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

Alpine and subalpine plant microbiome mediated plants adapt to the cold environment: A systematic review

With global climate change, ecosystems are affected, some of which are more vulnerable than others, such as alpine ecosystems. Microbes play an important role in environmental change in global ecosystems. Plants and microbes are tightly associated, and symbiotic or commensal microorganisms are crucial for plants to respond to stress, particularly for alpine plants. The current study of alpine and subalpine plant microbiome only stays at the community structure scale, but its ecological function and mechanism to help plants to adapt to the harsh environments have not received enough attention. Therefore, it is essential to systematically understand the structure, functions and mechanisms of the microbial community of alpine and subalpine plants, which will be helpful for the conservation of alpine and subalpine plants using synthetic microbial communities in the future. This review mainly summarizes the research progress of the alpine plant microbiome and its mediating mechanism of plant cold adaptation from the following three perspectives: (1) Microbiome community structure and their unique taxa of alpine and subalpine plants; (2) The role of alpine and subalpine plant microbiome in plant adaptation to cold stress; (3) Mechanisms by which the microbiome of alpine and subalpine plants promotes plant adaptation to low-temperature environments. Finally, we also discussed the future application of high-throughput technologies in the development of microbial communities for alpine and subalpine plants. The existing knowledge could improve our understanding of the important role of microbes in plant adaptation to harsh environments. In addition, perspective further studies on microbes' function confirmation and microbial manipulations in microbiome engineering were also discussed.

In silico characterization, molecular docking, and dynamic simulation of a novel fungal cell-death suppressing effector, MoRlpA as potential cathepsin B-like cysteine protease inhibitor during rice blast infection

The blast fungus Magnaporthe oryzae is one of the most notorious pathogens affecting rice production worldwide. The cereal killer employs a special class of small secreted proteins called effectors to manipulate and perturb the host metabolism. In turn, the host plants trigger effector-triggered immunity (ETI) via localized cell death and hypersensitive response (HR). We have identified and characterized a novel secreted effector MoRlpA from M. oryzae by extensive in silico methods. The localization studies suggested that it is exclusively secreted in the host apoplasts. Interestingly, MoRlpA interacts with a protease, cathepsin B from rice with highest affinity. The 3D structural models of both the proteins were generated. Cathepsin B-like cysteine proteases are usually involved in programmed cell death (PCD) and autophagy in plants which lead to generation of HR upon infection. Our results suggest that MoRlpA interacts with rice cathepsin B-like cysteine protease and demolish the host counter-attack by suppressing cell death and HR during an active blast infection. This was further validated by molecular docking and molecular dynamic simulation analyses. The important residues involved in the rice-blast pathogen interactions were deciphered. Overall, this research highlights stable interactions between MoRlpA-OsCathB during rice blast pathogenesis and providing an insight into how this novel RlpA protease inhibitor-cum-effector modulates the host's apoplast to invade the host tissues and establish a successful infection. Thus, this research will help to develop potential fungicide to block the binding region of MoRlpA target so that the cryptic pathogen would be recognized by the host. HIGHLIGHTSFor the first time, a novel secreted effector protein, MoRlpA has been identified and characterised from M. oryzae in silicoMoRlpA contains a rare lipoprotein A-like DPBB domain which is often an enzymatic domain in other systemsMoRlpA as an apoplastic effector interacts with the rice protease OsCathB to suppress the cell death and hypersensitive response during rice blast infectionThe three-dimensional structures of both the MoRlpA and OsCathB proteins were predictedMoRlpA-OsCathB interactions were analysed by molecular docking and molecular dynamic simulation studiesCommunicated by Ramaswamy H. Sarma.

Arabidopsis subtilases promote defense-related pectin methylesterase activity and robust immune responses to botrytis infection

Plants involve a fine modulation of pectin methylesterase (PME) activity against microbes. PME activity can promote the cell wall stiffening and the production of damage signals able to induce defense responses and plant resistance to pathogens. However, the molecular mechanisms underlying PME activation during disease remain largely unknown. In this study, we explored the role of subtilases (SBTs) as PME activators in Arabidopsis immunity. By using biochemical and reverse genetic approaches, we found that the expression of SBT3.3 and SBT3.5 influences the induction of defense-related PME activity and resistance to the fungus Botrytis cinerea. Arabidopsis sbt3.3 and sbt3.5 knockout mutants showed decreased induction of PME activity and increased susceptibility to the fungus. SBT3.3 expression was stimulated by oligogalacturonides. Overexpression of SBT3.3 overactivated PME activity during fungal infection and enhanced resistance to B. cinerea. A negative correlation was observed between SBT3.3 expression and cell wall methyl ester content in the genotypes analyzed after B. cinerea infection. Increased expression of defense-related genes, including PAD3, CYP81F2 and WAK2, was also revealed in SBT3.3 overexpressing lines. We also demonstrated that SBT3.3 and pro-PME17 are both secreted into the cell wall using distinct protein secretion pathways and different kinetics. Our results propose SBT3.3 and SBT3.5 as modulators of PME activity in Arabidopsis against Botrytis to promptly boost immunity limiting the growth-defense trade-off.

Pleiotropic effects of recombinant protease inhibitors in plants

Recombinant gene encoded protease inhibitors have been identified as some of the most effective antidigestive molecules to guard against proteolysis of essential proteins and plant attacking proteases from herbivorous pests and pathogenic microorganisms. Protease inhibitors (PIs) can be over expressed in transgenic plants to complement internal host defense systems, Bt toxins in genetically modified pest resistance and abiotic stress tolerance achieved through cystatins expression. Although the understanding of the role of proteolytic enzymes and their inhibitors encoded by both endogenous and transgenes expressed in crop plants has significantly advanced, their implication in biological systems still requires further elucidations. This paper, therefore, succinctly reviewed most recently published literature on recombinant proteases inhibitors (RPIs), focusing mainly on their unintended consequences in plants, other living organisms, and the environment. The review discusses major negative and unintended effects of RPIs involving the inhibitors' non-specificity on protease enzymes, non-target organisms and ubiquitous versatility in their mechanism of inhibition. The paper also discusses some direct and indirect effects of RPIs such as degradation by distinct classes of proteases, reduced functionality due to plant exposure to severe environmental stress and any other potential negative influences exerted on both the host plant as well as the environment. These pleiotropic effects must be decisively monitored to eliminate and prevent any potential adverse effects that transgenic plants carrying recombinant inhibitor genes may have on non-target organisms and biodiversity.

Differing Responses to Phytophthora cinnamomi Infection in Susceptible and Partially Resistant Persea americana (Mill.) Rootstocks: A Case for the Role of Receptor-Like Kinases and Apoplastic Proteases

The hemibiotrophic plant pathogen Phytophthora cinnamomi Rands is the most devastating pathogen of avocado (Persea americana Mill.) and, as such, causes significant annual losses in the industry. Although the molecular basis of P. cinnamomi resistance in avocado and P. cinnamomi virulence determinants have been the subject of recent research, none have yet attempted to compare the transcriptomic responses of both pathogen and host during their interaction. In the current study, the transcriptomes of both avocado and P. cinnamomi were explored by dual RNA sequencing. The basis for partial resistance was sought by the inclusion of both susceptible (R0.12) and partially resistant (Dusa®) rootstocks sampled at early (6, 12 and 24 hours post-inoculation, hpi) and late time-points (120 hpi). Substantial differences were noted in the number of differentially expressed genes found in Dusa® and R0.12, specifically at 12 and 24 hpi. Here, the partially resistant rootstock perpetuated defense responses initiated at 6 hpi, while the susceptible rootstock abruptly reversed course. Instead, gene ontology enrichment confirmed that R0.12 activated pathways related to growth and development, essentially rendering its response at 12 and 24 hpi no different from that of the mock-inoculated controls. As expected, several classes of P. cinnamomi effector genes were differentially expressed in both Dusa® and R0.12. However, their expression differed between rootstocks, indicating that P. cinnamomi might alter the expression of its effector arsenal based on the rootstock. Based on some of the observed differences, several P. cinnamomi effectors were highlighted as potential candidates for further research. Similarly, the receptor-like kinase (RLK) and apoplastic protease coding genes in avocado were investigated, focusing on their potential role in differing rootstock responses. This study suggests that the basis of partial resistance in Dusa® is predicated on its ability to respond appropriately during the early stages following P. cinnamomi inoculation, and that important components of the first line of inducible defense, apoplastic proteases and RLKs, are likely to be important to the observed outcome.

Decoy engineering of the receptor-like cytoplasmic kinase StPBS1 to defend against virus infection in potato

Potato virus Y (PVY) is an important pathogen of potato (Solanum tuberosum). Although the PBS1-RPS5 immune system is well documented in Arabidopsis thaliana, it has not been reported in potato. In Arabidopsis, the bacterial effector AvrPphB cleaves AtPBS1 to trigger an immune response. Here, we show that the AvrPphB-triggered immune response is mediated by StPBS1, a close homologue of AtPBS1 in potato. However, downstream signalling of StPBS1 was mediated by unknown resistance (R) proteins other than potato orthologues of AtRPS5 and HvPBR1, which is important for HvPBS1 signalling in barley. Immune signalling of StPBS1 is mediated by the AvrPphB C-terminal cleavage domain and an STKPQ motif, in contrast to AtPBS1-mediated immunity in which both AvrPphB cleavage fragments and an SEMPH motif are essential. The cleavage sequence of AvrPphB in StPBS1 was replaced with that of the PVY NIa-Pro protease to obtain StPBS1^NIa . StPBS1^NIa overexpression potato displayed stronger immunity to PVY infection than did the StPBS1 transgenic lines. StPBS1^NIa was cleaved at the expected target site by NIa-Pro protease from PVY. Thus, we characterized the function of StPBS1 in potato immunity and provide a biotechnology control method for PVY via transformation of decoy-engineered StPBS1^NIa .

The grapevine aspartic protease gene family: characterization and expression modulation in response to Plasmopara viticola

Grapevine aspartic proteases gene family is characterized and five VviAPs appear to be involved in grapevine defense against downy mildew. Grapevine (Vitis vinifera L.) is one of the most important crops worldwide. However, it is highly susceptible to the downy mildew disease caused by Plasmopara viticola (Berk. & Curt.) Berl. & De Toni. To minimize the use of fungicides used to control P. viticola, it is essential to gain a deeper comprehension on this pathosystem and proteases have gained particular interest in the past decade. Proteases were shown to actively participate in plant-pathogen interactions, not only in the processes that lead to plant cell death, stress responses and protein processing/degradation but also as components of the recognition and signalling pathways. The aim of this study was to identify and characterize the aspartic proteases (APs) involvement in grapevine defense against P. viticola. A genome-wide search and bioinformatics characterization of the V. vinifera AP gene family was conducted and a total of 81 APs proteins, coded by 65 genes, were found. VviAPs proteins can be divided into three categories, similar to those previously described for other plants. Twelve APs coding genes were selected, and expression analysis was conducted at several time-points after inoculation in both compatible and incompatible interactions. Five grapevine APs may be involved in grapevine tolerance against P. viticola. Our findings provide an overall understanding of the VviAPs gene family and establish better groundwork to further describe the roles of VviAPs in defense against P. viticola.

New genomic regions associated with white mold resistance in dry bean using a MAGIC population

Dry bean (Phaseolus vulgaris L.) production in many regions is threatened by white mold (WM) [Sclerotinia sclerotiorum (Lib.) de Bary]. Seed yield losses can be up to 100% under conditions favorable for the pathogen. The low heritability, polygenic inheritance, and cumbersome screening protocols make it difficult to breed for improved genetic resistance. Some progress in understanding genetic resistance and germplasm improvement has been accomplished, but cultivars with high levels of resistance are yet to be released. A WM multiparent advanced generation inter-cross (MAGIC) population (n = 1060) was developed to facilitate mapping and breeding efforts. A seedling straw test screening method provided a quick assay to phenotype the population for response to WM isolate 1980. Nineteen MAGIC lines were identified with improved resistance. For genome-wide association studies (GWAS), the data was transformed into three phenotypic distributions-quantitative, polynomial, and binomial-and coupled with ∼52,000 single-nucleotide polymorphisms (SNPs). The three phenotypic distributions identified 30 significant genomic intervals [-log₁₀ (P value) ≥ 3.0]. However, across distributions, four new genomic regions as well as two regions previously reported were found to be associated with resistance. Cumulative R² values were 57% for binomial distribution using 13 genomic intervals, 41% for polynomial using eight intervals, and 40% for quantitative using 11 intervals. New resistant germplasm as well as new genomic regions associated with resistance are now available for further investigation.

Alterations in the Transcriptome of Rye Plants following the Microdochium nivale Infection: Identification of Resistance/Susceptibility-Related Reactions Based on RNA-Seq Analysis

Microdochium nivale is a progressive and devastating phytopathogen that causes different types of cereal crop and grass diseases that are poorly characterized at the molecular level. Although rye (Secale cereale L.) is one of the most resistant crops to most of the phytopathogens, it is severely damaged by M. nivale. The recent high-quality chromosome-scale assembly of rye genome has improved whole-genome studies of this crop. In the present work, the first transcriptome study of the M. nivale-infected crop plant (rye) with the detailed functional gene classification was carried out, along with the physiological verification of the RNA-Seq data. The results revealed plant reactions that contributed to their resistance or susceptibility to M. nivale. Phytohormone abscisic acid was shown to promote plant tolerance to M. nivale. Flavonoids were proposed to contribute to plant resistance to this pathogen. The upregulation of plant lipase encoding genes and the induction of lipase activity in M. nivale-infected plants revealed in our study were presumed to play an important role in plant susceptibility to the studied phytopathogen. Our work disclosed important aspects of plant-M. nivale interactions, outlined the directions for future studies on poorly characterized plant diseases caused by this phytopathogen, and provided new opportunities to improve cereals breeding and food security strategies.

Advances in functional proteomics to study plant-pathogen interactions

Pathogen infection triggers complex signaling networks in plant cells that ultimately result in either susceptibility or resistance. We have made substantial progress in dissecting many of these signaling events, and it is becoming clear that changes in proteome composition and protein activity are major drivers of plant-microbe interactions. Here, we highlight different approaches to analyze the functional proteomes of hosts and pathogens and discuss how they have been used to further our understanding of plant disease. Global proteome profiling can quantify the dynamics of proteins, posttranslational modifications, and biological pathways that contribute to immune-related outcomes. In addition, emerging techniques such as enzyme activity-based profiling, proximity labeling, and kinase-substrate profiling are being used to dissect biochemical events that operate during infection. Finally, we discuss how these functional approaches can be integrated with other profiling data to gain a mechanistic, systems-level view of plant and pathogen signaling.

Phytocytokines function as immunological modulators of plant immunity

Plant plasma membrane-resident immune receptors regulate plant immunity by recognizing microbe-associated molecular patterns (MAMPs), damage-associated molecular patterns (DAMPs), and phytocytokines. Phytocytokines are plant endogenous peptides, which are usually produced in the cytosol and released into the apoplast when plant encounters pathogen infections. Phytocytokines regulate plant immunity through activating an overlapping signaling pathway with MAMPs/DAMPs with some unique features. Here, we highlight the current understanding of phytocytokine production, perception and functions in plant immunity, and discuss how plants and pathogens manipulate phytocytokine signaling for their own benefits during the plant-pathogen warfare.

A rhomboid-like protease gene from an interspecies translocation confers resistance to cyst nematodes

Plant-parasitic nematodes are severe pests in crop production worldwide. Chemical control of nematodes has been continuously reduced in recent decades owing to environmental and health concerns. Therefore, breeding nematode-resistant crops is an important aim if we are to secure harvests. The beet cyst nematode impairs root development and causes severe losses in sugar beet production. The only sources for resistance are distantly related wild species of the genus Patellifolia. Nematode resistance had been introduced into the beet genome via translocations from P. procumbens. We sequenced three translocations and identified the translocation breakpoints. By comparative sequence analysis of three translocations, we localized the resistance gene Hs4 within a region c. 230 kb in size. A candidate gene was characterized by CRISPR-Cas-mediated knockout and overexpression in susceptible roots. The gene encodes a rhomboid-like protease, which is predicted to be bound to the endoplasmic reticulum. Gene knockout resulted in complete loss of resistance, while overexpression caused resistance. The data confirm that the Hs4 gene alone protects against the pest. Thus, it constitutes a previously unknown mechanism of plants to combat parasitic nematodes. Its function in a nonrelated species suggests that the gene can confer resistance in crop species from different plant families.

Stress-induced activation of receptor signaling by protease-mediated cleavage

Plants encode a large number of proteases in activating intracellular signaling through proteolytic cleavages of various protein substrates. One type of the substrates is proligands, including peptide hormones, which are perceived by cell surface-resident receptors. The peptide hormones are usually first synthesized as propeptides, and then cleaved by specific proteases for activation. Accumulating evidence indicates that the protease-mediated cleavage of proligands can be triggered by environmental stresses and subsequently activates plant stress signaling. In this perspective, we highlight several recent publications and provide an update about stress-induced cleavage of propeptides and receptor-associated components by proteases in the activation of cell surface-resident receptor signaling in plants. We also discuss some questions and future challenges in the research of protease functions in plant stress response.

Cutting the line: manipulation of plant immunity by bacterial type III effector proteases

Pathogens and their hosts are engaged in an evolutionary arms race. Pathogen-derived effectors promote virulence by targeting components of a host's innate immune system, while hosts have evolved proteins that sense effectors and trigger a pathogen-specific immune response. Many bacterial effectors are translocated into host cells using type III secretion systems. Type III effector proteases irreversibly modify host proteins by cleavage of peptide bonds and are prevalent among both plant and animal bacterial pathogens. In plants, the study of model effector proteases has yielded important insights into the virulence mechanisms employed by pathogens to overcome their host's immune response, as well as into the mechanisms deployed by their hosts to detect these effector proteases and counteract their effects. In recent years, the study of a larger number of effector proteases, across a wider range of pathogens, has yielded novel insights into their functions and recognition. One key limitation that remains is the lack of methods to detect protease cleavage at the proteome-wide level. We review known substrates and mechanisms of plant pathogen type III effector proteases and compare their functions with those of known type III effector proteases of mammalian pathogens. Finally, we discuss approaches to uncover their function on a system-wide level.

SUMO mediated regulation of transcription factors as a mechanism for transducing environmental cues into cellular signaling in plants

Across all species, transcription factors (TFs) are the most frequent targets of SUMOylation. The effect of SUMO conjugation on the functions of transcription factors has been extensively studied in animal systems, with over 200 transcription factors being documented to be modulated by SUMOylation. This has resulted in the establishment of a number of paradigms that seek to explain the mechanisms by which SUMO regulates transcription factor functions. For instance, SUMO has been shown to modulate TF DNA binding activity; regulate both localization as well as the abundance of TFs and also influence the association of TFs with chromatin. With transcription factors being implicated as master regulators of the cellular signalling pathways that maintain phenotypic plasticity in all organisms, in this review, we will discuss how SUMO mediated regulation of transcription factor activity facilitates molecular pathways to mount an appropriate and coherent biological response to environmental cues.

Organ-specific expression and epigenetic traits of genes encoding digestive enzymes in the lance-leaf sundew (Drosera adelae)

Over the last two decades, extensive studies have been performed at the molecular level to understand the evolution of carnivorous plants. As fruits, the repertoire of protein components in the digestive fluids of several carnivorous plants have gradually become clear. However, the quantitative aspects of these proteins and the expression mechanisms of the genes that encode them are still poorly understood. In this study, using the Australian sundew Drosera adelae, we identified and quantified the digestive fluid proteins. We examined the expression and methylation status of the genes corresponding to major hydrolytic enzymes in various organs; these included thaumatin-like protein, S-like RNase, cysteine protease, class I chitinase, β-1, 3-glucanase, and hevein-like protein. The genes encoding these proteins were exclusively expressed in the glandular tentacles. Furthermore, the promoters of the β-1, 3-glucanase and cysteine protease genes were demethylated only in the glandular tentacles, similar to the previously reported case of the S-like RNase gene da-I. This phenomenon correlated with high expression of the DNA demethylase DEMETER in the glandular tentacles, strongly suggesting that it performs glandular tentacle-specific demethylation of the genes. The current study strengthens and generalizes the relevance of epigenetics to trap organ-specific gene expression in D. adelae. We also suggest similarities between the trap organs of carnivorous plants and the roots of non-carnivorous plants.

Defense and Offense Strategies: The Role of Aspartic Proteases in Plant-Pathogen Interactions

Plant aspartic proteases (APs; E.C.3.4.23) are a group of proteolytic enzymes widely distributed among different species characterized by the conserved sequence Asp-Gly-Thr at the active site. With a broad spectrum of biological roles, plant APs are suggested to undergo functional specialization and to be crucial in developmental processes, such as in both biotic and abiotic stress responses. Over the last decade, an increasing number of publications highlighted the APs' involvement in plant defense responses against a diversity of stresses. In contrast, few studies regarding pathogen-secreted APs and AP inhibitors have been published so far. In this review, we provide a comprehensive picture of aspartic proteases from plant and pathogenic origins, focusing on their relevance and participation in defense and offense strategies in plant-pathogen interactions.

Bowman-Birk Inhibitors: Insights into Family of Multifunctional Proteins and Peptides with Potential Therapeutical Applications

Bowman-Birk inhibitors (BBIs) are found primarily in seeds of legumes and in cereal grains. These canonical inhibitors share a highly conserved nine-amino acids binding loop motif CTP1SXPPXC (where P1 is the inhibitory active site, while X stands for various amino acids). They are natural controllers of plants' endogenous proteases, but they are also inhibitors of exogenous proteases present in microbials and insects. They are considered as plants' protective agents, as their elevated levels are observed during injury, presence of pathogens, or abiotic stress, i.a. Similar properties are observed for peptides isolated from amphibians' skin containing 11-amino acids disulfide-bridged loop CWTP1SXPPXPC. They are classified as Bowman-Birk like trypsin inhibitors (BBLTIs). These inhibitors are resistant to proteolysis and not toxic, and they are reported to be beneficial in the treatment of various pathological states. In this review, we summarize up-to-date research results regarding BBIs' and BBLTIs' inhibitory activity, immunomodulatory and anti-inflammatory activity, antimicrobial and insecticidal strength, as well as chemopreventive properties.

A fungal effector and a rice NLR protein have antagonistic effects on a Bowman-Birk trypsin inhibitor

Bowman-Birk trypsin inhibitors (BBIs) play important roles in animal and plant immunity, but how these protease inhibitors are involved in the immune system remains unclear. Here, we show that the rice (Oryza sativa) BBI protein APIP4 is a common target of a fungal effector and an NLR receptor for innate immunity. APIP4 exhibited trypsin inhibitor activity in vitro and in vivo. Knockout of APIP4 in rice enhanced susceptibility, and overexpression of APIP4 increased resistance to the fungal pathogen Magnaporthe oryzae. The M. oryzae effector AvrPiz-t interacted with APIP4 and suppressed APIP4 trypsin inhibitor activity. By contrast, the rice NLR protein Piz-t interacted with APIP4, enhancing APIP4 transcript and protein levels, and protease inhibitor activity. Our findings reveal a novel host defence mechanism in which a host protease inhibitor targeted by a fungal pathogen is protected by an NLR receptor.

Do plants use root-derived proteases to promote the uptake of soil organic nitrogen?

AIMS

The capacity of plant roots to directly acquire organic nitrogen (N) in the form of oligopeptides and amino acids from soil is well established. However, plants have poor access to protein, the central reservoir of soil organic N. Our question is: do plants actively secrete proteases to enhance the breakdown of soil protein or are they functionally reliant on soil microorganisms to undertake this role?

METHODS

Growing maize and wheat under sterile hydroponic conditions with and without inorganic N, we measured protease activity on the root surface (root-bound proteases) or exogenously in the solution (free proteases). We compared root protease activities to the rhizosphere microbial community to estimate the ecological significance of root-derived proteases.

RESULTS

We found little evidence for the secretion of free proteases, with almost all protease activity associated with the root surface. Root protease activity was not stimulated under N deficiency. Our findings suggest that cereal roots contribute one-fifth of rhizosphere protease activity.

CONCLUSIONS

Our results indicate that plant N uptake is only functionally significant when soil protein is in direct contact with root surfaces. The lack of protease upregulation under N deficiency suggests that root protease activity is unrelated to enhanced soil N capture.

Identification, characterization, and antifungal activity of cysteine peptidases from Calotropis procera latex

Cysteine peptidases (EC 3.4.22) are the most abundant enzymes in latex fluids. However, their physiological functions are still poorly understood, mainly related to defense against phytopathogens. The present study reports the cDNA cloning and sequencing of five undescribed cysteine peptidases from Calotropis procera (Aiton) Dryand (Apocynaceae) as well as some in silico analyses. Of these, three cysteine peptidases (CpCP1, CpCP2, and CpCP3) were purified. Their enzymatic kinetics were determined and they were assayed for their efficacy in inhibiting the hyphal growth of phytopathogenic fungi. The mechanism of action was investigated by fluorescence and atomic force microscopy as well as by induction of reactive oxygen species (ROS). The deduced amino acid sequences showed similar biochemical characteristics and high sequence homology with several other papain-like cysteine peptidases. Three-dimensional models showed two typical cysteine peptidase domains (L and R domains), forming a "V-shaped" active site containing the catalytic triad (Cys, His, and Asn). Proteolysis of CpCP1 was higher at pH 7.0, whereas for CpCP2 and CpCP3 it was higher at 7.5. All peptidases exhibited optimum activity at 35 °C and followed Michaelis-Menten kinetics. However, the major difference among them was that CpCP1 exhibited highest V_max, K_m, K_cat and catalytic efficiency. All peptidases were deleterious to the two fungi tested, with IC₅₀ of around 50 μg/mL. The peptidases promoted membrane permeabilization, morphological changes with leakage of cellular content, and induction of ROS in F. oxysporum spores. These results corroborate the hypothesis that latex cysteine peptidases play a role in defense against fungi.

Revisiting the Concept of Host Range of Plant Pathogens

Strategies to manage plant disease-from use of resistant varieties to crop rotation, elimination of reservoirs, landscape planning, surveillance, quarantine, risk modeling, and anticipation of disease emergences-all rely on knowledge of pathogen host range. However, awareness of the multitude of factors that influence the outcome of plant-microorganism interactions, the spatial and temporal dynamics of these factors, and the diversity of any given pathogen makes it increasingly challenging to define simple, all-purpose rules to circumscribe the host range of a pathogen. For bacteria, fungi, oomycetes, and viruses, we illustrate that host range is often an overlapping continuum-more so than the separation of discrete pathotypes-and that host jumps are common. By setting the mechanisms of plant-pathogen interactions into the scales of contemporary land use and Earth history, we propose a framework to assess the frontiers of host range for practical applications and research on pathogen evolution.

Laticifers, Latex, and Their Role in Plant Defense

Latex, a sap produced by cells called laticifers, occurs in plants of wide taxonomic diversity. Plants exude latex sap in response to physical damage. Questions about the function of latex or the underlying mechanisms persist, but a role in defense is likely. The presence of constitutive peptidases in latex sap in addition to inducible and de novo synthesized pathogenesis-related proteins (PR-proteins), raises the question about the role that each sap component plays to protect plants and how synergism occurs among sap proteins in the course of herbivory or infection. Here we discuss a variety of functions for laticifer and latex in plant defense. We propose that latex peptidases build the front line of defense against herbivores or pathogens.

Plant proteases during developmental programmed cell death

Proteases are among the key regulators of most forms of programmed cell death (PCD) in animals. Many PCD processes have also been associated with protease expression or activation in plants, However, functional evidence for the roles and actual modes of action of plant proteases in PCD remains surprisingly limited. In this review, we provide an update on protease involvement in the context of developmentally regulated plant PCD. To illustrate the diversity of protease functions, we focus on several prominent developmental PCD processes, including xylem and tapetum maturation, suspensor elimination, endosperm degradation, and seed coat formation, as well as plant senescence processes. Despite the substantial advances in the field, protease functions are often only correlatively linked to developmental PCD, and the specific molecular roles of proteases in many developmental PCD processes remain to be elucidated.

Infection Strategies Deployed by Botrytis cinerea, Fusarium acuminatum, and Rhizopus stolonifer as a Function of Tomato Fruit Ripening Stage

Worldwide, 20-25% of all harvested fruit and vegetables are lost annually in the field and throughout the postharvest supply chain due to rotting by fungal pathogens. Most postharvest pathogens exhibit necrotrophic or saprotrophic lifestyles, resulting in decomposition of the host tissues and loss of marketable commodities. Necrotrophic fungi can readily infect ripe fruit leading to the rapid establishment of disease symptoms. However, these pathogens generally fail to infect unripe fruit or remain quiescent until host conditions stimulate a successful infection. Previous research on infections of fruit has mainly been focused on the host's genetic and physicochemical factors that inhibit or promote disease. Here, we investigated if fruit pathogens can modify their own infection strategies in response to the ripening stage of the host. To test this hypothesis, we profiled global gene expression of three fungal pathogens that display necrotrophic behavior-Botrytis cinerea, Fusarium acuminatum, and Rhizopus stolonifer-during interactions with unripe and ripe tomato fruit. We assembled and functionally annotated the transcriptomes of F. acuminatum and R. stolonifer as no genomic resources were available. Then, we conducted differential gene expression analysis to compare each pathogen during inoculations versus in vitro conditions. Through characterizing patterns of overrepresented pathogenicity and virulence functions (e.g., phytotoxin production, cell wall degradation, and proteolysis) among the differentially expressed genes, we were able to determine shared strategies among the three fungi during infections of compatible (ripe) and incompatible (unripe) fruit tissues. Though each pathogen's strategy differed in the details, interactions with unripe fruit were commonly characterized by an emphasis on the degradation of cell wall components, particularly pectin, while colonization of ripe fruit featured more heavily redox processes, proteolysis, metabolism of simple sugars, and chitin biosynthesis. Furthermore, we determined that the three fungi were unable to infect fruit from the non-ripening (nor) tomato mutant, confirming that to cause disease, these pathogens require the host tissues to undergo specific ripening processes. By enabling a better understanding of fungal necrotrophic infection strategies, we move closer to generating accurate models of fruit diseases and the development of early detection tools and effective management strategies.

The Role of Proteases in the Virulence of Plant Pathogenic Bacteria

A pathogenic lifestyle is inextricably linked with the constant necessity of facing various challenges exerted by the external environment (both within and outside the host). To successfully colonize the host and establish infection, pathogens have evolved sophisticated systems to combat the host defense mechanisms and also to be able to withstand adverse environmental conditions. Proteases, as crucial components of these systems, are involved in a variety of processes associated with infection. In phytopathogenic bacteria, they play important regulatory roles and modulate the expression and functioning of various virulence factors. Secretory proteases directly help avoid recognition by the plant immune systems, and contribute to the deactivation of the defense response pathways. Finally, proteases are important components of protein quality control systems, and thus enable maintaining homeostasis in stressed bacterial cells. In this review, we discuss the known protease functions and protease-regulated signaling processes associated with virulence of plant pathogenic bacteria.

Dealing With Stress: A Review of Plant SUMO Proteases

The SUMO system is a rapid dynamic post-translational mechanism employed by eukaryotic cells to respond to stress. Plant cells experience hyperSUMOylation of substrates in response to stresses such as heat, ethanol, and drought. Many SUMOylated proteins are located in the nucleus, SUMOylation altering many nuclear processes. The SUMO proteases play two key functions in the SUMO cycle by generating free SUMO; they have an important role in regulating the SUMO cycle, and by cleaving SUMO off SUMOylated proteins, they provide specificity to which proteins become SUMOylated. This review summarizes the broad literature of plant SUMO proteases describing their catalytic activity, domains and structure, evolution, localization, and response to stress and highlighting potential new areas of research in the future.

Damage-Associated Molecular Pattern-Triggered Immunity in Plants

As a universal process in multicellular organisms, including animals and plants, cells usually emit danger signals when suffering from attacks of microbes and herbivores, or physical damage. These signals, termed as damage-associated molecular patterns (DAMPs), mainly include cell wall or extracellular protein fragments, peptides, nucleotides, and amino acids. Once exposed on cell surfaces, DAMPs are detected by plasma membrane-localized receptors of surrounding cells to regulate immune responses against the invading organisms and promote damage repair. DAMPs may also act as long-distance mobile signals to mediate systemic wounding responses. Generation, release, and perception of DAMPs, and signaling events downstream of DAMP perception are all rigorously modulated by plants. These processes integrate together to determine intricate mechanisms of DAMP-triggered immunity in plants. In this review, we present an extensive overview on our current understanding of DAMPs in plant immune system.