Overexpression of Yeast Arabinono-1,4-Lactone Oxidase Gene (ALO) Increases Tolerance to Oxidative Stress and Al Toxicity in Transgenic Tobacco Plants

L-Ascorbic acid (AsA) is the most abundant antioxidant and a major redox buffer that regulates plant responses to environmental stresses. AsA is also a precursor of oxalate in plants, while oxalate is associated with aluminum (Al) tolerance in some plant species. To enhance AsA in plants and increase tolerance against oxidative stress and Al toxicity, the yeast D-arabinono-1,4-lactone oxidase (ALO) gene was overexpressed in transgenic tobacco plants. Overexpression of ALO promoted synthesis of AsA and oxalate in leaves, but did not affect oxalate level in roots. Compared to the wild type, transgenic plants maintained higher levels of AsA, glutathione, maximal photochemical efficiency (F-v/F-m) and the actual PSII efficiency (Phi(PSII)) of photosystem II (PSII), and non-photochemical quenching (NPQ), and accumulated less H2O2 and malondialdehyde (MDA) in leaves in response to methyl viologen-and high-light-induced oxidative stress. In addition, transgenic plants showed higher AsA level and fresh weight of shoot and roots and lower levels of Al, H2O2, and MDA in root tips after Al treatment; however, there is no significant difference in organic acid (citrate and oxalate) exudation between the wild-type and transgenic plants in response to Al treatment. The results suggest that AsA and oxalate could be up-regulated by overexpressing ALO. The elevated AsA increased oxidative tolerance due to scavenging of reactive oxygen species (ROS) and induction of NPQ in leaves and enhanced Al tolerance as an antioxidant for avoiding Al-triggered oxidative damages in roots.