The zigzag model of plant-microbe interactions: is it time to move on?

Impact of Nutrient Stress on Plant Disease Resistance

Plants are constantly exposed to abiotic and biotic stresses that seriously affect crop yield and quality. A coordinated regulation of plant responses to combined abiotic/biotic stresses requires crosstalk between signaling pathways initiated by each stressor. Interconnected signaling pathways further finetune plant stress responses and allow the plant to respond to such stresses effectively. The plant nutritional status might influence disease resistance by strengthening or weakening plant immune responses, as well as through modulation of the pathogenicity program in the pathogen. Here, we discuss advances in our understanding of interactions between nutrient stress, deficiency or excess, and immune signaling pathways in the context of current agricultural practices. The introduction of chemical fertilizers and pesticides was a major component of the Green Revolution initiated in the 1960s that greatly boosted crop production. However, the massive application of agrochemicals also has adverse consequences on the environment and animal/human health. Therefore, an in-depth understanding of the connections between stress caused by overfertilization (or low bioavailability of nutrients) and immune responses is a timely and novel field of research with important implications for disease control in crop species. Optimizing nutrient management practices tailored to specific environmental conditions will be crucial in maximizing crop production using environmentally friendly systems.

Transcriptional dynamics during Heliothis zea nudivirus 1 infection in an ovarian cell line from Helicoverpa zea

Nudiviruses (family Nudiviridae) are double-stranded DNA viruses that infect various insects and crustaceans. Among them, Heliothis zea nudivirus 1 (HzNV-1) represents the rare case of a lepidopteran nudivirus inducing a sexual pathology. Studies about molecular pathological dynamics of HzNV-1 or other nudiviruses are scarce. Hence, this study aims to provide a transcriptomic profile of HzNV-1 in an ovary-derived cell line of Helicoverpa zea (HZ-AM1), during early (3, 6 and 9 h post-infection) and advanced (12 and 24 h post-infection) stages of infection. Total RNA was extracted from both virus- and mock-infected cells, and RNA-seq analysis was performed to examine both virus and host transcriptional dynamics. Hierarchical clustering was used to categorize viral genes, while differential gene expression analysis was utilized to pinpoint host genes that are significantly affected by the infection. Hierarchical clustering classified the 154 HzNV-1 genes into four temporal phases, with early phases mainly involving transcription and replication genes and later phases including genes for virion assembly. In addition, a novel viral promoter motif was identified in the upstream region of early-expressed genes. Host gene analysis revealed significant upregulation of heat shock protein genes and downregulation of histone genes. The identification of temporal patterns in viral gene expression enhances the molecular understanding of nudivirus pathology, while the identified differentially expressed host genes highlight the key pathways most hijacked by HzNV-1 infection.

Comparative genomics of Fusarium species causing Fusarium ear rot of maize

Fusarium ear rot (FER), caused by the fungal pathogen Fusarium verticillioides, stands as one of the most economically burdensome and pervasive diseases affecting maize worldwide. Its impact on food security is particularly pronounced due to the production of fumonisins, toxic secondary metabolites that pose serious health risks, especially for livestock. FER disease severity is complex and polygenic, with few resistance (R) genes being identified for use in breeding resistant varieties. While FER is the subject of several breeding programs, only a few studies have investigated entire populations of F. verticillioides with corresponding virulence data to better understand and characterize the pathogenomics. Here, we sequenced and compared the genomes of 50 Fusarium isolates (43 F. verticillioides and 7 other Fusarium spp.) that were used to inoculate a diverse maize population. Our objectives were to elucidate the genome size and composition of F. verticillioides, explore the variable relationship between fumonisin production and visual disease severity, and shed light on the phylogenetic relationships among the isolates. Additionally, we conducted a comparative analysis of the nucleotide variants (SNPs) and the isolates' effectoromes to uncover potential genetic determinants of pathogenicity. Our findings revealed several promising leads, notably the association of certain gene groups, such as pectate lyase, with disease severity. These genes should be investigated further as putative alleles for breeding resistant maize varieties. We suggest that, beyond validation of the alleles identified in this study, researchers validate each phenotypic dataset on an individual basis, particularly if considering fumonisin concentrations and when using diverse populations. Our study underscores the importance of genomic analysis in tackling FER and offers insights that could inform the development of resilient maize cultivars. By leveraging advances in genomics and incorporating pathogen populations into breeding programs, resistance to FER can be advanced.

Comparison of Root Transcriptomes against Clubroot Disease Pathogens in a Resistant Chinese Cabbage Cultivar (Brassica rapa cv. 'Akimeki')

Clubroot, caused by Plasmodiophora brassicae, is one of the diseases that causes major economic losses in cruciferous crops worldwide. Although prevention strategies, including soil pH adjustment and crop rotation, have been used, the disease's long persistence and devastating impact continuously remain in the soil. CR varieties were developed for clubroot-resistant (CR) Chinese cabbage, and 'Akimeki' is one of the clubroot disease-resistant cultivars. However, recent studies have reported susceptibility to several Korean pathotypes in Akimeki and the destruction of the resistance to P. brassicae in many Brassica species against CR varieties, requiring the understanding of more fine-tuned plant signaling by fungal pathogens. In this study, we focused on the early molecular responses of Akimeki during infection with two P. brassicae strains, Seosan (SS) and Hoengseong2 (HS2), using RNA sequencing (RNA-seq). Among a total of 2358 DEGs, 2037 DEGs were differentially expressed following SS and HS2 infection. Gene ontology (GO) showed that 1524 and 513 genes were up-regulated following SS and HS2 inoculations, respectively. Notably, the genes of defense response and jasmonic acid regulations were enriched in the SS inoculation condition, and the genes of water transport and light intensity response were enriched in the HS2 inoculation condition. Moreover, KEGG pathways revealed that the gene expression set were related to pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) mechanisms. The results will provide valuable information for developing CR cultivars in Brassica plants.

Specificity and breadth of plant specialized metabolite-microbe interactions

Plant specialized metabolites shape plant interactions with the environment including plant-microbe interactions. While we often group compounds into generic classes, it is the precise structure of a compound that creates a specific role in plant-microbe or-pathogen interactions. Critically, the structure guides definitive targets in individual interactions, yet single compounds are not limited to singular mechanistic targets allowing them to influence interactions across broad ranges of attackers, from bacteria to fungi to animals. Further, the direction of the effect can be altered by counter evolution within the interacting organism leading to single compounds being both beneficial and detrimental. Thus, the benefit of a single compound to a host needs to be assessed by measuring the net benefit across all interactions while in each specific interaction. Factoring this complexity for single compounds in plant-microbe interactions with the massive expansion in our identification of specialized metabolite pathways means that we need systematic studies to classify the full breadth of activities. Only with this full biological knowledge we can develop mechanistic, ecological, and evolutionary models to understand how plant specialized metabolites fully influence plant-microbe and plant-biotic interactions more broadly.

Insights to Gossypium defense response against Verticillium dahliae: the Cotton Cancer

The soil-borne pathogen Verticillium dahliae, also referred as "The Cotton Cancer," is responsible for causing Verticillium wilt in cotton crops, a destructive disease with a global impact. To infect cotton plants, the pathogen employs multiple virulence mechanisms such as releasing enzymes that degrade cell walls, activating genes that contribute to virulence, and using protein effectors. Conversely, cotton plants have developed numerous defense mechanisms to combat the impact of V. dahliae. These include strengthening the cell wall by producing lignin and depositing callose, discharging reactive oxygen species, and amassing hormones related to defense. Despite the efforts to develop resistant cultivars, there is still no permanent solution to Verticillium wilt due to a limited understanding of the underlying molecular mechanisms that drive both resistance and pathogenesis is currently prevalent. To address this challenge, cutting-edge technologies such as clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), host-induced gene silencing (HIGS), and gene delivery via nano-carriers could be employed as effective alternatives to control the disease. This article intends to present an overview of V. dahliae virulence mechanisms and discuss the different cotton defense mechanisms against Verticillium wilt, including morphophysiological and biochemical responses and signaling pathways including jasmonic acid (JA), salicylic acid (SA), ethylene (ET), and strigolactones (SLs). Additionally, the article highlights the significance of microRNAs (miRNAs), circular RNAs (circRNAs), and long non-coding RNAs (lncRNAs) in gene expression regulation, as well as the different methods employed to identify and functionally validate genes to achieve resistance against this disease. Gaining a more profound understanding of these mechanisms could potentially result in the creation of more efficient strategies for combating Verticillium wilt in cotton crops.

Natural immunity stimulation using ELICE16INDURES® plant conditioner in field culture of soybean

Recently, climate change has had an increasing impact on the world. Innate defense mechanisms operating in plants - such as PAMP-triggered Immunity (PTI) - help to reduce the adverse effects caused by various abiotic and biotic stressors. In this study, the effects of ELICE16INDURES® plant conditioner for organic farming, developed by the Research Institute for Medicinal Plants and Herbs Ltd. Budakalász Hungary, were studied in a soybean population in Northern Hungary. The active compounds and ingredients of this product were selected in such a way as to facilitate the triggering of general plant immunity without the presence and harmful effects of pathogens, thereby strengthening the healthy plant population and preparing it for possible stress effects. In practice, treatments of this agent were applied at two different time points and two concentrations. The conditioning effect was well demonstrated by using agro-drone and ENDVI determination in the soybean field. The genetic background of healthier plants was investigated by NGS sequencing, and by the expression levels of genes encoding enzymes involved in the catalysis of metabolic pathways regulating PTI. The genome-wide transcriptional profiling resulted in 13 contigs related to PAMP-triggered immunity and activated as a result of the treatments. Further analyses showed 16 additional PTI-related contigs whose gene expression changed positively as a result of the treatments. The gene expression values of genes encoded in these contigs were determined by in silico mRNA quantification and validated by RT-qPCR. Both - relatively low and high treatments - showed an increase in gene expression of key genes involving AOC, IFS, MAPK4, MEKK, and GST. Transcriptomic results indicated that the biosyntheses of jasmonic acid (JA), salicylic acid (SA), phenylpropanoid, flavonoid, phytoalexin, and cellular detoxification processes were triggered in the appropriate molecular steps and suggested that plant immune reactions may be activated also artificially, and innate immunity can be enhanced with proper plant biostimulants.

Progress and Challenges in Elucidating the Functional Role of Effectors in the Soybean-Phytophthora sojae Interaction

Phytophthora sojae, the agent responsible for stem and root rot, is one of the most damaging plant pathogens of soybean. To establish a compatible-interaction, P. sojae secretes a wide array of effector proteins into the host cell. These effectors have been shown to act either in the apoplastic area or the cytoplasm of the cell to manipulate the host cellular processes in favor of the development of the pathogen. Deciphering effector-plant interactions is important for understanding the role of P. sojae effectors in disease progression and developing approaches to prevent infection. Here, we review the subcellular localization, the host proteins, and the processes associated with P. sojae effectors. We also discuss the emerging topic of effectors in the context of effector-resistance genes interaction, as well as model systems and recent developments in resources and techniques that may provide a better understanding of the soybean-P. sojae interaction.

The immunity priming effect of the Arabidopsis phyllosphere resident yeast Protomyces arabidopsidicola strain C29

The phyllosphere is a complex habitat for diverse microbial communities. Under natural conditions, multiple interactions occur between host plants and phyllosphere resident microbes, such as bacteria, oomycetes, and fungi. Our understanding of plant associated yeasts and yeast-like fungi lags behind other classes of plant-associated microbes, largely due to a lack of yeasts associated with the model plant Arabidopsis, which could be used in experimental model systems. The yeast-like fungal species Protomyces arabidopsidicola was previously isolated from the phyllosphere of healthy wild-growing Arabidopsis, identified, and characterized. Here we explore the interaction of P. arabidopsidicola with Arabidopsis and found P. arabidopsidicola strain C29 was not pathogenic on Arabidopsis, but was able to survive in its phyllosphere environment both in controlled environment chambers in the lab and under natural field conditions. Most importantly, P. arabidopsidicola exhibited an immune priming effect on Arabidopsis, which showed enhanced disease resistance when subsequently infected with the fungal pathogen Botrytis cinerea. Activation of the mitogen-activated protein kinases (MAPK), camalexin, salicylic acid, and jasmonic acid signaling pathways, but not the auxin-signaling pathway, was associated with this priming effect, as evidenced by MAPK3/MAPK6 activation and defense marker expression. These findings demonstrate Arabidopsis immune defense priming by the naturally occurring phyllosphere resident yeast species, P. arabidopsidicola, and contribute to establishing a new interaction system for probing the genetics of Arabidopsis immunity induced by resident yeast-like fungi.

Influence of Elevated Temperatures on Resistance Against Phoma Stem Canker in Oilseed Rape

Cultivar resistance is an important tool in controlling pathogen-related diseases in agricultural crops. As temperatures increase due to global warming, temperature-resilient disease resistance will play an important role in crop protection. However, the mechanisms behind the temperature-sensitivity of the disease resistance response are poorly understood in crop species and little is known about the effect of elevated temperatures on quantitative disease resistance. Here, we investigated the effect of temperature increase on the quantitative resistance of Brassica napus against Leptosphaeria maculans. Field experiments and controlled environment inoculation assays were done to determine the influence of temperature on R gene-mediated and quantitative resistance against L. maculans; of specific interest was the impact of high summer temperatures on the severity of phoma stem canker. Field experiments were run for three consecutive growing seasons at various sites in England and France using twelve winter oilseed rape breeding lines or cultivars with or without R genes and/or quantitative resistance. Stem inoculation assays were done under controlled environment conditions with four cultivars/breeding lines, using avirulent and virulent L. maculans isolates, to determine if an increase in ambient temperature reduces the efficacy of the resistance. High maximum June temperature was found to be related to phoma stem canker severity. No temperature effect on stem canker severity was found for the cultivar ES Astrid (with only quantitative resistance with no known R genes). However, in the controlled environmental conditions, the cultivar ES Astrid had significantly smaller amounts of necrotic tissue at 20°C than at 25°C. This suggests that, under a sustained temperature of 25°C, the efficacy of quantitative resistance is reduced. Findings from this study show that temperature-resilient quantitative resistance is currently available in some oilseed cultivars and that efficacy of quantitative resistance is maintained at increased temperature but not when these elevated temperatures are sustained for a long period.

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

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

Polymorphisms in plants to restrict losses to pathogens: From gene family expansions to complex network evolution

Genetic polymorphisms are the basis of the natural diversity seen in all life on earth, also in plant-pathogen interactions. Initially, studies on plant-pathogen interaction focused on reporting phenotypic variation in resistance properties and on the identification of underlying major genes. Nowadays, the field of plant-pathogen interactions is moving from focusing on families of single dominant genes involved in gene-for-gene interactions to an understanding of the plant immune system in the context of a much more complex signaling network and quantitative resistance. Simultaneously, studies on pathosystems from the wild and genome analyses advanced, revealing tremendous variation in natural plant populations. It is now imperative to place studies on genetic diversity and evolution of plant-pathogen interactions in the appropriate molecular biological, as well as evolutionary, context.

How to win a tug-of-war: the adaptive evolution of Phytophthora effectors

The 'zigzag' model formulates some of the fundamental principles underpinning the dynamic interactions between pathogen effectors and plant immunity. As key virulence factors, effectors often exhibit a pattern of rapid evolution, presumably as a result of the host-pathogen arms race. Here, we summarize the current knowledge of mechanisms that may accelerate effector evolution in the highly successful Phytophthora pathogens. Recent findings on epigenetic regulation of effector genes that allows evasion of host recognition and maintenance of cost/benefit balance, and a conserved structural unit in effector proteins that may promote the evolution of virulence activities are highlighted.

Network biology to uncover functional and structural properties of the plant immune system

In the last two decades, advances in network science have facilitated the discovery of important systems' entities in diverse biological networks. This graph-based technique has revealed numerous emergent properties of a system that enable us to understand several complex biological processes including plant immune systems. With the accumulation of multiomics data sets, the comprehensive understanding of plant-pathogen interactions can be achieved through the analyses and efficacious integration of multidimensional qualitative and quantitative relationships among the components of hosts and their microbes. This review highlights comparative network topology analyses in plant-pathogen co-expression networks and interactomes, outlines dynamic network modeling for cell-specific immune regulatory networks, and discusses the new frontiers of single-cell sequencing as well as multiomics data integration that are necessary for unraveling the intricacies of plant immune systems.

Understanding and Exploiting Post-Translational Modifications for Plant Disease Resistance

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

Sulfur Deprivation Modulates Salicylic Acid Responses via Nonexpressor of Pathogenesis-Related Gene 1 in Arabidopsis thaliana

Mineral nutrients are essential for plant growth and reproduction, yet only a few studies connect the nutritional status to plant innate immunity. The backbone of plant defense response is mainly controlled by two major hormones: salicylic acid (SA) and jasmonic acid (JA). This study investigated changes in the macronutrient concentration (deficiency/excess of nitrogen, phosphorus, potassium, magnesium, and sulfur) on the expression of PR1, a well-characterized marker in the SA-pathway, and PDF1.2 and LOX2 for the JA-pathway, analyzing plants carrying the promoter of each gene fused to GUS as a reporter. After histochemical GUS assays, we determined that PR1 gene was strongly activated in response to sulfur (S) deficiency. Using RT-PCR, we observed that the induction of PR1 depended on the function of Non-expressor of Pathogenesis-Related gene 1 (NPR1) and SA accumulation, as PR1 was not expressed in npr1-1 mutant and NahG plants under S-deprived conditions. Plants treated with different S-concentrations showed that total S-deprivation was required to induce SA-mediated defense responses. Additionally, bioassays revealed that S-deprived plants, induced resistance to the hemibiotrophic pathogen Pseudomonas syringae pv. DC3000 and increase susceptibility to the necrotrophic Botrytis cinerea. In conclusion, we observed a relationship between S and SA/JA-dependent defense mechanisms in Arabidopsis.

The Critical Role of Small RNAs in Regulating Plant Innate Immunity

Plants, due to their sessile nature, have an innate immune system that helps them to defend against different pathogen infections. The defense response of plants is composed of a highly regulated and complex molecular network, involving the extensive reprogramming of gene expression during the presence of pathogenic molecular signatures. Plants attain proper defense against pathogens through the transcriptional regulation of genes encoding defense regulatory proteins and hormone signaling pathways. Small RNAs are emerging as versatile regulators of plant development and act in different tiers of plant immunity, including pathogen-triggered immunity (PTI) and effector-triggered immunity (ETI). The versatile regulatory functions of small RNAs in plant growth and development and response to biotic and abiotic stresses have been widely studied in recent years. However, available information regarding the contribution of small RNAs in plant immunity against pathogens is more limited. This review article will focus on the role of small RNAs in innate immunity in plants.

Engineering disease resistant plants through CRISPR-Cas9 technology

Plants are susceptible to phytopathogens, including bacteria, fungi, and viruses, which cause colossal financial shortfalls (pre- and post-harvest) and threaten global food safety. To combat with these phytopathogens, plant possesses two-layer of defense in the form of PAMP-triggered immunity (PTI), or Effectors-triggered immunity (ETI). The understanding of plant-molecular interactions and revolution of high-throughput molecular techniques have opened the door for innovations in developing pathogen-resistant plants. In this context, Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) has transformed genome editing (GE) technology and being harnessed for altering the traits. Here we have summarized the complexities of plant immune system and the use of CRISPR-Cas9 to edit the various components of plant immune system to acquire long-lasting resistance in plants against phytopathogens. This review also sheds the light on the limitations of CRISPR-Cas9 system, regulation of CRISPR-Cas9 edited crops and future prospective of this technology.

Dynamic Regulatory Event Mining by iDREM in Large-Scale Multi-omics Datasets During Biotic and Abiotic Stress in Plants

The system-wide complexity of genome regulation encoding the organism phenotypic diversity is well understood. However, a major challenge persists about the appropriate method to describe the systematic dynamic genome regulation event utilizing enormous multi-omics datasets. Here, we describe Interactive Dynamic Regulatory Events Miner (iDREM) which reconstructs gene-regulatory networks from temporal transcriptome, proteome, and epigenome datasets during stress to envisage "master" regulators by simulating cascades of temporal transcription-regulatory and interactome events. The iDREM is a Java-based software that integrates static and time-series transcriptomics and proteomics datasets, transcription factor (TF)-target interactions, microRNA (miRNA)-target interaction, and protein-protein interactions to reconstruct temporal regulatory network and identify significant regulators in an unsupervised manner. The hidden Markov model detects specialized manipulated pathways as well as genes to recognize statistically significant regulators (TFs/miRNAs) that diverge in temporal activity. This method can be translated to any biotic or abiotic stress in plants and animals to predict the master regulators from condition-specific multi-omics datasets including host-pathogen interactions for comprehensive understanding of manipulated biological pathways.

[Plant resistance to pathogens: A review describing the vertical and horizontal resistance concepts]

Understanding plant resistance requires an interdisciplinary effort between biological and agricultural sciences. In this setting, phytopathology has experienced an upsurge of interest from researchers and scholars in the disciplines of ecology, evolution and molecular biology. This encounter did neither avoid misunderstandings among scholars nor the use of similar concepts with different meanings. The purpose of this paper is to offer a modern comprehensive view of plant resistance against pathogens using a classical phytopathology concept as framework: Van der Plank s concept of horizontal and vertical resistance. This concept is used in other agricultural science disciplines (plant breeding and genetics), supporting why it is a proper framework for explaining plant resistance. Within this frame, other classical phytopathologycal concepts are explained in combination with modern model descriptions of plant-pathogen interactions and how all these concepts are related with quantitative and field resistance. This review is written in Spanish because it serves an additional purpose. In the Spanish-speaking America, besides interdisciplinarity, phytopathology as an academic discipline faces another challenge: the students' low-English language proficiency. In this regard, this review intends to become a companion guide for plant-pathology teachers in the region interested in providing an insight into the modern concepts of plant resistance.

A systematic analysis of apple root resistance traits to Pythium ultimum infection and the underpinned molecular regulations of defense activation

Apple replant disease (ARD), caused by a pathogen complex, significantly impacts apple orchard establishment. The molecular regulation on ARD resistance has not been investigated until recently. A systematic phenotyping effort and a series of transcriptomic analyses were performed to uncover the underpinned molecular mechanism of apple root resistance to P. ultimum, a representative member in ARD pathogen complex. Genotype-specific plant survival rates and biomass reduction corresponded with microscopic features of necrosis progression patterns along the infected root. The presence of defined boundaries separating healthy and necrotic sections likely caused delayed necrosis expansion in roots of resistant genotypes compared with swift necrosis progression and profuse hyphae growth along infected roots of susceptible genotypes. Comprehensive datasets from a series of transcriptome analyses generated the first panoramic view of genome-wide transcriptional networks of defense activation between resistant and susceptible apple roots. Earlier and stronger molecular defense activation, such as pathogen perception and hormone signaling, may differentiate resistance from susceptibility in apple root. Delayed and interrupted activation of multiple defense pathways could have led to an inadequate resistance response. Using the panel of apple rootstock germplasm with defined resistant and susceptible phenotypes, selected candidate genes are being investigated by transgenic manipulation including CRISPR/Cas9 tools for their specific roles during apple root defense toward P. ultimum infection. Individual apple genes with validated functions regulating root resistance responses can be exploited for developing molecular tools for accurate and efficient incorporation of resistance traits into new apple rootstocks.

Microbial Effector Proteins - A Journey through the Proteolytic Landscape

In the evolutionary arms race between pathogens and plants, pathogens evolved effector molecules that they secrete into the host to subvert plant cellular responses in a process termed the effector-targeted pathway (ETP). During recent years the repertoire of ETPs has increased and mounting evidence indicates that the proteasome and autophagy pathways are central hubs of microbial effectors. Both degradation pathways are implicated in a broad array of cellular responses and thus constitute an attractive target for effector proteins to have a broader impact on the host. In this article we first summarize recent findings on how effectors from various pathogens modulate proteolytic pathways and then provide a network analysis of established effector targets implicated in proteolytic degradation machineries. With this network we emphasize the idea that effectors targeting proteolytic degradation pathways will affect the protein synthesis-transport and degradation triangle. We put in perspective that, in utilizing the effector diversity of microbes, we produce excellent tools to study diverse cellular pathways and their possible interplay with each other.

Elicitor and Receptor Molecules: Orchestrators of Plant Defense and Immunity

Pathogen-associated molecular patterns (PAMPs), microbe-associated molecular patterns (MAMPs), herbivore-associated molecular patterns (HAMPs), and damage-associated molecular patterns (DAMPs) are molecules produced by microorganisms and insects in the event of infection, microbial priming, and insect predation. These molecules are then recognized by receptor molecules on or within the plant, which activates the defense signaling pathways, resulting in plant’s ability to overcome pathogenic invasion, induce systemic resistance, and protect against insect predation and damage. These small molecular motifs are conserved in all organisms. Fungi, bacteria, and insects have their own specific molecular patterns that induce defenses in plants. Most of the molecular patterns are either present as part of the pathogen’s structure or exudates (in bacteria and fungi), or insect saliva and honeydew. Since biotic stresses such as pathogens and insects can impair crop yield and production, understanding the interaction between these organisms and the host via the elicitor–receptor interaction is essential to equip us with the knowledge necessary to design durable resistance in plants. In addition, it is also important to look into the role played by beneficial microbes and synthetic elicitors in activating plants’ defense and protection against disease and predation. This review addresses receptors, elicitors, and the receptor–elicitor interactions where these components in fungi, bacteria, and insects will be elaborated, giving special emphasis to the molecules, responses, and mechanisms at play, variations between organisms where applicable, and applications and prospects.

The PTI to ETI Continuum in Phytophthora-Plant Interactions

Phytophthora species are notorious pathogens of several economically important crop plants. Several general elicitors, commonly referred to as Pathogen-Associated Molecular Patterns (PAMPs), from Phytophthora spp. have been identified that are recognized by the plant receptors to trigger induced defense responses in a process termed PAMP-triggered Immunity (PTI). Adapted Phytophthora pathogens have evolved multiple strategies to evade PTI. They can either modify or suppress their elicitors to avoid recognition by host and modulate host defense responses by deploying hundreds of effectors, which suppress host defense and physiological processes by modulating components involved in calcium and MAPK signaling, alternative splicing, RNA interference, vesicle trafficking, cell-to-cell trafficking, proteolysis and phytohormone signaling pathways. In incompatible interactions, resistant host plants perceive effector-induced modulations through resistance proteins and activate downstream components of defense responses in a quicker and more robust manner called effector-triggered-immunity (ETI). When pathogens overcome PTI-usually through effectors in the absence of R proteins-effectors-triggered susceptibility (ETS) ensues. Qualitatively, many of the downstream defense responses overlap between PTI and ETI. In general, these multiple phases of Phytophthora-plant interactions follow the PTI-ETS-ETI paradigm, initially proposed in the zigzag model of plant immunity. However, based on several examples, in Phytophthora-plant interactions, boundaries between these phases are not distinct but are rather blended pointing to a PTI-ETI continuum.

Temporal Quantitative Changes in the Resistant and Susceptible Wheat Leaf Apoplastic Proteome During Infection by Wheat Leaf Rust (Puccinia triticina)

Wheat leaf rust caused by the pathogenic fungus, Puccinia triticina, is a serious threat to bread wheat and durum production in many areas of the world. This plant-pathogen interaction has been studied extensively at the molecular genetics level however, proteomics data are still relatively scarce. The present study investigated temporal changes in the abundance of the apoplastic fluid proteome of resistant and susceptible wheat leaves infected with P. triticina race-1, using a label-free LC-MS-based approach. In general, there was very little difference between inoculated and control apoplastic proteomes in either host, until haustoria had become well established in the susceptible host, although the resistant host responds to pathogen challenge sooner. In the earlier samplings (up to 72 h after inoculation) there were just 46 host proteins with significantly changing abundance, and pathogen proteins were detected only rarely and not reproducibly. This is consistent with the biotrophic lifestyle of P. triticina, where the invading pathogen initially causes little tissue damage or host cell death, which occur only later during the infection cycle. The majority of the host proteins with altered abundance up to 72 h post-inoculation were pathogen-response-related, including peroxidases, chitinases, β-1-3-endo-glucanases, and other PR proteins. Five days after inoculation with the susceptible apoplasm it was possible to detect 150 P. triticina proteins and 117 host proteins which had significantly increased in abundance as well as 33 host proteins which had significantly decreased in abundance. The latter represents potential targets of pathogen effectors and included enzymes which could damage the invader. The pathogen-expressed proteins-seen most abundantly in the incompatible interaction-were mostly uncharacterized proteins however, many of their functions could be inferred through homology-matching with pBLAST. Pathogen proteins also included several candidate effector proteins, some novel, and some which have been reported previously. All MS data have been deposited in the PRIDE archive (www.ebi.ac.uk/pride/archive/) under Project PXD012586.

Identification of Candidate Ergosterol-Responsive Proteins Associated with the Plasma Membrane of Arabidopsis thaliana

The impact of fungal diseases on crop production negatively reflects on sustainable food production and overall economic health. Ergosterol is the major sterol component in fungal membranes and regarded as a general elicitor or microbe-associated molecular pattern (MAMP) molecule. Although plant responses to ergosterol have been reported, the perception mechanism is still unknown. Here, Arabidopsis thaliana protein fractions were used to identify those differentially regulated following ergosterol treatment; additionally, they were subjected to affinity-based chromatography enrichment strategies to capture and categorize ergosterol-interacting candidate proteins using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). Mature plants were treated with 250 nM ergosterol over a 24 h period, and plasma membrane-associated fractions were isolated. In addition, ergosterol was immobilized on two different affinity-based systems to capture interacting proteins/complexes. This resulted in the identification of defense-related proteins such as chitin elicitor receptor kinase (CERK), non-race specific disease resistance/harpin-induced (NDR1/HIN1)-like protein, Ras-related proteins, aquaporins, remorin protein, leucine-rich repeat (LRR)- receptor like kinases (RLKs), G-type lectin S-receptor-like serine/threonine-protein kinase (GsSRK), and glycosylphosphatidylinositol (GPI)-anchored protein. Furthermore, the results elucidated unknown signaling responses to this MAMP, including endocytosis, and other similarities to those previously reported for bacterial flagellin, lipopolysaccharides, and fungal chitin.

Computing Weakly Reversible Deficiency Zero Network Translations Using Elementary Flux Modes

We present a computational method for performing structural translation, which has been studied recently in the context of analyzing the steady states and dynamical behavior of mass-action systems derived from biochemical reaction networks. Our procedure involves solving a binary linear programming problem where the decision variables correspond to interactions between the reactions of the original network. We call the resulting network a reaction-to-reaction graph and formalize how such a construction relates to the original reaction network and the structural translation. We demonstrate the efficacy and efficiency of the algorithm by running it on 508 networks from the European Bioinformatics Institutes' BioModels database. We also summarize how this work can be incorporated into recently proposed algorithms for establishing mono- and multistationarity in biochemical reaction systems.

Systems Biology and Machine Learning in Plant-Pathogen Interactions

Systems biology is an inclusive approach to study the static and dynamic emergent properties on a global scale by integrating multiomics datasets to establish qualitative and quantitative associations among multiple biological components. With an abundance of improved high throughput -omics datasets, network-based analyses and machine learning technologies are playing a pivotal role in comprehensive understanding of biological systems. Network topological features reveal most important nodes within a network as well as prioritize significant molecular components for diverse biological networks, including coexpression, protein-protein interaction, and gene regulatory networks. Machine learning techniques provide enormous predictive power through specific feature extraction from biological data. Deep learning, a subtype of machine learning, has plausible future applications because a domain expert for feature extraction is not needed in this algorithm. Inspired by diverse domains of biology, we here review classic systems biology techniques applied in plant immunity thus far. We also discuss additional advanced approaches in both graph theory and machine learning, which may provide new insights for understanding plant-microbe interactions. Finally, we propose a hybrid approach in plant immune systems that harnesses the power of both network biology and machine learning, with a potential to be applicable to both model systems and agronomically important crop plants.

Plant immunity in natural populations and agricultural fields: Low presence of pathogenesis-related proteins in Solanum leaves

Plant immunity has mainly been studied under controlled conditions, limiting our knowledge regarding the regulation of immunity under natural conditions where plants grow in association with multiple microorganisms. Plant pathology theory, based on laboratory data, predicts complex biochemical plant-pathogen interactions leading to coevolution of pathogen infectivity vs. plant recognition of microbes in multiple layers over time. However, plant immunity is currently not evaluated in relation to ecological time-scales and field conditions. Here we report status of immunity in plants without visible disease symptoms in wild populations of nightshades, Solanum dulcamara and Solanum nigrum, and in agricultural fields of potato, Solanum tuberosum. We analysed presence of pathogenesis-related proteins in over 500 asymptomatic leaf samples collected in the field in June, July and August over three years. Pathogenesis-related proteins were present in only one-third of the collected samples, suggesting low activity of the immune system. We could also detect an increase in pathogenesis-related proteins later in the growing season, particularly in S. tuberosum. Our findings, based on pathogenesis-related protein markers, indicate major gaps in our knowledge regarding the status and regulation of plant immunity under field conditions.

Plastic Transcriptomes Stabilize Immunity to Pathogen Diversity: The Jasmonic Acid and Salicylic Acid Networks within the Arabidopsis/Botrytis Pathosystem

To respond to pathogen attack, selection and associated evolution has led to the creation of plant immune system that are a highly effective and inducible defense system. Central to this system are the plant defense hormones jasmonic acid (JA) and salicylic acid (SA) and crosstalk between the two, which may play an important role in defense responses to specific pathogens or even genotypes. Here, we used the Arabidopsis thaliana-Botrytis cinerea pathosystem to test how the host's defense system functions against genetic variation in a pathogen. We measured defense-related phenotypes and transcriptomic responses in Arabidopsis wild-type Col-0 and JA- and SA-signaling mutants, coi1-1 and npr1-1, individually challenged with 96 diverse B. cinerea isolates. Those data showed genetic variation in the pathogen influences on all components within the plant defense system at the transcriptional level. We identified four gene coexpression networks and two vectors of defense variation triggered by genetic variation in B. cinerea This showed that the JA and SA signaling pathways functioned to constrain/canalize the range of virulence in the pathogen population, but the underlying transcriptomic response was highly plastic. These data showed that plants utilize major defense hormone pathways to buffer disease resistance, but not the metabolic or transcriptional responses to genetic variation within a pathogen.

The Role of Soil Microorganisms in Plant Mineral Nutrition-Current Knowledge and Future Directions

In their natural environment, plants are part of a rich ecosystem including numerous and diverse microorganisms in the soil. It has been long recognized that some of these microbes, such as mycorrhizal fungi or nitrogen fixing symbiotic bacteria, play important roles in plant performance by improving mineral nutrition. However, the full range of microbes associated with plants and their potential to replace synthetic agricultural inputs has only recently started to be uncovered. In the last few years, a great progress has been made in the knowledge on composition of rhizospheric microbiomes and their dynamics. There is clear evidence that plants shape microbiome structures, most probably by root exudates, and also that bacteria have developed various adaptations to thrive in the rhizospheric niche. The mechanisms of these interactions and the processes driving the alterations in microbiomes are, however, largely unknown. In this review, we focus on the interaction of plants and root associated bacteria enhancing plant mineral nutrition, summarizing the current knowledge in several research fields that can converge to improve our understanding of the molecular mechanisms underpinning this phenomenon.

Advances on plant-pathogen interactions from molecular toward systems biology perspectives

In the past 2 decades, progress in molecular analyses of the plant immune system has revealed key elements of a complex response network. Current paradigms depict the interaction of pathogen-secreted molecules with host target molecules leading to the activation of multiple plant response pathways. Further research will be required to fully understand how these responses are integrated in space and time, and exploit this knowledge in agriculture. In this review, we highlight systems biology as a promising approach to reveal properties of molecular plant-pathogen interactions and predict the outcome of such interactions. We first illustrate a few key concepts in plant immunity with a network and systems biology perspective. Next, we present some basic principles of systems biology and show how they allow integrating multiomics data and predict cell phenotypes. We identify challenges for systems biology of plant-pathogen interactions, including the reconstruction of multiscale mechanistic models and the connection of host and pathogen models. Finally, we outline studies on resistance durability through the robustness of immune system networks, the identification of trade-offs between immunity and growth and in silico plant-pathogen co-evolution as exciting perspectives in the field. We conclude that the development of sophisticated models of plant diseases incorporating plant, pathogen and climate properties represent a major challenge for agriculture in the future.

Plant-parasitic nematodes: towards understanding molecular players in stress responses

BACKGROUND

Plant-parasitic nematode interactions occur within a vast molecular plant immunity network. Following initial contact with the host plant roots, plant-parasitic nematodes (PPNs) activate basal immune responses. Defence priming involves the release in the apoplast of toxic molecules derived from reactive species or secondary metabolism. In turn, PPNs must overcome the poisonous and stressful environment at the plant-nematode interface. The ability of PPNs to escape this first line of plant immunity is crucial and will determine its virulence.

SCOPE

Nematodes trigger crucial regulatory cytoprotective mechanisms, including antioxidant and detoxification pathways. Knowledge of the upstream regulatory components that contribute to both of these pathways in PPNs remains elusive. In this review, we discuss how PPNs probably orchestrate cytoprotection to resist plant immune responses, postulating that it may be derived from ancient molecular mechanisms. The review focuses on two transcription factors, DAF-16 and SKN-1 , which are conserved in the animal kingdom and are central regulators of cell homeostasis and immune function. Both regulate the unfolding protein response and the antioxidant and detoxification pathways. DAF-16 and SKN-1 target a broad spectrum of Caenorhabditis elegans genes coding for numerous protein families present in the secretome of PPNs. Moreover, some regulatory elements of DAF-16 and SKN-1 from C. elegans have already been identified as important genes for PPN infection.

CONCLUSION

DAF-16 and SKN-1 genes may play a pivotal role in PPNs during parasitism. In the context of their hub status and mode of regulation, we suggest alternative strategies for control of PPNs through RNAi approaches.

Climate change and the ash dieback crisis

Beyond the direct influence of climate change on species distribution and phenology, indirect effects may also arise from perturbations in species interactions. Infectious diseases are strong biotic forces that can precipitate population declines and lead to biodiversity loss. It has been shown in forest ecosystems worldwide that at least 10% of trees are vulnerable to extinction and pathogens are increasingly implicated. In Europe, the emerging ash dieback disease caused by the fungus Hymenoscyphus fraxineus, commonly called Chalara fraxinea, is causing a severe mortality of common ash trees (Fraxinus excelsior); this is raising concerns for the persistence of this widespread tree, which is both a key component of forest ecosystems and economically important for timber production. Here, we show how the pathogen and climate change may interact to affect the future spatial distribution of the common ash. Using two presence-only models, seven General Circulation Models and four emission scenarios, we show that climate change, by affecting the host and the pathogen separately, may uncouple their spatial distribution to create a mismatch in species interaction and so a lowering of disease transmission. Consequently, as climate change expands the ranges of both species polewards it may alleviate the ash dieback crisis in southern and occidental regions at the same time.

Mechanisms of quantitative disease resistance in plants

Quantitative disease resistance (QDR) causes the reduction, but not absence, of disease, and is a major type of disease resistance for many crop species. QDR results in a continuous distribution of disease scores across a segregating population, and is typically due to many genes with small effects. It may also be a source of durable resistance. The past decade has seen significant progress in cloning genes underlying QDR. In this review, we focus on these recently cloned genes and identify new themes of QDR emerging from these studies.

Magnaporthe oryzae effectors MoHEG13 and MoHEG16 interfere with host infection and MoHEG13 counteracts cell death caused by Magnaporthe-NLPs in tobacco

KEY MESSAGE

Adapted pathogens are able to modulate cell responses of their hosts most likely due to the activity of secreted effector molecules thereby enabling colonisation by ostensible nonhost pathogens. It is postulated that host and nonhost pathogens of a given plant species differ in their repertoire of secreted effector molecules that are able to suppress plant resistance. We pursued the strategy of identifying novel effectors of Magnaporthe oryzae, the causal agent of blast disease, by comparing the infection process of closely related host vs. nonhost Magnaporthe species on barley (Hordeum vulgare L.). When both types of pathogen simultaneously attacked the same cell, the nonhost isolate became a successful pathogen possibly due to potent effectors secreted by the host isolate. Microarray studies led to a set of M. oryzae Hypothetical Effector Genes (MoHEGs) which were classified as Early- and LateMoHEGs according to the maximal transcript abundance during colonization of barley. Interestingly, orthologs of these MoHEGs from a nonhost pathogen were similarly regulated when investigated in a host situation, suggesting evolutionary conserved functions. Knockout mutants of MoHEG16 from the group of EarlyMoHEGs were less virulent on barley and microscopic studies revealed an attenuated transition from epidermal to mesophyll colonization. MoHEG13, a LateMoHEG, was shown to antagonize cell death induced by M. oryzae Necrosis-and ethylene-inducing-protein-1 (Nep1)-like proteins in Nicotiana benthamiana. MoHEG13 has a virulence function as a knockout mutant showed attenuated disease progression when inoculated on barley.

Polarized Defense Against Fungal Pathogens Is Mediated by the Jacalin-Related Lectin Domain of Modular Poaceae-Specific Proteins

Modular proteins are an evolutionary answer to optimize performance of proteins that physically interact with each other for functionality. Using a combination of genetic and biochemical experiments, we characterized the rice protein OsJAC1, which consists of a jacalin-related lectin (JRL) domain predicted to bind mannose-containing oligosaccharides, and a dirigent domain which might function in stereoselective coupling of monolignols. Transgenic overexpression of OsJAC1 in rice resulted in quantitative broad-spectrum resistance against different pathogens including bacteria, oomycetes, and fungi. Overexpression of this gene or its wheat ortholog TAJA1 in barley enhanced resistance against the powdery mildew fungus. Both protein domains of OsJAC1 are required to establish resistance as indicated by single or combined transient expression of individual domains. Expression of artificially separated and fluorescence-tagged protein domains showed that the JRL domain is sufficient for targeting the powdery mildew penetration site. Nevertheless, co-localization of the lectin and the dirigent domain occurred. Phylogenetic analyses revealed orthologs of OsJAC1 exclusively within the Poaceae plant family. Dicots, by contrast, only contain proteins with either JRL or dirigent domain(s). Altogether, our results identify OsJAC1 as a representative of a novel type of resistance protein derived from a plant lineage-specific gene fusion event for better function in local pathogen defense.

PBL13 Is a Serine/Threonine Protein Kinase That Negatively Regulates Arabidopsis Immune Responses

Receptor-like cytoplasmic kinases (RLCKs) are a subset of plant receptor-like kinases lacking both extracellular and transmembrane domains. Some of the 46 members in the Arabidopsis (Arabidopsis thaliana) RLCK subfamily VII have been linked to plant innate immunity; however, most remain uncharacterized. Thus, multiple subfamily VII members are expected to be involved in plant immune signaling. Here, we investigate the role of AvrPphB SUSCEPTIBLE1-LIKE13 (PBL13), a subfamily VII RLCK with unique domain architecture. Unlike other characterized RLCKs, PBL13 transfer DNA insertion lines exhibit enhanced disease resistance after inoculation with virulent Pseudomonas syringae. The pbl13-2 knockout also exhibits elevated basal-level expression of the PATHOGENESIS-RELATED GENE1 defense marker gene, enhanced reactive oxygen species (ROS) burst in response to perception of bacterial microbial patterns, and accelerated flagellin-induced activation of mitogen-activated protein kinases. Recombinant PBL13 is an active kinase, and its primary autophosphorylated sites map to a 15-amino acid repeat motif unique to PBL13. Complementation of pbl13-2 with PBL13-3xFLAG converts the enhanced resistance and elevated ROS phenotypes back to wild-type levels. In contrast, kinase-dead PBL13(K111A)-3xFLAG was unable to rescue pbl13-2 disease phenotypes. Consistent with the enhanced ROS burst in the pbl13-2 knockout, PBL13 is able to associate with the nicotinamide adenine dinucleotide phosphate, reduced oxidase RESPIRATORY BURST OXIDASE HOMOLOG PROTEIN D (RBOHD) by split-luciferase complementation assay, and this association is disrupted by flagellin treatment. We conclude that the PBL13 kinase negatively regulates plant innate immunity to pathogenic bacteria and can associate with RBOHD before pathogen perception. These data are consistent with the hypothesis that PBL13 acts to prevent inappropriate activation of defense responses in the absence of pathogen challenge.

RNA-Seq Analysis of the Expression of Genes Encoding Cell Wall Degrading Enzymes during Infection of Lupin (Lupinus angustifolius) by Phytophthora parasitica

RNA-Seq analysis has shown that over 60% (12,962) of the predicted transcripts in the Phytophthora parasitica genome are expressed during the first 60 h of lupin root infection. The infection transcriptomes included 278 of the 431 genes encoding P. parasitica cell wall degrading enzymes. The transcriptome data provide strong evidence of global transcriptional cascades of genes whose encoded proteins target the main categories of plant cell wall components. A major cohort of pectinases is predominantly expressed early but as infection progresses, the transcriptome becomes increasingly dominated by transcripts encoding cellulases, hemicellulases, β-1,3-glucanases and glycoproteins. The most highly expressed P. parasitica carbohydrate active enzyme gene contains two CBM1 cellulose binding modules and no catalytic domains. The top 200 differentially expressed genes include β-1,4-glucosidases, β-1,4-glucanases, β-1,4-galactanases, a β-1,3-glucanase, an α-1,4-polygalacturonase, a pectin deacetylase and a pectin methylesterase. Detailed analysis of gene expression profiles provides clues as to the order in which linkages within the complex carbohydrates may come under attack. The gene expression profiles suggest that (i) demethylation of pectic homogalacturonan occurs before its deacetylation; (ii) cleavage of the backbone of pectic rhamnogalacturonan I precedes digestion of its side chains; (iii) early attack on cellulose microfibrils by non-catalytic cellulose-binding proteins and enzymes with auxiliary activities may facilitate subsequent attack by glycosyl hydrolases and enzymes containing CBM1 cellulose-binding modules; (iv) terminal hemicellulose backbone residues are targeted after extensive internal backbone cleavage has occurred; and (v) the carbohydrate chains on glycoproteins are degraded late in infection. A notable feature of the P. parasitica infection transcriptome is the high level of transcription of genes encoding enzymes that degrade β-1,3-glucanases during middle and late stages of infection. The results suggest that high levels of β-1,3-glucanases may effectively degrade callose as it is produced by the plant during the defence response.

Growth versus immunity--a redirection of the cell cycle?

Diseases caused by plant pathogens significantly reduce growth and yield in agricultural crop production. Raising immunity in crops is therefore a major aim in breeding programs. However, efforts to enhance immunity are challenged by the occurrence of growth inhibition triggered by immunity that can be as detrimental as diseases. In this review, we will propose molecular models to explain the inhibitory growth-immunity crosstalk. We will briefly discuss why the resource reallocation model might not represent the driving force for the observed growth-immunity trade-offs. We suggest a model in which immunity redirects and initiates hormone signalling activities that can impair plant growth by antagonising cell cycle regulation and meristem activities.

Nuclear processes associated with plant immunity and pathogen susceptibility

Plants are sessile organisms that have evolved exquisite and sophisticated mechanisms to adapt to their biotic and abiotic environment. Plants deploy receptors and vast signalling networks to detect, transmit and respond to a given biotic threat by inducing properly dosed defence responses. Genetic analyses and, more recently, next-generation -omics approaches have allowed unprecedented insights into the mechanisms that drive immunity. Similarly, functional genomics and the emergence of pathogen genomes have allowed reciprocal studies on the mechanisms governing pathogen virulence and host susceptibility, collectively allowing more comprehensive views on the processes that govern disease and resistance. Among others, the identification of secreted pathogen molecules (effectors) that modify immunity-associated processes has changed the plant-microbe interactions conceptual landscape. Effectors are now considered both important factors facilitating disease and novel probes, suited to study immunity in plants. In this review, we will describe the various mechanisms and processes that take place in the nucleus and help regulate immune responses in plants. Based on the premise that any process required for immunity could be targeted by pathogen effectors, we highlight and describe a number of functional assays that should help determine effector functions and their impact on immune-related processes. The identification of new effector functions that modify nuclear processes will help dissect nuclear signalling further and assist us in our bid to bolster immunity in crop plants.

Understanding plant immunity as a surveillance system to detect invasion

Various conceptual models to describe the plant immune system have been presented. The most recent paradigm to gain wide acceptance in the field is often referred to as the zigzag model, which reconciles the previously formulated gene-for-gene hypothesis with the recognition of general elicitors in a single model. This review focuses on the limitations of the current paradigm of molecular plant-microbe interactions and how it too narrowly defines the plant immune system. As such, we discuss an alternative view of plant innate immunity as a system that evolves to detect invasion. This view accommodates the range from mutualistic to parasitic symbioses that plants form with diverse organisms, as well as the spectrum of ligands that the plant immune system perceives. Finally, how this view can contribute to the current practice of resistance breeding is discussed.