Powdery Mildew In Arabidopsis Research Articles

TaATG8 Is Involved in the Response to Abiotic Stresses and Powdery Mildew in Arabidopsis

TaATG8 Is Involved in the Response to Abiotic Stresses and Powdery Mildew in Arabidopsis

Chimeric mutations in grapevine ENHANCED DISEASE RESISTANCE1 improve resistance to powdery mildew without growth penalty.

Grapevine (Vitis vinifera L.) incurs severe quality degradation and yield loss from powdery mildew, a major fungal disease caused by Erysiphe necator. ENHANCED DISEASE RESISTANCE1 (EDR1), a Raf-like mitogen-activated protein kinase kinase kinase, negatively regulates defense responses against powdery mildew in Arabidopsis (Arabidopsis thaliana). However, little is known about the role of the putatively orthologous EDR1 gene in grapevine. In this study, we obtained grapevine VviEDR1-edited lines using CRISPR/Cas9. Plantlets containing homozygous and bi-allelic indels in VviEDR1 developed leaf lesions shortly after transplanting into the soil and died at the seedling stage. Transgenic plants expressing wild-type VviEDR1 and mutant Vviedr1 alleles as chimera (designated as VviEDR1-chi) developed normally and displayed enhanced resistance to powdery mildew. Interestingly, VviEDR1-chi plants maintained a spatiotemporally distinctive pattern of VviEDR1 mutagenesis: while almost no mutations were detected from terminal buds, ensuring normal function of the apical meristem, mutations occurred in young leaves and increased as leaves matured, resulting in resistance to powdery mildew. Further analysis showed that the resistance observed in VviEDR1-chi plants was associated with callose deposition, increased production of salicylic acid and ethylene, H2O2 production and accumulation, and host cell death. Surprisingly, no growth penalty was observed with VviEDR1-chi plants. Hence, this study demonstrated a role of VviEDR1 in the negative regulation of resistance to powdery mildew in grapevine and provided an avenue for engineering powdery mildew resistance in grapevine.

Spatial regulation of immunity: unmasking the secrets of abaxial immunity to powdery mildew.

This article comments on: Wu Y, Sexton WK, Zhang Q, Bloodgood D, Wu Y, Hooks C, Coker F, Vasquez A, Wei C-I, Xiao S. 2024. Leaf abaxial immunity to powdery mildew in Arabidopsis is conferred by multiple defense mechanisms. Journal of Experimental Botany 75, 1465-1478.

Leaf abaxial immunity to powdery mildew in Arabidopsis is conferred by multiple defense mechanisms.

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.

Silicon modulates multi-layered defense against powdery mildew in Arabidopsis

Silicon (Si) has been widely employed in agriculture to enhance resistance against pathogens in many crop plants. However, the underlying molecular mechanisms of Si-mediated resistance remain elusive. In this study, the Arabidopsis-powdery mildew pathosystem was employed to investigate possible defense mechanisms of Si-mediated resistance. Because Arabidopsis lacks efficient Si transporters and thus is a low Si-accumulator, two heterologous Si influx transporters (from barley and muskmelon) were individually expressed in wild-type Arabidopsis Col-0 and a panel of mutants defective in different immune signaling pathways. Results from infection tests showed that while very low leaf Si content slightly induced salicylic acid (SA)-dependent resistance, high Si promoted PAD4-dependent but largely EDS1- and SA-independent resistance against the adapted powdery mildew isolate Golovinomyces cichoracearum UCSC1. Intriguingly, our results also showed that high Si could largely reboot non-host resistance in an immune-compromised eds1/pad4/sid2 triple mutant background against a non-adapted powdery mildew isolate G. cichoracearum UMSG1. Taken together, our results suggest that assimilated Si modulates distinct, multi-layered defense mechanisms to enhance plant resistance against adapted and no-adapted powdery mildew pathogens, possibly via synergistic interaction with defense-induced callose.

A core effector UV_1261 promotes Ustilaginoidea virens infection via spatiotemporally suppressing plant defense

False smut is a destructive grain disease of rice worldwide, characterized by false smut balls formed in rice flowers. Here we identified a small secreted protein UV_1261 contributing to virulence of Ustilaginoidea virens, the causal agent of this disease. The sequence of UV_1261 was highly conserved among isolates of U. virens and absent in other fungi. UV_1261 encodes a protein targeted to plant chloroplasts. Its expression exhibited a bimodal pattern during pathogenesis. Ectopic expression of UV_1261 in Nicotiana benthamiana and Arabidopsis led to suppression of flg22-induced ROS burst, callose deposition, and expression of defense-related genes, as well as enhanced susceptibility to powdery mildew in Arabidopsis. Down-regulation of UV_1261 via exogenous siRNA treatment resulted in reduced number of false smut balls. Consistently, stably knocking-down UV_1261 caused less number of false smut balls associated with higher expression of defense-related genes in rice flower. Taken together, our data demonstrate that UV_1261 is a core effector of U. virens essential for virulence and suppressing defense in rice flower, and thus may serve as a potential molecular target for controlling rice false smut disease.

Transcriptome analysis reveals pathways facilitating the growth of tobacco powdery mildew in Arabidopsis

Powdery mildew (PM) fungi are biotrophic pathogens that rely on living hosts to survive and thrive. However, their colonization is restricted by host defenses at both the penetration and post-penetration stages. The tobacco PM strain Golovinomyces cichoracearum (Gc) SICAU1 has overcome penetration resistance of Arabidopsis but its growth is arrested by post-penetration resistance. While Gc SICAU1 only poorly grows in Arabidopsis Col-0 wild-type plants, it can sustainably grow for more than 20 days on the same infected leaves of the double mutant pad4–1 sid2–1 that is defective in both the synthesis and signaling of salicylic acid (SA). To understand the underlying molecular mechanisms, we conducted a comparative transcriptome analysis between Col-0 and pad4–1 sid2–1 in response to Gc SICAU1. We found that 4811 genes were differentially expressed more than four-fold between any two of the measured seven time points (0, 1, 3, 6, 8, 10 and 12 days post-inoculation). Gene expression pattern analysis suggests that differential expression of 348 genes and 190 genes may explain resistance in Col-0 and susceptibility in pad4–1 sid2–1, respectively. Gene Ontology (GO) analysis suggests that Gc SICAU1 might be arrested in Col-0 by both pattern-triggered immunity and SA-dependent defense. By contrast, its sustained growth in pad4–1 sid2–1 may be attributable to the activation of a detoxification pathway that is normally repressed by the SA-signaling pathway. Taken together, our results suggest that multiple distinct, yet interconnected pathways control the growth of tobacco powdery mildew in Arabidopsis.

Proper expression of AS1 is required for RPW8.1-mediated defense against powdery mildew in Arabidopsis

The ectopic expression of Arabidopsis gene RESISTANCE TO POWDERY MILDEW 8.1 (R1Y4 plant line) triggers a hypersensitive response (HR) providing defense potentially effective against different pathogens. In order to understand the regulation of RPW8.1-mediated signaling, an approach based on ethyl methanesulfonate (EMS) mutagenesis was adopted to identify mutants that enhance or impair RPW8.1-mediated cell death. Here we describe the identification of mutants b10-6 and b9-1, point mutations that were induced in the At2g37630 locus resulting in an amino acid substitution or a premature stop codon in ASYMMETRIC LEAVES 1 (AS1), respectively. Loss-of-function in AS1 altered the transcription of RPW8.1, RPW8.1-dependent necrotic lesions in leaves and resistance to powdery mildew. In the mutant b10-6, RPW8.1-mediated cell death and disease resistance was compromised, however. in b9-1 RPW8.1-mediated cell death and resistance to powdery mildew was slightly enhanced. Expression of AS1-eYFP within b10-6 background suppressed RPW8.1-mediated cell death and disease resistance in the short-day condition, but largely restored the phenotypes of b10-6 to that of R1Y4 in the long-day condition. These data imply that proper expression of AS1 is required for full function of RPW8.1 in triggering cell death and resistance to powdery mildew.

EDS1-mediated basal defense and SA-signaling contribute to post-invasion resistance against tobacco powdery mildew in Arabidopsis

Arabidopsis-powdery mildew is a unique pathosystem in dissecting the molecular mechanism of plant-biotrophic fungus interactions. To increase the diversity of powdery mildew pathogens, we identified a tobacco powdery mildew, designated Gc SICAU1, which belonged to Golovinomyces cichoracearum (Gc) based on the internal transcribed spacer (ITS) rDNA fragment sequences. Inoculation test in laboratory conditions revealed that Gc SICAU1 could colonize on wild type Arabidopsis Col-0 leaves, but failed to grow as abundantly on tobacco leaves, indicating that Gc SICAU1 has completely overcome the pre-invasion resistance and its colonization is hindered by the post-invasion resistance in Arabidopsis. The growth of Gc SICAU1 in the fls2, efr-1 and pen1-1 mutants displayed no obvious difference from that of wild type plants. In contrast, salicylic acid (SA)-signaling deficient mutants such as pad4-1 and eds5-1, but not ethylene-signaling mutants, exhibited increased susceptibility to Gc SICAU1. Moreover, eds1-2 and pad4-1/sid2-1 were fully susceptible to Gc SICAU1 and the disease symptom was comparable to Gc SICAU1 on tobacco leaves. These data indicate that EDS1-mediated basal defense and SA-signaling play critical roles for post-invasion resistance against this pathogen. Taken together, our data demonstrated that Gc SICAU1 is a pathogen weakly virulent to Arabidopsis and can be used as a tool to finely dissect post-invasion resistance mechanisms.

Dominant negative RPW8.2 fusion proteins reveal the importance of haustorium-oriented protein trafficking for resistance against powdery mildew in Arabidopsis

Powdery mildew fungi form feeding structures called haustoria inside epidermal cells of host plants to extract photosynthates for their epiphytic growth and reproduction. The haustorium is encased by an interfacial membrane termed the extrahaustorial membrane (EHM). The atypical resistance protein RPW8.2 from Arabidopsis is specifically targeted to the EHM where RPW8.2 activates haustorium-targeted (thus broad-spectrum) resistance against powdery mildew fungi. EHM-specific localization of RPW8.2 suggests the existence of an EHM-oriented protein/membrane trafficking pathway during EHM biogenesis. However, the importance of this specific trafficking pathway for host defense has not been evaluated via a genetic approach without affecting other trafficking pathways. Here, we report that expression of EHM-oriented, nonfunctional RPW8.2 chimeric proteins exerts dominant negative effect over functional RPW8.2 and potentially over other EHM-localized defense proteins, thereby compromising both RPW8.2-mediated and basal resistance to powdery mildew. Thus, our results highlight the importance of the EHM-oriented protein/membrane trafficking pathway for host resistance against haustorium-forming pathogens such as powdery mildew fungi.

Elevated Early Callose Deposition Results in Complete Penetration Resistance to Powdery Mildew in Arabidopsis

A common response by plants to fungal attack is deposition of callose, a (1,3)-β-glucan polymer, in the form of cell wall thickenings called papillae, at site of wall penetration. While it has been generally believed that the papillae provide a structural barrier to slow fungal penetration, this idea has been challenged in recent studies of Arabidopsis (Arabidopsis thaliana), where fungal resistance was found to be independent of callose deposition. To the contrary, we show that callose can strongly support penetration resistance when deposited in elevated amounts at early time points of infection. We generated transgenic Arabidopsis lines that express POWDERY MILDEW RESISTANT4 (PMR4), which encodes a stress-induced callose synthase, under the control of the constitutive 35S promoter. In these lines, we detected callose synthase activity that was four times higher than that in wild-type plants 6 h post inoculation with the virulent powdery mildew Golovinomyces cichoracearum. The callose synthase activity was correlated with enlarged callose deposits and the focal accumulation of green fluorescent protein-tagged PMR4 at sites of attempted fungal penetration. We observed similar results from infection studies with the nonadapted powdery mildew Blumeria graminis f. sp. hordei. Haustoria formation was prevented in resistant transgenic lines during both types of powdery mildew infection, and neither the salicylic acid-dependent nor jasmonate-dependent pathways were induced. We present a schematic model that highlights the differences in callose deposition between the resistant transgenic lines and the susceptible wild-type plants during compatible and incompatible interactions between Arabidopsis and powdery mildew.

PAPP2C Interacts with the Atypical Disease Resistance Protein RPW8.2 and Negatively Regulates Salicylic Acid-Dependent Defense Responses in Arabidopsis

Many fungal and oomycete pathogens differentiate a feeding structure named the haustorium to extract nutrition from the plant epidermal cell. The atypical resistance (R) protein RPW8.2 activates salicylic acid (SA)-dependent, haustorium-targeted defenses against Golovinomyces spp., the causal agents of powdery mildew diseases on multiple plant species. How RPW8.2 activates defense remains uncharacterized. Here, we report that RPW8.2 interacts with the phytochrome-associated protein phosphatase type 2C (PAPP2C) in yeast and in planta as evidenced by co-immunoprecipitation and bimolecular fluorescence complementation assays. Down-regulation of PAPP2C by RNA interference (RNAi) in Col-0 plants lacking RPW8.2 leads to leaf spontaneous cell death and enhanced disease resistance to powdery mildew via the SA-dependent signaling pathway. Moreover, down-regulation of PAPP2C by RNAi in the RPW8.2 background results in strong HR-like cell death, which correlates with elevated RPW8.2 expression. We further demonstrate that hemagglutinin (HA)-tagged PAPP2C prepared from tobacco leaf cells transiently transformed with HA-PAPP2C possesses phosphatase activity. In addition, silencing a rice gene (Os04g0452000) homologous to PAPP2C also results in spontaneous cell death in rice. Combined, our results suggest that RPW8.2 is functionally connected with PAPP2C and that PAPP2C negatively regulates SA-dependent basal defense against powdery mildew in Arabidopsis.

Evaluation of Silicon for Managing Powdery Mildew on Gerbera Daisy

ABSTRACT Powdery mildew, caused by Erysiphe cichoracearum or Podosphaera fusca, is a common disease of gerbera daisy (Gerbera jamesonii) grown in Florida. Previous studies demonstrated that silicon reduces powdery mildew in Arabidopsis, cucumber, grape, strawberry, and wheat. In this study, two silicon (Si) sources, calcium silicate and potassium silicate, were evaluated for their ability to reduce powdery mildew in gerbera ‘Snow White’. The effect of calcium silicate in flower quality and the silicon uptake was also determined. Calcium silicate was not effective in reducing powdery mildew or in improving flower quality. The silicon content of gerberas treated with potassium silicate was slightly greater than that in plants treated with calcium silicate. However, the severity of powdery mildew was not reduced by potassium silicate. The results suggest that silicon may not be useful for managing this disease of gerbera daisy, possibly due to the low accumulation of silicon by gerbera leaves.