Heteroplasmy and Response Against Azoxystrobin in Cercospora
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The quinone outside inhibitor (QoI) or Strobilurin is one of the most important fungicides used to control fungal and some Oomycetes pathogens in agricultural crops. This class of fungicide was first isolated from a wood-rotting fungus called Strobilurus tenacellus. Several chemically modified derivatives of natural fungicide, Strobilurin A, are available which are more stable, efficacious, less harmful to human and environment. These fungicides are commercially available with different names and active ingredients: azoxystrobin (Syngenta), fenamidone (Bayer), fluoxastrobin (Arysta), kresoxim methyl (Cheminova), pyraclostrobin (BASF) and trifloxystrobin (Bayer) (Bartlett et al. 2002; Vincelli 2012).
QoI fungicides exhibit both translaminar (across leaf blade) and weak systemic movement within the plant. All QoI fungicides have the same mode of action which disrupt mitochondrial respiration and prevent energy production inside fungal cells (Vincelli 2012). The disruption of ATP generation occurs because of binding of strobilurin at Qo site of cytochrome b hence preventing electron transport from cytochrome b to cytochrome c1 (Bartlett et al. 2002).
QoI fungicides are applied to control a broad range of plant pathogens including fungi, water molds, downy mildews, powdery mildews and rusts (Vincelli 2012). They are mainly used as protective and curative fungicides because of effective action against spore germination and penetration (Balba 2007). The eradicative property has also been reported by preventing sporulation of fungal pathogen (Anesiadis et al. 2003). More than 50 species of plant pathogens resistant to QoI fungicides has been reported and there is a high risk of selecting resistant isolates in the field (Fungicide Resistant Action Committee 2013). Three different point mutation in mitochondrial cytochrome b gene has been associated with resistant mechanism against QoI fungicide. The primary mechanism of resistance is by amino acid substitution from glycine to alanine at 143rd codon (G143A) (Bartlett et al. 2002). Other two point mutation at cytochrome b gene is the substitution of phenylalanine with leucine at position 129 (F129L) and glycine with arginine at position 137 (G137R) which confer QoI resistance (Fernández-Ortuño et al. 2010). Another mechanism has also been identified that can bypass the blockage of electron transfer. Alternative oxidase (AOX) is a strobilurin-insensitive terminal oxidase which can bypass electron transfer in Complex III and Salicylhydroxamic acid (SHAM) is an active inhibitor of AOX (Wood and Hollomon 2003).
Resistant mechanism of C. sojina against QoI fungicides is associated with a mitochondrial genome which is present in multiple copies within a single cell. The coexistence of wild and mutated alleles in QoI resistant/sensitive locus has been reported in several other fungal pathogens such as Corynespora cassiicola, Collectotrichumgloeosporioides, Venturia inequalis and Mycovellosiella nattrassii (Ishii et al. 2007; Villani and Cox 2014). The proportion of wild and mutant allele in the mitochondrial genome has a major role for quantitative resistance (Villani and Cox 2014). Protective efficacy of the full dose of azoxystrobin against powdery and downy mildew has been found to decrease as populations contained 10% resistant isolates (Ishii et al. 2007). There have been reports of loss of resistance stability in the absence of selection pressure and vice versa (Fraaije et al. 2002; Ishii et al. 2007). The main objectives of this study are to i) identify heteroplasmy in Cercospora sojina; ii) monitor the proportion of resistant and sensitive allele in the presence of selection pressure in the laboratory; and, iii) study the sensitivity of C. sojina against azoxystrobin.
Materials and Methods
Isolate selection and development of single spore cultures
Isolates of C. sojina were screened for resistant and sensitive allele using Taqman assay. After screening, three isolates each having resistant and sensitive alleles were chosen for single spore cultures. Isolates were transferred to V8-RA media and grown in dark cabinet to enhance sporulation. After three weeks, plated were flooded with water and filtered with muslin filter cloth. Water was observed under dissecting microscope to identify single spores. Sterilized needed were used to pick single spore and transferred to new V8-RA plates. Culture was left at room temperature, mycelium harvested, lyophilized and DNA was extracted.
Radial growth study
A total of two isolates: 158-1 (resistant) and 312-1 (sensitive) were selected for fungicide sensitivity and radial growth study. Four different concentrations of azoxystrobin including control were used to culture both isolates in two replications. Technical grade formulation of azoxystrobin (0.104 gm) (96% a.i.; Syngenta Crop Protection) was used to make 100,000 µg a.i./ml stock in 1 ml acetone. Serial dilution was done to make four different concentration stocks: 10,000, 1000, 100 and 100 µg a.i./ml. V8 media was prepared with four different concentrations (10, 1, 0.1, 0.01 µg a.i./ml) by adding 1ml of respective fungicide stock in 1 liter of media. All four media along with control was amended with salicylhydroxamic acid (SHAM) at 60 µg a.i./ml.
Two straight line at 90o were drawn at the center of the plate. For resistant and sensitive isolates, a 5 mm mycelium disc was taken and placed at the center of amended plates in two replications. For each plate, diameters of growth were measured at the interval of 11, 21 and 30 days. Mycelium disc from amended plates was again transferred to the newly amended plate after 10 days. Diameters were measured similarly for three generations.
Taqman assay and Sanger sequencing
The G/C point mutation in cytochrome b gene will be discriminated by Taqman assay consisting of two dyes. VIC can detect resistant allele 'C' and FAM can detect sensitive allele 'G'. Threshold cycle or Ct of two dyes will be used in detecting the presence of two alleles in a single spore culture. Ct value is the cycle number at which the fluorescence generated crosses the threshold fluorescence and is inversely proportional to the amount of nucleic acid. Lower Ct indicates higher copies in the sample.
Sanger sequencing will be done to confirm the presence of both alleles in a single spore. Two primers pairs (Forward: 5' CTCATTAAATTAGTAATAACTGTGGC 3' and Reverse: 5' TAATACAGCTTCAGCATTTTTCTTCT 3'
) will be used to amplify a part of cytochrome b gene. PCR reaction will be done in a total volume of 25 µl consisting of 1.25 µl (10 µM) of each primer, 12.5 µl of 2x Veriseq PCR mix (Enzymatics Inc.), 1.25 µl DNA and 8.5 µl water and run in following settings: initial denaturation at 94° C for 2 min followed by 29 cycles of denaturation at 94° C for 20 s, annealing at 55° C for 25 s, extension at 72° C for 1 min and final extension at 72° C for 10 min.
Sequences derived from Sanger sequencing will be aligned to publicly available cytochrome b gene of C. sojina. The QoI resistant/sensitive point mutation locus will be observed for Heterozygosity. The proportions of resistant and sensitive alleles will be calculated based on Ct values and statistical analysis will be performed to compare among different generations.
The percent growth inhibition will be calculated as: ([colony diameter on control media - 5 mm] - [colony diameter on fungicide amended media - 5 mm]) / ([colony diameter on control media - 5 mm]) x 100. Further, radial growth of the same isolate among three generations and four different treatments will be compared statistically.
This study will help to explore if heteroplasmy exists in C. sojina as in other Cercospora species. The proportion of resistant and sensitive isolates determines the extent of disease, so it is important to know this ratio. In vitro assay to check the sensitivity of isolates against azoxystrobin at different concentration in a different generation will help to understand the effect of selection pressure. Further measurement of resistant and sensitive proportion with qPCR would help to determine the change occurred in following generations. Genetic study after fungicide treatment will also contribute in identifying changes due to selection pressure.
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