The Many Faces of Stress Response : The Multi-Level Response to Cold and Waterlogging Stress in Nicotiana tabacum L., and Defining the Antioxidant Gene Network in Cannabis sativa L.

Abstract

In their natural environment, plants are often exposed to a wide array of stressful conditions that frequently occur in combination, and which can cause complex responses. There is a lack of research into many of these combinations, including low temperature and waterlogging, which is relevant to early spring conditions in northern climates around the world. The current study focused on examining this stress combination, using Nicotiana tabacum as a model system. Tobacco plants were exposed to this combination for seven days, followed by a seven-day recovery period, and the effects on the physiology, the photosynthetic apparatus and metabolism were examined. It was determined that a low oxygen 'escape' strategy, characterized by continued rapid growth dominated in plants that were waterlogged at optimal growth temperatures. In contrast, exposure to low temperature induced a 'quiescence' strategy, causing decreased rates of photosynthesis and growth, regardless of whether a plant was waterlogged or not. Once the stresses were removed and plants were allowed to recover, plants exposed to low temperatures exhibited a more robust ability to recover, even when they had also been exposed to waterlogging stress. As such, it was concluded that simultaneous exposure to low temperature and waterlogging stresses is not more damaging than exposure to low temperature stress alone. Cannabis sativa is a dioecious plant known for the accumulation of secondary metabolites (cannabinoids) with known psychoactive and medicinal properties. Until recently, cultivation and usage of this plant was illegal and thus, much of its biology remains understudied. The second part of this thesis focuses on some of the first steps taken to study how cannabis responds to stress, including a detailed review of the recent literature on this topic. The genetic basis of stress resilience in this plant was also examined by characterizing the antioxidant gene family. These enzymes scavenge reactive oxygen species (ROS), to maintain healthy levels of these molecules. Under stress however, ROS can accumulate at high and damaging concentrations. Subsequently, increased accumulation and activity of antioxidant enzymes to manage ROS is a well-known hallmark of plant stress. It has been determined that H₂O₂ (a major plant ROS) is a by-product of cannabinoid synthesis. Accumulation of high cannabinoid levels has been associated with decreases in biomass and increased tissue damage. This may potentially be attributed to increased ROS levels due to cannabinoid synthesis. A genome-wide identification of antioxidant genes in two published C. sativa genomes was conducted. This screen identified ~40 antioxidant genes in both cannabis strains, ~25% of which localized to the female X chromosome. The increased abundance of antioxidant genes on the X chromosome may be a mechanism to increase antioxidant amounts in female plants that naturally accumulate high cannabinoid levels, and therefore produce increased ROS. Thus, the future selection of cannabis strains with increased antioxidant capacity may lead to development of more productive cultivars.