Submitted to: Journal of Applied Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2007
Publication Date: 6/15/2007
Citation: Wang, Y., Meilan, R., Wisniewski, M.E., Cui, M., Fuchigami, L. 2007. Transgenic tomato (Lycopersicon esculentum) that overexpress cAPX exhibits enhanced tolerance to UV-B and heat stress. Journal of Applied Horticulture. 8:1-4. Interpretive Summary: Unfavorable environmental conditions result in reduced crop yields and loss of vigor for perennial crops, such as fruit crops. Exposure of plants to environmental stress results in the formation of reactive forms of oxygen, such as hydrogen peroxide and superoxide radicals. Collectively, these types of oxygen molecules are referred to Reactive Oxygen Species (ROS). ROS can be very toxic and result in injury to plant membranes, proteins, and DNA. Understanding how plants manage and deal with oxidative stress is a key objective of the ARS National Program 302 which addresses Plant Molecular and Biological Processes. Increased resistance to the ROS could result in plants being less injured by many different types of environmental stress. The current research examined whether or not the overexpression of an ascorbate peroxidase (APX) gene in tomato could improve its resistance to heat and ultraviolet (UV-B) stress. APX is part of an antioxidant system in plants that has evolved to help plants cope with oxidative stress. This enzyme is responsible for converting hydrogen peroxide to water and ascorbate. Results indicated that higher levels of APX were present in the transgenic lines of tomato compared to the non-transformed wild-type (WT) tomatoes. The transgenic tomato lines were less injured when exposed to heat and UV-B light stress compared to the WT tomatoes. These results indicate that overexpression of APX is an approach that can be used to produce plants more resistant to environmental stress. Alternatively, this information can be used as a breeding tool. Those plants with naturally higher levels of APX could be used a breeding material to develop new cultivars of crops that have increased resistance to environmental stress. Further research will explore the effect of overexpression of APX on other antioxidant enzymes.
Technical Abstract: Reactive oxygen species (ROS), such as hydrogen peroxide, superoxide and hydroxyl radicals, are by-products of biological redox reactions. ROS can denature enzymes and damage important cellular components. Plants have developed antioxidant enzymes, such as superoxide dismutase (SOD) and ascorbate peroxidase (APX), to scavenge ROS and detoxify them. The effect of increased cytosolic ascorbate peroxidase (cAPX) on heat and UV-B stress tolerance was studied using transformed tomato (Lycopersicon esculentum cv. Zhongshu No. 5) plants. This research demonstrates, in either laboratory or field tests, the potential to enhance tolerance to heat, UV-B and sunscald stress by gene transfer. Overexpression of cAPX in transgenic tomato enhanced resistance to heat (40 deg C) and UV-B stress compared to wild-type plants. When leaf disks were placed at 40 deg C for 13 hours, the electrolyte leakage of disks from wild-type were 93%, whereas, two tested transgenic lines (A9, A16) exhibited 24% and 52% leakage, respectively. When fruits of wild-type and transgenic plants were exposed to UV-B (2.5mW/cm2) for five days, the extent of browning was 95%, 33%, and 37%, respectively. In field tests, the detached fruits from field-grown transgenic plants showed more resistance to exposure to direct sunlight than fruits from wild-type plants. APX activity in leaves of cAPX transgenic plants was several folds higher than in leaves of wild-type plants when exposed to heat, UV-B and drought stresses.