Genetic basis of resistance to benzimidazole and dicarboximide fungicides in Botryotinia fuckeliana (Botrytis cinerea)

Fungicide Resistance in Botrytis spp. and Regional Strategies for Its Management in Northern European Strawberry Production

Grey mould, caused by Botrytis cinerea and other Botrytis spp., is a major cause of fruit rot in strawberries and other fruit crops worldwide. Repeated fungicide applications are essential in order to secure harvests. However, resistance to all currently registered single-site fungicides is widespread. The rising importance of strains with multiple resistance to most or all fungicides is of particular concern. These strains may be introduced into fields via contaminated nursery plants and/or by immigration from adjacent plots. On the basis of research conducted in northern German and Danish strawberry production, a concept to manage fungicide resistance under northern European conditions has been developed and put into regional strawberry production practice. This principally includes the testing of nursery plants for fungicide-resistant Botrytis strains prior to planting; the restricted and specific use of fungicides at flowering in the production fields, taking account of the resistance spectrum within the local Botrytis population; and crop sanitation measures such as the removal of rotting fruits at the beginning of harvest. Further options such as protected cultivation, reduced fertilisation and biological control are also discussed. The practical implementation of such a strategy in northern Germany and Denmark has been shown to reduce the occurrence of multi-resistant strains to a tolerable steady-state level.

6-deoxy-6-amino chitosan: a preventative treatment in the tomato/Botrytis cinerea pathosystem

6-deoxy-6-amino chitosan (aminochitosan) is a water-soluble chitosan derivative with an additional amine group at the C-6 position. This modification has improved aqueous solubility, in vitro antifungal activity and is hypothesized to have enhanced in vivo antifungal activity compared to native chitosan. Gray mold disease in tomatoes is caused by the fungus, Botrytis cinerea, and poses a severe threat both pre- and post-harvest. To investigate the optimal concentration of aminochitosan and its lower molecular weight fractions for antifungal and priming properties in the tomato/B. cinerea pathosystem, different concentrations of aminochitosan were tested in vitro on B. cinerea growth and sporulation and in vivo as a foliar pre-treatment in tomato leaves. The leaves were monitored for photosynthetic changes using multispectral imaging and hydrogen peroxide accumulation using DAB. Despite batch-to-batch variations in aminochitosan, it displayed significantly greater inhibition of B. cinerea in vitro than native chitosan at a minimum concentration of 1 mg/mL. A concentration-dependent increase in the in vitro antifungal activities was observed for radial growth, sporulation, and germination with maximum in vitro inhibition for all the biopolymer batches and lower MW fractions at 2.5 and 5 mg/mL, respectively. However, the inhibition threshold for aminochitosan was identified as 1 mg/mL for spores germinating in vivo, compared to the 2.5 mg/mL threshold in vitro. The pre-treatment of leaves displayed efficacy in priming direct and systemic resistance to B. cinerea infection at 4, 6 and 30 days post-inoculation by maintaining elevated Fv/Fm activity and chlorophyll content due to a stronger and more rapid elicitation of the defense systems at earlier time points. Moreover, these defense systems appear to be ROS-independent at higher concentrations (1 and 2.5 mg/mL). In addition, aminochitosan accumulates in the cell membrane and therefore acts to increase the membrane permeability of cells after foliar spray. These observations corroborate the notion that aminochitosan biopolymers can exert their effects through both direct mechanisms of action and indirect immunostimulatory mechanisms. The contrast between in vitro and in vivo efficacy highlights the bimodal mechanisms of action of aminochitosan and the advantageous role of primed plant defense systems.

The phenylpropanoid pathway inhibitor piperonylic acid induces broad-spectrum pest and disease resistance in plants

Although many phenylpropanoid pathway-derived molecules act as physical and chemical barriers to pests and pathogens, comparatively little is known about their role in regulating plant immunity. To explore this research field, we transiently perturbed the phenylpropanoid pathway through application of the CINNAMIC ACID-4-HYDROXYLASE (C4H) inhibitor piperonylic acid (PA). Using bioassays involving diverse pests and pathogens, we show that transient C4H inhibition triggers systemic, broad-spectrum resistance in higher plants without affecting growth. PA treatment enhances tomato (Solanum lycopersicum) resistance in field and laboratory conditions, thereby illustrating the potential of phenylpropanoid pathway perturbation in crop protection. At the molecular level, transcriptome and metabolome analyses reveal that transient C4H inhibition in tomato reprograms phenylpropanoid and flavonoid metabolism, systemically induces immune signalling and pathogenesis-related genes, and locally affects reactive oxygen species metabolism. Furthermore, C4H inhibition primes cell wall modification and phenolic compound accumulation in response to root-knot nematode infection. Although PA treatment induces local accumulation of the phytohormone salicylic acid, the PA resistance phenotype is preserved in tomato plants expressing the salicylic acid-degrading NahG construct. Together, our results demonstrate that transient phenylpropanoid pathway perturbation is a conserved inducer of plant resistance and thus highlight the crucial regulatory role of this pathway in plant immunity.

Chitosan primes plant defence mechanisms against Botrytis cinerea, including expression of Avr9/Cf-9 rapidly elicited genes

Current crop protection strategies against the fungal pathogen Botrytis cinerea rely on a combination of conventional fungicides and host genetic resistance. However, due to pathogen evolution and legislation in the use of fungicides, these strategies are not sufficient to protect plants against this pathogen. Defence elicitors can stimulate plant defence mechanisms through a phenomenon known as defence priming. Priming results in a faster and/or stronger expression of resistance upon pathogen recognition by the host. This work aims to study defence priming by a commercial formulation of the elicitor chitosan. Treatments with chitosan result in induced resistance (IR) in solanaceous and brassicaceous plants. In tomato plants, enhanced resistance has been linked with priming of callose deposition and accumulation of the plant hormone jasmonic acid (JA). Large-scale transcriptomic analysis revealed that chitosan primes gene expression at early time-points after infection. In addition, two novel tomato genes with a characteristic priming profile were identified, Avr9/Cf-9 rapidly elicited protein 75 (ACRE75) and 180 (ACRE180). Transient and stable over-expression of ACRE75, ACRE180 and their Nicotiana benthamiana homologs, revealed that they are positive regulators of plant resistance against B. cinerea. This provides valuable information in the search for strategies to protect Solanaceae plants against B. cinerea.

Timing of light quality affects susceptibility to Botrytis cinerea in strawberry leaves

Light plays an essential role in the outcome of plant-pathogen interactions. In this study, the effect of pre- and post-inoculation light quality on Botrytis cinerea disease development in strawberry leaves was investigated. Circadian variations in leaf susceptibility to Botrytis cinerea were tested by inoculating at 5 h, 12 h, or 16 h after the start of photoperiod. The effects of 4 h and/or 12 h of red or blue light combined with white light in a 16 h photoperiod on leaf susceptibility and metabolites were studied. Light presence significantly decreased leaf susceptibility to Botrytis irrespective of light quality. Leaf susceptibility to B. cinerea showed significant circadian variations. Susceptibility was highest in leaves inoculated 12 h after the start of photoperiod followed by incubation in the dark. When disease development took place under the light regime, the shorter the light period after inoculation the stronger the disease symptoms. Twelve hours of red light radiation in a 16 h photoperiod increased, while 12 h blue light radiation decreased leaf resistance, resulting in alterations in the circadian variation of leaf susceptibility. These alterations correlated with changes in leaf metabolites, such as hydrogen peroxide, total phenolics and hexoses. Overall, we conclude that twelve hours of red light followed by white light in a 16 h photoperiod efficiently increased leaf resistance to B. cinerea.

Sensitivity of Botrytis cinerea From Nectarine/Cherry in China to Six Fungicides and Characterization of Resistant Isolates

Botrytis cinerea, the causal agent of gray mold, can result in considerable preharvest and postharvest losses in many economically valuable plant species. Fungicides were widely used to minimize such losses, but fungicide resistances were detected frequently. In the present study, we collected 164 isolates from nectarine and cherry in China and tested the sensitivity to six fungicides. Among the tested isolates, 71 (43.3%) were resistant to azoxystrobin, 14 (8.5%) to cyprodinil, 7 (4.3%) to boscalid, 4 (2.4%) to carbendazim, 1 (0.6%) to iprodione, and no isolates were found to be resistant to fludioxonil. The EC₅₀ value and resistance factor (RF) of resistant isolates were determined. Fitness analysis showed that there were no significant differences between sensitive and resistant isolates for osmotic stress and pathogenicity, while more conidia production was observed for some resistant isolates. Control efficacy of fungicides showed that the resistant isolates could not be controlled efficiently by using corresponding fungicides. The point mutation G143A was detected in the Cyt b gene of the isolates resistant to azoxystrobin, while the point mutation H272R of SdhB gene was confirmed in boscalid-resistant isolates, and mutations E198V/A of TUB2 gene and mutation I365S of BcOs1 occurred in carbendazim-resistant and iprodione-resistant isolates, respectively. These results indicate that the occurrence of fungicide resistance greatly threatens the management of gray mold on stone fruits nectarine and cherry.

Detection and Molecular Characterization of Resistance to the Dicarboximide and Benzamide Fungicides in Botrytis cinerea From Tomato in Hubei Province, China

Altogether, 192 Botrytis cinerea isolates collected from tomato greenhouses at different locations in Hubei Province were evaluated for their sensitivity to fungicides procymidone and zoxamide. The mean effective concentration to cause 50% growth inhibition (EC₅₀) values of procymidone for sensitive and resistant isolates were 0.25 and 3.60 μg/ml, respectively. The frequency of procymidone-resistant (Pro^R) isolates was 18%, and the highest frequency was recorded in Jingmen. Positive cross-resistance was observed for Pro^R isolates to other dicarboximide fungicides but not to phenylpyrroles. Significant differences were observed for fitness parameters (i.e., mycelial growth, osmotic sensitivity, and virulence between sensitive and resistant isolates). Amino acid sequence of the Bos1 gene revealed that Pro^R isolates carried either point mutations at codon 365 (I365S) or a pair of point mutations at codons 369 (Q369P) and 373 (N373S). For zoxamide, the mean EC₅₀ values for sensitive and resistant isolates were 0.22 and 5.32 μg/ml, respectively. Approximately 14% of the isolates were found to be resistant to zoxamide, and the highest frequency of resistance was also observed in Jingmen. There was positive cross-resistance for zoxamide-resistant (Zox^R) isolates to carbendazim. No significant differences were observed for fitness parameters between zoxamide-sensitive and Zox^R isolates. Sequence analysis of the β-tubulin gene of Botrytis cinerea revealed two previously reported point mutations (E198A and E198K) and one new point mutation (T351I). This new mutation was detected in only those isolates which possessed the E198K but not E198A substitution. This study allows for a better understanding of the resistance development profile in Hubei Province. Results will be useful for the improvement of fungicide resistance management strategies.

Effects of a NTG-based chemical mutagenesis on the propamocarb-tolerance of the nematophagous fungus Lecanicillium attenuatum

Lecanicillium attenuatum is an important nematophagous fungus with potential as a biopesticide for control of plant-pathogenic nematodes. However, relatively low fungicide-tolerance limits its application in the field. To improve the propamocarb-tolerance of L. attenuatum, a NTG-based mutagenesis system was established. Among different combinations of NTG concentration and treatment time in the first-round NTG treatment, the treatment of 1.0mg/ml NTG for 60min gave a proper conidial lethality rate of 84.6% and the highest positive mutation rate of 7.7%, and then produced the highest propamocarb-tolerant mutant LA-C-R1-T4-M whose EC₅₀ value reached to 1050.0μg/ml. The positive mutation range was 105.1% in the first-round NTG treatment. Multiple-round NTG treatment was further employed to enhance the propamocarb tolerance of L. attenuatum. The positive mutation range was significantly accumulated to 179.3% on the third-round NTG treatment, and then appeared to level-off and remained constant. These results indicated that multiple-round NTG treatment had a significant accumulative effect on fungal tolerance to propamocarb. Among all chemical-mutants, the LA-C-R3-M was the highest tolerant to propamocarb, whose EC₅₀ value was increased 2.79-fold compared to the wild-type strain, and it was mitotic stable after 20 passages on PDA medium. Colony growth, conidia yield and conidial germination on plates, and parasitism of nematode eggs of M. incognita and H. glycines were not significantly changed by the NTG-based mutagenesis compared to the wild-type strain in either single- or multiple-round NTG treatment. In conclusion, we succeeded in improving the propamocarb tolerance of L. attenuatum via the optimized NTG-based mutagenesis system. The improved strain LA-C-R3-M could be potentially applied with propamocarb in the field.

Optimizing Chemically Induced Resistance in Tomato Against Botrytis cinerea

Resistance-inducing chemicals can offer broad-spectrum disease protection in crops, but can also affect plant growth and interactions with plant-beneficial microbes. We have evaluated different application methods of β-aminobutyric acid (BABA) and jasmonic acid (JA) for long-lasting induced resistance in tomato against Botrytis cinerea. In addition, we have studied nontarget effects on plant growth and root colonization by arbuscular mycorrhizal fungi (AMF). Germinating seeds for 1 week in BABA- or JA-containing solutions promoted seed germination efficiency, did not affect plant growth, and induced resistance in 4-week-old plants. When formulating BABA and JA in carboxymethyl cellulose seed coating, only BABA was able to induce resistance in 4-week-old plants. Root treatment of 1-week-old seedlings with BABA or JA also induced resistance in 4-week-old plants. However, this seedling treatment repressed plant growth at higher concentrations of the chemicals, which was particularly pronounced in hydroponically grown plants after BABA treatment. Both seed coating with BABA, and seedling treatments with BABA or JA, did not affect AMF root colonization in soil-grown tomato. Our study has identified commercially feasible application methods of BABA and JA, which induce durable disease resistance in tomato without concurrent impacts on plant growth or colonization by plant-beneficial AMF.

Proteomic analysis on zoxamide-induced sensitivity changes in Phytophthora cactorum

Zoxamide is an important fungicide for oomycete disease management. In this study, we established the baseline sensitivity of Phytophthora cactorum to zoxamide and assessed the risk of developing resistance to zoxamide using ultraviolet irradiation and fungicide taming methods. All 73 studied isolates were sensitive to zoxamide, with effective concentrations for 50% inhibition of mycelial growth ranging from 0.04 to 0.29 mg/L and mean of 0.15 mg/L. Stable zoxamide-resistant mutants of P. cactorum were not obtained from four arbitrarily selected isolates by either treating mycelial cultures with ultraviolet irradiation or adapting mycelial cultures to the addition of increasing zoxamide concentrations. However, the sensitivity of the isolates to zoxamide could be easily reduced by successive zoxamide treatments. In addition to displaying decreased sensitivity to zoxamide, these isolates also showed decreased sensitivity to the fungicides flumorph and cymoxanil. Proteomic analysis indicated that some proteins involved in antioxidant detoxification, ATP-dependent multidrug resistance, and anti-apoptosis activity, are likely responsible for the induced decrease in the sensitivity of P. cactorum to zoxamide compared to controls. Further semi-quantitative PCR analysis demonstrated that the gene expression profiles of most of above proteins were consistent with the proteomic analysis. Based on the above results, P. cactorum shows low resistance risk to zoxamide; however, the fungicidal effect of zoxamide might be decreased due to induced resistance when this fungicide is continuously applied.

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.

Agrobacterium tumefaciens-mediated transformation in the entomopathogenic fungus Lecanicillium lecanii and development of benzimidazole fungicide resistant strains

Lecanicillium lecanii has been used in the biological control of several insects in agricultural practice. Since the gene manipulation tools for this entomopathogenic fungus have not been sufficiently developed, Agrobacterium tumefaciens-mediated transformation (ATMT) in L. lecanii was investigated in this study, using the wild-type isolate FZ9906 as a progenitor strain and the hygromycin B resistance (hph) gene as a selection marker. Furthermore, a field carbendazim-resistant (mrt) gene from Botrytis cinerea was expressed in L. lecanii FZ9906 via the ATMT system. The results revealed that the frequency of transformation surpassed 25transformants/10(6) conidia, most of the putative transformants contained a single copy of T-DNA, and the T-DNA inserts were stably inherited after five generations. All putative transformants had indistinguishable biological characteristics relative to the wild-type strain, excepting two transformants with altered growth habits or virulence. Moreover, the resistance of the putative transformants to carbendazim (MBC) was improved, and the highest one was 380-fold higher than the wild-type strain. In conclusion, ATMT is an effective and suitable system for L. lecanii transformation, and will be a useful tool for the basic and application research of gene functions and gene modifications of this strain.

Proteomic analysis of zoxamide-induced changes in Phytophthora cactorum

In this study, the global proteomic response of Phytophthora cactorum to zoxamide was evaluated using a two-dimensional gel electrophoresis (2-DE)-based proteomic approach. Among the 21 proteins identified by matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry (MALDI-TOF/TOF MS), four cytoskeleton-related proteins were down-regulated upon addition of zoxamide. Five detoxification metabolism enzymes, seven sugar metabolism proteins and one mitochondria-related protein were up-regulated by more than 2-fold in response to zoxamide. Taken together, these results suggest that zoxamide can decrease the expression of cytoskeleton-related proteins of P. cactorum, resulting in cell death; however, the up-regulation of detoxification metabolism-related enzymes may protect P. cactorum against zoxamide, and the up-regulation of proteins related to sugar metabolism and mitochondria may lead to the generation of more energy for detoxification metabolism. These data also suggest that proteomics may be useful not only in exploring the mode of action of fungicides but also for gaining insight into the resistance mechanisms that pathogens employ against fungicides.

Two non-target recessive genes confer resistance to the anti-oomycete microtubule inhibitor zoxamide in Phytophthora capsici

This study characterized isolates of P. capsici that had developed a novel mechanism of resistance to zoxamide, which altered the minimum inhibition concentration (MIC) but not the EC50. Molecular analysis revealed that the β-tubulin gene of the resistant isolates contained no mutations and was expressed at the same level as in zoxamide-sensitive isolates. This suggested that P. capsici had developed a novel non-target-site-based resistance to zoxamide. Analysis of the segregation ratio of zoxamide-resistance in the sexual progeny of the sensitive isolates PCAS1 and PCAS2 indicated that the resistance to zoxamide was controlled by one or more recessive nuclear genes. Furthermore, the segregation of resistance in the F1, F2, and BC1 progeny was in accordance with the theoretical ratios of the χ(2) test (P>0.05), which suggested that the resistance to zoxamide was controlled by two recessive genes, and that resistance to zoxamide occurred when at least one pair of these alleles was homozygous. This implies that the risk of zoxamide-resistance in P. capsici is low to moderate. Nevertheless this potential for resistance should be monitored closely, especially if two compatible mating types co-exist in the same field.

Concurrent overactivation of the cytosolic glutamine synthetase and the GABA shunt in the ABA-deficient sitiens mutant of tomato leads to resistance against Botrytis cinerea

Deficiency of abscisic acid (ABA) in the sitiens mutant of tomato (Solanum lycopersicum) culminates in increased resistance to Botrytis cinerea through a rapid epidermal hypersensitive response (HR) and associated phenylpropanoid pathway-derived cell wall fortifications. This study focused on understanding the role of primary carbon : nitrogen (C : N) metabolism in the resistance response of sitiens to B. cinerea. How alterations in C : N metabolism are linked with the HR-mediated epidermal arrest of the pathogen has been also investigated. Temporal alterations in the γ-aminobutyric acid (GABA) shunt, glutamine synthetase/glutamate synthase (GS/GOGAT) cycle and phenylpropanoid pathway were transcriptionally, enzymatically and metabolically monitored in both wild-type and sitiens plants. Virus-induced gene silencing, microscopic analyses and pharmacological assays were used to further confirm the data. Our results on the sitiens-B. cinerea interaction favor a model in which cell viability in the cells surrounding the invaded tissue is maintained by a constant replenishment of the tricarboxylic acid (TCA) cycle through overactivation of the GS/GOGAT cycle and the GABA shunt, resulting in resistance through both tightly controlling the defense-associated HR and slowing down the pathogen-induced senescence. Collectively, this study shows that maintaining cell viability via alterations in host C : N metabolism plays a vital role in the resistance response against necrotrophic pathogens.

Genetic analysis and molecular characterisation of laboratory and field mutants of Botryotinia fuckeliana (Botrytis cinerea) resistant to QoI fungicides

BACKGROUND

QoI fungicides, inhibitors of mitochondrial respiration, are considered to be at high risk of resistance development. In several phytopathogenic fungi, resistance is caused by mutations (most frequently G143A) in the mitochondrial cytochrome b (cytb) gene. The genetic and molecular basis of QoI resistance were investigated in laboratory and field mutants of Botryotinia fuckeliana (de Bary) Whetz. exhibiting in vitro reduced sensitivity to trifloxystrobin.

RESULTS

B. fuckeliana mutants highly resistant to trifloxystrobin were obtained in the laboratory by spontaneous mutations in wild-type strains, or from naturally infected plants on a medium amended with 1-3 mg L(-1) trifloxystrobin and 2 mM salicylhydroxamic acid, an inhibitor of alternative oxidase. No point mutations were detected, either in the complete nucleotide sequences of the cytb gene or in those of the aox and Rieske protein genes of laboratory mutants, whereas all field mutants carried the G143A mutation in the mitochondrial cytb gene. QoI resistance was always maternally inherited in ascospore progeny of sexual crosses of field mutants with sensitive reference strains.

CONCLUSIONS

The G143A mutation in cytb gene is confirmed to be responsible for field resistance to QoIs in B. fuckeliana. Maternal inheritance of resistance to QoIs in progeny of sexual crosses confirmed that it is caused by extranuclear genetic determinants. In laboratory mutants the heteroplasmic state of mutated mitochondria could likely hamper the G143A detection, otherwise other gene(s) underlying different mechanisms of resistance could be involved.

Treating seeds with activators of plant defence generates long-lasting priming of resistance to pests and pathogens

• Priming of defence is a strategy employed by plants exposed to stress to enhance resistance against future stress episodes with minimal associated costs on growth. Here, we test the hypothesis that application of priming agents to seeds can result in plants with primed defences. • We measured resistance to arthropod herbivores and disease in tomato (Solanum lycopersicum) plants grown from seed treated with jasmonic acid (JA) and/or β-aminobutryric acid (BABA). • Plants grown from JA-treated seed showed increased resistance against herbivory by spider mites, caterpillars and aphids, and against the necrotrophic fungal pathogen, Botrytis cinerea. BABA seed treatment provided primed defence against powdery mildew disease caused by the biotrophic fungal pathogen, Oidium neolycopersici. Priming responses were long-lasting, with significant increases in resistance sustained in plants grown from treated seed for at least 8 wk, and were associated with enhanced defence gene expression during pathogen attack. There was no significant antagonism between different forms of defence in plants grown from seeds treated with a combination of JA and BABA. • Long-term defence priming by seed treatments was not accompanied by reductions in growth, and may therefore be suitable for commercial exploitation.

Abscisic acid deficiency causes changes in cuticle permeability and pectin composition that influence tomato resistance to Botrytis cinerea

A mutant of tomato (Solanum lycopersicum) with reduced abscisic acid (ABA) production (sitiens) exhibits increased resistance to the necrotrophic fungus Botrytis cinerea. This resistance is correlated with a rapid and strong hydrogen peroxide-driven cell wall fortification response in epidermis cells that is absent in tomato with normal ABA production. Moreover, basal expression of defense genes is higher in the mutant compared with the wild-type tomato. Given the importance of this fast response in sitiens resistance, we investigated cell wall and cuticle properties of the mutant at the chemical, histological, and ultrastructural levels. We demonstrate that ABA deficiency in the mutant leads to increased cuticle permeability, which is positively correlated with disease resistance. Furthermore, perturbation of ABA levels affects pectin composition. sitiens plants have a relatively higher degree of pectin methylesterification and release different oligosaccharides upon inoculation with B. cinerea. These results show that endogenous plant ABA levels affect the composition of the tomato cuticle and cell wall and demonstrate the importance of cuticle and cell wall chemistry in shaping the outcome of this plant-fungus interaction.

The mitogen-activated protein kinase kinase BOS5 is involved in regulating vegetative differentiation and virulence in Botrytis cinerea

We present a characterization of bos5 from Botrytis cinerea, a gene that encodes a mitogen-activated protein kinase kinase (MAPKK), which is homologous to OS-5 of Neurospora crassa. The bos5 gene deletion mutant exhibited reduced vegetative growth and strongly impaired conidiation. The mutant also exhibited increased sensitivity to the dicarboximide fungicide iprodione and to osmotic stress mediated by NaCl or KCl. Western-blot analysis showed that the BcSAK1 protein, the putative downstream component of BOS5, was not phosphorylated in the mutant. Plant inoculation tests showed that the mutants were unable to infect cucumber leaves. All of these defects were restored by genetic complementation of the Deltabcos5-21 mutant with the wild-type bos5 gene. These results indicated that BOS5 is involved in the regulation of vegetative differentiation, virulence, adaptation to iprodione and ionic stress in B. cinerea.

Geographic Distribution of Fungicide-Insensitive Sclerotinia homoeocarpa Isolates from Golf Courses in the Northeastern United States

Chemical management of dollar spot in turf may lead to the development of Sclerotinia homoeocarpa populations with reduced fungicide sensitivity. The objective of this study was to determine the scope of S. homoeocarpa insensitivity to fungicides commonly used to control dollar spot on golf courses in the northeastern United States. A total of 965 and 387 isolates of S. homoeocarpa from intensively or individually sampled sites, respectively, were evaluated for in vitro sensitivity to iprodione, propiconazole, and thiophanate-methyl. Mean baseline sensitivities to iprodione and propiconazole were 0.2763 and 0.0016 μg a.i. ml^-1, respectively, and all baseline isolates were sensitive to thiophanate-methyl at 1,000 μg a.i. ml^-1. When compared with the baseline population, 14 and 18 of 20 total populations were less sensitive to iprodione and propiconazole, respectively. Individually sampled isolates obtained from fairways, putting greens, or tees were less sensitive to iprodione and propiconazole when compared with the baseline. For thiophanate-methyl, five populations were sensitive, six were resistant, and the remaining nine populations contained various proportions (2 to 92%) of resistant isolates. Individually sampled isolates obtained from fairways and putting greens were evaluated for associations in sensitivity among the three fungicides. A weak but positive correlation in sensitivity to iprodione and propiconazole was observed for isolates resistant to thiophanate-methyl but correlations for sensitive isolates were not significant. Furthermore, isolates with highly reduced sensitivity to iprodione clustered in a narrow range of propiconazole sensitivity. These data suggest the possible existence of resistance mechanisms common to diverse fungicide classes. Overall, results indicate that insensitivity of S. homoeocarpa to iprodione, propiconazole, and thiophanate-methyl exists in varying degrees on golf courses in the northeastern United States.

Mutations in a beta-tubulin confer resistance of Gibberella zeae to benzimidazole fungicides

ABSTRACT Wheat head blight caused by Gibberella zeae (anamorph: Fusarium graminearum) is a threat to food safety in China because of mycotoxin contamination of the harvested grain, the frequent occurrence of the disease, and the failure of chemical control in some areas due to benzimidazole resistance in the pathogen population. The molecular resistance mechanism, however, of G. zeae to benzimidazole fungicides (especially carbendazim; active ingredient: methyl benzimidazol-2-yl carbamate [MBC]) is poorly understood. DNA sequences of a beta-tubulin gene (beta(2)tub) (GenBank access number FG06611.1) in G. zeae were analyzed. Mutations in beta(2)tub in moderately resistant strains (MBC(MR)) included TTT (Phe)-->TAT (Tyr) at codon 167 or TTC (Phe)-->TAC (Tyr) at codon 200. A highly resistant strain (MBC(HR)) had two point mutations, one at codon 73, CAG (Gln)-->CGG (Arg), and the other at codon 198, GAG (Glu)-->CTG (Leu). To confirm that mutations in the beta(2)tub confer resistance to benzimidazole fungicides, the entire beta(2)tub locus was deleted from MBC(MR) and MBC(HR) strains of G. zeae. The resulting Deltabeta(2)tub mutants from both MBC(MR) and MBC(HR) strains grew normally on MBC-free potato dextrose agar medium and were supersensitive to MBC. Complementation of the Deltabeta(2)tub mutants by transformation with a copy of the intact beta(2)tub locus from their parent strains exhibited less resistance than the original strains, and complementation of the Deltabeta(2)tub mutants by transformation with a copy of the intact beta(2)tub locus from sensitive strains restored MBC sensitivity. The results indicated that the mutations in the beta(2)tub gene conferred resistance of G. zeae to benzimidazole fungicides and this gene can be used as a genetic marker in G. zeae.

Hexanoic acid-induced resistance against Botrytis cinerea in tomato plants

We have demonstrated that root treatment with hexanoic acid protects tomato plants against Botrytis cinerea. Hexanoic acid-induced resistance (Hx-IR) was blocked in the jasmonic acid (JA)-insensitive mutant jai1 (a coi1 homolog) and in the abscisic acid (ABA)-deficient mutant flacca (flc). Upon infection, the LoxD gene as well as the oxylipin 12-oxo-phytodienoic acid and the bioactive molecule JA-Ile were clearly induced in treated plants. However, the basal ABA levels were not altered. Hexanoic acid primed callose deposition against B. cinerea in a cultivar-dependent manner. Treated plants from Ailsa Craig, Moneymaker, and Rheinlands Ruhm showed increased callose deposition but not from Castlemart. Hexanoic acid did not prime callose accumulation in flc plants upon B. cinerea infection; therefore, ABA could act as a positive regulator of Hx-IR by enhancing callose deposition. Furthermore, although hexanoic acid protected the JA-deficient mutant defensless1 (def1), the priming for callose was higher than in the wild type. This suggests a link between JA and callose deposition in tomato. Hence, the obtained results support the idea that callose, oxylipins, and the JA-signaling pathway are involved in Hx-IR against B. cinerea. Moreover our data support the relevance of JA-signaling for basal defense against this necrotroph in tomato. Hexanoic acid also protected against Pseudomonas syringae, indicating a broad-spectrum effect for this new inducer.

Effect of carbendazim resistance on trichothecene production and aggressiveness of Fusarium graminearum

Fusarium graminearum (teleomorph, Gibberella zeae) causes head blight of cereals and contaminates grains with trichothecene mycotoxins that are harmful to humans and domesticated animals. Control of Fusarium head blight relies on carbendazim (MBC) in China, but resistance to MBC in F. graminearum is now widespread. Sixty-seven strains were evaluated for trichothecene production in shake culture or in the field. The strains included 60 wild-type strains (30 MBC-resistant and 30 MBC-sensitive), three MBC-resistant site-directed mutants at codon 167 in beta(2)-tubulin, three MBC-sensitive site-directed mutants at codon 240 in beta(2)-tubulin, and their MBC-sensitive wild-type progenitor strain ZF21. The incidence of infected spikelets and the amount of F. graminearum DNA in field grain (AFgDNA) also were evaluated for all strains. MBC resistance increased trichothecene production in shake culture or in the field. Although MBC resistance did not change the incidence of infected spikelets, it did increase AFgDNA. Tri5 gene expression increased in MBC-resistant strains grown in shake culture. We found a significant exponential relationship between trichothecene production and Tri5 gene expression in shake culture and a linear relationship between the incidence of infected spikelets or AFgDNA and trichothecene production in field grain.

Occurrence and Characterization of Dimethachlon Insensitivity in Sclerotinia sclerotiorum in Jiangsu Province of China

Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is the main fungal disease of oilseed rape (Brassica napus) in China. Numerous fungicide applications are required for control. Dimethachlon, one of the dicarboximide fungicides, has been the major fungicide for disease control after benzimidazole resistance became widespread. Fungal populations were collected throughout Jiangsu Province between 2006 and 2007 in order to determine their sensitivity to dimethachlon. A total of 1,066 single-sclerotium isolates of S. sclerotiorum were collected, and most of the isolates were considered sensitive to dimethachlon. Five isolates collected in Yancheng and Changzhou showed normal growth at 5 μg/ml dimethachlon with the resistance factor ≈10 (resistance factor was estimated as ratios between the EC₅₀ values of resistant isolates and the average EC₅₀ values of sensitive ones) compared to the sensitive isolates (EC₅₀ is the concentration of fungicide causing 50% reduction in growth). Through in vitro selection for resistance to the fungicide, 25 dimethachlon-resistant mutants were derived from 10 wild-type isolates of S. sclerotiorum. The resistance factors for the isolates ranged from 198 to 484, and the isolates were considered highly resistant to dimethachlon. Therefore, at least two different mechanisms of resistance seem to be involved: one that may provide a moderate resistance (insensitivity) and a second that may give a high resistance level under laboratory conditions. There was positive cross-resistance between dimethachlon and other dicarboximide fungicides, such as iprodione and procymidone, in these S. sclerotiorum isolates. The field dimethachlon-insensitive and the laboratory-induced dimethachlon-resistant isolates appeared to have mycelial growth, sclerotial production, and pathogenicity comparable to their wild-type parental isolates. Also, results of osmotic tests showed that there were no significant difference in mycelial radial growth between the field dimethachlon-sensitive and field dimethachlon-insensitive isolates on potato dextrose agar plates amended with 2, 4, 6, or 8% (wt/vol) NaCl, but the laboratory-induced dimethachlon-resistant isolates grew significantly more slowly than their wild-type sensitive parents under all concentrations of NaCl. Because these studies yielded a high frequency of laboratory resistance in S. sclerotiorum, together with the occurrence of field insensitivity, appropriate precautions against resistance development in natural populations should be taken.

Characterization and fitness of carbendazim-resistant strains of Fusarium graminearum (wheat scab)

BACKGROUND

Carbendazim (MBC) has failed to control wheat scab, caused by Fusarium graminearum Schwabe, on the eastern coast of China in recent years after about 30 years of application.

RESULTS

MBC resistance was found to be common in pathogen populations on the eastern coast and along areas of the Yangtze River. EC(50) and minimum inhibitive concentration (MIC) values of MBC inhibiting mycelium growth of wild-type isolates were less than 0.9 and 1.4 microg mL(-1) respectively, while EC(50) values of resistant collections averaged 7.02 +/- 11.86 microg mL(-1). The slope of the MBC dosage-response curve (DRC) for resistant isolates of F. graminearum was flat: 1 < b < 2.8 for resistant isolates and 3.5 < b < 11 for sensitive isolates). Both field resistant and sensitive MBC strains shared similar temperature sensitivity, fitness and virulence on ears. Field resistant strains and UV-induced mutants showed positive cross-resistance to other benzimidazole derivatives and were mainly at intermediate MBC resistance level. Highly resistant field MBC strains rarely appeared, but only some of the highly resistant MBC UV mutants were insensitive to N-phenylaminecarbamates. No mutation in beta-tubulin was found in F. graminearum, in contrast to mutation in this tubulin which has led to MBC resistance in other plant pathogens.

CONCLUSION

MBC(R) isolates have high fitness and competition in field, conferred by a novel molecular mechanism.

Botrytis cinerea: the cause of grey mould disease

INTRODUCTION

Botrytis cinerea (teleomorph: Botryotinia fuckeliana) is an airborne plant pathogen with a necrotrophic lifestyle attacking over 200 crop hosts worldwide. Although there are fungicides for its control, many classes of fungicides have failed due to its genetic plasticity. It has become an important model for molecular study of necrotrophic fungi.

TAXONOMY

Kingdom: Fungi, phylum: Ascomycota, subphylum: Pezizomycotina, class: Leotiomycetes, order: Helotiales, family: Sclerotiniaceae, genus: Botryotinia.

HOST RANGE AND SYMPTOMS

Over 200 mainly dicotyledonous plant species, including important protein, oil, fibre and horticultural crops, are affected in temperate and subtropical regions. It can cause soft rotting of all aerial plant parts, and rotting of vegetables, fruits and flowers post-harvest to produce prolific grey conidiophores and (macro)conidia typical of the disease.

PATHOGENICITY

B. cinerea produces a range of cell-wall-degrading enzymes, toxins and other low-molecular-weight compounds such as oxalic acid. New evidence suggests that the pathogen triggers the host to induce programmed cell death as an attack strategy. Resistance: There are few examples of robust genetic host resistance, but recent work has identified quantitative trait loci in tomato that offer new approaches for stable polygenic resistance in future.

USEFUL WEBSITES

http://www.phi-base.org/query.php, http://www.broad.mit.edu/annotation/genome/botrytis_cinerea/Home.html, http://urgi.versailles.inra.fr/projects/Botrytis/, http://cogeme.ex.ac.uk.

Resistance to Botrytis cinerea in sitiens, an abscisic acid-deficient tomato mutant, involves timely production of hydrogen peroxide and cell wall modifications in the epidermis

Plant defense mechanisms against necrotrophic pathogens, such as Botrytis cinerea, are considered to be complex and to differ from those that are effective against biotrophs. In the abscisic acid-deficient sitiens tomato (Solanum lycopersicum) mutant, which is highly resistant to B. cinerea, accumulation of hydrogen peroxide (H(2)O(2)) was earlier and stronger than in the susceptible wild type at the site of infection. In sitiens, H(2)O(2) accumulation was observed from 4 h postinoculation (hpi), specifically in the leaf epidermal cell walls, where it caused modification by protein cross-linking and incorporation of phenolic compounds. In wild-type tomato plants, H(2)O(2) started to accumulate 24 hpi in the mesophyll layer and was associated with spreading cell death. Transcript-profiling analysis using TOM1 microarrays revealed that defense-related transcript accumulation prior to infection was higher in sitiens than in wild type. Moreover, further elevation of sitiens defense gene expression was stronger than in wild type 8 hpi both in number of genes and in their expression levels and confirmed a role for cell wall modification in the resistant reaction. Although, in general, plant defense-related reactive oxygen species formation facilitates necrotrophic colonization, these data indicate that timely hyperinduction of H(2)O(2)-dependent defenses in the epidermal cell wall can effectively block early development of B. cinerea.

Resistance of Botrytis cinerea Isolates from Vegetable Crops to Anilinopyrimidine, Phenylpyrrole, Hydroxyanilide, Benzimidazole, and Dicarboximide Fungicides

During February 2005, 55 single-spore isolates of Botrytis cinerea were collected at the end of the season from vegetable crops grown in 18 greenhouses on the island of Crete, Greece. They were tested for sensitivity to the anilinopyrimidine fungicides pyrimethanil and cyprodinil, the hydroxyanilide fungicide fenhexamid, the phenylpyrrole fungicide fludioxonil, the dicarboximide fungicide iprodione, and the benzimidazole fungicide carbendazim. Results of the study showed the existence of benzimidazole- and dicarboximide-resistant strains at frequencies of 61.8 and 18%, respectively. Moreover, for first time, the development of resistance to anilinopyrimidine fungicides by B. cinerea was detected in greenhouse vegetable crops on the island of Crete. High resistance frequencies of 49.1 and 57.4% were observed for pyrimethanil and cyprodinil, respectively. In addition, one isolate was found to be resistant to the hydroxyanilide fungicide fenhexamid, while no strains resistant to the phenylpyrrole fungicide were detected. Among the 55 isolates tested, 13 were resistant only to carbendazim, 6 were resistant only to anilinopyrimidines, 3 were resistant to both benzimidazoles and dicarboximides, 17 were resistant to both benzimidazoles and anilinopyrimidines, 6 were resistant to both dicarboximides and anilinopyrimidines, 1 was simultaneously resistant to benzimidazoles, dicarboximides, and anilinopyrimidines, 1 was resistant to both anilinopyrimidines and hydroxyanilides, and 8 were sensitive to all fungicides tested. A strong cross-resistance relationship was found between the two anilinopyrimidine fungicides tested when log transformed EC₅₀ values of the isolates were subjected to a linear regression analysis (r = 0.71). Despite the detection of several phenotypes with simultaneous resistance to chemically unrelated active ingredients, in none of the remaining possible fungicide pairs was there observed any kind of cross-resistance relationship.

A class III histidine kinase acts as a novel virulence factor in Botrytis cinerea

Filamentous ascomycetes contain large numbers of histidine kinases (HK) that belong to eleven classes. Members of class III from different species were previously shown to be involved in osmoregulation and resistance to dicarboximide and phenylpyrrole fungicides. We have inactivated the gene encoding the single group III HK, BOS1, in the economically important plant pathogen Botrytis cinerea. BOS1 inactivation had pleiotropic effects on the fungus. Besides the expected osmosensitivity and resistance to fungicides, null mutants presented additional characteristics indicating that BOS1 is necessary for normal macroconidiation and full virulence. On standard culture media, null mutants very rarely formed conidiophores and those few conidiophores failed to produce conidia. This defect could be partially restored with 1 M sorbitol, suggesting that another BOS1-independent signal cascade may be involved in macroconidiation. The mutants were not found to be hypersensitive to various oxidative stresses but were more resistant to menadione. Finally, pathogenicity tests showed that bos1-null mutants were significantly reduced in the ability to infect host plants. Appressorium morphogenesis was not altered; however, in planta growth was severely reduced. To our knowledge, this is the first class III HK characterized as a pathogenicity factor in a plant-pathogenic ascomycete.

Group III histidine kinase is a positive regulator of Hog1-type mitogen-activated protein kinase in filamentous fungi

We previously reported that the group III histidine kinase Dic1p in the maize pathogen Cochliobolus heterostrophus is involved in resistance to dicarboximide and phenylpyrrole fungicides and in osmotic adaptation. In addition, exposure to the phenylpyrrole fungicide fludioxonil led to improper activation of Hog1-type mitogen-activated protein kinases (MAPKs) in some phytopathogenic fungi, including C. heterostrophus. Here we report, for the first time, the relationship between the group III histidine kinase and Hog1-related MAPK: group III histidine kinase is a positive regulator of Hog1-related MAPK in filamentous fungi. The phosphorylation pattern of C. heterostrophus BmHog1p (Hog1-type MAPK) was analyzed in wild-type and dic1-deficient strains by Western blotting. In the wild-type strain, phosphorylated BmHog1p was detected after exposure to both iprodione and fludioxonil at a concentration of 1 microg/ml. In the dic1-deficient strains, phosphorylated BmHog1p was not detected after exposure to 10 microg/ml of the fungicides. In response to osmotic stress (0.4 M KCl), a trace of phosphorylated BmHog1p was found in the dic1-deficient strains, whereas the band representing active BmHog1p was clearly detected in the wild-type strain. Similar results were obtained for Neurospora crassa Os-2p MAPK phosphorylation in the mutant of the group III histidine kinase gene os-1. These results indicate that group III histidine kinase positively regulates the activation of Hog1-type MAPKs in filamentous fungi. Notably, the Hog1-type MAPKs were activated at high fungicide (100 microg/ml) and osmotic stress (0.8 M KCl) levels in the histidine kinase mutants of both fungi, suggesting that another signaling pathway activates Hog1-type MAPKs in these conditions.

Control of the phytopathogen Botrytis cinerea using adipic acid monoethyl ester

The in vitro and in vivo antifungal activity of adipic acid monoethyl ester (AAME) on the necrotrophic pathogen Botrytis cinerea has been studied. This chemical effectively controlled this important phytopathogen, inhibited spore germination and mycelium development at non-phytotoxic concentrations. The effectiveness of AAME treatment is concentration-dependent and influenced by pH. Spore germination in the presence of AAME is stopped at a very early stage, preventing germ tube development. In addition, cytological changes such as retraction of the conidial cytoplasm in the fungus are observed. AAME was also found to act on membrane integrity, affecting permeability without exhibiting lytic activity, as described previously for other antifungal compounds. Polyamine content in the mycelium of B. cinerea was also affected in response to AAME treatment, resulting in putrescine reduction and spermine accumulation similar to a number of antifungal agents. Microscopic observation of treated conidia after inoculation on tomato leaves suggested that inhibited spores are not able to attach to and penetrate the leaf. Finally, AAME completely suppressed the grey mould disease of tomato fruits under controlled inoculation conditions, providing evidence for its efficacy in a biological context and for the potential use of this chemical as an alternative fungicide treatment.

Determinants of Pseudomonas putida WCS358 involved in inducing systemic resistance in plants

SUMMARY Pseudomonas putida WCS358 is a plant growth-promoting rhizobacterium originally isolated from the rhizosphere of potato. It can suppress soil-borne plant diseases by siderophore-mediated competition for iron, but it has also been reported to result in induced systemic resistance (ISR) in Arabidopsis thaliana. Bacterial determinants of this strain involved in inducing systemic resistance in Arabidopsis were investigated using a Tn5 transposon mutant defective in biosynthesis of the fluorescent siderophore pseudobactin, a non-motile Tn5 mutant lacking flagella, and a spontaneous phage-resistant mutant lacking the O-antigenic side chain of the lipopolysaccharides (LPS). When using Pseudomonas syringae pv. tomato as the challenging pathogen, purified pseudobactin, flagella and LPS all triggered ISR. However, the mutants were all as effective as the parental strain, suggesting redundancy in ISR-triggering traits in WCS358. The Botrytis cinerea-tomato, B. cinerea-bean and Colletotrichum lindemuthianum-bean model systems were used to test further the potential of P. putida WCS358 to induce ISR. Strain WCS358 significantly reduced disease development in all three systems, indicating that also on tomato and bean WCS358 can trigger ISR. In both tomato and bean, the LPS mutant had lost the ability to induce resistance, whereas the flagella mutant was still effective. In bean, the pseudobactin mutant was still effective, whereas this mutant has lost its effectivity in tomato. In both bean and tomato, flagella isolated from the parental strain were not effective, whereas LPS or pseudobactin did induce systemic resistance.

Evolution of an Osmosensing Histidine Kinase in Field Strains of Botryotinia fuckeliana (Botrytis cinerea) in Response to Dicarboximide Fungicide Usage

ABSTRACT DNA sequence polymorphisms in the putative two-component histidine protein kinase encoded by the Daf1 gene have been identified within a sample of 5 sensitive and 27 dicarboximide-resistant field strains of Botryotinia fuckeliana (anamorph Botrytis cinerea). The gene of 3948 bp is predicted to encode a 1315-amino acid protein comprising an N-terminal region, an amino acid repeat region, which has been hypothesized to be the binding site for dicarboximide fungicide, and a C-terminal region encompassing kinase and response regulator domains. Two amino acid variants were distinguished among the sensitive strains characterized by alanine (group 1), or threonine (group 2), at position 1259 in the C-terminal region. All resistant strains could be classified into either group 1 or group 2 but, in addition, all showed changes in the second amino acid repeat region. On the basis of the differences in this repeat region, four classes of resistant strains were recognized; class 1 characterized by an isoleucine to serine mutation, class 2 by an isoleucine to asparagine mutation, class 3 by an isoleucine to arginine mutation (all at position 365), and class 4 by an isoleucine to serine mutation (position 365) as well as a glutamine to proline mutation (position 369). All classes showed similar low levels of resistance to iprodione and to vinclozolin, except for class 3 and class 4 strains, which show low resistance to iprodione but moderate (class 3) or high (class 4) resistance to vinclozolin. The classes as a group did not differ from sensitive strains in osmotic sensitivity measured as mycelial growth response, but some class 1 strains showed an abnormal morphology on osmotically amended medium. The evolution of the amino acid differences is discussed in relation to field observations. It is proposed that class 1 and class 2 strains arose by single mutations within the sensitive population, whereas classes 3 and 4 arose by single mutations within a resistant population.

Dicarboximide resistance in field isolates of Alternaria alternata is mediated by a mutation in a two-component histidine kinase gene

Isolates of Alternaria alternata collected from a field site which had previously been treated with the dicarboximide fungicide iprodione were found to demonstrate a high level of resistance to iprodione and the phenylpyrrole fungicide, fludioxonil in plate assays. In order to determine the genetic basis for this fungicide resistance a partial length clone of a two-component histidine kinase (HK) was isolated from genomic DNA of a fungicide-sensitive A. alternata isolate using degenerate primers by PCR. Analysis of the AaHK1 gene structure indicates the presence of six 90 amino acid repeat domains upstream of a kinase domain as found in the homologous HK genes from other fungal species. Comparison of nucleic acid sequences from the fungicide-sensitive and fungicide-resistant A. alternata isolates confirmed the presence of mutations leading to premature termination of the translated HK protein. The possible role of the two-component HK in the development of dicarboximide resistance in A. alternata is discussed.

Cloning and characterization of the histidine kinase gene Dic1 from Cochliobolus heterostrophus that confers dicarboximide resistance and osmotic adaptation

A gene for a putative two-component histidine kinase, which is homologous to os-1 from Neurospora crassa, was cloned and sequenced from the plant-pathogenic fungus Cochliobolus heterostrophus. The predicted protein possessed the conserved histidine kinase domain, the response regulator domain, and six tandem repeats of 92-amino-acids at the N-terminal end that are found in histidine kinases from other filamentous fungi. Introduction of the histidine kinase gene complemented the deficiency of the C. heterostrophus dic1 mutant, suggesting that the Dic1 gene product is a histidine kinase. Dic1 mutants are resistant to dicarboximide and phenylpyrrole fungicides, and they are sensitive to osmotic stress. We previously classified dic1 alleles into three types, based on their phenotypes. To explain the phenotypic differences among the dic1 mutant alleles, we cloned and sequenced the mutant dic1 genes and compared their sequences with that of the wild-type strain. Null mutants for Dic1, and mutants with a deletion or point mutation in the N-terminal repeat region, were highly sensitive to osmotic stress and highly resistant to both fungicides. A single amino acid change within the kinase domain or the regulator domain altered the sensitivity to osmotic stress and conferred moderate resistance to the fungicides. These results suggest that this predicted protein, especially its repeat region, has an important function in osmotic adaptation and fungicide resistance.

Ethylene-insensitive tobacco shows differentially altered susceptibility to different pathogens

ABSTRACT Transgenic tobacco plants (Tetr) expressing the mutant etr1-1 gene from Arabidopsis thaliana are insensitive to ethylene and develop symptoms of wilting and stem rot when grown in nonautoclaved soil. Several isolates of Fusarium, Thielaviopsis, and Pythium were recovered from stems of diseased Tetr plants. Inoculation with each of these isolates of 6-week-old plants growing in autoclaved soil caused disease in Tetr plants but not in nontransformed plants. Also, when 2-week-old seedlings were used, nontransformed tobacco appeared nonsusceptible to the Fusarium isolates, whereas Tetr seedlings did develop disease. Tetr seedlings were not susceptible to several nonhost Fusarium isolates. In contrast to results with Fusarium isolates, inoculation of 2-week-old seedlings with a Thielaviopsis isolate resulted in equal symptom development of nontransformed and Tetr tobacco. In order to explore the potential range of pathogens to which Tetr tobacco plants display enhanced susceptibility, the pathogenicity of several root and leaf pathogens was tested. Tetr plants were more susceptible to the necrotrophic fungi Botrytis cinerea and Cercospora nicotianae and the bacterium Erwinia carotovora, but only marginally more to the bacterium Ralstonia solanacearum. In contrast, the biotrophic fungus Oidium neolycopersici, the oomycete Peronospora tabacina, and Tobacco mosaic virus caused similar or less severe symptoms on Tetr plants than on nontransformed plants. Total peroxidase activity of Tetr plants was lower than that of nontransformed plants, suggesting a role for peroxidases in resistance against necrotrophic microorganisms. A comparable range of pathogens was examined on Arabidopsis and its ethylene-insensitive mutants etr1-1 and ein2-1. With the exception of one Fusarium isolate, ethylene insensitivity increased susceptibility of Arabidopsis plants to a similar spectrum of necrotizing pathogens as in tobacco. Thus, both ethylene-insensitive tobacco and Arabidopsis plants appear to be impaired in their resistance to necrotrophic pathogens.

Induction of systemic resistance to Botrytis cinerea in tomato by Pseudomonas aeruginosa 7NSK2: role of salicylic acid, pyochelin, and pyocyanin

The rhizobacterium Pseudomonas aeruginosa 7NSK2 produces secondary metabolites such as pyochelin (Pch), its precursor salicylic acid (SA), and the phenazine compound pyocyanin. Both 7NSK2 and mutant KMPCH (Pch-negative, SA-positive) induced resistance to Botrytis cinerea in wild-type but not in transgenic NahG tomato. SA-negative mutants of both strains lost the capacity to induce resistance. On tomato roots, KMPCH produced SA and induced phenylalanine ammonia lyase activity, while this was not the case for 7NSK2. In 7NSK2, SA is probably very efficiently converted to Pch. However, Pch alone appeared not to be sufficient to induce resistance. In mammalian cells, Fe-Pch and pyocyanin can act synergistically to generate highly reactive hydroxyl radicals that cause cell damage. Reactive oxygen species are known to play an important role in plant defense. To study the role of pyocyanin in induced resistance, a pyocyanin-negative mutant of 7NSK2, PHZ1, was generated. PHZ1 is mutated in the phzM gene encoding an O-methyltransferase. PHZ1 was unable to induce resistance to B. cinerea, whereas complementation for pyocyanin production or co-inoculation with mutant 7NSK2-562 (Pch-negative, SA-negative, pyocyanin-positive) restored induced resistance. These results suggest that pyocyanin and Pch, rather than SA, are the determinants for induced resistance in wild-type P. aeruginosa 7NSK2.

Mechanisms of resistance to fungicides in field strains of Botrytis cinerea

Field strains of Botrytis cinerea Pers ex Fr, the causal agent of grey mould diseases, were collected from French vineyards between 1993 and 2000. Several phenotypes have been characterized according to the inhibitory effects of fungicides towards germ-tube elongation and mycelial growth. Two types of benzimidazole-resistant strains (Ben R1 and Ben R2) could be detected; negative cross-resistance to phenylcarbamates (e.g. diethofencarb) was only found in Ben R1. Benzimidazole resistance was related to point mutations at codon 198 (Ben R1) or 200 (Ben R2) of the beta-tubulin gene. Most dicarboximide-resistant strains were also weakly resistant to aromatic hydrocarbon fungicides (e.g. dicloran) but remained sensitive to phenylpyrroles (e.g. fludioxonil). These resistant field strains (Imi R1) contained a single base pair mutation at position 365 in a two-component histidine kinase gene, probably involved in the fungal osmoregulation. Three anilinopyrimidine-resistant phenotypes have been identified. In the most resistant one (Ani R1), resistance was restricted to anilinopyrimidines, but no differences were observed in the amino-acid sequences of cystathionine beta-lyase (the potential target site of these fungicides) from Ani R1 or wild-type strains. In the two other phenotypes (Ani R2 and Ani R3), resistance extended to various other groups of fungicide, including dicarboximides, phenylpyrroles and sterol biosynthesis inhibitors. This multi-drug resistance was probably determined by over-production of ATP-binding cassette transporters. The hydroxyanilide fenhexamid is a novel botryticide whose primary target site is the 3-keto reductase involved in sterol C-4 demethylations. Apart from the multi-drug-resistant strain Ani R3, three other fenhexamid-resistant phenotypes have been recognized. For two of them (Hyd R1 and Hyd R2) fenhexamid-resistance seemed to result from P450-mediated detoxification. Reduced sensitivity of the target site could be the putative resistance mechanism operating in the third resistant phenotype (Hyd R3). Increased sensitivity to inhibitors of sterol 14 alpha-demethylase recorded in Hyd R1 strains was related to two amino-acid changes at positions 15 and 105 of this enzyme.

An osmosensing histidine kinase mediates dicarboximide fungicide resistance in Botryotinia fuckeliana (Botrytis cinerea)

A two-component histidine protein kinase gene, homologous to os-1 from Neurospora crassa, was cloned and sequenced from a single ascospore isolate of Botryotinia fuckeliana. A series of nine spontaneous mutants resistant to dicarboximide fungicides was selected from this strain and characterized with respect to fungicide resistance and osmotic sensitivity. Genetic crosses of the mutants with an authentic Daf1 strain showed that the phenotypes mapped to this locus. Single point mutations (seven transitions, one transversion, and one short deletion) were detected in the alleles of the nine mutants sequenced. The mutational changes were shown to cosegregate with the dicarboximide resistance and osmotic sensitivity phenotypes in progeny obtained from crossing selected resistant strains with a sensitive strain. All mutations detected are predicted to result in amino acid changes in the coiled-coil region of the putative Daf1 histidine kinase, and it is proposed that dicarboximide fungicides target this domain.

Abscisic acid determines basal susceptibility of tomato to Botrytis cinerea and suppresses salicylic acid-dependent signaling mechanisms

Abscisic acid (ABA) is one of the plant hormones involved in the interaction between plants and pathogens. In this work, we show that tomato (Lycopersicon esculentum Mill. cv Moneymaker) mutants with reduced ABA levels (sitiens plants) are much more resistant to the necrotrophic fungus Botrytis cinerea than wild-type (WT) plants. Exogenous application of ABA restored susceptibility to B. cinerea in sitiens plants and increased susceptibility in WT plants. These results indicate that ABA plays a major role in the susceptibility of tomato to B. cinerea. ABA appeared to interact with a functional plant defense response against B. cinerea. Experiments with transgenic NahG tomato plants and benzo(1,2,3)thiadiazole-7-carbothioic acid demonstrated the importance of salicylic acid in the tomato-B. cinerea interaction. In addition, upon infection with B. cinerea, sitiens plants showed a clear increase in phenylalanine ammonia lyase activity, which was not observed in infected WT plants, indicating that the ABA levels in healthy WT tomato plants partly repress phenylalanine ammonia lyase activity. In addition, sitiens plants became more sensitive to benzo(1,2,3)thiadiazole-7-carbothioic acid root treatment. The threshold values for PR1a gene expression declined with a factor 10 to 100 in sitiens compared with WT plants. Thus, ABA appears to negatively modulate the salicylic acid-dependent defense pathway in tomato, which may be one of the mechanisms by which ABA levels determine susceptibility to B. cinerea.

Resistance to fungal pathogens triggered by the Cf9-Avr9 response in tomato and oilseed rape in the absence of hypersensitive cell death

summary In tomato and related species, the Cf9 resistance gene induces hypersensitive cell death and activates downstream defence pathways upon recognition of the Avr9 elicitor. We investigated whether the Cf9-Avr9 response without hypersensitive cell death symptoms increases resistance to several fungi. A low Avr9 dose that does not cause hypersensitive cell death was injected in Cf9 tomato and transgenic Cf9 oilseed rape plants. Subsequently, the injected leaves were infected with different fungal pathogens. The disease development of Botrytis cinerea was delayed in Cf9 tomato when the pathogen was inoculated on, or around, the Avr9 injection site. Disease development of Leptosphaeria maculans and Sclerotinia sclerotiorum was delayed on Cf9 oilseed rape plant parts located around the Avr9 injection site. Disease development of Oidium lycopersicum in Cf9 tomato or Erysiphe polygoni in Cf9 oilseed rape was not restricted on leaves injected with Avr9. The Avr9 injection induced systemic resistance to L. maculans and E. polygoni in Cf9 oilseed rape. F(1)(Cf9xAvr9) oilseed rape plants, obtained from crosses of transgenic Cf9x transgenic Avr9 oilseed rape, exhibited higher levels of resistance to L. maculans and E. polygoni but not to S. sclerotiorum, than wild-type plants. F(1)(Cf9xAvr9) plants treated with benzothiadiazole (BTH) did not show elevated levels of expression of some pathogenesis-related genes but developed higher levels of resistance to L. maculans than BTH-treated wild-type plants. This report demonstrates that the hypersensitive cell death which is associated with the Cf9-Avr9 response is not required for quantitative disease resistance.

The ABC transporter BcatrB from Botrytis cinerea is a determinant of the activity of the phenylpyrrole fungicide fludioxonil

This study demonstrates that the ATP-binding cassette (ABC) transporter BcatrB from Botrytis cinerea influences the activity of phenlpyrrole fungicides against the pathogen. This conclusion is based on toxicity assays and northern analysis experiments which show that BcatrB replacement mutants, which do not express the BcatrB gene, show an increased sensitivity to the phenylpyrrole fungicides fludioxonil and fenpiclonil. Mutants overexpressing BcatrB exhibit a decreased sensitivity to these fungicides. In addition, accumulation of fludioxonil by BcatrB replacement mutants was higher than by wild-type isolates. For mutants overexpressing BcatrB the reverse was observed. Additional ABC and major facilitator superfamily (MFS) transporter genes were identified in an expressed sequence tag (EST) database, suggesting that B cinerea has gene families of ABC and MFS transporters. Corresponding fragments of ten ABC (BcatrC-BcatrN) and three MFS transporter genes (Bcmfs1-4) were cloned and characterised. Fludioxonil affected the transcript level of some members of these gene families in germlings during a short treatment with the fungicide at sub-lethal concentrations. Hence, other ABC and MFS transporters may affect the activity of phenylpyrrole fungicides as well. Other fungicides such as the anilinopyrimidine fungicide cyprodinil, the azole fungicide tebuconazole, the dicarboximide fungicide iprodione and the strobilurin fungicide trifloxystrobin also induced transcription of some of the ABC and MFS transporter genes identified. Therefore, we propose that various ABC and MFS transporters function in protection of the fungus against fungicides and are involved in multi-drug resistance development.

The cAMP signal transduction pathway mediates resistance to dicarboximide and aromatic hydrocarbon fungicides in Ustilago maydis

The cAMP signal transduction pathway mediates the switch between yeast-like and filamentous growth and influences both sexual development and pathogenicity in the smut fungus Ustilago maydis. Signaling via cAMP may also play a role in fungicide resistance in U. maydis. In particular, the adr1 gene, which encodes the catalytic subunit of the U. maydis cAMP-dependent protein kinase (PKA), is implicated in resistance to the dicarboximide and aromatic hydrocarbon fungicides. In this study, we examined the sensitivity of PKA to vinclozolin and could not demonstrate direct inhibition of protein kinase activity. However, we did find that mutants with disruptions in the ubc1 gene, which encodes the regulatory subunit of PKA, were resistant to both vinclozolin and chloroneb. We also found that these fungicides altered the morphology of both wild-type and ubc1 mutant cells. In addition, strains that are defective in ubc1 display osmotic sensitivity, a property often associated with vinclozolin and chloroneb resistance in other fungi.

The ABC transporter BcatrB affects the sensitivity of Botrytis cinerea to the phytoalexin resveratrol and the fungicide fenpiclonil

During pathogenesis, fungal pathogens are exposed to a variety of fungitoxic compounds. This may be particularly relevant to Botrytis cinerea, a plant pathogen that has a broad host range and, consequently, is subjected to exposure to many plant defense compounds. In practice, the pathogen is controlled with fungicides belonging to different chemical groups. ATP-binding cassette (ABC) transporters might provide protection against plant defense compounds and fungicides by ATP-driven efflux mechanisms. To test this hypothesis, we cloned BcatrB, an ABC transporter-encoding gene from B. cinerea. This gene encodes a 1,439 amino acid protein with nucleotide binding fold (NBF) and transmembrane (TM) domains in a [NBF-TM6]2 topology. The amino acid sequence has 31 to 67% identity with ABC transporters from various fungi. The expression of BcatrB is up regulated by treatment of B. cinerea germlings with the grapevine phytoalexin resveratrol and the fungicide fenpiclonil. BcatrB replacement mutants are not affected in saprophytic growth on different media but are more sensitive to resveratrol and fenpiclonil than the parental isolate. Furthermore, virulence of deltaBcatrB mutants on grapevine leaves was slightly reduced. These results indicate that BcatrB is a determinant in sensitivity of B. cinerea to plant defense compounds and fungicides.

Phenotype Instability in Botrytis cinerea in the Absence of Benzimidazole and Dicarboximide Fungicides

ABSTRACT Stability of phenotypes of isolates of Botrytis cinerea that were sensitive or resistant to benzimidazole and dicarboximide fungicides was examined in the absence of fungicides in laboratory and growth room experiments. Twelve greenhouse isolates of B. cinerea were subcultured on potato dextrose agar (PDA) for 20 generations and on geranium seedlings for 15 generations. Three isolates of each of the following four phenotypes were used: sensitive to the fungicides thiophanate-methy1 (a benzimidazole) and vinclozolin (a dicarboximide) (S(T)S(V)), resistant to both fungicides (R(T)R(V)), resistant to thiophanate-methy1 and sensitive to vinclozolin (R(T)S(V)), and sensitive to thiophanate-methy1 and resistant to vinclozolin (S(T)R(V)). In three trials on PDA, 36 populations were subcultured; 8 populations changed phenotypes by the end of 20 generations, as determined by conidium germination on fungicide-amended medium. Five of the eight initially were S(T)R(V); the resulting phenotypes were S(T)S(V), R(T)S(V), and R(T)R(V). Populations from eight other isolates exhibited temporary changes in phenotype during intermediate generations on PDA but reverted to initial phenotypes by the twentieth generation; five of these populations changed to phenotype R(T)R(V). In two geranium seedling trials, each of the 12 greenhouse isolates was inoculated onto a set of three seedlings for each generation, and diseased tissue that developed was used to initiate the next generation. Therefore, a total of 72 populations of B. cinerea were subcultured in the two trials; 5 of these populations changed phenotype at the end of 15 generations. Three of the five initially were S(T)R(V); these changed to phenotypes S(T)S(V) or R(T)R(V). In each of the two trials on geranium seedlings, a population subcultured from one S(T)S(V) isolate changed phenotype one to phenotype R(T)R(V) and one to phenotype R(T)S(V). In all trials, no population resistant to thiophanate-methy1 changed to a thiophanate-methy1-sensitive phenotype, and no population changed to phenotype S(T)R(V). Random amplified polymorphic DNA (RAPD) fingerprints were generated with the 12 initial isolates and 49 isolates subcultured on PDA or geranium seedlings. Cluster analyses of RAPD markers showed that subcultured isolates exhibiting the same phenotype clustered together and that subcultured isolates derived from a common greenhouse isolate but with different phenotypes were in different clusters. Some populations that did not change phenotype exhibited considerable differences in RAPD marker patterns. The results of this study indicate that, in the absence of fungicides, sensitive populations of B. cinerea can develop resistance to thiophanate-methy1 and vinclozolin, and this resistance can be maintained in populations through multiple generations. Populations resistant only to vinclozolin (S(T)R(V)) exhibited a high frequency of phenotype change, and populations resistant to both fungicides (R(T)R(V)) were stable.

Resistance of Botryosphaeria dothidea from Pistachio to Iprodione

Out of 79 field-collected isolates of Botryosphaeria dothidea, causal agent of panicle and shoot blight of California pistachio, 1 had low resistance to the dicarboximide fungicide iprodione (effective fungicide concentration to inhibit 50% of mycelial growth [EC₅₀] = 2.726 μg/ml), whereas all the other isolates were sensitive (EC₅₀ < 1 μg/ml). B. dothidea isolates readily developed resistance to iprodione in vitro. Furthermore, these in vitro-derived iprodione-resistant (IR) isolates retained high virulence on pistachio. The EC₅₀ values for these IR isolates were greater than 25 μg/ml, whereas the EC₅₀ values for the original wild-type counterparts were less than 1 μg/ml. Iprodione resistance significantly declined when these IR isolates were propagated on pistachio leaves in the absence of the fungicide. IR isolates also were resistant to vinclozolin, another dicarboximide fungicide, but sensitive to tebuconazole and benomyl. IR isolates showed an increase in osmotic sensitivity on media amended with NaCl. Applications of iprodione at 500 μg/ml, which is effective against naturally sensitive isolates, failed to control disease caused by IR isolates in both the laboratory and greenhouse. The results indicate that, although naturally occurring IR isolates of B. dothidea may be rare in California pistachio orchards, the fungus readily develops resistance to iprodione in vitro and, more importantly, retains high levels of virulence on pistachio.