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Abstract In Arabidopsis thaliana, the POWDERY MILDEW RESISTANT4 (PMR4)/GLUCAN SYNTHASE LIKE5 (GSL5) callose synthase is required for pathogen-induced callose deposition in cell wall defense. Paradoxically, pmr4/gsl5 mutants exhibit strong resistance to both powdery and downy mildew. The powdery mildew resistance of pmr4/gsl5 has been attributed to upregulated salicylic acid (SA) signaling based on its dependance on PHYTOALEXIN DEFICIENT4 (PAD4), which controls SA accumulation, and its abolishment by bacterial NahG salicylate hydroxylase. Our study revealed that disruption of PMR4/GSL5 also leads to early senescence. Suppressor analysis uncovered that PAD4 and N-hydroxypipecolic acid (NHP) biosynthetic genes ABERRANT GROWTH AND DEATH2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1) and FLAVIN-DEPENDENT MONOXYGENASE1 (FMO1) are required for early senescence of pmr4/gsl5 mutants. The critical role of NHP in the early senescence of pmr4/gsl5 was supported by greatly increased accumulation of pipecolic acid in pmr4/gsl5 mutants. In contrast, disruption of the SA biosynthetic gene ISOCHORISMATE SYNTHASE1/SA-INDUCTION DIFFICIENT 2 (ICS1/SID2), which greatly reduces SA accumulation, had little effect on impaired growth of pmr4/gsl5. Furthermore, while disruption of PAD4 completely abolished the powdery mildew resistance in pmr4/gsl5, mutations in ICS1/SID2, ALD1, or FMO1 had only a minor effect on the resistance of the mutant plants. However, disruption of both ICS1/SID2 and FMO1 abolished the enhanced immunity of the callose synthase mutants against the fungal pathogen. Therefore, while NHP plays a crucial role in the early senescence of pmr4/gsl5 mutants, both SA and NHP have important roles in the strong powdery mildew resistance induced by the loss of the callose synthase. 

Summary Plants have evolved a sophisticated immunity system for specific detection of pathogens and rapid induction of measured defences. Over‐ or constitutive activation of defences would negatively affect plant growth and development. Hence, the plant immune system is under tight positive and negative regulation. MAP kinase phosphatase1 (MKP1) has been identified as a negative regulator of plant immunity in model plantArabidopsis. However, the molecular mechanisms by which MKP1 regulates immune signalling in wheat (Triticum aestivum) are poorly understood. In this study, we investigated the role of TaMKP1 in wheat defence against two devastating fungal pathogens and determined its subcellular localization. We demonstrated that knock‐down ofTaMKP1by CRISPR/Cas9 in wheat resulted in enhanced resistance to rust caused byPuccinia striiformisf. sp.tritici(Pst) and powdery mildew caused byBlumeria graminisf. sp.tritici(Bgt), indicating thatTaMKP1negatively regulates disease resistance in wheat. Unexpectedly, whileTamkp1mutant plants showed increased resistance to the two tested fungal pathogens they also had higher yield compared with wild‐type control plants without infection. Our results suggested that TaMKP1 interacts directly with dephosphorylated and activated TaMPK3/4/6, and TaMPK4 interacts directly with TaPAL. Taken together, we demonstrated TaMKP1 exert negative modulating roles in the activation of TaMPK3/4/6, which are required for MAPK‐mediated defence signalling. This facilitates our understanding of the important roles of MAP kinase phosphatases and MAPK cascades in plant immunity and production, and provides germplasm resources for breeding for high resistance and high yield. 

Abstract Grapevine (Vitis vinifera) is an economically important fruit crop worldwide. The widely cultivated grapevine is susceptible to powdery mildew caused by Erysiphe necator. In this study, we used CRISPR-Cas9 to simultaneously knock out VviWRKY10 and VviWRKY30 encoding two transcription factors reported to be implicated in defense regulation. We generated 53 wrky10 single mutant transgenic plants and 15 wrky10 wrky30 double mutant transgenic plants. In a 2-yr field evaluation of powdery mildew resistance, the wrky10 mutants showed strong resistance, while the wrky10 wrky30 double mutants showed moderate resistance. Further analyses revealed that salicylic acid (SA) and reactive oxygen species contents in the leaves of wrky10 and wrky10 wrky30 were substantially increased, as was the ethylene (ET) content in the leaves of wrky10. The results from dual luciferase reporter assays, electrophoretic mobility shift assays and chromatin immunoprecipitation (ChIP) assays demonstrated that VviWRKY10 could directly bind to the W-boxes in the promoter of SA-related defense genes and inhibit their transcription, supporting its role as a negative regulator of SA-dependent defense. By contrast, VviWRKY30 could directly bind to the W-boxes in the promoter of ET-related defense genes and promote their transcription, playing a positive role in ET production and ET-dependent defense. Moreover, VviWRKY10 and VviWRKY30 can bind to each other's promoters and mutually inhibit each other's transcription. Taken together, our results reveal a complex mechanism of regulation by VviWRKY10 and VviWRKY30 for activation of measured and balanced defense responses against powdery mildew in grapevine. 

Abstract Powdery mildew fungi are obligate biotrophic pathogens that only invade plant epidermal cells. There are two epidermal surfaces in every plant leaf: the adaxial (upper) side and the abaxial (lower) side. While both leaf surfaces can be susceptible to adapted powdery mildew fungi in many plant species, there have been observations of leaf abaxial immunity in some plant species including Arabidopsis. The genetic basis of such leaf abaxial immunity remains unknown. In this study, we tested a series of Arabidopsis mutants defective in one or more known defense pathways with the adapted powdery mildew isolate Golovinomyces cichoracearum UCSC1. We found that leaf abaxial immunity was significantly compromised in mutants impaired for both the EDS1/PAD4- and PEN2/PEN3-dependent defenses. Consistently, expression of EDS1–yellow fluorescent protein and PEN2–green fluorescent protein fusions from their respective native promoters in the respective eds1-2 and pen2-1 mutant backgrounds was higher in the abaxial epidermal cells than in the adaxial epidermal cells. Altogether, our results indicate that leaf abaxial immunity against powdery mildew in Arabidopsis is at least partially due to enhanced EDS1/PAD4- and PEN2/PEN3-dependent defenses. Such transcriptionally pre-programmed defense mechanisms may underlie leaf abaxial immunity in other plant species such as hemp and may be exploited for engineering adaxial immunity against powdery mildew fungi in crop plants. 

Abstract Nonhost resistance (NHR) refers to the immunity of most tested genotypes of a plant species to most tested variants of a pathogen species. Thus, NHR is broad spectrum and durable in nature and constitutes a major safety barrier against invasion of a myriad of potentially pathogenic microbes in any plants including domesticated crops. Genetic study of NHR is generally more difficult compared to host resistance mainly because NHR is genetically more complicated and often lacks intraspecific polymorphisms. Nevertheless, substantial progress has been made towards the understanding of the molecular basis of NHR in the past two decades using various approaches. Not surprisingly, molecular mechanisms of NHR revealed so far encompasses pathogen‐associated molecular pattern‐triggered immunity and effector‐triggered immunity. In this review, we briefly discuss the inherent difficulty in genetic studies of NHR and summarize the main approaches that have been taken to identify genes contributing to NHR. We also discuss new enabling strategies for dissecting multilayered NHR in model plants with a focus on NHR against filamentous pathogens, especially biotrophic pathogens such as powdery mildew and rust fungi. 

Abstract Study on the regulation of broad‐spectrum resistance is an active area in plant biology.RESISTANCE TO POWDERY MILDEW 8.1(RPW8.1) is one of a few broad‐spectrum resistance genes triggering the hypersensitive response (HR) to restrict multiple pathogenic infections. To address the question how RPW8.1 signaling is regulated, we performed a genetic screen and tried to identify mutations enhancing RPW8.1‐mediated HR. Here, we provided evidence to connect an annexin protein with RPW8.1‐mediated resistance inArabidopsisagainst powdery mildew. We isolated and characterizedArabidopsis b7‐6mutant. A point mutation inb7‐6at theAt5g12380locus resulted in an amino acid substitution in ANNEXIN 8 (AtANN8). Loss‐of‐function or RNA‐silencing ofAtANN8led to enhanced expression ofRPW8.1, RPW8.1‐dependent necrotic lesions in leaves, and defense against powdery mildew. Conversely, over‐expression ofAtANN8compromised RPW8.1‐mediated disease resistance and cell death. Interestingly, the mutation in AtANN8 enhanced RPW8.1‐triggered H₂O₂. In addition, mutation in AtANN8 led to hypersensitivity to salt stress. Together, our data indicate that AtANN8 is involved in multiple stress signaling pathways and negatively regulates RPW8.1‐mediated resistance against powdery mildew and cell death, thus linking ANNEXIN's function with plant immunity. 

Summary Biotrophic pathogens are believed to strategically manipulate sugar transport in host cells to enhance their access to carbohydrates. However, mechanisms of sugar translocation from host cells to biotrophic fungi such as powdery mildew across the plant–haustorium interface remain poorly understood.To investigate this question, systematic subcellular localisation analysis was performed for all the 14 members of the monosaccharide sugar transporter protein (STP) family inArabidopsis thaliana. The best candidate AtSTP8 was further characterised for its transport properties inSaccharomyces cerevisiaeand potential role in powdery mildew infection by gene ablation and overexpression in Arabidopsis.Our results showed that AtSTP8 was mainly localised to the endoplasmic reticulum (ER) and appeared to be recruited to the host‐derived extrahaustorial membrane (EHM) induced by powdery mildew. Functional complementation assays inS. cerevisiaesuggested that AtSTP8 can transport a broad spectrum of hexose substrates. Moreover, transgenic Arabidopsis plants overexpressingAtSTP8showed increased hexose concentration in leaf tissues and enhanced susceptibility to powdery mildew.Our data suggested that the ER‐localised sugar transporter AtSTP8 may be recruited to the EHM where it may be involved in sugar acquisition by haustoria of powdery mildew from host cells in Arabidopsis. 

Recessive mutations in the mildew locus O (MLO) gene were first identified as key factors conferring broad-spectrum resistance to powdery mildew in barley. This discovery inspired extensive research on MLOs and novel breeding strategies for powdery mildew resistance by targetingMLOgenes in various crops. Over the past two decades, studies have revealed broader roles for MLOs beyond powdery mildew susceptibility, including regulating interactions with diverse pathogens and symbionts, root thigmomorphogenesis, and reproductive development. Recent findings identify MLOs as calcium channels, offering a unifying molecular framework for understanding their diverse biological functions. However, significant challenges remain in comprehensively understanding the cellular and molecular mechanisms underlying MLO functions. In this review, we examine the MLO-related literature to delineate the multifaceted roles of MLOs in plant immunity and development. By integrating published phylogenetic, genetic, biochemical, and molecular studies with original in silico analyses, we propose mechanistic models to contextualize the diverse functions of MLOs with a focus on plant immunity and susceptibility. 

Summary Calcium‐dependent protein kinases (CDPKs) play vital roles in metabolic regulations and stimuli responses in plants. However, little is known about their function in grapevine.Here, we report thatVpCDPK9andVpCDPK13, two paralogousCDPKsfromVitis pseudoreticulataaccession Baihe‐35‐1, appear to positively regulate powdery mildew resistance. The transcription of them in leaves of ‘Baihe‐35‐1’ were differentially induced upon powdery mildew infection. Overexpression ofVpCDPK9‐YFPorVpCDPK13‐YFPin theV. viniferasusceptible cultivar Thompson Seedless resulted in enhanced resistance to powdery mildew (YFP, yellow fluorescent protein). This might be due to elevation of SA and ethylene production, and excess accumulation of H₂O₂and callose in penetrated epidermal cells and/or the mesophyll cells underneath.Ectopic expression ofVpCDPK9‐YFPin Arabidopsis resulted in varied degrees of reduced stature, pre‐mature senescence and enhanced powdery mildew resistance. However, these phenotypes were abolished inVpCDPK9‐YFPtransgenic lines impaired in SA signaling (pad4sid2) or ethylene signaling (ein2). Moreover, both of VpCDPK9 and VpCDPK13 were found to interact with and potentially phosphorylate VpMAPK3, VpMAPK6, VpACS1 and VpACS2in vivo(ACS, 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthase; MAPK, mitogen‐activated protein kinase).These results suggest thatVpCDPK9andVpCDPK13contribute to powdery mildew resistance via positively regulating SA and ethylene signaling in grapevine. 

Crop diseases are responsible for substantial yield losses worldwide, thereby threatening global food security. In this Research Topic, a collection of high-quality articles reported recent research progress concerning genes, proteins, secondary metabolites involved in the interactions between crop plants and their pathogens as well as utilization of new synthetic chemicals in control of crop diseases. As co-editors of this research topic, we appreciate the contributions from the authors of the papers published under this topic and highlight the three themes drawn from their research findings.