Necrotrophic Exploitation and Subversion of Plant Defense: A Lifestyle or Just a Phase, and Implications in Breeding Resistance

Litchi aspartic protease LcAP1 enhances plant resistance via suppressing cell death triggered by the pectate lyase PlPeL8 from Peronophythora litchii

Plant cell death is regulated in plant-pathogen interactions. While some aspartic proteases (APs) participate in regulating programmed cell death or defense responses, the defense functions of most APs remain largely unknown. Here, we report on a virulence factor, PlPeL8, which is a pectate lyase found in the hemibiotrophic pathogen Peronophythora litchii. Through in vivo and in vitro assays, we confirmed the interaction between PlPeL8 and LcAP1 from litchi, and identified LcAP1 as a positive regulator of plant immunity. PlPeL8 induced cell death associated with NbSOBIR1 and NbMEK2. The 11 conserved residues of PlPeL8 were essential for inducing cell death and enhancing plant susceptibility. Twenty-three LcAPs suppressed cell death induced by PlPeL8 in Nicotiana benthamiana depending on their interaction with PlPeL8. The N-terminus of LcAP1 was required for inhibiting PlPeL8-triggered cell death and susceptibility. Furthermore, PlPeL8 led to higher susceptibility in NbAPs-silenced N. benthamiana than the GUS-control. Our results indicate the crucial roles of LcAP1 and its homologs in enhancing plant resistance via suppression of cell death triggered by PlPeL8, and LcAP1 represents a promising target for engineering disease resistance. Our study provides new insights into the role of plant cell death in the arms race between plants and hemibiotrophic pathogens.

Interactive transcriptome analyses of Northern Wild Rice (Zizania palustris L.) and Bipolaris oryzae show convoluted communications during the early stages of fungal brown spot development

Fungal diseases, caused mainly by Bipolaris spp., are past and current threats to Northern Wild Rice (NWR) grain production and germplasm preservation in both natural and cultivated settings. Genetic resistance against the pathogen is scarce. Toward expanding our understanding of the global gene communications of NWR and Bipolaris oryzae interaction, we designed an RNA sequencing study encompassing the first 12 h and 48 h of their encounter. NWR activated numerous plant recognition receptors after pathogen infection, followed by active transcriptional reprogramming of signaling mechanisms driven by Ca2+ and its sensors, mitogen-activated protein kinase cascades, activation of an oxidative burst, and phytohormone signaling-bound mechanisms. Several transcription factors associated with plant defense were found to be expressed. Importantly, evidence of diterpenoid phytoalexins, especially phytocassane biosynthesis, among expression of other defense genes was found. In B. oryzae, predicted genes associated with pathogenicity including secreted effectors that could target plant defense mechanisms were expressed. This study uncovered the early molecular communication between the NWR-B. oryzae pathosystem, which could guide selection for allele-specific genes to boost NWR defenses, and overall aid in the development of more efficient selection methods in NWR breeding through the use of the most virulent fungal isolates.

Transgressive segregation, hopeful monsters, and phenotypic selection drove rapid genetic gains and breakthroughs in predictive breeding for quantitative resistance to Macrophomina in strawberry

Two decades have passed since the strawberry (Fragaria x ananassa) disease caused by Macrophomina phaseolina, a necrotrophic soilborne fungal pathogen, began surfacing in California, Florida, and elsewhere. This disease has since become one of the most common causes of plant death and yield losses in strawberry. The Macrophomina problem emerged and expanded in the wake of the global phase-out of soil fumigation with methyl bromide and appears to have been aggravated by an increase in climate change-associated abiotic stresses. Here we show that sources of resistance to this pathogen are rare in gene banks and that the favorable alleles they carry are phenotypically unobvious. The latter were exposed by transgressive segregation and selection in populations phenotyped for resistance to Macrophomina under heat and drought stress. The genetic gains were immediate and dramatic. The frequency of highly resistant individuals increased from 1% in selection cycle 0 to 74% in selection cycle 2. Using GWAS and survival analysis, we found that phenotypic selection had increased the frequencies of favorable alleles among 10 loci associated with resistance and that favorable alleles had to be accumulated among four or more of these loci for an individual to acquire resistance. An unexpectedly straightforward solution to the Macrophomina disease resistance breeding problem emerged from our studies, which showed that highly resistant cultivars can be developed by genomic selection per se or marker-assisted stacking of favorable alleles among a comparatively small number of large-effect loci.

Characterization of a Host-Specific Toxic Activity Produced by Bipolaris cookei, Causal Agent of Target Leaf Spot of Sorghum

Target leaf spot (TLS) of sorghum, caused by the necrotrophic fungus Bipolaris cookei, can cause severe yield loss in many parts of the world. We grew B. cookei in liquid culture and observed that the resulting culture filtrate (CF) was differentially toxic when infiltrated into the leaves of a population of 288 diverse sorghum lines. In this population, we found a significant correlation between high CF sensitivity and susceptibility to TLS. This suggests that the toxin produced in culture may play a role in the pathogenicity of B. cookei in the field. We demonstrated that the toxic activity is light sensitive and, surprisingly, insensitive to pronase, suggesting that it is not proteinaceous. We identified the two sorghum genetic loci most associated with the response to CF in this population. Screening seedlings with B. cookei CF could be a useful approach for prescreening germplasm for TLS resistance.

A leucine-rich repeat receptor kinase gene confers quantitative susceptibility to maize southern leaf blight

Southern leaf blight (SLB), caused by the necrotrophic fungal pathogen Cochliobolus heterostrophus (anamorph Bipolaris maydis), is a major foliar disease which causes significant yield losses in maize worldwide. A major quantitative trait locus, qSLB_3.04 , conferring recessive resistance to SLB was previously mapped on maize chromosome 3. Using a combination of map-based cloning, association analysis, ethyl methanesulfonate and transposon mutagenesis, and CRISPR-Cas9 editing, we demonstrate that a leucine-rich repeat receptor-like kinase gene which we have called ChSK1 (Cochliobolus heterostrophus Susceptibility Kinase 1) at qSLB_3.04 causes increased susceptibility to SLB. Genes of this type have generally been associated with the defense response. We present evidence that ChSK1 may be associated with suppression of the basal immune response. These findings contribute to our understanding of plant disease susceptibility genes and the potential to use them for engineering durable disease resistance.

The Necrotroph Botrytis cinerea BcSpd1 Plays a Key Role in Modulating Both Fungal Pathogenic Factors and Plant Disease Development

Botrytis cinerea is a necrotrophic microbe that causes gray mold disease in a broad range of hosts. In the present study, we conducted molecular microbiology and transcriptomic analyses of the host-B. cinerea interaction to investigate the plant defense response and fungal pathogenicity. Upon B. cinerea infection, plant defense responses changed from activation to repression; thus, the expression of many defense genes decreased in Arabidopsis thaliana. B. cinerea Zn(II)2Cys6 transcription factor BcSpd1 was involved in the suppression of plant defense as ΔBcSpd1 altered wild-type B05.10 virulence by recovering part of the defense responses at the early infection stage. BcSpd1 affected genes involved in the fungal sclerotium development, infection cushion formation, biosynthesis of melanin, and change in environmental pH values, which were reported to influence fungal virulence. Specifically, BcSpd1 bound to the promoter of the gene encoding quercetin dioxygenase (BcQdo) and positively affected the gene expression, which was involved in catalyzing antifungal flavonoid degradation. This study indicates BcSpd1 plays a key role in the necrotrophic microbe B. cinerea virulence toward plants by regulating pathogenicity-related compounds and thereby suppressing early plant defense.

Unraveling Plant Cell Death during Phytophthora Infection

Oomycetes form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms, of which several hundred organisms are considered among the most devastating plant pathogens—especially members of the genus Phytophthora. Phytophthora spp. have a large repertoire of effectors that aid in eliciting a susceptible response in host plants. What is of increasing interest is the involvement of Phytophthora effectors in regulating programed cell death (PCD)—in particular, the hypersensitive response. There have been numerous functional characterization studies, which demonstrate Phytophthora effectors either inducing or suppressing host cell death, which may play a crucial role in Phytophthora’s ability to regulate their hemi-biotrophic lifestyle. Despite several advances in techniques used to identify and characterize Phytophthora effectors, knowledge is still lacking for some important species, including Phytophthora cinnamomi. This review discusses what the term PCD means and the gap in knowledge between pathogenic and developmental forms of PCD in plants. We also discuss the role cell death plays in the virulence of Phytophthora spp. and the effectors that have so far been identified as playing a role in cell death manipulation. Finally, we touch on the different techniques available to study effector functions, such as cell death induction/suppression.

The necrotroph Botrytis cinerea promotes disease development in Panax ginseng by manipulating plant defense signals and antifungal metabolites degradation

Background

Panax ginseng Meyer is one of the most valuable medicinal plants which is enriched in anti-microbe secondary metabolites and widely used in traditional medicine. Botrytis cinerea is a necrotrophic fungus that causes gray mold disease in a broad range of hosts. B. cinerea could overcome the ginseng defense and cause serious leaf and root diseases with unknown mechanism.

Methods

We conducted simultaneous transcriptomic and metabolomic analysis of the host to investigate the defense response of ginseng affected by B. cinerea. The gene deletion and replacement were then performed to study the pathogenic gene in B. cinerea during ginseng - fungi interaction.

Results

Upon B. cinerea infection, ginseng defense responses were switched from the activation to repression, thus the expression of many defense genes decreased and the biosynthesis of antifungal metabolites were reduced. Particularly, ginseng metabolites like kaempferol, quercetin and luteolin which could inhibit fungi growth were decreased after B. cinerea infection. B. cinerea quercetin dioxygenase (Qdo) involved in catalyzing flavonoids degradation and △BcQdo mutants showed increased substrates accumulation and reduced disease development.

Conclusion

This work indicates the flavonoids play a role in ginseng defense and BcQdo involves in B. cinerea virulence towards the P. ginseng. B. cinerea promotes disease development in ginseng by suppressing of defense related genes expression and reduction of antifungal metabolites biosynthesis.

Prediction of effector proteins and their implications in pathogenicity of phytopathogenic filamentous fungi: A review

Plant pathogenic fungi encode and secrete effector proteins to promote pathogenesis. In recent years, the important role of effector proteins in fungi and plant host interactions has become increasingly prominent. In this review, the functional characterization and molecular mechanisms by which fungal effector proteins modulate biological processes and suppress the defense of plant hosts are discussed, with an emphasis on cell localization during fungal infection. This paper also provides a comprehensive review of bioinformatic and experimental methods that are currently available for the identification of fungal effector proteins. We additionally summarize the secretion pathways and the methods for verifying the presence effector proteins in plant host cells. For future research, comparative genomic studies of different pathogens with varying life cycles will allow comprehensive and systematic identification of effector proteins. Additionally, functional analysis of effector protein interactions with a wider range of hosts (especially non-model crops) will provide more detailed repertoires of fungal effectors. Identifying effector proteins and verifying their functions will improve our understanding of their role in causing disease and in turn guide future strategies for combatting fungal infections.

Review Update on the Life Cycle, Plant–Microbe Interaction, Genomics, Detection and Control Strategies of the Oil Palm Pathogen Ganoderma boninense

Plant pathogens are key threats to agriculture and global food security, causing various crop diseases that lead to massive economic losses. Palm oil is a commodity export of economic importance in Southeast Asia, especially in Malaysia and Indonesia. However, the sustainability of oil palm plantations and production is threatened by basal stem rot (BSR), a devastating disease predominantly caused by the fungus Ganoderma boninense Pat. In Malaysia, infected trees have been reported in nearly 60% of plantation areas, and economic losses are estimated to reach up to ~USD500 million a year. This review covers the current knowledge of the mechanisms utilized by G. boninense during infection and the methods used in the disease management to reduce BSR, including cultural practices, chemical treatments and antagonistic microorganism manipulations. Newer developments arising from multi-omics technologies such as whole-genome sequencing (WGS) and RNA sequencing (RNA-Seq) are also reviewed. Future directions are proposed to increase the understanding of G. boninense invasion mechanisms against oil palm. It is hoped that this review can contribute towards an improved disease management and a sustainable oil palm production in this region.

An Arabidopsis nonhost resistance gene, IMPORTIN ALPHA 2 provides immunity against rice sheath blight pathogen, Rhizoctonia solani

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Rice sheath blight is caused by necrotrophic dreadful fungus Rhizoctonia solani.

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Forward genetics tools identified RSS1 (IMPA2; IMPORTIN ALPHA 2) as a NHR gene.

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Mutation in RSS1 at P65S in first exon compromise the immunity to R. solani.

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rss1 shows enhanced cell death, ROS, callose deposition and developmental defect.

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RSS1 activates early salicylic acid mediated defense response against R. solani.

There is neither resistant rice cultivar nor any control measure against Rhizoctonia solani AG-1 IA (RS), causal of sheath blight and a major threat to global rice production. Rice is a host and Arabidopsis is a nonhost with underlying nonhost resistance (NHR) gene which is largely untested. Using approaches of forward genetics and tools, cytology, and molecular biology, we identified homozygous mutants in Arabidopsis, mapped the NHR gene, and functionally characterized it in response to RS. Rss1 was mapped on Ch 4 between JAERI18 and Ch4_9.18 (844.6 Kb) and identified IMPORTIN ALPHA 2 as the candidate RSS1 gene. We found that breach of immunity in rss1 by RS activates defense responses whereas photosynthetic pigment biosynthesis and developmental processes are negatively regulated. In addition, a gradual decrease in PR1 by 3 dpi revealed that RSS1 positively regulated early SA-mediated resistance. Whereas increased expression of PDF1.2 by 3 dpi supported switching to necrotrophy, SA-mediated defense in Col-0 leading to immune response. Enhanced expression of ATG8a in rss1 supported autophagic cell death. IMPA2, IMPA1, and RAN1 function together to provide NHR against RS. These findings demonstrate that IMPA2 provides NHR against RS in Col-0 that evoke SA-mediated early immunity with boulevard for potential biotechnological application.

Genomic prediction of strawberry resistance to postharvest fruit decay caused by the fungal pathogen Botrytis cinerea

Gray mold, a disease of strawberry (Fragaria × ananassa) caused by the ubiquitous necrotroph Botrytis cinerea, renders fruit unmarketable and causes economic losses in the postharvest supply chain. To explore the feasibility of selecting for increased resistance to gray mold, we undertook genetic and genomic prediction studies in strawberry populations segregating for fruit quality and shelf life traits hypothesized to pleiotropically affect susceptibility. As predicted, resistance to gray mold was heritable but quantitative and genetically complex. While every individual was susceptible, the speed of symptom progression and severity differed. Narrow-sense heritability ranged from 0.38 to 0.71 for lesion diameter (LD) and 0.39 to 0.44 for speed of emergence of external mycelium (EM). Even though significant additive genetic variation was observed for LD and EM, the phenotypic ranges were comparatively narrow and genome-wide analyses did not identify any large-effect loci. Genomic selection (GS) accuracy ranged from 0.28 to 0.59 for LD and 0.37 to 0.47 for EM. Additive genetic correlations between fruit quality and gray mold resistance traits were consistent with prevailing hypotheses: LD decreased as titratable acidity increased, whereas EM increased as soluble solid content decreased and firmness increased. We concluded that phenotypic and GS could be effective for reducing LD and increasing EM, especially in long shelf life populations, but that a significant fraction of the genetic variation for resistance to gray mold was caused by the pleiotropic effects of fruit quality traits that differ among market and shelf life classes.

Biology and molecular interactions of Parastagonospora nodorum blotch of wheat

Parastagonospora nodorum is one of the important necrotrophic pathogens of wheat which causes severe economical loss to crop yield. So far, a number of effectors of Parastagonospora nodorum origin and their target interacting genes on the host plant have been characterized. Since targeting effector-sensitive gene carefully can be helpful in breeding for resistance. Therefore, constant efforts are required to further characterize the effectors, their interacting genes, and underlying biochemical pathways. Furthermore, to develop effective counter-strategies against emerging diseases, continuous efforts are required to determine the qualitative resistance that demands to screen of diverse genotypes for host resistance. Stagonospora nodorum blotch also refers to as Stagonospora glume blotch and leaf is caused by Parastagonospora nodorum. The pathogen deploys necrotrophic effectors for the establishment and development on wheat plants. The necrotrophic effectors and their interaction with host receptors lead to the establishment of infection on leaves and extensive lesions formation which either results in host cell death or suppression/activation of host defence mechanisms. The wheat Stagonospora nodorum interaction involves a set of nine host gene-necrotrophic effector interactions. Out of these, Snn1-SnTox1, Tsn1-SnToxA and Snn-SnTox3 are one of the most studied interaction, due to its role in the suppression of reactive oxygen species production, regulating the cytokinin content through ethylene-dependent wayduring initial infection stage. Further, although the molecular basis is not fully unveiled, these effectors regulate the redox state and influence the ethylene biosynthesis in infected wheat plants. Here, we have discussed the biology of the wheat pathogen Parastagonospora nodorum, role of its necrotrophic effectors and their interacting sensitivity genes on the redox state, how they hijack the resistance mechanisms, hormonal regulated immunity and other signalling pathways in susceptible wheat plants. The information generated from effectors and their corresponding sensitivity genes and other biological processes could be utilized effectively for disease management strategies.

Pathogens pulling the strings: Effectors manipulating salicylic acid and phenylpropanoid biosynthesis in plants

During evolution, plants have developed sophisticated ways to cope with different biotic and abiotic stresses. Phytohormones and secondary metabolites are known to play pivotal roles in defence responses against invading pathogens. One of the key hormones involved in plant immunity is salicylic acid (SA), of which the role in plant defence is well established and documented. Plants produce an array of secondary metabolites categorized in different classes, with the phenylpropanoids as major players in plant immunity. Both SA and phenylpropanoids are needed for an effective immune response by the plant. To successfully infect the host, pathogens secrete proteins, called effectors, into the plant tissue to lower defence. Secreted effectors can interfere with several metabolic or signalling pathways in the host to facilitate infection. In this review, we will focus on the different strategies pathogens have developed to affect the levels of SA and phenylpropanoids to increase plant susceptibility.

Photosynthesis and chloroplast redox signaling in the age of global warming: stress tolerance, acclimation, and developmental plasticity

Contemporary climate change is characterized by the increased intensity and frequency of environmental stress events such as floods, droughts, and heatwaves, which have a debilitating impact on photosynthesis and growth, compromising the production of food, feed, and biofuels for an expanding population. The need to increase crop productivity in the context of global warming has fueled attempts to improve several key plant features such as photosynthetic performance, assimilate partitioning, and tolerance to environmental stresses. Chloroplast redox metabolism, including photosynthetic electron transport and CO2 reductive assimilation, are primary targets of most stress conditions, leading to excessive excitation pressure, photodamage, and propagation of reactive oxygen species. Alterations in chloroplast redox poise, in turn, provide signals that exit the plastid and modulate plant responses to the environmental conditions. Understanding the molecular mechanisms involved in these processes could provide novel tools to increase crop yield in suboptimal environments. We describe herein various interventions into chloroplast redox networks that resulted in increased tolerance to multiple sources of environmental stress. They included manipulation of endogenous components and introduction of electron carriers from other organisms, which affected not only stress endurance but also leaf size and longevity. The resulting scenario indicates that chloroplast redox pathways have an important impact on plant growth, development, and defense that goes beyond their roles in primary metabolism. Manipulation of these processes provides additional strategies for the design of crops with improved performance under destabilized climate conditions as foreseen for the future.

Genes Associated with Foliar Resistance to Septoria Nodorum Blotch of Hexaploid Wheat (Triticum aestivum L.)

The genetic control of host response to the fungal necrotrophic disease Septoria nodorum blotch (SNB) in bread wheat is complex, involving many minor genes. Quantitative trait loci (QTL) controlling SNB response were previously identified on chromosomes 1BS and 5BL. The aim of this study, therefore, was to align and compare the genetic map representing QTL interval on 1BS and 5BS with the reference sequence of wheat and identify resistance genes (R-genes) associated with SNB response. Alignment of QTL intervals identified significant genome rearrangements on 1BS between parents of the DH population EGA Blanco, Millewa and the reference sequence of Chinese Spring with subtle rearrangements on 5BL. Nevertheless, annotation of genomic intervals in the reference sequence were able to identify and map 13 and 12 R-genes on 1BS and 5BL, respectively. R-genes discriminated co-located QTL on 1BS into two distinct but linked loci. NRC1a and TFIID mapped in one QTL on 1BS whereas RGA and Snn1 mapped in the linked locus and all were associated with SNB resistance but in one environment only. Similarly, Tsn1 and WK35 were mapped in one QTL on 5BL with NETWORKED 1A and RGA genes mapped in the linked QTL interval. This study provided new insights on possible biochemical, cellular and molecular mechanisms responding to SNB infection in different environments and also addressed limitations of using the reference sequence to identify the full complement of functional R-genes in modern varieties.

Surviving the odds: From perception to survival of fungal phytopathogens under host-generated oxidative burst

Fungal phytopathogens pose a serious threat to global crop production. Only a handful of strategies are available to combat these fungal infections, and the increasing incidence of fungicide resistance is making the situation worse. Hence, the molecular understanding of plant-fungus interactions remains a primary focus of plant pathology. One of the hallmarks of host-pathogen interactions is the overproduction of reactive oxygen species (ROS) as a plant defense mechanism, collectively termed the oxidative burst. In general, high accumulation of ROS restricts the growth of pathogenic organisms by causing localized cell death around the site of infection. To survive the oxidative burst and achieve successful host colonization, fungal phytopathogens employ intricate mechanisms for ROS perception, ROS neutralization, and protection from ROS-mediated damage. Together, these countermeasures maintain the physiological redox homeostasis that is essential for cell viability. In addition to intracellular antioxidant systems, phytopathogenic fungi also deploy interesting effector-mediated mechanisms for extracellular ROS modulation. This aspect of plant-pathogen interactions is significantly under-studied and provides enormous scope for future research. These adaptive responses, broadly categorized into "escape" and "exploitation" mechanisms, are poorly understood. In this review, we discuss the oxidative stress response of filamentous fungi, their perception signaling, and recent insights that provide a comprehensive understanding of the distinct survival mechanisms of fungal pathogens in response to the host-generated oxidative burst.

What are the Top 10 Unanswered Questions in Molecular Plant-Microbe Interactions?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.The past few decades have seen major discoveries in the field of molecular plant-microbe interactions. As the result of technological and intellectual advances, we are now able to answer questions at a level of mechanistic detail that we could not have imagined possible 20 years ago. The MPMI Editorial Board felt it was time to take stock and reassess. What big questions remain unanswered? We knew that to identify the fundamental, overarching questions that drive our research, we needed to do this as a community. To reach a diverse audience of people with different backgrounds and perspectives, working in different areas of plant-microbe interactions, we queried the more than 1,400 participants at the 2019 International Congress on Molecular Plant-Microbe Interactions meeting in Glasgow. This group effort resulted in a list of ten, broad-reaching, fundamental questions that influence and inform our research. Here, we introduce these Top 10 unanswered questions, giving context and a brief description of the issues. Each of these questions will be the subject of a detailed review in the coming months. We hope that this process of reflecting on what is known and unknown and identifying the themes that underlie our research will provide a framework to use going forward, giving newcomers a sense of the mystery of the big questions and inspiring new avenues and novel insights.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

Genome-wide association analysis of the strength of the MAMP-elicited defense response and resistance to target leaf spot in sorghum

Plants have the capacity to respond to conserved molecular features known as microbe-associated molecular patterns (MAMPs). The goal of this work was to assess variation in the MAMP response in sorghum, to map loci associated with this variation, and to investigate possible connections with variation in quantitative disease resistance. Using an assay that measures the production of reactive oxygen species, we assessed variation in the MAMP response in a sorghum association mapping population known as the sorghum conversion population (SCP). We identified consistent variation for the response to chitin and flg22-an epitope of flagellin. We identified two SNP loci associated with variation in the flg22 response and one with the chitin response. We also assessed resistance to Target Leaf Spot (TLS) disease caused by the necrotrophic fungus Bipolaris cookei in the SCP. We identified one strong association on chromosome 5 near a previously characterized disease resistance gene. A moderately significant correlation was observed between stronger flg22 response and lower TLS resistance. Possible reasons for this are discussed.

New tools for characterizing early brown stem rot disease resistance signaling in soybean

Brown stem rot (BSR) reduces soybean [Glycine max (L.) Merr.] yield by up to 38%. The BSR causal agent is Phialophora gregata f. sp. sojae, a slow-growing, necrotrophic fungus whose life cycle includes latent and pathogenic phases, each lasting several weeks. Brown stem rot foliar symptoms are often misdiagnosed as other soybean diseases or nutrient stress, making BSR resistance especially difficult to phenotype. To shed light on the genes and networks contributing to P. gregata resistance, we conducted RNA sequencing (RNA-seq) of a resistant genotype (PI 437970, Rbs3). Leaf, stem, and root tissues were collected 12, 24, and 36 h after stab inoculation with P. gregata, or mock infection, in the plant stem. By using multiple tissues and time points, we could see that leaves, stems, and roots use the same defense pathways. Our analyses suggest that P. gregata induces a biphasic defense response, with pathogen-associated molecular pattern (PAMP) triggered immunity observed in leaves at 12 and 24 h after infection (HAI) and effector triggered immunity detected at 36 h after infection in the stems. Gene networks associated with defense, photosynthesis, nutrient homeostasis, DNA replication, and growth are the hallmarks of resistance to P. gregata. While P. gregata is a slow-growing pathogen, our results demonstrate that pathogen recognition occurs hours after infection. By exploiting the genes and networks described here, we will be able to develop novel diagnostic tools to facilitate breeding and screening for BSR resistance.

LsGRP1, a class II glycine-rich protein of Lilium, confers plant resistance via mediating innate immune activation and inducing fungal programmed cell death

Defence-related LsGRP1 is a leaf-specific plant class II glycine-rich protein (GRP) involved in salicylic acid-induced systemic resistance against grey mould caused by necrotrophic Botrytis elliptica in lily (Lilium) cultivar Stargazer. The C-terminal region of LsGRP1 (LsGRP1^C ) can inhibit fungal growth in vitro via a mechanism of inducing fungal apoptosis programmed cell death (PCD). In this study, the role of LsGRP1 in induced defence mechanism was investigated using LsGRP1-silenced Stargazer lily and LsGRP1-transgenic Arabidopsis thaliana. LsGRP1 silencing in lily was found to slightly inhibit plant growth and greatly increase the susceptibility to B. elliptica by suppressing callose deposition and early reactive oxygen species (ROS) accumulation. In contrast, LsGRP1-transgenic Arabidopsis showed higher resistance to Botrytis cinerea and also to Pseudomonas syringae pv. tomato DC3000 as compared to the wild type, accompanied with the enhancement of callose deposition and ROS accumulation. Additionally, LsGRP1 silencing increased plant cell death caused by B. elliptica secretion and reduced pathogen-associated molecular pattern (PAMP)-triggered defence activation in Stargazer lily. Consistently, LsGRP1 expression boosted PAMP-triggered defence responses and effector recognition-induced hypersensitive response in Arabidopsis. Moreover, fungal apoptosis PCD triggered by LsGRP1 in an LsGRP1^C -dependent manner was demonstrated by leaf infiltration with LsGRP1^C -containing recombinant proteins in Stargazer lily. Based on these results, we presume that LsGRP1 plays roles in plant defence via functioning as a pathogen-inducible switch for plant innate immune activation and acting as a fungal apoptosis PCD inducer to combat pathogen attack.

Expression of Thioredoxin/Thioredoxin Reductase System Genes in Aphid-Challenged Maize Seedlings

Thioredoxins (Trxs) and thioredoxin reductases (TrxRs) encompass a highly complex network involved in sustaining thiol-based redox homeostasis in plant tissues. The purpose of the study was to gain a new insight into transcriptional reprogramming of the several genes involved in functioning of Trx/TrxR system in maize (Zea mays L.) seedlings, exposed to the bird cherry-oat aphid (Rhopalosiphum padi L.) or the rose-grass aphid (Metopolophium dirhodum Walk.) infestation. The biotests were performed on two maize genotypes (susceptible Złota Karłowa and relatively resistant Waza). The application of real-time qRT-PCR technique allowed to identify a molecular mechanism triggered in more resistant maize plants, linked to upregulation of thioredoxins-encoding genes (Trx-f, Trx-h, Trx-m, Trx-x) and thioredoxin reductase genes (Ftr1, Trxr2). Significant enhancement of TrxR activity in aphid-infested Waza seedlings was also demonstrated. Furthermore, we used an electrical penetration graph (EPG) recordings of M. dirhodum stylet activities in seedlings of the two studied maize varieties. Duration of phloem phase (E1 and E2 models) of rose-grass aphids was about three times longer while feeding in Waza plants, compared to Złota Karłowa cv. The role of activation of Trx/TrxR system in maintaining redox balance and counteracting oxidative-induced damages of macromolecules in aphid-stressed maize plants is discussed.

Subcellular Roles of Glutathione in Mediating Plant Defense during Biotic Stress

Glutathione and reactive oxygen species (ROS) play important roles, within different cell compartments, in activating plant defense and the development of resistance. In mitochondria, the accumulation of ROS and the change of glutathione towards its oxidized state leads to mitochondrial dysfunction, activates cell death, and triggers resistance. The accumulation of glutathione in chloroplasts and peroxisomes at the early stages of plant pathogen interactions is related to increased tolerance and resistance. The collapse of the antioxidative system in these two cell compartments at the later stages leads to cell death through retrograde signaling. The cytosol can be considered to be the switchboard during biotic stress where glutathione is synthesized, equally distributed to, and collected from different cell compartments. Changes in the redox state of glutathione and the accumulation of ROS in the cytosol during biotic stress can initiate the activation of defense genes in nuclei through pathways that involve salicylic acid, jasmonic acid, auxins, and abscisic acid. This review dissects the roles of glutathione in individual organelles during compatible and incompatible bacterial, fungal, and viral diseases in plants and explores the subcelluar roles of ROS, glutathione, ascorbate, and related enzymes in the development of resistance.

Iron homeostasis and plant immune responses: Recent insights and translational implications

Iron metabolism and the plant immune system are both critical for plant vigor in natural ecosystems and for reliable agricultural productivity. Mechanistic studies of plant iron home-ostasis and plant immunity have traditionally been carried out in isolation from each other; however, our growing understanding of both processes has uncovered significant connections. For example, iron plays a critical role in the generation of reactive oxygen intermediates during immunity and has been recently implicated as a critical factor for immune-initiated cell death via ferroptosis. Moreover, plant iron stress triggers immune activation, suggesting that sensing of iron depletion is a mechanism by which plants recognize a pathogen threat. The iron deficiency response engages hormone signaling sectors that are also utilized for plant immune signaling, providing a probable explanation for iron-immunity cross-talk. Finally, interference with iron acquisition by pathogens might be a critical component of the immune response. Efforts to address the global burden of iron deficiency-related anemia have focused on classical breeding and transgenic approaches to develop crops biofortified for iron content. However, our improved mechanistic understanding of plant iron metabolism suggests that such alterations could promote or impede plant immunity, depending on the nature of the alteration and the virulence strategy of the pathogen. Effects of iron biofortification on disease resistance should be evaluated while developing plants for iron biofortification.

Identification of QTL for Target Leaf Spot resistance in Sorghum bicolor and investigation of relationships between disease resistance and variation in the MAMP response

Target leaf spot (TLS) of sorghum, a foliar disease caused by the necrotrophic fungus Bipolaris cookei (also known as Bipolaris sorghicola), can affect grain yield in sorghum by causing premature drying of leaves and defoliation. Two sorghum recombinant inbred line (RIL) populations, BTx623/BTx642 and BTx623/SC155-14E, were assessed for TLS resistance in replicated trials. Using least square mean trait data, four TLS resistance QTL were identified, two in each population. Of these, three were previously unidentified while a major QTL on chromosome 5 in the BTx623/BTx642 RIL population corresponded to the previously identified TLS resistance gene ds1. A set of sorghum lines were assessed for production of reactive oxygen species induced by treatment with the microbe-associated molecular pattern (MAMP) flg22 (a derivative of flagellin). Flg22-induced ROS production varied between lines in a consistent fashion. One QTL associated with variation in the flg22 response was detected in the RIL populations. No evidence was found to link variation in the MAMP response to variation in TLS resistance.

The plant hypersensitive response: concepts, control and consequences

The hypersensitive defence response is found in all higher plants and is characterized by a rapid cell death at the point of pathogen ingress. It is usually associated with pathogen resistance, though, in specific situations, it may have other consequences such as pathogen susceptibility, growth retardation and, over evolutionary timescales, speciation. Due to the potentially severe costs of inappropriate activation, plants employ multiple mechanisms to suppress inappropriate activation of HR and to constrain it after activation. The ubiquity of this response among higher plants despite its costs suggests that it is an extremely effective component of the plant immune system.