Aldehyde Dehydrogenases in Arabidopsis thaliana: Biochemical Requirements, Metabolic Pathways, and Functional Analysis

Aldehyde dehydrogenases (ALDHs) are a family of enzymes which catalyze the oxidation of reactive aldehydes to their corresponding carboxylic acids. Here we summarize molecular genetic and biochemical analyses of selected ArabidopsisALDH genes. Aldehyde molecules are very reactive and are involved in many metabolic processes but when they accumulate in excess they become toxic. Thus activity of aldehyde dehydrogenases is important in regulating the homeostasis of aldehydes. Overexpression of some ALDH genes demonstrated an improved abiotic stress tolerance. Despite the fact that several reports are available describing a role for specific ALDHs, their precise physiological roles are often still unclear. Therefore a number of genetic and biochemical tools have been generated to address the function with an emphasis on stress-related ALDHs. ALDHs exert their functions in different cellular compartments and often in a developmental and tissue specific manner. To investigate substrate specificity, catalytic efficiencies have been determined using a range of substrates varying in carbon chain length and degree of carbon oxidation. Mutational approaches identified amino acid residues critical for coenzyme usage and enzyme activities.

Comparative analysis of LEA-like 11-24 gene expression and regulation in related plant species within the Linderniaceae that differ in desiccation tolerance

The resurrection plant Craterostigma plantagineum is able to withstand desiccation of its vegetative tissues and is found in areas with variable water availability. The closely related species Lindernia brevidens and Lindernia subracemosa are both endemic to montane rainforests of coastal Africa, but remarkably L. brevidens is tolerant to desiccation. We studied the regulation of the desiccation-related LEA-like 11-24 gene at multiple levels in closely related species in order to investigate the conservation of mechanisms involved in desiccation tolerance. The dehydration-responsive transcription of the LEA-like 11-24 gene is differentially regulated in these plants. Comparison of the LEA-like 11-24 core promoter regions revealed that promoters have different activities, but some functional cis-acting elements are conserved between species. Upon dehydration, LEA-like 11-24 proteins are phosphorylated at different levels and phosphorylation sites are not conserved among the three LEA-like 11-24 proteins. Differences in the regulation of the LEA-like 11-24 gene in the studied plant species appear to be the result of mutations that occurred during evolution. We postulate that L. brevidens will eventually lose the ability to survive vegetative desiccation, given that this trait appears not to be essential for survival.

Polyamine metabolic canalization in response to drought stress in Arabidopsis and the resurrection plant Craterostigma plantagineum

In this work, we have studied the transcriptional profiles of polyamine biosynthetic genes and analyzed polyamine metabolic fluxes during a gradual drought acclimation response in Arabidopsis thaliana and the resurrection plant Craterostigma plantagineum. The analysis of free putrescine, spermidine and spermine titers in Arabidopsis arginine decarboxylase (adc1-3, adc2-3), spermidine synthase (spds1-2, spds2-3) and spermine synthase (spms-2) mutants during drought stress, combined with the quantitative expression of the entire polyamine biosynthetic pathway in the wild-type, has revealed a strong metabolic canalization of putrescine to spermine induced by drought. Such canalization requires spermidine synthase 1 (SPDS1) and spermine synthase (SPMS) activities and, intriguingly, does not lead to spermine accumulation but to a progressive reduction in spermidine and spermine pools in the wild-type. Our results suggest the participation of the polyamine back-conversion pathway during the drought stress response rather than the terminal catabolism of spermine. The putrescine to spermine canalization coupled to the spermine to putrescine back-conversion confers an effective polyamine recycling-loop during drought acclimation. Putrescine to spermine canalization has also been revealed in the desiccation tolerant plant C. plantagineum, which conversely to Arabidopsis, accumulates high spermine levels which associate with drought tolerance. Our results provide a new insight to the polyamine homeostasis mechanisms during drought stress acclimation in Arabidopsis and resurrection plants.

Betaine aldehyde dehydrogenase genes from Arabidopsis with different sub-cellular localization affect stress responses

Arabidopsis thaliana belongs to those plants that do not naturally accumulate glycine betaine (GB), although its genome contains two genes, ALDH10A8 and ALDH10A9 that code for betaine aldehyde dehydrogenases (BADHs). BADHs were initially known to catalyze the last step of the biosynthesis of GB in plants. But they can also oxidize metabolism-derived aminoaldehydes to their corresponding amino acids in some cases. This study was carried out to investigate the functional properties of Arabidopsis BADH genes. Here, we have shown that ALDH10A8 and ALDH10A9 proteins are targeted to leucoplasts and peroxisomes, respectively. The expression patterns of ALDH10A8 and ALDH10A9 genes have been analysed under abiotic stress conditions. Both genes are expressed in the plant and weakly induced by ABA, salt, chilling (4°C), methyl viologen and dehydration. The role of the ALDH10A8 gene was analysed using T-DNA insertion mutants. There was no phenotypic difference between wild-type and mutant plants in the absence of stress. But ALDH10A8 seedlings and 4-week-old plants were more sensitive to dehydration and salt stress than wild-type plants. The recombinant ALDH10A9 enzyme was shown to oxidize betaine aldehyde, 4-aminobutyraldehyde and 3-aminopropionaldehyde to their corresponding carboxylic acids. We hypothesize that ALDH10A8 or ALDH10A9 may serve as detoxification enzymes controlling the level of aminoaldehydes, which are produced in cellular metabolism under stress conditions.

Transcriptomes of the desiccation-tolerant resurrection plant Craterostigma plantagineum

Studies of the resurrection plant Craterostigma plantagineum have revealed some of the mechanisms which these desiccation-tolerant plants use to survive environments with extreme dehydration and restricted seasonal water. Most resurrection plants are polyploid with large genomes, which has hindered efforts to obtain whole genome sequences and perform mutational analysis. However, the application of deep sequencing technologies to transcriptomics now permits large-scale analyses of gene expression patterns despite the lack of a reference genome. Here we use pyro-sequencing to characterize the transcriptomes of C. plantagineum leaves at four stages of dehydration and rehydration. This reveals that genes involved in several pathways, such as those required for vitamin K and thiamin biosynthesis, are tightly regulated at the level of gene expression. Our analysis also provides a comprehensive picture of the array of cellular responses controlled by gene expression that allow resurrection plants to survive desiccation.

A role for a cell wall localized glycine-rich protein in dehydration and rehydration of the resurrection plant Boea hygrometrica

The acquisition of desiccation tolerance in dicotyledonous angiosperms requires the induction of a co-ordinated programme of genetic and biochemical processes during drying and the adaptive mechanisms are primarily protoplasmic in nature. Recent studies have shown that changes in cell wall structure and composition are also important for recovery after drying, however, the molecular mechanisms that underpin these adaptive responses are largely unknown. Here, the desiccation-tolerant plant Boea hygrometrica was used as a model system to investigate the changes in gene expression and cell wall adaptation that take place during extreme dehydration. A cDNA macroarry analysis of dehydration-inducible genes led to the identification of a gene encoding a glycine-rich protein (BhGRP1). The corresponding transcript was up-regulated during drying in B. hygrometrica leaves. In silico analysis revealed that BhGRP1 is targeted to the cell wall and this was confirmed in planta. Morphological changes in the cell wall architecture were also observed during the process of drying and re-watering. Concomitant with this observation, cell wall profiling by Fourier transform infrared spectroscopy indicated that protein levels increased upon desiccation and remained broadly similar upon re-watering. These findings suggest that the deposition of BhGRP1 may play a role in cell wall maintenance and repair during dehydration and rehydration in B. hygrometrica.

Reassessing the role of phospholipase D in the Arabidopsis wounding response

Plants respond to wounding by means of a multitude of reactions, with the purpose of stifling herbivore assault. Phospholipase D (PLD) has previously been implicated in the wounding response. Arabidopsis (Arabidopsis thaliana) AtPLDalpha1 has been proposed to be activated in intact cells, and the phosphatidic acid (PA) it produces to serve as a precursor for jasmonic acid (JA) synthesis and to be required for wounding-induced gene expression. Independently, PLD activity has been reported to have a bearing on wounding-induced MAPK activation. However, which PLD isoforms are activated, where this activity takes place (in the wounded or non-wounded cells) and what exactly the consequences are is a question that has not been comprehensively addressed. Here, we show that PLD activity during the wounding response is restricted to the ruptured cells using (32)P(i)-labelled phospholipid analyses of Arabidopsis pld knock-out mutants and PLD-silenced tomato cell-suspension cultures. pldalpha1 knock-out lines have reduced wounding-induced PA production, and the remainder is completely eliminated in a pldalpha1/delta double knock-out line. Surprisingly, wounding-induced protein kinase activation, AtLOX2 gene expression and JA biosynthesis were not affected in these knock-out lines. Moreover, larvae of the Cabbage White butterfly (Pieris rapae) grew equally well on wild-type and the pld knock-out mutants.

Evaluation of portal venous gas detected by ultrasound examination for diagnosis of necrotising enterocolitis

BACKGROUND

Early diagnosis of necrotising enterocolitis (NEC) is difficult but essential for timely therapy. The diagnostic hallmarks and specific radiological signs for NEC are pneumatosis intestinalis (PI) and portal venous gas (PVG), but PVG in abdominal ultrasound (PVG-US) has been proposed as an effective tool in the diagnosis of NEC as well.

OBJECTIVE

To prospectively assess the value of PVG-US for the diagnosis of NEC.

METHODS

The study screened 352 neonates for PVG-US (n = 796 routine examinations) and performed 48 additional screenings in 34/352 neonates with suspected (stage I, n = 28) or definite NEC (stage > or =II, n = 20). Sensitivity and specificity of PVG-US for detection of NEC were computed by using NEC stage > or =II as the reference standard.

RESULTS

PVG-US was only present in cases of suspected or definite NEC. The study observed PVG-US in 4/28 NEC stage I and in 9/20 NEC stage > or =II episodes corresponding to a 86% specificity and a 45% sensitivity for diagnosis of NEC stage > or =II. However, 7/20 patients with NEC stage > or =II showed intraoperative findings other than NEC and another 3/20 infants presented with radiologically unspecific intestinal dilatation. None of these 10 infants had detectable PVG-US. Thus, with application of specific radiological signs the sensitivity of PVG-US for diagnosis of NEC stage > or =II increased to 90%.

CONCLUSION

Screening for PVG-US is a useful, easy and quick bedside test with a high specificity for NEC. Moreover, these results question the value of the Walsh criteria in the diagnosis of NEC.

Subcellular localization and enzymatic properties of differentially expressed transketolase genes isolated from the desiccation tolerant resurrection plant Craterostigma plantagineum

The desiccation tolerant resurrection plant Craterostigma plantagineum encodes three classes of transketolase transcripts, which are distinguished by their gene structures and their expression patterns. One class, represented by tkt3, is constitutively expressed and two classes, represented by tkt7 and tkt10, are upregulated upon rehydration of desiccated C. plantagineum plants. The objective of this work was to characterize the differentially expressed transketolase isoforms with respect to subcellular localization and enzymatic activity. Using GFP fusion constructs and enzymatic activity assays, we demonstrate that C. plantagineum has novel forms of transketolase which localize not to the chloroplast, but mainly to the cytoplasm and which are distinct in the enzymatic properties from the transketolase enzymes active in the Calvin cycle or oxidative pentose phosphate pathway. A transketolase preparation from rehydrated leaves was able to synthesize the unusual C8 carbon sugar octulose when glucose-6-phosphate and hydroxy-pyruvate were used as acceptor and donor molecules in in vitro assays. This suggests that a transketolase catalyzed reaction is likely to be involved in the octulose biosynthesis in C. plantagineum.

Multiple PLDs required for high salinity and water deficit tolerance in plants

High salinity and drought have received much attention because they severely affect crop production worldwide. Analysis and comprehension of the plant's response to excessive salt and dehydration will aid in the development of stress-tolerant crop varieties. Signal transduction lies at the basis of the response to these stresses, and numerous signaling pathways have been implicated. Here, we provide further evidence for the involvement of phospholipase D (PLD) in the plant's response to high salinity and dehydration. A tomato (Lycopersicon esculentum) alpha-class PLD, LePLDalpha1, is transcriptionally up-regulated and activated in cell suspension cultures treated with salt. Gene silencing revealed that this PLD is indeed involved in the salt-induced phosphatidic acid production, but not exclusively. Genetically modified tomato plants with reduced LePLDalpha1 protein levels did not reveal altered salt tolerance. In Arabidopsis (Arabidopsis thaliana), both AtPLDalpha1 and AtPLDdelta were found to be activated in response to salt stress. Moreover, pldalpha1 and plddelta single and double knock-out mutants exhibited enhanced sensitivity to high salinity stress in a plate assay. Furthermore, we show that both PLDs are activated upon dehydration and the knock-out mutants are hypersensitive to hyperosmotic stress, displaying strongly reduced growth.

Analysis of desiccation-induced candidate phosphoproteins from Craterostigma plantagineum isolated with a modified metal oxide affinity chromatography procedure

Reversible protein phosphorylation/dephosphorylation is crucial for regulation of many cellular events, and increasing evidence indicates that this post-translational modification is also involved in the complex process of acquisition of desiccation tolerance. To analyze the phosphoproteome of the desiccation tolerant resurrection plant Craterostigma plantagineum, MOAC-enriched proteins from leaves at different stages of a de-/rehydration cycle were separated by 2-D PAGE and detected by phosphoprotein-specific staining. Using this strategy 20 putative phosphoproteins were identified by MALDI-TOF MS and MS/MS, which were not detected when total proteins were analyzed. The characterized desiccation-related phosphoproteins CDeT11-24 and CDeT6-19 were used as internal markers to validate the specificity of the analyses. For 16 of the identified proteins published evidence suggests that they are phosphoproteins. Comparative analysis of the 2-D gels showed that spot intensities of most identified putative phosphoproteins change during the de-/rehydration cycle. This suggests an involvement of these proteins in desiccation tolerance. Nearly all changes in the phosphoproteome of C. plantagineum, which are triggered by dehydration, are reversed within 4 days of rehydration, which is in agreement with physiological observations. Possible functions of selected proteins are discussed in the context of the de-/rehydration cycle.

Lindernia brevidens: a novel desiccation-tolerant vascular plant, endemic to ancient tropical rainforests

A particular adaptation to survival under limited water availability has been realized in the desiccation-tolerant resurrection plants, which tend to grow in a habitat with seasonal rainfall and long dry periods. One of the best-studied examples is Craterostigma plantagineum. Here we report an unexpected finding: Lindernia brevidens, a close relative of C. plantagineum, exhibits desiccation tolerance, even though it is endemic to the montane rainforests of Tanzania and Kenya, where it never experiences seasonal dry periods. L. brevidens has been found exclusively in two fragments of the ancient Eastern Arc Mountains, which were protected from the devastating Pleistocene droughts by the stable Indian Ocean temperature. Analysis of the microhabitat reveals that L. brevidens is found in the same habitat as hygrophilous plant species, which further indicates that the plant never dries out completely. The objective of this investigation was to address whether C. plantagineum and L. brevidens have desiccation-related pathways in common, or whether L. brevidens has acquired novel pathways. A third, closely related, desiccation-sensitive species, Lindernia subracemosa, has been included for comparison. Mechanisms that confer cellular protection during extreme water loss are well conserved between C. plantagineum and L. brevidens, including the interconversion of 2-octulose to sucrose within the two desiccation-tolerant species. Furthermore, transcriptional control regions of desiccation-related genes belonging to the late embryogenesis abundant (LEA) protein family are also highly conserved. We propose that L. brevidens is a neoendemic species that has retained desiccation tolerance through genome stability, despite tolerance being superfluous to environmental conditions.

Current status and implications of engineering drought tolerance in plants using transgenic approaches

Abstract

Water deficit is a consequence of several different stresses including drought, salinity and extreme temperatures. It severely limits plant growth and productivity. Traditional plant breeding programmes have contributed significantly to the generation of stress-tolerant plants, but the process is rather time-consuming, expensive and hampered by the multigenic nature of the stress. Techniques developed in the field of plant molecular biology have the potential to modify plants for growth under unfavourable conditions and to provide new genotypes faster than traditional breeding is able to. The engineering of metabolic pathways associated with drought stress has emerged as a promising way to improve tolerance in model and crop plants. In recent years, efforts in this area have led to improvements in drought stress tolerance. Attempts to engineer improved drought tolerance using single- or multi-gene transfer of candidate genes offer rapid improvements in drought tolerance, although such genetic engineering strategies are limited by an incomplete understanding of stress tolerance mechanisms. Major efforts are required in molecular dissection of drought tolerance mechanisms in important crop plants. This review summarizes recent advances in elucidating stress response mechanisms and their biotechnological applications. Emphasis is placed on transgenic plants that exhibit improved drought tolerance. Various strategies to obtain drought-tolerant plants will be described and discussed.

Dehydrins in Lupinus albus: pattern of protein accumulation in response to drought

Dehydrins (DHNs) are proteins that accumulate abundantly in various plant tissues in response to environmental stresses and during seed maturation, possibly assisting cells in tolerating dehydration. White lupins (Lupinus albus L.) are able to withstand periods of severe water deficit (WD) and previous work suggested that the stem plays a central role as a survival structure. To investigate DHNs involvement in this strategy, we studied tissue specific protein accumulation of a RAB16-like DHN in lupin during a progressive WD and early recovery. Differences were found between leaves, stems and roots. In leaves and roots, the accumulation of the RAB16-like DHN was independent of the water status whereas in the stem (cortex and stele), DHNs were only detected under severe plant WD (stele relative water content, RWC, reduction of 6-7% and cortex RWC reduction of 20%). DHN mRNA analysis by RT-PCR, showed the presence of one DHN mRNA regardless of the tissue or the plant water status.

Retrotransposons and siRNA have a role in the evolution of desiccation tolerance leading to resurrection of the plant Craterostigma plantagineum

* Craterostigma plantagineum can lose up to 96% of its water content but fully recover within hours after rehydration. The callus tissue of the plant becomes desiccation tolerant upon pre-incubation with abscisic acid (ABA). In callus and vegetative organs, ABA addition and water depletion induce a set of dehydration-responsive genes. * Previously, activation tagging led to the isolation of Craterostigma desiccation tolerant (CDT-1), a dehydration-related ABA-inducible gene which renders callus desiccation tolerant without ABA pre-treatment. This gene belongs to a family of retroelements, members of which are inducible by dehydration. * Craterostigma plantagineum transformation with mutated versions of CDT-1 indicated that protein is not required for the induction of callus desiccation tolerance. Northern analysis and protoplast transfection indicated that CDT-1 directs the synthesis of a double-stranded 21-bp short interfering RNA (siRNA), which opens the metabolic pathway for desiccation tolerance. * Via transposition, these retroelements have progressively increased the capacity of the species to synthesize siRNA and thus recover after dehydration. This may be a case of evolution towards the acquisition of a new trait, stimulated by the environment acting directly on intra-genomic DNA replication.

Analysis of a LEA gene promoter via Agrobacterium-mediated transformation of the desiccation tolerant plant Lindernia brevidens

An Agrobacterium tumefaciens-based transformation procedure was developed for the desiccation tolerant species Lindernia brevidens. Leaf explants were infected with A. tumefaciens strain GV3101 harbouring a binary vector that carried the hygromycin resistance gene and an eGFP reporter gene under the control of a native dehydration responsive LEA promoter (Lb2745pro). PCR analysis of the selected hygromycin-resistant plants revealed that the transformation rates were high (14/14) and seeds were obtained from 13/14 of the transgenic lines. A combination of RNA gel blot and microscopic analyses demonstrated that eGFP expression was induced upon dehydration and ABA treatment. Comparison with existing procedures used to transform the well studied resurrection plant and close relative, Craterostigma plantagineum, revealed that the transformation process is both rapid and leads to the production of viable seed thus making L. brevidens a candidate species for functional genomics approaches to determine the genetic basis of desiccation tolerance.

Comparison of SSFP and IR GRE Techniques for Measurement of Total Myocardial Mass-Influence of Applied Contrast Dosage and Implication for Relative Infarct Size Assessment

OBJECTIVE

To compare total left ventricular mass assessment using steady state free precession (SSFP) and inversion recovery fast gradient echo (IR GRE) imaging and further to assess the influence of contrast dosage on mass by IR GRE and its implications on relative infarct size assessment with both methods.

METHODS

Forty-three patients with first documented myocardial infarction and single vessel disease underwent measurement of total myocardial mass using SSFP technique and an IR GRE sequence. As part of a Phase 2 multi-center dose ranging study for infarct identification patients received 1 of 4 possible dosages (0.05, 0.1, 0.2 or 0.3 mmol/kg body weight) of the contrast agent gadoversetamide (OptiMARK, Tyco Healthcare Mallinckrodt, St. Louis, MO, USA).

RESULTS

Left ventricular mass assessment using IR GRE resulted in a slightly greater detection of myocardial mass than from the SSFP images (160.1 and 156.4 g, respectively, p < 0.001). The overall good correlation of both methods (R2 = 0.97 for the total study group, p < 0.001) was further improved by using gadoversetamide at doses of 0.2 or 0.3 mmol/kg (R2= 0.99, p < 0.001), mainly as a result of a considerably higher blood-myocardial contrast-to-noise ratio (CNR) in the IR GRE images. Bland-Altman analysis in these subgroups showed very little scatter of the residuals over the mean (3.5 +/- 5.4 g and 1.3 +/- 6.9 g respectively, 95% confidence interval). The observed differences in total mass calculation, while statistically significant, were not correlated with clinically relevant differences in estimation of relative infarct size.

CONCLUSION

Total LV mass calculations using SSFP and IR GRE techniques are interchangeable when using appropriate contrast media, such as gadoversetamide. Late gadolinium enhancement results in good blood myocardial CNR. Hence, for relative infarct size assessment either method for calculation of total myocardial mass can be used.

The role of small RNAs in abiotic stress

It was recently discovered that plants respond to environmental stress not only with a specific gene expression programme at the mRNA and protein level but also small RNAs as response modulators play an important role. The small RNAs lead to cleavage or translational inhibition of mRNAs via complementary target sites. Different examples are described where small RNAs have been shown to be involved in stress responses. A link between hormonal action and small RNA activities has frequently been observed thus coupling exogenous factors with endogenous transmitters. Using the CDT-1 gene from the desiccation tolerant plant Craterostigma plantagineum as an example, it is discussed that generation of novel small RNAs could be an evolutionary pathway in plants to adapt to extreme environments.

Desiccation of the resurrection plant Craterostigma plantagineum induces dynamic changes in protein phosphorylation

Reversible phosphorylation of proteins is an important mechanism by which organisms regulate their reactions to external stimuli. To investigate the involvement of phosphorylation during acquisition of desiccation tolerance, we have analysed dehydration-induced protein phosphorylation in the desiccation tolerant resurrection plant Craterostigma plantagineum. Several dehydration-induced proteins were shown to be transiently phosphorylated during a dehydration and rehydration (RH) cycle. Two abundantly expressed phosphoproteins are the dehydration- and abscisic acid (ABA)-responsive protein CDeT11-24 and the group 2 late embryogenesis abundant (LEA) protein CDeT6-19. Although both proteins accumulate in leaves and roots with similar kinetics in response to dehydration, their phosphorylation patterns differ. Several phosphorylation sites were identified on the CDeT11-24 protein using liquid chromatography-tandem mass spectrometry (LCMS/MS). The coincidence of phosphorylation sites with predicted coiled-coil regions leads to the hypothesis that CDeT11-24 phosphorylations influence the stability of coiled-coil interactions with itself and possibly other proteins.