UV-B priming for disease resistance : the use of UV-B light to reduce susceptibility of lettuce plants to downy mildew disease : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Pathology at Massey University, Manawatū, New Zealand

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Biotrophic disease is one of the largest causes of decreased yield in horticulture. Integrated Pest Management (IPM) systems are required to control disease in a manner which is effective and sustainable, yet there are still a limited number of new approaches available. Pretreatments of UV-B light (280-320 nm) have been previously observed to reduce plant susceptibility to disease, and may be a potential disease control tool to use as part of an IPM approach. Here, I characterised the capability of UV-B LED technology to reduce susceptibility of a range of lettuce (Lactuca sativa) cultivars to downy mildew disease caused by the obligate biotroph Bremia lactucae. Reductions in disease susceptibility of UV-B-pretreated plants was observed as: delayed disease incidence, reduced visual disease rating and lower B. lactucae conidia count. UV-B-induced reductions to conidia counts were sufficient to reduce the infectivity of the diseased plant. Secondary infections caused by UV-B-pretreated plants exhibited yet further reduced disease severity. UV-B light has been observed to induce a similar gene expression profile to that of disease defence in plants. To determine the mechanism of a UV-B-induced disease defence, similarities between UV-B and disease defence pathways were identified. Analysis of previously published gene expression data revealed similarities in flavonoid-related gene expression between exposure to UV-B light in Arabidopsis thaliana, and resistance to downy mildew (Hyaloperonospora arabidopsidis). The specific role of flavonoids in UV-induced defence was further investigated, with B. lactucae conidia counts of lettuce plants negatively correlated with flavonoid level in a UV-B-dependent manner. LC-MS was used to identify metabolic features which contribute to this correlation, and of these, quercetin 3-O-(6″-O-malonyl)-β-D-glucoside had the strongest negative correlation with B. lactucae conidia count. The direct effect of quercetin 3-O-(6″-O-malonyl)-β-D-glucoside was tested through infiltration into lettuce leaves followed by subsequent downy mildew infection. Decreased B. lactucae conidia count was observed in two lettuce cultivars infiltrated with quercetin 3-O-(6″-O-malonyl)-β-D-glucoside concentrations similar to those induced by a UV-B-treatment. It was concluded that UV-B-pretreatments can decrease disease susceptibility to downy mildew in lettuce, and that this defence is underpinned in part by UV-B-induced phenolics. These findings highlight the opportunity for UV-B morphogenesis to be exploited in the development of next-generation, sustainable disease control tools.
These Figures were removed for copyright reasons: 1.1 (=Bent & Mackey, 2007 Fig 1a ), 1.4 (=Camagna & Takemoto, 2018 Fig 3) & 1.7 (=Redovnikovic et al., 2008 Fig 4).
Lettuce, Disease and pest resistance, Downy mildew of lettuce, Bremia lactucae, Plants, Effect of ultraviolet radiation on