How plants differ in toxin-sensitivity

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.

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.

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.

The Effector Protein CgNLP1 of Colletotrichum gloeosporioides Affects Invasion and Disrupts Nuclear Localization of Necrosis-Induced Transcription Factor HbMYB8-Like to Suppress Plant Defense Signaling

Fungi secrete numerous effectors to modulate host defense systems. Understanding the molecular mechanisms by which fungal effectors regulate plant defense is of great importance for the development of novel strategies for disease control. In this study, we identified necrosis- and ethylene-inducing protein 1 (Nep1)-like protein (NLP) effector gene, CgNLP1, which contributed to conidial germination, appressorium formation, invasive growth, and virulence of Colletotrichum gloeosporioides to the rubber tree. Transient expression of CgNLP1 in the leaves of Nicotiana benthamiana induced ethylene production in plants. Ectopic expression of CgNLP1 in Arabidopsis significantly enhanced the resistance to Botrytis cinerea and Alternaria brassicicola. An R2R3 type transcription factor HbMYB8-like of rubber tree was identified as the target of CgNLP1.HbMYB8-like, localized on the nucleus, and induced cell death in N. benthamiana. CgNLP1 disrupted nuclear accumulation of HbMYB8-like and suppressed HbMYB8-like induced cell death, which is mediated by the salicylic acid (SA) signal pathway. This study suggested a new strategy whereby C. gloeosporioides exploited the CgNLP1 effector to affect invasion and suppress a host defense regulator HbMYB8-like to facilitate infection.

Deciphering the role of plant plasma membrane lipids in response to invasion patterns: how could biology and biophysics help?

Plants have to constantly face pathogen attacks. To cope with diseases, they have to detect the invading pathogen as early as possible via the sensing of conserved motifs called invasion patterns. The first step of perception occurs at the plasma membrane. While many invasion patterns are perceived by specific proteinaceous immune receptors, several studies have highlighted the influence of the lipid composition and dynamics of the plasma membrane in the sensing of invasion patterns. In this review, we summarize current knowledge on how some microbial invasion patterns could interact with the lipids of the plasma membrane, leading to a plant immune response. Depending on the invasion pattern, different mechanisms are involved. This review outlines the potential of combining biological with biophysical approaches to decipher how plasma membrane lipids are involved in the perception of microbial invasion patterns.

Fighting salt or enemies: shared perception and signaling strategies

Plants react to a myriad of biotic and abiotic environmental signals through specific cellular mechanisms required for survival under stress. Although pathogen perception has been widely studied and characterized, salt stress perception and signaling remain largely elusive. Recent observations, obtained in the model plant Arabidopsis thaliana, show that perception of specific features of pathogens also allows plants to mount salt stress resilience pathways, highlighting the possibility that salt sensing and pathogen perception mechanisms partially overlap. We discuss these overlapping strategies and examine the emerging role of A. thaliana cell wall and plasma membrane components in activating both salt- and pathogen-induced responses, as part of exquisite mechanisms underlying perception of damage and danger. This knowledge helps understanding the complexity of plant responses to pathogens and salinity, leading to new hypotheses that could explain why plants evolved similar strategies to respond to these, at first sight, very different types of stimuli.

RLP23 is required for Arabidopsis immunity against the grey mould pathogen Botrytis cinerea

Necrosis- and ethylene-inducing-like proteins (NLPs) are secreted by fungi, oomycetes and bacteria. Conserved nlp peptides derived from NLPs are recognized as pathogen-associated molecular patterns (PAMPs), leading to PAMP-triggered immune responses. RLP23 is the receptor of the nlp peptides in Arabidopsis thaliana; however, its actual contribution to plant immunity is unclear. Here, we report that RLP23 is required for Arabidopsis immunity against the necrotrophic fungal pathogen Botrytis cinerea. Arabidopsis rlp23 mutants exhibited enhanced susceptibility to B. cinerea compared with the wild-type plants. Notably, microscopic observation of the B. cinerea infection behaviour indicated the involvement of RLP23 in pre-invasive resistance to the pathogen. B. cinerea carried two NLP genes, BcNEP1 and BcNEP2; BcNEP1 was expressed preferentially before/during invasion into Arabidopsis, whereas BcNEP2 was expressed at the late phase of infection. Importantly, the nlp peptides derived from both BcNEP1 and BcNEP2 induced the production of reactive oxygen species in an RLP23-dependent manner. In contrast, another necrotrophic fungus Alternaria brassicicola did not express the NLP gene in the early infection phase and exhibited no enhanced virulence in the rlp23 mutants. Collectively, these results strongly suggest that RLP23 contributes to Arabidopsis pre-invasive resistance to B. cinerea via NLP recognition at the early infection phase.

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

Filamentous pathogens, such as phytopathogenic oomycetes and fungi, secrete a remarkable diversity of apoplastic effector proteins to facilitate infection, many of which are able to induce cell death in plants. Over the past decades, over 177 apoplastic cell death-inducing proteins (CDIPs) have been identified in filamentous oomycetes and fungi. An emerging number of studies have demonstrated the role of many apoplastic CDIPs as essential virulence factors. At the same time, apoplastic CDIPs have been documented to be recognized by plant cells as pathogen-associated molecular patterns (PAMPs). The recent findings of extracellular recognition of apoplastic CDIPs by plant leucine-rich repeat-receptor-like proteins (LRR-RLPs) have greatly advanced our understanding of how plants detect them and mount a defense response. This review summarizes the latest advances in identifying apoplastic CDIPs of plant pathogenic oomycetes and fungi, and our current understanding of the dual roles of apoplastic CDIPs in plant-filamentous pathogen interactions.

Ecotoxicological characteristics and ecological risk assessment of trace elements in the bottom sediments of the Rożnów reservoir (Poland)

The bottom sediments in catchment areas behind dams play a significant role in water ecosystems. On the other hand, the structure of sediments makes them a natural geosorbent, in which pollutants introduced to the aquatic environment accumulate. The use of biotests is recognised as an important approach for the assessment of the quality of bottom sediments, as the chemical analysis of sediment samples alone does not provide evidence of the impact of contaminants on biota. The aim of the study was to apply the chemical and ecological indices to determine the potential risk posed by trace elements in the bottom sediments and to evaluate sediment toxicity using organisms belonging to two taxonomic groups, i.e., plants (Phytotoxkit) and crustaceans (Rapidtoxkit). The 46 sediment samples were taken from the Rożnów Dam Reservoir in Southern Poland. The mean concentration of the trace elements in the sediments was 5.22 mg As; 0.26 mg Cd; 63.23 mg Cr; 28.65 mg Cu; 37.11 mg Ni; 11.15 mg Pb; 69.69 mg Zn and 0.09 mg Hg ∙ kg^-1 d.m. The mean probable effect concentration quotient (PECq) value among different sampling sites ranged between 0.04 and 0.33 suggested moderate potential toxicity to the biological communities in bottom sediments. The Ni was potentially the most toxic element for biota in the Rożnów Reservoir. The sensitivity of organisms formed the following order: Thamnocephalus platyurus >Lepidium sativum >Sinapis alba >Sorghum saccharatum. For the plants, the stimulating effect of bottom sediments on root growth was often indicated, while a toxic effect was demonstrated for T. platyurus in 80% of the samples. However, the correlation analysis and PCA results showed that trace elements that originated from similar sources were associated to the toxicity of sediments towards T. platyurus, while ecotoxicity for plants could not be explained by the content of trace elements in bottom sediments. T. platyurus is a good indicator for predicting the toxicity of bottom sediments from the Rożnów Reservoir. However, our study found that both chemical and ecotoxicological analyses are important for a comprehensive evaluation of the quality of bottom sediments.

Apoplastic invasion patterns triggering plant immunity: plasma membrane sensing at the frontline

Plants are able to effectively cope with invading pathogens by activating an immune response based on the detection of invasion patterns (IPs) originating from the pathogen or released by the plant after infection. At a first level, this perception takes place at the plasma membrane through cell surface immune receptors and although the involvement of proteinaceous pattern recognition receptors (PRRs) is well established, increasing data are also pointing out the role of membrane lipids in the sensing of IPs. In this review, we discuss the evolution of various conceptual models describing plant immunity and present an overview of well-characterized IPs from different natures and origins. We summarize the current knowledge on how they are perceived by plants at the plasma membrane, highlighting the increasingly apparent diversity of sentinel-related systems in plants.

Activity and Phylogenetics of the Broadly Occurring Family of Microbial Nep1-Like Proteins

Necrosis- and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP) have an extremely broad taxonomic distribution; they occur in bacteria, fungi, and oomycetes. NLPs come in two forms, those that are cytotoxic to eudicot plants and those that are noncytotoxic. Cytotoxic NLPs bind to glycosyl inositol phosphoryl ceramide (GIPC) sphingolipids that are abundant in the outer leaflet of plant plasma membranes. Binding allows the NLP to become cytolytic in eudicots but not monocots. The function of noncytotoxic NLPs remains enigmatic, but the expansion of NLP genes in oomycete genomes suggests they are important. Several plant species have evolved the capacity to recognize NLPs as molecular patterns and trigger plant immunity, e.g., Arabidopsis thaliana detects nlp peptides via the receptor-like protein RLP23. In this review, we provide a historical perspective from discovery to understanding of molecular mechanisms and describe the latest developments in the NLP field to shed light on these fascinating microbial proteins.

Papers of note in Science 358 (6369)

This week’s articles explain why broad-leaved plants are susceptible to a microbial toxin that does not affect monocots; identify the enzymes that detyrosinate tubulin; and present synthetic biological systems for recording environmental signals and for translating a biochemical reaction into dynamic cellular behavior.