Arabidopsis and tobacco plants ectopically expressing the soybean antiquitin-like ALDH7 gene display enhanced tolerance to drought, salinity, and oxidative stress

Despite extensive studies in eukaryotic aldehyde dehydrogenases, functional information about the ALDH7 antiquitin-like proteins is lacking. A soybean antiquitin homologue gene, designated GmTP55, has been isolated which encodes a dehydrogenase motif-containing 55 kDa protein induced by dehydration and salt stress. GmTP55 is closely related to the stress-induced plant antiquitin-like proteins that belong to the ALDH7 family. Transgenic tobacco (Nicotiana tabacum) and Arabidopsis (Arabidopsis thaliana) plants constitutively expressing GmTP55 have been obtained in order to examine the physiological role of this enzyme under a variety of stress conditions. Ectopic expression of GmTP55 in both Arabidopsis and tobacco conferred tolerance to salinity during germination and to water deficit during plant growth. Under salt stress, the germination efficiency of both transgenic tobacco and Arabidopsis seeds was significantly higher than that of their control counterparts. Likewise, under progressive drought, the transgenic tobacco lines apparently kept the shoot turgidity to a normal level, which contrasted with the leaf wilt phenotype of control plants. The transgenic plants also exhibited an enhanced tolerance to H(2)O(2)- and paraquat-induced oxidative stress. Both GmTP55-expressing Arabidopsis and tobacco seeds germinated efficiently in medium supplemented with H(2)O(2), whereas the germination of control seeds was drastically impaired. Similarly, transgenic tobacco leaf discs treated with paraquat displayed a significant reduction in the necrotic lesions as compared with control leaves. These transgenic lines also exhibited a lower concentration of lipid peroxidation-derived reactive aldehydes under oxidative stress. These results suggest that antiquitin may be involved in adaptive responses mediated by a physiologically relevant detoxification pathway in plants.     

Oxidative stress activates ATMPK6, an Arabidopsis homologue of MAP kinase

Mitogen-activated protein kinase (MAPK) cascades function in biotic and abiotic stress responses in plants. We analysed effect of oxidative stress on the activation of ATMPK6, an Arabidopsis thaliana MAPK, in Arabidopsis T87 cultured cells and rosette leaves using anti-ATMPK6 specific antibody. ATMPK6 in T87 cells was strongly activated by reactive oxygen species (ROS) such as H(2)O(2) and KO(2). In leaves, ATMPK6 was activated by paraquat and 3-amino-1,2,4-triazole (a catalase inhibitor). These results indicate that ATMPK6 is one of the candidates for signal mediators in response to abiotic or biotic sources for ROS in Arabidopsis.     

Overexpression of an Arabidopsis peroxisomal ascorbate peroxidase gene in tobacco increases protection against oxidative stress.

The Arabidopsis gene APX3 that encodes a putative peroxisomal membrane-bound ascorbate peroxidase was expressed in transgenic tobacco plants. APX3-expressing lines had substantial levels of APX3 mRNA and protein. The H2O2 can be converted to more reactive toxic molecules, e.g. .OH, if it is not quickly removed from plant cells. The expression of APX3 in tobacco could protect leaves from oxidative stress damage caused by aminotriazole which inhibits catalase activity that is found mainly in glyoxysomes and peroxisomes and leads to accumulation of H2O2 in those organelles. However, these plants did not show increased protection from oxidative damage caused by paraquat which leads to the production of reactive oxygen species in chloroplasts. Therefore, protection provided by the expression of APX3 seems to be specific against oxidative stress originated from peroxisomes, not from chloroplasts, which is consistent with the hypothesis that APX3 is a peroxisomal membrane-bound antioxidant enzyme.     

Generation of hydrogen peroxide in chloroplasts of Arabidopsis overexpressing glycolate oxidase as an inducible system to study oxidative stress

Arabidopsis (Arabidopsis thaliana) overexpressing glycolate oxidase (GO) in chloroplasts accumulates both hydrogen peroxide (H(2)O(2)) and glyoxylate. GO-overexpressing lines (GO plants) grown at 75 micromol quanta m(-2) s(-1) show retarded development, yellowish rosettes, and impaired photosynthetic performance, while at 30 micromol quanta m(-2) s(-1), this phenotype virtually disappears. The GO plants develop oxidative stress lesions under photorespiratory conditions but grow like wild-type plants under nonphotorespiratory conditions. GO plants coexpressing enzymes that further metabolize glyoxylate but still accumulate H(2)O(2) show all features of the GO phenotype, indicating that H(2)O(2) is responsible for the GO phenotype. The GO plants can complete their life cycle, showing that they are able to adapt to the stress conditions imposed by the accumulation of H(2)O(2) during the light period. Moreover, the data demonstrate that a response to oxidative stress is installed, with increased expression and/or activity of known oxidative stress-responsive components. Hence, the GO plants are an ideal noninvasive model system in which to study the effects of H(2)O(2) directly in the chloroplasts, because H(2)O(2) accumulation is inducible and sustained perturbations can reproducibly be provoked by exposing the plants to different ambient conditions.     

Expression of a vegetative-storage-protein gene from Arabidopsis is regulated by copper, senescence and ozone

Emerging data suggest that the mechanisms regulating plant copper homeostasis could be implicated in stress and senescence signal transduction pathways. To gain insight into copper-modulated patterns of gene expression, copper-treated Arabidopsis thaliana (L.) Heynh. plants were analysed by mRNA differential display. The experimental conditions were selected using aggregation of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) as a molecular sensor to monitor copper-induced oxidative stress. Two copper-induced messengers encoding a vegetative storage protein (VSP2) were isolated by this technique. Both clones differed in the length of their 3'-untranslated region according to the presence of two polyadenylation signals in this region. VSP2 expression was further studied under natural senescence and various conditions causing oxidative stress, such as ozone exposure, paraquat and H2O2 treatments. The expression of other messengers related to copper homeostasis and detoxification processes was followed in parallel to that of VSP2. Here, we describe specific gene-expression responses to copper treatment, and present arguments connecting copper homeostasis, senescence and antioxidative responses in plants. Our results are consistent with the role of VSPs as temporary nitrogen-storage proteins which accumulate if nutrients are abundant, either in developing organs or in cotyledons and mature leaves subjected to generalized protein mobilization, such as those conditions created under severe oxidative stress.     

Low oleic acid-derived repression of jasmonic acid-inducible defense responses requires the WRKY50 and WRKY51 proteins

Signaling induced upon a reduction in oleic acid (18:1) levels simultaneously up-regulates salicylic acid (SA)-mediated responses and inhibits jasmonic acid (JA)-inducible defenses, resulting in enhanced resistance to biotrophs but increased susceptibility to necrotrophs. SA and the signaling component Enhanced Disease Susceptibility1 function redundantly in this low-18:1-derived pathway to induce SA signaling but do not function in the repression of JA responses. We show that repression of JA-mediated signaling under low-18:1 conditions is mediated via the WRKY50 and WRKY51 proteins. Knockout mutations in WRKY50 and WRKY51 lowered SA levels but did not restore pathogenesis-related gene expression or pathogen resistance to basal levels in the low-18:1-containing Arabidopsis (Arabidopsis thaliana) mutant, suppressor of SA insensitivity2 (ssi2). In contrast, both JA-inducible PDF1.2 (defensin) expression and basal resistance to Botrytis cinerea were restored. Simultaneous mutations in both WRKY genes (ssi2 wrky50 wrky51) did not further enhance the JA or Botrytis-related responses. The ssi2 wrky50 and ssi2 wrky51 plants contained high levels of reactive oxygen species and exhibited enhanced cell death, the same as ssi2 plants. This suggested that high reactive oxygen species levels or increased cell death were not responsible for the enhanced susceptibility of ssi2 plants to B. cinerea. Exogenous SA inhibited JA-inducible PDF1.2 expression in the wild type but not in wrky50 or wrky51 mutant plants. These results show that the WRKY50 and WRKY51 proteins mediate both SA- and low-18:1-dependent repression of JA signaling.     

SNM-dependent recombinational repair of oxidatively induced DNA damage in Arabidopsis thaliana

Two different roles for SNM (sensitive to nitrogen mustard) proteins have already been described: the SNM1/PSO2-related proteins are involved in the repair of the interstrand crosslink (ICL) and the ARTEMIS proteins are involved in the V(D)J recombination process. Our study shows that an Arabidopsis SNM protein, although structurally closer to the SNM1/PSO2 members, shares some properties with ARTEMIS but also has novel characteristics. Arabidopsis plants defective for the expression of AtSNM1 did not show hypersensitivity to the ICL-forming agents but to the chemotherapeutic agent bleomycin and to H(2)O(2). AtSNM1 mutant plants are delayed in the repair of oxidative damage and did not show enhancement of the frequency of somatic homologous recombination on exposure to H(2)O(2) and to the bacterial elicitor flagellin, both inducing oxidative stress, as observed in the control plants. Therefore, our results suggest the existence, in plants, of a novel SNM-dependent recombinational repair process of oxidatively induced DNA damage.     

AtGLB1 enhances the tolerance of Arabidopsis to hydrogen peroxide stress

Hydrogen peroxide (H2O2) as a widespread molecule plays an important role in plant stress responses. Here, we showed that an Arabidopsis line overexpressing hemoglobin 1 (AtGLB1) can enhance its tolerance to severe hypoxic stress. In our research, Arabidopsis lines with different hemoglobin levels were employed to study the relationship between H2O2 level and the tolerance to hypoxic stress. The relatively low endogenous H2O2 level of AtGLB1-overexpressing plants could be one of the main factors for the increased tolerance of plants to hypoxic stress. Further investigation indicated that the activity of the antioxidant system involved in scavenging H2O2 increased in all three lines examined during hypoxic treatment, while only the line overexpressing AtGLB1 could retain these relatively high levels up to 48 h of the treatment, suggesting that the antioxidant system might play a role in the low H2O2 level of Arabidopsis overexpressing AtGLB1.