Phytophthora infestans small phospholipase D-like proteins elicit plant cell death and promote virulence

Pectate Lyase from Fusarium sacchari Induces Plant Immune Responses and Contributes to Virulence

Fusarium sacchari is one of the primary pathogens causing Pokkah Boeng disease (PBD) in sugarcane in China. Pectate lyases (PL), which play a critical role in pectin degradation and fungal virulence, have been extensively studied in major bacterial and fungal pathogens of a wide range of plant species. However, only a few PLs have been functionally investigated. In this study, we analyzed the function of the pectate lyase gene, FsPL, from F. sacchari. FsPL is a key virulence factor of F. sacchari and can induce plant cell death. FsPL also triggers the pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) response in Nicotiana benthamiana, as reflected by increases in reactive oxygen species (ROS) production, electrolyte leakage, and callose accumulation, as well as the upregulation of defense response genes. In addition, our study also found that the signal peptide of FsPL was necessary for induced cell death and PTI responses. Virus-induced gene silencing showed that FsPL-induced cell death in Nicotiana benthamiana was mediated by leucine-rich repeat (LRR) receptor-like kinases BAK1 and SOBIR1. Thus, FsPL may not only be a critical virulence factor for F. sacchari but may also induce plant defense responses. These findings provide new insights into the functions of pectate lyase in host-pathogen interactions. IMPORTANCE Pokkah Boeng disease (PBD) is one of the main diseases affecting sugarcane in China, seriously damaging sugarcane production and economic development. Therefore, it is important to clarify the pathogenic mechanisms of this disease and to provide a theoretical basis for the breeding of PBD-resistant sugarcane strains. The present study aimed to analyze the function of FsPL, a recently identified pectate lyase gene from F. sacchari. FsPL is a key virulence factor of F. sacchari that induces plant cell death. Our results provide new insights into the function of pectate lyase in host-pathogen interactions.

Genome-wide screening and identification of nuclear Factor-Y family genes and exploration their function on regulating abiotic and biotic stress in potato (Solanum tuberosum L.)

The Nuclear Factor-Y (NF-Y) transcription factor (TF), which includes three distinct subunits (NF-YA, NF-YB and NF-YC), is known to manipulate various aspects of plant growth, development, and stress responses. Although the NF-Y gene family was well studied in many species, little is known about their functions in potato. In this study, a total of 37 potato NF-Y genes were identified, including 11 StNF-YAs, 20 StNF-YBs, and 6 StNF-YCs. The genetic features of these StNF-Y genes were investigated by comparing their evolutionary relationship, intron/exon organization and motif distribution pattern. Multiple alignments showed that all StNF-Y proteins possessed clearly conserved core regions that were flanked by non-conserved sequences. Gene duplication analysis indicated that nine StNF-Y genes were subjected to tandem duplication and eight StNF-Ys arose from segmental duplication events. Synteny analysis suggested that most StNF-Y genes (33 of 37) were orthologous to potato's close relative tomato (Solanum lycopersicum L.). Tissue-specific expression of the StNF-Y genes suggested their potential roles in controlling potato growth and development. The role of StNF-Ys in regulating potato responses to abiotic stress (ABA, drought and salinity) was also confirmed: twelve StNF-Y genes were up-regulated and another two were down-regulated under different abiotic treatments. In addition, genes responded differently to pathogen challenges, suggesting that StNF-Y genes may play distinct roles under certain biotic stress. In summary, insights into the evolution of NF-Y family members and their functions in potato development and stress responses are provided.

The Molecular Mechanisms of Phytophthora infestans in Response to Reactive Oxygen Species Stress

Reactive oxygen species (ROSs) are critical for the growth, development, proliferation, and pathogenicity of microbial pathogens; however, excessive levels of ROSs are toxic. Little is known about the signaling cascades in response to ROS stress in oomycetes such as Phytophthora infestans, the causal agent of potato late blight. Here, P. infestans was used as a model system to investigate the mechanism underlying the response to ROS stress in oomycete pathogens. Results showed severe defects in sporangium germination, mycelium growth, appressorium formation, and virulence of P. infestans in response to H₂O₂ stress. Importantly, these phenotypes mimic those of P. infestans treated with rapamycin, the inhibitor of target of rapamycin (TOR, 1-phosphatidylinositol-3-kinase). Strong synergism occurred when P. infestans was treated with a combination of H₂O₂ and rapamycin, suggesting that a crosstalk exists between ROS stress and the TOR signaling pathway. Comprehensive analysis of transcriptome, proteome, and phosphorylation omics showed that H₂O₂ stress significantly induced the operation of the TOR-mediated autophagy pathway. Monodansylcadaverine staining showed that in the presence of H₂O₂ and rapamycin, the autophagosome level increased in a dosage-dependent manner. Furthermore, transgenic potatoes containing double-stranded RNA of TOR in P. infestans (PiTOR) displayed high resistance to P. infestans. Therefore, TOR is involved in the ROS response and is a potential target for control of oomycete diseases, because host-mediated silencing of PiTOR increases potato resistance to late blight.

Comparative Secretome Analyses of Toxigenic and Atoxigenic Rathayibacter Species

Phytopathogenic Rathayibacter species are unique bacterial plant pathogens because they are obligately vectored by plant parasitic anguinid nematodes to the developing seedheads of forage grasses and cereals. This understudied group of plant-associated Actinomycetes includes the neurotoxigenic plant pathogen R. toxicus, which causes annual ryegrass toxicity in grazing livestock. R. toxicus is currently endemic to Australia and is listed as a plant pathogen select agent by the U.S. Department of Agriculture-Animal and Plant Health Inspection Service. The complex Rathayibacter disease cycle requires intimate interactions with the nematode vector and plant hosts, which warrants an increased understanding of the secretory and surface-associated proteins that mediate these diverse eukaryotic interactions. Here we present the first comparative secretome analysis for this complex, nematode-vectored Rathayibacter genus that compares the three agronomically damaging toxigenic and atoxigenic Rathayibacter species, R. toxicus, R. iranicus, and R. tritici. The exoproteomic comparison identified 1,423 unique proteins between the three species via liquid chromatography-tandem mass spectrometry, leading to the identification of putative pathogenicity-related proteins and proteins that may mediate nematode attachment. Of the uniquely identified proteins, 94 homologous proteins were conserved between the three Rathayibacter exoproteomes and comprised between 43.4 and 58.6% of total protein abundance. Comparative analyses revealed both conserved and uniquely expressed extracellular proteins, which, interestingly, had more similarities to extracellular proteins commonly associated with bacterial animal pathogens than classic plant pathogens. This comparative exoproteome analysis will facilitate the characterization of proteins essential for vector attachment and host colonization and assist in the development of serological diagnostic assays.

Mining oomycete proteomes for metalloproteases leads to identification of candidate virulence factors in Phytophthora infestans

Pathogens deploy a wide range of pathogenicity factors, including a plethora of proteases, to modify host tissue or manipulate host defences. Metalloproteases (MPs) have been implicated in virulence in several animal and plant pathogens. Here we investigated the repertoire of MPs in 46 stramenopile species including 37 oomycetes, 5 diatoms, and 4 brown algae. Screening their complete proteomes using hidden Markov models (HMMs) trained for MP detection resulted in over 4,000 MPs, with most species having between 65 and 100 putative MPs. Classification in clans and families according to the MEROPS database showed a highly diverse MP repertoire in each species. Analyses of domain composition, orthologous groups, distribution, and abundance within the stramenopile lineage revealed a few oomycete-specific MPs and MPs potentially related to lifestyle. In-depth analyses of MPs in the plant pathogen Phytophthora infestans revealed 91 MPs, divided over 21 protein families, including 25 MPs with a predicted signal peptide or signal anchor. Expression profiling showed different patterns of MP gene expression during pre-infection and infection stages. When expressed in leaves of Nicotiana benthamiana, 12 MPs changed the sizes of lesions caused by inoculation with P. infestans; with 9 MPs the lesions were larger, suggesting a positive effect on the virulence of P. infestans, while 3 MPs had a negative effect, resulting in smaller lesions. To the best of our knowledge, this is the first systematic inventory of MPs in oomycetes and the first study pinpointing MPs as potential pathogenicity factors in Phytophthora.

Devastating intimacy: the cell biology of plant-Phytophthora interactions

An understanding of the cell biology underlying the burgeoning molecular genetic and genomic knowledge of oomycete pathogenicity is essential to gain the full context of how these pathogens cause disease on plants. An intense research focus on secreted Phytophthora effector proteins, especially those containing a conserved N-terminal RXLR motif, has meant that most cell biological studies into Phytophthora diseases have focussed on the effectors and their host target proteins. While these effector studies have provided novel insights into effector secretion and host defence mechanisms, there remain many unanswered questions about fundamental processes involved in spore biology, host penetration and haustorium formation and function.

Peptide Extracts from Seven Medicinal Plants Discovered to Inhibit Oomycete Phytophthora infestans, a Causative Agent of Potato Late Blight Disease

We report the inhibitory effect of peptide extracts obtained from seven medicinal plants against a causative agent of late blight disease Phytophthora infestans. We find that all the extracts possess inhibitory activity toward the zoospores output, zoosporangium germination, and the development of P. infestans on potato disc tubers at different quantitative levels. Based on the biological effects detected, an extract of common horsetail (Equisetum arvense) biomass is recognized as the most effective and is selected for further structural analysis. We perform a combination of amino acid analysis and MALDI-TOF mass spectrometry, which reveal the presence of Asn/Asp- and Gln/Glu-rich short peptides with molecular masses in the range of 500-900 Da and not exceeding 1500 Da as the maximum. Analytical anion-exchange HPLC is successfully applied for separation of the peptide extract from common horsetail (E. arvense). We collect nine dominant components that are combined in two groups with differences in retention times. The N-terminal amino acid sequence of the prevalent compounds after analytical ion-exchange HPLC allows us to identify them as peptide fragments of functionally active proteins associated with photosynthesis, aquatic transport, and chitin binding. The anti-oomycete effects may be associated with the conversion of ribulose-1,5-bisphosphate carboxylase/oxygenase to produce a number of biologically active anionic peptides with possible regulatory functions. These data inform our knowledge regarding biologically active peptide fragments; they are the components of programmed or induced proteolysis of plant proteins and can realize secondary antimicrobial functions.

Phytophthora nicotianae Infection of Citrus Leaves and Host Defense Activation Compared to Root Infection

Currently, little is known about the host pathogen interaction between Phytophthora spp. and citrus roots versus leaves. Therefore, we compared the molecular events occurring in citrus roots and leaves after zoospore inoculation with Phytophthora nicotianae. We analyzed the physical characteristics and genetic responses to P. nicotianae infection of leaves and roots for susceptible and tolerant citrus rootstocks to examine the potential for leaves to model root responses to P. nicotianae infection. Leaves responded faster and stronger to P. nicotianae infection than roots, and leaves showed greater differential response than roots. In addition to differences in hormonal responses, sugar, phospholipase D (PLD), and phospholipase A (PLA) involvement in the interaction between citrus and P. nicotianae was identified. This work, for the first time, creates a solid P. nicotianae zoospore infection protocol, reports P. nicotianae infection on citrus leaves through stomata, and provides evidence that different host organs respond to the pathogen differentially in timing and magnitude. This work identifies the hormones, sugars, pathogenesis-related genes, PLDs, and PLAs that are involved in the molecular events occurring in citrus under infection of P. nicotianae zoospore, and advances our understanding of the mechanisms underlying the interaction.

Two Phytophthora parasitica cysteine protease genes, PpCys44 and PpCys45, trigger cell death in various Nicotiana spp. and act as virulence factors

Proteases secreted by pathogens have been shown to be important virulence factors modifying plant immunity, and cysteine proteases have been demonstrated to participate in different pathosystems. However, the virulence functions of the cysteine proteases secreted by Phytophthora parasitica are poorly understood. Using a publicly available genome database, we identified 80 cysteine proteases in P. parasitica, 21 of which were shown to be secreted. Most of the secreted cysteine proteases are conserved among different P. parasitica strains and are induced during infection. The secreted cysteine protease proteins PpCys44/45 (proteases with identical protein sequences) and PpCys69 triggered cell death on the leaves of different Nicotiana spp. A truncated mutant of PpCys44/45 lacking a signal peptide failed to trigger cell death, suggesting that PpCys44/45 functions in the apoplastic space. Analysis of three catalytic site mutants showed that the enzyme activity of PpCys44/45 is required for its ability to trigger cell death. A virus-induced gene silencing assay showed that PpCys44/45 does not induce cell death on NPK1 (Nicotiana Protein Kinase 1)-silenced Nicotiana benthamiana plants, indicating that the cell death phenotype triggered by PpCys44/45 is dependent on NPK1. PpCys44- and PpCys45-deficient double mutants showed decreased virulence, suggesting that PpCys44 and PpCys45 positively promote pathogen virulence during infection. PpCys44 and PpCys45 are important virulence factors of P. parasitica and trigger NPK1-dependent cell death in various Nicotiana spp.