Apoplastic Cell Death-Inducing Proteins of Filamentous Plant Pathogens: Roles in Plant-Pathogen Interactions

The study of iron- and copper-binding proteome of Fusarium oxysporum and its effector candidates

Fusarium oxysporum f.sp. lycopersici is a phytopathogen which causes vascular wilt disease in tomato plants. The survival tactics of both pathogens and hosts depend on intricate interactions between host plants and pathogenic microbes. Iron-binding proteins (IBPs) and copper-binding proteins (CBPs) play a crucial role in these interactions by participating in enzyme reactions, virulence, metabolism, and transport processes. We employed high-throughput computational tools at the sequence and structural levels to investigate the IBPs and CBPs of F. oxysporum. A total of 124 IBPs and 37 CBPs were identified in the proteome of Fusarium. The ranking of amino acids based on their affinity for binding with iron is Glu > His> Asp > Asn > Cys, and for copper is His > Asp > Cys respectively. The functional annotation, determination of subcellular localization, and Gene Ontology analysis of these putative IBPs and CBPs have unveiled their potential involvement in a diverse array of cellular and biological processes. Three iron-binding glycosyl hydrolase family proteins, along with four CBPs with carbohydrate-binding domains, have been identified as potential effector candidates. These proteins are distinct from the host Solanum lycopersicum proteome. Moreover, they are known to be located extracellularly and function as enzymes that degrade the host cell wall during pathogen-host interactions. The insights gained from this report on the role of metal ions in plant-pathogen interactions can help develop a better understanding of their fundamental biology and control vascular wilt disease in tomato plants.

Plant triterpenoid saponins function as susceptibility factors to promote the pathogenicity of Botrytis cinerea

The gray mold fungus Botrytis cinerea is a necrotrophic pathogen that causes diseases in hundreds of plant species, including high-value crops. Its polyxenous nature and pathogenic success are due to its ability to perceive host signals in its favor. In this study, we found that laticifer cells of Euphorbia lathyris are a source of susceptibility factors required by B. cinerea to cause disease. Consequently, poor-in-latex (pil) mutants, which lack laticifer cells, show full resistance to this pathogen, whereas lot-of-latex mutants, which produce more laticifer cells, are hypersusceptible. These S factors are triterpenoid saponins, which are widely distributed natural products of vast structural diversity. The downregulation of laticifer-specific oxydosqualene cyclase genes, which encode the first committed step enzymes for triterpene and, therefore, saponin biosynthesis, conferred disease resistance to B. cinerea. Likewise, the Medicago truncatula lha-1 mutant, compromised in triterpenoid saponin biosynthesis, showed enhanced resistance. Interestingly, the application of different purified triterpenoid saponins pharmacologically complemented the disease-resistant phenotype of pil and hla-1 mutants and enhanced disease susceptibility in different plant species. We found that triterpenoid saponins function as plant cues that signal transcriptional reprogramming in B. cinerea, leading to a change in its growth habit and infection strategy, culminating in the abundant formation of infection cushions, the multicellular appressoria apparatus dedicated to plant penetration and biomass destruction in B. cinerea. Taken together, these results provide an explanation for how plant triterpenoid saponins function as disease susceptibility factors to promote B. cinerea pathogenicity.

Template-Based Modelling of the Structure of Fungal Effector Proteins

The discovery of new fungal effector proteins is necessary to enable the screening of cultivars for disease resistance. Sequence-based bioinformatics methods have been used for this purpose, but only a limited number of functional effector proteins have been successfully predicted and subsequently validated experimentally. A significant obstacle is that many fungal effector proteins discovered so far lack sequence similarity or conserved sequence motifs. The availability of experimentally determined three-dimensional (3D) structures of a number of effector proteins has recently highlighted structural similarities amongst groups of sequence-dissimilar fungal effectors, enabling the search for similar structural folds amongst effector sequence candidates. We have applied template-based modelling to predict the 3D structures of candidate effector sequences obtained from bioinformatics predictions and the PHI-BASE database. Structural matches were found not only with ToxA- and MAX-like effector candidates but also with non-fungal effector-like proteins-including plant defensins and animal venoms-suggesting the broad conservation of ancestral structural folds amongst cytotoxic peptides from a diverse range of distant species. Accurate modelling of fungal effectors were achieved using RaptorX. The utility of predicted structures of effector proteins lies in the prediction of their interactions with plant receptors through molecular docking, which will improve the understanding of effector-plant interactions.

The Botrytis cinerea transglycosylase BcCrh4 is a cell death-inducing protein with cell death-promoting and -suppressing domains

Botrytis cinerea is a necrotrophic fungal plant pathogen that causes grey mould and rot diseases in many crops. Here, we show that the B. cinerea BcCrh4 transglycosylase is secreted during plant infection and induces plant cell death and pattern-triggered immunity (PTI), fulfilling the characteristics of a cell death-inducing protein (CDIP). The CDIP activity of BcCrh4 is independent of the transglycosylase enzymatic activity, it takes place in the apoplast and does not involve the receptor-like kinases BAK1 and SOBIR1. During saprophytic growth, BcCrh4 is localized in the endoplasmic reticulum and in vacuoles, but during plant infection, it accumulates in infection cushions (ICs) and is then secreted to the apoplast. Two domains within the BcCrh4 protein determine the CDIP activities: a 20aa domain at the N' end activates intense cell death and PTI, while a stretch of 52aa in the middle of the protein induces a weaker response and suppresses the activity of the 20aa N' domain. Deletion of bccrh4 affected fungal development and IC formation in particular, resulting in reduced virulence. Collectively, our findings demonstrate that BcCrh4 is required for fungal development and pathogenicity, and hint at a dual mechanism that balances the virulence activity of this, and potentially other CDIPs.

Identification of common fungal extracellular membrane (CFEM) proteins in Fusarium sacchari that inhibit plant immunity and contribute to virulence

IMPORTANCE

Common fungal extracellular membrane (CFEM) domain-containing protein has long been considered an essential effector, playing a crucial role in the interaction of pathogens and plant. Strategies aimed at understanding the pathogenicity mechanism of F. sacchari are eagerly anticipated to ultimately end the spread of pokkah boeng disease. Twenty FsCFEM proteins in the genome of F. sacchari have been identified, and four FsCFEM effector proteins have been found to suppress BCL2-associated X protein-triggered programmed cell death in N. benthamiana. These four effector proteins have the ability to enter plant cells and inhibit plant immunity. Furthermore, the expression of these four FsCFEM effector proteins significantly increases during the infection stage, with the three of them playing an essential role in achieving full virulence. These study findings provide a direction toward further exploration of the immune response in sugarcane. By applying these discoveries, we can potentially control the spread of disease through techniques such as host-induced gene silencing.

The secret password: Cell death-inducing proteins in filamentous phytopathogens - As versatile tools to develop disease-resistant crops

Cell death-inducing proteins (CDIPs) are some of the secreted effector proteins manifested by filamentous oomycetes and fungal pathogens to invade the plant tissue and facilitate infection. Along with their involvement in different developmental processes and virulence, CDIPs play a crucial role in plant-pathogen interactions. As the name implies, CDIPs cause necrosis and trigger localised cell death in the infected host tissues by the accumulation of higher concentrations of hydrogen peroxide (H2O2), oxidative burst, accumulation of nitric oxide (NO), and electrolyte leakage. They also stimulate the biosynthesis of defense-related phytohormones such as salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and ethylene (ET), as well as the expression of pathogenesis-related (PR) genes that are important in disease resistance. Altogether, the interactions result in the hypersensitive response (HR) in the host plant, which might confer systemic acquired resistance (SAR) in some cases against a vast array of related and unrelated pathogens. The CDIPs, due to their capability of inducing host resistance, are thus unique among the array of proteins secreted by filamentous plant pathogens. More interestingly, a few transgenic plant lines have also been developed expressing the CDIPs with added resistance. Thus, CDIPs have opened an interesting hot area of research. The present study critically reviews the current knowledge of major types of CDIPs identified across filamentous phytopathogens and their modes of action in the last couple of years. This review also highlights the recent breakthrough technologies in studying plant-pathogen interactions as well as crop improvement by enhancing disease resistance through CDIPs.

The completed genome sequence of Pestalotiopsis versicolor, a pathogenic ascomycete fungus with implications for bayberry production

The pathogenic fungus Pestalotiopsis versicolor is a major etiological agent of fungal twig blight disease affecting bayberry trees. However, the lack of complete genome sequence information for this crucial pathogenic fungus hinders the molecular and genetic investigation of its pathogenic mechanism. To address this knowledge gap, we have generated the complete genome sequence of P. versicolor strain XJ27, employing a combination of Illumina, PacBio, and Hi-C sequencing technologies. This comprehensive genome sequence, comprising 7 chromosomes with an N50 contig size of 7,275,017 bp, a GC content ratio of 50.16%, and a total size of 50.80 Mb, encompasses 13,971 predicted coding genes. By performing comparative genomic analysis between P. versicolor and the genomes of eleven plant-pathogenic fungi, as well as three closely related fungi within the same group, we have gained initial insights into its evolutionary trajectory, particularly through gene family analysis. These findings shed light on the distinctive characteristics and evolutionary history of P. versicolor. Importantly, the availability of this high-quality genetic resource will serve as a foundational tool for investigating the biology, molecular pathogenesis, and virulence of P. versicolor. Furthermore, it will facilitate the development of more potent antifungal medications by uncovering potential vulnerabilities in its genetic makeup.

Elucidating the zinc-binding proteome of Fusarium oxysporum f. sp. lycopersici with particular emphasis on zinc-binding effector proteins

Fusarium oxysporum f. sp. lycopersici is a soil-borne phytopathogenic species which causes vascular wilt disease in the Solanum lycopersicum (tomato). Due to the continuous competition for zinc usage by Fusarium and its host during infection makes zinc-binding proteins a hotspot for focused investigation. Zinc-binding effector proteins are pivotal during the infection process, working in conjunction with other essential proteins crucial for its biological activities. This work aims at identifying and analysing zinc-binding proteins and zinc-binding proteins effector candidates of Fusarium. We have identified three hundred forty-six putative zinc-binding proteins; among these proteins, we got two hundred and thirty zinc-binding proteins effector candidates. The functional annotation, subcellular localization, and Gene Ontology analysis of these putative zinc-binding proteins revealed their probable role in wide range of cellular and biological processes such as metabolism, gene expression, gene expression regulation, protein biosynthesis, protein folding, cell signalling, DNA repair, and RNA processing. Sixteen proteins were found to be putatively secretory in nature. Eleven of these were putative zinc-binding protein effector candidates may be involved in pathogen-host interaction during infection. The information obtained here may enhance our understanding to design, screen, and apply the zinc-metal ion-based antifungal agents to protect the S. lycopersicum and control the vascular wilt caused by F. oxysporum.

Identification and Characterization of Novel Candidate Effector Proteins from Magnaporthe oryzae

The fungal pathogen Magnaporthe oryzae secretes a large number of effector proteins to facilitate infection, most of which are not functionally characterized. We selected potential candidate effector genes from the genome of M. oryzae, field isolate P131, and cloned 69 putative effector genes for functional screening. Utilizing a rice protoplast transient expression system, we identified that four candidate effector genes, GAS1, BAS2, MoCEP1 and MoCEP2 induced cell death in rice. In particular, MoCEP2 also induced cell death in Nicotiana benthamiana leaves through Agrobacteria-mediated transient gene expression. We further identified that six candidate effector genes, MoCEP3 to MoCEP8, suppress flg22-induced ROS burst in N. benthamiana leaves upon transient expression. These effector genes were highly expressed at a different stage after M. oryzae infection. We successfully knocked out five genes in M. oryzae, MoCEP1, MoCEP2, MoCEP3, MoCEP5 and MoCEP7. The virulence tests suggested that the deletion mutants of MoCEP2, MoCEP3 and MoCEP5 showed reduced virulence on rice and barley plants. Therefore, those genes play an important role in pathogenicity.

A Pythium myriotylum Small Cysteine-Rich Protein Triggers Immune Responses in Diverse Plant Hosts

The oomycete Pythium myriotylum is a necrotrophic pathogen that infects many crop species worldwide, including ginger, soybean, tomato, and tobacco. Here, we identified a P. myriotylum small cysteine-rich protein, PmSCR1, that induces cell death in Nicotiana benthamiana by screening small, secreted proteins that were induced during infection of ginger and did not have a predicted function at the time of selection. Orthologs of PmSCR1 were found in other Pythium species, but these did not have cell death-inducing activity in N. benthamiana. PmSCR1 encodes a protein containing an auxiliary activity 17 family domain and triggers multiple immune responses in host plants. The elicitor function of PmSCR1 appears to be independent of enzymatic activity, because the heat inactivation of PmSCR1 protein did not affect PmSCR1-induced cell death or other defense responses. The elicitor function of PmSCR1 was also independent of BAK1 and SOBIR1. Furthermore, a small region of the protein, PmSCR^186-211, is sufficient for inducing cell death. A pretreatment using the full-length PmSCR1 protein promoted the resistance of soybean and N. benthamiana to Phytophthora sojae and Phytophthora capsici infection, respectively. These results reveal that PmSCR1 is a novel elicitor from P. myriotylum, which exhibits plant immunity-inducing activity in multiple host plants. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Killing softly: a roadmap of Botrytis cinerea pathogenicity

Botrytis cinerea, a widespread plant pathogen with a necrotrophic lifestyle, causes gray mold disease in many crops. Massive secretion of enzymes and toxins was long considered to be the main driver of infection, but recent studies have uncovered a rich toolbox for B. cinerea pathogenicity. The emerging picture is of a multilayered infection process governed by the exchange of factors that collectively contribute to disease development. No plant shows complete resistance against B. cinerea, but pattern-triggered plant immune responses have the potential to significantly reduce disease progression, opening new possibilities for producing B. cinerea-tolerant plants. We examine current B. cinerea infection models, highlight knowledge gaps, and suggest directions for future studies.

Advances in Fungal Elicitor-Triggered Plant Immunity

There is an array of pathogenic fungi in the natural environment of plants, which produce some molecules including pathogen-associated molecular patterns (PAMPs) and effectors during infection. These molecules, which can be recognized by plant specific receptors to activate plant immunity, including PTI (PAMP-triggered immunity) and ETI (effector-triggered immunity), are called elicitors. Undoubtedly, identification of novel fungal elicitors and their plant receptors and comprehensive understanding about fungal elicitor-triggered plant immunity will be of great significance to effectively control plant diseases. Great progress has occurred in fungal elicitor-triggered plant immunity, especially in the signaling pathways of PTI and ETI, in recent years. Here, recent advances in fungal elicitor-triggered plant immunity are summarized and their important contribution to the enlightenment of plant disease control is also discussed.

Transcriptome Analysis of Fusarium–Tomato Interaction Based on an Updated Genome Annotation of Fusarium oxysporum f. sp. lycopersici Identifies Novel Effector Candidates That Suppress or Induce Cell Death in Nicotiana benthamiana

Fusarium oxysporum f. sp. lycopersici (Fol) causes vascular wilt disease in tomato. Upon colonization of the host, Fol secretes many small effector proteins into the xylem sap to facilitate infection. Besides known SIX (secreted in xylem) proteins, the identity of additional effectors that contribute to Fol pathogenicity remains largely unexplored. We performed a deep RNA-sequencing analysis of Fol race 2-infected tomato, used the sequence data to annotate a published genome assembly generated via PacBio SMRT sequencing of the Fol race 2 reference strain Fol4287, and analysed the resulting transcriptome to identify Fol effector candidates among the newly annotated genes. We examined the Fol-infection expression profiles of all 13 SIX genes present in Fol race 2 and identified 27 new candidate effector genes that were likewise significantly upregulated upon Fol infection. Using Agrobacterium-mediated transformation, we tested the ability of 22 of the new candidate effector genes to suppress or induce cell death in leaves of Nicotiana benthamiana. One effector candidate designated Fol-EC19, encoding a secreted guanyl-specific ribonuclease, was found to trigger cell death and two effector candidates designated Fol-EC14 and Fol-EC20, encoding a glucanase and a secreted trypsin, respectively, were identified that can suppress Bax-mediated cell death. Remarkably, Fol-EC14 and Fol-EC20 were also found to suppress I-2/Avr2- and I/Avr1-mediated cell death. Using the yeast secretion trap screening system, we showed that these three biologically-active effector candidates each contain a functional signal peptide for protein secretion. Our findings provide a basis for further understanding the virulence functions of Fol effectors.

A Secreted Lignin Peroxidase Required for Fungal Growth and Virulence and Related to Plant Immune Response

Botryosphaeria spp. are important phytopathogenic fungi that infect a wide range of woody plants, resulting in big losses worldwide each year. However, their pathogenetic mechanisms and the related virulence factors are rarely addressed. In this study, seven lignin peroxidase (LiP) paralogs were detected in Botryosphaeria kuwatsukai, named BkLiP1 to BkLiP7, respectively, while only BkLiP1 was identified as responsible for the vegetative growth and virulence of B. kuwatsukai as assessed in combination with knock-out, complementation, and overexpression approaches. Moreover, BkLiP1, with the aid of a signal peptide (SP), is translocated onto the cell wall of B. kuwatsukai and secreted into the apoplast space of plant cells as expressed in the leaves of Nicotiana benthamiana, which can behave as a microbe-associated molecular pattern (MAMP) to trigger the defense response of plants, including cell death, reactive oxygen species (ROS) burst, callose deposition, and immunity-related genes up-regulated. It supports the conclusion that BkLiP1 plays an important role in the virulence and vegetative growth of B. kuwatsukai and alternatively behaves as an MAMP to induce plant cell death used for the fungal version, which contributes to a better understanding of the pathogenetic mechanism of Botryosphaeria fungi.

Botrytis cinerea BcSSP2 protein is a late infection phase, cytotoxic effector

Botrytis cinerea is a broad-host-range necrotrophic phytopathogen responsible for serious diseases in leading crops. To facilitate infection, B. cinerea secretes a large number of effectors that induce plant cell death. In screening secretome data of B. cinerea during infection stage, we identified a phytotoxic protein (BcSSP2) that can also induce immune resistance in plants. BcSSP2 is a small, cysteine-rich protein without any known domains. Transient expression of BcSSP2 in leaves caused chlorosis that intensifies with time and eventually leads to death. Point mutations in eight of 10 cysteine residues abolished phytotoxicity, but residual toxic activity remained after heating treatment, suggesting contribution of unknown epitopes to protein phytotoxicity. The expression of bcssp2 was low during the first 36 h after inoculation and increased sharply upon transition to late infection stage. Deletion of bcssp2 did not cause statistically significant changes in lesions size on bean and tobacco leaves. Further analyses indicated that the phytotoxicity of BcSSP2 is negatively regulated by the receptor-like kinases BAK1 and SOBIR1. Collectively, our findings show that BcSSP2 is an effector protein that toxifies the host cells, but is also recognized by the plant immune system.

Characterization of Salivary Secreted Proteins That Induce Cell Death From Riptortus pedestris (Fabricius) and Their Roles in Insect-Plant Interactions

Riptortus pedestris (Fabricius) is a polyphagous hemipteran crop pest that mainly feeds on the leguminous plants, resulting in shriveled and dimpled seeds. With recent several outbreaks in the Huang-Huai-Hai region of China, as well as in South Korea and Japan, this species has caused enormous economic losses to soybean crops. In the present study, we found that R. pedestris feeding results in local lesions at the infestation sites. To identify the key effectors that induce plant damage during feeding, the salivary glands of R. pedestris were dissected for transcriptome sequencing, and 200 putative secreted proteins were transiently expressed in N. benthamiana. Among them, three intracellular effectors (RP191, RP246, and RP302) and one apoplastic effector (RP309) were identified as necrosis-inducing proteins (NIPs), which also triggered the reactive oxidative burst. Yeast signal sequence trap and qRT-PCR analysis suggested that these proteins might be secreted into plant tissue during R. pedestris infestation. Pathogenicity assays revealed that RP191, 246, and 302 promote Phytophthora capsici infection or induce Spodoptera litura feeding by inhibiting plant immunity. RP302 is localized to the cytoplasm and nuclei, while RP191 and 246 are endoplasmic reticulum (ER) resident proteins. RP309 stimulates the expression of PTI marker genes, and its induced cell death depends on co-receptors NbBAK1 and NbSOBIR1, indicating that it is a HAMP. Bioinformatics analysis demonstrated that four NIPs are recently evolved effectors and only conserved in the Pentatomidae. In this study, saliva-secreted proteins were used as the starting point to preliminarily analyze the harm mechanism of R. pedestris, which might provide a new idea and theoretical basis for this species control.

Clathrin Is Important for Virulence Factors Delivery in the Necrotrophic Fungus Botrytis cinerea

Fungi are the most prevalent plant pathogens, causing annually important damages. To infect and colonize their hosts, they secrete effectors including hydrolytic enzymes able to kill and macerate plant tissues. These secreted proteins are transported from the Endoplasmic Reticulum and the Golgi apparatus to the extracellular space through intracellular vesicles. In pathogenic fungi, intracellular vesicles were described but their biogenesis and their role in virulence remain unclear. In this study, we report the essential role of clathrin heavy chain (CHC) in the pathogenicity of Botrytis cinerea, the agent of gray mold disease. To investigate the importance of this protein involved in coat vesicles formation in eukaryotic cells, a T-DNA insertional mutant reduced in the expression of the CHC-encoding gene, and a mutant expressing a dominant-negative form of CHC were studied. Both mutants were strongly affected in pathogenicity. Characterization of the mutants revealed altered infection cushions and an important defect in protein secretion. This study demonstrates the essential role of clathrin in the infectious process of a plant pathogenic fungus and more particularly its role in virulence factors delivery.

FoEG1, a secreted glycoside hydrolase family 12 protein from Fusarium oxysporum, triggers cell death and modulates plant immunity

Fusarium oxysporum is an important soilborne fungal pathogen with many different formae speciales that can colonize the plant vascular system and cause serious crop wilt disease worldwide. We found a glycoside hydrolase family 12 protein FoEG1, secreted by F. oxysporum, that acted as a pathogen-associated molecular pattern (PAMP) targeting the apoplast of plants to induce cell death. Purified FoEG1 protein triggered cell death in different plants and induced the plant defence response to enhance the disease resistance of plants. The ability of FoEG1 to induce cell death was mediated by leucine-rich repeat (LRR) receptor-like kinases BAK1 and SOBIR1, and this ability was independent of its hydrolase activity. The mutants of cysteine residues did not affect the ability of FoEG1 to induce cell death, and an 86 amino acid fragment from amino acid positions 144 to 229 of FoEG1 was sufficient to induce cell death in Nicotiana benthamiana. In addition, the expression of FoEG1 was strongly induced in the early stage of F. oxysporum infection of host plants, and FoEG1 deletion or loss of enzyme activity reduced the virulence of F. oxysporum. Therefore, our results suggest that FoEG1 can contribute to the virulence of F. oxysporum depending on its enzyme activity and can also act as a PAMP to induce plant defence responses.

Quantitative Proteomics Reveals the Dynamic Regulation of the Tomato Proteome in Response to Phytophthora infestans

Late blight (LB) disease is a major threat to potato and tomato production. It is caused by the hemibiotrophic pathogen, Phytophthora infestans. P. infestans can destroy all of the major organs in plants of susceptible crops and result in a total loss of productivity. At the early pathogenesis stage, this hemibiotrophic oomycete pathogen causes an asymptomatic biotrophic infection in hosts, which then progresses to a necrotrophic phase at the later infection stage. In this study, to examine how the tomato proteome is regulated by P. infestans at different stages of pathogenesis, a data-independent acquisition (DIA) proteomics approach was used to trace the dynamics of the protein regulation. A comprehensive picture of the regulation of tomato proteins functioning in the immunity, signaling, defense, and metabolism pathways at different stages of P. infestans infection is revealed. Among the regulated proteins, several involved in mediating plant defense responses were found to be differentially regulated at the transcriptional or translational levels across different pathogenesis phases. This study increases understanding of the pathogenesis of P. infestans in tomato and also identifies key transcriptional and translational events possibly targeted by the pathogen during different phases of its life cycle, thus providing novel insights for developing a new strategy towards better control of LB disease in tomato.

Cross Kingdom Immunity: The Role of Immune Receptors and Downstream Signaling in Animal and Plant Cell Death

Both plants and animals are endowed with sophisticated innate immune systems to combat microbial attack. In these multicellular eukaryotes, innate immunity implies the presence of cell surface receptors and intracellular receptors able to detect danger signal referred as damage-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs). Membrane-associated pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), C-type lectin receptors (CLRs), receptor-like kinases (RLKs), and receptor-like proteins (RLPs) are employed by these organisms for sensing different invasion patterns before triggering antimicrobial defenses that can be associated with a form of regulated cell death. Intracellularly, animals nucleotide-binding and oligomerization domain (NOD)-like receptors or plants nucleotide-binding domain (NBD)-containing leucine rich repeats (NLRs) immune receptors likely detect effectors injected into the host cell by the pathogen to hijack the immune signaling cascade. Interestingly, during the co-evolution between the hosts and their invaders, key cross-kingdom cell death-signaling macromolecular NLR-complexes have been selected, such as the inflammasome in mammals and the recently discovered resistosome in plants. In both cases, a regulated cell death located at the site of infection constitutes a very effective mean for blocking the pathogen spread and protecting the whole organism from invasion. This review aims to describe the immune mechanisms in animals and plants, mainly focusing on cell death signaling pathways, in order to highlight recent advances that could be used on one side or the other to identify the missing signaling elements between the perception of the invasion pattern by immune receptors, the induction of defenses or the transmission of danger signals to other cells. Although knowledge of plant immunity is less advanced, these organisms have certain advantages allowing easier identification of signaling events, regulators and executors of cell death, which could then be exploited directly for crop protection purposes or by analogy for medical research.

The infection cushion of Botrytis cinerea: a fungal 'weapon' of plant-biomass destruction

The necrotrophic plant-pathogen fungus Botrytis cinerea produces multicellular appressoria dedicated to plant penetration, named infection cushions (IC). A microarray analysis was performed to identify genes upregulated in mature IC. The expression data were validated by RT-qPCR analysis performed in vitro and in planta, proteomic analysis of the IC secretome and biochemical assays. 1231 upregulated genes and 79 up-accumulated proteins were identified. The data support the secretion of effectors by IC: phytotoxins, ROS, proteases, cutinases, plant cell wall-degrading enzymes and plant cell death-inducing proteins. Parallel upregulation of sugar transport and sugar catabolism-encoding genes would indicate a role of IC in nutrition. The data also reveal a substantial remodelling of the IC cell wall and suggest a role for melanin and chitosan in IC function. Lastly, mutagenesis of two upregulated genes in IC identified secreted fasciclin-like proteins as actors in the pathogenesis of B. cinerea. These results support the role of IC in plant penetration and also introduce other unexpected functions for this fungal organ, in colonization, necrotrophy and nutrition of the pathogen.