The role of the rhizosphere microbial community in plant chemistry and aphid herbivory in Brassica oleracea

Soil microbial communities can influence plant productivity, chemistry and even diversity. Intensive farming practices have caused widespread soil degradation, raising concerns regarding soil health and need for sustainable agriculture. Although soil microbe-plant interactions have been extensively studied, the relationships between soil microbial communities and higher trophic levels, such as herbivorous insects, are poorly understood. This thesis reports the findings of a series of mesocosm experiments which used a model system of Derby Day cabbages (Brassica oleracea L. var. capitata), peach-potato aphids (Myzus persicae), and soil sourced from an agricultural field site. Firstly, I conducted an exploratory study of the soil microbial community response to different fertiliser regimes and cabbage growth using Next Generation Sequencing (NGS) of the 16S rRNA gene. This was complemented by concomitant measurements of the plant and aphid performance in order to identify potential soil-plant-insect relationships. The results revealed that the diversity and composition of bacterial communities were more strongly influenced by the cabbage age and fertiliser treatment than aphid herbivory. Several bacteria exhibited enhanced abundance in rhizosphere of older cabbages, including sulphur-oxidising bacteria (SOB) of the Thiobacillus genus. A member of this genus was then selected as an inoculant in the subsequent experiment to test its plant growth promoting potential for B. oleracea. Brassica plants produce a class of secondary metabolites called glucosinolates, which have multiple beneficial properties including anti-herbivory and anti-carcinogenic attributes. As this compound is rich in sulphur (S), it was hypothesised that soil inoculation with the SOB Thiobacillus thioparus may enhance glucosinolate production in B. oleracea, thereby improving its defence against aphid feeding. Analysis revealed a promotional effect of enhanced SOB populations on glucosinolate content of leaves. Although an accompanying significant effect on aphid populations was not detected, this experiment shows the potential for the modulation of plant chemical defences by the soil microbial community