A maize cystatin suppresses host immunity by inhibiting apoplastic cysteine proteases

BRASSINOSTEROID-SIGNALING KINASE1 associates with and is required for cysteine protease RESPONSE TO DEHYDRATION 19-mediated disease resistance in Arabidopsis

The receptor-like cytoplasmic kinase BRASSINOSTEROID-SIGNALING KINASE1 (BSK1) interacts with pattern recognition receptor (PRR) FLAGELLIN SENSING2 (FLS2) and positively regulates plant innate immunity in Arabidopsis thaliana. However, the molecular components involved in BSK1-mediated immune signaling remain largely unknown. To further explore the molecular mechanism underlying BSK1-mediated disease resistance, we screened two cysteine proteases, RESPONSE TO DEHYDRATION 19 (RD19) and RD19-LIKE 2 (RDL2), as BSK1-binding partners. Overexpression of RD19, but not RDL2, displayed an autoimmune phenotype, presenting programmed cell death and enhanced resistance to multiple pathogens. Interestingly, RD19-mediated immune activation depends on BSK1, as knockout of BSK1 in RD19-overexpressing plants rescued their autoimmunity and abolished the increased resistance. Furthermore, we found that BSK1 plays a positive role in maintaining RD19 protein abundance in Arabidopsis. Our results provide new insights into BSK1-mediated immune signaling and reveal a potential mechanism by which BSK1 stabilizes RD19 to promote effective immune output.

Decoding the molecular mechanism underlying salicylic acid (SA)-mediated plant immunity: an integrated overview from its biosynthesis to the mode of action

Salicylic acid (SA) is an important phytohormone, well-known for its regulatory role in shaping plant immune responses. In recent years, significant progress has been made in unravelling the molecular mechanisms underlying SA biosynthesis, perception, and downstream signalling cascades. Through the concerted efforts employing genetic, biochemical, and omics approaches, our understanding of SA-mediated defence responses has undergone remarkable expansion. In general, following SA biosynthesis through Avr effectors of the pathogens, newly synthesized SA undergoes various biochemical changes to achieve its active/inactive forms (e.g. methyl salicylate). The activated SA subsequently triggers signalling pathways associated with the perception of pathogen-derived signals, expression of defence genes, and induction of systemic acquired resistance (SAR) to tailor the intricate regulatory networks that coordinate plant immune responses. Nonetheless, the mechanistic understanding of SA-mediated plant immune regulation is currently limited because of its crosstalk with other signalling networks, which makes understanding this hormone signalling more challenging. This comprehensive review aims to provide an integrated overview of SA-mediated plant immunity, deriving current knowledge from diverse research outcomes. Through the integration of case studies, experimental evidence, and emerging trends, this review offers insights into the regulatory mechanisms governing SA-mediated immunity and signalling. Additionally, this review discusses the potential applications of SA-mediated defence strategies in crop improvement, disease management, and sustainable agricultural practices.

NIa-Pro of sugarcane mosaic virus targets Corn Cysteine Protease 1 (CCP1) to undermine salicylic acid-mediated defense in maize

Papain-like cysteine proteases (PLCPs) play pivotal roles in plant defense against pathogen invasions. While pathogens can secrete effectors to target and inhibit PLCP activities, the roles of PLCPs in plant-virus interactions and the mechanisms through which viruses neutralize PLCP activities remain largely uncharted. Here, we demonstrate that the expression and activity of a maize PLCP CCP1 (Corn Cysteine Protease), is upregulated following sugarcane mosaic virus (SCMV) infection. Transient silencing of CCP1 led to a reduction in PLCP activities, thereby promoting SCMV infection in maize. Furthermore, the knockdown of CCP1 resulted in diminished salicylic acid (SA) levels and suppressed expression of SA-responsive pathogenesis-related genes. This suggests that CCP1 plays a role in modulating the SA signaling pathway. Interestingly, NIa-Pro, the primary protease of SCMV, was found to interact with CCP1, subsequently inhibiting its protease activity. A specific motif within NIa-Pro termed the inhibitor motif was identified as essential for its interaction with CCP1 and the suppression of its activity. We have also discovered that the key amino acids responsible for the interaction between NIa-Pro and CCP1 are crucial for the virulence of SCMV. In conclusion, our findings offer compelling evidence that SCMV undermines maize defense mechanisms through the interaction of NIa-Pro with CCP1. Together, these findings shed a new light on the mechanism(s) controlling the arms races between virus and plant.

Plant cathepsin B, a versatile protease

Plant proteases are essential enzymes that play key roles during crucial phases of plant life. Some proteases are mainly involved in general protein turnover and recycle amino acids for protein synthesis. Other proteases are involved in cell signalling, cleave specific substrates and are key players during important genetically controlled molecular processes. Cathepsin B is a cysteine protease that can do both because of its exopeptidase and endopeptidase activities. Animal cathepsin B has been investigated for many years, and much is known about its mode of action and substrate preferences, but much remains to be discovered about this potent protease in plants. Cathepsin B is involved in plant development, germination, senescence, microspore embryogenesis, pathogen defence and responses to abiotic stress, including programmed cell death. This review discusses the structural features, the activity of the enzyme and the differences between the plant and animal forms. We discuss its maturation and subcellular localisation and provide a detailed overview of the involvement of cathepsin B in important plant life processes. A greater understanding of the cell signalling processes involving cathepsin B is needed for applied discoveries in plant biotechnology.

Pathogen-induced methylglyoxal negatively regulates rice bacterial blight resistance by inhibiting OsCDR1 protease activity

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight (BB), a globally devastating disease of rice (Oryza sativa) that is responsible for significant crop loss. Sugars and sugar metabolites are important for pathogen infection, providing energy and regulating events associated with defense responses; however, the mechanisms by which they regulate such events in BB are unclear. As an inevitable sugar metabolite, methylglyoxal (MG) is involved in plant growth and responses to various abiotic stresses, but the underlying mechanisms remain enigmatic. Whether and how MG functions in plant biotic stress responses is almost completely unknown. Here, we report that the Xoo strain PXO99 induces OsWRKY62.1 to repress transcription of OsGLY II genes by directly binding to their promoters, resulting in overaccumulation of MG. MG negatively regulates rice resistance against PXO99: osglyII2 mutants with higher MG levels are more susceptible to the pathogen, whereas OsGLYII2-overexpressing plants with lower MG content show greater resistance than the wild type. Overexpression of OsGLYII2 to prevent excessive MG accumulation confers broad-spectrum resistance against the biotrophic bacterial pathogens Xoo and Xanthomonas oryzae pv. oryzicola and the necrotrophic fungal pathogen Rhizoctonia solani, which causes rice sheath blight. Further evidence shows that MG reduces rice resistance against PXO99 through CONSTITUTIVE DISEASE RESISTANCE 1 (OsCDR1). MG modifies the Arg97 residue of OsCDR1 to inhibit its aspartic protease activity, which is essential for OsCDR1-enhanced immunity. Taken together, these findings illustrate how Xoo promotes infection by hijacking a sugar metabolite in the host plant.

XYLEM CYSTEINE PEPTIDASE 1 and its inhibitor CYSTATIN 6 regulate pattern-triggered immunity by modulating the stability of the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D

Plants produce a burst of reactive oxygen species (ROS) after pathogen infection to successfully activate immune responses. During pattern-triggered immunity (PTI), ROS are primarily generated by the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD). RBOHD is degraded in the resting state to avoid inappropriate ROS production; however, the enzyme mediating RBOHD degradation and how to prevent RBOHD degradation after pathogen infection is unclear. In this study, we identified an Arabidopsis (Arabidopsis thaliana) vacuole-localized papain-like cysteine protease, XYLEM CYSTEINE PEPTIDASE 1 (XCP1), and its inhibitor CYSTATIN 6 (CYS6). Pathogen-associated molecular pattern-induced ROS burst and resistance were enhanced in the xcp1 mutant but were compromised in the cys6 mutant, indicating that XCP1 and CYS6 oppositely regulate PTI responses. Genetic and biochemical analyses revealed that CYS6 interacts with XCP1 and depends on XCP1 to enhance PTI. Further experiments showed that XCP1 interacts with RBOHD and accelerates RBOHD degradation in a vacuole-mediated manner. CYS6 inhibited the protease activity of XCP1 toward RBOHD, which is critical for RBOHD accumulation upon pathogen infection. As CYS6, XCP1, and RBOHD are conserved in all plant species tested, our findings suggest the existence of a conserved strategy to precisely regulate ROS production under different conditions by modulating the stability of RBOHD.

A Trojan Horse Approach Using Ustilago maydis to Study Apoplastic Maize (Zea mays) Peptides In Situ

Plant peptides are important signaling components in many parts of the plant lifecycle, e.g., development, reproduction, environmental stress response, and plant pathogen defenses. Yet, in maize, one of the most grown crops worldwide, only a few peptides have been identified and studied. In general, molecular research is severely impacted by time-consuming and costly maize transformation, and external application of purified peptides does not allow functional analysis in deeper cell layers due to the thickness of the tissue. In an attempt to bypass these problems while studying the function of small secreted proteins in maize, we established the Trojan Horse approach. Here, tagged peptides are delivered into the maize apoplast in a highly localized fashion by using a genetically modified version of the biotrophic pathogen Ustilago maydis. This technique offers the possibility of rapid testing of predicted maize peptides for in situ functions.

Combination of transcriptomic, proteomic, and degradomic profiling reveals common and distinct patterns of pathogen-induced cell death in maize

Regulated cell death (RCD) is crucial for plant development, as well as in decision-making in plant-microbe interactions. Previous studies revealed components of the molecular network controlling RCD, including different proteases. However, the identity, the proteolytic network as well as molecular components involved in the initiation and execution of distinct plant RCD processes, still remain largely elusive. In this study, we analyzed the transcriptome, proteome, and N-terminome of Zea mays leaves treated with the Xanthomonas effector avrRxo1, the mycotoxin Fumonisin B1 (FB1), or the phytohormone salicylic acid (SA) to dissect plant cellular processes related to cell death and plant immunity. We found highly distinct and time-dependent biological processes being activated on transcriptional and proteome levels in response to avrRxo1, FB1, and SA. Correlation analysis of the transcriptome and proteome identified general, as well as trigger-specific markers for cell death in Zea mays. We found that proteases, particularly papain-like cysteine proteases, are specifically regulated during RCD. Collectively, this study characterizes distinct RCD responses in Z. mays and provides a framework for the mechanistic exploration of components involved in the initiation and execution of cell death.

Maize Phytocytokines Modulate Pro-Survival Host Responses and Pathogen Resistance

Phytocytokines are signaling peptides that alert plant cells of danger. However, the downstream responses triggered by phytocytokines and their effect on plant survival are still largely unknown. Here, we have identified three biologically active maize orthologues of phytocytokines previously described in other plants. The maize phytocytokines show common features with microbe-associated molecular patterns (MAMPs), including the induction of immune-related genes and activation of papain-like cysteine proteases. In contrast to MAMPs, phytocytokines do not promote cell death in the presence of wounding. In infection assays with two fungal pathogens, we found that phytocytokines affect the development of disease symptoms, likely due to the activation of phytohormonal pathways. Collectively, our results show that phytocytokines and MAMPs trigger unique and antagonistic features of immunity. We propose a model in which phytocytokines activate immune responses partially similar to MAMPs but, in contrast to microbial signals, they act as danger and survival molecules to the surrounding cells. Future studies will focus on the components determining the divergence of signaling outputs upon phytocytokine activation. [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.

XCP1 cleaves Pathogenesis-related protein 1 into CAPE9 for systemic immunity in Arabidopsis

Proteolytic activation of cytokines regulates immunity in diverse organisms. In animals, cysteine-dependent aspartate-specific proteases (caspases) play central roles in cytokine maturation. Although the proteolytic production of peptide cytokines is also essential for plant immunity, evidence for cysteine-dependent aspartate-specific proteases in regulating plant immunity is still limited. In this study, we found that the C-terminal proteolytic processing of a caspase-like substrate motif "CNYD" within Pathogenesis-related protein 1 (PR1) generates an immunomodulatory cytokine (CAPE9) in Arabidopsis. Salicylic acid enhances CNYD-targeted protease activity and the proteolytic release of CAPE9 from PR1 in Arabidopsis. This process involves a protease exhibiting caspase-like enzyme activity, identified as Xylem cysteine peptidase 1 (XCP1). XCP1 exhibits a calcium-modulated pH-activity profile and a comparable activity to human caspases. XCP1 is required to induce systemic immunity triggered by pathogen-associated molecular patterns. This work reveals XCP1 as a key protease for plant immunity, which produces the cytokine CAPE9 from the canonical salicylic acid signaling marker PR1 to activate systemic immunity.

Proteomics profiling and in silico analysis of peptides identified during Fusarium oxysporum infection in castor (Ricinus communis)

Castor is industrially important non-edible oil seeds crop severely affected by soil borne pathogen Fusarium oxysporum f. sp. ricini which causes heavy economic losses among the castor growing states in India and worldwide. The development of Fusarium wilt resistant varieties in castor is also challenging because the genes identified for resistance are recessive in nature. Unlike transcriptomics and genomics, proteomics is always a method of choice for quick identification of novel proteins expressed during biological events. Therefore, comparative proteomic approach was employed for identification of proteins released in resistant genotype during Fusarium infection. Protein was extracted from inoculated 48-1 resistant and JI-35 susceptible genotype and subjected to 2D-gel electrophoresis coupled with RPLC-MS/MS. This analysis resulted in 18 unique peptides in resistant genotype and 8 unique peptides in susceptible genotype were identified through MASCOT search database. The real time expression study showed that 5 genes namely CCR 1, Germin like protein 5-1, RPP8, Laccase 4 and Chitinase like 6 was found highly up-regulated during Fusarium oxysporum infection. Furthermore, end point PCR analysis of c-DNA showed amplification of three genes namely Chitinase 6 like, RPP8 and β-glucanase exclusively in resistant genotype indicating that these genes may be involved in resistance phenomenon in castor. Up-regulation of CCR-1 and Laccase 4 involved in lignin biosynthesis provides mechanical strength and may help to prevent the entry of fungal mycelia and protein Germin like 5-1 helps to neutralized ROS by SOD activity. The clear role of these genes can be further confirmed through functional genomics for castor improvement and also for development of transgenic in different crops for wilt resistance.

Mechanisms controlling plant proteases and their substrates

In plants, proteolysis is emerging as an important field of study due to a growing understanding of the critical involvement of proteases in plant cell death, disease and development. Because proteases irreversibly modify the structure and function of their target substrates, proteolytic activities are stringently regulated at multiple levels. Most proteases are produced as dormant isoforms and only activated in specific conditions such as altered ion fluxes or by post-translational modifications. Some of the regulatory mechanisms initiating and modulating proteolytic activities are restricted in time and space, thereby ensuring precision activity, and minimizing unwanted side effects. Currently, the activation mechanisms and the substrates of only a few plant proteases have been studied in detail. Most studies focus on the role of proteases in pathogen perception and subsequent modulation of the plant reactions, including the hypersensitive response (HR). Proteases are also required for the maturation of coexpressed peptide hormones that lead essential processes within the immune response and development. Here, we review the known mechanisms for the activation of plant proteases, including post-translational modifications, together with the effects of proteinaceous inhibitors.

Peptidome and Transcriptome Analysis of Plant Peptides Involved in Bipolaris maydis Infection of Maize

Southern corn leaf blight (SCLB) caused by Bipolaris maydis threatens maize growth and yield worldwide. In this study, TMT-labeled comparative peptidomic analysis was established between infected and uninfected maize leaf samples using liquid-chromatography-coupled tandem mass spectrometry. The results were further compared and integrated with transcriptome data under the same experimental conditions. Plant peptidomic analysis identified 455 and 502 differentially expressed peptides (DEPs) in infected maize leaves on day 1 and day 5, respectively. A total of 262 common DEPs were identified in both cases. Bioinformatic analysis indicated that the precursor proteins of DEPs are associated with many pathways generated by SCLB-induced pathological changes. The expression profiles of plant peptides and genes in maize plants were considerably altered after B. maydis infection. These findings provide new insights into the molecular mechanisms of SCLB pathogenesis and offer a basis for the development of maize genotypes with SCLB resistance.

Multi Omics Analysis Revealed a Resistance Mechanism of Tibetan Barley (Hordeum vulgare L., Qingke) Infected by Ustilago hordei

Tibetan barley (Hordeum vulgare L., qingke) is the principal cereal cultivated on Tibet. Ustilago hordei causing covered smut is a serious disease that limits the yield of qingke. Here, based on multi omics study including metabolome, proteome and transcriptome, we show that during infection, primary metabolisms such as carbohydrate, amino acid, and lipids were significantly changed. Jasmonic acid, which perform as a biotic stress signaler, was significantly repressed, and related genes or proteins also showed different expression in infected qingke. In addition, other defense-related compounds such as riboflavin, ascorbic acid, and protease inhibitors were also detected in omics data. Our results revealed a preliminary biological profile of qingke infected by U. hordei and provide a resource for further research.

Nicotiana benthamiana as a model plant host for Xylella fastidiosa: Control of infections by transient expression and endotherapy with a bifunctional peptide

Transient expression of genes encoding peptides BP134 and BP178 by means of a Potato virus X (PVX) based-vector system, and treatment with synthetic peptides by endotherapy, were evaluated in the control of Xylella fastidiosa infections, in the model plant Nicotiana benthamiana. Transient production of BP178 significantly decreased disease severity compared to PVX and non-treated control (NTC) plants, without adverse effects. Plants treated with synthetic BP134 and BP178 showed consistently lower levels of disease than NTC plants. However, the coinfection with PVX-BP134 and X. fastidiosa caused detrimental effects resulting in plant death. The levels of X. fastidiosa in three zones sampled, upwards and downwards of the inoculation/treatment point, significantly decreased compared to the NTC plants, after the treatment with BP178, but not when BP178 was produced transiently. The effect of treatment and transient production of BP178 in the induction of defense-related genes was also studied. Synthetic BP178 applied by endotherapy induced the expression of ERF1, PR1a, PAL, PALII and WRKY25, while the transient expression of BP178 overexpressed the Cath, Cyc, PR4a, 9-LOX and Endochitinase B genes. Both treatments upregulated the expression of PR1, PR3, PR4 and CycT9299 genes compared to the NTC or PVX plants. It was concluded that the effect of BP178, either by endotherapy or by transient expression, on the control of the X. fastidiosa infections in N. benthamiana, was due in part to the induction of the plant defense system in addition to its bactericidal activity reported in previous studies. However, the protection observed when BP178 was transiently produced seems mainly mediated by the induction of plant defense, because the levels of X. fastidiosa were not significantly affected.

A Puccinia striiformis f. sp. tritici effector inhibits high-temperature seedling-plant resistance in wheat

Resistance to Pseudomonas syringae pv. maculicola 1 (RPM1)-induced protein kinase (RIPK) in Arabidopsis belongs to the receptor-like cytoplasmic kinase (RLCK) family and plays a vital role in immunity. However, the role of RLCKs in the high-temperature seedling-plant (HTSP) resistance of wheat (Triticum aestivum) to Puccinia striiformis f. sp. tritici (Pst), the stripe rust pathogen, remains unclear. Here, we identified a homologous gene of RIPK in wheat, namely TaRIPK. Expression of TaRIPK was induced by Pst inoculation and high temperatures. Silencing of TaRIPK reduced the expression level of TaRPM1, resulting in weaker HTSP resistance. Moreover, TaRIPK interacts with and phosphorylates papain-like cysteine protease 1 (TaPLCP1). Meanwhile, we found that the Pst-secreted protein PSTG_01766 targets TaPLCP1. Transient expression of PSTG_01766 inhibited basal immunity in tobacco (Nicotiana benthamiana) and wheat. The role of PSTG_01766 as an effector involved in HTSP resistance was further supported by host-induced gene silencing and bacterial type three secretion system-mediated delivery into wheat. PSTG_01766 inhibited the TaRIPK-induced phosphorylation of TaPLCP1. Furthermore, PSTG_01766 has the potential to influence the subcellular localization of TaPLCP1. Overall, we suggest that the TaRIPK-TaPLCP1-TaRPM1 module fits the guard model for disease resistance, participating in HTSP resistance. PSTG_01766 decreases HTSP resistance via targeting TaPLCP1. Guarded by wheat and attacked by Pst, TaPLCP1 may serve as a central hub of the defense response. Our findings improve the understanding of the molecular mechanism of wheat HTSP resistance, which may be an important strategy for controlling stripe rust in the face of global warming.

Mycorrhiza-induced mycocypins of Laccaria bicolor are potent protease inhibitors with nematotoxic and collembola antifeedant activity

Fungivory of mycorrhizal hyphae has a significant impact on fungal fitness and, by extension, on nutrient transfer between fungi and host plants in natural ecosystems. Mycorrhizal fungi have therefore evolved an arsenal of chemical compounds that are hypothesized to protect the hyphal tissues from being eaten, such as the protease inhibitors mycocypins. The genome of the ectomycorrhizal fungus Laccaria bicolor has an unusually high number of mycocypin-encoding genes. We have characterized the evolution of this class of proteins, identified those induced by symbiosis with a host plant and characterized the biochemical properties of two upregulated L. bicolor mycocypins. More than half of L. bicolor mycocypin-encoding genes are differentially expressed during symbiosis or fruiting body formation. We show that two L. bicolor mycocypins that are strongly induced during symbiosis are cysteine protease inhibitors and exhibit similar but distinct localization in fungal tissues at different developmental stages and during interaction with a host plant. Moreover, we show that these L. bicolor mycocypins have toxic and feeding deterrent effect on nematodes and collembolans, respectively. Therefore, L. bicolor mycocypins may be part of a mechanism by which this species deters grazing by different members of the soil food web.

Integrative Analysis of Genes Involved in the Global Response to Potato Wart Formation

Synchytrium endobioticum, the causal agent of potato wart disease, poses a major threat to commercial potato production. Understanding the roles of transcriptionally regulated genes following pathogen infection is necessary for understanding the system-level host response to pathogen. Although some understanding of defense mechanisms against S. endobioticum infection has been gained for incompatible interactions, the genes and signaling pathways involved in the compatible interaction remain unclear. Based on the collection of wart diseased tubers of a susceptible cultivar, we performed phenotypic and dual RNA-Seq analyses of wart lesions in seven stages of disease progression. We totally detected 5,052 differentially expressed genes (DEGs) by comparing the different stages of infection to uninfected controls. The tendency toward differential gene expression was active rather than suppressed under attack by the pathogen. The number of DEGs step-up along with the development of the disease and the first, third and seventh of the disease stages showed substantially increase of DEGs in comparison of the previous stage. The important functional groups identified via Gene ontology (GO) and KEGG enrichment were those responsible for plant-pathogen interaction, fatty acid elongation and phenylpropanoid biosynthesis. Gene coexpression networks, composed of 17 distinct gene modules that contained between 25 and 813 genes, revealed high interconnectivity of the induced response and led to the identification of a number of hub genes enriched at different stages of infection. These results provide a comprehensive perspective on the global response of potato to S. endobioticum infection and identify a potential transcriptional regulatory network underlying this susceptible response, which contribute to a better understanding of the potato-S. endobioticum pathosystem.

Ascorbate peroxidase 1 confers resistance to southern corn leaf blight in maize

Southern corn leaf blight (SCLB), caused by Bipolaris maydis, is one of the most devastating diseases affecting maize production. However, only one SLCB resistance gene, conferring partial resistance, is currently known, underscoring the importance of isolating new SCLB resistance-related genes. Here, we performed a comparative proteomic analysis and identified 258 proteins showing differential abundance during the maize response to B. maydis. These proteins included an ascorbate peroxidase (Zea mays ascorbate peroxidase 1 (ZmAPX1)) encoded by a gene located within the mapping interval of a previously identified quantitative trait locus associated with SCLB resistance. ZmAPX1 overexpression resulted in lower H₂ O₂ accumulation and enhanced resistance against B. maydis. Jasmonic acid (JA) contents and transcript levels for JA biosynthesis and responsive genes increased in ZmAPX1-overexpressing plants infected with B. maydis, whereas Zmapx1 mutants showed the opposite effects. We further determined that low levels of H₂ O₂ are accompanied by an accumulation of JA that enhances SCLB resistance. These results demonstrate that ZmAPX1 positively regulates SCLB resistance by decreasing H₂ O₂ accumulation and activating the JA-mediated defense signaling pathway. This study identified ZmAPX1 as a potentially useful gene for increasing SCLB resistance. Furthermore, the generated data may be relevant for clarifying the functions of plant APXs.

Transcriptional repression of TaNOX10 by TaWRKY19 compromises ROS generation and enhances wheat susceptibility to stripe rust

Reactive oxygen species (ROS) are vital for plant immunity and regulation of their production is crucial for plant health. While the mechanisms that elicit ROS production have been relatively well studied, those that repress ROS generation are less well understood. Here, via screening Brachypodium distachyon RNA interference mutants, we identified BdWRKY19 as a negative regulator of ROS generation whose knockdown confers elevated resistance to the rust fungus Puccinia brachypodii. The three wheat paralogous genes TaWRKY19 are induced during infection by virulent P. striiformis f. sp. tritici (Pst) and have partially redundant roles in resistance. The stable overexpression of TaWRKY19 in wheat increased susceptibility to an avirulent Pst race, while mutations in all three TaWRKY19 copies conferred strong resistance to Pst by enhancing host plant ROS accumulation. We show that TaWRKY19 is a transcriptional repressor that binds to a W-box element in the promoter of TaNOX10, which encodes an NADPH oxidase and is required for ROS generation and host resistance to Pst. Collectively, our findings reveal that TaWRKY19 compromises wheat resistance to the fungal pathogen and suggest TaWRKY19 as a potential target to improve wheat resistance to the commercially important wheat stripe rust fungus.

Versatile effectors of phytopathogenic fungi target host immunity

Phytopathogenic fungi secrete a large arsenal of effector molecules, including proteinaceous effectors, small RNAs, phytohormones and derivatives thereof. The pathogenicity of fungal pathogens is primarily determined by these effectors that are secreted into host cells to undermine innate immunity, as well as to facilitate the acquisition of nutrients for their in planta growth and proliferation. After conventional and non-conventional secretion, fungal effectors are translocated into different subcellular compartments of the host cells to interfere with various biological processes. In extracellular spaces, apoplastic effectors cope with physical and chemical barriers to break the first line of plant defenses. Intracellular effectors target essential immune components on the plasma membrane, in the cytosol, including cytosolic organelles, and in the nucleus to suppress host immunity and reprogram host physiology, favoring pathogen colonization. In this review, we comprehensively summarize the recent advances in fungal effector biology, with a focus on the versatile virulence functions of fungal effectors in promoting pathogen infection and colonization. A perspective of future research on fungal effector biology is also discussed.

"The PLCP gene family of grapevine (Vitis vinifera L.): characterization and differential expression in response to Plasmopara Viticola"

BACKGROUND

Papain-like cysteine proteases (PLCPs), a large group of cysteine proteases, are structurally related to papain. The members belonging to PLCPs family contribute to plant immunity, senescence, and defense responses in plants. The PLCP gene family has been identified in Arabidopsis, rice, soybean, and cotton. However, no systematic analysis of PLCP genes has been undertaken in grapevine. Since Plasmopara viticola as a destructive pathogen could affect immunity of grapes in the field, we considered that the members belonged to PLCPs family could play a crucial role in defensive mechanisms or programmed cell death. We aimed to evaluate the role of PLCPs in 2 different varieties of grapevines and compared the changes of their expressions with the transcriptional data in response to P. viticola.

RESULTS

In this study, 23 grapevine PLCP (VvPLCP) genes were identified by comprehensive bioinformatics analysis. Subsequently, the chromosomal localizations, gene structure, conserved domains, phylogenetic relationship, gene duplication, and cis-acting elements were analyzed. Numerous cis-acting elements related to plant development, hormone, and stress responses were identified in the promoter of the VvPLCP genes. Phylogenetic analysis grouped the VvPLCP genes into nine subgroups. The transcription of VvPLCP in different inoculation time points and varieties indicated that VvPLCP may have vital functions in grapevine defense against Plasmopara viticola. According to transcriptome data and qPCR analysis, we observed the increasing expression levels of VvRD21-1 at 72 h after inoculation in resistant variety, inferring that it was related to grape downy mildew resistance. Meanwhile, 3 genes including VvXBCP1, VvSAG12-1, and VvALP1 showed higher expression at 24 h after pathogen inoculation in the susceptible variety and might be related to the downy mildew phenotype. We nominated these four genes to function during hypersensitive response (HR) process, inferring that these genes could be associated with downy mildew resistance in grapes.

CONCLUSIONS

Our results provide the reference for functional studies of PLCP gene family, and highlight its functions in grapevine defense against P. viticola. The results help us to better understand the complexity of the PLCP gene family in plant immunity and provide valuable information for future functional characterization of specific genes in grapevine.

Genome-Wide Identification and Characterization of the Cystatin Gene Family in Bread Wheat (Triticum aestivum L.)

Cystatins, as reversible inhibitors of papain-like and legumain proteases, have been identified in several plant species. Although the cystatin family plays crucial roles in plant development and defense responses to various stresses, this family in wheat (Triticum aestivum L.) is still poorly understood. In this study, 55 wheat cystatins (TaCystatins) were identified. All TaCystatins were divided into three groups and both the conserved gene structures and peptide motifs were relatively conserved within each group. Homoeolog analysis suggested that both homoeolog retention percentage and gene duplications contributed to the abundance of the TaCystatin family. Analysis of duplication events confirmed that segmental duplications played an important role in the duplication patterns. The results of codon usage pattern analysis showed that TaCystatins had evident codon usage bias, which was mainly affected by mutation pressure. TaCystatins may be regulated by cis-acting elements, especially abscisic acid and methyl jasmonate responsive elements. In addition, the expression of all selected TaCystatins was significantly changed following viral infection and cold stress, suggesting potential roles in response to biotic and abiotic challenges. Overall, our work provides new insights into TaCystatins during wheat evolution and will help further research to decipher the roles of TaCystatins under diverse stress conditions.

A clubroot pathogen effector targets cruciferous cysteine proteases to suppress plant immunity

Plant pathogen effector proteins are key to pathogen virulence. In susceptible host Brassicas, the clubroot pathogen, Plasmodiophora brassicae, induces the production of nutrient-sink root galls, at the site of infection. Among a list of 32 P. brassiae effector candidates previously reported by our group, we identified SSPbP53 as a putative apoplastic cystatin-like protein highly expressed during the secondary infection. Here we found that SSPbP53 encoding gene is conserved among several P. brassicae pathotypes and that SSPbP53 is an apoplastic protein able to directly interact with and inhibit cruciferous papain-like cysteine proteases (PLCPs), specifically Arabidopsis XYLEM CYSTEINE PEPTIDASE 1 (AtXCP1). The severity of clubroot disease is greatly reduced in the Arabidopsis xcp1 null mutant (AtΔxcp1) after infection with P. brassicae resting spores, indicating that the interaction of P. brassicae SSPbP53 with XCP1 is important to clubroot susceptibility. SSPbP53 is the first cystatin-like effector identified and characterized for a plant pathogenic protist.

Host apoplastic cysteine protease activity is suppressed during the mutualistic association of Lolium perenne and Epichloë festucae

Plants secrete various defence-related proteins into the apoplast, including proteases. Papain-like cysteine proteases (PLCPs) are central components of the plant immune system. To overcome plant immunity and successfully colonize their hosts, several plant pathogens secrete effector proteins inhibiting plant PLCPs. We hypothesized that not only pathogens, but also mutualistic microorganisms interfere with PLCP-meditated plant defences to maintain endophytic colonization with their hosts. Epichloë festucae forms mutualistic associations with cool season grasses and produces a range of secondary metabolites that protect the host against herbivores. In this study, we performed a genome-wide identification of Lolium perenne PLCPs, analysed their evolutionary relationship, and classified them into nine PLCP subfamilies. Using activity-based protein profiling, we identified four active PLCPs in the apoplast of L. perenne leaves that are inhibited during endophyte interactions. We characterized the L. perenne cystatin LpCys1 for its inhibitory capacity against ryegrass PLCPs. LpCys1 abundance is not altered during the mutualistic interaction and it mainly inhibits LpCP2. However, since the activity of other L. perenne PLCPs is not sensitive to LpCys1, we propose that additional inhibitors, likely of fungal origin, are involved in the suppression of apoplastic PLCPs during E. festucae infection.

Papain-like cysteine proteases are required for the regulation of photosynthetic gene expression and acclimation to high light stress

Chloroplasts are considered to be devoid of cysteine proteases. Using transgenic Arabidopsis lines expressing the rice cystatin, oryzacystatin I (OC-I), in the chloroplasts (PC lines) or cytosol (CYS lines), we explored the hypothesis that cysteine proteases regulate photosynthesis. The CYS and PC lines flowered later than the wild type (WT) and accumulated more biomass after flowering. In contrast to the PC rosettes, which accumulated more leaf chlorophyll and carotenoid pigments than the WT, the CYS lines had lower amounts of leaf pigments. High-light-dependent decreases in photosynthetic carbon assimilation and the abundance of the Rubisco large subunit protein, the D1 protein, and the phosphorylated form of D1 proteins were attenuated in the CYS lines and reversed in the PC lines relative to the WT. However, the transgenic lines had higher amounts of LHC, rbcs, pasbA, and pasbD transcripts than the WT, and also showed modified chloroplast to nucleus signalling. We conclude that cysteine proteases accelerate the reconfiguration of the chloroplast proteome after flowering and in response to high-light stress. Inhibition of cysteine proteases, such as AtCEP1, slows chloroplast protein degradation and stimulates photosynthetic gene expression and chloroplast to nucleus signalling, enhancing stress tolerance traits.

The front line of defence: a meta-analysis of apoplastic proteases in plant immunity

Secreted proteases act at the front line of defence and play pivotal roles in disease resistance. However, the criteria for apoplastic immune proteases are not always defined and followed. Here, we critically reviewed 46 apoplastic proteases that function in plant defence. We found that most apoplastic immune proteases are induced upon infection, and 17 proteases are genetically required for the immune response. Proteolytic activity has been confirmed for most of the proteases but is rarely shown to be required for biological function, and the apoplastic location of proteases can be subjective and dynamic. Pathogen-derived inhibitors have only been described for cysteine and serine proteases, and the selection pressure acting on immune proteases is rarely investigated. We discuss six different mechanisms by which these proteases mediate plant immunity and summarize the challenges for future research.

Brassica juncea leaf cuticle proteome analysis shows myrosinase protein, antifreeze activity, and post-translationally modified secretory proteins

Plant cuticle, the site of perception of stress signals, is an extracellular hydrophobic barrier that covers the epidermis of the above-ground parts. This lipidic layer has been explored for its cutin and wax composition. However, reports on the cuticle proteins are scanty. Therefore, leaf cuticle proteins of Brassica juncea isolated using organic solvents (chloroform-methanol, 2:1(v/v)) were analyzed using gel based and quantitative shotgun proteomics. Out of 615 proteins identified, 27% (169) had signal peptides supporting extracellular localization. Bioinformatics tool, QuickGO predicted the involvement of these proteins in catabolism (21%), peptidase activity (13%), oxidoreductase (12%), defense response (9%), fatty acid binding (9%), nutrient reservoir activity (8%), chitin binding (7%) and lipid transport (2%). Myrosinase-catalyzed glucosinolate hydrolysis releases bioactive compounds, which contribute to plant defense. This system is termed as "mustard oil bomb". Myrosinase and its associating protein, GDSL esterase/lipase ESM1 (involved in cuticle structuring and defense) were detected in the cuticle. GDSL-esterase/lipase ESM1 and β-glucanase (an antifreeze protein) showed in vitro activity. Analysis of cuticle extract by nanoliter osmometer-phase contrast microscopy detected antifreeze activity due to non-protein component. Post-translational modification analysis using PTM viewer predicted N-glycosylation (66%), N-terminal proteolysis (40%), and phosphorylation (32%) to be the dominant modification in the classical secretory proteins. N-glycosylation of myrosinase and GDSL esterase/lipase, ESM1 was confirmed by Con A affinoblotting. This study not only identified leaf cuticle proteins, but also laid the foundation for exploring the extracellular glucosinolate-myrosinase system, PTM crosstalk, and antifreeze activity as stress adaptive strategies in B. juncea.

Comparative proteomic analysis reveals insights into the dynamic responses of maize (Zea mays L.) to Setosphaeria turcica infection

Maize (Zea mays L.) production is severely affected by northern corn leaf blight (NCLB), which is a destructive foliar disease caused by Setosphaeria turcica. In recent years, studies on the interaction between maize and S. turcica have been focused at the transcription level, with no research yet at the protein level. Here, we applied tandem mass tag labelling and liquid chromatography-tandem mass spectrometry to investigate the proteomes of maize leaves at 24 h and 72 h post-inoculation (hpi) with S. turcica. In total, 4740 proteins encoded by 4711 genes were quantified in this study. Clustering analyses provided an understanding of the dynamic reprogramming of leaves proteomes by revealing the functions of different proteins during S. turcica infection. Screening and classification of differentially expressed proteins (DEPs) revealed that numerous defense-related proteins, including defense marker proteins and proteins related to the phenylpropanoid lignin biosynthesis, benzoxazine biosynthesis and the jasmonic acid signalling pathway, participated in the defense responses of maize to S. turcica infection. Furthermore, the earlier induction of GST family proteins contributed to the resistance to S. turcica. In addition, the protein-protein interaction network of DEPs suggests that some defense-related proteins, for example, ZmGEB1, a hub node, play key roles in defense responses against S. turcica infection. Our study findings provide insight into the complex responses triggered by S. turcica at the protein level and lay the foundation for studying the interaction process between maize and S. turcica infection.

The Ustilago hordei-Barley Interaction Is a Versatile System for Characterization of Fungal Effectors

Obligate biotrophic fungal pathogens, such as Blumeria graminis and Puccinia graminis, are amongst the most devastating plant pathogens, causing dramatic yield losses in many economically important crops worldwide. However, a lack of reliable tools for the efficient genetic transformation has hampered studies into the molecular basis of their virulence or pathogenicity. In this study, we present the Ustilago hordei-barley pathosystem as a model to characterize effectors from different plant pathogenic fungi. We generate U. hordei solopathogenic strains, which form infectious filaments without the presence of a compatible mating partner. Solopathogenic strains are suitable for heterologous expression system for fungal virulence factors. A highly efficient Crispr/Cas9 gene editing system is made available for U. hordei. In addition, U. hordei infection structures during barley colonization are analyzed using transmission electron microscopy, showing that U. hordei forms intracellular infection structures sharing high similarity to haustoria formed by obligate rust and powdery mildew fungi. Thus, U. hordei has high potential as a fungal expression platform for functional studies of heterologous effector proteins in barley.

Engineering Smut Resistance in Maize by Site-Directed Mutagenesis of LIPOXYGENASE 3

Biotic stresses caused by microbial pathogens impair crop yield and quality if not restricted by expensive and often ecologically problematic pesticides. For a sustainable agriculture of tomorrow, breeding or engineering of pathogen-resistant crop varieties is therefore a major cornerstone. Maize is one of the four most important cereal crops in the world. The biotrophic fungal pathogen Ustilago maydis causes galls on all aerial parts of the maize plant. Biotrophic pathogens like U. maydis co-evolved with their host plant and depend during their life cycle on successful manipulation of the host's cellular machinery. Therefore, removing or altering plant susceptibility genes is an effective and usually durable way to obtain resistance in plants. Transcriptional time course experiments in U. maydis-infected maize revealed numerous maize genes being upregulated upon establishment of biotrophy. Among these genes is the maize LIPOXYGENASE 3 (LOX3) previously shown to be a susceptibility factor for other fungal genera as well. Aiming to engineer durable resistance in maize against U. maydis and possibly other pathogens, we took a Cas endonuclease technology approach to generate loss of function mutations in LOX3. lox3 maize mutant plants react with an enhanced PAMP-triggered ROS burst implicating an enhanced defense response. Based on visual assessment of disease symptoms and quantification of relative fungal biomass, homozygous lox3 mutant plants exposed to U. maydis show significantly decreased susceptibility. U. maydis infection assays using a transposon mutant lox3 maize line further substantiated that LOX3 is a susceptibility factor for this important maize pathogen.

Fungal factors involved in host immune evasion, modulation and exploitation during infection

Human and plant pathogenic fungi have a major impact on public health and agriculture. Although these fungi infect very diverse hosts and are often highly adapted to specific host niches, they share surprisingly similar mechanisms that mediate immune evasion, modulation of distinct host targets and exploitation of host nutrients, highlighting that successful strategies have evolved independently among diverse fungal pathogens. These attributes are facilitated by an arsenal of fungal factors. However, not a single molecule, but rather the combined effects of several factors enable these pathogens to establish infection. In this review, we discuss the principles of human and plant fungal pathogenicity mechanisms and discuss recent discoveries made in this field.

Apoplastic Proteases: Powerful Weapons against Pathogen Infection in Plants

Plants associate with diverse microbes that exert beneficial, neutral, or pathogenic effects inside the host. During the initial stages of invasion, the plant apoplast constitutes a hospitable environment for invading microbes, providing both water and nutrients. In response to microbial infection, a number of secreted proteins from host cells accumulate in the apoplastic space, which is related to microbial association or colonization processes. However, the molecular mechanisms underlying plant modulation of the apoplast environment and how plant-secreted proteases are involved in pathogen resistance are still poorly understood. Recently, several studies have reported the roles of apoplastic proteases in plant resistance against bacteria, fungi, and oomycetes. On the other hand, microbe-secreted proteins directly and/or indirectly inhibit host-derived apoplastic proteases to promote infection. These findings illustrate the importance of apoplastic proteases in plant-microbe interactions. Therefore, understanding the protease-mediated apoplastic battle between hosts and pathogens is of fundamental importance for understanding plant-pathogen interactions. Here, we provide an overview of plant-microbe interactions in the apoplastic space. We define the apoplast, summarize the physical and chemical properties of these structures, and discuss the roles of plant apoplastic proteases and pathogen protease inhibitors in host-microbe interactions. Challenges and future perspectives for research into protease-mediated apoplastic interactions are discussed, which may facilitate the engineering of resistant crops.

Genome-wide transcriptional response of papain-like cysteine protease-mediated resistance against Xanthomonas oryzae pv. oryzae in rice

Transgenic rice overexpressing PLCP attenuated the virulence of Xanthomonas oryzae pv. oryzae through extensive activation of transduction signal and transcription activities that orchestrate downstream responses including the biosynthesis of secondary metabolites and up-regulation of several pathogenesis-related proteins. High-throughput transcriptome investigations of plant immunity highlight the complexity of gene networks leading to incompatible interaction with the pathogen. Accumulating findings implicate papain-like cysteine proteases (PLCPs) as a central hub in plant defense. While diverse roles of PLCPs in different pathosystems have become more evident, information on gene networks and signaling pathways necessary to orchestrate downstream responses are lacking. To understand the biological significance of cysteine protease against Xanthomonas oryzae pv. oryzae, PLCP overexpression and knockout rice lines were generated. The pathogenicity test revealed the attenuation of Xanthomonas oryzae pv. oryzae race K3a virulence in transgenic lines which is ascribed to high hydrogen peroxide and free salicylic acid accumulation. Next-generation sequencing of RNA from transgenic and wild-type plants identified 1597 combined differentially expressed genes, 1269 of which were exclusively regulated in the transgenic libraries. It was found that PLCP aids rice to circumvent infection through the extensive activation of transduction signal and transcription factors that orchestrate downstream responses, including up-regulation of multiple pathogenesis-related proteins and biosynthesis of secondary metabolites.

Smut fungal strategies for the successful infection

The smut fungi include a large number of plant pathogens that establish obligate biotrophic relationships with their host. Throughout the whole life inside plant tissue, smut fungi keep plant cells alive and acquire nutrients via biotrophic interfaces. This mini-review mainly summarizes the interactions between smut fungi and their host plants during the infection process. Despite various strategies recruited by plants to defense invading pathogens, smut fungi successfully evolved an arsenal for colonization. Mating of two compatible haploids gives rise to parasitic mycelium, which can sense plant surface cues such as fatty acids and hydrophobic surface, and induce the formation of appressoria for surface penetration. Plants can recognize fungal invading and activate defense response, including callose and lignin deposition, programmed cell death, and SA signaling pathway. To suppress plant immunity and alter the metabolic pathway of host plants, a cocktail of effectors is secreted by smut fungi depending on the plant organ and cell type that is infected.

Comparative proteomics combined with analyses of transgenic plants reveal ZmREM1.3 mediates maize resistance to southern corn rust

Southern corn rust (SCR), which is a destructive disease caused by Puccinia polysora Underw. (P. polysora), commonly occurs in warm-temperate and tropical regions. To identify candidate proteins related to SCR resistance and characterize the molecular mechanisms underlying the maize-P. polysora interaction, a comparative proteomic analysis of susceptible and resistant maize lines was performed. Statistical analyses revealed 1489 differentially abundant proteins in the resistant line, as well as 1035 differentially abundant proteins in the susceptible line. After the P. polysora infection, the abundance of one remorin protein (ZmREM1.3) increased in the resistant genotype, but decreased in the susceptible genotype. Plant-specific remorins are important for responses to microbial infections as well as plant signalling processes. In this study, transgenic maize plants overexpressing ZmREM1.3 exhibited enhanced resistance to the biotrophic P. polysora. In contrast, homozygous ZmREM1.3 UniformMu mutant plants were significantly more susceptible to P. polysora than wild-type plants. Additionally, the ZmREM1.3-overexpressing plants accumulated more salicylic acid (SA) and jasmonic acid (JA). Moreover, the expression levels of defence-related genes were higher in ZmREM1.3-overexpressing maize plants than in non-transgenic control plants in response to the P. polysora infection. Overall, our results provide evidence that ZmREM1.3 positively regulates maize defences against P. polysora likely via SA/JA-mediated defence signalling pathways. This study represents the first large-scale proteomic analysis of the molecular mechanisms underlying the maize-P. polysora interaction. This is also the first report confirming the remorin protein family affects plant resistance to SCR.

Maize phenylalanine ammonia-lyases contribute to resistance to Sugarcane mosaic virus infection, most likely through positive regulation of salicylic acid accumulation

Sugarcane mosaic virus (SCMV) is a pathogen of worldwide importance that causes dwarf mosaic disease on maize (Zea mays). Until now, few maize genes/proteins have been shown to be involved in resistance to SCMV. In this study, we characterized the role of maize phenylalanine ammonia-lyases (ZmPALs) in accumulation of the defence signal salicylic acid (SA) and in resistance to virus infection. SCMV infection significantly increased SA accumulation and expression of SA-responsive pathogenesis-related protein genes (PRs). Interestingly, exogenous SA treatment decreased SCMV accumulation and enhanced resistance. Both reverse transcription-coupled quantitative PCR and RNA-Seq data confirmed that expression levels of at least four ZmPAL genes were significantly up-regulated upon SCMV infection. Knockdown of ZmPAL expression led to enhanced SCMV infection symptom severity and virus multiplication, and simultaneously resulted in decreased SA accumulation and PR gene expression. Intriguingly, application of exogenous SA to SCMV-infected ZmPAL-silenced maize plants decreased SCMV accumulation, showing that ZmPALs are required for SA-mediated resistance to SCMV infection. In addition, lignin measurements and metabolomic analysis showed that ZmPALs are also involved in SCMV-induced lignin accumulation and synthesis of other secondary metabolites via the phenylpropanoid pathway. In summary, our results indicate that ZmPALs are required for SA accumulation in maize and are involved in resistance to virus infection by limiting virus accumulation and moderating symptom severity.

Molecular Interactions Between Smut Fungi and Their Host Plants

Smut fungi are a large group of biotrophic plant pathogens that infect mostly monocot species, including economically relevant cereal crops. For years, Ustilago maydis has stood out as the model system to study the genetics and cell biology of smut fungi as well as the pathogenic development of biotrophic plant pathogens. The identification and functional characterization of secreted effectors and their role in virulence have particularly been driven forward using the U. maydis-maize pathosystem. Today, advancing tools for additional smut fungi such as Ustilago hordei and Sporisorium reilianum, as well as an increasing number of available genome sequences, provide excellent opportunities to investigate in parallel the effector function and evolution associated with different lifestyles and host specificities. In addition, genome analyses revealed similarities in the genomic signature between pathogenic smuts and epiphytic Pseudozyma species. This review elaborates on how knowledge about fungal lifestyles, genome biology, and functional effector biology has helped in understanding the biology of this important group of fungal pathogens. We highlight the contribution of the U. maydis model system but also discuss the differences from other smut fungi, which raises the importance of comparative genomic and genetic analyses in future research.

Cell type specific transcriptional reprogramming of maize leaves during Ustilago maydis induced tumor formation

Ustilago maydis is a biotrophic pathogen and well-established genetic model to understand the molecular basis of biotrophic interactions. U. maydis suppresses plant defense and induces tumors on all aerial parts of its host plant maize. In a previous study we found that U. maydis induced leaf tumor formation builds on two major processes: the induction of hypertrophy in the mesophyll and the induction of cell division (hyperplasia) in the bundle sheath. In this study we analyzed the cell-type specific transcriptome of maize leaves 4 days post infection. This analysis allowed identification of key features underlying the hypertrophic and hyperplasic cell identities derived from mesophyll and bundle sheath cells, respectively. We examined the differentially expressed (DE) genes with particular focus on maize cell cycle genes and found that three A-type cyclins, one B-, D- and T-type are upregulated in the hyperplasic tumorous cells, in which the U. maydis effector protein See1 promotes cell division. Additionally, most of the proteins involved in the formation of the pre-replication complex (pre-RC, that assure that each daughter cell receives identic DNA copies), the transcription factors E2F and DPa as well as several D-type cyclins are deregulated in the hypertrophic cells.

A fungal substrate mimicking molecule suppresses plant immunity via an inter-kingdom conserved motif

Ustilago maydis is a biotrophic fungus causing corn smut disease in maize. The secreted effector protein Pit2 is an inhibitor of papain-like cysteine proteases (PLCPs) essential for virulence. Pit2 inhibitory function relies on a conserved 14 amino acids motif (PID14). Here we show that synthetic PID14 peptides act more efficiently as PLCP inhibitors than the full-length Pit2 effector. Mass spectrometry shows processing of Pit2 by maize PLCPs, which releases an inhibitory core motif from the PID14 sequence. Mutational analysis demonstrates that two conserved residues are essential for Pit2 function. We propose that the Pit2 effector functions as a substrate mimicking molecule: Pit2 is a suitable substrate for apoplastic PLCPs and its processing releases the embedded inhibitor peptide, which in turn blocks PLCPs to modulate host immunity. Remarkably, the PID14 core motif is present in several plant associated fungi and bacteria, indicating the existence of a conserved microbial inhibitor of proteases (cMIP).

Manipulation of Phytohormone Pathways by Effectors of Filamentous Plant Pathogens

Phytohormones regulate a large variety of physiological processes in plants. In addition, salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are responsible for primary defense responses against abiotic and biotic stresses, while plant growth regulators, such as auxins, brassinosteroids (BRs), cytokinins (CKs), abscisic acid (ABA), and gibberellins (GAs), also contribute to plant immunity. To successfully colonize plants, filamentous pathogens like fungi and oomycetes have evolved diverse strategies to interfere with phytohormone pathways with the help of secreted effectors. These include proteins, toxins, polysaccharides as well as phytohormones or phytohormone mimics. Such pathogen effectors manipulate phytohormone pathways by directly altering hormone levels, by interfering with phytohormone biosynthesis, or by altering or blocking important components of phytohormone signaling pathways. In this review, we outline the various strategies used by filamentous phytopathogens to manipulate phytohormone pathways to cause disease.

Proteases Underground: Analysis of the Maize Root Apoplast Identifies Organ Specific Papain-Like Cysteine Protease Activity

Plant proteases are key regulators of plant cell processes such as seed development, immune responses, senescence and programmed cell death (PCD). Apoplastic papain-like cysteine proteases (PL) are hubs in plant-microbe interactions and play an important role during abiotic stresses. The apoplast is a crucial interface for the interaction between plant and microbes. So far, apoplastic maize PL and their function have been mostly described for aerial parts. In this study, we focused on apoplastic PLCPs in the roots of maize plants. We have analyzed the phylogeny of maize PLCPs and investigated their protein abundance after salicylic acid (SA) treatment. Using activity-based protein profiling (ABPP) we have identified a novel root-specific PLCP belonging to the RD21-like subfamily, as well as three SA activated PLCPs. The root specific PLCP CP1C shares sequence and structural similarities to known CP1-like proteases. Biochemical analysis of recombinant CP1C revealed different substrate specificities and inhibitor affinities compared to the related proteases. This study characterized a root-specific PLCP and identifies differences between the SA-dependent activation of PLCPs in roots and leaves.

Differential response of silencing HvIcy2 barley plants against Magnaporthe oryzae infection and light deprivation

BACKGROUND

Phytocystatins (PhyCys) act as endogenous regulators of cysteine proteases (CysProt) involved in various physiological processes. Besides, PhyCys are involved in plant reactions to abiotic stresses like drought or darkness and have been used as effective molecules against different pests and pathogens. The barley PhyCys-CysProt system is considered a model of protease-inhibitor regulation of protein turnover. Thirteen barley cystatins (HvCPI-1 to HvCPI-13) have been previously identified and characterized. Among them HvCPI-2 has been shown to have a relevant role in plant responses to pathogens and pests, as well as in the plant response to drought.

RESULTS

The present work explores the multiple role of this barley PhyCys in response to both, biotic and abiotic stresses, focusing on the impact of silencing this gene. HvIcy-2 silencing lines behave differentially against the phytopathogenic fungus Magnaporthe oryzae and a light deprivation treatment. The induced expression of HvIcy-2 by the fungal stress correlated to a higher susceptibility of silencing HvIcy-2 plants. In contrast, a reduction in the expression of HvIcy-2 and in the cathepsin-L and -B like activities in the silencing HvIcy-2 plants was not accompanied by apparent phenotypical differences with control plants in response to light deprivation.

CONCLUSION

These results highlight the specificity of PhyCys in the responses to diverse external prompts as well as the complexity of the regulatory events leading to the response to a particular stress. The mechanism of regulation of these stress responses seems to be focused in maintaining the balance of CysProt and PhyCys levels, which is crucial for the modulation of physiological processes induced by biotic or abiotic stresses.

The cloak, dagger, and shield: proteases in plant-pathogen interactions

Plants sense the presence of pathogens or pests through the recognition of evolutionarily conserved microbe- or herbivore-associated molecular patterns or specific pathogen effectors, as well as plant endogenous danger-associated molecular patterns. This sensory capacity is largely mediated through plasma membrane and cytosol-localized receptors which trigger complex downstream immune signaling cascades. As immune signaling outputs are often associated with a high fitness cost, precise regulation of this signaling is critical. Protease-mediated proteolysis represents an important form of pathway regulation in this context. Proteases have been widely implicated in plant-pathogen interactions, and their biochemical mechanisms and targets continue to be elucidated. During the plant and pathogen arms race, specific proteases are employed from both the plant and the pathogen sides to contribute to either defend or invade. Several pathogen effectors have been identified as proteases or protease inhibitors which act to functionally defend or camouflage the pathogens from plant proteases and immune receptors. In this review, we discuss known protease functions and protease-regulated signaling processes involved in both sides of plant-pathogen interactions.

Genome wide association study for gray leaf spot resistance in tropical maize core

Gray leaf spot is a maize foliar disease with worldwide distribution and can drastically reduce the production in susceptible genotypes. Published works indicate that resistance to gray leaf spot is a complex trait controlled by multiple genes, with additive effect and influenced by environment. The aim of this study was to identify genomic regions, including putative genes, associated with resistance to gray leaf spot under natural conditions of disease occurrence. A genome wide association study was conducted with 355,972 single nucleotide polymorphism markers on a phenotypic data composed by 157 tropical maize inbred lines, evaluated at Maringá -Brazil. Seven single nucleotide polymorphisms were significantly associated with gray leaf spot, some of which were localized to previously reported quantitative trait loci regions. Three gene models linked to the associated single nucleotide polymorphism were expressed at flowering time and tissue related with gray leaf spot infection, explaining a considerable proportion of the phenotypic variance, ranging from 0.34 to 0.38. The gene model GRMZM2G073465 (bin 10.07) encodes a cysteine protease3 protein, gene model GRMZM2G007188 (bin 1.02) expresses a rybosylation factor-like protein and the gene model GRMZM2G476902 (bin 4.08) encodes an armadillo repeat protein. These three proteins are related with plant defense pathway. Once these genes are validated in next studies, they will be useful for marker-assisted selection and can help improve the understanding of maize resistance to gray leaf spot.

Pathogen Trojan Horse Delivers Bioactive Host Protein to Alter Maize Anther Cell Behavior in Situ

Small proteins are crucial signals during development, host defense, and physiology. The highly spatiotemporal restricted functions of signaling proteins remain challenging to study in planta. The several month span required to assess transgene expression, particularly in flowers, combined with the uncertainties from transgene position effects and ubiquitous or overexpression, makes monitoring of spatiotemporally restricted signaling proteins lengthy and difficult. This situation could be rectified with a transient assay in which protein deployment is tightly controlled spatially and temporally in planta to assess protein functions, timing, and cellular targets as well as to facilitate rapid mutagenesis to define functional protein domains. In maize (Zea mays), secreted ZmMAC1 (MULTIPLE ARCHESPORIAL CELLS1) was proposed to trigger somatic niche formation during anther development by participating in a ligand-receptor module. Inspired by Homer's Trojan horse myth, we engineered a protein delivery system that exploits the secretory capabilities of the maize smut fungus Ustilago maydis, to allow protein delivery to individual cells in certain cell layers at precise time points. Pathogen-supplied ZmMAC1 cell-autonomously corrected both somatic cell division and differentiation defects in mutant Zmmac1-1 anthers. These results suggest that exploiting host-pathogen interactions may become a generally useful method for targeting host proteins to cell and tissue types to clarify cellular autonomy and to analyze steps in cell responses.

An apoplastic peptide activates salicylic acid signalling in maize

Localized control of cell death is crucial for the resistance of plants to pathogens. Papain-like cysteine proteases (PLCPs) regulate plant defence to drive cell death and protection against biotrophic pathogens. In maize (Zea mays), PLCPs are crucial in the orchestration of salicylic acid (SA)-dependent defence signalling. Despite this central role in immunity, it remains unknown how PLCPs are activated, and which downstream signals they induce to trigger plant immunity. Here, we discover an immune signalling peptide, Z. mays immune signalling peptide 1 (Zip1), which is produced after salicylic acid (SA) treatment. In vitro studies demonstrate that PLCPs are required to release bioactive Zip1 from its propeptide precursor. Conversely, Zip1 treatment strongly elicits SA accumulation in leaves. Moreover, transcriptome analyses revealed that Zip1 and SA induce highly overlapping transcriptional changes. Consequently, Zip1 promotes the infection of the necrotrophic fungus Botrytis cinerea, while it reduces virulence of the biotrophic fungus Ustilago maydis. Thus, Zip1 represents the previously missing signal that is released by PLCPs to activate SA defence signalling.

Indispensable Role of Proteases in Plant Innate Immunity

Plant defense is achieved mainly through the induction of microbe-associated molecular patterns (MAMP)-triggered immunity (MTI), effector-triggered immunity (ETI), systemic acquired resistance (SAR), induced systemic resistance (ISR), and RNA silencing. Plant immunity is a highly complex phenomenon with its own unique features that have emerged as a result of the arms race between plants and pathogens. However, the regulation of these processes is the same for all living organisms, including plants, and is controlled by proteases. Different families of plant proteases are involved in every type of immunity: some of the proteases that are covered in this review participate in MTI, affecting stomatal closure and callose deposition. A large number of proteases act in the apoplast, contributing to ETI by managing extracellular defense. A vast majority of the endogenous proteases discussed in this review are associated with the programmed cell death (PCD) of the infected cells and exhibit caspase-like activities. The synthesis of signal molecules, such as salicylic acid, jasmonic acid, and ethylene, and their signaling pathways, are regulated by endogenous proteases that affect the induction of pathogenesis-related genes and SAR or ISR establishment. A number of proteases are associated with herbivore defense. In this review, we summarize the data concerning identified plant endogenous proteases, their effect on plant-pathogen interactions, their subcellular localization, and their functional properties, if available, and we attribute a role in the different types and stages of innate immunity for each of the proteases covered.

Role of Papain-Like Cysteine Proteases in Plant Development

Papain-like cysteine proteases (PLCP) are prominent peptidases found in most living organisms. In plants, PLCPs was divided into nine subgroups based on functional and structural characterization. They are key enzymes in protein proteolysis and involved in numerous physiological processes. In this paper, we reviewed the updated achievements of physiological roles of plant PLCPs in germination, development, senescence, immunity, and stress responses.