dramatic changes in the proteomic pattern of kiwifruit. Psa infection triggered the activation of numerous resistance events, including the MAPK cascade, phenylpropanoid biosynthesis, and hormone signaling transduction. In most cases, the differential expression of a number of genes involved in the SA signaling pathway played a significant role in kiwifruit's responses to Psa. Moreover, SA treatment upregulated numerous resistance-related proteins, which functioned in defense responses to Psa, including phenylpropanoid biosynthesis, the MAPK cascade, and the upregulation of pathogenesis-related proteins. We also found that SA treatment could facilitate timely defense responses to Psa infection and enhance the activation of defense responses that were downregulated in kiwifruit during infection with Psa. Thus, our research deciphered the potential mechanisms of SA in promoting Psa resistance in kiwifruit and can provide a basis for the use of SA to enhance kiwifruit resistance and effectively control the occurrence of kiwifruit bacterial cankers.

Investigating Protein-Protein Interactions Between Grapevine Leafroll-Associated Virus 3 and Vitis vinifera

Grapevine leafroll disease (GLD) is a globally important disease that affects the metabolic composition and biomass of grapes, leading to a reduction in grape yield and quality of wine produced. Grapevine leafroll-associated virus 3 (GLRaV-3) is the main causal agent for GLD. This study aimed to identify protein-protein interactions between GLRaV-3 and its host. A yeast two-hybrid (Y2H) library was constructed from Vitis vinifera mRNA and screened against GLRaV-3 open reading frames encoding structural proteins and those potentially involved in systemic spread and silencing of host defense mechanisms. Five interacting protein pairs were identified, three of which were demonstrated in planta. The minor coat protein of GLRaV-3 was shown to interact with 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase 02, a protein involved in primary carbohydrate metabolism and the biosynthesis of aromatic amino acids. Interactions were also identified between GLRaV-3 p20A and an 18.1-kDa class I small heat shock protein, as well as MAP3K epsilon protein kinase 1. Both proteins are involved in the response of plants to various stressors, including pathogen infections. Two additional proteins, chlorophyll a-b binding protein CP26 and a SMAX1-LIKE 6 protein, were identified as interacting with p20A in yeast but these interactions could not be demonstrated in planta. The findings of this study advance our understanding of the functions of GLRaV-3-encoded proteins and how the interaction between these proteins and those of V. vinifera could lead to GLD.

Unlocking Cowpea's Defense Responses: Conserved Transcriptional Signatures in the Battle against CABMV and CPSMV Viruses

Cowpea aphid-borne mosaic virus (CABMV) and Cowpea severe mosaic virus (CPSMV) threaten cowpea commercial production. This study aimed to analyze Conserved Transcriptional Signatures (CTS) in cowpea's genotypes that are resistant to these viruses. CTS covered up- (UR) or down-regulated (DR) cowpea transcripts in response to CABMV and CPSMV mechanical inoculations. The conservation of cowpea's UR defense response was primarily observed with the one hpi treatments, with decreased CTS representatives as time elapsed. This suggests that cowpea utilizes generic mechanisms during its early interaction with the studied viruses, and subsequently employs more specialized strategies for each viral agent. The potential action of the CTS-UR emphasizes the importance of redox balance, ethylene and jasmonic acid pathways. Additionally, the CTS-UR provides evidence for the involvement of R genes, PR proteins, and PRRs receptors-extensively investigated in combating bacterial and fungal pathogens-in the defense against viral inoculation. AP2-ERF, WRKY, and MYB transcription factors, as well as PIP aquaporins and MAPK cascades, also emerged as significant molecular players. The presented work represents the first study investigating conserved mechanisms in the cowpea defense response to viral inoculations, highlighting relevant processes for initial defense responses.

Aloe Vera Extract Gel-Biosynthesized Selenium Nanoparticles Enhance Disease Resistance in Lettuce by Modulating the Metabolite Profile and Bacterial Endophytes Composition

The use of nanotechnology to suppress crop diseases has attracted significant attention in agriculture. The present study investigated the antifungal mechanism by which aloe vera extract gel-biosynthesized (AVGE) selenium nanoparticles (Se NPs) suppressed Fusarium-induced wilt disease in lettuce (Lactuca sativa). AVGE Se NPs were synthesized by utilizing sodium selenite as a Se source and AVGE as a biocompatible capping and reducing agent. Over 21 d, 2.75% of total AVGE Se NPs was dissolved into Se ions, which was more than 8-fold greater than that of bare Se NPs (0.34%). Upon exposure to soil applied AVGE Se NPs at 50 mg/kg, fresh shoot biomass was significantly increased by 61.6 and 27.8% over the infected control and bare Se NPs, respectively. As compared to the infected control, the shoot levels of citrate, isocitrate, succinate, malate, and 2-oxo-glutarate were significantly upregulated by 0.5-3-fold as affected by both Se NPs. In addition, AVGE Se NPs significantly increased the shoot level of khelmarin D, a type of coumarin, by 4.40- and 0.71-fold over infected controls and bare Se NPs, respectively. Additionally, AVGE Se NPs showed greater upregulation of jasmonic acid and downregulation of abscisic acid content relative to bare Se NPs in diseased shoots. Moreover, the diversity of bacterial endophytes was significantly increased by AVGE Se NPs, with the values of Shannon index 40.2 and 9.16% greater over the infected control and bare Se NPs. Collectively, these findings highlight the significant potential of AVGE Se NPs as an effective and biocompatible strategy for nanoenabled sustainable crop protection.

Deficiency of the Arabidopsis mismatch repair MSH6 attenuates Pseudomonas syringae invasion

The MutS homolog 6 (MSH6) is a nuclear DNA mismatch repair (MMR) gene that encodes the MSH6 protein. MSH6 interacts with MSH2 to form the MutSα heterodimer. MutSα corrects DNA mismatches and unpaired nucleotides arising during DNA replication, deamination of 5-methylcytosine, and recombination between non-identical DNA sequences. In addition to correcting DNA biosynthetic errors, MutSα also recognizes chemically damaged DNA bases. Here, we show that inactivation of MSH6 affects the basal susceptibility of Arabidopsis thaliana to Pseudomonas syringae pv tomato DC3000. The msh6 T-DNA insertional mutant exhibited a reduced susceptibility to the bacterial invasion. This heightened basal resistance of msh6 mutants appears to be dependent on an increased stomatal closure, an accumulation of H₂O₂ and double-strand breaks (DSBs) and a constitutive expression of pathogenesis-related (NPR1 and PR1) and DNA damage response (RAD51D and SOG1) genes. Complementation of this mutant with the MSH6 wild type allele under the control of its own promoter resulted in reversal of the basal bacterial resistance phenotype and the stomatal closure back to wild type levels. Taken together, these results demonstrate that inactivation of MSH6 increases Arabidopsis basal susceptibility to the bacterial pathogen and suggests a link between DNA repair and stress signaling in plants.

GDSL Esterase/Lipase GELP1 Involved in the Defense of Apple Leaves against Colletotrichum gloeosporioides Infection

GDSL esterases/lipases are a subclass of lipolytic enzymes that play critical roles in plant growth and development, stress response, and pathogen defense. However, the GDSL esterase/lipase genes involved in the pathogen response of apple remain to be identified and characterized. Thus, in this study, we aimed to analyze the phenotypic difference between the resistant variety, Fuji, and susceptible variety, Gala, during infection with C. gloeosporioides, screen for anti-disease-associated proteins in Fuji leaves, and elucidate the underlying mechanisms. The results showed that GDSL esterase/lipase protein GELP1 contributed to C. gloeosporioides infection defense in apple. During C. gloeosporioides infection, GELP1 expression was significantly upregulated in Fuji. Fuji leaves exhibited a highly resistant phenotype compared with Gala leaves. The formation of infection hyphae of C. gloeosporioides was inhibited in Fuji. Moreover, recombinant His:GELP1 protein suppressed hyphal formation during infection in vitro. Transient expression in Nicotiana benthamiana showed that GELP1-eGFP localized to the endoplasmic reticulum and chloroplasts. GELP1 overexpression in GL-3 plants increased resistance to C. gloeosporioides. MdWRKY15 expression was upregulated in the transgenic lines. Notably, GELP1 transcript levels were elevated in GL-3 after salicylic acid treatment. These results suggest that GELP1 increases apple resistance to C. gloeosporioides by indirectly regulating salicylic acid biosynthesis.

The Role of Ascorbate-Glutathione System and Volatiles Emitted by Insect-Damaged Lettuce Roots as Navigation Signals for Insect and Slug Parasitic Nematodes

The effect of wireworm-damaged lettuce roots on the antioxidative defense system (ascorbate-glutathione cycle, photosynthetic pigments) and movement of insect/slug parasitic nematodes towards determined root exudates was studied in a glasshouse experiment. Lettuce seedlings were grown in a substrate soil in the absence/presence of wireworms (Elateridae). The ascorbate-glutathione system and photosynthetic pigments were analyzed by HPLC, while volatile organic compounds (VOC) emitted by lettuce roots were investigated by GC-MS. Herbivore-induced root compounds, namely 2,4-nonadienal, glutathione, and ascorbic acid, were selected for a chemotaxis assay with nematodes Steinernema feltiae, S. carpocapsae, Heterorhabditis bacteriophora, Phasmarhabditis papillosa, and Oscheius myriophilus. Root pests had a negative effect on the content of photosynthetic pigments in the leaves of infested plants, indicating that they reacted to the presence of reactive oxygen species (ROS). Using lettuce as a model plant, we recognized the ascorbate-glutathione system as a redox hub in defense response against wireworms and analyzed its role in root-exudate-mediated chemotaxis of nematodes. Infected plants also demonstrated increased levels of volatile 2,4-nonadienal. Entomopathogenic nematodes (EPNs, S. feltiae, S. carpocapsae, and H. bacteriophora) proved to be more mobile than parasitic nematodes O. myriophilus and P. papillosa towards chemotaxis compounds. Among them, 2,4-nonadienal repelled all tested nematodes. Most exudates that are involved in belowground tritrophic interactions remain unknown, but an increasing effort is being made in this field of research. Understanding more of these complex interactions would not only allow a better understanding of the rhizosphere but could also offer ecologically sound alternatives in the pest management of agricultural systems.

Detection and differentiation of herbicide stresses in roses by Raman spectroscopy

Herbicide application is a critical component of modern horticulture. Misuse of herbicides can result in damage to economically important plants. Currently, such damage can be detected only at symptomatic stages by subjective visual inspection of plants, which requires substantial biological expertise. In this study, we investigated the potential of Raman spectroscopy (RS), a modern analytical technique that allows sensing of plant health, for pre-symptomatic diagnostics of herbicide stresses. Using roses as a model plant system, we investigated the extent to which stresses caused by Roundup (Glyphosate) and Weed-B-Gon (2, 4-D, Dicamba and Mecoprop-p (WBG), two of the most commonly used herbicides world-wide, can be diagnosed at pre- and symptomatic stages. We found that spectroscopic analysis of rose leaves enables ~90% accurate detection of Roundup- and WBG-induced stresses one day after application of these herbicides on plants. Our results also show that the accuracy of diagnostics of both herbicides at seven days reaches 100%. Furthermore, we show that RS enables highly accurate differentiation between the stresses induced by Roundup- and WBG. We infer that this sensitivity and specificity arises from the differences in biochemical changes in plants that are induced by both herbicides. These findings suggest that RS can be used for a non-destructive surveillance of plant health to detect and identify herbicide-induced stresses in plants.

Pipecolic acid synthesis is required for systemic acquired resistance and plant-to-plant-induced immunity in barley

Defense responses in plants are based on complex biochemical processes. Systemic acquired resistance (SAR) helps to fight infections by (hemi-)biotrophic pathogens. One important signaling molecule in SAR is pipecolic acid (Pip), accumulation of which is dependent on the aminotransferase ALD1 in Arabidopsis. While exogenous Pip primes defense responses in the monocotyledonous cereal crop barley (Hordeum vulgare), it is currently unclear if endogenous Pip plays a role in disease resistance in monocots. Here, we generated barley ald1 mutants using CRISPR/Cas9, and assessed their capacity to mount SAR. Endogenous Pip levels were reduced after infection of the ald1 mutant, and this altered systemic defense against the fungus Blumeria graminis f. sp. hordei. Furthermore, Hvald1 plants did not emit nonanal, one of the key volatile compounds that are normally emitted by barley plants after the activation of SAR. This resulted in the inability of neighboring plants to perceive and/or respond to airborne cues and prepare for an upcoming infection, although HvALD1 was not required in the receiver plants to mediate the response. Our results highlight the crucial role of endogenous HvALD1 and Pip for SAR, and associate Pip, in particular together with nonanal, with plant-to-plant defense propagation in the monocot crop barley.

Recovery from virus infection: plant's armory in action

MAIN CONCLUSION

This review focuses on different factors involved in promoting symptom recovery in plants post-virus infection such as epigenetics, transcriptional reprogramming, phytohormones with an emphasis on RNA silencing as well as role of abiotic factors such as temperature on symptom recovery. Plants utilize several different strategies to defend themselves in the battle against invading viruses. Most of the viral proteins interact with plant proteins and interfere with molecular dynamics in a cell which eventually results in symptom development. This initial symptom development is countered by the plant utilizing various factors including the plant's adaptive immunity to develop a virus tolerant state. Infected plants can specifically target and impede the transcription of viral genes as well as degrade the viral transcripts to restrict their proliferation by the production of small-interfering RNA (siRNA) generated from the viral nucleic acid, known as virus-derived siRNA (vsiRNA). To further escalate the degradation of viral nucleic acid, secondary siRNAs are generated. The production of virus-activated siRNA (vasiRNA) from the host genome causes differential regulation of the host transcriptome which plays a major role in establishing a virus tolerant state within the infected plant. The systemic action of vsiRNAs, vasiRNA, and secondary siRNAs with the help of defense hormones like salicylic acid can curb viral proliferation, and thus the newly emerged leaves develop fewer symptoms, maintaining a state of tolerance.

Rhizobacteria control damping-off and promote growth of lima bean with and without co-inoculation with Rhizobium tropici CIAT899

Rhizoctonia solani compromises the production of lima bean, an alternative and low-input food source in many tropical regions. Inoculation of bacterial strains has been used, but research on their biocontrol and growth promotion potential on lima bean is scarce. The objective of this study was to evaluate the effects of inoculation with rhizobacterial strains of the genera Bacillus, Brevibacillus, Paenibacillus, Burkholderia, Pseudomonas, and Rhizobium in combination or not with N₂-fixing Rhizobium tropici on the control of damping-off disease and growth promotion in lima bean plants. Greenhouse experiments were conducted to evaluate the inoculation with bacterial strains with biocontrol potential in combination or not with R. tropici in substrate infected with R. solani CML 1846. Growth promotion of these strains was also assessed. Strains of Brevibacillus (UFLA 02-286), Pseudomonas (UFLA 02-281 and UFLA 04-885), Rhizobium (UFLA 04-195), and Burkholderia (UFLA 04-227) co-inoculated with the strain CIAT 899 (Rhizobium tropici) were the most effective in controlling R. solani, reducing the disease incidence in 47-60% on lima bean. The promising strains used in the biocontrol assays were also responsive in promoting growth of lima bean under disease and sterile conditions. A positive synergistic effect of co-inoculation of different genera contributed to plant growth, and these outcomes are important first steps to improve lima bean production.

Control of white mold (Sclerotinia sclerotiorum) through plant-mediated RNA interference

The causative agent of white mold, Sclerotinia sclerotiorum, is capable of infecting over 600 plant species and is responsible for significant crop losses across the globe. Control is currently dependent on broad-spectrum chemical agents that can negatively impact the agroecological environment, presenting a need to develop alternative control measures. In this study, we developed transgenic Arabidopsis thaliana (AT1703) expressing hairpin (hp)RNA to silence S. sclerotiorum ABHYDROLASE-3 and slow infection through host induced gene silencing (HIGS). Leaf infection assays show reduced S. sclerotiorum lesion size, fungal load, and ABHYDROLASE-3 transcript abundance in AT1703 compared to wild-type Col-0. To better understand how HIGS influences host-pathogen interactions, we performed global RNA sequencing on AT1703 and wild-type Col-0 directly at the site of S. sclerotiorum infection. RNA sequencing data reveals enrichment of the salicylic acid (SA)-mediated systemic acquired resistance (SAR) pathway, as well as transcription factors predicted to regulate plant immunity. Using RT-qPCR, we identified predicted interacting partners of ABHYDROLASE-3 in the polyamine synthesis pathway of S. sclerotiorum that demonstrate co-reduction with ABHYDROLASE-3 transcript levels during infection. Together, these results demonstrate the utility of HIGS technology in slowing S. sclerotiorum infection and provide insight into the role of ABHYDROLASE-3 in the A. thaliana-S. sclerotiorum pathosystem.

Changes in polyamine contents during Fusarium graminearum and Fusarium verticillioides inoculation in maize seedlings with or without seed-priming

Maize (Zea mays L.) is the most produced field crop all over the world. One of its most critical diseases that results in economic loss is ear rot caused by various Fusarium species. Previous researches have shown that polyamines, found in all living cells, play crucial role in biotic stress responses. At the same time, biosynthesis of polyamines is of paramount importance not only for plants but also for their pathogens to promote stress tolerance and pathogenicity. In our study, we investigated the polyamine content changes induced in the seedlings of two maize genotypes of different susceptibility by isolates of Fusarium verticillioides and Fusarium graminearum, two Fusarium species of different lifestyles. Apart from that, it was examined how infection efficiency and changes in polyamine contents were modified by salicylic acid or putrescine seed soaking pre-treatments. Our observations confirmed that initial and stress-induced changes in the polyamine contents are not directly related to tolerance in either coleoptile or radicle. However, the two pathogens with different lifestyles induced remarkably distinct changes in the polyamine contents. The effect of the seed soaking pre-treatments depended on the pathogens and plant resistance as well: both salicylic acid and putrescine seed soaking had positive results against F. verticillioides, while in the case of infection with F. graminearum, seed soaking with distilled water alone affected biomass parameters positively in the tolerant genotype.

Transcriptome Dynamics Underlying Planticine®-Induced Defense Responses of Tomato (Solanum lycopersicum L.) to Biotic Stresses

The induction of natural defense mechanisms in plants is considered to be one of the most important strategies used in integrated pest management (IPM). Plant immune inducers could reduce the use of chemicals for plant protection and their harmful impacts on the environment. Planticine® is a natural plant defense biostimulant based on oligomers of α(1→4)-linked D-galacturonic acids, which are biodegradable and nontoxic. The aim of this study was to define the molecular basis of Planticine’s biological activity and the efficacy of its use as a natural plant resistance inducer in greenhouse conditions. Three independent experiments with foliar application of Planticine® were carried out. The first experiment in a climatic chamber (control environment, no pest pressure) subjected the leaves to RNA-seq analysis, and the second and third experiments in greenhouse conditions focused on efficacy after a pest infestation. The result was the RNA sequencing of six transcriptome libraries of tomatoes treated with Planticine® and untreated plants; a total of 3089 genes were found to be differentially expressed genes (DEGs); among them, 1760 and 1329 were up-regulated and down-regulated, respectively. DEG analysis indicated its involvement in such metabolic pathways and processes as plant-pathogen interaction, plant hormone signal transduction, MAPK signaling pathway, photosynthesis, and regulation of transcription. We detected up-regulated gene-encoded elicitor and effector recognition receptors (ELRR and ERR), mitogen-activated protein kinase (MAPKs) genes, and transcription factors (TFs), i.e., WRKY, ERF, MYB, NAC, bZIP, pathogenesis-related proteins (PRPs), and resistance-related metabolite (RRMs) genes. In the greenhouse trials, foliar application of Planticine® proved to be effective in reducing the infestation of tomato leaves by the biotrophic pathogen powdery mildew and in reducing feeding by thrips, which are insect herbivores. Prophylactic and intervention use of Planticine® at low infestation levels allows the activation of plant defense mechanisms.

Decrypting the multi-functional biological activators and inducers of defense responses against biotic stresses in plants

Plant diseases are still the main problem for the reduction in crop yield and a threat to global food security. Additionally, excessive usage of chemical inputs such as pesticides and fungicides to control plant diseases have created another serious problem for human and environmental health. In view of this, the application of plant growth-promoting rhizobacteria (PGPR) for controlling plant disease incidences has been identified as an eco-friendly approach for coping with the food security issue. In this review, we have identified different ways by which PGPRs are capable of reducing phytopathogenic infestations and enhancing crop yield. PGPR suppresses plant diseases, both directly and indirectly, mediated by microbial metabolites and signaling components. Microbial synthesized anti-pathogenic metabolites such as siderophores, antibiotics, lytic enzymes, hydrogen cyanide, and several others act directly on phytopathogens. The indirect mechanisms of reducing plant disease infestation are caused by the stimulation of plant immune responses known as initiation of systemic resistance (ISR) which is mediated by triggering plant immune responses elicited through pathogen-associated molecular patterns (PAMPs). The ISR triggered in the infected region of the plant leads to the development of systemic acquired resistance (SAR) throughout the plant making the plant resistant to a wide range of pathogens. A number of PGPRs including Pseudomonas and Bacillus genera have proven their ability to stimulate ISR. However, there are still some challenges in the large-scale application and acceptance of PGPR for pest and disease management. Further, we discuss the newly formulated PGPR inoculants possessing both plant growth-promoting activities and plant disease suppression ability for a holistic approach to sustaining plant health and enhancing crop productivity.

Salicylic Acid and Mobile Regulators of Systemic Immunity in Plants: Transport and Metabolism

Systemic acquired resistance (SAR) occurs when primary infected leaves produce several SAR-inducing chemical or mobile signals that are transported to uninfected distal parts via apoplastic or symplastic compartments and activate systemic immunity. The transport route of many chemicals associated with SAR is unknown. Recently, it was demonstrated that pathogen-infected cells preferentially transport salicylic acid (SA) through the apoplasts to uninfected areas. The pH gradient and deprotonation of SA may lead to apoplastic accumulation of SA before it accumulates in the cytosol following pathogen infection. Additionally, SA mobility over a long distance is essential for SAR, and transpiration controls the partitioning of SA into apoplasts and cuticles. On the other hand, glycerol-3-phosphate (G3P) and azelaic acid (AzA) travel via the plasmodesmata (PD) channel in the symplastic route. In this review, we discuss the role of SA as a mobile signal and the regulation of SA transport in SAR.

β-D-XYLOSIDASE 4 modulates systemic immune signaling in Arabidopsis thaliana

Pectin- and hemicellulose-associated structures of plant cell walls participate in defense responses against pathogens of different parasitic lifestyles. The resulting immune responses incorporate phytohormone signaling components associated with salicylic acid (SA) and jasmonic acid (JA). SA plays a pivotal role in systemic acquired resistance (SAR), a form of induced resistance that - after a local immune stimulus - confers long-lasting, systemic protection against a broad range of biotrophic invaders. β-D-XYLOSIDASE 4 (BXL4) protein accumulation is enhanced in the apoplast of plants undergoing SAR. Here, two independent Arabidopsis thaliana mutants of BXL4 displayed compromised systemic defenses, while local resistance responses to Pseudomonas syringae remained largely intact. Because both phloem-mediated and airborne systemic signaling were abrogated in the mutants, the data suggest that BXL4 is a central component in SAR signaling mechanisms. Exogenous xylose, a possible product of BXL4 enzymatic activity in plant cell walls, enhanced systemic defenses. However, GC-MS analysis of SAR-activated plants revealed BXL4-associated changes in the accumulation of certain amino acids and soluble sugars, but not xylose. In contrast, the data suggest a possible role of pectin-associated fucose as well as of the polyamine putrescine as regulatory components of SAR. This is the first evidence of a central role of cell wall metabolic changes in systemic immunity. Additionally, the data reveal a so far unrecognized complexity in the regulation of SAR, which might allow the design of (crop) plant protection measures including SAR-associated cell wall components.

Geminiviral C4/AC4 proteins: An emerging component of the viral arsenal against plant defence

Virus infection triggers a plethora of defence reactions in plants to incapacitate the intruder. Viruses, in turn, have added additional functions to their genes so that they acquire capabilities to neutralize the above defence reactions. In plant-infecting viruses, the family Geminiviridae comprises members, majority of whom encode 6-8 genes in their small single-stranded DNA genomes. Of the above genes, one which shows the most variability in its amino acid sequence is the C4/AC4. Recent studies have uncovered evidence, which point towards a wide repertoire of functions performed by C4/AC4 revealing its role as a major player in suppressing plant defence. This review summarizes the various plant defence mechanisms against viruses and highlights how C4/AC4 has evolved to counter most of them.

Resistance Inducers for the Protection of Pedunculate Oak (Quercus robur L.) Seedlings against Powdery Mildew Erysiphe alphitoides

Oak powdery mildew caused by Erysiphe alphitoides (Griffon and Maubl.; U. Braun & S. Takam.) is a common disease in European forests. One of the most susceptible species is the pedunculate oak (Quercus robur L.). Presently, a few methods are available to control powdery mildew, e.g., the use of fungicides (e.g., based on citric acid), antagonistic fungi or bacteria, chemical treatments (e.g., sulphur, potassium bicarbonate) or genetic resistance. In our study, we aimed to check the effects of using chitosan derivatives and novel active substances inducing the plants' natural resistance: benzodiathiadiazole (both in neutral and salt form). 84 pedunculate oak seedlings were subjected to the experiment in three treatment variants (plus positive and negative controls). The plants were treated with active substances and inoculated with E. alphitoides. Although the powdery mildew symptoms appeared in all variants, they were manifested mainly by the mycelium in the form of small spots. The experiment indicated that the highest limitation of powdery mildew mycelium was achieved by applying N-methyl-N-methoxyamide-7-carboxybenzo(1,2,3)thiadiazole (BTHWA). The application of BTHWA reduced disease development by 88.9% when compared to the effects of the other variants.

Regulation of Salicylic Acid and N-Hydroxy-Pipecolic Acid in Systemic Acquired Resistance

In plants, salicylic acid (SA) is a central immune signal that is involved in both local and systemic acquired resistance (SAR). In addition to SA, several other chemical signals are also involved in SAR and these include N-hydroxy-pipecolic acid (NHP), a newly discovered plant metabolite that plays a crucial role in SAR. Recent discoveries have led to a better understanding of the biosynthesis of SA and NHP and their signaling during plant defense responses. Here, I review the recent progress in role of SA and NHP in SAR. In addition, I discuss how these signals cooperate with other SAR-inducing chemicals to regulate SAR.

Plant Disease Resistance-Related Signaling Pathways: Recent Progress and Future Prospects

Plant-pathogen interactions induce a signal transmission series that stimulates the plant's host defense system against pathogens and this, in turn, leads to disease resistance responses. Plant innate immunity mainly includes two lines of the defense system, called pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). There is extensive signal exchange and recognition in the process of triggering the plant immune signaling network. Plant messenger signaling molecules, such as calcium ions, reactive oxygen species, and nitric oxide, and plant hormone signaling molecules, such as salicylic acid, jasmonic acid, and ethylene, play key roles in inducing plant defense responses. In addition, heterotrimeric G proteins, the mitogen-activated protein kinase cascade, and non-coding RNAs (ncRNAs) play important roles in regulating disease resistance and the defense signal transduction network. This paper summarizes the status and progress in plant disease resistance and disease resistance signal transduction pathway research in recent years; discusses the complexities of, and interactions among, defense signal pathways; and forecasts future research prospects to provide new ideas for the prevention and control of plant diseases.

POWERDRESS positively regulates systemic acquired resistance in Arabidopsis

PWR, an epigenetic regulator, and PIF4, a transcription factor coordinately regulate both local resistance and systemic acquired resistance in Arabidopsis. A plant that gets infected once becomes resistant to subsequent infections through the development of systemic acquired resistance (SAR). Primary-infected tissues generate mobile signals that travel to systemic tissues and cause epigenetic changes associated with the SAR activation. Epigenetic regulators and the process of infection memory development are largely obscure for plants. POWERDRESS (PWR), a SANT domain-containing histone deacetylation (HDAC) promoting gene, is essential for thermomorphogenesis. Here we show that PWR is required for the SAR activation in Arabidopsis. The pwr mutants in Ler and Col-0 background possess normal local resistance but are defective in SAR. PHYTOCHROME-INTERACTING FACTOR 4 (PIF4) genetically interacts with PWR for flowering and thermomorphogenesis and is a negative regulator of basal immunity. We found a cooperative function for suppressing basal immunity and SAR activation by PIF4 and PWR, respectively. PWR promotes the expression of SA biosynthesis genes and the accumulation of SA in the systemic tissues. RSI1/FLD, which influences histone methylation and acetylation, is essential to infection memory development in Arabidopsis. Our results show that PWR and RSI1 positively regulate each other's expression. Exogenous application of HDAC inhibitor sodium butyrate abolishes SAR-mediated SA accumulation, expression of PR1 gene, and protection against pathogens after challenge inoculation. The results indicate the possibility of the involvement of HDAC activity of PWR in the formation of infection memory development in Arabidopsis.

Tissue-specific mRNA profiling of the Brassica napus-Sclerotinia sclerotiorum interaction uncovers novel regulators of plant immunity

White mold is caused by the fungal pathogen Sclerotinia sclerotiorum and leads to rapid and significant loss in plant yield. Among its many brassicaceous hosts, including Brassica napus (canola) and Arabidopsis, the response of individual tissue layers directly at the site of infection has yet to be explored. Using laser microdissection coupled with RNA sequencing, we profiled the epidermis, mesophyll, and vascular leaf tissue layers of B. napus in response to S. sclerotiorum. High-throughput tissue-specific mRNA sequencing increased the total number of detected transcripts compared with whole-leaf assessments and provided novel insight into the conserved and specific roles of ontogenetically distinct leaf tissue layers in response to infection. When subjected to pathogen infection, the epidermis, mesophyll, and vasculature activate both specific and shared gene sets. Putative defense genes identified through transcription factor network analysis were then screened for susceptibility against necrotrophic, hemi-biotrophic, and biotrophic pathogens. Arabidopsis deficient in PR5-like RECEPTOR KINASE (PR5K) mRNA levels were universally susceptible to all pathogens tested and were further characterized to identify putative interacting partners involved in the PR5K signaling pathway. Together, these data provide insight into the complexity of the plant defense response directly at the site of infection.

Phosphorylation-mediated inactivation of C3H14 by MPK4 enhances bacterial-triggered immunity in Arabidopsis

Perception of pathogen-associated molecular patterns (PAMPs) triggers mitogen-activated protein (MAP) kinase 4 (MPK4)-mediated phosphorylation and induces downstream transcriptional reprogramming, but the mechanisms of the MPK4 defense pathway are poorly understood. Here, we showed that phosphorylation-mediated inactivation of the CCCH protein C3H14 by MPK4 positively regulates the immune response in Arabidopsis (Arabidopsis thaliana). Compared with wild-type plants, loss-of-function mutations in C3H14 and its paralog C3H15 resulted in enhanced defense against Pst DC3000 in infected leaves and the development of systemic acquired resistance (SAR), whereas C3H14 or C3H15 overexpression enhanced susceptibility to this pathogen and failed to induce SAR. The functions of C3H14 in PAMP-triggered immunity (PTI) and SAR were dependent on MPK4-mediated phosphorylation. Challenge with Pst DC3000 or the flagellin peptide flg22 enhanced the phosphorylation of C3H14 by MPK4 in the cytoplasm, relieving C3H14-inhibited expression of PTI-related genes and attenuating C3H14-activated expression of its targets NIM1-INTERACTING1 (NIMIN1) and NIMIN2, two negative regulators of SAR. Salicylic acid (SA) affected the MPK4-C3H14-NIMIN1/2 cascades in immunity, but SA signaling mediated by the C3H14-NIMIN1/2 cascades was independent of MPK4 phosphorylation. Our study suggests that C3H14 might be a negative component of the MPK4 defense signaling pathway.

The Banana MaWRKY18, MaWRKY45, MaWRKY60 and MaWRKY70 Genes Encode Functional Transcription Factors and Display Differential Expression in Response to Defense Phytohormones

WRKY transcription factors (TFs) play key roles in plant defense responses through phytohormone signaling pathways. However, their functions in tropical fruit crops, especially in banana, remain largely unknown. Several WRKY genes from the model plants rice (OsWRKY45) and Arabidopsis (AtWRKY18, AtWRKY60, AtWRKY70) have shown to be attractive TFs for engineering disease resistance. In this study, we isolated four banana cDNAs (MaWRKY18, MaWRKY45, MaWRKY60, and MaWRKY70) with homology to these rice and ArabidopsisWRKY genes. The MaWRKY cDNAs were isolated from the wild banana Musa acuminata ssp. malaccensis, which is resistant to several diseases of this crop and is a progenitor of most banana cultivars. The deduced amino acid sequences of the four MaWRKY cDNAs revealed the presence of the conserved WRKY domain of ~60 amino acids and a zinc-finger motif at the N-terminus. Based on the number of WRKY repeats and the structure of the zinc-finger motif, MaWRKY18 and MaWRKY60 belong to group II of WRKY TFs, while MaWRKY45 and MaWRKY70 are members of group III. Their corresponding proteins were located in the nuclei of onion epidermal cells and were shown to be functional TFs in yeast cells. Moreover, expression analyses revealed that the majority of these MaWRKY genes were upregulated by salicylic acid (SA) or methyl jasmonate (MeJA) phytohormones, although the expression levels were relatively higher with MeJA treatment. The fact that most of these banana WRKY genes were upregulated by SA or MeJA, which are involved in systemic acquired resistance (SAR) or induced systemic resistance (ISR), respectively, make them interesting candidates for bioengineering broad-spectrum resistance in this crop.

Current advances and research prospects for agricultural and industrial uses of microbial strains available in world collections

Microorganisms are an important component of the ecosystem and have an enormous impact on human lives. Moreover, microorganisms are considered to have desirable effects on other co-existing species in a variety of habitats, such as agriculture and industries. In this way, they also have enormous environmental applications. Hence, collections of microorganisms with specific traits are a crucial step in developing new technologies to harness the microbial potential. Microbial culture collections (MCCs) are a repository for the preservation of a large variety of microbial species distributed throughout the world. In this context, culture collections (CCs) and microbial biological resource centres (mBRCs) are vital for the safeguarding and circulation of biological resources, as well as for the progress of the life sciences. Ex situ conservation of microorganisms tagged with specific traits in the collections is the crucial step in developing new technologies to harness their potential. Type strains are mainly used in taxonomic study, whereas reference strains are used for agricultural, biotechnological, pharmaceutical research and commercial work. Despite the tremendous potential in microbiological research, little effort has been made in the true sense to harness the potential of conserved microorganisms. This review highlights (1) the importance of available global microbial collections for man and (2) the use of these resources in different research and applications in agriculture, biotechnology, and industry. In addition, an extensive literature survey was carried out on preserved microorganisms from different collection centres using the Web of Science (WoS) and SCOPUS. This review also emphasizes knowledge gaps and future perspectives. Finally, this study provides a critical analysis of the current and future roles of microorganisms available in culture collections for different sustainable agricultural and industrial applications. This work highlights target-specific potential microbial strains that have multiple important metabolic and genetic traits for future research and use.

Plant growth-promoting microorganisms as biocontrol agents of plant diseases: Mechanisms, challenges and future perspectives

Plant diseases and pests are risk factors that threaten global food security. Excessive chemical pesticide applications are commonly used to reduce the effects of plant diseases caused by bacterial and fungal pathogens. A major concern, as we strive toward more sustainable agriculture, is to increase crop yields for the increasing population. Microbial biological control agents (MBCAs) have proved their efficacy to be a green strategy to manage plant diseases, stimulate plant growth and performance, and increase yield. Besides their role in growth enhancement, plant growth-promoting rhizobacteria/fungi (PGPR/PGPF) could suppress plant diseases by producing inhibitory chemicals and inducing immune responses in plants against phytopathogens. As biofertilizers and biopesticides, PGPR and PGPF are considered as feasible, attractive economic approach for sustainable agriculture; thus, resulting in a "win-win" situation. Several PGPR and PGPF strains have been identified as effective BCAs under environmentally controlled conditions. In general, any MBCA must overcome certain challenges before it can be registered or widely utilized to control diseases/pests. Successful MBCAs offer a practical solution to improve greenhouse crop performance with reduced fertilizer inputs and chemical pesticide applications. This current review aims to fill the gap in the current knowledge of plant growth-promoting microorganisms (PGPM), provide attention about the scientific basis for policy development, and recommend further research related to the applications of PGPM used for commercial purposes.

Molecular mechanisms of resistance to Myzus persicae conferred by the peach Rm2 gene: A multi-omics view

The transcriptomic and metabolomic responses of peach to Myzus persicae infestation were studied in Rubira, an accession carrying the major resistance gene Rm2 causing antixenosis, and GF305, a susceptible accession. Transcriptome and metabolome showed both a massive reconfiguration in Rubira 48 hours after infestation while GF305 displayed very limited changes. The Rubira immune system was massively stimulated, with simultaneous activation of genes encoding cell surface receptors involved in pattern-triggered immunity and cytoplasmic NLRs (nucleotide-binding domain, leucine-rich repeat containing proteins) involved in effector-triggered immunity. Hypersensitive reaction featured by necrotic lesions surrounding stylet punctures was supported by the induction of cell death stimulating NLRs/helpers couples, as well as the activation of H2O2-generating metabolic pathways: photorespiratory glyoxylate synthesis and activation of the futile P5C/proline cycle. The triggering of systemic acquired resistance was suggested by the activation of pipecolate pathway and accumulation of this defense hormone together with salicylate. Important reduction in carbon, nitrogen and sulphur metabolic pools and the repression of many genes related to cell division and growth, consistent with reduced apices elongation, suggested a decline in the nutritional value of apices. Finally, the accumulation of caffeic acid conjugates pointed toward their contribution as deterrent and/or toxic compounds in the mechanisms of resistance.

Identification and analysis of proline-rich proteins and hybrid proline-rich proteins super family genes from Sorghum bicolor and their expression patterns to abiotic stress and zinc stimuli

Systematic genome-wide analysis of Sorghum bicolor revealed the identification of a total of 48 homologous genes comprising 21 proline-rich proteins (PRPs) and 27 hybrid proline-rich proteins (HyPRPs). Comprehensive scrutiny of these gene homologs was conducted for gene structure, phylogenetic investigations, chromosome mapping, and subcellular localization of proteins. Promoter analysis uncovered the regions rich with phosphorous- (BIHD), ammonium-, sulfur-responsive (SURE), and iron starvation-responsive (IRO2) along with biotic, abiotic, and development-specific cis-elements. Further, PRPs exhibit more methylation and acetylation sites in comparison with HyPRPs. miRNAs have been predicted which might play a role in cleavage and translation inhibition. Several of the SbPRP genes were stimulated in a tissue-specific manner under drought, salt, heat, and cold stresses. Additionally, exposure of plants to abscisic acid (ABA) and zinc (Zn) also triggered PRP genes in a tissue-dependent way. Among them, SbPRP17 has been found upregulated markedly in all tissues irrespective of the stress imposed. The expressions of SbHyPRPs, especially SbHyPRP2, SbHyPRP6, and SbHyPRP17 were activated under all stresses in all three tissues. On the other hand, SbHyPRP8 (root only) and SbHyPRP12 (all three tissues) were highly responsive to cold stress and ABA while SbHyPRP26 was induced by drought and Zn in the stem. Taken together, this study indicates the critical roles that SbPRPs and SbHyPRPs play during diverse abiotic stress conditions and notably the plausible roles that these genes play upon exposure to zinc, the crucial micronutrient in plants.