Breaching by a new strain of Leptosphaeria maculans of anatomical barriers in cotyledons of Brassica napus cultivar Surpass 400 with resistance based on a single dominant gene |
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Authors: | Hua Li Vanessa Stone Neree Dean Krishnapillai Sivasithamparam Martin J Barbetti |
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Institution: | (1) School of Earth and Geographical Sciences, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Nedlands, WA, 6009, Australia;(2) School of Plant Biology, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Nedlands, WA, 6009, Australia |
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Abstract: | Infection processes were examined to investigate the breach by a strain of Leptosphaeria maculans of anatomical barriers in cv. Surpass 400, a cultivar containing single dominant gene-based resistance (SDGBR). Two strains,
UWA 192 and UWA P11, were used to inoculate cvs. Surpass 400 and Westar. The pre-penetration and penetration behaviour of
both strains was similar in both cultivars. However, they differed significantly after penetration. When UWA P11 infected
cv. Surpass 400 through stomata, guard cells rapidly died within a few hours and the surrounding mesophyll cells became necrotic,
constituting a hypersensitive reaction (HR). Hyphal growth continued, albeit slowly, through the intercellular palisade mesophyll
and spongy mesophyll spaces, but hyphae rarely spread beyond the HR region, and did not sporulate. Polyphenolic compounds
accumulated in the area bordering the HR. However, when UWA 192 infected through stomata, symptoms were not evident until
10–12 days postinoculation (dpi) and were typically characterized by pale tan to grey circular lesions in which abundant pycnidia
were produced by 14 dpi. Subsequently, hyphae extensively spread beyond the lesion border, reaching the veins and progressing
down the petiole towards the stem. Where the SDGBR remained effective (i.e. against strain UWA P11) death of cells was restricted
to a few palisade cells within the HR, even though hyphae were present in the lower tissue layers of the cotyledon. In contrast,
where the SDGBR was not present (cv. Westar) or was overcome (cv. Surpass 400 with UWA 192), extensive death of epidermal
and upper and lower palisade cells occurred throughout infected areas of the cotyledon, with subsequent abundant production
of pycnidia. Polyphenolic compounds, which are also associated with resistance, did not accumulate in this instance. It was
evident that the ability of the host to instigate the HR mechanism displayed by cv. Surpass 400 was lost with UWA 192 resulting
in a “normal” susceptible response. This is the first study of the specific processes involved in the breaching of the HR
in cv. Surpass 400. |
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Keywords: | Oilseed rape Blackleg disease Phoma Host resistance |
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