Inflorescences vs leaves: a distinct modulation of carbon metabolism process during Botrytis infection

Plant growth and survival depends critically on photo assimilates. Pathogen infection leads to changes in carbohydrate metabolism of plants. In this study, we monitored changes in the carbohydrate metabolism in the grapevine inflorescence and leaves using Botrytis cinerea and Botrytis pseudo cinerea. Fluctuations in gas exchange were correlated with variations in chlorophyll a fluorescence. During infection, the inflorescences showed an increase in net photosynthesis (Pn) with a stomatal limitation. In leaves, photosynthesis decreased, with a non-stomatal limitation. A decrease in the effective photosystem II (PSII) quantum yield (ΦPSII) was accompanied by an increase in photochemical quenching (qP) and non-photochemical quenching (qN). The enhancement of qP and ΦPSII could explain the observed increase in Pn. In leaves, the significant decline in ΦPSII and qP, and increase in qN suggest that energy was mostly oriented toward heat dissipation instead of CO2 fixation. The accumulation of glucose and sucrose in inflorescences and glucose and fructose in the leaves during infection indicate that the plant's carbon metabolism is differently regulated in these two organs. While a strong accumulation of starch was observed at 24 and 48 hours post-inoculation (hpi) with both species of Botrytis in the inflorescences, a significant decrease with B. cinerea at 24 hpi and a significant increase with B. pseudo cinerea at 48 hpi were observed in the leaves. On the basis of these results, it can be said that during pathogen attack, the metabolism of grapevine inflorescence and leaf is modified suggesting distinct mechanisms modifying gas exchange, PSII activity and sugar contents in these two organs.

Transcriptome and metabolome reprogramming in Vitis vinifera cv. Trincadeira berries upon infection with Botrytis cinerea

Vitis vinifera berries are sensitive towards infection by the necrotrophic pathogen Botrytis cinerea, leading to important economic losses worldwide. The combined analysis of the transcriptome and metabolome associated with fungal infection has not been performed previously in grapes or in another fleshy fruit. In an attempt to identify the molecular and metabolic mechanisms associated with the infection, peppercorn-sized fruits were infected in the field. Green and veraison berries were collected following infection for microarray analysis complemented with metabolic profiling of primary and other soluble metabolites and of volatile emissions. The results provided evidence of a reprogramming of carbohydrate and lipid metabolisms towards increased synthesis of secondary metabolites involved in plant defence, such as trans-resveratrol and gallic acid. This response was already activated in infected green berries with the putative involvement of jasmonic acid, ethylene, polyamines, and auxins, whereas salicylic acid did not seem to be involved. Genes encoding WRKY transcription factors, pathogenesis-related proteins, glutathione S-transferase, stilbene synthase, and phenylalanine ammonia-lyase were upregulated in infected berries. However, salicylic acid signalling was activated in healthy ripening berries along with the expression of proteins of the NBS-LRR superfamily and protein kinases, suggesting that the pathogen is able to shut down defences existing in healthy ripening berries. Furthermore, this study provided metabolic biomarkers of infection such as azelaic acid, a substance known to prime plant defence responses, arabitol, ribitol, 4-amino butanoic acid, 1-O-methyl- glucopyranoside, and several fatty acids that alone or in combination can be used to monitor Botrytis infection early in the vineyard.

Elicitin-like proteins Oli-D1 and Oli-D2 from Pythium oligandrum trigger hypersensitive response in Nicotiana benthamiana and induce resistance against Botrytis cinerea in tomato

The biocontrol agent Pythium oligandrum and its elicitin-like proteins oligandrins have been shown to induce disease resistance in a range of plants. In the present study, the ability of two oligandrins, Oli-D1 and Oli-D2, to induce an immune response and the possible molecular mechanism regulating the defence responses in Nicotiana benthamiana and tomato were investigated. Infiltration of recombinant Oli-D1 and Oli-D2 proteins induced a typical immune response in N. benthamiana including the induction of a hypersensitive response (HR), accumulation of reactive oxygen species and production of autofluorescence. Agrobacterium-mediated transient expression assays revealed that full-length Oli-D1 and Oli-D2 were required for full HR-inducing activity in N. benthamiana, and virus-induced gene silencing-mediated knockdown of some of the signalling regulatory genes demonstrated that NbSGT1 and NbNPR1 were required for Oli-D1 and Oli-D2 to induce HR in N. benthamiana. Subcellular localization analyses indicated that both Oli-D1 and Oli-D2 were targeted to the plasma membrane of N. benthamiana. When infiltrated or transiently expressed in leaves, Oli-D1 and Oli-D2 induced resistance against Botrytis cinerea in tomato and activated the expression of a set of genes involved in the jasmonic acid/ethylene (JA/ET)-mediated signalling pathway. Our results demonstrate that Oli-D1 and Oli-D2 are effective elicitors capable of inducing immune responses in plants, probably through the JA/ET-mediated signalling pathway, and that both Oli-D1 and Oli-D2 have potential for the development of bioactive formulae for crop disease control in practice.

Independent Emergence of Resistance to Seven Chemical Classes of Fungicides in Botrytis cinerea

Gray mold, caused by the fungal pathogen Botrytis cinerea, is one of the most destructive diseases of small fruit crops and control is largely dependent on the application of fungicides. As part of a region-wide resistance-monitoring program that investigated 1,890 B. cinerea isolates from 189 fields in 10 states of the United States, we identified seven isolates (0.4%) from five locations in four different states with unprecedented resistance to all seven Fungicide Resistance Action Committee (FRAC) codes with single-site modes of action including FRAC 1, 2, 7, 9, 11, 12, and 17 registered in the United States for gray mold control. Resistance to thiophanate-methyl, iprodione, boscalid, pyraclostrobin, and fenhexamid was based on target gene mutations that conferred E198A and F200Y in β-tubulin, I365N/S in Bos1, H272R/Y in SdhB, G143A in Cytb, and T63I and F412S in Erg27. Isolates were grouped into MDR1 and MDR1h phenotypes based on sensitivity to fludioxonil and variations in transcription factor mrr1. MDR1h isolates had a previously described 3-bp deletion at position 497 in mrr1. Expression of ABC transporter atrB was increased in MDR1 isolates but highest in MDR1h isolates. None of the isolates with seven single resistances (SR) had identical nucleotide variations in target genes, indicating that they emerged independently. Multifungicide resistance phenotypes did not exhibit significant fitness penalties for the parameters used in this study, but MDR1h isolates produced more sclerotia at low temperatures and exhibited increased sensitivity to salt stress. In this study we show that current resistance management strategies have not been able to prevent the geographically independent development of resistance to all seven site-specific fungicides currently registered for gray mold control in the United States and document the presence of MDR1h in North America.

The response regulator BcSkn7 is required for vegetative differentiation and adaptation to oxidative and osmotic stresses in Botrytis cinerea

The high-osmolarity glycerol pathway plays an important role in the responses of fungi to various environmental stresses. Saccharomyces cerevisiae Skn7 is a response regulator in the high-osmolarity glycerol pathway, which regulates the oxidative stress response, cell cycle and cell wall biosynthesis. In this study, we characterized an Skn7 orthologue BcSkn7 in Botrytis cinerea. BcSKN7 can partly restore the growth defects of S. cerevisiae SKN7 mutant and vice versa. The BcSKN7 mutant (ΔBcSkn7-1) revealed increased sensitivity to ionic osmotic and oxidative stresses and to ergosterol biosynthesis inhibitors. In addition, ΔBcSkn7-1 was also impaired dramatically in conidiation and sclerotial formation. Western blot analysis showed that BcSkn7 positively regulated the phosphorylation of BcSak1 (the orthologue of S. cerevisiae Hog1) under osmotic stress, indicating that BcSkn7 is associated with the high-osmolarity glycerol pathway in B. cinerea. In contrast with BcSak1, BcSkn7 is not involved in the regulation of B. cinerea virulence. All of the phenotypic defects of ΔBcSkn7-1 are restored by genetic complementation of the mutant with the wild-type BcSKN7. The results of this study indicate that BcSkn7 plays an important role in the regulation of vegetative differentiation and in the response to various stresses in B. cinerea.

Antagonism between phytohormone signalling underlies the variation in disease susceptibility of tomato plants under elevated CO2

Increasing CO2 concentrations ([CO2]) have the potential to disrupt plant-pathogen interactions in natural and agricultural ecosystems, but the research in this area has often produced conflicting results. Variations in phytohormone salicylic acid (SA) and jasmonic acid (JA) signalling could be associated with variations in the responses of pathogens to plants grown under elevated [CO2]. In this study, interactions between tomato plants and three pathogens with different infection strategies were compared. Elevated [CO2] generally favoured SA biosynthesis and signalling but repressed the JA pathway. The exposure of plants to elevated [CO2] revealed a lower incidence and severity of disease caused by tobacco mosaic virus (TMV) and by Pseudomonas syringae, whereas plant susceptibility to necrotrophic Botrytis cinerea increased. The elevated [CO2]-induced and basal resistance to TMV and P. syringae were completely abolished in plants in which the SA signalling pathway nonexpressor of pathogenesis-related genes 1 (NPR1) had been silenced or in transgenic plants defective in SA biosynthesis. In contrast, under both ambient and elevated [CO2], the susceptibility to B. cinerea highly increased in plants in which the JA signalling pathway proteinase inhibitors (PI) gene had been silenced or in a mutant affected in JA biosynthesis. However, plants affected in SA signalling remained less susceptible to this disease. These findings highlight the modulated antagonistic relationship between SA and JA that contributes to the variation in disease susceptibility under elevated [CO2]. This information will be critical for investigating how elevated CO2 may affect plant defence and the dynamics between plants and pathogens in both agricultural and natural ecosystems.

AIR12, a b-type cytochrome of the plasma membrane of Arabidopsis thaliana is a negative regulator of resistance against Botrytis cinerea

AIR12 (Auxin Induced in Root culture) is a single gene of Arabidopsis that codes for a mono-heme cytochrome b. Recombinant AIR12 from Arabidopsis accepted electrons from ascorbate or superoxide, and donated electrons to either monodehydroascorbate or oxygen. AIR12 was found associated in vivo to the plasma membrane. Though linked to the membrane by a glycophosphatidylinositol anchor, AIR12 is a hydrophilic and glycosylated protein predicted to be fully exposed to the apoplast. The expression pattern of AIR12 in Arabidopsis is developmentally regulated and correlated to sites of controlled cell separation (e.g. micropilar endosperm during germination, epidermal cells surrounding the emerging lateral root) and cells around wounds. Arabidopsis (Landsberg erecta-0) mutants with altered levels of AIR12 did not show any obvious phenotype. However, AIR12-overexpressing plants accumulated ROS (superoxide, hydrogen peroxide) and lipid peroxides in leaves, indicating that AIR12 may alter the redox state of the apoplast under particular conditions. On the other hand, AIR12-knock out plants displayed a strongly decreased susceptibility to Botrytis cinerea infection, which in turn induced AIR12 expression in susceptible wild type plants. Altogether, the results suggest that AIR12 plays a role in the regulation of the apoplastic redox state and in the response to necrotrophic pathogens. Possible relationships between these functions are discussed.

Wounding induces local resistance but systemic susceptibility to Botrytis cinerea in pepper plants

Cotyledon wounding in pepper caused the early generation of hydrogen peroxide both locally (cotyledons) and systemically (upper true leaves). However, 72 h later there is a different wound response between local and systemic organs, as shown by resistance to the pathogenic fungus Botrytis cinerea, that increased locally and decreased systemically. Signaling by ethylene and jasmonic acid was assessed by using two inhibitors: 1-methylcyclopropene (MCP, inhibitor of ethylene receptors) and ibuprofen (inhibitor of jasmonate biosynthesis). MCP did not affect the modulation of resistance levels to Botrytis by wounding, ruling out the involvement of ethylene signaling. Ibuprofen did not inhibit wound-induced resistance at the local level, but inhibited wound-induced systemic susceptibility. Moreover, changes of biochemical and structural defenses in response to wounding were studied. Peroxidase activity and the expression of a peroxidase gene (CAPO1) increased locally as a response to wounding, but no changes were observed systemically. Lignin deposition was induced in wounded cotyledons, but was repressed in systemic leaves of wounded plants, whereas soluble phenolics did not change locally and decreased systemically. The expression of two other genes involved in plant defense (CABPR1 and CASC1) was also differentially regulated locally and systemically, pointing to a generalized increase in plant defenses at the local level and a systemic decrease as a response to wounding. Wound-induced defenses at the local level coincided with resistance to the necrotroph fungus B. cinerea, whereas depleted defenses in systemic leaves of wounded plants correlated to induced susceptibility against this pathogen. It may be that the local response acts as a sink of energy resources to mount a defense against pathogens, whereas in systemic organs the resources for defense are lower.

Vitamin B6 contributes to disease resistance against Pseudomonas syringae pv. tomato DC3000 and Botrytis cinerea in Arabidopsis thaliana

Vitamin B6 (VB6) is an important cofactor for numerous enzymatic reactions and plays an important role in abiotic stress tolerance. However, direct molecular evidence supporting a role for VB6 in plant disease resistance remains lacking. In this study, we explored the possible function of VB6 in disease resistance by analyzing disease phenotypes of Arabidopsis mutants with defects in de novo biosynthetic pathway and salvage pathway of VB6 biosynthesis against Pseudomonas syringae pv. tomato (Pst) DC3000 and Botrytis cinerea. Mutations in AtPDX1.2 and AtPDX1.3 genes involved in the de novo pathway, and in AtSOS4 gene involved in the salvage pathway led to increased levels of diseases caused by Pst DC3000 and B. cinerea. The pdx1.2 and pdx1.3 plants had reduced VB6 contents and showed a further reduction in VB6 contents after infection by Pst DC3000 or B. cinerea. Our preliminary results suggest an important role for VB6 in plant disease resistance against different types of pathogens.

The transcription factor VvWRKY33 is involved in the regulation of grapevine (Vitis vinifera) defense against the oomycete pathogen Plasmopara viticola

Grapevine (Vitis vinifera ssp. vinifera) is one of the most important fruit species; however, it is highly susceptible to various pathogens, which can cause severe crop losses in viticulture. It has been shown that several WRKY class transcription factors (TFs) are part of the signal transduction cascade, which leads to the activation of plant defense reactions against various pathogens. In the present investigation, a full-length cDNA was isolated from V. vinifera leaf tissue encoding a predicted protein, designated VvWRKY33, which shows the characteristics of group I WRKY protein family. VvWRKY33 induction correlates with the expression of VvPR10.1 (pathogenesis-related 10.1) gene in the leaves of the resistant cultivar 'Regent' after infection with Plasmopara viticola, whereas in the susceptible cultivar 'Lemberger' VvWRKY33 and VvPR10.1 are not induced. Corresponding expression of the TF and VvPR10.1 was even obtained in uninfected ripening berries. In planta, analysis of VvWRKY33 has been performed by ectopic expression of VvWRKY33 in grapevine leaves of greenhouse plants mediated via Agrobacterium tumefaciens transformation. In consequence, VvWRKY33 strongly increases resistance to P. viticola in the susceptible cultivar 'Shiraz' and reduces pathogen sporulation of about 50-70%, indicating a functional role for resistance in grapevine. Complementation of the resistance-deficient Arabidopsis thaliana Columbia-0 (Col-0) mutant line wrky33-1 by constitutive expression of VvWRKY33 restores resistance against Botrytis cinerea to wild-type level and in some complemented mutant lines even exceeds the resistance level of the parental line Col-0. Our results support the involvement of VvWRKY33 in the defense reaction of grapevine against different pathogens.

Epigallocatechin-3-gallate functions as a physiological regulator by modulating the jasmonic acid pathway

Flavonoids, a class of plant polyphenols derived from plant secondary metabolism, play important roles in plant development and have beneficial effects on human health. Epigallocatechin-3-gallate (EGCG) is the most abundant polyphenol, and its molecular and biochemical mechanism have been followed with interest. The shared signaling heritage or convergence of organisms has allowed us to extend this research into the model plant, Arabidopsis thaliana. Here, we showed that EGCG could promote jasmonic acid (JA) signaling in A. thaliana. EGCG not only inhibited seed germination but also elevated the resistance to necrotrophic Botrytis cinerea, partly by altering the relative strength of JA signaling. Accordingly, JA marker gene induction, seed germination inhibition and the increased resistance to B. cinerea were attenuated in the JA-insensitive coi1-2 mutant. The coi1-2 mutant was partially insensitive to the treatment of EGCG, further implicating the function of EGCG in JA signaling and/or perception. Our results indicate that EGCG, a member of the flavonoid class of polyphenols, affects signal processing in seed development and disease susceptibility via modulation of JA signaling.

Influence of Fungal Strain, Temperature, and Wetness Duration on Infection of Grapevine Inflorescences and Young Berry Clusters by Botrytis cinerea

The effect of temperature and wetness duration on infection of Vitis vinifera inflorescences (from "inflorescence clearly visible" to "end of flowering" stages) and young berry clusters (at "fruit swelling" and "berries groat-sized" stages) by Botrytis cinerea was investigated. Artificial inoculations were carried out using conidial suspensions of eight B. cinerea strains belonging to the transposon genotypes transposa and vacuma. Infection incidence was significantly affected by strain but not by transposon genotype (transposon genotype accounted for only 6.5% of the variance). Infection incidence was also affected by the interaction between strain and growth stage of the inflorescence or berry cluster (overall accounting for approximately 57% of the experimental variance). Thus, under our experimental conditions, the ability to cause infection was a strain rather than a transposon genotype attribute. Across all strains, infection incidence was lowest when inflorescences were clearly visible or fully developed, highest at flowering (from beginning to end of flowering), and intermediate at the postflowering fruit stages (fruit swelling and berries groat-sized). One transposa strain, however, was highly virulent on all grapevine growth stages tested. The effects of temperature and wetness duration on infection incidence were similar for all fungal strains and grapevine growth stages; infection incidence was highest at 20°C and lowest at 30°C, and was also low at 5°C. Similar results were obtained for mycelial growth and conidial germination. Based on the pooled data for all strains and grapevine growth stages, an equation was developed that accounted for the combined effects of temperature and wetness duration on relative infection incidence. This equation should be useful for developing decision-making systems concerning B. cinerea control at early grapevine growth stages.

Role of dioxygenase α-DOX2 and SA in basal response and in hexanoic acid-induced resistance of tomato (Solanum lycopersicum) plants against Botrytis cinerea

Resistance of tomato (Solanum Lycopersicum) to the fungal pathogen Botrytis cinerea requires complex interplay between hormonal signalling. In this study, we explored the involvement of new oxylipins in the tomato basal and induced response to this necrotroph through the functional analysis of the tomato α-dioxygenase2 (α-DOX2)-deficient mutant divaricata. We also investigated the role of SA in the defence response against this necrotrophic fungus using SA-deficient tomato nahG plants. The plants lacking dioxigenase α-DOX2, which catalyses oxylipins production from fatty acids, were more susceptible to Botrytis, and hexanoic acid-induced resistance (Hx-IR) was impaired; hence α-DOX2 is required for both tomato defence and the enhanced protection conferred by natural inducer hexanoic acid (Hx) against B. cinerea. The divaricata plants accumulated less pathogen-induced callose and presented lower levels of jasmonic acid (JA) and 12-oxo-phytodienoic acid (OPDA) upon infection if compared to the wild type. Glutathion-S-transferase (GST) gene expression decreased and ROS production significantly increased in Botrytis-infected divaricata plants. These results indicate that absence of α-DOX2 influences the hormonal changes, oxidative burst and callose deposition that occur upon Botrytis infection in tomato. The study of SA-deficient nahG tomato plants showed that the plants with low SA levels displayed increased resistance to Botrytis, but were unable to display Hx-IR. This supports the involvement of SA in Hx-IR. NaghG plants displayed reduced callose and ROS accumulation upon infection and an increased GST expression. This reflects a positive relationship between SA and these defensive mechanisms in tomato. Finally, Hx boosted the pathogen-induced callose in nahG plants, suggesting that this priming mechanism is SA-independent. Our results support the involvement of the oxylipins pathway and SA in tomato response to Botrytis, probably through complex crosstalk of the hormonal balance with callose and ROS accumulation, and reinforce the role of the oxidative stress in the outcome of the plant-Botrytis interaction.

BcGs1, a glycoprotein from Botrytis cinerea, elicits defence response and improves disease resistance in host plants

In this study, a necrosis-inducing protein was purified from the culture filtrate of the necrotrophic fungus Botrytis cinerea BC-98 strain. Secreted proteins were collected and fractionated by liquid chromatography. The fraction with the highest necrosis-inducing activity was further purified. A glycoprotein named BcGs1 was identified by 2D electrophoresis and mass spectrometry. The BcGs1 protein consisted of 672 amino acids with a theoretical molecular weight of 70.487 kDa. Functional domain analysis indicated that BcGs1 was a glucan 1,4-alpha-glucosidase, a cell wall-degrading enzyme, with a Glyco_hydro_15 domain and a CBM20_glucoamylase domain. The BcGs1 protein caused necrotic lesions that mimicked a typical hypersensitive response and H2O2 production in tomato and tobacco leaves. BcGs1-treated plants exhibited resistance to B. cinerea, Pseudomonas syringae pv. tomato DC3000 and tobacco mosaic virus in systemic leaves. In addition, BcGs1 triggered elevation of the transcript levels of the defence-related genes PR-1a, TPK1b and Prosystemin. This is the first report of a Botrytis glucan 1,4-alpha-glucosidase triggering host plant immunity as an elicitor. These results lay a foundation for further study of the comprehensive interaction between plants and necrotrophic fungi.

The F-box protein MAX2 contributes to resistance to bacterial phytopathogens in Arabidopsis thaliana

BACKGROUND

The Arabidopsis thaliana F-box protein MORE AXILLARY GROWTH2 (MAX2) has previously been characterized for its role in plant development. MAX2 appears essential for the perception of the newly characterized phytohormone strigolactone, a negative regulator of polar auxin transport in Arabidopsis.

RESULTS

A reverse genetic screen for F-box protein mutants altered in their stress responses identified MAX2 as a component of plant defense. Here we show that MAX2 contributes to plant resistance against pathogenic bacteria. Interestingly, max2 mutant plants showed increased susceptibility to the bacterial necrotroph Pectobacterium carotovorum as well as to the hemi-biotroph Pseudomonas syringae but not to the fungal necrotroph Botrytis cinerea. max2 mutant phenotype was associated with constitutively increased stomatal conductance and decreased tolerance to apoplastic ROS but also with alterations in hormonal balance.

CONCLUSIONS

Our results suggest that MAX2 previously characterized for its role in regulation of polar auxin transport in Arabidopsis, and thus plant development also significantly influences plant disease resistance. We conclude that the increased susceptibility to P. syringae and P. carotovorum is due to increased stomatal conductance in max2 mutants promoting pathogen entry into the plant apoplast. Additional factors contributing to pathogen susceptibility in max2 plants include decreased tolerance to pathogen-triggered apoplastic ROS and alterations in hormonal signaling.

The Physcomitrella patens unique alpha-dioxygenase participates in both developmental processes and defense responses

BACKGROUND

Plant α-dioxygenases catalyze the incorporation of molecular oxygen into polyunsaturated fatty acids leading to the formation of oxylipins. In flowering plants, two main groups of α-DOXs have been described. While the α-DOX1 isoforms are mainly involved in defense responses against microbial infection and herbivores, the α-DOX2 isoforms are mostly related to development. To gain insight into the roles played by these enzymes during land plant evolution, we performed biochemical, genetic and molecular analyses to examine the function of the single copy moss Physcomitrella patens α-DOX (Ppα-DOX) in development and defense against pathogens.

RESULTS

Recombinant Ppα-DOX protein catalyzed the conversion of fatty acids into 2-hydroperoxy derivatives with a substrate preference for α-linolenic, linoleic and palmitic acids. Ppα-DOX is expressed during development in tips of young protonemal filaments with maximum expression levels in mitotically active undifferentiated apical cells. In leafy gametophores, Ppα-DOX is expressed in auxin producing tissues, including rhizoid and axillary hairs. Ppα-DOX transcript levels and Ppα-DOX activity increased in moss tissues infected with Botrytis cinerea or treated with Pectobacterium carotovorum elicitors. In B. cinerea infected leaves, Ppα-DOX-GUS proteins accumulated in cells surrounding infected cells, suggesting a protective mechanism. Targeted disruption of Ppα-DOX did not cause a visible developmental alteration and did not compromise the defense response. However, overexpressing Ppα-DOX, or incubating wild-type tissues with Ppα-DOX-derived oxylipins, principally the aldehyde heptadecatrienal, resulted in smaller moss colonies with less protonemal tissues, due to a reduction of caulonemal filament growth and a reduction of chloronemal cell size compared with normal tissues. In addition, Ppα-DOX overexpression and treatments with Ppα-DOX-derived oxylipins reduced cellular damage caused by elicitors of P. carotovorum.

CONCLUSIONS

Our study shows that the unique α-DOX of the primitive land plant P. patens, although apparently not crucial, participates both in development and in the defense response against pathogens, suggesting that α-DOXs from flowering plants could have originated by duplication and successive functional diversification after the divergence from bryophytes.

Biological control of the grapevine diseases 'grey mold' and 'powdery mildew' by Bacillus B27 and B29 strains

Uncinula necator and Botrytis cinerea are the most destructive pathogens of the grapevine in Tunisia and elsewhere. We used two strains of Bacillus subtilis group, B27 and B29 to control powdery mildew and the grey mold disease of the grapevine. Green house experiments showed that B29 and B27 strains of the bacteria efficiently reduced the severity of powdery mildew up to 50% and 60%, respectively. Further, they decreased Botrytis cinerea development on grape leaf by 77% and 99%, respectively. The mode of action has been shown to be chitinolytic. These two bacteria showed significant production of total proteins discharged into the culture medium. Determination of some chitinolytic enzymes revealed the involvement of N-acetyl glucosaminidase (Nagase), the chitin-1,4-chitobiosidase (Biase) and endochitinase in degrading the mycelium of B. cinerea.

Drought stress tolerance in grapevine involves activation of polyamine oxidation contributing to improved immune response and low susceptibility to Botrytis cinerea

Environmental factors including drought stress may modulate plant immune responses and resistance to pathogens. However, the relationship between mechanisms of drought tolerance and resistance to pathogens remained unknown. In this study, the effects of drought stress on polyamine (PA) homeostasis and immune responses were investigated in two grapevine genotypes differing in their drought tolerance; Chardonnay (CHR), as sensitive and Meski (MSK), as tolerant. Under drought conditions, MSK plants showed the lowest leaf water loss and reduction of photosynthetic efficiency, and expressed a lower level of NCED2, a gene involved in abscisic acid biosynthesis, compared with CHR plants. The improved drought tolerance in MSK was also coincident with the highest change in free PAs and up-regulation of the genes encoding arginine decarboxylase (ADC), copper amine-oxidase (CuAO), and PA-oxidases (PAO) and their corresponding enzyme activities. MSK plants also accumulated the highest level of amino acids, including Arg, Glu, Gln, Pro, and GABA, emphasizing the participation of PA-related amino acid homeostasis in drought tolerance. Importantly, drought-tolerant plants also exhibited enhanced phytoalexin accumulation and up-regulation of PR genes, especially PR-2 and Chit4c, compared with the sensitive plants. This is consistent with a lower susceptibility of MSK than CHR to Botrytis cinerea. Data suggest a possible connection between water stress tolerance and immune response in grapevine. Pharmacological experiments revealed that under drought conditions CuAO and PAO pathways were involved in the regulation of photosynthetic efficiency, and also of immune response and resistance of grapevine to a subsequent pathogen attack. These results open new views to improve our understanding of crosstalk between drought tolerance mechanisms and immune response.

Characterization of miRNAs associated with Botrytis cinerea infection of tomato leaves

BACKGROUND

Botrytis cinerea Pers. Fr. is an important pathogen causing stem rot in tomatoes grown indoors for extended periods. MicroRNAs (miRNAs) have been reported as gene expression regulators related to several stress responses and B. cinerea infection in tomato. However, the function of miRNAs in the resistance to B. cinerea remains unclear.

RESULTS

The miRNA expression patterns in tomato in response to B. cinerea stress were investigated by high-throughput sequencing. In total, 143 known miRNAs and seven novel miRNAs were identified and their corresponding expression was detected in mock- and B. cinerea-inoculated leaves. Among those, one novel and 57 known miRNAs were differentially expressed in B. cinerea-infected leaves, and 8 of these were further confirmed by quantitative reverse-transcription PCR (qRT-PCR). Moreover, five of these eight differentially expressed miRNAs could hit 10 coding sequences (CDSs) via CleaveLand pipeline and psRNAtarget program. In addition, qRT-PCR revealed that four targets were negatively correlated with their corresponding miRNAs (miR319, miR394, and miRn1).

CONCLUSION

Results of sRNA high-throughput sequencing revealed that the upregulation of miRNAs may be implicated in the mechanism by which tomato respond to B. cinerea stress. Analysis of the expression profiles of B. cinerea-responsive miRNAs and their targets strongly suggested that miR319, miR394, and miRn1 may be involved in the tomato leaves' response to B. cinerea infection.

Silencing of OPR3 in tomato reveals the role of OPDA in callose deposition during the activation of defense responses against Botrytis cinerea

Cis-(+)-12-oxo-phytodienoic acid (OPDA) is likely to play signaling roles in plant defense that do not depend on its further conversion to the phytohormone jasmonic acid. To elucidate the role of OPDA in Solanum lycopersicum (tomato) plant defense, we have silenced the 12-oxophytodienoate reductase 3 (OPR3) gene. Two independent transgenic tomato lines (SiOPR3-1 and SiOPR3-2) showed significantly reduced OPR3 expression upon infection with the necrotrophic pathogen Botrytis cinerea. Moreover, SiOPR3 plants are more susceptible to this pathogen, and this susceptibility is accompanied by a significant decrease in OPDA levels and by the production of JA-Ile being almost abolished. OPR3 silencing also leads to a major reduction in the expression of other genes of the jasmonic acid (JA) synthesis and signaling pathways after infection. These results confirm that in tomato plants, as in Arabidopsis, OPR3 determines OPDA availability for JA biosynthesis. In addition, we show that an intact JA biosynthetic pathway is required for proper callose deposition, as its pathogen-induced accumulation is reduced in SiOPR3 plants. Interestingly, OPDA, but not JA, treatment restored basal resistance to B. cinerea and induced callose deposition in SiOPR3-1 and SiOPR3-2 transgenic plants. These results provide clear evidence that OPDA by itself plays a major role in the basal defense of tomato plants against this necrotrophic pathogen.

Molecular characterisation and detection of resistance to succinate dehydrogenase inhibitor fungicides in Botryotinia fuckeliana (Botrytis cinerea)

BACKGROUND

Succinate dehydrogenase inhibitors (SDHIs), interfering with fungal respiration, are considered to be fungicides at medium to high risk of resistance. Boscalid was the first molecule belonging to the SDHIs that was introduced for the control of Botryotinia fuckeliana. A range of different target-site mutations leading to boscalid resistance have been found in field populations of the fungus. The different types of mutation confer different cross-resistance profiles towards novel SDHIs, such as the recently introduced fungicide fluopyram. This study combines the determination of cross-resistance profiles and the setting-up of methods for fast molecular detection of the mutations.

RESULTS

By means of in vitro tests, a range of SdhB mutations were characterised for resistance levels towards boscalid and fluopyram. SdhB mutations conferring P225L and P225F substitutions conferred high resistance to boscalid and high or moderate resistance to fluopyram respectively. Mutants carrying the N230I replacement were moderately resistant to both SDHIs. Substitutions at position H272 responsible for a high level of resistance to boscalid conferred sensitivity (H272R), hypersensitivity (H272Y) or moderate resistance (H272V) to fluopyram. Allele-specific (AS) PCR was developed and used for genotyping 135 B. fuckeliana isolates. The assay confirmed the strict association between resistance profiles and allelic variants of the SdhB gene. Real-time AS-PCR proved to be sensitive and specific for quantitative detection of different SDHI-resistant genotypes.

CONCLUSION

Fluopyram-resistant mutants are currently rarely detected in the field sprayed with boscalid, but this may change with intensive exposure of the fungal population to fluopyram. PCR assays/methods developed in the study provide tools for fast monitoring of field populations and observing possible changes in population composition following fluopyram introduction, useful for the setting-up of appropriate preventive measures.

Occurrence of fungicide resistance in populations of Botryotinia fuckeliana (Botrytis cinerea) on table grape and strawberry in southern Italy

BACKGROUND

Botryotinia fuckeliana (Botrytis cinerea) is a pathogen with a high risk of development of resistance to fungicides. Fungicide resistance was monitored during 2008-2011 in B. fuckeliana populations from both table-grape vineyards and greenhouse-grown strawberries in southern Italy.

RESULTS

Isolates showing different levels of resistance to anilinopyrimidines (APs) were detected at high frequency (up to 98%) in fields treated intensively with APs (4-7 sprays season(-1) ). A slight decrease in sensitivity to fludioxonil, always combined with AP resistance, was generally found at lower frequencies. The repeated use of fenhexamid on grapevine (3-8 sprays season(-1) ) led to a strong selection of highly resistant isolates (up to 100%). Boscalid-resistant mutants were detected at very variable frequencies (0-73%). Occurrence of resistance to quinone outside inhibitors (QoIs) was also ascertained. Multiple fungicide resistance to 2-6 different modes of action were frequently recovered. Single nucleotide polymorphisms (SNPs) in the target genes Erg27, SdhB and cytb were associated with resistance to fenehexamid, boscalid and QoIs respectively.

CONCLUSION

Resistance to the fungicides commonly used against grey mould on table grape and strawberry is quite common in southern Italy. This is an outcome of the incorrect use of fungicides, often because of the maximum number of detectable residues of plant protection products imposed by big international retailers, and underlines the crucial role of antiresistance strategies in integrated pest management.

The SWEET family of sugar transporters in grapevine: VvSWEET4 is involved in the interaction with Botrytis cinerea

During plant development, sugar export is determinant in multiple processes such as nectar production, pollen development and long-distance sucrose transport. The plant SWEET family of sugar transporters is a recently identified protein family of sugar uniporters. In rice, SWEET transporters are the target of extracellular bacteria, which have evolved sophisticated mechanisms to modify their expression and acquire sugars to sustain their growth. Here we report the characterization of the SWEET family of sugar transporters in Vitis vinifera. We identified 17 SWEET genes in the V. vinifera 40024 genome and show that they are differentially expressed in vegetative and reproductive organs. Inoculation with the biotrophic pathogens Erysiphe necator and Plasmopara viticola did not result in significant induction of VvSWEET gene expression. However, infection with the necrotroph Botrytis cinerea triggered a strong up-regulation of VvSWEET4 expression. Further characterization of VvSWEET4 revealed that it is a glucose transporter localized in the plasma membrane that is up-regulated by inducers of reactive oxygen species and virulence factors from necrotizing pathogens. Finally, Arabidopsis knockout mutants in the orthologous AtSWEET4 were found to be less susceptible to B. cinerea. We propose that stimulation of expression of a developmentally regulated glucose uniporter by reactive oxygen species production and extensive cell death after necrotrophic fungal infection could facilitate sugar acquisition from plant cells by the pathogen.

Botrytis species: relentless necrotrophic thugs or endophytes gone rogue?

Plant pathology has a long-standing tradition of classifying microbes as pathogens, endophytes or saprophytes. Lifestyles of pathogens are categorized as biotrophic, necrotrophic or hemibiotrophic. Botrytis species are considered by many to be archetypal examples of necrotrophic fungi, with B. cinerea being the most extensively studied species because of its broad host range and economic impact. In this review, we discuss recent work which illustrates that B. cinerea is capable of colonizing plants internally, presumably as an endophyte, without causing any disease or stress symptoms. The extent of the facultative endophytic behaviour of B. cinerea and its relevance in the ecology and disease epidemiology may be vastly underestimated. Moreover, we discuss the recent discovery of a novel Botrytis species, B. deweyae, which normally grows as an endophyte in ornamental daylilies (Hemerocallis), but displays facultative pathogenic behaviour, and is increasingly causing economic damage. We propose that the emergence of endophytes 'gone rogue' as novel diseases may be related to increased inbreeding of hybrid lines and reduced genetic diversity. These observations lead us to argue that the sometimes inflexible classification of pathogenic microbes by their lifestyles requires serious reconsideration. There is much more variety to the interactions of Botrytis with its hosts than the eye (or the plant pathologist) can see, and this may be true for other microbes interacting with plants.

β-Aminobutyric acid (BABA)-induced resistance in Arabidopsis thaliana: link with iron homeostasis

β-Aminobutyric acid (BABA) is a nonprotein amino acid inducing resistance in many different plant species against a wide range of abiotic and biotic stresses. Nevertheless, how BABA primes plant natural defense reactions remains poorly understood. Based on its structure, we hypothesized and confirmed that BABA is able to chelate iron (Fe) in vitro. In vivo, we showed that it led to a transient Fe deficiency response in Arabidopsis thaliana plants exemplified by a reduction of ferritin accumulation and disturbances in the expression of genes related to Fe homeostasis. This response was not correlated to changes in Fe concentrations, suggesting that BABA affects the availability or the distribution of Fe rather than its assimilation. The phenotype of BABA-treated plants was similar to those of plants cultivated in Fe-deficient conditions. A metabolomic analysis indicated that both BABA and Fe deficiency induced the accumulation of common metabolites, including p-coumaroylagmatine, a metabolite previously shown to be synthesized in several plant species facing pathogen attack. Finally, we showed that the protective effect induced by BABA against Botrytis cinerea was mimicked by Fe deficiency. In conclusion, the Fe deficiency response caused by BABA could bring the plant to a defense-ready state, participating in the plant resistance against the pathogens.

Features of air masses associated with the deposition of Pseudomonas syringae and Botrytis cinerea by rain and snowfall

Clarifying the role of precipitation in microbial dissemination is essential for elucidating the processes involved in disease emergence and spread. The ecology of Pseudomonas syringae and its presence throughout the water cycle makes it an excellent model to address this issue. In this study, 90 samples of freshly fallen rain and snow collected from 2005-2011 in France were analyzed for microbiological composition. The conditions favorable for dissemination of P. syringae by this precipitation were investigated by (i) estimating the physical properties and backward trajectories of the air masses associated with each precipitation event and by (ii) characterizing precipitation chemistry, and genetic and phenotypic structures of populations. A parallel study with the fungus Botrytis cinerea was also performed for comparison. Results showed that (i) the relationship of P. syringae to precipitation as a dissemination vector is not the same for snowfall and rainfall, whereas it is the same for B. cinerea and (ii) the occurrence of P. syringae in precipitation can be linked to electrical conductivity and pH of water, the trajectory of the air mass associated with the precipitation and certain physical conditions of the air mass (i.e. temperature, solar radiation exposure, distance traveled), whereas these predictions are different for B. cinerea. These results are pertinent to understanding microbial survival, emission sources and atmospheric processes and how they influence microbial dissemination.

The transcription factor Ste12 mediates the regulatory role of the Tmk1 MAP kinase in mycoparasitism and vegetative hyphal fusion in the filamentous fungus Trichoderma atroviride

Mycoparasitic species of the fungal genus Trichoderma are potent antagonists able to combat plant pathogenic fungi by direct parasitism. An essential step in this mycoparasitic fungus-fungus interaction is the detection of the fungal host followed by activation of molecular weapons in the mycoparasite by host-derived signals. The Trichoderma atroviride MAP kinase Tmk1, a homolog of yeast Fus3/Kss1, plays an essential role in regulating the mycoparasitic host attack, aerial hyphae formation and conidiation. However, the transcription factors acting downstream of Tmk1 are hitherto unknown. Here we analyzed the functions of the T. atroviride Ste12 transcription factor whose orthologue in yeast is targeted by the Fus3 and Kss1 MAP kinases. Deletion of the ste12 gene in T. atroviride not only resulted in reduced mycoparasitic overgrowth and lysis of host fungi but also led to loss of hyphal avoidance in the colony periphery and a severe reduction in conidial anastomosis tube formation and vegetative hyphal fusion events. The transcription of several orthologues of Neurospora crassa hyphal fusion genes was reduced upon ste12 deletion; however, the Δste12 mutant showed enhanced expression of mycoparasitism-relevant chitinolytic and proteolytic enzymes and of the cell wall integrity MAP kinase Tmk2. Based on the comparative analyses of Δste12 and Δtmk1 mutants, an essential role of the Ste12 transcriptional regulator in mediating outcomes of the Tmk1 MAPK pathway such as regulation of the mycoparasitic activity, hyphal fusion and carbon source-dependent vegetative growth is suggested. Aerial hyphae formation and conidiation, in contrast, were found to be independent of Ste12.

Tomato SR/CAMTA transcription factors SlSR1 and SlSR3L negatively regulate disease resistance response and SlSR1L positively modulates drought stress tolerance

BACKGROUND

The SR/CAMTA proteins represent a small family of transcription activators that play important roles in plant responses to biotic and abiotic stresses. Seven SlSR/CAMTA genes were identified in tomato as tomato counterparts of SR/CAMTA; however, the involvement of SlSRs/CAMTAs in biotic and abiotic stress responses is not clear. In this study, we performed functional analysis of the SlSR/CAMTA family for their possible functions in defense response against pathogens and tolerance to drought stress.

RESULTS

Expression of SlSRs was induced with distinct patterns by Botrytis cinerea and Pseudomonas syringae pv. tomato (Pst) DC3000. Virus-induced gene silencing (VIGS)-based knockdown of either SlSR1 or SlSR3L in tomato resulted in enhanced resistance to B. cinerea and Pst DC3000 and led to constitutive accumulation of H2O2, elevated expression of defense genes, marker genes for pathogen-associated molecular pattern-triggered immunity, and regulatory genes involved in the salicylic acid- and ethylene-mediated signaling pathways. Furthermore, the expression of SlSR1L and SlSR2L in detached leaves and whole plants was significantly induced by drought stress. Silencing of SlSR1L led to decreased drought stress tolerance, accelerated water loss in leaves, reduced root biomass and attenuated expression of drought stress responsive genes in tomato. The SlSR1 and SlSR3L proteins were localized in the nucleus of plant cells when transiently expressed in Nicotiana benthamiana and had transcriptional activation activity in yeast.

CONCLUSIONS

VIGS-based functional analyses demonstrate that both SlSR1 and SlSR3L in the tomato SlSR/CAMTA family are negative regulators of defense response against B. cinerea and Pst DC3000 while SlSR1L is a positive regulator of drought stress tolerance in tomato.

CYCLIN-DEPENDENT KINASE8 differentially regulates plant immunity to fungal pathogens through kinase-dependent and -independent functions in Arabidopsis

CYCLIN-DEPENDENT KINASE8 (CDK8) is a widely studied component of eukaryotic Mediator complexes. However, the biological and molecular functions of plant CDK8 are not well understood. Here, we provide evidence for regulatory functions of Arabidopsis thaliana CDK8 in defense and demonstrate its functional and molecular interactions with other Mediator and non-Mediator subunits. The cdk8 mutant exhibits enhanced resistance to Botrytis cinerea but susceptibility to Alternaria brassicicola. The contributions of CDK8 to the transcriptional activation of defensin gene PDF1.2 and its interaction with MEDIATOR COMPLEX SUBUNIT25 (MED25) implicate CDK8 in jasmonate-mediated defense. Moreover, CDK8 associates with the promoter of AGMATINE COUMAROYLTRANSFERASE to promote its transcription and regulate the biosynthesis of the defense-active secondary metabolites hydroxycinnamic acid amides. CDK8 also interacts with the transcription factor WAX INDUCER1, implying its additional role in cuticle development. In addition, overlapping functions of CDK8 with MED12 and MED13 and interactions between CDK8 and C-type cyclins suggest the conserved configuration of the plant Mediator kinase module. In summary, while CDK8's positive transcriptional regulation of target genes and its phosphorylation activities underpin its defense functions, the impaired defense responses in the mutant are masked by its altered cuticle, resulting in specific resistance to B. cinerea.

Characterization of the Dual Subcellular Localization of Lilium LsGRP1, a Plant Class II Glycine-Rich Protein

The defense-related gene LsGRP1 exhibits an increased level of expression in Lilium spp. after being infected by Botrytis elliptica, the fungal pathogen of lily leaf blight. In this study, the expression profile of the LsGRP1 protein (a plant class II glycine-rich protein) was characterized biochemically and its subcellular localization in lily leaves was evaluated using immunohistochemistry, enhanced green fluorescent protein (EGFP) imaging, and protein extraction analysis. Using an LsGRP1-specific antibody, LsGRP1 was found to be most abundant in epidermal cells and phloem tissues. Leaves from lily plants at different growth stages demonstrated similar levels of 14- and 16-kDa LsGRP1 and a decreased amount of 23-kDa LsGRP1 at the senescence stage. LsGRP1-EGFP imaging and protein extraction assays revealed that 14-kDa LsGRP1 was located in the plasma membrane whereas 16- and 23-kDa LsGRP1 was weakly bound to the cell wall. The time course analyses of LsGRP1 expression in response to salicylic acid treatment or B. elliptica infection showed an increased accumulation of 14- and 23-kDa LsGRP1 over time. Because 23-kDa LsGRP1 could be detected by an ubiquitin antibody, conversion of 14-kDa to 23-kDa LsGRP1 via mono-ubiquitination was presumed, which is a phenomenon that has not been reported for a plant class II glycine-rich protein.

Linking phytochrome to plant immunity: low red : far-red ratios increase Arabidopsis susceptibility to Botrytis cinerea by reducing the biosynthesis of indolic glucosinolates and camalexin

Shade-intolerant plants respond to low red : far-red (R : FR) ratios, which signal the proximity of potential competitors, by down-regulating immune responses. Here we investigated the mechanisms underlying this immune suppression in Arabidopsis. We used genetic, transcriptomic and metabolomic approaches to examine the functional connections between R : FR ratio and Arabidopsis resistance to the fungus Botrytis cinerea. Low R : FR ratios reduced the concentration of indol-3-ylmethyl glucosinolate (I3M) (an indolic glucosinolate, iGS) and camalexin in plants inoculated with B. cinerea, and attenuated the I3M response triggered by jasmonate elicitation. These effects on metabolite abundance correlated with reduced expression of iGS and camalexin biosynthetic genes. Furthermore, the effect of low R : FR increasing Arabidopsis susceptibility to B. cinerea was not present in mutants deficient in the biosynthesis of camalexin (pad3) or metabolism of iGS (pen2). Finally, in a mutant deficient in the JASMONATE ZIM DOMAIN-10 (JAZ10) protein, which does not respond to low R : FR with increased susceptibility to B. cinerea, supplemental FR failed to down-regulate iGS production. These results indicate that suppression of Arabidopsis immunity against B. cinerea by low R : FR ratios is mediated by reduced levels of Trp-derived defenses, and provide further evidence for a functional role of JAZ10 in the link between phytochrome and jasmonate signaling.

PIRIN2 stabilizes cysteine protease XCP2 and increases susceptibility to the vascular pathogen Ralstonia solanacearum in Arabidopsis

PIRIN (PRN) is a member of the functionally diverse cupin protein superfamily. There are four members of the Arabidopsis thaliana PRN family, but the roles of these proteins are largely unknown. Here we describe a function of the Arabidopsis PIRIN2 (PRN2) that is related to susceptibility to the bacterial plant pathogen Ralstonia solanacearum. Two prn2 mutant alleles displayed decreased disease development and bacterial growth in response to R.  solanacearum infection. We elucidated the underlying molecular mechanism by analyzing PRN2 interactions with the papain-like cysteine proteases (PLCPs) XCP2, RD21A, and RD21B, all of which bound to PRN2 in yeast two-hybrid assays and in Arabidopsis protoplast co-immunoprecipitation assays. We show that XCP2 is stabilized by PRN2 through inhibition of its autolysis on the basis of PLCP activity profiling assays and enzymatic assays with recombinant protein. The stabilization of XCP2 by PRN2 was also confirmed in planta. Like prn2 mutants, an xcp2 single knockout mutant and xcp2 prn2 double knockout mutant displayed decreased susceptibility to R. solanacearum, suggesting that stabilization of XCP2 by PRN2 underlies susceptibility to R. solanacearum in Arabidopsis.

Biological characteristics and resistance analysis of the novel fungicide SYP-1620 against Botrytis cinerea

SYP-1620, a quinone-outside-inhibitor (QoI), is a novel broad-spectrum fungicide. In this study, 108 isolates of Botrytis cinerea from different geographical regions in Jiangsu Province of China were characterized for baseline sensitivity to SYP-1620. The curves of baseline sensitivity were unimodal with a mean EC50 value of 0.0130±0.0109 μg/mL for mycelial growth, 0.01147±0.0062 μg/mL for spore germination, respectively. The biological characterization of SYP-1620 against B. cinerea was determined in vitro. The results indicated that SYP-1620 has a strong inhibiting effect on spore germination, mycelial growth, and respiration. The protective and curative test of SYP-1620 suggested that protective effect was better than curative either on strawberry leaves or on cucumber leaves in vivo. In addition, the biological characterization of SYP-1620-resistant mutants of B. cinerea was investigated. SYP-1620 has no cross-resistance with other types of fungicide. Compared to the sensitive isolates, the resistant mutants had lower mycelial growth and virulence but not differ in mycelial dry weight. Sequencing indicated that SYP-1620 resistance was associated with a single point mutation (G143A) in the cytochrome b gene.

MicroRNA400-guided cleavage of Pentatricopeptide repeat protein mRNAs Renders Arabidopsis thaliana more susceptible to pathogenic bacteria and fungi

Although a large number of microRNAs (miRNAs) have been identified in different plant species, the functional roles and targets of the majority of miRNAs have not yet been determined. Here, Arabidopsis thaliana miRNA400 (miR400) was investigated for its functional role in the defense response to diverse pathogens. Transgenic Arabidopsis plants that overexpress MIR400 (35S::MIR400) displayed much more severe disease symptoms than the wild-type plants when infected with the bacterium Pseudomonas syringae pv. tomato DC3000 or the fungus Botrytis cinerea. MiR400 guided the cleavage of two genes (At1g06580 and At1g62720) encoding pentatricopeptide repeat (PPR) proteins. To confirm further that the miR400-mediated defense response was due to the cleavage of PPR mRNAs, loss-of-function mutant and artificial miRNA-mediated knockdown mutants of PPR were generated, and their disease responses were analyzed upon pathogen challenge. Similar to the 35S::MIR400 plants, the ppr mutants displayed much more severe disease symptoms than the wild-type plants when challenged with the pathogens, indicating that miR400 affects the defense response by cleaving PPR mRNAs. Expression of miR400 was down-regulated, whereas the PPR1 and PPR2 transcripts increased upon pathogen challenge. Collectively, the present study reveals that miR400-mediated dysfunction of PPR proteins renders Arabidopsis more susceptible to pathogenic bacteria and fungi, which emphasizes the importance of PPR proteins in plant defense against diverse pathogens.

The endo-arabinanase BcAra1 is a novel host-specific virulence factor of the necrotic fungal phytopathogen Botrytis cinerea

The plant cell wall is one of the first physical interfaces encountered by plant pathogens and consists of polysaccharides, of which arabinan is an important constituent. During infection, the necrotrophic plant pathogen Botrytis cinerea secretes a cocktail of plant cell-wall-degrading enzymes, including endo-arabinanase activity, which carries out the breakdown of arabinan. The roles of arabinan and endo-arabinanases during microbial infection were thus far elusive. In this study, the gene Bcara1 encoding for a novel α-1,5-L-endo-arabinanase was identified and the heterologously expressed BcAra1 protein was shown to hydrolyze linear arabinan with high efficiency whereas little or no activity was observed against the other oligo- and polysaccharides tested. The Bcara1 knockout mutants displayed reduced arabinanase activity in vitro and severe retardation in secondary lesion formation during infection of Arabidopsis leaves. These results indicate that BcAra1 is a novel endo-arabinanase and plays an important role during the infection of Arabidopsis. Interestingly, the level of Bcara1 transcript was considerably lower during the infection of Nicotiana benthamiana compared with Arabidopsis and, consequently, the ΔBcara1 mutants showed the wild-type level of virulence on N. benthamiana leaves. These results support the conclusion that the expression of Bcara1 is host dependent and is a key determinant of the disease outcome.

Redox systems in Botrytis cinerea: impact on development and virulence

The thioredoxin system is of great importance for maintenance of cellular redox homeostasis. Here, we show that it has a severe influence on virulence of Botrytis cinerea, demonstrating that redox processes are important for host-pathogen interactions in this necrotrophic plant pathogen. The thioredoxin system is composed of two enzymes, the thioredoxin and the thioredoxin reductase. We identified two genes encoding for thioredoxins (bctrx1, bctrx2) and one gene encoding for a thioredoxin reductase (bctrr1) in the genome of B. cinerea. Knockout mutants of bctrx1 and bctrr1 were severely impaired in virulence and more sensitive to oxidative stress. Additionally, Δbctrr1 showed enhanced H2O2 production and retarded growth. To investigate the impact of the second major cellular redox system, glutathione, we generated deletion mutants for two glutathione reductase genes. The effects were only marginal; deletion of bcglr1 resulted in reduced germination and, correspondingly, to retarded infection as well as reduced growth on minimal medium, whereas bcglr2 deletion had no distinctive phenotype. In summary, we showed that the balanced redox status maintained by the thioredoxin system is essential for development and pathogenesis of B. cinerea, whereas the second major cellular redox system, the glutathione system, seems to have only minor impact on these processes.

Loss of ceramide kinase in Arabidopsis impairs defenses and promotes ceramide accumulation and mitochondrial H2O2 bursts

Arabidopsis thaliana plants that lack ceramide kinase, encoded by ACCELERATED CELL DEATH5 (ACD5), display spontaneous programmed cell death late in development and accumulate substrates of ACD5. Here, we compared ceramide accumulation kinetics, defense responses, ultrastructural features, and sites of reactive oxygen species (ROS) production in wild-type and acd5 plants during development and/or Botrytis cinerea infection. Quantitative sphingolipid profiling indicated that ceramide accumulation in acd5 paralleled the appearance of spontaneous cell death, and it was accompanied by autophagy and mitochondrial ROS accumulation. Plants lacking ACD5 differed significantly from the wild type in their responses to B. cinerea, showing earlier and higher increases in ceramides, greater disease, smaller cell wall appositions (papillae), reduced callose deposition and apoplastic ROS, and increased mitochondrial ROS. Together, these data show that ceramide kinase greatly affects sphingolipid metabolism and the site of ROS accumulation during development and infection, which likely explains the developmental and infection-related cell death phenotypes. The acd5 plants also showed an early defect in restricting B. cinerea germination and growth, which occurred prior to the onset of cell death. This early defect in B. cinerea restriction in acd5 points to a role for ceramide phosphate and/or the balance of ceramides in mediating early antifungal responses that are independent of cell death.

Tomato NAC transcription factor SlSRN1 positively regulates defense response against biotic stress but negatively regulates abiotic stress response

Biotic and abiotic stresses are major unfavorable factors that affect crop productivity worldwide. NAC proteins comprise a large family of transcription factors that play important roles in plant growth and development as well as in responses to biotic and abiotic stresses. In a virus-induced gene silencing-based screening to identify genes that are involved in defense response against Botrytis cinerea, we identified a tomato NAC gene SlSRN1 (Solanum lycopersicumStress-related NAC1). SlSRN1 is a plasma membrane-localized protein with transactivation activity in yeast. Expression of SlSRN1 was significantly induced by infection with B. cinerea or Pseudomonas syringae pv. tomato (Pst) DC3000, leading to 6–8 folds higher than that in the mock-inoculated plants. Expression of SlSRN1 was also induced by salicylic acid, jasmonic acid and 1-amino cyclopropane-1-carboxylic acid and by drought stress. Silencing of SlSRN1 resulted in increased severity of diseases caused by B. cinerea and Pst DC3000. However, silencing of SlSRN1 resulted in increased tolerance against oxidative and drought stresses. Furthermore, silencing of SlSRN1 accelerated accumulation of reactive oxygen species but attenuated expression of defense genes after infection by B. cinerea. Our results demonstrate that SlSRN1 is a positive regulator of defense response against B. cinerea and Pst DC3000 but is a negative regulator for oxidative and drought stress response in tomato.

Expression of Arabidopsis sugar transport protein STP13 differentially affects glucose transport activity and basal resistance to Botrytis cinerea

Botrytis cinerea is the causing agent of the grey mold disease in more than 200 crop species. While signaling pathways leading to the basal resistance against this fungus are well described, the role of the import of sugars into host cells remains to be investigated. In Arabidopsis thaliana, apoplastic hexose retrieval is mediated by the activity of sugar transport proteins (STPs). Expression analysis of the 14 STP genes revealed that only STP13 was induced in leaves challenged with B. cinerea. STP13-modified plants were produced and assayed for their resistance to B. cinerea and glucose transport activity. We report that STP13-deficient plants exhibited an enhanced susceptibility and a reduced rate of glucose uptake. Conversely, plants with a high constitutive level of STP13 protein displayed an improved capacity to absorb glucose and an enhanced resistance phenotype. The correlation between STP13 transcripts, protein accumulation, glucose uptake rate and resistance level indicates that STP13 contributes to the basal resistance to B. cinerea by limiting symptom development and points out the importance of the host intracellular sugar uptake in this process. We postulate that STP13 would participate in the active resorption of hexoses to support the increased energy demand to trigger plant defense reactions and to deprive the fungus by changing sugar fluxes toward host cells.

Reversal of senescence by N resupply to N-starved Arabidopsis thaliana: transcriptomic and metabolomic consequences

Leaf senescence is a developmentally controlled process, which is additionally modulated by a number of adverse environmental conditions. Nitrogen shortage is a well-known trigger of precocious senescence in many plant species including crops, generally limiting biomass and seed yield. However, leaf senescence induced by nitrogen starvation may be reversed when nitrogen is resupplied at the onset of senescence. Here, the transcriptomic, hormonal, and global metabolic rearrangements occurring during nitrogen resupply-induced reversal of senescence in Arabidopsis thaliana were analysed. The changes induced by senescence were essentially in keeping with those previously described; however, these could, by and large, be reversed. The data thus indicate that plants undergoing senescence retain the capacity to sense and respond to the availability of nitrogen nutrition. The combined data are discussed in the context of the reversibility of the senescence programme and the evolutionary benefit afforded thereby. Future prospects for understanding and manipulating this process in both Arabidopsis and crop plants are postulated.

Resistance to fludioxonil in Botrytis cinerea isolates from blackberry and strawberry

Site-specific fungicides, including the phenylpyrrole fludioxonil, are frequently used for gray mold control but are at risk for the development of resistance. In this study, field isolates that were low-resistant (LR) and moderately resistant (MR) to fludioxonil from blackberry and strawberry fields of North Carolina, South Carolina, and Virginia were characterized. Genes involved in osmoregulation, including bcsak1, BcOS4, bos5, and BRRG-1, were cloned and sequenced to detect potential target gene alterations; however, none were found. A previously described mutation (R632I) in transcription factor Mrr1, which is known to increase the expression of ATP-binding cassette transporter AtrB, was found in MR but not in sensitive (S) or LR isolates. Expression of atrB in MR isolates was ≈200-fold increased compared with an S isolate; however, 30- to 100-fold overexpression was also detected in LR isolates. Both MR isolates exhibited increased sensitivity to salt stress in the form of mycelial growth inhibition at 4% NaCl, indicating a disruption of osmoregulatory processes in those strains. However, the glycerol content was indistinguishable between S, LR, and MR isolates with and without exposure to fludioxonil, suggesting that the glycerol synthesis pathway may not be a part of the resistance mechanism in LR or MR strains. An investigation into the origin of LR and MR isolates from blackberry revealed two insertions in the mrr1 gene consistent with those found in the Botrytis clade group S. The emergence of strains overexpressing atrB in European and now in North American strawberry fields underscores the importance of this resistance mechanism for development of resistance to fludioxonil in Botrytis cinerea.

Cucumber Pti1-L is a cytoplasmic protein kinase involved in defense responses and salt tolerance

Homologs of the cytoplasmic protein kinase Pti1 are found in diverse plant species. A clear role of Pti1 in plant defense response has not been established. We identified a Pti1 homolog in cucumber (CsPti1-L). CsPti1-L expression was induced when cucumber plants were challenged with the fungal pathogen Sphaerotheca fuliginea or with salt treatment. CsPti1-L expression in cucumber leaves also was induced by methyl jasmonate, salicylic acid, and abscisic acid. CsPti1-L exhibited autophosphorylation activity and was targeted to the cytoplasm. Transgenic Nicotiana benthamiana expressing CsPti1-L exhibited greater cell death and increased ion leakage in response to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000, resistance to Botrytis cinerea infection, and higher tolerance to salt stress. RT-PCR analysis of transgenic N. benthamiana overexpressing CsPti1-L revealed constitutive upregulation of multiple genes involved in plant-defense and osmotic-stress responses. Our results suggest a functional role for CsPti1-L as a positive regulator of pathogen-defense and salt-stress responses.

Tomato SlMKK2 and SlMKK4 contribute to disease resistance against Botrytis cinerea

BACKGROUND

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signaling modules that mediate the transduction of extracellular stimuli via receptors/sensors into intracellular responses and play key roles in plant immunity against pathogen attack. However, the function of tomato MAPK kinases, SlMKKs, in resistance against Botrytis cinerea remains unclear yet.

RESULTS

A total of five SlMKK genes with one new member, SlMKK5, were identified in tomato. qRT-PCR analyses revealed that expression of SlMKK2 and SlMKK4 was strongly induced by B. cinerea and by jasmonic acid and ethylene precursor 1-amino cyclopropane-1-carboxylic acid. Virus-induced gene silencing (VIGS)-based knockdown of individual SlMKKs and disease assays identified that SlMKK2 and SlMKK4 but not other three SlMKKs (SlMKK1, SlMKK3 and SlMKK5) are involved in resistance against B. cinerea. Silencing of SlMKK2 or SlMKK4 resulted in reduced resistance to B. cinerea, increased accumulation of reactive oxygen species and attenuated expression of defense genes after infection of B. cinerea in tomato plants. Furthermore, transient expression of constitutively active phosphomimicking forms SlMKK2DD and SlMKK4DD in leaves of Nicotiana benthamiana plants led to enhanced resistance to B. cinerea and elevated expression of defense genes.

CONCLUSIONS

VIGS-based knockdown of SlMKK2 and SlMKK4 expression in tomato and gain-of-function transient expression of constitutively active phosphomimicking forms SlMKK2DD and SlMKK2DD in N. benthamiana demonstrate that both SlMKK2 and SlMKK4 function as positive regulators of defense response against B. cinerea.

A novel Botrytis species is associated with a newly emergent foliar disease in cultivated Hemerocallis

Foliar tissue samples of cultivated daylilies (Hemerocallis hybrids) showing the symptoms of a newly emergent foliar disease known as 'spring sickness' were investigated for associated fungi. The cause(s) of this disease remain obscure. We isolated repeatedly a fungal species which proved to be member of the genus Botrytis, based on immunological tests. DNA sequence analysis of these isolates, using several different phyogenetically informative genes, indicated that they represent a new Botrytis species, most closely related to B. elliptica (lily blight, fire blight) which is a major pathogen of cultivated Lilium. The distinction of the isolates was confirmed by morphological analysis of asexual sporulating cultures. Pathogenicity tests on Hemerocallis tissues in vitro demonstrated that this new species was able to induce lesions and rapid tissue necrosis. Based on this data, we infer that this new species, described here as B. deweyae, is likely to be an important contributor to the development of 'spring sickness' symptoms. Pathogenesis may be promoted by developmental and environmental factors that favour assault by this necrotrophic pathogen. The emergence of this disease is suggested to have been triggered by breeding-related changes in cultivated hybrids, particularly the erosion of genetic diversity. Our investigation confirms that emergent plant diseases are important and deserve close monitoring, especially in intensively in-bred plants.

Spatial distribution of single-nucleotide polymorphisms related to fungicide resistance and implications for sampling

Spatial distribution of single-nucleotide polymorphisms (SNPs) related to fungicide resistance was studied for Botrytis cinerea populations in vineyards and for B. squamosa populations in onion fields. Heterogeneity in this distribution was characterized by performing geostatistical analyses based on semivariograms and through the fitting of discrete probability distributions. Two SNPs known to be responsible for boscalid resistance (H272R and H272Y), both located on the B subunit of the succinate dehydrogenase gene, and one SNP known to be responsible for dicarboximide resistance (I365S) were chosen for B. cinerea in grape. For B. squamosa in onion, one SNP responsible for dicarboximide resistance (I365S homologous) was chosen. One onion field was sampled in 2009 and another one was sampled in 2010 for B. squamosa, and two vineyards were sampled in 2011 for B. cinerea, for a total of four sampled sites. Cluster sampling was carried on a 10-by-10 grid, each of the 100 nodes being the center of a 10-by-10-m quadrat. In each quadrat, 10 samples were collected and analyzed by restriction fragment length polymorphism polymerase chain reaction (PCR) or allele specific PCR. Mean SNP incidence varied from 16 to 68%, with an overall mean incidence of 43%. In the geostatistical analyses, omnidirectional variograms showed spatial autocorrelation characterized by ranges of 21 to 1 m. Various levels of anisotropy were detected, however, with variograms computed in four directions (at 0°, 45°, 90°, and 135° from the within-row direction used as reference), indicating that spatial autocorrelation was prevalent or characterized by a longer range in one direction. For all eight data sets, the β-binomial distribution was found to fit the data better than the binomial distribution. This indicates local aggregation of fungicide resistance among sampling units, as supported by estimates of the parameter θ of the β-binomial distribution of 0.09 to 0.23 (overall median value = 0.20). On the basis of the observed spatial distribution patterns of SNP incidence, sampling curves were computed for different levels of reliability, emphasizing the importance of sample size for the detection of mutation incidence below the risk threshold for control failure.

Influence of weeds on Argentine ant (Hymenoptera: Formicidae) and obscure mealybug (Hemiptera: Pseudococcidae) in a central California vineyard

Obscure mealybug is a pest of grapes in the cool climate regions of coastal California, is found on some vineyard weeds, and is tended by the Argentine ant. A study was conducted at a vineyard in Arroyo Grande, CA, to evaluate the impact of weeds on ant activity on grapevines, and the role that ants and weeds have on obscure mealybug infestation in grape clusters. The incidence of the fungus Botrytis cinerea was recorded as well. Treatments were weed exclusion versus the presence of weeds, and ant exclusion versus the presence of ants. Ant activity was evaluated weekly using sugar-based monitoring stations, and mealybug infestation and Botrytis incidence of clusters were evaluated at harvest. Ant exclusion reduced the overall number of ant visits by 82%, and ants increased mealybug infestation of clusters by 53%. Ant activity was 33% higher in the weeds treatment, but there was no impact of weeds on mealybug infestation. We suggest that the higher ant activity recorded in the weeds treatment may have been an artifact of the sugar-based sampling method. Botrytis incidence was three times higher with ants, but did not differ between weeds and weed exclusion treatments. The study supports other research showing a relationship between mealybug infestation and the presence of ants, as well as the lack of impact of floor vegetation on mealybug infestation of grape clusters. It is the first report of a relationship between ants and Botrytis, although it is more likely that the higher Botrytis incidence found here is a result of increased mealybug density than a direct effect by ants.

Knocking out Bcsas1 in Botrytis cinerea impacts growth, development, and secretion of extracellular proteins, which decreases virulence

Pathogenic fungi usually secrete a series of virulence factors to the extracellular environment to facilitate infection. Rab GTPases play a central role in the secretory pathway. To explore the function of Rab/GTPase in filamentous fungi, we knocked out a Rab/GTPase family gene, Bcsas1, in Botrytis cinerea, an aggressive fungal pathogen that infects more than 200 plant species. A detailed analysis was conducted on the virulence and the secretory capability of the mutants. The results indicated that knockout of Bcsas1 inhibited hyphal development and reduced sporulation of B. cinerea on potato dextrose agar plates resulting in reduced virulence on various fruit hosts. Knocking out the Bcsas1 gene led to an accumulation of transport vesicles at the hyphal tip, significantly reduced extracellular protein content, and lowered the activity of polygalacturonase and xylanase in the extracellular medium. However, mutation of Bcsas1 did not affect the expression of genes encoding polygalacturonase and xylanase, suggesting the secretion of these two family enzymes was suppressed in the mutant. Moreover, a comparative analysis of the secretome provided further evidence that the disruption of Bcsas1 in mutant strains significantly depressed the secretion of polysaccharide hydrolases and proteases. The results indicate that Bcsas1, the Rab8/SEC4-like gene, plays a crucial role in development, protein secretion, and virulence of B. cinerea.

The Arabidopsis LYSIN MOTIF-CONTAINING RECEPTOR-LIKE KINASE3 regulates the cross talk between immunity and abscisic acid responses

Transmembrane receptor-like kinases characterized by the presence of one or more lysin motif (LysM) domains in the extracytoplasmic portion (LysM-containing receptor-like kinases [LYKs]) mediate recognition of symbiotic and pathogenic microorganisms in plants. The Arabidopsis (Arabidopsis thaliana) genome encodes five putative LYKs; among them, AtLYK1/CHITIN ELICITOR RECEPTOR KINASE1 is required for response to chitin and peptidoglycan, and AtLYK4 contributes to chitin perception. More recently, AtLYK3 has been shown to be required for full repression, mediated by Nod factors, of Arabidopsis innate immune responses. In this work, we show that AtLYK3 also negatively regulates basal expression of defense genes and resistance to Botrytis cinerea and Pectobacterium carotovorum infection. Enhanced resistance of atlyk3 mutants requires PHYTOALEXIN-DEFICIENT3, which is crucial for camalexin biosynthesis. The expression of AtLYK3 is strongly repressed by elicitors and fungal infection and is induced by the hormone abscisic acid (ABA), which has a negative impact on resistance against B. cinerea and P. carotovorum. Plants lacking a functional AtLYK3 also show reduced physiological responses to ABA and are partially resistant to ABA-induced inhibition of PHYTOALEXIN-DEFICIENT3 expression. These results indicate that AtLYK3 is important for the cross talk between signaling pathways activated by ABA and pathogens.

Fitness and competitive ability of Botrytis cinerea field isolates with dual resistance to SDHI and QoI fungicides, associated with several sdhB and the cytb G143A mutations

Respiration inhibitors such as the succinate dehydrogenase inhibitors (SDHIs) and the quinone outside inhibitors (QoIs) are fungicide classes with increasing relevance in gray mold control. However, recent studies have shown that dual resistance to both fungicide classes is a common trait in Botrytis cinerea populations from several hosts throughout the world. Resistance of B. cinerea to SDHIs is associated with several mutations in the sdhB, sdhC, and sdhD genes, while resistance to QoIs, in most cases, is associated with the G143A mutation in the cytb gene. The objective of the current study was to investigate the fitness and the competitive ability of B. cinerea field strains possessing one of the H272Y/R/L, N230I, or P225F sdhB substitutions and the G143A mutation of cytb. Fitness parameters measured were (i) mycelial growth and conidia germination in vitro, (ii) aggressiveness and sporulation capacity in vivo, (iii) sclerotia production in vitro and sclerotia viability under different storage conditions, and (iv) sensitivity to oxidative stress imposed by diquat treatments. The competitive ability of the resistant isolates was measured in the absence and presence of the SDHI fungicides boscalid and fluopyram selection pressure. The measurements of individual fitness components showed that the H272R/G143A isolates had the lower differences compared with the sensitive isolates. In contrast, the groups of H272Y/L/G143A, N230I/G143A, and P225F/G143A isolates showed reduced fitness values compared with the sensitive isolates. Isolates possessing only the cytb G143A substitution did not show any fitness cost. The competition experiments showed that, in the absence of fungicide selection pressure, after four disease cycles on apple fruit, the sensitive isolates dominated in the population in all the mixtures tested. In contrast, when the competition experiment was conducted under the selection pressure of boscalid, a gradual decrease in the frequency of sensitive isolates was observed, whereas the frequency of H272L and P225F isolates was increased. When the competition experiment was conducted in the presence of fluopyram, the sensitive isolates were eliminated even after the first disease cycle and the P225F mutants dominated in the population. Such results suggest that the sdhB mutations may have adverse effects on the mutants. The observed dominance of sensitive isolates in the competition experiments conducted in the absence of fungicides suggest that the application of SDHIs in alternation schemes may delay the selection or reduce the frequency of SDHI-resistant mutants.

Characterization of iprodione resistance in Botrytis cinerea from strawberry and blackberry

Gray mold, caused by the fungal pathogen Botrytis cinerea, is one of the most destructive diseases of strawberry. Control of the disease in commercial fields is largely dependent on the application of fungicides, including the dicarboximide iprodione. Single-spore isolates were collected from strawberry fields in Florida, North Carolina, and South Carolina and subjected to an assay using conidial germination that distinguished sensitive (S) isolates from isolates with various levels of resistance to iprodione. Of the 245 isolates, 1 was highly resistant (HR), 5 were moderately resistant (MR), and 43 had low resistance (LR) to iprodione. LR and MR strains were found in the Florida population and in 9 of 11 locations from North Carolina and South Carolina, indicating that resistance was widespread but accounted for only a relatively small percentage of the B. cinerea population. Sequence analysis of the target gene bos1, which codes for a class III histidine kinase, revealed that the MR phenotype was associated with Q369P and N373S mutations and that the LR phenotype was associated with either a I365S or a I365N mutation. The I365S and I365N mutations were also present in five additionally included HR isolates from North Carolina and South Carolina blackberry fields and one HR isolate from a Virginia strawberry field but no mutation or mutation combinations in bos1 were uniquely associated with the HR phenotype. Expression analysis of bos1 in S and HR isolates did not reveal convincing evidence of the gene's involvement in HR resistance either. The six HR isolates had three different phenotypes with respect to their sensitivity to fludioxonil; two were S, two were LR, and two were MR. The fludioxonil LR and MR isolates were also resistant to tolnaftate, an indication of multidrug efflux pump activity. These data suggest that, in addition to point mutations in bos1, drug efflux pump activity and potentially a third mechanism of resistance may be contributing to the iprodione HR phenotype. Detached fruit studies showed that field rates of Rovral 4 Flowable (iprodione) did not control iprodione MR and HR isolates.