Senescence and oxidative stress in wallflowers and Arabidopsis

Reactive oxygen species (ROS) together with other signalling molecules including phytohormones regulate plant growth and senescence as well as responses to abiotic stress and pathogens. One aim of this thesis was to investigate how ROS and phytohormones regulate progression of senescence in wallflower petals. A second aim of this thesis was to investigate how SAG21, previously found to be elevated in wallflower petals, functions during development, senescence and stress in the model plant, Arabidopsis thaliana. In wallflowers (Chapter 3), onset of petal senescence was characterized by a rise in ethylene and auxin, followed by ROS accumulation. In parallel, transcript levels for markers of senescence, ethylene biosynthesis and auxin-response also increased with age. However, SAG21 peaked earlier, before visible senescence. Treatments that aimed to increase cytokinin levels or reduce ethylene delayed progression of senescence and time to petal abscission compared to controls. In contrast, ethylene hastened both processes. Marker genes analysed displayed differential expression patterns. Patterns of ROS-related enzyme activity (catalase, ascorbate peroxidase and superoxide dismutase) were also investigated in relation to wallflower petal and leaf senescence. SAG21 (previously named AtLEA5 in Arabidopsis) belongs to the late embryogenesis- associated (LEA) protein family, implicated in growth and redox responses. A SAG21-YFP fusion was localized to Arabidopsis mitochondria (Chapter 4). Over-expression (OEX) of SAG21 in Arabidopsis resulted in increased root and shoot biomass, whereas antisense (AS) lines exhibited reduced biomass. Root and shoot development, flowering and senescence were altered in transgenic lines. Transgenic lines exhibited marked root hair phenotypes (Chapter 5). Abiotic stress induced stress induced SAG21 expression in roots but not leaves. Growth of B. cinerea and of a virulent bacterial pathogen {Pseudomonas syringae pv. tomato) was affected by SA