Cytotoxic activity of Nep1-like proteins on monocots

Plant sphingolipids: Subcellular distributions and functions

Sphingolipids are common membrane components that maintain membrane structural integrity and function as signaling molecules. Different sphingolipids have specific functions and are unevenly distributed across the membranes of various organelles and subcellular compartments. In this review, we survey the sphingolipidomes of different subcellular structures in Arabidopsis (Arabidopsis thaliana) cells and provide a detailed account of the functions of specific sphingolipids at each location. For example, glycosphingolipids, including glucosylceramide and glycosyl inositol phosphoceramide, mainly function in membranes, whereas simple sphingolipids, including free long-chain bases and ceramide, may have important signaling roles in the plasma membrane, mitochondria, and nucleus during plant stress responses and cell death. This review thus offers a broad perspective of the multifaceted roles of plant sphingolipids in different locations in the plant cell.

Validation of Monilinia fructicola Putative Effector Genes in Different Host Peach (Prunus persica) Cultivars and Defense Response Investigation

Monilinia fructicola is the most common and destructive brown rot agent on peaches. Knowledge of gene expression mediating host-pathogen interaction is essential to manage fungal plant diseases. M. fructicola putative virulence factors have been predicted by genome investigations. The pathogen interaction with the host was validated. Five M. fructicola isolates were inoculated on two cultivars (cv.s) of peach (Prunus persica (L.) Batsch) 'Royal Summer' and 'Messapia' with intermediate and late ripening periods, respectively. The expression pattern of 17 candidate effector genes of M. fructicola with functions linked to host invasion and fungal life, and seven peach genes involved in the immune defense system were monitored at 0, 2, 6, 10, and 24 h-post inoculation (hpi). All fungal isolates induced similar brown rot lesions on both cv.s whereas the modulation of effector genes was regulated mainly at 2, 6, and 10 hpi, when disease symptoms appeared on the fruit surface, confirming the involvement of effector genes in the early infection stage. Although differences were observed among the fungal isolates, the principal component investigation identified the main differences linked to the host genotype. The salicylic acid and jasmonate/ethylene signaling pathways were differently modulated in the host independent from the fungal isolate used for inoculation. On plants susceptible to brown rot, the pathogen may have adapted to the host's physiology by modulating its effectors as weapons.

Evaluation of cell death-inducing activity of Monilinia spp. effectors in several plants using a modified TRV expression system

Introduction

Brown rot is the most important fungal disease affecting stone fruit and it is mainly caused by Monilinia fructicola, M. laxa and M. fructigena. Monilinia spp. are necrotrophic plant pathogens with the ability to induce plant cell death by the secretion of different phytotoxic molecules, including proteins or metabolites that are collectively referred to as necrotrophic effectors (NEs).

Methods

We exploited the genomes of M. fructicola, M. laxa and M. fructigena to identify their common group of secreted effector proteins and tested the ability of a selected set of effectors to induce cell death in Nicotiana benthamiana, Solanum lycopersicum and Prunus spp. leaves.

Results

Fourteen candidate effector genes of M. fructicola, which displayed high expression during infection, were transiently expressed in plants by agroinfiltration using a modified Tobacco Rattle Virus (TRV)-based expression system. Some, but not all, effectors triggered leaf discoloration or cell death in N. benthamiana and S. lycopersicum, which are non-hosts for Monilinia and in Prunus spp., which are the natural hosts. The effector MFRU_030g00190 induced cell death in almost all Prunus genotypes tested, but not in the Solanaceous plants, while MFRU_014g02060, which is an ortholog to BcNep1, caused necrosis in all plant species tested.

Conclusion

This method provides opportunities for screening Prunus germplasm with Monilinia effector proteins, to serve as a tool for identifying genetic loci that confer susceptibility to brown rot disease.

The function of sphingolipids in membrane trafficking and cell signaling in plants, in comparison with yeast and animal cells

Sphingolipids are essential membrane components involved in a wide range of cellular, developmental and signaling processes. Sphingolipids are so essential that knock-out mutation often leads to lethality. In recent years, conditional or weak allele mutants as well as the broadening of the pharmacological catalog allowed to decipher sphingolipid function more precisely in a less invasive way. This review intends to provide a discussion and point of view on the function of sphingolipids with a main focus on endomembrane trafficking, Golgi-mediated protein sorting, cell polarity, cell-to-cell communication and cell signaling at the plasma membrane. While our main angle is the plant field research, we will constantly refer to and compare with the advances made in the yeast and animal field. In this review, we will emphasize the role of sphingolipids not only as a membrane component, but also as a key player at a center of homeostatic regulatory networks involving direct or indirect interaction with other lipids, proteins and ion fluxes.

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.

Overexpression of a GIPC glycosyltransferase gene, OsGMT1, suppresses plant immunity and delays heading time in rice

Glycosylinositol phosphorylceramides (GIPCs) are the major sphingolipids in the plant plasma membrane. In Arabidopsis, mutations of genes involved in the synthesis of GIPCs affect many physiological aspects of plants, including growth, pollen fertility, defense, and stress signaling. Loss of function of the GIPC MANNOSYL-TRANSFERASE1 (AtGMT1) results in GIPC misglycosylation and induces plant immune responses accompanied by a severely dwarfed phenotype, thus indicating that GIPCs play important roles in plant immunity. Here, we investigated the enzymatic activity and phenotypes of transgenic lines of OsGMT1, the ortholog of AtGMT1. Sphingolipidomic analysis indicated that OsGMT1 retained the enzymatic activity of GIPC hexose (Hex) glycosylation, but the knockout lines did not accumulate H₂O₂. In contrast, the OsGMT1 overexpression lines showed significant down-regulation of several defense-associated or cell wall synthesis-associated genes, and enhanced sensitivity to rice blast. Furthermore, we first demonstrated the sensitivity of rice cells to MoNLP1 protein through calcein AM release assays using rice protoplasts, thus legitimizing the presence of MoNLPs in rice blast fungus. In addition, yeast two-hybrid screens using OsGMT1 as bait revealed that OsGMT1 may regulate heading time through the OsHAP5C signaling pathway. Together, our findings suggested clear physiological functional differentiation of GMT1 orthologs between rice and Arabidopsis.

Disruption of plant plasma membrane by Nep1-like proteins in pathogen-plant interactions

Lipid membrane destruction by microbial pore-forming toxins (PFTs) is a ubiquitous mechanism of damage to animal cells, but is less prominent in plants. Nep1-like proteins (NLPs) secreted by phytopathogens that cause devastating crop diseases, such as potato late blight, represent the only family of microbial PFTs that effectively damage plant cells by disrupting the integrity of the plant plasma membrane. Recent research has elucidated the molecular mechanism of NLP-mediated membrane damage, which is unique among microbial PFTs and highly adapted to the plant membrane environment. In this review, we cover recent insight into how NLP cytolysins damage plant membranes and cause cell death.

Characteristics, Roles and Applications of Proteinaceous Elicitors from Pathogens in Plant Immunity

In interactions between pathogens and plants, pathogens secrete many molecules that facilitate plant infection, and some of these compounds are recognized by plant pattern recognition receptors (PRRs), which induce immune responses. Molecules in both pathogens and plants that trigger immune responses in plants are termed elicitors. On the basis of their chemical content, elicitors can be classified into carbohydrates, lipopeptides, proteinaceous compounds and other types. Although many studies have focused on the involvement of elicitors in plants, especially on pathophysiological changes induced by elicitors in plants and the mechanisms mediating these changes, there is a lack of up-to-date reviews on the characteristics and functions of proteinaceous elicitors. In this mini-review, we provide an overview of the up-to-date knowledge on several important families of pathogenic proteinaceous elicitors (i.e., harpins, necrosis- and ethylene-inducing peptide 1 (nep1)-like proteins (NLPs) and elicitins), focusing mainly on their structures, characteristics and effects on plants, specifically on their roles in plant immune responses. A solid understanding of elicitors may be helpful to decrease the use of agrochemicals in agriculture and gardening, generate more resistant germplasms and increase crop yields.

Analysis of plant cell death-inducing proteins of the necrotrophic fungal pathogens Botrytis squamosa and Botrytis elliptica

Fungal plant pathogens secrete proteins that manipulate the host in order to facilitate colonization. Necrotrophs have evolved specialized proteins that actively induce plant cell death by co-opting the programmed cell death machinery of the host. Besides the broad host range pathogen Botrytis cinerea, most other species within the genus Botrytis are restricted to a single host species or a group of closely related hosts. Here, we focused on Botrytis squamosa and B. elliptica, host specific pathogens of onion (Allium cepa) and lily (Lilium spp.), respectively. Despite their occurrence on different hosts, the two fungal species are each other's closest relatives. Therefore, we hypothesize that they share a considerable number of proteins to induce cell death on their respective hosts. In this study, we first confirmed the host-specificity of B. squamosa and B. elliptica. Then we sequenced and assembled high quality genomes. The alignment of these two genomes revealed a high level of synteny with few balanced structural chromosomal arrangements. To assess the cell death-inducing capacity of their secreted proteins, we produced culture filtrates of B. squamosa and B. elliptica that induced cell death responses upon infiltration in host leaves. Protein composition of the culture filtrate was analysed by mass spectrometry, and we identified orthologous proteins that were present in both samples. Subsequently, the expression of the corresponding genes during host infection was compared. RNAseq analysis showed that the majority of the orthogroups of the two sister species display similar expression patterns during infection of their respective host. The analysis of cell death-inducing proteins of B. squamosa and B. elliptica provides insights in the mechanisms used by these two Botrytis species to infect their respective hosts.

The Secreted Ribonuclease SRE1 Contributes to Setosphaeria turcica Virulence and Activates Plant Immunity

During the plant infection process, pathogens can secrete several effectors. Some of the effectors are well-known for their roles in regulating plant immunity and promoting successful pathogen colonization. However, there are few studies on the ribonuclease (RNase) effectors secreted by fungi. In the present study, we discovered a secretable RNase (SRE1) in the secretome of Setosphaeria turcica that was significantly upregulated during the early stages of S. turcica infection in maize. Knockdown of SRE1 significantly reduced the virulence of S. turcica. SRE1 can induce cell death in maize and Nicotiana benthamiana. However, unlike the conventional hypersensitive response (HR) caused by other effectors, SRE1 is not dependent on its signal peptide (SP) or plant receptor kinases (such as BAK1 and SOBIR1). SRE1-induced cell death depends upon its enzymatic activity and the N-terminal β-hairpin structure. SRE1 relies on its N-terminal β-hairpin structure to enter cells, and then degrades plant's RNA through its catalytic activity causing cytotoxic effects. Additionally, SRE1 enhances N. benthamiana's resistance to pathogenic fungi and oomycetes. In summary, SRE1 promotes the virulence of S. turcica, inducing plant cell death and activating plant immune responses.