Cytosolic proline is required for basal freezing tolerance in Arabidopsis

The amino acid proline accumulates in many plant species under abiotic stress conditions, and various protective functions have been proposed. During cold stress, however, proline content in Arabidopsis thaliana does not correlate with freezing tolerance. Freezing sensitivity of a starchless plastidic phosphoglucomutase mutant (pgm) indicated that localization of proline in the cytosol might stabilize the plasma membrane during freeze-thaw events. Here, we show that re-allocation of proline from cytosol to vacuole was similar in the pyrroline-5-carboxylate synthase 2-1 (p5cs2-1) mutant and the pgm mutant and caused similar reduction of basal freezing tolerance. In contrast, the starch excess 1-1 mutant (sex1-1) had even lower freezing tolerance than pgm but did not affect sub-cellular localization of proline. Freezing sensitivity of sex1-1 mutants affected primarily the photosynthetic electron transport and was enhanced in a sex1-1::p5cs2-1 double mutant. These findings indicate that several independent factors determine basal freezing tolerance. In a pgm::p5cs2-1 double mutant, freezing sensitivity and proline allocation to the vacuole were the same as in the parental lines, indicating that the lack of cytosolic proline was the common cause of reduced basal freezing tolerance in both mutants. We conclude that cytosolic proline is an important factor in freezing tolerance of non-acclimated plants.

Probing the function of a ligand-modulated dynamic tunnel in bifunctional proline utilization A (PutA)

In many bacteria, the reactions of proline catabolism are catalyzed by the bifunctional enzyme known as proline utilization A (PutA). PutA catalyzes the two-step oxidation of l-proline to l-glutamate using distinct proline dehydrogenase (PRODH) and l-glutamate-γ-semialdehyde dehydrogenase (GSALDH) active sites, which are separated by over 40 Å and connected by a complex tunnel system. The tunnel system consists of a main tunnel that connects the two active sites and functions in substrate channeling, plus six ancillary tunnels whose functions are unknown. Here we used tunnel-blocking mutagenesis to probe the role of a dynamic ancillary tunnel (tunnel 2a) whose shape is modulated by ligand binding to the PRODH active site. The 1.90 Å resolution crystal structure of Geobacter sulfurreducens PutA variant A206W verified that the side chain of Trp206 cleanly blocks tunnel 2a without perturbing the surrounding structure. Steady-state kinetic measurements indicate the mutation impaired PRODH activity without affecting the GSALDH activity. Single-turnover experiments corroborated a severe impairment of PRODH activity with flavin reduction decreased by nearly 600-fold in A206W relative to wild-type. Substrate channeling is also significantly impacted as A206W exhibited a 3000-fold lower catalytic efficiency in coupled PRODH-GSALDH activity assays, which measure NADH formation as a function of proline. The structure suggests that Trp206 inhibits binding of the substrate l-proline by preventing the formation of a conserved glutamate-arginine ion pair and closure of the PRODH active site. Our data are consistent with tunnel 2a serving as an open space through which the glutamate of the ion pair travels during the opening and closing of the active site in response to binding l-proline. These results confirm the essentiality of the conserved ion pair in binding l-proline and support the hypothesis that the ion pair functions as a gate that controls access to the PRODH active site.

Identification of C3H2C3-type RING E3 ubiquitin ligase in grapevine and characterization of drought resistance function of VyRCHC114

BACKGROUND

RING is one of the largest E3 ubiquitin ligase families and C3H2C3 type is the largest subfamily of RING, which plays an important role in plant growth and development, and growth and responses to biotic and abiotic stresses.

RESULTS

A total of 143 RING C3H2C3-type genes (RCHCs) were discovered from the grapevine genome and separated into groups (I-XI) according to their phylogenetic analysis, and these genes named according to their positions on chromosomes. Gene replication analysis showed that tandem duplications play a predominant role in the expansion of VvRCHCs family together. Structural analysis showed that most VvRCHCs (67.13 %) had no more than 2 introns, while genes clustered together based on phylogenetic trees had similar motifs and evolutionarily conserved structures. Cis-acting element analysis showed the diversity of VvRCHCs regulation. The expression profiles of eight DEGs in RNA-Seq after drought stress were like the results of qRT-PCR analysis. In vitro ubiquitin experiment showed that VyRCHC114 had E3 ubiquitin ligase activity, overexpression of VyRCHC114 in Arabidopsis improved drought tolerance. Moreover, the transgenic plant survival rate increased by 30 %, accompanied by electrolyte leakage, chlorophyll content and the activities of SOD, POD, APX and CAT were changed. The quantitative expression of AtCOR15a, AtRD29A, AtERD15 and AtP5CS1 showed that they participated in the response to drought stress may be regulated by the expression of VyRCHC114.

CONCLUSIONS

This study provides valuable new information for the evolution of grapevine RCHCs and its relevance for studying the functional characteristics of grapevine VyRCHC114 genes under drought stress.

Arabidopsis OSMOTIN 34 Functions in the ABA Signaling Pathway and Is Regulated by Proteolysis

Plants have evolutionarily established resistance responses to a variety of abiotic stress conditions, in which ABA mediates the integrated regulation of these stress responses. Numerous proteins function at the transcription level or at the protein level when contributing to controls of the ABA signaling process. Although osmotin is identified as a salt-inducible protein, its function in the abiotic stress response is yet to be elucidated. To examine the role of Arabidopsis OSMOTIN 34 (OSM34) in the ABA signaling pathway, a deletion mutant osm34 generated by a CRISPR/Cas9 system and the double mutant osm34 osml (osmotin 34-like) were analyzed for various ABA responses. Both osm34 and osm34 osml showed reduced levels of ABA responses in seeds and leaves. Moreover, proline level and expression of the proline biosynthesis gene P5CS1 was significantly reduced in osm34 osml. Interestingly, OSM34 binds to SKP2A, an F-Box protein whose transcription is induced by ABA. The protein stability of OSM34 was determined to be under the control of the 26S proteasome. In conclusion, our data suggest that OSM34 functions as a positive regulator in the generation of ABA responses and is under post-translational control.

Abscisic acid-responsive element binding transcription factors contribute to proline synthesis and stress adaptation in Arabidopsis

Proline accumulation is one of the most common adaptive responses of higher plants against abiotic stresses like drought. It plays multiple roles in osmotic adjustment, cell homeostasis and stress recovery. Genetic regulation of proline accumulation under drought is complex, and transcriptional cascades modulating proline is poorly understood. Here, we employed quadruple mutant (abf1 abf2 abf3 abf4) to dissect the role of ABA-responsive elements (ABREs) binding transcription factors (ABFs) in modulating proline accumulation across varying stress scenarios. ABREs are present across the promoter of the P5CS1 gene, whose upregulation is considered a hallmark for drought inducible proline accumulation. Upon ABA treatment, P5CS1 mRNA expression and proline content in the shoot were significantly higher in Col-0 compared to the quadruple mutant. Similar results were found at 2 h and 3 h after acute dehydration. We quantified proline at different time points after drought stress treatment. The proline content was higher in wild type (Col-0) than the quadruple mutant at the early stage of drought. Notably, the proline accumulation in wild type increased at a slower rate than the quadruple mutant 7 d after drought stress. Besides, the quadruple mutant displayed significant oxidative damage, low tissue turgidity and higher membrane damage under terminal drought stress. Both terminal drought stress and long-term constant water stress revealed substantial differences in growth rate between wild type and quadruple mutant. The study provides evidence that ABFs are involved in drought stress response, such as proline biosynthesis in Arabidopsis.

Overexpression of Isoprene Synthase Affects ABA- and Drought-Related Gene Expression and Enhances Tolerance to Abiotic Stress

Isoprene is the most abundant single biogenic volatile compound emitted by plants. Despite the relevance of this molecule to plant abiotic resistance and its impact on global atmospheric chemistry, little is known about the details of its mechanism of action. Here, we characterized through both physiological and molecular methods the mechanisms of action of isoprene using model transgenic arabidopsis lines overexpressing a monocot isoprene synthase gene. Our results demonstrated the effect that isoprene had on ABA signaling at different tissue-specific, spatial, and temporal scales. In particular, we found that isoprene enhanced stomatal sensitivity to ABA through upregulation of RD29B signaling gene. By contrast, isoprene decreased sensitivity to ABA in germinating seeds and roots, suggesting tissue-specific mechanisms of action. In leaves, isoprene caused the downregulation of COR15A and P5CS genes, suggesting that the enhanced tolerance to water-deprivation stress observed in isoprene-emitting plants may be mediated chiefly by an enhanced membrane integrity and tolerance to osmotic stress.

Overexpression of the Lolium perenne L. delta1-pyrroline 5-carboxylate synthase (LpP5CS) gene results in morphological alterations and salinity tolerance in switchgrass (Panicum virgatum L.)

In plants, Δ1-pyrroline- 5-carboxylate synthase (P5CS) is the rate-limiting enzyme in proline biosynthesis. In this study, we introduced the LpP5CS (Lolium perenne L.) gene into switchgrass by Agrobacterium-mediated transformation. The transgenic lines (TG) were classified into two groups based on their phenotypes and proline levels. The group I lines (TG4 and TG6) had relatively high proline levels and improved biomass yield. The group II lines (TG1 and TG2) showed low proline levels, severely delayed flowering, stunted growth and reduced biomass yield. Additionally, we used RNA-seq analysis to detect the most significant molecular changes, and we analyzed differentially expressed genes, such as flowering-related and CYP450 family genes. Moreover, the biomass yield, physiological parameters, and expression levels of reactive oxygen species scavenger-related genes under salt stress all indicated that the group I plants exhibited significantly increased salt tolerance compared with that of the control plants, in contrast to the group II plants. Thus, genetic improvement of switchgrass by overexpressing LpP5CS to increase proline levels is feasible for increasing plant stress tolerance.

Proline synthesis in developing microspores is required for pollen development and fertility

BACKGROUND

In many plants, the amino acid proline is strongly accumulated in pollen and disruption of proline synthesis caused abortion of microspore development in Arabidopsis. So far, it was unclear whether local biosynthesis or transport of proline determines the success of fertile pollen development.

RESULTS

We analyzed the expression pattern of the proline biosynthetic genes PYRROLINE-5-CARBOXYLATE SYNTHETASE 1 & 2 (P5CS1 & 2) in Arabidopsis anthers and both isoforms were strongly expressed in developing microspores and pollen grains but only inconsistently in surrounding sporophytic tissues. We introduced in a p5cs1/p5cs1 p5cs2/P5CS2 mutant background an additional copy of P5CS2 under the control of the Cauliflower Mosaic Virus (CaMV) 35S promoter, the tapetum-specific LIPID TRANSFER PROTEIN 12 (Ltp12) promoter or the pollen-specific At5g17340 promoter to determine in which site proline biosynthesis can restore the fertility of proline-deficient microspores. The specificity of these promoters was confirmed by β-glucuronidase (GUS) analysis, and by direct proline measurement in pollen grains and stage-9/10 anthers. Expression of P5CS2 under control of the At5g17340 promoter fully rescued proline content and normal morphology and fertility of mutant pollen. In contrast, expression of P5CS2 driven by either the Ltp12 or CaMV35S promoter caused only partial restoration of pollen development with little effect on pollen fertility.

CONCLUSIONS

Overall, our results indicate that proline transport is not able to fulfill the demand of the cells of the male germ line. Pollen development and fertility depend on local proline biosynthesis during late stages of microspore development and in mature pollen grains.

Control of proline accumulation under drought via a novel pathway comprising the histone methylase CAU1 and the transcription factor ANAC055

Proline plays a crucial role in the drought stress response in plants. However, there are still gaps in our knowledge about the molecular mechanisms that regulate proline metabolism under drought stress. Here, we report that the histone methylase encoded by CAU1, which is genetically upstream of P5CS1 (encoding the proline biosynthetic enzyme Δ1-pyrroline-5-carboxylate synthetase 1), plays a crucial role in proline-mediated drought tolerance. We determined that the transcript level of CAU1 decreased while that of ANAC055 (encoding a transcription factor) increased in wild-type Arabidopsis under drought stress. Further analyses showed that CAU1 bound to the promoter of ANAC055 and suppressed its expression via H4R3sme2-type histone methylation in the promoter region. Thus, under drought stress, a decreased level of CAU1 led to an increased transcript level of ANAC055, which induced the expression of P5CS1 and increased proline level independently of CAS. Drought tolerance and the level of proline were found to be decreased in the cau1 anac055 double-mutant, while proline supplementation restored drought sensitivity in the anac055 mutant. Our results reveal the details of a novel pathway leading to drought tolerance mediated by CAU1.

Control of proline accumulation under drought via a novel pathway comprising the histone methylase CAU1 and the transcription factor ANAC055

Proline plays a crucial role in the drought stress response in plants. However, there are still gaps in our knowledge about the molecular mechanisms that regulate proline metabolism under drought stress. Here, we report that the histone methylase encoded by CAU1, which is genetically upstream of P5CS1 (encoding the proline biosynthetic enzyme Δ1-pyrroline-5-carboxylate synthetase 1), plays a crucial role in proline-mediated drought tolerance. We determined that the transcript level of CAU1 decreased while that of ANAC055 (encoding a transcription factor) increased in wild-type Arabidopsis under drought stress. Further analyses showed that CAU1 bound to the promoter of ANAC055 and suppressed its expression via H4R3sme2-type histone methylation in the promoter region. Thus, under drought stress, a decreased level of CAU1 led to an increased transcript level of ANAC055, which induced the expression of P5CS1 and increased proline level independently of CAS. Drought tolerance and the level of proline were found to be decreased in the cau1 anac055 double-mutant, while proline supplementation restored drought sensitivity in the anac055 mutant. Our results reveal the details of a novel pathway leading to drought tolerance mediated by CAU1.

Proline Accumulation Is Regulated by Transcription Factors Associated with Phosphate Starvation

Pro accumulation in plants is a well-documented physiological response to osmotic stress caused by drought or salinity. In Arabidopsis (Arabidopsis thaliana), the stress and ABA-induced Δ1-PYRROLINE-5-CARBOXYLATE SYNTHETASE1 (P5CS1) gene was previously shown to control Pro biosynthesis in such adverse conditions. To identify regulatory factors that control the transcription of P5CS1, Y1H screens were performed with a genomic fragment of P5CS1, containing 1.2-kB promoter and 0.8-kb transcribed regions. The myeloblastosis (MYB)-type transcription factors PHOSPHATE STARVATION RESPONSE1 (PHR1) and PHR1-LIKE1 (PHL1) were identified to bind to P5CS1 regulatory sequences in the first intron, which carries a conserved PHR1-binding site (P1BS) motif. Binding of PHR1 and PHL1 factors to P1BS was confirmed by Y1H, electrophoretic mobility assay and chromatin immunoprecipitation. Phosphate starvation led to gradual increase in Pro content in wild-type Arabidopsis plants as well as transcriptional activation of P5CS1 and PRO DEHYDROGENASE2 genes. Induction of P5CS1 transcription and Pro accumulation during phosphate deficiency was considerably reduced by phr1 and phl1 mutations and was impaired in the ABA-deficient aba2-3 and ABA-insensitive abi4-1 mutants. Growth and viability of phr1phl1 double mutant was significantly reduced in phosphate-depleted medium, while growth was only marginally affected in the aba2-3 mutants, suggesting that ABA is implicated in growth retardation in such nutritional stress. Our results reveal a previously unknown link between Pro metabolism and phosphate nutrition and show that Pro biosynthesis is target of cross talk between ABA signaling and regulation of phosphate homeostasis through PHR1- and PHL1-mediated transcriptional activation of the P5CS1 gene.

Synthesis of the compatible solute proline by Bacillus subtilis: point mutations rendering the osmotically controlled proHJ promoter hyperactive

The ProJ and ProH enzymes of Bacillus subtilis catalyse together with ProA (ProJ-ProA-ProH), osmostress-adaptive synthesis of the compatible solute proline. The proA-encoded gamma-glutamyl phosphate reductase is also used for anabolic proline synthesis (ProB-ProA-ProI). Transcription of the proHJ operon is osmotically inducible whereas that of the proBA operon is not. Targeted and quantitative proteome analysis revealed that the amount of ProA is not limiting for the interconnected anabolic and osmostress-responsive proline production routes. A key player for enhanced osmostress-adaptive proline production is the osmotically regulated proHJ promoter. We used site-directed mutagenesis to study the salient features of this stress-responsive promoter. Two important features were identified: (i) deviations of the proHJ promoter from the consensus sequence of SigA-type promoters serve to keep transcription low under non-inducing growth conditions, while still allowing a finely tuned induction of transcriptional activity when the external osmolarity is increased and (ii) a suboptimal spacer length for SigA-type promoters of either 16-bp (the natural proHJ promoter), or 18-bp (a synthetic promoter variant) is strictly required to allow regulation of promoter activity in proportion to the external salinity. Collectively, our data suggest that changes in the local DNA structure at the proHJ promoter are important determinants for osmostress-inducibility of transcription.

Capturing Environmental Plant Memories in DNA, with a Little Help from Chromatin

Plants are eukaryotes living mostly immotile in harsh environments. On occasion, it is beneficial for their survival to maintain a transcriptional response to an environmental stress longer than the stress lasts (transcriptional memory) and even to reiterate such a response more quickly or more strongly when the same stress is re-encountered (priming memory). In eukaryotes, transcription takes place in the context of chromatin, the packaging material of DNA. Chromatin regulation is often invoked when it comes to environmental transcriptional and priming memory in plants, but rarely chromatin-based regulation can be accurately assigned to a given aspect of transcription in vivo. The conserved eukaryotic chromatin-modifying system Polycomb/Trithorax can support both long-term stability and flexibility of gene expression in Drosophila. The main principles of Polycomb/Trithorax regulation will be outlined and illustrated with the best-studied case of environmental memory from Arabidopsis. Despite being complex, the Polycomb/Trithorax system relies on experimentally tractable elements in the form of DNA, termed Polycomb/Trithorax Responsive Elements. PREs/TREs are essentially memory DNA elements. Here, relevant information to identify PRE/TRE-like elements in plants is highlighted. Examples of priming memory in plants are discussed in relation to the first two reported putative memory DNA elements. Arguably, similar cases from plants can be conducive in dissecting the contribution of DNA-based from chromatin-based regulation of transcription, when two types of DNA elements are defined: those representing binding sites for the transcription factors determining the environmental response and those controlling memory by regulating chromatin modification dynamics, ultimately maintaining the corresponding transcriptional state.

Robust root growth in altered hydrotropic response1 (ahr1) mutant of Arabidopsis is maintained by high rate of cell production at low water potential gradient

Hydrotropism is the directional root growth response determined by water stimulus. In a water potential gradient system (WPGS) the roots of the Arabidopsis wild type have a diminished root growth compared to normal medium (NM). In contrast, the altered hydrotropic response1 (ahr1) mutant roots maintain their robust growth in the same WPGS. The aims of this work were to ascertain how ahr1 roots could sustain growth in the WPGS, with a special focus on the integration of cellular processes involved in the signaling that determines root growth during abiotic stress and their relation to hydrotropism. Cellular analysis of the root apical meristem of ahr1 mutant contrary to the wild type showed an absence of changes in the meristem length, the elongation zone length, the length of fully elongated cells, and the cell cycle duration. The robust and steady root growth of ahr1 seedlings in the WPGS is explained by the mutant capacity to maintain cell production and cell elongation at the same level as in the NM. Analysis of auxin response at a transcriptional level showed that roots of the ahr1 mutant had a lower auxin response when grown in the WPGS, compared to wild type, indicating that auxin signaling participates in attenuation of root growth under water stress conditions. Also, wild type plants exhibited a high increase in proline content while ahr1 mutants showed minimum changes in the Normal Medium→Water Stress Medium (NM→WSM), a lower water potential gradient system than the WPGS. Accordingly, in this condition, gene expression of Δ1-6 Pyrroline-5-Carboxylate Synthetase1 (P5CS1) involved in proline synthesis strongly increased in wild type but not in ahr1 seedlings. The ahr1 phenotype shows unique features since the mutant root cells continue to proliferate and grow in the presence of a progressively negative water potential gradient at a level comparable to wild type growing in the NM. As such, it represents an exceptional resource for understanding hydrotropism.

[Evaluation of Salt Tolerance of Transgenic Tobacco Plants Bearing with P5CS1 Gene of Arabidopsis thaliana]

Arabidopsis thaliana delta1-pyrroline-5-carhoxylate synthase 1 gene (P5CS1) cDNA was cloned under the control of the potent constitutive 35S RNA promoter of the cauliflower mosaic virus and transferred into genome of tobacco cv. Petit Havana SR-1 (Nicotiana tabacum L.) plants. It is shown that the constitutive level of proline in the transgenic plants T0 exceeds that of the SR1 reference line by 1.5 to 4 times. Under conditions of salt stress (200, 300 mM NaCl) T1-generation transgenic plants in early stages of development formed a large biomass, developed more quickly, and had a higher rate of root growth compared to the control, which confirms the involvement of the P5CS1 gene in molecular mechanisms of stress resistance in plants.

Stress physiology functions of the Arabidopsis histidine kinase cytokinin receptors

Cytokinin signaling has complex effects on abiotic stress responses that remain to be fully elucidated. The Arabidopsis histidine kinases (AHKs), AHK2, AHK3 and CRE1 (cytokinin response1/AHK4) are the principle cytokinin receptors of Arabidopsis. Using a set of ahk mutants, we found dramatic differences in response to low water potential and salt stress among the AHKs. ahk3-3 mutants had increased root elongation after transfer to low water potential media. Conversely ahk2-2 was hypersensitive to salt stress in terms of root growth and fresh weight and accumulated higher than wild-type levels of proline specifically under salt stress. Strongly reduced proline accumulation in ahk double mutants after low water potential treatment indicated a more general role of cytokinin signaling in proline metabolism. Reduced P5CS1 (Δ(1) -pyrroline-5-carboxylate synthetase1) gene expression may have contributed to this reduced proline accumulation. Low water potential phenotypes of ahk mutants were not caused by altered abscisic acid (ABA) accumulation as all ahk mutants had wild-type ABA levels, despite the observation that ahk double mutants had reduced NCED3 (9-cis-epoxycartenoid dioxygenase3) expression when exposed to low water potential. No difference in osmoregulatory solute accumulation was detected in any of the ahk mutants indicating that they do not affect drought responsive osmotic adjustment. Overall, our examination of ahk mutants found specific phenotypes associated with AHK2 and AHK3 as well as a general function of cytokinin signaling in proline accumulation and low water potential induction of P5CS1 and NCED3 expression. These results show the stress physiology function of AHKs at a new level of detail.

Effect of drought and combined drought and heat stress on polyamine metabolism in proline-over-producing tobacco plants

The roles of proline and polyamines (PAs) in the drought stress responses of tobacco plants were investigated by comparing the responses to drought alone and drought in combination with heat in the upper and lower leaves and roots of wild-type tobacco plants and transformants that constitutively over-express a modified gene for the proline biosynthetic enzyme Δ1-pyrroline-5-carboxylate synthetase (P5CSF129A; EC 2.7.2.11/1.2.1.41). In both genotypes, drought stress coincided with a decrease in relative water content (RWC) that was much less severe in the upper leaves than elsewhere in the plant. The drought also increased proline levels in both genotypes. A brief period of heat stress (2 h at 40 °C) at the end of the drought period did not significantly influence the proline levels in the upper leaves and roots but caused a further increase in the lower leaves of both genotypes. The rate at which these elevated proline levels returned to normal during the post-stress recovery period was slower in the transformants and plants that had been subjected to the combined stress. In both genotypes, drought stress significantly reduced the levels of spermidine (Spd) and putrescine (Put) in the leaves and roots relative to those for controls, and increased the levels of spermine (Spm) and diaminopropane (Dap, formed by the oxidative deamination of Spd and Spm). Spd levels may have declined due to its consumption in Spm biosynthesis and/or oxidation by polyamine oxidase (PAO; EC 1.5.3.11) to form Dap, which became more abundant during drought stress. During the rewatering period, the plants' Put and Spd levels recovered quickly and the activity of the PA biosynthesis enzymes in their leaves and roots increased substantially; this increase was more pronounced in transformants than WT plants. The high levels of Spm observed in drought stressed plants persisted even after the 24 h recovery and rewatering phase. The malondialdehyde (MDA) contents of the lower leaves of WTs increased substantially during the drought stress period; a less pronounced increase occurred in the transformants and after the application of the combined stress. After the post-stress recovery period, the MDA contents in the leaves of both genotypes were higher than those in the corresponding controls. The MDA contents of the upper leaves in plants of both genotypes remained relatively constant throughout, indicating that these leaves are preferentially protected against the adverse effects of oxidative stress and demonstrating the efficiency of the plants' induced antioxidative defense mechanisms.

Analysis by virus induced gene silencing of the expression of two proline biosynthetic pathway genes in Nicotiana benthamiana under stress conditions

Proline accumulation is responsible for stress adaptation in many plants. To distinguish the involvement of two proline synthetic pathways, the virus induced gene silencing (VIGS) system that silenced the expression of genes encoding Δ(1)-pyrroline-5-carboxylate synthetase (P5CS; EC:1.5.1.12) and ornithine-δ-aminotransferase (OAT; EC 2.6.1.13) was performed, separately or concomitantly, in four-week-old Nicotiana benthamiana. Leaf discs of VIGS-treated tobacco were subjected to the treatment of drought, abscisic acid (ABA), or polyethylene glycol (PEG). The treated leaf discs were then collected for the determination of mRNA, chlorophyll, proline and polyamine level. Under drought stress or PEG treatment, most proline accumulation was inhibited in P5CS-silenced plants and only a small portion was inhibited in OAT-silenced plants under drought stress and no inhibition was observed under PEG treatment. Under ABA treatment, proline accumulation was inhibited completely in P5CS-silenced plants but unaffected in OAT-silenced plants. The degradation of chlorophyll was enhanced in P5CS-silenced plants but retarded in OAT-silenced plants under PEG treatment. Under ABA treatment, the degradation of chlorophyll was unaffected in both P5CS-silenced and OAT-silenced plants. The increase of polyamine level was unaffected in P5CS-silenced plants but increased in OAT-silenced plants under PEG treatment. Under ABA treatment, the increase of polyamine level was unaffected in P5CS-silenced plants but the polyamine level was increased later in OAT-silenced plants. Therefore, P5CS plays a major role in proline accumulation under drought, PEG, or ABA treatment, while OAT plays a minor role in drought or PEG treatment and does not participate in ABA treatment. OAT appears to have a close relationship with the regulation of polyamine levels in PEG and ABA treatments.

Elevation of free proline and proline-rich protein levels by simultaneous manipulations of proline biosynthesis and degradation in plants

Proline-rich proteins (PRP) are cell wall and plasma membrane-anchored factors involved in cell wall maintenance and its stress-induced fortification. Here we compare the synthesis of P5C as the proline (Pro) precursor in the cytosol and chloroplast by an introduced alien system and evaluate correlation between PRP synthesis and free Pro accumulation in plants. We developed a Pro over-producing system by generating transgenic tobacco plants overexpressing E. coli P5C biosynthetic enzymes; Pro-indifferent gamma-glutamyl kinase 74 (GK74) and gamma-glutamylphosphate reductase (GPR), as well as antisensing proline dehydrogenase (ProDH) transcription. GK74 and GPR enzymes were targeted either to the cytosol or plastids. Molecular analyses indicated that the two bacterial enzymes are efficiently expressed in plant cells, correctly targeted to the cytosol or chloroplasts, and processed to active enzymatic complexes in the two compartments. Maximal Pro increase is obtained when GK74 and GPR are active in chloroplasts, and ProDH mRNA level is reduced by anti-sense silencing, resulting in more than 50-fold higher Pro content compared to that of wild type tobacco plants. The Pro over-producing system efficiently works in tobacco and Arabidopsis. The elevation of Pro levels promotes accumulation of ectopically expressed Cell Wall Linker Protein (AtCWLP), a membrane protein with an external Pro-rich domain. These results suggest that the Pro-generating system can support endogenous or alien PRP production in plants.

The dominant glutamic acid metabolic flux to produce γ-amino butyric acid over proline in Nicotiana tabacum leaves under water stress relates to its significant role in antioxidant activity

γ-Amino butyric acid (GABA) and proline play a crucial role in protecting plants during various environmental stresses. Their synthesis is from the common precursor glutamic acid, which is catalyzed by glutamate decarboxylase and Δ(1) -pyrroline-5-carboxylate synthetase respectively. However, the dominant pathway under water stress has not yet been established. To explore this, excised tobacco leaves were used to simulate a water-stress condition. The results showed GABA content was much higher than that of proline in leaves under water-deficit and non-water-deficit conditions. Specifically, the amount of GABA significantly increased compared to proline under continuous water loss for 16 h, indicating that GABA biosynthesis is the dominant pathway from glutamic acid metabolism under these conditions. Quantitative reverse transcription polymerase chain reaction and protein Western gel-blot analysis further confirmed this. To explore the function of GABA accumulation, a system producing superoxide anion (O(2) (-) ), peroxide hydrogen (H(2) O(2) ), and singlet oxygen ((1) O(2) ) was employed to investigate the scavenging role on free-radical production. The results demonstrated that the scavenging ability of GABA for O(2) (-) , H(2) O(2) , and (1) O(2) was significantly higher than that of proline. This indicated that GABA acts as an effective osmolyte to reduce the production of reactive oxygen species under water stress.

The development and evaluation of single cell suspension from wheat and barley as a model system; a first step towards functional genomics application

BACKGROUND

The overall research objective was to develop single cell plant cultures as a model system to facilitate functional genomics of monocots, in particular wheat and barley. The essential first step towards achieving the stated objective was the development of a robust, viable single cell suspension culture from both species.

RESULTS

We established growth conditions to allow routine culturing of somatic cells in 24 well microtiter plate format. Evaluation of the wheat and barley cell suspension as model cell system is a multi step process. As an initial step in the evaluation procedure we chose to study the impact of selected abiotic stress elicitors at the physiological, biochemical and molecular level. We report the results of osmotic stress imposed by NaCl and PEG. As proline is an important osmoprotectant of the cereal cells, colorimetric assay for proline detection was developed for small volumes (200 μl). We performed RT-PCR experiments to study the change in the expression of the genes encoding Δ1-pyrroline-5-carboxylate synthetase (P5CS) and Δ1-pyrroline-5-carboxylate reductase (PC5R) in response to abiotic stress.

CONCLUSIONS

We found differences between the wheat and barley suspension cultures, barley being more tolerant to the applied osmotic stresses. We suggested a model to explain the obtained differences in stress tolerance between the two species. The suspension cell cultures have proven useful for determining changes in proline concentration and expression level of genes (P5CS, P5CR) under various treatments and we suggest that the cells can be used as a model host system to study gene expression and regulation in monocots.

The proline biosynthetic genes P5CS1 and P5CS2 play overlapping roles in Arabidopsis flower transition but not in embryo development

Overexpression of the proline biosynthetic gene P5CS1 results in early flowering in Arabidopsis. However, the p5cs1 loss-of-function mutant exhibits a modest delay in flowering, suggesting that P5CS2, a duplicated P5CS1 gene present in the Arabidopsis, may also play a role in flower transition. In situ mRNA hybridizations and quantitative reverse transcription-polymerase chain reaction (RT-PCR) revealed that P5CS1 and P5CS2 are expressed at similar levels and with the same pattern of expression in vegetative and floral shoot apical meristems as well as in axillary meristems. Arabidopsis lines homozygous for the p5cs1 mutant and simultaneously heterozygous for the p5cs2 mutation showed a stronger late-flowering phenotype than p5cs1 single mutants, confirming that also P5CS2 plays a role in flower transition and supporting the notion of overlapping functions of the two P5CS genes in this developmental process. P5CS1 and P5CS2 have identical messenger RNA (mRNA) distributions also in embryos, but only p5cs2 mutant embryos exhibit alterations of the cellular division planes and consequently stop developing. This suggests a specific role of P5CS2 in embryogenesis and an involvement of proline in cell division. Accordingly, exogenous proline accelerated organ growth and meristem formation, and stimulated expression of the cell cycle-related protein CYCB1;1.

Salt stress increases the expression of p5cs gene and induces proline accumulation in cactus pear

Proline (Pro) is one of the most accumulated osmolytes in salinity and water deficit conditions in plants. In the present study, we measured the Pro content, the activity and the expression level of delta 1-pyrroline-5-carboxylate synthetase (P5CS: gamma-glutamyl kinase, EC 2.7.2.11 and glutamate-5-semialdehyde dehydrogenase, EC 1.2.1.41), a key regulatory enzyme involved in the biosynthesis of Pro, in cactus pear (Opuntia streptacantha) subjected to 6, 9 and 11 days of salt stress. Treatment with NaCl of O. streptacantha young plants resulted in a decrease in the cladode thickness and root length, and in a significant and gradual accumulation of Pro in young cladodes, in a time- and concentration-dependent manner. P5CS activity, studied as gamma-glutamyl kinase, was reduced at all times as a consequence of salt treatment, except at the sixth day at 75 and 150mM of NaCl, where a slight increase was observed. We isolated an open reading frame (ORF) fragment of p5cs gene. The deduced amino acid sequence of the P5CS protein exhibited 90.4% of identity with the P5CS protein from Mesembryanthemum crystallinum. RT-PCR analysis revealed that the Osp5cs gene of O. streptacantha was induced by salt stress at 9 and 11 days of treatment. Furthermore, ABA-induced Osp5cs gene expression was observed in cladodes of cactus pear young plants. We observed an evident correlation between the transcript up-regulation and the Pro accumulation under salt stress; however, these results do not parallel with the changes in P5CS enzymatic activity. This Pro accumulation might function as an osmolyte for the intracellular osmotic adjustment and might be playing a critical role in protecting photosynthetic activity in O. streptacantha plants under salt stress.

Salinity-dependent switching of osmolyte strategies in a moderately halophilic bacterium: glutamate induces proline biosynthesis in Halobacillus halophilus

The moderately halophilic bacterium Halobacillus halophilus copes with the salinity in its environment by the production of compatible solutes. At intermediate salinities of around 1 M NaCl, cells produce glutamate and glutamine in a chloride-dependent manner (S. H. Saum, J. F. Sydow, P. Palm, F. Pfeiffer, D. Oesterhelt, and V. Müller, J. Bacteriol. 188:6808-6815, 2006). Here, we report that H. halophilus switches its osmolyte strategy and produces proline as the dominant solute at higher salinities (2 to 3 M NaCl). The proline biosynthesis genes proH, proJ, and proA were identified. They form a transcriptional unit and encode the pyrroline-5-carboxylate reductase, the glutamate-5-kinase, and the glutamate-5-semialdehyde dehydrogenase, respectively, catalyzing proline biosynthesis from glutamate. Expression of the genes was clearly salinity dependent and reached a maximum at 2.5 M NaCl, indicating that the pro operon is involved in salinity-induced proline biosynthesis. To address the role of anions in the process of pro gene activation and proline biosynthesis, we used a cell suspension system. Chloride salts lead to the highest accumulation of proline. Interestingly, chloride could be substituted to a large extent by glutamate salts. This unexpected finding was further analyzed on the transcriptional level. The cellular mRNA levels of all three pro genes were increased up to 90-fold in the presence of glutamate. A titration revealed that a minimal concentration of 0.2 M glutamate already stimulated pro gene expression. These data demonstrate that the solute glutamate is involved in the switch of osmolyte strategy from glutamate to proline as the dominant compatible solute during the transition from moderate to high salinity.

Directed evolution of an artificial bifunctional enzyme, γ-glutamyl kinase/γ-glutamyl phosphate reductase, for improved osmotic tolerance ofEscherichia colitransformants

To produce the artificial bifunctional enzyme gamma-glutamyl kinase/gamma-glutamyl phosphate reductase, a mutant library of the proBA fusion gene from Bacillus subtilis was created by error-prone PCR. Selecting by functional complementation of the proline auxotroph Escherichia coli JM83 and NaCl tolerance, we isolated a mutant of the proBA fusion gene that improved the osmotolerance of host cells of E. coli JM83. A single amino acid replacement (Asn177Asp) located in a conserved domain in gamma-glutamyl kinase leads to overproduction of proline by host cells. The mutated gamma-glutamyl kinase/gamma-glutamyl phosphate reductase enzyme was rendered about 100-fold less sensitive to proline-mediated feedback inhibition than the control.

Correlation between the induction of a gene for delta 1-pyrroline-5-carboxylate synthetase and the accumulation of proline in Arabidopsis thaliana under osmotic stress

The isolation and characterization is reported of a cDNA for delta 1-pyrroline-5-carboxylate (P5C) synthetase (cAtP5CS), an enzyme involved in the biosynthesis of proline, from a cDNA library prepared from a dehydrated rosette plant of Arabidopsis thaliana. Southern blot analysis suggested that only one copy of the corresponding gene (AtP5CS) is present in A. thaliana. The deduced amino acid sequence of the P5CS protein (AtP5CS) from A. thaliana exhibited 74% homology to that of the P5CS from Vigna aconitifolia. Northern blot analysis revealed that the gene for P5CS was induced by dehydration, high salt and treatment with ABA, while it was not induced by heat or cold treatment. Moreover, the simultaneous accumulation of proline was observed as a result of the former treatments in A. thaliana. A cDNA for P5C reductase (cAtP5CR) was also isolated from A. thaliana and Northern blot analysis was performed. The AtP5CR gene was not induced to a significant extent by dehydration or high-salt stress. These observations suggest that the AtP5CS gene plays a principal role in the biosynthesis of proline in A. thaliana under osmotic stress.