Molecular and physiological characterization of transgenic <i>Arabidopsis</i> plants expressing different aldehyde dehydrogenase (ALDH) genes

Abstract

Various reactive molecules such as aldehydes and their intermediates accumulate in plants exposed to environmental stress conditions. These molecules are highly toxic and can cause peroxidation of cellular lipids, protein and nucleic acid modifications. Due to the potential cytotoxicity of these molecules in living cells, various aldehyde dehydrogenase (ALDH) proteins are involved in maintaining a careful balance of cellular accumulation of the toxic aldehyde molecules by converting them into their non-toxic corresponding carboxylic acids. To investigate the biological role of plant-ALDHs and their involvement in abiotic stress tolerance mechanisms, several transgenic <i>Arabidopsis</i> plants containing different <i>Arabidopsis</i>- and <i>Craterostigma-ALDH</i>-cDNA constructs have been generated and characterized under various abiotic stress conditions. Cellular and tissue specific localization of <i>ALDH</i> gene expression <i>via GUS</i> reporter gene fusion showed that <i>ALDH3I1, ALDH3H1 </i>and<i> Cp-ALDH</i> are stress inducible genes. The experiments also revealed that <i>ALDH3I1 </i>and<i> Cp-ALDH</i> expression is leaf specific, while the stress-inducible expression of <i>ALDH3H1</i> is restricted to roots. Immunological experiments showed that <i>ALDH3I1</i> protein accumulations were triggered by ABA, paraquat (methyl viologen, a chemical that induces oxidative stress), and H2O2 treatment, indicating that the signal transduction leading to <i>ALDH</i> gene expression is responsive to ABA and reactive oxygen species (H₂O₂). The overexpression of <i>ALDH</i> genes controls in return the excessive accumulation of ROS, which occurs as a result of environmental stress. This confers thereby an enhanced tolerance to stress. Molecular and biochemical characterizations of selected transgenic plants exposed to stress treatments revealed that transgenic plants overexpressiong the <i>ALDH</i> genes showed significant tolerance to a wide range of abiotic stress conditions especially dehydration, salt stress (NaCl, KCl), heavy metal toxicity (copper and cadmium) and low temperature exposure in comparison to the wild-type plants. The loss of <i>ALDH</i> gene functions or a repression of endogenous <i>ALDH</i> gene expression in kock-out and antisense transgenic plants respectively correlates with sensitivity to various abiotic stress treatments. The overexpression of <i>ALDH</i> genes was found to significantly reduce the level of lipid peroxidation, and the amounts of reactive oxygen species (H₂O₂, O₂^-) in plants exposed to dehydration and salt stress conditions. These findings suggest that aldehyde dehydrogenase genes play a crucial role in aldehyde detoxification and antioxidant systems of plants exposed to abiotic stress conditions. Understanding the regulatory mechanisms of <i>ALDH</i> gene expressions in plants could prove to be a promising way to generate transgenic plants that can cope with multiple abiotic and even biotic stress conditions.