A Cytochrome B5-Like Heme/Steroid Binding Domain Protein, PlCB5L1, Regulates Mycelial Growth, Pathogenicity and Oxidative Stress Tolerance in Peronophythora litchii

UvCYP503 is required for stress response and pathogenicity in Ustilaginoidea virens

The fungus Ustilaginoidea virens, which impacts rice spikes, causes rice false smut (RFS), a significant prevalent disease in rice cultivation regions globally. Cytochrome P450 genes are known to be involved in secondary metabolism and pathogenesis in various species, but studies on CYP450 genes in U. virens are limited. In this research, a P450 family gene, CYP503, was found up-regulated during invasion stage of U. virens. Observation of fluorescence indicated that UvCYP503-GFP is situated within cytoplasm of hyphae. Disruption of CYP503 led to decreased hyphal development, conidiation, and pathogenicity. Additional RNA-seq assay revealed that UvCYP503 affects the transcript of genes associated with pathogenicity, various stress responses, and other CYP450 genes. In alignment with RNA-seq results, compared with wild-type, ΔUvcyp503 mutants showed increased sensitivity to cell wall stresses, but reduced sensitivity to osmotic and hyperosmotic stressors. Moreover, ΔUvcyp503 mutants exhibited decreased sensitivity to the fungicides difenoconazole and tebuconazole. This study represents a phenome-based functional analysis of a CYP503 gene in U. virens and provides valuable genetic resources for further research in filamentous fungi and other plant pathogens.

A RACK1 family protein regulates pathogenicity of Peronophythora litchii by acting as a scaffold for MAPK signal modules

Litchi downy blight caused by Peronophythora litchii is the most destructive disease of litchi (Litchi chinensis). RACK1 (Receptor for activated C kinase 1) is a group of scaffold proteins, mainly involved in the regulation of various signaling pathways by interacting with signal transduction proteins and affecting the activity of these proteins. In this study, a RACK1 homologue identified in P. litchii, and named PlRACK1. The protein was found to interact with the mitogen-activated protein kinases, PlMAPK1 and PlMAPK2. CRISPR/Cas9-mediated genome editing technology was used to knock out PlRACK1, and we found that it was involved in mycelial growth, cell wall integrity, ROS metabolism, laccase activity, and pathogenicity of P. litchii. PlMAPK1 interacted with RACK1, and they jointly regulated sporangiophore branching of P. litchii. Transcriptome analysis showed that P. litchii MAPK Phosphatase 1 (PlMKP1) and beta-glucoside (PlBglX) were regulated by PlRACK1, both of which were also required for the pathogenicity of P. litchii. As well, PlMKP1 also interacted with PlMAPK1 and PlMAPK2. These results provide insights into the direct interactions between RACK1, MAPKs, and MKP, and their functions in growth, development, and pathogenesis in a plant pathogenic oomycete.

Litchi aspartic protease LcAP1 enhances plant resistance via suppressing cell death triggered by the pectate lyase PlPeL8 from Peronophythora litchii

Plant cell death is regulated in plant-pathogen interactions. While some aspartic proteases (APs) participate in regulating programmed cell death or defense responses, the defense functions of most APs remain largely unknown. Here, we report on a virulence factor, PlPeL8, which is a pectate lyase found in the hemibiotrophic pathogen Peronophythora litchii. Through in vivo and in vitro assays, we confirmed the interaction between PlPeL8 and LcAP1 from litchi, and identified LcAP1 as a positive regulator of plant immunity. PlPeL8 induced cell death associated with NbSOBIR1 and NbMEK2. The 11 conserved residues of PlPeL8 were essential for inducing cell death and enhancing plant susceptibility. Twenty-three LcAPs suppressed cell death induced by PlPeL8 in Nicotiana benthamiana depending on their interaction with PlPeL8. The N-terminus of LcAP1 was required for inhibiting PlPeL8-triggered cell death and susceptibility. Furthermore, PlPeL8 led to higher susceptibility in NbAPs-silenced N. benthamiana than the GUS-control. Our results indicate the crucial roles of LcAP1 and its homologs in enhancing plant resistance via suppression of cell death triggered by PlPeL8, and LcAP1 represents a promising target for engineering disease resistance. Our study provides new insights into the role of plant cell death in the arms race between plants and hemibiotrophic pathogens.

A plant cell death-inducing protein from litchi interacts with Peronophythora litchii pectate lyase and enhances plant resistance

Cell wall degrading enzymes, including pectate lyases (PeLs), released by plant pathogens, break down protective barriers and/or activate host immunity. The direct interactions between PeLs and plant immune-related proteins remain unclear. We identify two PeLs, PlPeL1 and PlPeL1-like, critical for full virulence of Peronophythora litchii on litchi (Litchi chinensis). These proteins enhance plant susceptibility to oomycete pathogens in a PeL enzymatic activity-dependent manner. However, LcPIP1, a plant immune regulator secreted by litchi, binds to PlPeL1/PlPeL1-like, and attenuates PlPeL1/PlPeL1-like induced plant susceptibility to Phytophthora capsici. LcPIP1 also induces cell death and various immune responses in Nicotiana benthamiana. Conserved in plants, LcPIP1 homologs bear a conserved "VDMASG" motif and exhibit immunity-inducing activity. Furthermore, SERK3 interacts with LcPIP1 and is required for LcPIP1-induced cell death. NbPIP1 participates in immune responses triggered by the PAMP protein INF1. In summary, our study reveals the dual roles of PlPeL1/PlPeL1-like in plant-pathogen interactions: enhancing pathogen virulence through PeL enzymatic activity while also being targeted by LcPIP1, thus enhancing plant immunity.

A C2H2 Zinc Finger Protein PlCZF1 Is Necessary for Oospore Development and Virulence in Peronophythora litchii

C₂H₂ zinc finger is one of the most common motifs found in the transcription factors (TFs) in eukaryotes organisms, which have a broad range of functions, such as regulation of growth and development, stress tolerance and pathogenicity. Here, PlCZF1 was identified to encode a C₂H₂ zinc finger in the litchi downy blight pathogen Peronophythora litchii. PlCZF1 is conserved in P. litchii and Phytophthora species. In P. litchii, PlCZF1 is highly expressed in sexual developmental and early infection stages. We generated Δplczf1 mutants using the CRISPR/Cas9 method. Compared with the wild type, the Δplczf1 mutants showed no significant difference in vegetative growth and asexual reproduction, but were defective in oospore development and virulence. Further experiments revealed that the transcription of PlM90, PlLLP and three laccase encoding genes were down-regulated in the Δplczf1 mutant. Our results demonstrated that PlCZF1 is a vital regulator for sexual development and pathogenesis in P. litchii.

Autophagy-Related Gene PlATG6a Is Involved in Mycelial Growth, Asexual Reproduction and Tolerance to Salt and Oxidative Stresses in Peronophythora litchii

Autophagy is ubiquitously present in eukaryotes. During this process, intracellular proteins and some waste organelles are transported into lysosomes or vacuoles for degradation, which can be reused by the cell to guarantee normal cellular metabolism. However, the function of autophagy-related (ATG) proteins in oomycetes is rarely known. In this study, we identified an autophagy-related gene, PlATG6a, encoding a 514-amino-acid protein in Peronophythora litchii, which is the most destructive pathogen of litchi. The transcriptional level of PlATG6a was relatively higher in mycelium, sporangia, zoospores and cysts. We generated PlATG6a knockout mutants using CRISPR/Cas9 technology. The P. litchii Δplatg6a mutants were significantly impaired in autophagy and vegetative growth. We further found that the Δplatg6a mutants displayed decreased branches of sporangiophore, leading to impaired sporangium production. PlATG6a is also involved in resistance to oxidative and salt stresses, but not in sexual reproduction. The transcription of peroxidase-encoding genes was down-regulated in Δplatg6a mutants, which is likely responsible for hypersensitivity to oxidative stress. Compared with the wild-type strain, the Δplatg6a mutants showed reduced virulence when inoculated on the litchi leaves using mycelia plugs. Overall, these results suggest a critical role for PlATG6a in autophagy, vegetative growth, sporangium production, sporangiophore development, zoospore release, pathogenesis and tolerance to salt and oxidative stresses in P. litchii.