Time to Fight: Molecular Mechanisms of Age-Related Resistance

Leaf abaxial and adaxial surfaces differentially affect the interaction of Botrytis cinerea across several eudicots

Eudicot plant species have leaves with two surfaces: the lower abaxial and the upper adaxial surface. Each surface varies in a diversity of components and molecular signals, resulting in potentially different degrees of resistance to pathogens. We tested how Botrytis cinerea, a necrotroph fungal pathogen, interacts with the two different leaf surfaces across 16 crop species and 20 Arabidopsis genotypes. This showed that the abaxial surface is generally more susceptible to the pathogen than the adaxial surface. In Arabidopsis, the differential lesion area between leaf surfaces was associated with jasmonic acid (JA) and salicylic acid (SA) signaling and differential induction of defense chemistry across the two surfaces. When infecting the adaxial surface, leaves mounted stronger defenses by producing more glucosinolates and camalexin defense compounds, partially explaining the differential susceptibility across surfaces. Testing a collection of 96 B. cinerea strains showed the genetic heterogeneity of growth patterns, with a few strains preferring the adaxial surface while most are more virulent on the abaxial surface. Overall, we show that leaf-Botrytis interactions are complex with host-specific, surface-specific, and strain-specific patterns.

Ontogenic stage-associated SA response contributes to leaf age-dependent resistance in Arabidopsis and cotton

Introduction

As leaves grow, they transition from a low-microbe environment embedded in shoot apex to a more complex one exposed to phyllosphere microbiomes. Such change requires a coordinated reprogramming of cellular responses to biotic stresses. It remains unclear how plants shift from fast growth to robust resistance during organ development.

Results

Here, we reported that salicylic acid (SA) accumulation and response were temporarily increased during leaf maturation in herbaceous annual Arabidopsis. Leaf primordia undergoing active cell division were insensitive to the elicitor-induced SA response. This age-dependent increase in SA response was not due to prolonged exposure to environmental microbes. Autoimmune mutants with elevated SA levels did not alter the temporal pattern dependent on ontogenic stage. Young Arabidopsis leaves were more susceptible than mature leaves to Pseudomonas syringae pv. tomato (Pto) DC3000 cor- infection. Finally, we showed a broadly similar pattern in cotton, a woody perennial, where young leaves with reduced SA signaling were preferentially invaded by a Xanthomonas pathogen after leaf surface infection.

Discussion

Through this work, we provided insights in the SA-mediated ontogenic resistance in Arabidopsis and tomato.

Pathogenicity and induced resistance in Larix kaempferi and Larix olgensis inoculated with Endoconidiophora fujiensis

With climate warming and economic globalization, insect-microbe assemblages are becoming increasingly responsible for various devastating forest diseases worldwide. Japanese larch (Larix kaempferi) is extensively cultivated in China because of its high survival rate, rapid maturation and robust mechanical properties. Endoconidiophora fujiensis, an ophiostomatoid fungus associated with Ips subelongatus, has been identified as a lethal pathogen of L. kaempferi in Japan. However, there is a dearth of research on the pathogenicity of E. fujiensis in larches in China. Therefore, we investigated the pathogenicity of E. fujiensis in introduced L. kaempferi and indigenous larch (Larix olgensis) trees and compared the induced resistance responses to the pathogen in both tree species in terms of physiology and gene expression. Five-year-old saplings and 25-year-old adult trees of L. olgensis and L. kaempferi were inoculated in parallel during the same growing season. Endoconidiophora fujiensis exhibited high pathogenicity in both larch species, but particularly in L. kaempferi compared with L. olgensis adult trees; adult L. olgensis was more resistant to E. fujiensis than adult L. kaempferi, which was reflected in higher accumulation of defensive monoterpenes, such as myrcene, 3-carene and limonene and the earlier induction of defense genes catalase (CAT) and pathogenesis-related protein 1 (PR1). This study contributes to our understanding of the interactions between bark beetle-associated ophiostomatoid fungi and host larches, from phenotypic responses to alterations in secondary metabolites via defense- and metabolism-related gene activation, providing a valuable foundation for the management of larch diseases and pests in the future.

Phosphate deficiency increases plant susceptibility to Botrytis cinerea infection by inducing the abscisic acid pathway

Plants have evolved finely regulated defense systems to counter biotic and abiotic threats. In the natural environment, plants are typically challenged by simultaneous stresses and, amid such conditions, crosstalk between the activated signaling pathways becomes evident, ultimately altering the outcome of the defense response. As an example of combined biotic and abiotic stresses, inorganic phosphate (Pi) deficiency, common in natural and agricultural environments, can occur along with attack by the fungus Botrytis cinerea, a devastating necrotrophic generalist pathogen responsible for massive crop losses. We report that Pi deficiency in Arabidopsis thaliana increases its susceptibility to infection by B. cinerea by influencing the early stages of pathogen infection, namely spore adhesion and germination on the leaf surface. Remarkably, Pi-deficient plants are more susceptible to B. cinerea despite displaying the appropriate activation of the jasmonic acid and ethylene signaling pathways, as well as producing secondary defense metabolites and reactive oxygen species. Conversely, the callose deposition in response to B. cinerea infection is compromised under Pi-deficient conditions. The levels of abscisic acid (ABA) are increased in Pi-deficient plants, and the heightened susceptibility to B. cinerea observed under Pi deficiency can be reverted by blocking ABA biosynthesis. Furthermore, high level of leaf ABA induced by overexpression of NCED6 in Pi-sufficient plants also resulted in greater susceptibility to B. cinerea infection associated with increased spore adhesion and germination, and reduced callose deposition. Our findings reveal a link between the enhanced accumulation of ABA induced by Pi deficiency and an increased sensitivity to B. cinerea infection.

Molecular Basis of Plant-Pathogen Interactions in the Agricultural Context

Biotic stressors pose significant threats to crop yield, jeopardizing food security and resulting in losses of over USD 220 billion per year by the agriculture industry. Plants activate innate defense mechanisms upon pathogen perception and invasion. The plant immune response comprises numerous concerted steps, including the recognition of invading pathogens, signal transduction, and activation of defensive pathways. However, pathogens have evolved various structures to evade plant immunity. Given these facts, genetic improvements to plants are required for sustainable disease management to ensure global food security. Advanced genetic technologies have offered new opportunities to revolutionize and boost plant disease resistance against devastating pathogens. Furthermore, targeting susceptibility (S) genes, such as OsERF922 and BnWRKY70, through CRISPR methodologies offers novel avenues for disrupting the molecular compatibility of pathogens and for introducing durable resistance against them in plants. Here, we provide a critical overview of advances in understanding disease resistance mechanisms. The review also critically examines management strategies under challenging environmental conditions and R-gene-based plant genome-engineering systems intending to enhance plant responses against emerging pathogens. This work underscores the transformative potential of modern genetic engineering practices in revolutionizing plant health and crop disease management while emphasizing the importance of responsible application to ensure sustainable and resilient agricultural systems.

Critical parameters for robust Agrobacterium-mediated transient transformation and quantitative promoter assays in Catharanthus roseus seedlings

Agrobacterium-mediated transient expression methods are widely used to study gene function in both model and non-model plants. Using a dual-luciferase assay, we quantified the effect of Agrobacterium-infiltration parameters on the transient transformation efficiency of Catharanthus roseus seedlings. We showed that transformation efficiency is highly sensitive to seedling developmental state and a pre- and post-infiltration dark incubation and is less sensitive to the Agrobacterium growth stage. For example, 5 versus 6 days of germination in the dark increased seedling transformation efficiency by seven- to eight-fold while a dark incubation pre- and post-infiltration increased transformation efficiency by five- to 13-fold. Agrobacterium in exponential compared with stationary phase increased transformation efficiency by two-fold. Finally, we quantified the variation in our Agrobacterium-infiltration method in replicate infiltrations and experiments. Within a given experiment, significant differences of up to 2.6-fold in raw firefly luciferase (FLUC) and raw Renilla luciferase (RLUC) luminescence occurred in replicate infiltrations. These differences were significantly reduced when FLUC was normalized to RLUC values, highlighting the utility of including a reference reporter to minimize false positives. Including a second experimental replicate further reduced the potential for false positives. This optimization and quantitative validation of Agrobacterium infiltration in C. roseus seedlings will facilitate the study of this important medicinal plant and will expand the application of Agrobacterium-mediated transformation methods in other plant species.

Natural variation in the pattern-triggered immunity response in plants: Investigations, implications and applications

The pattern-triggered immunity (PTI) response is triggered at the plant cell surface by the recognition of microbe-derived molecules known as microbe- or pathogen-associated molecular patterns or molecules derived from compromised host cells called damage-associated molecular patterns. Membrane-localized receptor proteins, known as pattern recognition receptors, are responsible for this recognition. Although much of the machinery of PTI is conserved, natural variation for the PTI response exists within and across species with respect to the components responsible for pattern recognition, activation of the response, and the strength of the response induced. This review describes what is known about this variation. We discuss how variation in the PTI response can be measured and how this knowledge might be utilized in the control of plant disease and in developing plant varieties with enhanced disease resistance.

Leaf abaxial immunity to powdery mildew in Arabidopsis is conferred by multiple defense mechanisms

Powdery mildew fungi are obligate biotrophic pathogens that only invade plant epidermal cells. There are two epidermal surfaces in every plant leaf: the adaxial (upper) side and the abaxial (lower) side. While both leaf surfaces can be susceptible to adapted powdery mildew fungi in many plant species, there have been observations of leaf abaxial immunity in some plant species including Arabidopsis. The genetic basis of such leaf abaxial immunity remains unknown. In this study, we tested a series of Arabidopsis mutants defective in one or more known defense pathways with the adapted powdery mildew isolate Golovinomyces cichoracearum UCSC1. We found that leaf abaxial immunity was significantly compromised in mutants impaired for both the EDS1/PAD4- and PEN2/PEN3-dependent defenses. Consistently, expression of EDS1-yellow fluorescent protein and PEN2-green fluorescent protein fusions from their respective native promoters in the respective eds1-2 and pen2-1 mutant backgrounds was higher in the abaxial epidermal cells than in the adaxial epidermal cells. Altogether, our results indicate that leaf abaxial immunity against powdery mildew in Arabidopsis is at least partially due to enhanced EDS1/PAD4- and PEN2/PEN3-dependent defenses. Such transcriptionally pre-programmed defense mechanisms may underlie leaf abaxial immunity in other plant species such as hemp and may be exploited for engineering adaxial immunity against powdery mildew fungi in crop plants.

The Homeodomain-Leucine Zipper Subfamily I Contributes to Leaf Age- and Time-Dependent Resistance to Pathogens in Arabidopsis thaliana

In Arabidopsis thaliana (Arabidopsis), nonhost resistance (NHR) is influenced by both leaf age and the moment of inoculation. While the circadian clock and photoperiod have been linked to the time-dependent regulation of NHR in Arabidopsis, the mechanism underlying leaf age-dependent NHR remains unclear. In this study, we investigated leaf age-dependent NHR to Pyricularia oryzae in Arabidopsis. Our findings revealed that this NHR type is regulated by both miR156-dependent and miR156-independent pathways. To identify the key players, we utilized rice-FOX Arabidopsis lines and identified the rice HD-Zip I OsHOX6 gene. Notably, OsHOX6 expression confers robust NHR to P. oryzae and Colletotrichum nymphaeae in Arabidopsis, with its effect being contingent upon leaf age. Moreover, we explored the role of AtHB7 and AtHB12, the Arabidopsis closest homologues of OsHOX6, by studying mutants and overexpressors in Arabidopsis-C. higginsianum interaction. AtHB7 and AtHB12 were found to contribute to both penetration resistance and post-penetration resistance to C. higginsianum in a leaf age- and time-dependent manner. These findings highlight the involvement of HD-Zip I AtHB7 and AtHB12, well-known regulators of development and abiotic stress responses, in biotic stress responses in Arabidopsis.

Molecular perspectives on age-related resistance of plants to (viral) pathogens

Age-related resistance to microbe invasion is a commonly accepted concept in plant pathology. However, the impact of such age-dependent interactive phenomena is perhaps not yet sufficiently recognized by the broader plant science community. Toward cataloging an understanding of underlying mechanisms, this review explores recent molecular studies and their relevance to the concept. Examples describe differences in genetic background, transcriptomics, hormonal balances, protein-mediated events, and the contribution by short RNA-controlled gene silencing events. Throughout, recent findings with viral systems are highlighted as an illustration of the complexity of the interactions. It will become apparent that instead of uncovering a unifying explanation, we unveiled only trends. Nevertheless, with a degree of confidence, we propose that the process of plant age-related defenses is actively regulated at multiple levels. The overarching goal of this control for plants is to avoid a constitutive waste of resources, especially at crucial metabolically draining early developmental stages.

A new and effective method to induce infection of Phyllachora maydis into corn for tar spot studies in controlled environments

BACKGROUND

Tar spot of corn is a significant and spreading disease in the continental U.S. and Canada caused by the obligate biotrophic fungus Phyllachora maydis. As of 2023, tar spot had been reported in 18 U.S. states and one Canadian Province. The symptoms of tar spot include chlorotic flecking followed by the formation of black stromata where conidia and ascospores are produced. Advancements in research and management for tar spot have been limited by a need for a reliable method to inoculate plants to enable the study of the disease. The goal of this study was to develop a reliable method to induce tar spot in controlled conditions.

RESULTS

We induced infection of corn by P. maydis in 100% of inoculated plants with a new inoculation method. This method includes the use of vacuum-collection tools to extract ascospores from field-infected corn leaves, application of spores to leaves, and induction of the disease in the dark at high humidity and moderate temperatures. Infection and disease development were consistently achieved in four independent experiments on different corn hybrids and under different environmental conditions in a greenhouse and growth chamber. Disease induction was impacted by the source and storage conditions of spores, as tar spot was not induced with ascospores from leaves stored dry at 25 ºC for 5 months but was induced using ascospores from infected leaves stored at -20 ºC for 5 months. The time from inoculation to stromata formation was 10 to 12 days and ascospores were present 19 days after inoculation throughout our experiments. In addition to providing techniques that enable in-vitro experimentation, our research also provides fundamental insights into the conditions that favor tar spot epidemics.

CONCLUSIONS

We developed a method to reliably inoculate corn with P. maydis. The method was validated by multiple independent experiments in which infection was induced in 100% of the plants, demonstrating its consistency in controlled conditions. This new method facilitates research on tar spot and provides opportunities to study the biology of P. maydis, the epidemiology of tar spot, and for identifying host resistance.

Arabidopsis translation initiation factor binding protein CBE1 negatively regulates accumulation of the NADPH oxidase respiratory burst oxidase homolog D

Cell surface pattern recognition receptors sense invading pathogens by binding microbial or endogenous elicitors to activate plant immunity. These responses are under tight control to avoid excessive or untimely activation of cellular responses, which may otherwise be detrimental to host cells. How this fine-tuning is accomplished is an area of active study. We previously described a suppressor screen that identified Arabidopsis thaliana mutants with regained immune signaling in the immunodeficient genetic background bak1-5, which we named modifier of bak1-5 (mob) mutants. Here, we report that bak1-5 mob7 mutant restores elicitor-induced signaling. Using a combination of map-based cloning and whole-genome resequencing, we identified MOB7 as conserved binding of eIF4E1 (CBE1), a plant-specific protein that interacts with the highly conserved eukaryotic translation initiation factor eIF4E1. Our data demonstrate that CBE1 regulates the accumulation of respiratory burst oxidase homolog D, the NADPH oxidase responsible for elicitor-induced apoplastic reactive oxygen species production. Furthermore, several mRNA decapping and translation initiation factors colocalize with CBE1 and similarly regulate immune signaling. This study thus identifies a novel regulator of immune signaling and provides new insights into reactive oxygen species regulation, potentially through translational control, during plant stress responses.

Interaction of EXA1 and eIF4E Family Members Facilitates Potexvirus Infection in Arabidopsis thaliana

Plant viruses depend on a number of host factors for successful infection. Deficiency of critical host factors confers recessively inherited viral resistance in plants. For example, loss of Essential for poteXvirus Accumulation 1 (EXA1) in Arabidopsis thaliana confers resistance to potexviruses. However, the molecular mechanism of how EXA1 assists potexvirus infection remains largely unknown. Previous studies reported that the salicylic acid (SA) pathway is upregulated in exa1 mutants, and EXA1 modulates hypersensitive response-related cell death during EDS1-dependent effector-triggered immunity. Here, we show that exa1-mediated viral resistance is mostly independent of SA and EDS1 pathways. We demonstrate that Arabidopsis EXA1 interacts with three members of the eukaryotic translation initiation factor 4E (eIF4E) family, eIF4E1, eIFiso4E, and novel cap-binding protein (nCBP), through the eIF4E-binding motif (4EBM). Expression of EXA1 in exa1 mutants restored infection by the potexvirus Plantago asiatica mosaic virus (PlAMV), but EXA1 with mutations in 4EBM only partially restored infection. In virus inoculation experiments using Arabidopsis knockout mutants, EXA1 promoted PlAMV infection in concert with nCBP, but the functions of eIFiso4E and nCBP in promoting PlAMV infection were redundant. By contrast, the promotion of PlAMV infection by eIF4E1 was, at least partially, EXA1 independent. Taken together, our results imply that the interaction of EXA1-eIF4E family members is essential for efficient PlAMV multiplication, although specific roles of three eIF4E family members in PlAMV infection differ. IMPORTANCE The genus Potexvirus comprises a group of plant RNA viruses, including viruses that cause serious damage to agricultural crops. We previously showed that loss of Essential for poteXvirus Accumulation 1 (EXA1) in Arabidopsis thaliana confers resistance to potexviruses. EXA1 may thus play a critical role in the success of potexvirus infection; hence, elucidation of its mechanism of action is crucial for understanding the infection process of potexviruses and for effective viral control. Previous studies reported that loss of EXA1 enhances plant immune responses, but our results indicate that this is not the primary mechanism of exa1-mediated viral resistance. Here, we show that Arabidopsis EXA1 assists infection by the potexvirus Plantago asiatica mosaic virus (PlAMV) by interacting with the eukaryotic translation initiation factor 4E family. Our results imply that EXA1 contributes to PlAMV multiplication by regulating translation.

Long-read direct RNA sequencing reveals epigenetic regulation of chimeric gene-transposon transcripts in Arabidopsis thaliana

Transposable elements (TEs) are accumulated in both intergenic and intragenic regions in plant genomes. Intragenic TEs often act as regulatory elements of associated genes and are also co-transcribed with genes, generating chimeric TE-gene transcripts. Despite the potential impact on mRNA regulation and gene function, the prevalence and transcriptional regulation of TE-gene transcripts are poorly understood. By long-read direct RNA sequencing and a dedicated bioinformatics pipeline, ParasiTE, we investigated the transcription and RNA processing of TE-gene transcripts in Arabidopsis thaliana. We identified a global production of TE-gene transcripts in thousands of A. thaliana gene loci, with TE sequences often being associated with alternative transcription start sites or transcription termination sites. The epigenetic state of intragenic TEs affects RNAPII elongation and usage of alternative poly(A) signals within TE sequences, regulating alternative TE-gene isoform production. Co-transcription and inclusion of TE-derived sequences into gene transcripts impact regulation of RNA stability and environmental responses of some loci. Our study provides insights into TE-gene interactions that contributes to mRNA regulation, transcriptome diversity, and environmental responses in plants.

Host developmental stages shape the evolution of a plant RNA virus

Viruses are obligate pathogens that entirely rely on their hosts to complete their infectious cycle. The outcome of viral infections depends on the status of the host. Host developmental stage is an important but sometimes overlooked factor impacting host-virus interactions. This impact is especially relevant in a context where climate change and human activities are altering plant development. To better understand how different host developmental stages shape virus evolution, we experimentally evolved turnip mosaic virus (TuMV) on Arabidopsis thaliana at three different developmental stages: vegetative (juvenile), bolting (transition) and reproductive (mature). After infecting plants with an Arabidopsis-naive or an Arabidopsis-well-adapted TuMV isolate, we observed that hosts in later developmental stages were prone to faster and more severe infections. This observation was extended to viruses belonging to different genera. Thereafter, we experimentally evolved lineages of the naive and the well-adapted TuMV isolates in plants from each of the three developmental stages. All evolved viruses enhanced their infection traits, but this increase was more intense in viruses evolved in younger hosts. The genomic changes of the evolved viral lineages revealed mutation patterns that strongly depended on the founder viral isolate as well as on the developmental stage of the host wherein the lineages were evolved. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

Distinct function of SPL genes in age-related resistance in Arabidopsis

In plants, age-related resistance (ARR) refers to a gain of disease resistance during shoot or organ maturation. ARR associated with vegetative phase change, a transition from juvenile to adult stage, is a widespread agronomic trait affecting resistance against multiple pathogens. How innate immunity in a plant is differentially regulated during successive stages of shoot maturation is unclear. In this work, we found that Arabidopsis thaliana showed ARR against its bacterial pathogen Pseudomonas syringae pv. tomato DC3000 during vegetative phase change. The timing of the ARR activation was associated with a temporal drop of miR156 level. The microRNA miR156 maintains juvenile phase by inhibiting the accumulation and translation of SPL transcripts. A systematic inspection of the loss- and gain-of-function mutants of 11 SPL genes revealed that a subset of SPL genes, notably SPL2, SPL10, and SPL11, activated ARR in adult stage. The immune function of SPL10 was independent of its role in morphogenesis. Furthermore, the SPL10 mediated an age-dependent augmentation of the salicylic acid (SA) pathway partially by direct activation of PAD4. Disrupting SA biosynthesis or signaling abolished the ARR against Pto DC3000. Our work demonstrated that the miR156-SPL10 module in Arabidopsis is deployed to operate immune outputs over developmental timing.

Pathogen-triggered changes in plant development: Virulence strategies or host defense mechanism?

Plants, as sessile organisms, are constantly exposed to pathogens in nature. Plants rely on physical barriers, constitutive chemical defenses, and sophisticated inducible immunity to fight against pathogens. The output of these defense strategies is highly associated with host development and morphology. Successful pathogens utilize various virulence strategies to colonize, retrieve nutrients, and cause disease. In addition to the overall defense-growth balance, the host-pathogen interactions often lead to changes in the development of specific tissues/organs. In this review, we focus on recent advances in understanding the molecular mechanisms of pathogen-induced changes in plants' development. We discuss that changes in host development could be a target of pathogen virulence strategies or an active defense strategy of plants. Current and ongoing research about how pathogens shape plant development to increase their virulence and causes diseases could give us novel views on plant disease control.

Data science approaches provide a roadmap to understanding the role of abscisic acid in defence

Abscisic acid (ABA) is a plant hormone well known to regulate abiotic stress responses. ABA is also recognised for its role in biotic defence, but there is currently a lack of consensus on whether it plays a positive or negative role. Here, we used supervised machine learning to analyse experimental observations on the defensive role of ABA to identify the most influential factors determining disease phenotypes. ABA concentration, plant age and pathogen lifestyle were identified as important modulators of defence behaviour in our computational predictions. We explored these predictions with new experiments in tomato, demonstrating that phenotypes after ABA treatment were indeed highly dependent on plant age and pathogen lifestyle. Integration of these new results into the statistical analysis refined the quantitative model of ABA influence, suggesting a framework for proposing and exploiting further research to make more progress on this complex question. Our approach provides a unifying road map to guide future studies involving the role of ABA in defence.

Refining the emergence scenario of the invasive recombinant Tomato yellow leaf curl virus -IS76

TYLCV-IS76, a unique recombinant between tomato yellow leaf curl virus (TYLCV) and tomato yellow leaf curl Sardinia virus (TYLCSV), has replaced its parental viruses in southern Morocco. To refine its emergence scenario, its fitness was monitored experimentally in conditions aiming at reproducing natural situations, i.e. superinfection of plants already infected with parental viruses and competition with other TYLCV/TYLCSV recombinants (LSRec) automatically generated in plants coinfected with TYLCV and TYLCSV. TYLCV-IS76 accumulated significantly more than parental viruses regardless of plant age and superinfection delay. Although TYLCV-IS76 and LSRec both accumulated more than parental viruses in laboratory conditions, LSRec were displaced by TYLCV-IS76 in nature like parental viruses were. TYLCV-IS76 did not exhibit any vector transmission advantage over LSRec and TYLCV the most competitive parental virus. Thus, it is apparently only in the plant compartment that the recombination event that generated TYLCV-IS76, induced the competitiveness advantage by which the last became first.

The carbohydrate elicitor Riclinoctaose facilitates defense and growth of potato roots by inducing changes in transcriptional and metabolic profiles

Promoting both root growth and defense is conducive to the production of potatoes (Solanum tuberosum L.), while the role of elicitors in this topic hasn't been fully understood. To investigate the effect of Riclinoctaose (RiOc) on root growth and defense, potato tissue cuttings were cultivated with different concentration of RiOc (0, 50, 200 mg/L) for 5 weeks and changes in root morphology, transcription, enzymatic and metabolomic profiles were monitored over time. The results indicated that RiOc triggered the salicylic acid (SA)-mediated defense response and facilitated the growth of adventitious and lateral roots in a dose- and time-dependent manner. MPK3/MPK6, SA- and auxin-signaling pathways and transcription factors such as WUS, SCR and GRAS4/GRAS9 participated in this process. Moreover, the ¹H NMR based metabolome profiling demonstrated that potato roots altered the primary metabolism to respond to the RiOc elicitation and efficiency in production and allocation of defense and growth-related metabolites was improved. After 5-week treatment, the level of glucose, N-acetylglucosamine, glutamine, asparagine, isoleucine, valine, 3-hydroxyisovalerate and ferulate increased, while acetate, acetoacetate, fucose, and 2-hydroxyphenylacetate declined. In conclusion, RiOc played dual roles in activating the SA-mediated defense response and in promoting growth of potato roots by inducing changes in root transcription and metabolism.

Priming of Arabidopsis resistance to herbivory by insect egg deposition depends on the plant's developmental stage

While traits of plant resistance to herbivory often change during ontogeny, it is unknown whether the primability of this resistance depends on the plant's developmental stage. Resistance in non-flowering Arabidopsis thaliana against Pieris brassicae larvae is known to be primable by prior egg deposition on leaves. We investigated whether this priming effect is maintained in plants at the flowering stage. Larval performance assays revealed that flowering plants' resistance to herbivory was not primable by egg deposition. Accordingly, transcriptomes of flowering plants showed almost no response to eggs. In contrast, egg deposition on non-flowering plants enhanced the expression of genes induced by subsequent larval feeding. Strikingly, flowering plants showed constitutively high expression levels of these genes. Larvae performed generally worse on flowering than on non-flowering plants, indicating that flowering plants constitutively resist herbivory. Furthermore, we determined the seed weight in regrown plants that had been exposed to eggs and larvae during the non-flowering or flowering stage. Non-flowering plants benefitted from egg priming with a smaller loss in seed yield. The seed yield of flowering plants was unaffected by the treatments, indicating tolerance towards the larvae. Our results show that the primability of anti-herbivore defences in Arabidopsis depends on the plant's developmental stage.

Scaling-up to understand tree-pathogen interactions: A steep, tough climb or a walk in the park?

Plants have proficient tools that allow them to survive interactions with pathogens. Upon attack, they respond with specific countermeasures, which are controlled by the immune system. However, defences can fail and this failure exposes plants to fast-spreading devastation. Trees face similar challenges to other plants and their immune system allows them to mount defences against pathogens. However, their slow growth, longevity, woodiness, and size can make trees a challenging system to study. Here, we review scientific successes in plant systems, highlight the key challenges and describe the enormous opportunities for pathology research in trees. We discuss the benefits that scaling-up our understanding on tree-pathogen interactions can provide in the fight against plant pathogenic threats.

Manipulation of Vector Host Preference by Pathogens: Implications for Virus Spread and Disease Management

Some plant pathogens manipulate the behavior and performance of their vectors, potentially enhancing pathogen spread. The implications are evolutionary and epidemiological but also economic for pathogens that cause disease in crops. Here we explore with models the effects of vector manipulation on crop yield loss to disease and on the economic returns for vector suppression. We use two frameworks, one that simulates the proportional occurrence of the pathogen in the vector population with the option to eliminate vectors by a single insecticidal treatment, and one that includes vector population dynamics and the potential for multiple insecticidal sprays in a season to suppress vectors. We parameterize the models with published data on vector manipulation, crop yields as affected by the age of the plant at infection, commodity prices and costs of vector control for three pathosystems. Using the first framework, maximum returns for treating vectors are greater with vector manipulation than without it by approximately US$10 per acre (US$24.7/ha) in peas infected by Pea enation mosaic virus and Bean leaf roll virus, and approximately US$50 per acre (US$124/ha) for potatoes infected by Potato leaf roll virus. Using the second framework, maximum returns for controlling the psyllid vectors of Candidatus Liberibacter solanacearum are 50% greater (approximately US$400/acre, US$988/ha) but additional returns for multiple weekly sprays diminish more with vector manipulation than without it. These results suggest that the economics of vector manipulation can be substantial and provide a framework that can inform management decisions.

Imaging-Based Resistance Assay Using Enhanced Luminescence-Tagged Pseudomonas syringae Reveals a Complex Epigenetic Network in Plant Defense Signaling Pathways

High-throughput resistance assays in plants have a limited selection of suitable pathogens. In this study, we developed a Pseudomonas syringae strain chromosomally tagged with the Nanoluc luciferase (NL) from the deep-sea shrimp Oplophorus gracilirostris, a bioluminescent marker significantly brighter than the conventional firefly luciferase. Our reporter strain tagged with NL was more than 100 times brighter than P. syringae tagged with the luxCDABE operon from Photorhabdus luminescens, one of the existing luciferase-based strains. In planta imaging was improved by using the surfactant Silwet L-77, particularly at a lower reporter concentration. Using this imaging system, more than 30 epigenetic mutants were analyzed for their resistance traits because the defense signaling pathway is known to be epigenetically regulated. SWC1, a defense-related chromatin remodeling complex, was found to be a positive defense regulator, which supported one of two earlier conflicting reports. Compromises in DNA methylation in the CG context led to enhanced resistance against virulent Pseudomonas syringae pv. tomato. Dicer-like and Argonaute proteins, important in the biogenesis and exerting the effector function of small RNAs, respectively, showed modest but distinct requirements for effector-triggered immunity and basal resistance to P. syringae pv. tomato. In addition, the transcriptional expression of an epigenetic component was found to be a significant predictor of its immunity contribution. In summary, this study showcased how a high-throughput resistance assay enabled by a pathogen strain with an improved luminescent reporter could provide insightful knowledge about complex defense signaling pathways.[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.

Sweetpotato Root Development Influences Susceptibility to Black Rot Caused by the Fungal Pathogen Ceratocystis fimbriata

Black rot of sweetpotato, caused by Ceratocystis fimbriata, is an important reemerging disease threatening sweetpotato production in the United States. This study assessed disease susceptibility of the storage root surface, storage root cambium, and slips (vine cuttings) of 48 sweetpotato cultivars, advanced breeding lines, and wild relative accessions. We also characterized the effect of storage root development on susceptibility to C. fimbriata. None of the cultivars examined at the storage root level were resistant, with most cultivars exhibiting similar levels of susceptibility. In storage roots, Jewel and Covington were the least susceptible and significantly different from White Bonita, the most susceptible cultivar. In the slip, significant differences in disease incidence were observed for above- and below-ground plant structures among cultivars, advanced breeding lines, and wild relative accessions. Burgundy and Ipomoea littoralis displayed less below-ground disease incidence compared with NASPOT 8, Sunnyside, and LSU-417, the most susceptible cultivars. Correlation of black rot susceptibility between storage roots and slips was not significant, suggesting that slip assays are not useful to predict resistance in storage roots. Immature, early-developing storage roots were comparatively more susceptible than older, fully developed storage roots. The high significant correlation between the storage root cross-section area and the cross-sectional lesion ratio suggests the presence of an unfavorable environment for C. fimbriata as the storage root develops. Incorporating applications of effective fungicides at transplanting and during early-storage root development when sweetpotato tissues are most susceptible to black rot infection may improve disease management efforts.

Age-Dependent Abiotic Stress Resilience in Plants

Developmental age is a strong determinant of stress responses in plants. Differential susceptibility to various environmental stresses is widely observed at both the organ and whole-plant level. While it is clear that age determines stress susceptibility, the causes, regulatory mechanisms, and functions are only now beginning to emerge. Compared with concepts on age-related biotic stress resilience, advancements in the abiotic stress field are relatively limited. In this review, we focus on current knowledge of ontogenic resistance to abiotic stresses, highlighting examples at the organ (leaf) and plant level, preceded by an overview of the relevant concepts in plant aging. We also discuss age-related abiotic stress resilience mechanisms, speculate on their functional relevance, and outline outstanding questions.

Transcriptomic Analysis of Radish (Raphanus sativus L.) Spontaneous Tumor

Spontaneous tumors can develop in different organs of various plant species without any pathogen infection and, as a rule, appear in plants with a certain genotype: Mutants, interspecific hybrids, etc. In particular, among the inbred lines of radish (Raphanus sativus L.), lines that form spontaneous tumors on the taproot during the flowering period were obtained many years ago. In this work, we analyzed the differential gene expression in the spontaneous tumors of radish versus the lateral roots using the RNA-seq method. Data were obtained indicating the increased expression of genes associated with cell division and growth (especially genes that regulate G2-M transition and cytokinesis) in the spontaneous tumor. Among genes downregulated in the tumor tissue, genes participating in the response to stress and wounding, mainly involved in the biosynthesis of jasmonic acid and glucosinolates, were enriched. Our data will help elucidate the mechanisms of spontaneous tumor development in higher plants.

What is the Molecular Basis of Nonhost Resistance?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.Nonhost resistance is typically considered the ability of a plant species to repel all attempts of a pathogen species to colonize it and reproduce on it. Based on this common definition, nonhost resistance is presumed to be very durable and, thus, of great interest for its potential use in agriculture. Despite considerable research efforts, the molecular basis of this type of plant immunity remains nebulous. We here stress the fact that "nonhost resistance" is a phenomenological rather than a mechanistic concept that comprises more facets than typically considered. We further argue that nonhost resistance essentially relies on the very same genes and pathways as other types of plant immunity, of which some may act as bottlenecks for particular pathogens on a given plant species or under certain conditions. Thus, in our view, the frequently used term "nonhost genes" is misleading and should be avoided. Depending on the plant-pathogen combination, nonhost resistance may involve the recognition of pathogen effectors by host immune sensor proteins, which might give rise to host shifts or host range expansions due to evolutionary-conditioned gains and losses in respective armories. Thus, the extent of nonhost resistance also defines pathogen host ranges. In some instances, immune-related genes can be transferred across plant species to boost defense, resulting in augmented disease resistance. We discuss future routes for deepening our understanding of nonhost resistance and argue that the confusing term "nonhost resistance" should be used more cautiously in the light of a holistic view of plant immunity.

MiR172b-TOE1/2 module regulates plant innate immunity in an age-dependent manner

Plant innate immunity varies with age and plant developmental stages. Recently, we reported that Arabidopsis thaliana microRNA miR172b regulates FLS2 transcription through two transcription factors: TARGET OF EAT1 (TOE1) and TOE2. Although the flg22-triggered immune responses were investigated in 2-d-old or even younger toe1/toe2 mutant and miR172b over expression (OE) transgenic plants, the FLS2-mediated immune responses in older plants remain uncharacterized yet. In this work, we analyzed the flg22-triggered immune response in 6-d-old toe1/toe2 and miR172b OE plants. We found that unlike 2-d-old plants, 6-d-old Col-0, toe1/toe2 and miR172b OE plants exhibit comparable flg22-triggered immune responses. Strikingly, miR172b precursor in 6-d-old Col-0 plants upon flg22 treatment reached to a very high level, consequently, the TOE1/2 protein level under this condition was very low or almost undetectable, which explains why 6-d-old WT seedlings are very similar to toe1/toe2 seedlings or miR172b OE plants with respect to the flg22-triggered immune responses. Taken together, our study reveals that miR172b-TOE1/2 module regulates plant innate immunity in an age-dependent manner.

Developmentally regulated activation of defense allows for rapid inhibition of infection in age-related resistance to Phytophthora capsici in cucumber fruit

Background

Age-related resistance (ARR) is a developmentally regulated phenomenon conferring resistance to pathogens or pests. Although ARR has been observed in several host-pathogen systems, the underlying mechanisms are largely uncharacterized. In cucumber, rapidly growing fruit are highly susceptible to Phytophthora capsici but become resistant as they complete exponential growth. We previously demonstrated that ARR is associated with the fruit peel and identified gene expression and metabolomic changes potentially functioning as preformed defenses.

Results

Here, we compare the response to infection in fruit at resistant and susceptible ages using microscopy, quantitative bioassays, and weighted gene co-expression analyses. We observed strong transcriptional changes unique to resistant aged fruit 2–4 h post inoculation (hpi). Microscopy and bioassays confirmed this early response, with evidence of pathogen death and infection failure as early as 4 hpi and cessation of pathogen growth by 8–10 hpi. Expression analyses identified candidate genes involved in conferring the rapid response including those encoding transcription factors, hormone signaling pathways, and defenses such as reactive oxygen species metabolism and phenylpropanoid biosynthesis.

Conclusion

The early pathogen death and rapid defense response in resistant-aged fruit provide insight into potential mechanisms for ARR, implicating both pre-formed biochemical defenses and developmentally regulated capacity for pathogen recognition as key factors shaping age-related resistance.

Priming Melon Defenses with Acibenzolar-S-methyl Attenuates Infections by Phylogenetically Distinct Viruses and Diminishes Vector Preferences for Infected Hosts

Plant virus management is mostly achieved through control of insect vectors using insecticides. However, insecticides are only marginally effective for preventing virus transmission. Furthermore, it is well established that symptoms of virus infections often encourage vector visitation to infected hosts, which exacerbates secondary spread. Plant defense elicitors, phytohormone analogs that prime the plant immune system against attack, may be a viable approach for virus control that complements insecticide use by disrupting pathologies that attract vectors. To explore this, we tested the effect of a commercial plant elicitor, acibenzolar-S-methyl (ASM), on infection rates, virus titers, and symptom development in melon plants inoculated with one of two virus species, Cucumber mosaic virus (CMV) and Cucurbit yellow stunting disorder virus (CYSDV). We also conducted behavioral assays to assess the effect of ASM treatment and virus inoculation on vector behavior. For both pathogens, ASM treatment reduced symptom severity and delayed disease progression. For CYSDV, this resulted in the attenuation of symptoms that encourage vector visitation and virion uptake. We did observe slight trade-offs in growth vs. defense following ASM treatment, but these effects did not translate into reduced yields or plant performance in the field. Our results suggest that immunity priming may be a valuable tool for improving management of insect-transmitted plant viruses.

Effects of the Age-Related Resistance to Potato virus Y in Potato on the Systemic Spread of the Virus, Incidence of the Potato Tuber Necrotic Ringspot Disease, Tuber Yield, and Translocation Rates Into Progeny Tubers

The recombinant strain of potato virus Y (PVY), PVY^NTN, is the main cause of the potato tuber necrotic ringspot disease (PTNRD) in susceptible potato cultivars, which reduces the quality of potato tubers, in addition to the yield loss. Control of PVY has been the main challenge in most potato-producing areas. Here, the effects of the age-related resistance (ARR) were investigated in transplants of a potato cultivar Yukon Gold to the infection with PVY^NTN strain in greenhouse experiments. Within the first 3 weeks after transplanting into soil (week 1 [W1] to W3), Yukon Gold plants developed ARR that impaired the systemic movement of PVY^NTN into upper noninoculated leaves and concomitant translocation into progeny tubers starting from W4 after transplanting. The yield and quality of tubers from PVY-infected plants with the established ARR (W5 to W8) were comparable with the healthy controls, suggesting that late PVY infection would not significantly affect commercial potato production. Plants inoculated early (W1 to W2), before the establishment of the ARR, exhibited a 100% primary systemic infection with PVY^NTN and produced fewer tubers of smaller sizes, exhibiting PTNRD; this resulted ≤70% yield reduction compared with plants inoculated later in the season (W5 to W8). This ARR greatly restricted the systemic movement of PVY^NTN in the foliage and resulted in very limited translocation rates of the virus into tested progeny tubers: 7.8 and 4.1% in 2017 and 2018, respectively, of all plants inoculated later in the season (W5 to W8). This study suggests that PVY^NTN management programs in Yukon Gold seed potato should focus more on the early stages of the potato development before the onset of the ARR.

The Lifecycle of the Plant Immune System

Throughout their life span, plants confront an endless barrage of pathogens and pests. To successfully defend against biotic threats, plants have evolved a complex immune system responsible for surveillance, perception, and the activation of defense. Plant immunity requires multiple signaling processes, the outcome of which vary according to the lifestyle of the invading pathogen(s). In short, these processes require the activation of host perception, the regulation of numerous signaling cascades, and transcriptome reprograming, all of which are highly dynamic in terms of temporal and spatial scales. At the same time, the development of a single immune event is subjective to the development of plant immune system, which is co-regulated by numerous processes, including plant ontogenesis and the host microbiome. In total, insight into each of these processes provides a fuller understanding of the mechanisms that govern plant-pathogen interactions. In this review, we will discuss the "lifecycle" of plant immunity: the development of individual events of defense, including both local and distal processes, as well as the development and regulation of the overall immune system by ontogenesis regulatory genes and environmental microbiota. In total, we will integrate the output of recent discoveries and theories, together with several hypothetical models, to present a dynamic portrait of plant immunity.