SummaryIn plants, oxidative stress is one of the major causes of damage as a result of various environmental stresses. Oxidative stress is primarily because of the excessive accumulation of reactive oxygen species (ROS). The ampli®cation of ROS damage is further stimulated by the accumulation of toxic degradation products, i.e. aldehydes, arising from reactions of ROS with lipids and proteins. Previously, the isolation of dehydration-inducible genes encoding aldehyde dehydrogenases (ALDHs) was reported from the desiccation-tolerant plant Craterostigma plantagineum and Arabidopsis thaliana. ALDHs belong to a family of NAD(P) -dependent enzymes with a broad substrate speci®city that catalyze the oxidation of various toxic aldehydes to carboxylic acids. Analysis of transcript accumulation revealed that Ath-ALDH3 is induced in response to NaCl, heavy metals (Cu 2 and Cd 2 ), and chemicals that induce oxidative stress (methyl viologen (MV) and H 2 O 2 ). To investigate the physiological role and possible involvement of ALDHs in stress protection, we generated transgenic Arabidopsis plants overexpressing Ath-ALDH3. Transgenic lines show improved tolerance when exposed to dehydration, NaCl, heavy metals (Cu 2 and Cd 2 ), MV, and H 2 O 2 . Tolerance of transgenic plants is correlated with decreased accumulation of lipid peroxidation-derived reactive aldehydes (as measured by malondialdehyde) compared to wild-type plants. Increased activity of Ath-ALDH3 appears to constitute a detoxi®cation mechanism that limits aldehyde accumulation and oxidative stress, thus revealing a novel pathway of detoxi®cation in plants. We suggest that Ath-ALDH3 could be used to obtain plants with tolerance to diverse environmental stresses.

Aldehyde dehydrogenases (ALDHs) play a major role in the detoxification processes of aldehydes generated in plants when exposed to abiotic stress. In previous studies, we have shown that the Arabidopsis thaliana ALDH3I1 gene is transcriptionally activated by abiotic stress, and over-expression of the ALDH3I1 gene confers stress tolerance in transgenic plants. The A. thaliana genome contains 14 ALDH genes expressed in different sub-cellular compartments and are presumably involved in different reactions. The purpose of this study was to compare the potential of a cytoplasmic and a chloroplastic stress-inducible ALDH in conferring stress tolerance under different conditions. We demonstrated that constitutive or stress-inducible expression of both the chloroplastic ALDH3I1 and the cytoplasmic ALDH7B4 confers tolerance to osmotic and oxidative stress. Stress tolerance in transgenic plants is accompanied by a reduction of H2O2 and malondialdehyde (MDA) derived from cellular lipid peroxidation. Involvement of ALDHs in stress tolerance was corroborated by the analysis of ALDH3I1 and ALDH7B4 T-DNA knockout (KO) mutants. Both mutant lines exhibited higher sensitivity to dehydration and salt than wild-type (WT) plants. The results indicate that ALDH3I1 and ALDH7B4 not only function as aldehyde-detoxifying enzymes, but also as efficient reactive oxygen species (ROS) scavengers and lipid peroxidation-inhibiting enzymes. The potential of ALDHs to interfere with H2O2 was also shown for recombinant bacterial proteins.

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In recent years, there has been a significant increase in the number of completely sequenced plant genomes. The comparison of fully sequenced genomes allows for identification of new gene family members, as well as comprehensive analysis of gene family evolution. The aldehyde dehydrogenase (ALDH) gene superfamily comprises a group of enzymes involved in the NAD+- or NADP+-dependent conversion of various aldehydes to their corresponding carboxylic acids. ALDH enzymes are involved in processing many aldehydes that serve as biogenic intermediates in a wide range of metabolic pathways. In addition, many of these enzymes function as ‘aldehyde scavengers’ by removing reactive aldehydes generated during the oxidative degradation of lipid membranes, also known as lipid peroxidation. Plants and animals share many ALDH families, and many genes are highly conserved between these two evolutionarily distinct groups. Conversely, both plants and animals also contain unique ALDH genes and families. Herein we carried outgenome-wide identification of ALDH genes in a number of plant species—including Arabidopsis thaliana (thale crest), Chlamydomonas reinhardtii (unicellular algae), Oryza sativa (rice), Physcomitrella patens (moss), Vitis vinifera (grapevine) and Zea mays (maize). These data were then combined with previous analysis of Populus trichocarpa (poplar tree), Selaginella moellindorffii (gemmiferous spikemoss), Sorghum bicolor (sorghum) and Volvox carteri (colonial algae) for a comprehensive evolutionary comparison of the plant ALDH superfamily. As a result, newly identified genes can be more easily analyzed and gene names can be assigned according to current nomenclature guidelines; our goal is to clarify previously confusing and conflicting names and classifications that might confound results and prevent accurate comparisons between studies.

Aldehyde dehydrogenase (ALDH) genes have been identified in almost all organisms from prokaryotes to eukaryotes, but particularly in plants knowledge is very limited with respect to their function. The data presented here are a contribution towards a functional analysis of selected Arabidopsis ALDH genes by using expression profiles in wild types and mutants. The Arabidopsis thaliana genome contains 14 genes which represent 9 families. To gain insight into the possible roles of aldehyde dehydrogenases from Arabidopsis, the expression patterns of five selected ALDH genes were analyzed under defined physiological conditions. Three genes (ALDH3I1, 3H1 and ALDH7B4) that belong to two different families are differentially activated by dehydration, high salinity and ABA in a tissue-specific manner. The other two genes (ALDH3F1 and ALDH22A1) are constitutively expressed at a low level. Transcript analysis of ALDH3I1 and ALDH7B4 in Arabidopsis mutants suggests that stress responses are differentially controlled by the phytohormone ABA as well as by pathways that affect sugar metabolism and fatty acid composition of membrane lipids. Our results indicate that the stress-associated ALDH genes participate in several pathways and that their regulation involves diverged signal transduction pathways.