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First published online December 3, 2008; 10.1104/pp.108.130716 Plant Physiology 149:905-915 (2009) © 2009 American Society of Plant Biologists OPEN ACCESS ARTICLE
Rice Aldehyde Dehydrogenase7 Is Needed for Seed Maturation and Viability1,[W],[OA]National Research Laboratory, Department of Integrative Bioscience and Biotechnology, and Center for Functional Genomics, Pohang University of Science and Technology, Pohang 790–784, Republic of Korea
Aldehyde dehydrogenases (ALDHs) catalyze the irreversible oxidation of a wide range of reactive aldehydes to their corresponding carboxylic acids. Although the proteins have been studied from various organisms and at different growth stages, their roles in seed development have not been well elucidated. We obtained T-DNA insertional mutants in OsALDH7, which is remarkably inducible by oxidative and abiotic stresses. Interestingly, endosperms from the osaldh7 null mutants accumulated brown pigments during desiccation and storage. Extracts from the mutant seeds showed a maximum absorbance peak at 360 nm, the wavelength that melanoidin absorbs. Under UV light, those extracts also exhibited much stronger fluorescence than the wild type, suggesting that the pigments are melanoidin. These pigments started to accumulate in the late seed developmental stage, the time when OsALDH7 expression began to increase significantly. Purified OsALDH7 protein showed enzyme activities to malondialdehyde, acetaldehyde, and glyceraldehyde. These results suggest that OsALDH7 is involved in removing various aldehydes formed by oxidative stress during seed desiccation. The mutant seeds were more sensitive to our accelerated aging treatment and accumulated more malondialdehyde than the wild type. These data imply that OsALDH7 plays an important role in maintaining seed viability by detoxifying the aldehydes generated by lipid peroxidation.
The major regulatory factors that control seed aging are oxidative stress, lipid peroxidation, and respiration (Sun and Leopold, 1995  ; Bailly et al., 1996; Akimoto et al., 2004). Lipid peroxidation and respiration result in the formation of reactive aldehydes such as malondialdehyde (MDA) and acetaldehyde, which tend to react with proteins and amino acids (Mueller, 1998; Almeras et al., 2003; Weber et al., 2004). Those reactions cause aging and seed damage (Zhang et al., 1995, 1997). Until recently, a physiological approach has been taken in research on seed aging, and molecular and genetic studies have been seldom reported (Clerkx et al., 2004a).
Rice (Oryza sativa) is an important food crop, especially in Asia. Although stress tolerance has been extensively evaluated in an effort to develop advanced cultivars, the aging of seeds is an important economic problem that has been rarely examined (Devaiah et al., 2007
Aldehydes are intermediates in several fundamental metabolism pathways for carbohydrates, vitamins, steroids, amino acids, and lipids (Yoshida et al., 1998
Aldehyde dehydrogenases (ALDHs) are represented by a protein super-family that can be categorized into 21 families in eukaryotes (Perozich et al., 1999
Family 7 ALDHs (antiquitins) are comparatively distinguishable from other ALDH families, because they show low sequence identity (approximately 30%) to ALDHs from other families (Vaciliou et al., 1999
Overexpression of Arabidopsis (Arabidopsis thaliana) and soybean (Glycine max) ALDH7 confers tolerance to osmotic and oxidative stresses in transgenic plants (Kotchoni et al., 2006 In this study, we used null mutants in the rice ALDH7 gene to investigate the functional roles of ALDH7 during seed development and storage.
Isolation of a Mutant That Accumulates Brown Pigments in Mature Seeds Screening T-DNA insertional mutant populations for abnormality in their mature seeds resulted in the identification of a mutant that accumulates brown pigments (Fig. 1A ). These pigments were found in the pericarp as well as the inner endosperm (Fig. 1B). This pattern is unusual, because pigments are usually accumulated mainly in the pericarp in color-seed cultivars. The level of pigment increased as the storage time was extended (Fig. 1B). This implies that such accumulation is induced by a factor generated during seed maturation and the storage period.
During the late stage of seed development and in storage, the water content in rice seeds dropped to <20%, which caused stress to the cells that still survived. Post-harvest heating to dry those seeds was another source of stress for the aleurone and embryo cells. To examine whether pigments were generated by these stresses, we treated wild-type and mutant seeds for 2 months at 60°C. This exposure induced pigment accumulation in the wild-type seeds (Fig. 1C) while enhancing such accumulation in the mutants (Fig. 1D).
To analyze the components of this accumulated pigment, we scanned the absorption spectra of the aqueous extracts from wild-type and mutant seeds. Extracts from both genotypes peaked at 360 nm, although the mutant extract showed a peak that was up to 4 times higher (Fig. 2A
). Heat treatment of seeds for 2 months at 60°C increased the absorbance in both the mutant and wild type (Fig. 2A). Under UV light, the extracts displayed fluorescence, with intensity being much greater from the mutants (Fig. 2B). These results suggest that the pigment is a product of a Maillard reaction, which nonenzymatically produces melanoidin from carbonyl and amino compounds during storage (Adams et al., 2005
For further verification, we compared our rice extracts with those from the seeds of barley (Hordeum vulgare) and coffee (Coffea arabica), two well-known materials rich in melanoidin. Again, peaks occurred at 360 nm, with their heights being correlated with pigment intensity (Fig. 2C). Those extracts also contained fluorescent materials (Fig. 2D).
Amadori products are intermediates of the Maillard reaction (Sun and Leopold, 1995
Because the mutant phenotype cosegregated with T-DNA, we determined DNA sequences for the T-DNA flanking region by inverse PCR (An et al., 2003
To examine the subcellular region in which OsALDH7 functions, we conducted a localization experiment using protoplasts purified from Oc suspension cells. OsALDH7-GFP protein was localized in the cytoplasm, which was colocalized with mRFP, a well-known cytosol marker (Supplemental Fig. S1). This result is consistent with a previous report of Arabidopsis ALDH7B4 localization (Kotchoni et al., 2006 ).
Kotchoni et al. (2006)
Wild type and osaldh7-1 mutants were germinated and grown on Murashige and Skoog (MS) media. Under the normal growth conditions, the mutant plants were not different from wild type (Fig. 4F). When osaldh7-1 seedlings were treated with cold or high salt, they were more sensitive than the wild type (Fig. 4, G and H). Therefore, we conclude that OsALDH7 is needed for the detoxification of aldehydes that form under various stress conditions.
Pigment accumulation started in the late stage of seed development (35 d after pollination), mainly in pericarp tissues (Fig. 5A ), with a higher amount of pigment being accumulated at the fully mature stage (Fig. 5B). Stored seeds showed even further accumulation, especially in the mutant (Fig. 5C).
We looked for correlations between pigment accumulation and ALDH7 transcript levels during seed development. Real-time PCR analyses indicated that those levels were low early on but then significantly increased at the late stage (Fig. 5D). A sudden rise in OsALDH7 expression occurred at the time when pigment accumulation started. This timing coincides with a rapid decline in water content in seeds (Hoshikawa, 1989 ), suggesting that OsALDH7 is associated with desiccation. Therefore, the phenotypes observed in our osaldh7 mutants were likely due to a failure to remove the aldehydes generated by dehydration stress.
We examined whether OsALDH7 indeed encodes for an enzyme that catalyzes aldehydes. The OsALDH7 full-length cDNA clone was inserted into the pET-topo vector containing six His residues. This molecule was introduced into Escherichia coli strain BL21 (DE3), and OsALDH7-His tag fusion molecules were induced by isopropylthio-β-galactoside treatment. Using a His-tag column, we purified OsALDH7-His protein and performed immunoblots to demonstrate that the purified protein was OsALDH7 (data not shown). The purified protein had enzyme activity against MDA, a by-product formed in the nonenzymatic lipid peroxidation pathway (Fig. 6A ). MDAs are electrophilic, strongly binding to peptides and amino acids. Therefore, the MDA content in cells is evidence of their environmental conditions and can be used as a general index of oxidative stress. Here, OsALDH7 catalyzed other aldehydes as well, including acetaldehyde (Fig. 6B) and glyceraldehyde (Fig. 6C). Acetaldehyde is generated from the alcohol fermentation pathway and glyceraldehyde is related with carbohydrate metabolism. These results suggest that OsALDH7 has broad substrates and may participate in multiple functions such as metabolic pathways and abiotic stresses.
MDA Contents Increase in osaldh7 Seeds If one of the roles of OsALDH7 is to remove MDA, then MDA contents should be higher in the osaldh7 seeds than in the wild type. Therefore, we measured those contents in developing and mature seeds. During the former stage, seeds are exposed to severe oxidative stress due to desiccation. Such an environment promotes the acceleration of lipid peroxidation and an increase in MDA. Performing a thiobarbituric acid-reactive-substances assay, we found that, as expected, MDA contents were higher in the osaldh7 seeds (Fig. 7, A and B ). Although contents in wild-type seeds were somewhat elevated, those in the mutants were more significantly increased (up to 2-fold), especially after seed harvest. Patterns were similar with A360 (Fig. 7, C and D). These data demonstrate that OsALDH7 plays a pivotal role in the removal of MDA that forms during seed desiccation.
osaldh7 Seeds Show Accelerated Seed Aging
Because lipid peroxidation is involved in seed deterioration (Bailly et al., 1996
When ordinary seeds complete their maturation, they must become desiccated if long-term storage is to be successful (Murthy et al., 2003 ; Gilles et al., 2007). Their tolerance to dry conditions is based on their formation of desiccation-related proteins and materials (osmoprotectants), such as late embryogenesis abundant (LEA) proteins and sugars (Sun et al., 1994; Black et al., 1999; Wise and Tunnacliffe, 2004). LEA genes begin to be expressed in the mid- to late stage of seed development (Bartels et al., 1988; Hartwigsen and Goggi, 2002). Their overexpression enhances tolerance to salt, drought, and osmotic stresses in transgenic plants (Brini et al., 2007; Zhang et al., 2007). Here, we observed that levels of OsALDH7 transcripts started to increase at the late stage of maturation, similar to those of the LEA genes. Expression patterns for OsALDH7 implied that a relationship exists between OsALDH7 and seed desiccation. Kirch et al. (2005) have reported that expression of the Arabidopsis ALDH7B4 gene, in Family 7, is activated by abscisic acid, which regulates the dehydration stress response and the seed desiccation pathway. Other researchers have suggested that osmoprotection may be a major role of the plant antiquitins (Stroeher et al., 1995; Buchanan et al., 2005), compounds that oxidize aldehydes (Fong et al., 2006). Based on those previous studies and our results, we anticipated that OsALDH7 would function during seed desiccation and maturation. We observed that this gene was strongly expressed under abiotic stresses, e.g. drought, cold, and heat, and that osaldh7 mutant seedlings were more sensitive than the wild type. Our data, therefore, suggest that OsALDH7 can be used to confer tolerance to diverse stresses.
ALDHs detoxify the aldehydes generated by various metabolisms (Yoshida et al., 1998
The brown reaction, i.e. the Maillard reaction, occurs during storage or because of spontaneous heating (Murthy et al., 2003
It is difficult to investigate the effect of natural aging, because long-term storage is needed. Here, seed germination rates after short-term storage did not differ significantly between wild type and mutants. Our observation is consistent with that of Sattler et al. (2004)
Elleder (1981)
We tried to investigate the chemical nature of this pigment by liquid chromatography-mass spectrometry and gas chromatography-mass spectrometry. However, we were unable to identify any candidate peaks. This may have been because melanoidins are highly polymerized compounds. The structure of those pigments has been poorly defined (Adams et al., 2005 In conclusion, we suggest the following working model. Lipid peroxidation is accelerated by oxidative stress in late-stage seeds, which causes an increase in MDA and other aldehydes. The toxic materials are removed by OsALDH7, inhibiting the Maillard reaction. However, in the osaldh7 mutant, MDA and other aldehydes are not removed, and a Maillard reaction is promoted, forming brown pigments in mature seeds.
Plant Materials and Growing Conditions
Seeds of osaldh7 and segregant wild-type rice (Oryza sativa ‘Dongjin’) were sterilized with 50% hypochlorite for 30 min, washed three times with sterile distilled water, and placed on an MS medium (Murashige and Skoog, 1962
Defective seeds of the mutants were isolated from T-DNA tagging lines by screening for alterations in their shape or color. The T-DNA insertion position was determined by inverse PCR (An et al., 2003
One-week-old seedlings were grown in an MS medium. To induce cold stress, the seedlings were transferred to 4°C and incubated for 3 d. During this treatment, they were harvested daily (beginning at d 0) for RNA isolation and real-time PCR analysis. After the 3-d test period, the seedlings were allowed to recover at room temperature for 12 h before sampling. For the heat treatment, seedlings were held at 50°C and sampled at 0, 6, and 12 h. For drought experiments, water was blotted from the seedling roots with paper towels before the seedlings were placed on a paper towel at 30°C. These treated seedlings were sampled at 0.0, 0.5, 1.0, and 2.0 h. The paraquat treatment involved submerging the seedling roots in a 20 µM MV solution, then sampling them at 12-h intervals. After 1 d, the seedlings were transferred to distilled water and sampled after 12 h. For high salt treatment, seedling roots were submerged in 250 mM NaCl for 12 h. Afterward, they were allowed to recover at room temperature.
Hulled-whole seeds (about 100 mg) of the wild type and osaldh7 were ground in a milling machine, and the powder was dissolved in 1 mL of distilled water. After vigorous vortexing, the solution was mixed at 4°C for 24 h on a rotator. This solution was then centrifuged at 13,000 rpm for 15 min, and the supernatant was retrieved into a new Eppendorf tube. The solution was filtered using a 0.2-µm syringe filter (Sartorius). The extracts were scanned with a UV spectrophotometer (Shimadzu) over a spectral range of 200 to 700 nm. The extracts were also observed under UV light to determine whether the materials were fluorescent.
Stored seeds (10 mg) were ground with a milling machine before 1.2 mL of 50 mM phosphate buffer, pH 7.2, was added to the powder. Nucleic acids were removed by adding 200 µL of 10% streptomycin sulfate, then centrifuging at 15,000 rpm for 15 min. The supernatant was transferred to a new tube, and proteins were precipitated with ammonium sulfate (0.55 g mL–1). The proteins precipitated by centrifugation were dissolved in 50 mM phosphate buffer, pH 7.2, and the protein solution was further purified on 10-DG columns (Bio-Rad). Those purified proteins were then used for measurement of Amadori products by the nitro blue tetrazolium method, as described previously (Murthy and Sun, 2000
Samples were homogenized in a milling machine (Retsch), and total RNA was extracted using TRIzol reagent (Invitrogen). For first-strand cDNA synthesis, 2 µg of total RNA was reverse-transcribed in a total volume of 25 µL that contained 10 ng of oligo(dT)12-18 primer, 2.5 mM dNTPs, and 200 units of Moloney murine leukemia virus reverse transcriptase (Promega) in a reaction buffer. After RT at 37°C for 90 min, RT-PCR was conducted for 33 cycles in a 25-µL solution containing 20 pmol of gene-specific primers, 0.2 mM dNTPs, 1 unit of Taq DNA polymerase (Enzynomics), and 1x reaction buffer. Primers were designed at different exons, allowing the differentiation of cDNA products from genomic DNA contamination. Real-time PCR was performed using a Roche LightCycler II as previously described (Han et al., 2006
To construct our localization vectors, we performed PCR with the following primers: 5'-gcACTAGTatggggagcttcgcgaggaa-3' and 5'-gcACTAGTgccaaaatttattccttgt-3' (underlined parts indicate SpeI-recognized sequences). Full-length cDNA of OsALDH7 was inserted into the pBluescript SK+ vector (Stratagene), which was digested with EcoRV. The subclone was digested with SpeI and then introduced into the SpeI site in pGA3452, which contains the sGFP gene driven by the maize (Zea mays) ubiquitin promoter. Protoplasts prepared from the Oc cell line of rice were transformed with the localization vectors by methods described previously (Han et al., 2006
Full-length OsALDH7 cDNA was isolated by the primers 5'-caccatggggagcttcgcgagg-3' (forward primer) and 5'-ctagccaaaatttattcctt-3' (reverse primer). The cDNA clone was inserted into pET100/D-TOPO, and the resulting plasmid was transformed into Escherichia coli strain BL21 (DE3). Transformed cells were incubated at 37°C overnight, then transferred to 250 mL of a fresh medium (1.6% tryptone, 1.0% yeast extract, and 0.5% NaCl) and incubated in a 37°C shaker. Once the OD600 reached 0.5 to 0.8, 1 mM isopropylthio-β-galactoside was added, and incubation was continued at 30°C for another 8 h. For OsALDH7 assays, cells were harvested and resuspended in 10 mL of B-PER protein extraction reagent (Thermo Fisher Scientific). This solution was incubated on ice for 20 min and sonicated three times, for 10 s each, using a Branson Sonifier model 450 at maximum output. The lysate was centrifuged at 12,000 rpm for 20 min. After the supernatant was transferred to a new tube, 6x His-tagged OsALDH7 proteins were purified on a Ni-NTA Spin column (Qiagen). For our enzyme assay, 10 µg of purified protein was added to a reaction mixture containing 1.5 mM NAD (Sigma) and 0.1 M sodium pyrophosphate buffer, pH 8.5. The total volume was adjusted to 300 µL with water before 20 µg of an aldehyde (MDA, acetaldehyde, or glyceraldehyde) was added to the mixture. Finally, the emission fluorescence of NADH was recorded for up to 2 min, at 20-s intervals, on a SPECTRAMAX Gemini (Elsevier Biosoft).
Levels of lipid peroxidation were assayed according to the thiobarbituric acid test, which determines the amount of MDA as an end product of the reaction (Cakmak and Horst, 1991
To promote AA, the hulls from 50 seeds each of the wild type and osaldh7-1 and osaldh7-2 mutants were removed. The seeds were transferred to a high-humidity chamber (100% relative humidity) and incubated at 50°C for 24 and 36 h. These AA-treated seeds were then surface-sterilized with 50% hypochlorite and placed on an MS medium. Germination rates were scored 7 d after sowing. This experiment was repeated three times. Sequence data from this article can be found in the GenBank/EMBL data libraries under accession number AF323586.
The following materials are available in the online version of this article.
We thank Hea-kyung Jung, Hee-Jung Woo, Hyunsook Lee, Jeonghi Lee, Kyoungmi Han, and Kyung-Mi Kim for assisting in sequencing the T-DNA insertion sites; In-Soon Park and Kyungsook An for generating the T-DNA insertional lines; Nam-In Baek and Jong-Soo Yoo for analyses of pigment components; Yang-Seok Lee, Yongmin Woo, Sunok Moon, and Jong-jin Park for valuable discussion; and Priscilla Licht for critical reading of the manuscript. Received October 2, 2008; accepted November 22, 2008; published December 3, 2008.
1 This work was supported in part by the Crop Functional Genomic Center, the 21st Century Frontier Program (grant no. CG1111), by the Biogreen 21 Program (grant no. 20070401–034–001–007–03–00), Rural Development Administration, by the Korea Science and Engineering Foundation through the National Research Laboratory Program funded by the Ministry of Science and Technology (grant no. M10600000270–06J0000–27010), and by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund; grant no. KRF–2007–341–C00028).  The author responsible for the distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Gynheung An (genean{at}postech.ac.kr).
[W] The online version of this article contains Web-only data.
[OA] Open access articles can be viewed online without a subscription. www.plantphysiol.org/cgi/doi/10.1104/pp.108.130716 * Corresponding author; e-mail genean{at}postech.ac.kr.
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