Abstract
Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive diseases of wheat and has been largely managed using demethylation inhibitor (DMI) fungicide triadimefon in China. To determine the sensitivity of Pst, a Chinese Pst isolate and its sexually produced progeny isolates were tested with triadimefon using the detached leaf method. The half maximal effective concentration (EC50) values varied greatly among the progeny isolates, ranging from 0.06 mg L–1 to 7.89 mg L–1. Twenty-six of the 56 tested progeny isolates were less sensitive to triadimefon than the parental isolate. A single-nucleotide mutation at the 401 position resulting in an amino acid change from tyrosine (Y) to phenylalanine (F) in the 134th codon (Y134F) of the cytochrome P450 sterol 14a-demethylase enzyme (CYP51), the target gene of DMI fungicide, was identified in the parental isolate. The 87 tested progeny isolates segregated into 19 homozygous wild type (AA), 40 heterozygous (AT), and 28 homozygous mutant (TT) genotypes, fitting a 1:2:1 ratio (χ2 = 2.43; P = 0.30). The mutant isolates had higher EC50 values than the wild type isolates. Significant differences in logEC50 were found between the mutant isolates and the wild type isolates (P = 2.2e–16). However, homozygous and heterozygous mutant isolates were not significantly different (P = 0.21), indicating dominant mutation. Twenty-two progeny isolates were used to inoculate a susceptible wheat variety, and latency period and lesion growth were recorded to compare wild type and mutant isolates for the pathogenicity fitness components. A moderate but significant negative correlation was detected between lesion growth and sensitivity to triadimefon (r = −0.53; P = 0.01). No significant variation in lesion growth was found between homozygous and heterozygous mutant isolates (P = 0.83). In the case of latency period and triadimefon sensitivity, no significant correlation was found (P = 0.17). These results are useful for understanding reduced sensitivity in the pathogen population and improving stripe rust management.
Highlights
Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici Erikss. (Pst), is one of the most important diseases on wheat worldwide (Chen, 2005; Wellings, 2011)
In a cross between Demethylation inhibitor (DMI)-sensitive and resistant isolates of Blumeria graminis f. sp. hordei (Bgh), both mutations segregate with resistance, which was consistent with cytochrome P450 sterol 14a-demethylase enzyme (CYP51) controlling a major portion of DMI fungicide resistance (Robinson et al, 2002; Wyand and Brown, 2005)
The mutants in the CYP51 gene in the parental isolate and 87 progeny isolates were confirmed by genotyping using Kompetitive Allele Specific Polymerase chain reaction (PCR) (KASP) primers KaspF1 (5 -ga aggtgaccaagttcatgctCTGTATTCGGTACGGATGTAGTTTA-3 ), KaspF2 (5 -gaaggtcggagtcaacggattCTGTATTCGGTACGGATGT AGTTTT-3 ), and KaspR (5 -AAGATTGCGTTCGGGACAT3 ), in which the detective primer sequences of FAM and HEX were added to the 5 of forward primers
Summary
Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici Erikss. (Pst), is one of the most important diseases on wheat worldwide (Chen, 2005; Wellings, 2011). Three major molecular mechanisms have been found associated with resistance to azole compounds belonging to DMI fungicides in plant pathogenic fungi: (1) point mutations in the CYP51 sequence, (2) overexpression of the CYP51 enzyme, and (3) overexpression of genes encoding efflux pump proteins. In a cross between DMI-sensitive and resistant isolates of Bgh, both mutations segregate with resistance, which was consistent with CYP51 controlling a major portion of DMI fungicide resistance (Robinson et al, 2002; Wyand and Brown, 2005). We used a Pst sexual population, which was developed by selfing isolate PL17-7 on barberry plants (Tian et al, 2017), to determine the relationships between fungicide sensitivity and fitness components. The specific objective were to (1) determine the triadimefon sensitivity levels of the parental isolate and the sexually produced progeny isolates and its inheritance; (2) determine the genotypes at the CYP51 locus of the parental and progeny isolates, and compare the triadimefon fungicide sensitivity levels of isolates with the different CYP51 genotypes; and (3) determine the relationships between triadimefon resistance and fitness components such as latency period and lesion growth
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