ECTOPIC EXPRESSION OF A SPINACH SOD GENE IN YOUNG APPLE TREES ENHANCES ABIOTIC STRESS RESISTANCE

Abstract

Reactive oxygen species (ROS) are induced during both biotic and abiotic stress, either as signaling molecules or as result of stress injury. ROS are highly destructive to cell components and the injury resulting from these compounds is referred to as oxidative stress. Antioxidant enzymes, such as superoxide dismutase (SOD), scavenge oxygen radicals preventing the injury resulting from oxidative stress. The objective of the present research was to produce transgenic apple plants (Malus x domestica 'Royal Gala') with enhanced production of a cytosolic SOD. A full-length SOD cDNA was isolated from spinach by a combination of RT-PCR and conventional plaque lift screening of a pea cDNA library. The SOD gene was mobilized into a binary vector consisting of pBINPLUSARS and pRTL2 for Agrobacterium-mediated transformation of apple. The resulting SOD-overexpression (SOD-OX), blank-cassette, and un-transformed lines were evaluated for resistance to acute and prolonged exposure to high temperature, and freezing temperatures in non-acclimated and acclimated plants, by ion leakage assays of leaves and bark from one-year-old trees. Results indicated that SOD-OX leaves exhibited improved resistance to both acute (30 min) and longer-term exposure (2h to 24h) to elevated temperatures compared to the non SOD-OX lines. Cold tolerance of non-cold-acclimated SOD-OX tissues did not differ from the control plants. Cold acclimated (2 weeks exposure to a short day photoperiod at 4°C) leaves of SOD-OX trees, however, were more cold tolerant compared to the other lines, while bark was not. The overexpression of antioxidant enzymes is believed to help cells recover from post-injury ROS rather than directly increase stress tolerance. Therefore, the differences observed in increased stress tolerance in the transgenic apple plants may be a reflection of the type and extent of injury caused by heat vs. freezing stress.