Seed, Patrick
(2023)
Mapping and characterisation of resistance to Septoria tritici blotch in winter wheat.
PhD thesis, University of Nottingham.
Abstract
The resistance of cv. Cougar to Zymoseptoria tritici has been investigated in detail, involving Quantitative trait loci (QTL) analyses to map the resistance, various physiological measurements to investigate the mechanism of resistance and transcriptomic analyses to identify candidate resistance genes. To map the resistance in cv. Cougar, QTL analyses utilised visual septoria tritici blotch (STB) assessments, chlorophyll fluorescence measurements & hyperspectral reflectance measurements. Additionally, these physiological measurements were used to map other sources of resistance from both cv. Cougar and cv. Scout and to predict future STB disease within a growing season. Utilising a set of near isogenic lines (NILs) for the source of resistance in Cougar, and two incompatible Z.tritici isolates (HT-22/IPO323), transcriptomic analysis characterised the conserved incompatible wheat-Z.tritici transcriptome and identified candidate resistance gene(s) at the location identified on chromosome 4D. The Cougar resistance mechanism was investigated using a range of techniques including chlorophyll fluorescence measurements, hormone, carotenoid and xanthophyll enzyme quantification, superoxide and hydrogen peroxide accumulation, stomatal behaviour, stomatal morphology and Z.tritici spore germination rates. These techniques were used to investigate the effect of compatibility by utilising a Z.tritici isolate which has overcome the Cougar resistance (508, compatible) and an avirulent isolate (HT-22, incompatible). STB assessments and hyperspectral reflectance measurements mapped the cv. Cougar resistance to chromosome 4D in 2018, with a breakdown of this resistance in 2019, likely due to the evolution of pathogen genotypes with virulence to cv Cougar. NPQI (Pheophitinization of chlorophyll a to phaeophytin) was the best index at mapping the resistance to chromosome 4D and was unique to this QTL. This study also demonstrated the broader application of chlorophyll fluorescence and hyperspectral reflectance measurements in identifying novel sources of resistance, with indices such as D2 (chlorophyll fluorescence), ND705 (chlorophyll content) and ARI (anthocyanin content) tightly correlating with septoria tritici blotch (STB) disease in a manner consistent with previous studies. Transcriptomic analysis identified 3 candidate resistance genes at the location identified on chromosome 4D, with upregulation of protein kinases, F-box proteins and cytochrome P450 proteins amongst the main transcripts associated with incompatibility. Resistance was associated with a photoinhibitory and photoprotective mechanism involving both the qI and qE components of NPQ and a change in carotenoid composition reflected by changes in xanthophyll enzyme expression and a decrease in superoxide accumulation. Additionally, changes in SA, JA and 7 ABA content was associated with the Cougar defence response. Preformed defence traits such as stomatal morphology, behaviour and spore germination rates were not associated with compatibility. These results demonstrate that the major source of resistance in cv. Cougar resides on chromosome 4D and likely involves a form of effector triggered immunity which induces a photoinhibitory and photoprotective resistance mechanism involving altered carotenoid and hormone metabolism.
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