Golovinomyces Orontii Research Articles

First Report of Powdery Mildew Caused by Golovinomyces orontii on Lactuca sativa in Korea

First Report of Powdery Mildew Caused by <i>Golovinomyces orontii</i> on <i>Lactuca sativa</i> in Korea

First Report of Powdery Mildew Caused by Erysiphe sedi on Bryophyllum pinnatum in China

HomePlant DiseaseVol. 100, No. 2First Report of Powdery Mildew Caused by Erysiphe sedi on Bryophyllum pinnatum in China PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Powdery Mildew Caused by Erysiphe sedi on Bryophyllum pinnatum in ChinaS.-R. Tang, S.-Y. Liu, B.-R. Wang, M. Yu, and Y. LiS.-R. TangSearch for more papers by this author, S.-Y. LiuSearch for more papers by this author, B.-R. WangSearch for more papers by this author, M. YuSearch for more papers by this author, and Y. LiSearch for more papers by this authorAffiliationsAuthors and Affiliations S.-R. Tang S.-Y. Liu B.-R. Wang M. Yu Y. Li , Engineering Research Center of Chinese Ministry of Education for Edible and Medicinal Fungi, Jilin Agricultural University, Changchun 130118, Jilin Province, P. R. China. Published Online:30 Nov 2015https://doi.org/10.1094/PDIS-07-15-0799-PDNAboutSectionsSupplemental ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Bryophyllum pinnatum (Lam.) Oken (syn. Kalancoe pinnata, family Crassulaceae) is a succulent and popular houseplant in tropical and subtropical areas. In March 2015, severe powdery mildew infections were observed on B. pinnatum at Jilin Agricultural University (43.81° N; 125.41° E), Changchun, China. The disease incidence was about 60 to 90% on leaves and stems. A representative specimen was deposited in the Herbarium of Mycology of Jilin Agricultural University (HMJAU) under Accession No. HMJAU02281. Whitish colonies covered the surfaces of leaves. Mycelia were flexuous to straight, branched, 3.4 to 7.7 μm wide, and septate. Hyphal appressoria were well developed, lobed, 3.1 to 5.6 μm diameter, solitary or in opposite pairs. Conidiophores were erect and straight, 102 to 157 × 8 to 11 μm. Foot-cells were cylindrical, 24 to 48 × 8 to 11 μm, and followed by 1 to 3 short cells or by a cell of about the same length or slightly longer and an additional shorter cell. Conidia were oblong-elliptical, subcylindrical or oval, 30 to 49 × 13 to 22 μm, formed singly, and without fibrosin bodies. Pseudoidium-type germ tubes were produced at the end of conidia. The teleomorphic state was not observed in the collected samples. The morphological characteristics of asexual structures were consistent with descriptions of Erysiphe sedi U. Braun (Braun and Cook 2012). To verify the identification of the pathogen, the complete internal transcribed spacer (ITS) region of rDNA of the pathogen was amplified by seminested PCR with the primers ITS5/P3, followed by ITS1/ITS4, and then sequenced. The resulting 552-bp sequence (GenBank Accession No. KR091961) was 100% homologous with E. sedi from Kalanchoe blossfeldiana (JX173288). Based on morphological and molecular characteristics, the fungus was identified as E. sedi. Pathogenicity test was completed by gently pressing a diseased leaf onto excised leaves of asymptomatic living seedlings. Noninoculated living seedlings were used as controls. Inoculated plants were maintained in an incubator at 24°C. Symptoms developed 12 days after inoculation, whereas the control leaves remained symptomless (voucher specimens HMJAU02281-I). The morphology of the fungus on the inoculated leaves was identical to that observed on the originally diseased leaves. Powdery mildew on B. pinnatum has been reported as Erysiphe pisi var. pisi, Golovinomyces orontii, Oidium spp. (Amano 1986; Farr and Rossman 2015), E. polygoni (Tai 1979), and Sphaerotheca fuliginea (Hashioka 1938). To our knowledge, this is the first unequivocal report of powdery mildew caused by E. sedi on B. pinnatum in China. The results in this study extended the host range of E. sedi.

Network-Based Comparative Analysis of Arabidopsis Immune Responses to Golovinomyces orontii and Botrytis cinerea Infections.

A comprehensive exploration of common and specific plant responses to biotrophs and necrotrophs is necessary for a better understanding of plant immunity. Here, we compared the Arabidopsis defense responses evoked by the biotrophic fungus Golovinomyces orontii and the necrotrophic fungus Botrytis cinerea through integrative network analysis. Two time-course transcriptional datasets were integrated with an Arabidopsis protein-protein interaction (PPI) network to construct a G. orontii conditional PPI sub-network (gCPIN) and a B. cinerea conditional PPI sub-network (bCPIN). We found that hubs in gCPIN and bCPIN played important roles in disease resistance. Hubs in bCPIN evolved faster than hubs in gCPIN, indicating the different selection pressures imposed on plants by different pathogens. By analyzing the common network from gCPIN and bCPIN, we identified two network components in which the genes were heavily involved in defense and development, respectively. The co-expression relationships between interacting proteins connecting the two components were different under G. orontii and B. cinerea infection conditions. Closer inspection revealed that auxin-related genes were overrepresented in the interactions connecting these two components, suggesting a critical role of auxin signaling in regulating the different co-expression relationships. Our work may provide new insights into plant defense responses against pathogens with different lifestyles.

Nuclear Function of Subclass I Actin-Depolymerizing Factor Contributes to Susceptibility in Arabidopsis to an Adapted Powdery Mildew Fungus

Actin-depolymerizing factors (ADFs) are conserved proteins that function in regulating the structure and dynamics of actin microfilaments in eukaryotes. In this study, we present evidence that Arabidopsis (Arabidopsis thaliana) subclass I ADFs, particularly ADF4, functions as a susceptibility factor for an adapted powdery mildew fungus. The null mutant of ADF4 significantly increased resistance against the adapted powdery mildew fungus Golovinomyces orontii. The degree of resistance was further enhanced in transgenic plants in which the expression of all subclass I ADFs (i.e. ADF1-ADF4) was suppressed. Microscopic observations revealed that the enhanced resistance of adf4 and ADF1-4 knockdown plants (ADF1-4Ri) was associated with the accumulation of hydrogen peroxide and cell death specific to G. orontii-infected cells. The increased resistance and accumulation of hydrogen peroxide in ADF1-4Ri were suppressed by the introduction of mutations in the salicylic acid- and jasmonic acid-signaling pathways but not by a mutation in the ethylene-signaling pathway. Quantification by microscopic images detected an increase in the level of actin microfilament bundling in ADF1-4Ri but not in adf4 at early G. orontii infection time points. Interestingly, complementation analysis revealed that nuclear localization of ADF4 was crucial for susceptibility to G. orontii. Based on its G. orontii-infected-cell-specific phenotype, we suggest that subclass I ADFs are susceptibility factors that function in a direct interaction between the host plant and the powdery mildew fungus.

First Report of Powdery Mildew Caused by Golovinomyces orontii (Erisyphe orontii) on Vinca major in South Carolina

First Report of Powdery Mildew Caused by <i>Golovinomyces orontii (Erisyphe orontii)</i> on <i>Vinca major</i> in South Carolina

Quantitative analyses on dynamic changes in the organization of host Arabidopsis thaliana actin microfilaments surrounding the infection organ of the powdery mildew fungus Golovinomyces orontii.

Obligate biotrophic fungi that cause powdery mildew on host plants form a specialized infection organ called the haustorium in the host apoplast. It was previously reported that the haustorium is surrounded by host actin microfilaments (AFs). The previous study used fixed cells, in which AFs were stained with fluorescently labeled phalloidine, therefore the structural dynamics of haustorium-surrounding AFs has not been examined. In the present study, we performed a live imaging analysis to examine the dynamics and developmental changes in the organization of haustorium-surrounding host AFs using host Arabidopsis thaliana and A. thaliana-adapted powdery mildew fungus Golovinomyces orontii. Image correlation-based velocimetry analysis suggested that AFs around haustorium are rather static compared to the dynamicity of AFs at the cell surface. Quantification of AF density and bundling showed that the density, but not the level of bundling, of haustorium-surrounding AFs increased as the haustorium matures. The possible role of AFs around haustoria is discussed.

Genetic structure and phylogeny of Italian and Czech populations of the cucurbit powdery mildew fungus Golovinomyces orontii inferred by multilocus sequence typing

Cucurbit powdery mildew caused by Golovinomyces orontii is a serious disease that affects cucurbit crops in temperate areas. In northern Italy, the species is responsible for the early infections at the beginning of the growing season. However, chasmothecia have never been recorded in Italy and the impact of either asexual or sexual reproduction of G. orontii remains to be determined. To investigate and compare the genetic structure of Italian populations with those from the Czech Republic, where chasmothecia are sporadically recorded, seven housekeeping gene fragments (tef‐1α, csI, ITS, H3, tub2, IGS and mtLSU) were amplified and used in a multilocus sequence typing (MLST) approach. The four‐gamete test was performed to detect recombination between and within populations. Phylogeny was inferred using both Bayesian and minimum evolution analyses. Results from MLST revealed the presence of 141 single nucleotide polymorphisms and the existence of 13 different haplotypes. Phylogenetic analysis revealed the existence of two main distinct phylogenetic groups that were also highly genetically different, confirming that G. orontii is a species complex. Recombination was detected within both the phylogenetic groups and the geographical populations, indicating that sexual reproduction could have occurred. However, considering the lower haplotype diversity and the high frequency of ST3 (17 isolates) and ST7 (eight isolates) haplotypes, the Italian population could be predominantly clonal, while sexual reproduction may occasionally occur due to the introduction of genotypes similar to those from the Czech Republic.

Down-regulation of Arabidopsis DND1 orthologs in potato and tomato leads to broad-spectrum resistance to late blight and powdery mildew

Multiple susceptibility genes (S), identified in Arabidopsis, have been shown to be functionally conserved in crop plants. Mutations in these S genes result in resistance to different pathogens, opening a new way to achieve plant disease resistance. The aim of this study was to investigate the role of Defense No Death1 (DND1) in susceptibility of tomato and potato to late blight (Phytophthora infestans). In Arabidopsis, the dnd1 mutant has broad-spectrum resistance against several fungal, bacterial, and viral pathogens. However this mutation is also associated with a dwarfed phenotype. Using an RNAi approach, we silenced AtDND1 orthologs in potato and tomato. Our results showed that silencing of the DND1 ortholog in both crops resulted in resistance to the pathogenic oomycete P. infestans and to two powdery mildew species, Oidium neolycopersici and Golovinomyces orontii. The resistance to P. infestans in potato was effective to four different isolates although the level of resistance (complete or partial) was dependent on the aggressiveness of the isolate. In tomato, DND1-silenced plants showed a severe dwarf phenotype and autonecrosis, whereas DND1-silenced potato plants were not dwarfed and showed a less pronounced autonecrosis. Our results indicate that S gene function of DND1 is conserved in tomato and potato. We discuss the possibilities of using RNAi silencing or loss-of-function mutations of DND1 orthologs, as well as additional S gene orthologs from Arabidopsis, to breed for resistance to pathogens in crop plants.Electronic supplementary materialThe online version of this article (doi:10.1007/s11248-015-9921-5) contains supplementary material, which is available to authorized users.

Cucurbit powdery mildews: methodology for objective determination and denomination of races

Cucurbit powdery mildew (CPM), a disease on field and greenhouse cucurbit crops worldwide, is caused most frequently by two obligate erysiphaceous ectoparasites (Golovinomyces orontii s.l., Podosphaera xanthii) that are highly variable in their pathogenicity and virulence. Various independent systems of CPM race determination and denomination are used worldwide, having to date been differentiated on different cultivars or lines of melon (Cucumis melo L.). We briefly review historical perspectives and the current state of understanding of the virulence variation of the two CPM pathogens at the pathogenic race level, their differentiation and their designation. Furthermore, we propose for use by the international CPM research, breeding, seed and production community new tools to enhance research, communication and management of CPM. These tools consist of five components: 1) a set of 21 differential genotypes of Cucumis melo L. for the identification of CPM races; 2) a triple-part, septet code for meaningful, concise designation of CPM races; 3) protocols for maintaining CPM isolates and differential genotypes and for laboratory assays to examine the virulence of CPM isolates; 4) rules and principles of practical application of this system in breeding, seed production and cucurbit growing, including a proposal of a race denomination suitable for practical application; and 5) crucial activities leading to the implementation and running of new tools for CPM research and management. The five components of this package have equal importance, forming a compact system, and none of them can be omitted.

OCCURRENCE OF CHASMOTHECIA AND MATING TYPE DISTRIBUTION OF PODOSPHAERA XANTHII, A CAUSAL AGENT OF CUCURBIT POWDERY MILDEW IN NORTHERN ITALY

Powdery mildew is a widespread disease that causes important losses to cucurbit production. The main agents of the disease are Podosphaera xanthii and Golovinomyces orontii. To determine the occurrence of chasmothecia as overwintering forms of both fungal species in northern Italy, powdery mildew-infected samples from cultivated cucurbits were collected in different locations of the provinces of Bologna and Mantova during 2010, 2011 and 2012. Only the sexual stage of P. xanthii was found, indicating that in northern Italy, contrary to what reported from other areas of the Mediterranean basin, the pathogen overwinters as chasmothecia. In parallel, to determine the frequency and distribution of both MAT 1-1-1 and MAT 1-2-1 P. xanthii idiomorphs, a multiplex- PCR with MAT idiomorph-specific primers was carried out on 147 monoconidial isolates obtained from infected leaf samples. The obtained frequencies were tested for random mating. Results showed a MAT ratio that tended to 1:1, supporting the finding of the sexual stage thus suggesting the occurrence of actively mating populations and that sexual reproduction plays a significant role in the life cycle of P. xanthii in this area. The lack of G. orontii chasmothecia suggests that this species might have alternative overwintering strategies.

Seasonal variations in the occurrence ofGolovinomyces orontiiandPodosphaera xanthii, causal agents of cucurbit powdery mildew in Northern Italy

Powdery mildew, caused by Golovinomyces orontii and Podosphaera xanthii, is a widespread disease that causes important losses in cucurbit production. To determine the aetiology and the epidemiology of cucurbit powdery mildew disease in the North of Italy, observations on the occurrence of the main disease-causing fungal species were conducted during the 2010, 2011 and 2012 growing seasons. Samples of infected leaves of zucchini, melon and pumpkin plants, either from field or greenhouse crops, were collected every 15–18 days from May to September/October. To identify the fungal species, both morphological observations based on the asexual stage and molecular identifications by a Multiplex-PCR reaction with species-specific primers were performed. Climatic parameters of temperature and relative humidity were also monitored. Pearson's correlation coefficient and Principal Component Analysis showed a negative significant correlation between the two species, and a peculiar epidemiological behaviour was also observed: the earlier infections were caused by G. orontii, which was the predominant species till the end of June–middle of July. At this time, this species progressively decreased in frequency and was replaced by P. xanthii that became the main species infecting cucurbits till the end of the growing season. As the two species have different ecological requirements, these seasonal variations in the cucurbit powdery mildew species composition could possibly be explained by the influence of temperature and relative humidity on the pathogen epidemiology during the growing season but also by the different overwintering strategies adopted by the two species.

First Report of Powdery Mildew Caused by Golovinomyces orontii on Invasive Weed Lactuca serriola in Korea

First Report of Powdery Mildew Caused by <i>Golovinomyces orontii</i> on Invasive Weed <i>Lactuca serriola</i> in Korea

Convergent targeting of a common host protein-network by pathogen effectors from three kingdoms of life.

While conceptual principles governing plant immunity are becoming clear, its systems-level organization and the evolutionary dynamic of the host-pathogeninterface are still obscure. We generated a systematic protein-protein interaction network of virulence effectors from the ascomycete pathogen Golovinomyces orontii and Arabidopsis thaliana host proteins. We combined this data set with corresponding data for the eubacterial pathogen Pseudomonas syringae and the oomycete pathogen Hyaloperonospora arabidopsidis. The resulting network identifies host proteins onto which intraspecies and interspecies pathogen effectors converge. Phenotyping of 124 Arabidopsis effector-interactor mutants revealed a correlation between intraspecies and interspecies convergence and several altered immune response phenotypes. Several effectors and the most heavily targeted host protein colocalized in subnuclear foci. Products of adaptively selected Arabidopsis genes are enriched for interactions with effector targets. Our data suggest the existenceof a molecular host-pathogen interface that isconserved across Arabidopsis accessions, while evolutionary adaptation occurs in the immediate network neighborhood of effector targets.

The powdery mildew resistance protein RPW8.2 is carried on VAMP721/722 vesicles to the extrahaustorial membrane of haustorial complexes.

Plants employ multiple cell-autonomous defense mechanisms to impede pathogenesis of microbial intruders. Previously we identified an exocytosis defense mechanism in Arabidopsis against pathogenic powdery mildew fungi. This pre-invasive defense mechanism depends on the formation of ternary protein complexes consisting of the plasma membrane-localized PEN1 syntaxin, the adaptor protein SNAP33 and closely sequence-related vesicle-resident VAMP721 or VAMP722 proteins. The Arabidopsis thaliana resistance to powdery mildew 8.2 protein (RPW8.2) confers disease resistance against powdery mildews upon fungal entry into host cells and is specifically targeted to the extrahaustorial membrane (EHM), which envelops the haustorial complex of the fungus. However, the secretory machinery involved in trafficking RPW8.2 to the EHM is unknown. Here we report that RPW8.2 is transiently located on VAMP721/722 vesicles, and later incorporated into the EHM of mature haustoria. Resistance activity of RPW8.2 against the powdery mildew Golovinomyces orontii is greatly diminished in the absence of VAMP721 but only slightly so in the absence of VAMP722. Consistent with this result, trafficking of RPW8.2 to the EHM is delayed in the absence of VAMP721. These findings implicate VAMP721/722 vesicles as key components of the secretory machinery for carrying RPW8.2 to the plant-fungal interface. Quantitative fluorescence recovery after photobleaching suggests that vesicle-mediated trafficking of RPW8.2-yellow fluorescent protein (YFP) to the EHM occurs transiently during early haustorial development and that lateral diffusion of RPW8.2-YFP within the EHM exceeds vesicle-mediated replenishment of RPW8.2-YFP in mature haustoria. Our findings imply the engagement of VAMP721/722 in a bifurcated trafficking pathway for pre-invasive defense at the cell periphery and post-invasive defense at the EHM.

Interaction of a Blumeria graminis f. sp. hordei effector candidate with a barley ARF‐GAP suggests that host vesicle trafficking is a fungal pathogenicity target

Filamentous phytopathogens, such as fungi and oomycetes, secrete effector proteins to establish successful interactions with their plant hosts. In contrast with oomycetes, little is known about effector functions in true fungi. We used a bioinformatics pipeline to identify Blumeria effector candidates (BECs) from the obligate biotrophic barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh). BEC1-BEC5 are expressed at different time points during barley infection. BEC1, BEC2 and BEC4 have orthologues in the Arabidopsis thaliana-infecting powdery mildew fungus Golovinomyces orontii. Arabidopsis lines stably expressing the G. orontii BEC2 orthologue, GoEC2, are more susceptible to infection with the non-adapted fungus Erysiphe pisi, suggesting that GoEC2 contributes to powdery mildew virulence. For BEC3 and BEC4, we identified thiopurine methyltransferase, a ubiquitin-conjugating enzyme, and an ADP ribosylation factor-GTPase-activating protein (ARF-GAP) as potential host targets. Arabidopsis knockout lines of the respective HvARF-GAP orthologue (AtAGD5) allowed higher entry levels of E. pisi, but exhibited elevated resistance to the oomycete Hyaloperonospora arabidopsidis. We hypothesize that ARF-GAP proteins are conserved targets of powdery and downy mildew effectors, and we speculate that BEC4 might interfere with defence-associated host vesicle trafficking.

First Report of Powdery Mildew Caused by Golovinomyces magnicellulatus var. magnicellulatus on Physalis alkekengi var. franchetii in China.

Physalis alkekengi var. franchetii (Mast.) Makino, known as Chinese lantern, belonging to Solanaceae, is cultivated for its fruits of medicinal value in East Asia (4). Since July 2010, a powdery mildew has been continuously observed on this plant in Shenyang City in northeastern China. More than 90% of the plants in a garden were affected. Symptoms first appeared as circular to irregular white patches, which progressed to abundant mycelial growth on both sides of leaves and young stems. In the middle of August, chasmothecia were formed abundantly, especially on the lower leaf surface. Voucher specimens were deposited in the herbarium of Qingdao Agricultural University (HMQAU10014, 12047, and 12144). Conidiophores produced 2 to 5 conidia in chains with a sinuate outline, followed by 2 to 3 cells. Foot-cells of conidiophores were straight and 46 to 65 × 9 to 11 μm. Conidia were ellipsoid to barrel-shaped, 26 to 32 × 13 to 15.5 μm, lacked distinct fibrosin bodies, and produced germ tubes on the subterminal position. Chasmothecia were amphigenous, also cauligenous, 100 to 140 μm across, and contained 10 to 25 asci. Appendages were mycelioid, 0.5 to 4 times as long as the chasmothecial diameter, and 1- to 3-septate. Asci were short-stalked, 45 to 64 × 26 to 34 μm, and contained two ascospores of 18 to 25 × 12.5 to 15 μm. The internal transcribed spacer (ITS) region of rDNA was amplified (3) and sequenced. The resulting 600-bp sequence was deposited in GenBank (Accession No. KC488260). A GenBank BLAST search of complete ITS sequence showed 100% identity with that of Golovinomyces orontii on P. alkekengi var. franchetii (AB077647 ex Japan) and >99% similarity with those of G. magnicellulatus on Phlox paniculata (AB077621 ex Japan, AF011303 ex the United States, and GU945756 and GU945757 ex Korea). G. orontii is currently confined to the Golovinomyces isolates on Cichorioideae (1). On the basis of the morphological characteristics and ITS sequence data, the fungus was identified as G. magnicellulatus var. magnicellulatus (U. Braun) V.P. Heluta (1). It was already noted that Golovinomyces isolates on Physalis and Phlox are phylogenetically close each other (3). A pathogenicity test was conducted by gently pressing a diseased leaf onto leaves of five healthy Chinese lanterns. Five non-inoculated plants served as controls. Inoculated plants developed symptoms after 8 days, whereas the control plants remained symptomless. The fungus present on the inoculated plants was morphologically identical to that originally observed on diseased plants, fulfilling Koch's postulates. Powdery mildews of Chinese lantern associated with Golovinomyces species have been known in Korea and Japan (2). A Korean material of Golovinomyces sp. on P. alkekengi var. franchetii was identified as G. magnicellulatus var. magnicellulatus based on morphological characteristics and 100% ITS sequence identity with a Chinese isolate (Shin, unpublished data). To our knowledge, this is the first report of powdery mildew caused by G. magnicellulatus var. magnicellulatus on Chinese lantern in China.

Analyses of wrky18 wrky40 Plants Reveal Critical Roles of SA/EDS1 Signaling and Indole-Glucosinolate Biosynthesis for Golovinomyces orontii Resistance and a Loss-of Resistance Towards Pseudomonas syringae pv. tomato AvrRPS4

Simultaneous mutation of two WRKY-type transcription factors, WRKY18 and WRKY40, renders otherwise susceptible wild-type Arabidopsis plants resistant towards the biotrophic powdery mildew fungus Golovinomyces orontii. Resistance in wrky18 wrky40 double mutant plants is accompanied by massive transcriptional reprogramming, imbalance in salicylic acid (SA) and jasmonic acid (JA) signaling, altered ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1) expression, and accumulation of the phytoalexin camalexin. Genetic analyses identified SA biosynthesis and EDS1 signaling as well as biosynthesis of the indole-glucosinolate 4MI3G as essential components required for loss-of-WRKY18 WRKY40-mediated resistance towards G. orontii. The analysis of wrky18 wrky40 pad3 mutant plants impaired in camalexin biosynthesis revealed an uncoupling of pre- from postinvasive resistance against G. orontii. Comprehensive infection studies demonstrated the specificity of wrky18 wrky40-mediated G. orontii resistance. Interestingly, WRKY18 and WRKY40 act as positive regulators in effector-triggered immunity, as the wrky18 wrky40 double mutant was found to be strongly susceptible towards the bacterial pathogen Pseudomonas syringae DC3000 expressing the effector AvrRPS4 but not against other tested Pseudomonas strains. We hypothesize that G. orontii depends on the function of WRKY18 and WRKY40 to successfully infect Arabidopsis wild-type plants while, in the interaction with P. syringae AvrRPS4, they are required to mediate effector-triggered immunity.

Host Cell Ploidy Underlying the Fungal Feeding Site Is a Determinant of Powdery Mildew Growth and Reproduction

Golovinomyces orontii is an obligate biotrophic powdery mildew (PM) that colonizes Arabidopsis thaliana and agronomic species. It establishes a specialized feeding structure in epidermal cells to fuel its extensive surface hyphal growth and reproduction. Previously, endoreduplication was identified in Arabidopsis mesophyll cells underlying the fungal feeding site, presumably to meet the metabolic demands imposed by the fungus. Furthermore, the cell cycle transcription factor MYB3R4 was shown to regulate this process. Herein, PM-induced endoreduplication is further characterized and three additional factors influencing host ploidy in cells underlying the fungal feeding site are identified. While mutations in PUX2 and PMR6 reduce basal ploidy, mutations in PMR5 (and MYB3R4) abrogate the PM-induced ploidy increase. Moreover, analysis of pmr5 microarray data suggests that PMR5 acts upstream of a MYB3R transcription factor such as MYB3R4 to control PM-induced ploidy. Induced endoreduplication occurs exclusively in mesophyll cells underlying the fungal feeding site at 5 days postinoculation, concomitant with PM reproduction. Gene copy number increases and chromatin remains decondensed, suggesting active, elevated gene expression. Cell ploidy underlying the fungal feeding site is highly correlated with the extent of PM growth and reproduction for these mutants, indicating that (induced) mesophyll cell ploidy is a PM susceptibility determinant.

First Report of Powdery Mildew Caused by Golovinomyces sp. on Plantago australis in Brazil.

The plantain Plantago australis Lam. (Plantaginaceae) is a herbaceous species native to southern Brazil that is known for the analgesic, antibiotic, and anti-inflammatory properties of its leaf extracts (2). Powdery mildew was observed on wild P. australis plants in the cities of Tapejara, Jari, and Santa Maria (State of Rio Grande do Sul, Brazil) during the summer of 2011. Affected plants were more often observed in shaded areas. Signs included sparse to abundant white powdery masses of conidia and mycelium on pseudo-petioles and leaves, mostly on the adaxial surface. Severely affected plants (≥80% of foliar area affected) had small chlorotic leaves and reduced size compared to healthy ones. Mycelia were superficial and presented nipple-shaped appressoria. Conidiophores were often curved at the base, unbranched, cylindrical, 81 to 125 μm long (average 97.3 ± 14.9 μm) and composed of a cylindrical foot cell 52 to 73 μm long (average 65.4 ± 7.5 μm) and 9 to 14 μm wide (average 11.6 ± 1.5 μm) followed by one to two shorter cells 17 to 29 μm long (average 23.4 ± 3.6 μm). Conidia were produced in chains of up to eight cells, did not contain fibrosin bodies, ranged from ellipsoid-ovoid to subcylindrical, and measured 24 to 35 μm long (average 30.5 ± 3.7 μm) and 12 to 19 μm wide (average 15.8 ± 1.7 μm). Germ tubes were produced apically (reticuloidium type). Chasmothecia were not observed on sampled leaves. Genomic DNA was extracted from conidia, conidiophores, and mycelium and used to amplify the internal transcribed spacer (ITS) (ITS1-5.8s-ITS2) region using the ITS1 and ITS4 primers. The resulting sequence (558 bp) was deposited under accession number JX312220 in GenBank. Searches with the BLASTn algorithm revealed similarity of 100% with Golovinomyces orontii (Castagne) V.P. Heluta 1988 from Veronica arvensis L. (AB077652.1) (3), 99% with G. orontii from Galium spurium L. and Galium aparine L. (AB430818.1 and AB430813.1) (2) and 99% with G. sordidus (L. Junell) V.P. Heluta 1988 from P. lanceolata L. (AB077665.1) (3). Based on morphological characteristics and sequence analysis of the ITS region, the fungus was identified as belonging to Golovinomyces sp. To fulfill Koch's postulates, five cultivated plants of P. australis with four to five expanded leaves were inoculated by dusting conidia (10 to 15 conidia cm-2) on their leaves. Inoculated and non-inoculated control plants were kept in a greenhouse at 27 ± 5°C and relative humidity of 80 ± 15%. Powdery mildew symptoms identical to those of wild plants were observed 8 to 10 days after in inoculated plants. Although G. sordidus was previously reported on P. australis subsp. hirtella in Argentina and on several species of Plantago in others world regions (1), to our knowledge, Golovinomyces sp. has not been previously reported as a pathogen of P. australis in Brazil. Although the economic impact of the disease is limited, the reduction in plant size and leaves affects biomass production used in the extraction of pharmaceutical compounds.

The role of nitric oxide in the interaction of Arabidopsis thaliana with the biotrophic fungi, Golovinomyces orontii and Erysiphe pisi

Powdery mildews are a diverse group of pathogenic fungi that can infect a large number of plant species, including many economically important crops. However, basic and applied research on these devastating diseases has been hampered by the obligate biotrophic lifestyle of the pathogens, which require living host cells for growth and reproduction, and lacking genetic and molecular tools for important host plants. The establishment of Arabidopsis thaliana as a host of different powdery mildew species allowed pursuing new strategies to study the molecular mechanisms governing these complex plant–pathogen interactions. Nitric oxide (NO) has emerged as an important signaling molecule in plants, which is produced upon infection and involved in activation of plant immune responses. However, the source and pathway of NO production and its precise function in the regulatory network of reactions leading to resistance is still unknown. We studied the response of Arabidopsis thaliana to infection with the adapted powdery mildew, Golovinomyces orontii (compatible interaction) and the non-adapted, Erysiphe pisi (incompatible interaction). We observed that NO accumulated rapidly and transiently at infection sites and we established a correlation between the resistance phenotype and the amount and timing of NO production. Arabidopsis mutants with defective immune response accumulated lower NO levels compared to wild type. Conversely, increased NO levels, generated by treatment with chemicals or expression of a NO-synthesizing enzyme, resulted in enhanced resistance, but only sustained NO production prevented excessive leaf colonization by the fungus, which was not achieved by a short NO burst although this reduced the initial penetration success. By contrast, lowered NO levels did not impair the ultimate resistance phenotype. Although our results suggest a function of NO in mediating plant immune responses, a direct impact on pathogen growth and development cannot be excluded.