Plant Immunity Research Articles

Research Progress on miRNAs and Artificial miRNAs in Insect and Disease Resistance and Breeding in Plants.

MicroRNAs (miRNAs) are small, non-coding RNAs that are expressed in a tissue- and temporal-specific manner during development. They have been found to be highly conserved during the evolution of different species. miRNAs regulate the expression of several genes in various organisms, with some regulating the expression of multiple genes with similar or completely unrelated functions. Frequent disease and insect pest infestations severely limit agricultural development. Thus, cultivating resistant crops via miRNA-directed gene regulation in plants, insects, and pathogens is an important aspect of modern breeding practices. To strengthen the application of miRNAs in sustainable agriculture, plant endogenous or exogenous miRNAs have been used for plant breeding. Consequently, the development of biological pesticides based on miRNAs has become an important avenue for future pest control methods. However, selecting the appropriate miRNA according to the desired target traits in the target organism is key to successfully using this technology for pest control. This review summarizes the progress in research on miRNAs in plants and other species involved in regulating plant disease and pest resistance pathways. We also discuss the molecular mechanisms of relevant target genes to provide new ideas for future research on pest and disease resistance and breeding in plants.

Methyl jasmonate induced tolerance effect of Pinus koraiensis to Bursaphelenchus xylophilus.

Methyl jasmonate (MeJA) can affect the balance of hormones and regulate the disease resistance of plants. Exploring the application and mechanism of MeJA in inducing the tolerance of Pinus koraiensis to pine wood nematode (PWN) infection is of great significance for developing new strategies for pine wilt disease control. Different concentrations (0.1, 1, 5 and 10 mm) of MeJA treatment groups showed differences in relative tolerance index and relative anti-nematode index of P. koraiensis seedlings to PWN infection. The treatment of 5 mm MeJA solution induced the best tolerance effect, followed by the 1 mm MeJA solution. Transcriptome analysis indicated that many plant defense-related genes upregulated after treatment with 1, 5 and 10 mm MeJA solutions. Among them, genes such as jasmonate ZIM domain-containing protein, phenylalanine ammonia-lyase and peroxidase also continuously upregulated after PWN infection. Metabolome analysis indicated that jasmonic acid (JA) was significantly increased at 7 days postinoculation with PWN, and after treatment with both 1 and 5 mm MeJA solutions. Integrated analysis of transcriptome and metabolome indicated that differences in JA accumulation might lead to ubiquitin-mediated proteolysis, and expression changes in trans-caffeic acid and trans-cinnamic acid-related genes, leading to the abundance differences of these two metabolisms and the formation of multiple lignin and glucosides. MeJA treatment could activate the expression of defense-related genes that correlated with JA, regulate the abundance of defense-related secondary metabolites, and improve the tolerance of P. koraiensis seedlings to PWN infection. © 2024 Society of Chemical Industry.

Understanding the role of Bacillus subtilis GEB1 in enhancing guava plant immunity against Meloidogyne enterolobii: A multifaceted approach

Guava root-knot nematode, Meloidogyne enterolobii has emerged as a significant threat to guava cultivation, impacting production worldwide. By integrating the molecular, histopathological and in silico approaches the current study explores the efficacy of Bacillus subtilis GEB1 in enhancing the resistance of guava against M. enterolobii. The gene expression studies reveal that guava plants treated with B. subtilis and challenged with nematodes at 72 h showed 2 fold increase in phytoene synthase, 2.1 fold increase in mitogen-activated protein kinase, 1.54 fold increase in phenylalanine ammonia-lyase and 1.82 fold increase in peroxidase genes when compared to untreated control. The Scanning electron microscope analysis of guava roots treated with B. subtilis revealed the presence of spores in the rhizoplane region which led to deter the invading nematodes. The comparative histological studies with guava roots treated with B. subtilis revealed lesser damage in the cortex, vascular regions with limited hypertrophy and hyperplasia, lesser adult females, and feeding traces with minimum root, stele and cortex diameter and thickness than those of untreated infested roots. Attempts were made to dock the plant functional proteins with B. subtilis peptides to understand their interaction. The results revealed that bacillobactin showed a greater binding affinity of −9.0 kcal/mol with phytoene synthase, plipstatin (−16.0 kcal/mol), and fengycin (−15.9 kcal/mol) with MAPK proteins of guava. Overall, the current study offers insight into the intricate mechanisms of plant-nematode-bacteria interactions and underscores the importance of B. subtilis GEB-1 as a biocontrol agent to combat Meloidogyne enterolobii infection in guava.

The stripe rust effector Pst3180.3 inhibits the transcriptional activity of TaMYB4L to modulate wheat immunity and analyzes the key active sites of the interaction conformation

Puccinia striiformis f. sp. tritici (Pst) has a wide range and serious damage, which severely threatens global wheat production. In this study, we focused on an effector protein Pst3180.3, which was induced to be highly expressed during the Pst infection stage. The N-terminal 19 amino acid of Pst3180.3 was verified to function as a signal peptide and transferred to cytoplasm and nucleus of wheat following Pst infection. Transient overexpression of Pst3180.3 in Nicotiana benthamiana inhibited programmed cell death triggered via BAX. The instantaneous silencing of Pst3180.3 by BSMV- HIGS significantly reduced the number of uredinia and increased accumulation of reactive oxygen species. Those results indicated that Pst3180.3 is an important pathogenic factor of Pst. Interaction of Pst3180.3 with a transcription factor TaMYB4L in host was confirmed through yeast two-hybrid, luciferase complementation, and co-immunoprecipitation. Virus-induced gene silencing of TaMYB4L weakened the resistance to Pst, indicated that TaMYB4L may be involved in the positive regulation of plant immunity. Dual-luciferase assays revealed that Pst3180.3 inhibited the transcriptional activity of TaMYB4L. Meanwhile, molecular docking analysis identified the key residue sites for the interaction and binding between Pst3180.3 and MYB4L. Those results demonstrated that Pst3180.3 binds to TaMYB4L and interacts to inhibit wheat resistance to Pst infection.

The Role of Salicylic, Jasmonic Acid and Ethylene in the Development of the Resistance/Susceptibility of Wheat to the SnTox1-Producing Isolate of the Pathogenic Fungus Stagonospora nodorum (Berk.).

The SnTox1 effector is a virulence factor of the fungal pathogen Stagonospora nodorum (Berk.), which interacts with the host susceptibility gene Snn1 in a gene-for-gene manner and causes necrosis on the leaves of sensitive wheat genotypes. It is known that salicylic acid (SA), jasmonic acid (JA) and ethylene are the key phytohormones involved in plant immunity. To date, effectors of various pathogens have been discovered that can manipulate plant hormonal pathways and even use hormone crosstalk to promote disease development. However, the role of SnTox1 in manipulating hormonal pathways has not been studied in detail. We studied the redox status and the expression of twelve genes of hormonal pathways and two MAPK genes in six bread wheat cultivars sensitive and insensitive to SnTox1 with or without treatment by SA, JA and ethephon (ethylene-releasing agent) during infection with the SnTox1-producing isolate S. nodorum 1SP. The results showed that SnTox1 controls the antagonism between the SA and JA/ethylene signaling pathways. The SA pathway was involved in the development of susceptibility, and the JA/ethylene pathways were involved in the development of wheat plants resistance to the Sn1SP isolate in the presence of a SnTox1-Snn1 interaction. SnTox1 hijacked the SA pathway to suppress catalase activity, increase hydrogen peroxide content and induce necrosis formation; it simultaneously suppresses the JA and ethylene hormonal pathways by SA. To do this, SnTox1 reprogrammed the expression of the MAPK genes TaMRK3 and TaMRK6 and the TF genes TaWRKY13, TaEIN3 and TaWRKY53b. This study provides new data on the role of SnTox1 in manipulating hormonal pathways and on the role of SA, JA and ethylene in the pathosystem wheat S. nodorum.

SDE19, a SEC-dependent effector from 'Candidatus Liberibacter asiaticus' suppresses plant immunity and targets Citrus sinensis Sec12 to interfere with vesicle trafficking.

Citrus huanglongbing (HLB), which is caused by the phloem-colonizing bacteria Candidatus Liberibacter asiaticus (CLas), poses a significant threat to citrus production worldwide. The pathogenicity mechanism of HLB remains poorly understood. SEC-dependent effectors (SDEs) have been suggested to play critical roles in the interaction between citrus and CLas. Here, we explored the function of CLIBASIA_05320 (SDE19), a core SDE from CLas, and its interaction with its host target. Our data revealed that SDE19 is expressed at higher level during infection of citrus than that during infection of the Asian citrus psyllid. Subcellular localization assays showed that SDE19 is localized in the nucleus and cytoplasm and is capable of moving from cell to cell in Nicotiana benthamiana. To investigate whether SDE19 facilitates pathogen infection, we generated transgenic Arabidopsis thaliana and citrus plants overexpressing SDE19. Transgenic A. thaliana and citrus plants were more susceptible to Pseudomonas syringae pv. tomato (Pst) and Xanthomonas citri subsp. citri (Xcc), respectively. In addition, RNA-seq analysis demonstrated that overexpression of SDE19 resulted in a reprogramming of expression of genes related to biotic stimulus responses. SDE19 interacts with Citrus sinensis Sec12, a guanine nucleotide exchange factor responsible for the assembly of plant COPII (coat protein II)-coated vesicles, which mediate vesicle trafficking from the ER to the Golgi. SDE19 colocalizes with Sec12 in the ER by binding to its N-terminal catalytic region, affecting the stability of Sec12 through the 26S proteasome. This interaction hinders the secretion of apoplastic defense-related proteins such as PR1, P69B, GmGIP1, and RCR3. Furthermore, the secretion of PR1 and callose deposition is decreased in SDE19-transgenic A. thaliana. Taken together, SDE19 is a novel virulent SDE secreted by CLas that interacts with Sec12 to disrupt vesicle trafficking, inhibit defense-related proteins secretion, and promote bacterial infection. This study sheds light on how CLas manipulates the host vesicle trafficking pathway to suppress the secretion of defense-related proteins and interfere with plant immunity.

A secreted fungal laccase targets the receptor kinase OsSRF3 to inhibit OsBAK1–OsSRF3-mediated immunity in rice

The identification effector targets and characterization of their functions are crucial for understanding pathogen infection mechanisms and components of plant immunity. Here, we identify the effector UgsL, a ustilaginoidin synthetase with a key role in regulating virulence of the rice false smut fungus Ustilaginoidea virens. Heterologous expression of UgsL in rice (Oryza sativa) enhances plant susceptibility to multiple pathogens, and host-induced gene silencing of UgsL enhances plant resistance to U. virens, indicating that UgsL inhibits rice immunity. UgsL interacts with STRUBBELIG RECEPTOR KINASE 3 (OsSRF3). Genome editing and overexpression of OsSRF3 demonstrate that OsSRF3 plays a pivotal role in the resistance of rice to multiple pathogens. Remarkably, overexpressing OsSRF3 enhances resistance without adversely affecting plant growth or yield. We show that BRASSINOSTEROID RECEPTOR-ASSOCIATED KINASE 1 (OsBAK1) interacts with and phosphorylates OsSRF3 to activate pathogen-triggered immunity, inducing the mitogen-activated protein kinase cascade, a reactive oxygen species burst, callose deposition, and expression of defense-related genes. UgsL interferes with the phosphorylation of OsSRF3 by OsBAK1. Furthermore, UgsL mediates OsSRF3 degradation by facilitating its association with the ubiquitin-26S proteasome. Our results reveal that OsSRF3 positively regulates immunity in rice and that UgsL mediates its degradation, thereby inhibiting the activation of OsBAK1–OsSRF3-mediated immune pathways.

Plant Immunity Modulation in Arbuscular Mycorrhizal Symbiosis and Its Impact on Pathogens and Pests

Arbuscular mycorrhizal (AM) symbiosis is the oldest and most widespread mutualistic association on Earth and involves plants and soil fungi belonging to Glomeromycotina. A complex molecular, cellular, and genetic developmental program enables partner recognition, fungal accommodation in plant tissues, and activation of symbiotic functions such as transfer of phosphorus in exchange for carbohydrates and lipids. AM fungi, as ancient obligate biotrophs, have evolved strategies to circumvent plant defense responses to guarantee an intimate and long-lasting mutualism. They are among those root-associated microorganisms able to boost plants’ ability to cope with biotic stresses leading to mycorrhiza-induced resistance (MIR), which can be effective across diverse hosts and against different attackers. Here, we examine the molecular mechanisms underlying the modulation of plant immunity during colonization by AM fungi and at the onset and display of MIR against belowground and aboveground pests and pathogens. Understanding the MIR efficiency spectrum and its regulation is of great importance to optimizing the biotechnological application of these beneficial microbes for sustainable crop protection.

Phytopathogens Reprogram Host Alternative mRNA Splicing.

Alternative splicing (AS) is an evolutionarily conserved cellular process in eukaryotes in which multiple messenger RNA (mRNA) transcripts are produced from a single gene. The concept that AS adds to transcriptome complexity and proteome diversity introduces a new perspective for understanding how phytopathogen-induced alterations in host AS cause diseases. Recently, it has been recognized that AS represents an integral component of the plant immune system during parasitic, commensalistic, and symbiotic interactions. Here, I provide an overview of recent progress detailing the reprogramming of plant AS by phytopathogens and the functional implications on disease phenotypes. Additionally, I discuss the vital function of AS of immune receptors in regulating plant immunity and how phytopathogens use effector proteins to target key components of the splicing machinery and exploit alternatively spliced variants of immune regulators to negate defense responses. Finally, the functional association between AS and nonsense-mediated mRNA decay in the context of plant-pathogen interface is recapitulated.

Plant-Driven Assembly of Disease-Suppressive Soil Microbiomes

Plants have coevolved together with the microbes that surround them and this assemblage of host and microbes functions as a discrete ecological unit called a holobiont. This review outlines plant-driven assembly of disease-suppressive microbiomes. Plants are colonized by microbes from seed, soil, and air but selectively shape the microbiome with root exudates, creating microenvironment hot spots where microbes thrive. Using plant immunity for gatekeeping and surveillance, host-plant genetic properties govern microbiome assembly and can confer adaptive advantages to the holobiont. These advantages manifest in disease-suppressive soils, where buildup of specific microbes inhibits the causal agent of disease, that typically develop after an initial disease outbreak. Based on disease-suppressive soils such as take-all decline, we developed a conceptual model of how plants in response to pathogen attack cry for help and recruit plant-protective microbes that confer increased resistance. Thereby, plants create a soilborne legacy that protects subsequent generations and forms disease-suppressive soils.

Activation of an atypical plant NLR with an N-terminal deletion initiates cell death at the vacuole

Plants evolve nucleotide-binding leucine-rich repeat receptors (NLRs) to induce immunity. Activated coiled-coil (CC) domain containing NLRs (CNLs) oligomerize and form apparent cation channels promoting calcium influx and cell death, with the alpha-1 helix of the individual CC domains penetrating the plasma membranes. Some CNLs are characterized by putative N-myristoylation and S-acylation sites in their CC domain, potentially mediating permanent membrane association. Whether activated Potentially Membrane Localized NLRs (PMLs) mediate cell death and calcium influx in a similar way is unknown. We uncovered the cell-death function at the vacuole of an atypical but conserved Arabidopsis PML, PML5, which has a significant deletion in its CCG10/GA domain. Active PML5 oligomers localize in Golgi membranes and the tonoplast, alter vacuolar morphology, and induce cell death, with the short N-terminus being sufficient. Mutant analysis supports a potential role of PMLs in plant immunity. PML5-like deletions are found in several Brassicales paralogs, pointing to the evolutionary importance of this innovation. PML5, with its minimal CC domain, represents the first identified CNL utilizing vacuolar-stored calcium for cell death induction.

A conserved protein family in mirid bug Riptortus pedestris plays dual roles in regulating plant immunity.

The mirid bug (Riptortus pedestris), a major soybean pest, migrates into soybean fields during the pod filling stage and causes staygreen syndrome, which leads to substantial yield losses. The mechanism by which R. pedestris elicits soybean (Glycine max) defenses and counter-defenses remains largely unexplored. In this study, we characterized a protein family from R. pedestris, designated Riptortus pedestris HAMP 1 (RPH1) and its putative paralogs (RPH1L1, 2, 3, 4, and 5), whose members exhibit dual roles in triggering and inhibiting plant immunity. RPH1 and RPH1L1 function as herbivore-associated molecular patterns (HAMPs), activating pattern-triggered immunity (PTI) in tobacco (Nicotiana benthamiana) and G. max. Furthermore, RPH1 stimulates jasmonic acid and ethylene biosynthesis in G. max, thereby enhancing its resistance to R. pedestris feeding. Additionally, RPH1 homologs are universally conserved across various herbivorous species, with many homologs also acting as HAMPs that trigger plant immunity. Interestingly, the remaining RPH1 putative paralogs (RPH1L2-5) serve as effectors that counteract RPH1-induced PTI, likely by disrupting the extracellular perception of RPH1. This research uncovers a HAMP whose homologs are conserved in both chewing and piercing-sucking insects. Moreover, it unveils an extracellular evasion mechanism utilized by herbivores to circumvent plant immunity using functionally differentiated paralogs.

Virulent Effector Hasp155 of Puccinia striiformis f. sp. tritici Suppresses Plant Immunity and Promotes Fungus Infection.

As a kind of obligate biotrophic fungus, Puccinia striiformis f. sp. tritici (Pst) secretes vast effectors via haustoria to host cells during the infection to inhibit host defense responses and promote fungal invasion. In this study, based on the completion of genome sequencing and haustorial transcriptome sequencing of Pst, we identified a Pst effector (Hasp155) that is significantly induced in the early stage of Pst infection to wheat. The 18 N-terminal amino acids of Hasp155 encoded a signal peptide with a secretory function. Transient expression of Hasp155 in Nicotiana benthamiana inhibited Bax-induced cell death as well as chitin-triggered callose deposition and defense-related gene expression. Moreover, delivery of the Hasp155 protein into wheat cells via type three secretion systems (TTSS) led to reduced plant immunity to nonpathogenic bacteria and to the avirulent Pst race with decreased H2O2 accumulation and promoted Pst development. Furthermore, transgenic overexpression of Hasp155 significantly renders wheat resistance susceptible, resulting in a decreased defense response and increased Pst pathogenicity. Overall, these results indicate that Hasp155 is an important effector of Pst pathogenicity by suppressing plant immunity.

Medicago truncatula SOBIR1 controls pathogen immunity and specificity in the Rhizobium-legume symbiosis.

Medicago truncatula Nod Factor Perception (MtNFP)plays a role in both the Rhizobium-Legume (RL)symbiosis and plant immunity, and evidence suggests that the immune-related function of MtNFP is relevant for symbiosis. To better understand these roles of MtNFP, we sought to identify new interacting partners. We screened a yeast-2-hybrid cDNA library from Aphanomyces euteiches infected and noninfected M. truncatula roots. The M. truncatula leucine-rich repeat (LRR)receptor-like kinase SUPPRESSOR OF BIR1 (MtSOBIR1)was identified as an interactor of MtNFP and was characterised for kinase activity, and potential roles in symbiosis and plant immunity. We showed that the kinase domain of MtSOBIR1 is active and can transphosphorylate the pseudo-kinase domain of MtNFP. MtSOBIR1 could functionally complement Atsobir1 and Nbsobir1/sobir1-like mutants for defence activation, and Mtsobir1 mutants were defective in immune responses to A. euteiches. For symbiosis, we showed that Mtsobir1 mutant plants had both a strong, early infection defect and defects in the defence suppression in nodules, and both effects were plant genotype- and rhizobial strain-specific. This work highlights a conserved function for MtSOBIR1 in activating defence responses to pathogen attack, and potentially novel symbiotic functions of downregulating defence in association with the control of symbiotic specificity.

Linear β-1,2-glucans trigger immune hallmarks and enhance disease resistance in plants.

Immune responses in plants are triggered by molecular patterns or elicitors, recognized by plant pattern recognition receptors. Such molecular patterns are consequence of host-pathogen interactions and the response cascade activated after their perception is known as pattern-triggered immunity (PTI). Glucans have emerged as key players in PTI, but the ability of certain glucans to stimulate defensive responses in plants remains understudied. This work focused on identifying novel glucan oligosaccharides as molecular patterns. The ability of various microorganism-derived glucans to prompt PTI responses was tested, revealing that specific microbial-derived molecules, such as short linear β-1,2-glucans, trigger this response in plants by increasing the production of reactive oxygen species (ROS), MAP kinase phosphorylation, and differential expression of defence-related genes in Arabidopsis thaliana. Pretreatments with β-1,2-glucan trisaccharide (B2G3) improved Arabidopsis defence against bacterial and fungal infections in a hypersusceptible genotype. The knowledge generated was then transferred to the monocotyledonous model species maize and wheat, confirming that these plants also respond to β-1,2-glucans, with increased ROS production and improved protection against fungal infections following B2G3 pretreatments. In summary, as with other β-glucans, plants perceive β-1,2-glucans as warning signals and stimulate defence responses against phytopathogens.

Comparative transcriptome analysis reveals defense responses against soft rot induced by Pectobacterium aroidearum and Pectobacterium carotovorum in Pinellia ternata

Pectobacterium carotovorum and Pectobacterium aroidearum represent the primary pathogens causing variable soft rot disease. However, the fundamental defense responses of Pinellia ternata to pathogens remain unclear. Our investigation demonstrated that the disease produced by P. carotovorum is more serious than P. aroidearum. RNA-seq analysis indicated that many cell wall-related genes, receptor-like kinase genes, and resistance-related genes were induced by P. aroidearum and P. carotovorum similarly. But many different regulatory pathways exert a crucial function in plant immunity against P. aroidearum and P. carotovorum, including hormone signaling, whereas more auxin-responsive genes were responsive to P. carotovorum, while more ethylene and gibberellin-responsive genes were responsive to P. aroidearum. 12 GDSL esterase/lipase genes and 3 fasciclin-like arabinogalactan protein genes were specifically upregulated by P. carotovorum, whereas 11 receptor-like kinase genes and 8 disease resistance genes were up-regulated only by P. aroidearum. Among them, a lectin gene (part1transcript/39001) was induced by P. carotovorum and P. aroidearum simultaneously. Transient expression in N. benthamiana demonstrated that the lectin gene improves plant resistance to P. carotovorum. This study offers a comprehensive perspective on P. ternata immunity produced by different soft rot pathogens and reveals the importance of lectin in anti-soft rot of P. ternata for the first time.

Carbon flow from roots to rhizobacterial networks: Grafting effects

Plants recruit microorganisms from bulk soil by secreting easily available organic carbon into the rhizosphere. Grafting often increases the disease resistance of agricultural plants by modifying this carbon flow from roots into rhizosphere and by recruiting active microorganisms that suppress pathogens. Here, we continuously labeled grafted and ungrafted watermelon plants in a 13CO2 atmosphere to identify the active microorganisms assimilating root exudates. Multi-omics associated technologies (amplicon sequencing, metagenomics and metabolomics) combined with 13C tracing were used to examine the carbon flows, microbial utilization and transformation in the rhizosphere. The number of potentially active bacterial species recruited in the rhizosphere of grafted plants and utilizing root exudates was four times more than in ungrafted plants. These potentially active species matched to metagenome-assembled-genomes (MAGs) mainly belonging to Sphingomonas in the rhizosphere of ungrafted plants, and to Sphingomonas, Chitinophaga, Dyadobacter and Pseudoxanthomonas in the rhizosphere of grafted plants. Sphingomonas possesses the functional potential to metabolize a plant self-toxic substance, namely 4-hydroxybenzoic acid. Furthermore, grafting shaped the complex metabolic interactions and changed the original metabolic dependence between the potentially active bacterial species. Grafting plants diversified belowground carbon flows, activating a greater number of beneficial microbes.

Recreating or Rcr3ating? Bioengineering homologs of the Rcr3 protease to activate plant immunity.

Recreating or Rcr3ating? Bioengineering homologs of the Rcr3 protease to activate plant immunity.

Complex of Defense Polypeptides of Wheatgrass (Elytrigia elongata) Associated with Plant Immunity to Biotic and Abiotic Stress Factors.

Plant defense polypeptides play a crucial role in providing plants with constitutive immunity against various biotic and abiotic stressors. In this study, we explored a complex of proteins from wheatgrass (Elytrigia elongata) spikelets to estimate their role in the plant's tolerance to various environmental factors. The current research shows that in vitro protein extracts from E. elongata spikelets possess antifungal activity against certain Fusarium species, which are specific cereal pathogens, at concentrations of 1-2 mg/mL. In this study, we reproduced these antifungal activities using a 4 mg/mL extract in artificial fungal infection experiments on wheat grain (Triticum aestivum) under controlled laboratory conditions. Furthermore, the tested extract demonstrated a protective effect on Saccharomyces cerevisiae exposed to hyper-salinity stress at a concentration of 2 mg/mL. A combined scheme of fractionation and structural identification was applied for the estimation of the diversity of defense polypeptides. Defensins, lipid-transfer proteins, hydrolase inhibitors (cereal bifunctional trypsin/alpha-amylase inhibitors from a Bowman-Birk trypsin inhibitor), and high-molecular-weight disease resistance proteins were isolated from the extract. Thus, wheatgrass spikelets appear to be a reservoir of defense polypeptides. Our findings contribute to a deeper understanding of plant defense proteins and peptides and their involvement in the adaptation to various stress factors, and they reveal the regulatory effect at the ecosystem level.

Sphingolipid homeostasis--how do cells know when enough is enough? Implications for plant pathogen responses.

Sphingolipid homeostatic regulation is important for balancing plant life and death. Plant cells finely tune sphingolipid biosynthesis to ensure sufficient levels to support growth through their basal functions as major components of endomembranes and the plasma membrane. Conversely, accumulation of sphingolipid biosynthetic intermediates, long-chain bases (LCBs) and ceramides, is associated with programmed cell death (PCD). Limiting these apoptotic intermediates is important for cell viability; while overriding homeostatic regulation permits cells to generate elevated LCBs and ceramides to respond to pathogens to elicit the hypersensitive response in plant immunity. Key to sphingolipid homeostasis is serine palmitoyltransferase (SPT), an ER-associated, multi-subunit enzyme catalyzing the first step in the biosynthesis of LCBs, the defining feature of sphingolipids. Across eukaryotes, SPT interaction with its negative regulator ORM is critical for sphingolipid biosynthesis. The recent cryo-electron microscopy structure of the Arabidopsis SPT complex indicates that ceramides bind ORMs to competitively inhibit SPT activity. This system provides a sensor for intracellular ceramide concentrations for sphingolipid homeostatic regulation. Combining the newly elucidated Arabidopsis SPT structure and mutant characterization, we present a model for the role of the two functionally divergent Arabidopsis ceramide synthase classes to produce ceramides that form repressive (trihydroxy LCB-ceramides) or non-repressive (dihydroxy LCB-ceramides) ORM interactions to influence SPT activity. We describe how sphingolipid biosynthesis is regulated by the interplay of ceramide synthases with ORM-SPT when "enough is enough" and override homeostatic suppression when "enough is not enough" to respond to environmental stimuli such as microbial pathogen attack.