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

Abstract

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.