Overexpression of OsRAN2 in rice and Arabidopsis renders transgenic plants hypersensitive to salinity and osmotic stress

Functional and Transcriptome Analysis Reveal Specific Roles of Dimocarpus longan DlRan3A and DlRan3B in Root Hair Development, Reproductive Growth, and Stress Tolerance

Ran GTPases play essential roles in plant growth and development. Our previous studies revealed the nuclear localization of DlRan3A and DlRan3B proteins and proposed their functional redundancy and distinction in Dimocarpus longan somatic embryogenesis, hormone, and abiotic stress responses. To further explore the possible roles of DlRan3A and DlRan3B, gene expression analysis by qPCR showed that their transcripts were both more abundant in the early embryo and pulp in longan. Heterologous expression of DlRan3A driven by its own previously cloned promoter led to stunted growth, increased root hair density, abnormal fruits, bigger seeds, and enhanced abiotic stress tolerance. Conversely, constitutive promoter CaMV 35S (35S)-driven expression of DlRan3A, 35S, or DlRan3B promoter-controlled expression of DlRan3B did not induce the alterations in growth phenotype, while they rendered different hypersensitivities to abiotic stresses. Based on the transcriptome profiling of longan Ran overexpression in tobacco plants, we propose new mechanisms of the Ran-mediated regulation of genes associated with cell wall biosynthesis and expansion. Also, the transgenic plants expressing DlRan3A or DlRan3B genes controlled by 35S or by their own promoter all exhibited altered mRNA levels of stress-related and transcription factor genes. Moreover, DlRan3A overexpressors were more tolerant to salinity, osmotic, and heat stresses, accompanied by upregulation of oxidation-related genes, possibly involving the Ran-RBOH-CIPK network. Analysis of a subset of selected genes from the Ran transcriptome identified possible cold stress-related roles of brassinosteroid (BR)-responsive genes. The marked presence of genes related to cell wall biosynthesis and expansion, hormone, and defense responses highlighted their close regulatory association with Ran.

Humic acids enhance salt stress tolerance associated with pyrroline 5-carboxylate synthetase gene expression and hormonal alteration in perennial ryegrass (Lolium perenne L.)

Humic acid (HA) has been used as an important component in biostimulant formulations to enhance plant tolerance to salt stress, but the mechanisms underlying are not fully understood. This study was to investigate the physiological and molecular mechanisms of HA's impact on salt stress tolerance in perennial ryegrass (Lolium perenne L.). The two types of HA were extracted from weathered coal samples collected from Wutai County (WTH) and Jingle County (JLH) of Shanxi Province, China. The grass seedlings subjected to salt stress (250 mM NaCl) were treated with HA solutions containing 0.01% WTH (W/V) or 0.05% JLH (W/V), respectively. The HA treatments improved leaf photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs) and reduced leaf oxidative injury (lower malondialdehyde content) and Pro and intercellular CO2 concentrations in salt-stressed perennial ryegrass. The HA treatments also reversed the decline in antioxidative enzymes ascorbate peroxidase (APX), catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) activity and improved growth and anti-senescence hormones indole-3-acetic acid (IAA) and brassinosteroid (BR). The HA treatments reduced the relative expression of P5CS and its downstream products proline (Pro) and the stress defense hormones abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), and polyamines (PA). The results of this study indicate that the application of HAs may improve salt stress tolerance by regulating P5CS gene expression related to osmotic adjustment and increasing the activity of antioxidant enzymes and anti-senescence hormones in perennial ryegrass.

Emerging role of small GTPases and their interactome in plants to combat abiotic and biotic stress

Plants are frequently subjected to abiotic and biotic stress which causes major impediments in their growth and development. It is emerging that small guanosine triphosphatases (small GTPases), also known as monomeric GTP-binding proteins, assist plants in managing environmental stress. Small GTPases function as tightly regulated molecular switches that get activated with the aid of guanosine triphosphate (GTP) and deactivated by the subsequent hydrolysis of GTP to guanosine diphosphate (GDP). All small GTPases except Rat sarcoma (Ras) are found in plants, including Ras-like in brain (Rab), Rho of plant (Rop), ADP-ribosylation factor (Arf) and Ras-like nuclear (Ran). The members of small GTPases in plants interact with several downstream effectors to counteract the negative effects of environmental stress and disease-causing pathogens. In this review, we describe processes of stress alleviation by developing pathways involving several small GTPases and their associated proteins which are important for neutralizing fungal infections, stomatal regulation, and activation of abiotic stress-tolerant genes in plants. Previous reviews on small GTPases in plants were primarily focused on Rab GTPases, abiotic stress, and membrane trafficking, whereas this review seeks to improve our understanding of the role of all small GTPases in plants as well as their interactome in regulating mechanisms to combat abiotic and biotic stress. This review brings to the attention of scientists recent research on small GTPases so that they can employ genome editing tools to precisely engineer economically important plants through the overexpression/knock-out/knock-in of stress-related small GTPase genes.

Ran-GTP/-GDP-dependent nuclear accumulation of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 and TGACG-BINDING FACTOR2 controls salicylic acid-induced leaf senescence

Leaf senescence is the final stage of leaf development and can be triggered by various external factors, such as hormones and light deprivation. In this study, we demonstrate that the overexpression of the GTP-bound form of Arabidopsis (Arabidopsis thaliana) Ran1 (a Ras-related nuclear small G-protein, AtRan1) efficiently promotes age-dependent and dark-triggered leaf senescence, while Ran-GDP has the opposite effect. Transcriptome analysis comparing AtRan1-GDP- and AtRan1-GTP-overexpressing transgenic plants (Ran1T27Nox and Ran1G22Vox, respectively) revealed that differentially expressed genes (DEGs) related to the senescence-promoting hormones salicylic acid (SA), jasmonic acid, abscisic acid, and ethylene (ET) were significantly upregulated in dark-triggered senescing leaves of Ran1G22Vox, indicating that these hormones are actively involved in Ran-GTP/-GDP-dependent, dark-triggered leaf senescence. Bioinformatic analysis of the promoter regions of DEGs identified diverse consensus motifs, including the bZIP motif, a common binding site for TGACG-BINDING FACTOR (TGA) transcription factors. Interestingly, TGA2 and its interactor, NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1), which are two positive transcriptional regulators of SA signaling, differed in their extent of accumulation in the nucleus versus cytoplasm of Ran1T27Nox and Ran1G22Vox plants. Moreover, SA-induced, Ran-GTP-/-GDP-dependent functions of NPR1 included genome-wide global transcriptional reprogramming of genes involved in cell death, aging, and chloroplast organization. Furthermore, the expression of AtRan1-GTP in SA signaling-defective npr1 and SA biosynthesis-deficient SA-induction deficient2 genetic backgrounds abolished the effects of AtRan1-GTP, thus retarding age-promoted leaf senescence. However, ET-induced leaf senescence was not mediated by Ran machinery-dependent nuclear shuttling of ETHYLENE-INSENSITIVE3 and ETHYLENE-INSENSITIVE3-LIKE1 proteins. We conclude that Ran-GTP/-GDP-dependent nuclear accumulation of NPR1 and TGA2 represents another regulatory node for SA-induced leaf senescence.

A ras-related nuclear protein Ran participates in the 20E signaling pathway and is essential for the growth and development of Locusta migratoria

The small GTPase Ran is a member of the Ras superfamily of small GTP-binding proteins, which plays a key role in the translocation of RNA and proteins through the nuclear pore complex. In this study, the full-length cDNA sequence of LmRan gene was obtained, which consists of 648-nucleotides open reading frame (ORF) and encodes 215 amino acids. RT-qPCR results revealed that LmRan was expressed in all developmental days and tissues investigated. Injection of dsLmRan into 4th and 5th instar nymphs, resulted in a significant down-regulation of LmRan transcripts, respectively. All dsLmRan-injected nymphs died before molting. Further hematoxylin and eosin staining of the integument showed that there was no apolysis occurred after silencing LmRan. In addition, the weight of dsLmRan-injected nymphs was significantly lower than that of the control group, and the gastric caecum and midgut was severely smaller. Especiallly, the mRNA level of LmCYP302a1, LmCYP315a1 and LmCYP314a1 responsible for 20E synthesis, LmE75 and LmE74 genes involved in the 20E signaling pathway, LmGfat, LmUAP1 and LmCHT10 genes involved in chitin metabolism pathway were dramatically decreased in the dsLmRan-injected nymphs. Together, the results indicated that LmRan participate in the 20E signaling pathway, which is essential for the growth and development of locusts.

Acclimation to cold stress reduces injury from low temperature and bispyribac-sodium on rice

BACKGROUND

In subtropical areas, early planting exposes rice seedlings to cold stress, impairing seedling growth and making them more vulnerable to other stresses including herbicide injury. The objectives of this work were: to evaluate the effect of cold stress on bispyribac-sodium selectivity in rice; to determine the mechanisms of cold tolerance in sensitive ('Epagri 109') and tolerant ('IRGA 424') rice cultivars; and to ascertain that cold acclimatization influences bispyribac-sodium selectivity in rice.

RESULTS

Prolonged cold stress caused high lipid peroxidation, increased rice injury, and stunted growth. Short-term acclimation with cold stress reduced rice injury with bispyribac-sodium. Total phenols were upregulated in rice exposed to cold stress. Prolonged cold stress increased the superoxide dismutase and catalase activity in IRGA 424. Antioxidant activity was higher in the cold-tolerant than in the cold-sensitive cultivar. Only catalase activity was responsive to bispyribac-sodium. OsRAN2, OsGSTL2, and CYP72A21 were upregulated by cold and herbicide stress in both cultivars. OsGSTL2 was upregulated more in IRGA 424 than in Epagri 109. OsFAD8 was upregulated in cold-sensitive rice exposed to short-duration cold stress but was not responsive to bispyribac-sodium.

CONCLUSION

Cold stress reduces bispyribac-sodium selectivity in rice. Short-term acclimation to cold stress reduces the effect of cold stress and enhances bispyribac-sodium selectivity. The tolerance of rice (IRGA 424) to cold stress is due to differential induction of protection genes CYP72A21 and OsGSTL2 associated with herbicide metabolism, together with the accumulation of total phenols and higher activity of antioxidant enzymes.

Nuclear transport factor GmNTF2B-1 enhances soybean drought tolerance by interacting with oxidoreductase GmOXR17 to reduce reactive oxygen species content

Drought is a critical abiotic stressor that modulates soybean yield. Drought stress drastically enhances reactive oxygen species (ROS) formation, and maintaining ROS content above a cytostatic level but below a cytotoxic level is essential for normal biology processes in plants. At present, most of the known ROS-scavenging systems are in the cytoplasm, and the mechanism of ROS regulation in the nucleus remains unclear. GmNTF2B-1 is a member of the IV subgroup in the nucleus transporter family. Its expression is localized to the roots and is stimulated by drought stress. In this study, the overexpression of GmNTF2B-1 was found to improve the drought tolerance of transgenic soybean by influencing the ROS content in plants. An oxidoreductase, GmOXR17, was identified to interact with GmNTF2B-1 in the nucleus through the yeast two-hybrid, coimmunoprecipitation and bimolecular fluorescence complementation assays. The drought tolerance of GmOXR17 transgenic soybean was similar to that of GmNTF2B-1. GmNTF2B-1 was expressed in both cytoplasm and nucleus, and GmOXR17 transferred from the cytoplasm to the nucleus under drought stress. The overexpression of GmNTF2B-1 enhanced the nuclear entry of GmOXR17, and the overexpression of GmNTF2B-1 or GmOXR17 could decrease the H2 O2 content and oxidation level in the nucleus. In conclusion, the interaction between GmNTF2B-1 and GmOXR17 may enhance the nuclear entry of GmOXR17, thereby enhancing nuclear ROS scavenging to improve the drought resistance of soybean.

DEFECTIVE EMBRYO AND MERISTEMS genes are required for cell division and gamete viability in Arabidopsis

The DEFECTIVE EMBRYO AND MERISTEMS 1 (DEM1) gene encodes a protein of unknown biochemical function required for meristem formation and seedling development in tomato, but it was unclear whether DEM1’s primary role was in cell division or alternatively, in defining the identity of meristematic cells. Genome sequence analysis indicates that flowering plants possess at least two DEM genes. Arabidopsis has two DEM genes, DEM1 and DEM2, which we show are expressed in developing embryos and meristems in a punctate pattern that is typical of genes involved in cell division. Homozygous dem1 dem2 double mutants were not recovered, and plants carrying a single functional DEM1 allele and no functional copies of DEM2, i.e. DEM1/dem1 dem2/dem2 plants, exhibit normal development through to the time of flowering but during male reproductive development, chromosomes fail to align on the metaphase plate at meiosis II and result in abnormal numbers of daughter cells following meiosis. Additionally, these plants show defects in both pollen and embryo sac development, and produce defective male and female gametes. In contrast, dem1/dem1 DEM2/dem2 plants showed normal levels of fertility, indicating that DEM2 plays a more important role than DEM1 in gamete viability. The increased importance of DEM2 in gamete viability correlated with higher mRNA levels of DEM2 compared to DEM1 in most tissues examined and particularly in the vegetative shoot apex, developing siliques, pollen and sperm. We also demonstrate that gamete viability depends not only on the number of functional DEM alleles inherited following meiosis, but also on the number of functional DEM alleles in the parent plant that undergoes meiosis. Furthermore, DEM1 interacts with RAS-RELATED NUCLEAR PROTEIN 1 (RAN1) in yeast two-hybrid and pull-down binding assays, and we show that fluorescent proteins fused to DEM1 and RAN1 co-localize transiently during male meiosis and pollen development. In eukaryotes, RAN is a highly conserved GTPase that plays key roles in cell cycle progression, spindle assembly during cell division, reformation of the nuclear envelope following cell division, and nucleocytoplasmic transport. Our results demonstrate that DEM proteins play an essential role in cell division in plants, most likely through an interaction with RAN1.

Up to half of the genes predicted from genome projects lack a known biological and biochemical function. Many of these genes are likely to play essential roles but it is difficult to reveal their function because minor changes in the genetic sequence can result in lethality and genetic redundancy can obscure analysis. Genome projects predict that flowering plants have at least two DEM genes that encode a protein of unknown cellular and biochemical function. In this paper, we use multiple combinations of dem mutants in Arabidopsis to show that DEM genes are essential for cell division and gamete viability. Interestingly, gamete viability depends not only on the number of functional copies of DEM genes in the gametes, but also on the number of functional copies of DEM genes in the parent plant that produces the gametes. We also show that DEM proteins interact with RAN, a highly conserved protein that controls cell division in all eukaryotic organisms.

Genome-wide identification of Ran GTPase family genes from wheat (T. aestivum) and their expression profile during developmental stages and abiotic stress conditions

Maintenance of growth is important for sustaining yield under stress conditions. Hence, identification of genes involved in cell division and growth under abiotic stress is utmost important. Ras-related nuclear protein (Ran) is a small GTPase required for nucleocytoplasmic transport, mitotic progression, and nuclear envelope assembly in plants. In the present study, two Ran GTPase genes TaRAN1 and TaRAN2 were identified though genome-wide analysis in wheat (T. aestivum). Comparative analysis of Ran GTPases from wheat, barley, rice, maize, sorghum, and Arabidopsis revealed similar gene structure within phylogenetic clades and highly conserved protein structure. Expression analysis from expVIP platform showed ubiquitous expression of TaRAN genes across tissues and developmental stages. Under biotic and abiotic stresses, TaRAN1 expression was largely unaltered, while TaRAN2 showed stress specific response. In qRT-PCR analysis, TaRAN1 showed significantly higher expression as compared to TaRAN2 in shoot and root at seedling, vegetative, and reproductive stages. During progressive drought stress, TaRAN1 and TaRAN2 expression increase during early stress and restored to control level expression at higher stress levels in shoot. The steady-state level of transcripts was maintained to that of control in roots under drought stress. Under cold stress, expression of both the TaRAN genes decreased significantly at 3 h and became similar to control at 6 h in shoots, while salt stress significantly reduced the expression of TaRAN genes in shoots. The analysis suggests differential regulation of TaRAN genes under developmental stages and abiotic stresses. Delineating the molecular functions of Ran GTPases will help unravel the mechanism of stress induced growth inhibition in wheat.

The Arabidopsis small G-protein AtRAN1 is a positive regulator in chitin-induced stomatal closure and disease resistance

Chitin, a fungal microbial-associated molecular pattern, triggers various defence responses in several plant systems. Although it induces stomatal closure, the molecular mechanisms of its interactions with guard cell signalling pathways are unclear. Based on screening of public microarray data obtained from the ATH1 Affymetrix and Arabidopsis eFP browser, we isolated a cDNA encoding a Ras-related nuclear protein 1 AtRAN1. AtRAN1 expression was enriched in guard cells in a manner consistent with involvement in the control of the stomatal movement. AtRAN1 mutation impaired chitin-induced stomatal closure and accumulation of reactive oxygen species and nitric oxide in guard cells. In addition, Atran1 mutant plants exhibited compromised chitin-enhanced plant resistance to both bacterial and fungal pathogens due to changes in defence-related genes. Furthermore, Atran1 mutant plants were hypersensitive to drought stress compared to Col-0 plants, and had lower levels of stress-responsive genes. These data demonstrate a previously uncharacterized signalling role for AtRAN1, mediating chitin-induced signalling.

Efficient Plant Production of Recombinant NS1 Protein for Diagnosis of Dengue

Dengue fever is endemic in more than 120 countries, which account for 3.9 billion people at risk of infection worldwide. The absence of a vaccine with effective protection against the four serotypes of this virus makes differential molecular diagnosis the key step for the correct treatment of the disease. Rapid and efficient diagnosis prevents progression to a more severe stage of this disease. Currently, the limiting factor in the manufacture of dengue (DENV) diagnostic kits is the lack of large-scale production of the non-structural 1 (NS1) protein (antigen) to be used in the capture of antibodies from the blood serum of infected patients. In this work, we use plant biotechnology and genetic engineering as tools for the study of protein production for research and commercial purposes. Gene transfer, integration and expression in plants is a valid strategy for obtaining large-scale and low-cost heterologous protein production. The authors produced NS1 protein of the dengue virus serotype 2 (NS1DENV2) in the Arabidopsis thaliana plant. Transgenic plants obtained by genetic transformation expressed the recombinant protein that was purified and characterized for diagnostic use. The yield was 203 μg of the recombinant protein per gram of fresh leaf. By in situ immunolocalization, transgenic protein was observed within the plant tissue, located in aggregates bodies. These antigens showed high sensitivity and specificity to both IgM (84.29% and 91.43%, respectively) and IgG (83.08% and 87.69%, respectively). The study goes a step further to validate the use of plants as a strategy for obtaining large-scale and efficient protein production to be used in dengue virus diagnostic tests.

Systematic Analysis of Cold Stress Response and Diurnal Rhythm Using Transcriptome Data in Rice Reveals the Molecular Networks Related to Various Biological Processes

Rice (Oryza sativa L.), a staple crop plant that is a major source of calories for approximately 50% of the human population, exhibits various physiological responses against temperature stress. These responses are known mechanisms of flexible adaptation through crosstalk with the intrinsic circadian clock. However, the molecular regulatory network underlining this crosstalk remains poorly understood. Therefore, we performed systematic transcriptome data analyses to identify the genes involved in both cold stress responses and diurnal rhythmic patterns. Here, we first identified cold-regulated genes and then identified diurnal rhythmic genes from those (119 cold-upregulated and 346 cold-downregulated genes). We defined cold-responsive diurnal rhythmic genes as CD genes. We further analyzed the functional features of these CD genes through Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses and performed a literature search to identify functionally characterized CD genes. Subsequently, we found that light-harvesting complex proteins involved in photosynthesis strongly associate with the crosstalk. Furthermore, we constructed a protein-protein interaction network encompassing four hub genes and analyzed the roles of the Stay-Green (SGR) gene in regulating crosstalk with sgr mutants. We predict that these findings will provide new insights in understanding the environmental stress response of crop plants against climate change.

Mobile TERMINAL FLOWER1 determines seed size in Arabidopsis

Seed size is a pivotal agronomic trait that links plant sexual reproduction and subsequent seedling establishment, and is affected by the timing of endosperm cellularization following endosperm proliferation after double fertilization. The molecular switch that controls the timing of endosperm cellularization has so far been largely unclear. Here, we report that the Arabidopsis TERMINAL FLOWER1 (TFL1) is a mobile regulator generated in the chalazal endosperm, and moves to the syncytial peripheral endosperm to mediate timely endosperm cellularization and seed size through stabilizing ABSCISIC ACID INSENSITIVE 5. We further show that Ras-related nuclear GTPases interact with TFL1 and regulate its trafficking to the syncytial peripheral endosperm. Our findings reveal TFL1 as an essential molecular switch for regulating endosperm cellularization and seed size. Generation of mobile TFL1 in the chalazal endosperm, which is close to maternal vascular tissues, could provide a hitherto-unknown means to control seed development by mother plants.

Nitrogen fertilizer-regulated plant-fungi interaction is related to root invertase-induced hexose generation

The mechanisms underlying nitrogen (N)-regulated plant-fungi interactions are not well understood. N application modulates plant carbohydrate (C) sinks and is involved in the overall plant-fungal association. We hypothesized that N regulates plant-fungi interactions by influencing the carbohydrate metabolism. The mutualistic fungus Phomopsis liquidambaris was found to prioritize host hexose resources through in vitro culture assays and in planta inoculation. Rice-Ph. liquidambaris systems were exposed to N gradients ranging from N-deficient to N-abundant conditions to study whether and how the sugar composition was involved in the dynamics of N-mediated fungal colonization. We found that root soluble acid invertases were activated, resulting in increased hexose fluxes in inoculated roots. These fluxes positively influenced fungal colonization, especially under N-deficient conditions. Further experiments manipulating the carbohydrate composition and root invertase activity through sugar feeding, chemical treatments and the use of different soil types revealed that the external disturbance of root invertase could reduce endophytic colonization and eliminate endophyte-induced host benefits under N-deficient conditions. Collectively, these results suggest that the activation of root invertase is related to N deficiency-enhanced endophytic colonization via increased hexose generation. Certain combinations of farmland ecosystems with suitable N inputs could be implemented to maximize the benefits of plant-fungi associations.

The Small GTPase Superfamily in Plants: A Conserved Regulatory Module with Novel Functions

Small GTP-binding proteins represent a highly conserved signaling module in eukaryotes that regulates diverse cellular processes such as signal transduction, cytoskeletal organization and cell polarity, cell proliferation and differentiation, intracellular membrane trafficking and transport vesicle formation, and nucleocytoplasmic transport. These proteins function as molecular switches that cycle between active and inactive states, and this cycle is linked to GTP binding and hydrolysis. In this review, the roles of the plant complement of small GTP-binding proteins in these cellular processes are described, as well as accessory proteins that control their activity, and current understanding of the functions of individual members of these families in plants-with a focus on the model organism Arabidopsis-is presented. Some potential novel roles of these GTPases in plants, relative to their established roles in yeast and/or animal systems, are also discussed.

The Saltol QTL-localized transcription factor OsGATA8 plays an important role in stress tolerance and seed development in Arabidopsis and rice

GATA represents a highly conserved family of transcription factors reported in organisms ranging from fungi to angiosperms. A member of this family, OsGATA8, localized within the Saltol QTL in rice, has been reported to be induced by salinity, drought, and ABA. However, its precise role in stress tolerance has not yet been elucidated. Using genetic, molecular, and physiological analyses, in this study we show that OsGATA8 increases seed size and tolerance to abiotic stresses in both Arabidopsis and rice. Transgenic lines of rice were generated with 3-fold overexpression of OsGATA8 compared to the wild-type together with knockdown lines with 2-fold lower expression. The overexpressing lines showed higher biomass accumulation and higher photosynthetic efficiency in seedlings compared to the wild-type and knockdown lines under both normal and salinity-stress conditions. OsGATA8 appeared to be an integrator of diverse cellular processes, including K+/Na+ content, photosynthetic efficiency, relative water content, Fv/Fm ratio, and the stability to sub-cellular organelles. It also contributed to maintaining yield under stress, which was ~46% higher in overexpression plants compared with the wild-type. OsGATA8 produced these effects by regulating the expression of critical genes involved in stress tolerance, scavenging of reactive oxygen species, and chlorophyll biosynthesis.

Rice Senescence-Induced Receptor-Like Kinase (OsSRLK) Is Involved in Phytohormone-Mediated Chlorophyll Degradation

Chlorophyll breakdown is a vital catabolic process of leaf senescence as it allows the recycling of nitrogen and other nutrients. In the present study, we isolated rice senescence-induced receptor-like kinase (OsSRLK), whose transcription was upregulated in senescing rice leaves. The detached leaves of ossrlk mutant (ossrlk) contained more green pigment than those of the wild type (WT) during dark-induced senescence (DIS). HPLC and immunoblot assay revealed that degradation of chlorophyll and photosystem II proteins was repressed in ossrlk during DIS. Furthermore, ultrastructural analysis revealed that ossrlk leaves maintained the chloroplast structure with intact grana stacks during dark incubation; however, the retained green color and preserved chloroplast structures of ossrlk did not enhance the photosynthetic competence during age-dependent senescence in autumn. In ossrlk, the panicles per plant was increased and the spikelets per panicle were reduced, resulting in similar grain productivity between WT and ossrlk. By transcriptome analysis using RNA sequencing, genes related to phytohormone, senescence, and chlorophyll biogenesis were significantly altered in ossrlk compared to those in WT during DIS. Collectively, our findings indicate that OsSRLK may degrade chlorophyll by participating in a phytohormone-mediated pathway.

OsSPL10, a SBP-Box Gene, Plays a Dual Role in Salt Tolerance and Trichome Formation in Rice (Oryza sativa L.)

Salinity is one of the major abiotic stress factors limiting rice production. Glabrousness is a trait of agronomic importance in rice (Oryza sativa L.). We previously found a single-gene recessive mutant sst, which displayed increased salt tolerance and glabrous leaf and glume without trichomes, and identified an SBP-box gene OsSPL10 as the candidate of the SST gene. In this study, OsSPL10-knockout and OsSPL10-overexpression mutants were created to check the function of the gene. The knockout mutants exhibited enhanced salt tolerance and glabrous leaves and glumes as expected, while the overexpression mutants showed opposite phenotypes, in which both salt sensitivity and trichome density on leaf and glume were increased. These results clearly confirmed that OsSPL10 is SST, and suggested that OsSPL10 controls the initiation rather than the elongation of trichomes. In addition, expression analysis indicated that OsSPL10 was preferentially expressed in young panicle and stem, and protein OsSPL10 was localized in nucleus. Taken together, OsSPL10 negatively controls salt tolerance but positively controls trichome formation in rice.

Nicotiana benthamiana RanBP1-1 Is Involved in the Induction of Disease Resistance via Regulation of Nuclear-Cytoplasmic Transport of Small GTPase Ran

Plant cells enhance the tolerances to abiotic and biotic stresses via recognition of the stress, activation and nuclear import of signaling factors, up-regulation of defense genes, nuclear export of mRNA and translation of defense proteins. Nuclear pore-mediated transports should play critical roles in these processes, however, the regulatory mechanisms of nuclear-cytoplasmic transport during stress responses are largely unknown. In this study, a regulator of nuclear export of RNA and proteins, NbRanBP1-1 (Ran-binding protein1-1), was identified as an essential gene for the resistance of Nicotiana benthamiana to potato blight pathogen Phytophthora infestans. NbRanBP1-1-silenced plants showed delayed accumulation of capsidiol, a sesquiterpenoid phytoalexin, in response to elicitor treatment, and reduced resistance to P. infestans. Abnormal accumulation of mRNA was observed in NbRanBP1-1-silenced plants, indicating that NbRanBP1-1 is involved in the nuclear export of mRNA. In NbRanBP1-1-silenced plants, elicitor-induced expression of defense genes, NbEAS and NbWIPK, was not affected in the early stage of defense induction, but the accumulation of NbWIPK protein was reduced. Nuclear export of the small G-protein NbRan1a was activated during the induction of plant defense, whereas this process was compromised in NbRanBP1-1-silenced plants. Silencing of genes encoding the nuclear pore proteins, Nup75 and Nup160, also caused abnormal nuclear accumulation of mRNA, defects in the nuclear export of NbRan1a, and reduced production of capsidiol, resulting in decreased resistance to P. infestans. These results suggest that nuclear export of NbRan is a key event for defense induction in N. benthamiana, and both RanBP1-1 and nucleoporins play important roles in the process.

Plant Hormone Signaling Crosstalks between Biotic and Abiotic Stress Responses

In the natural environment, plants are often bombarded by a combination of abiotic (such as drought, salt, heat or cold) and biotic (necrotrophic and biotrophic pathogens) stresses simultaneously. It is critical to understand how the various response pathways to these stresses interact with one another within the plants, and where the points of crosstalk occur which switch the responses from one pathway to another. Calcium sensors are often regarded as the first line of response to external stimuli to trigger downstream signaling. Abscisic acid (ABA) is a major phytohormone regulating stress responses, and it interacts with the jasmonic acid (JA) and salicylic acid (SA) signaling pathways to channel resources into mitigating the effects of abiotic stresses versus defending against pathogens. The signal transduction in these pathways are often carried out via GTP-binding proteins (G-proteins) which comprise of a large group of proteins that are varied in structures and functions. Deciphering the combined actions of these different signaling pathways in plants would greatly enhance the ability of breeders to develop food crops that can thrive in deteriorating environmental conditions under climate change, and that can maintain or even increase crop yield.

Differential response to heat stress in outer and inner onion bulb scales

The formation of brown protective skin in onion bulbs can be induced by rapid post-harvest heat treatment. Onions that are peeled to different depths and are exposed to heat stress show that only the outer scales form the dry brown skin, whereas the inner scales maintain high water content and do not change color. Our study demonstrates that browning of the outer scale during heat treatment is due to an enzymatic process that is associated with high levels of oxidation components, such as peroxidase and quercetin glucoside. De novo transcriptome analysis revealed differential molecular responses of the outer and inner scales to heat stress. Genes involved in lipid metabolism, oxidation pathways, and cell-wall modification were highly expressed in the outer scale during heating. Defense response-related genes such as those encoding heat-shock proteins, antioxidative stress defense, or production of osmoprotectant metabolites were mostly induced in the inner scale in response to heat exposure. These transcriptomic data led to a conceptual model that suggests sequential processes for the development of browning and desiccation of the outer scale versus processes associated with defense response and heat tolerance in the inner scales.

The nuclear pore complex: a strategic platform for regulating cell signaling

Contents Summary 25 I Introduction 25 II. Structural organization of the NPC 26 III. The role of NPCs in immune signaling 26 IV. The role of NPCs in hormone signaling 28 V. Conclusions 29 Acknowledgements 29 References 29 SUMMARY: Nuclear pore complexes (NPCs) are fundamental components of the eukaryotic cell. They perforate the nuclear envelope and serve as highly selective transport gates that enable bi-directional macromolecule exchange between the nucleus and cytoplasm. Recent studies illustrate that the NPC is not a static structural channel but a flexible environment and strategic player during nuclear signaling. The constitutional and conformational dynamics of the NPC allow it to tailor nucleocytoplasmic transport activities and define specific signaling output in response to various cellular and environmental cues. In this Insight, we review the roles of NPC constituents in immune activation and hormone signaling in plants, and discuss the possible role of the NPC as a legitimate platform for regulating cell signaling.

TaNTF2, a contributor for wheat resistance to the stripe rust pathogen

Nuclear Transport Factor 2 (NTF2) functions as a critical regulator in balancing the GTP-and GDP-bound forms of Ran, a class of evolutionarily conserved small GTP-binding protein. During the incompatible interaction between wheat-Puccinia striiformis f. sp. tritici (Pst), a cDNA fragment encoding a putative wheat NTF2 gene was found to be significantly induced, suggesting a potential role in wheat resistance to Pst. In this work, the full length of TaNTF2 was obtained, with three copies located on 7A, 7B and 7D chromosomes, respectively. QRT-PCR further verified the up-regulated expression of TaNTF2 in response to avirulent Pst. In addition, TaNTF2 was also induced by exogenous hormone applications, especially JA treatment. Transient expression of TaNTF2 in tobacco cells confirmed its subcellular localization in the cytoplasm, perinuclear area and nucleus. And virus induced gene silencing (VIGS) was used to identify the function of TaNTF2 during an incompatible wheat-Pst interaction. When TaNTF2 was knocked down, resistance of wheat to avirulentPst was decreased, with a bigger necrotic spots, and higher numbers of hyphal branches and haustorial mother cells. Our results demonstrated that TaNTF2 was a contributor for wheat resistance to the stripe rust pathogen, which will help to comprehensively understand the NTF2/Ran modulating mechanism in wheat-Pst interaction.

Proteomics and Phosphoproteomics of Heat Stress-Responsive Mechanisms in Spinach

Elevated temperatures limit plant growth and reproduction and pose a growing threat to agriculture. Plant heat stress response is highly conserved and fine-tuned in multiple pathways. Spinach (Spinacia oleracea L.) is a cold tolerant but heat sensitive green leafy vegetable. In this study, heat adaptation mechanisms in a spinach sibling inbred heat-tolerant line Sp75 were investigated using physiological, proteomic, and phosphoproteomic approaches. The abundance patterns of 911 heat stress-responsive proteins, and phosphorylation level changes of 45 phosphoproteins indicated heat-induced calcium-mediated signaling, ROS homeostasis, endomembrane trafficking, and cross-membrane transport pathways, as well as more than 15 transcription regulation factors. Although photosynthesis was inhibited, diverse primary and secondary metabolic pathways were employed for defense against heat stress, such as glycolysis, pentose phosphate pathway, amino acid metabolism, fatty acid metabolism, nucleotide metabolism, vitamin metabolism, and isoprenoid biosynthesis. These data constitute a heat stress-responsive metabolic atlas in spinach, which will springboard further investigations into the sophisticated molecular mechanisms of plant heat adaptation and inform spinach molecular breeding initiatives.

Plant Translation Initiation Complex eIFiso4F Directs Pokeweed Antiviral Protein to Selectively Depurinate Uncapped Tobacco Etch Virus RNA

Pokeweed antiviral protein (PAP) is a ribosome inactivating protein (RIP) that depurinates the sarcin/ricin loop (SRL) of rRNA, inhibiting protein synthesis. PAP depurinates viral RNA, and in doing so, lowers the infectivity of many plant viruses. The mechanism by which PAP accesses uncapped viral RNA is not known, impeding scientists from developing effective antiviral agents for the prevention of the diseases caused by uncapped RNA viruses. Kinetic rates of PAP interacting with tobacco etch virus (TEV) RNA, in the presence and absence of eIFiso4F, were examined, addressing how the eIF affects selective PAP targeting and depurination of the uncapped viral RNA. PAP-eIFs copurification assay and fluorescence resonance energy transfer demonstrate that PAP forms a ternary complex with the eIFiso4G and eIFiso4E, directing the depurination of uncapped viral RNA. eIFiso4F selectively targets PAP to depurinate TEV RNA by increasing PAP's specificity constant for uncapped viral RNA 12-fold, when compared to the depurination of an oligonucleotide RNA that mimics the SRL of large rRNA, and cellular capped luciferase mRNA. This explains how PAP is able to lower infectivity of pokeweed viruses, while preserving its own ribosomes and cellular RNA from depurination: PAP utilizes cellular eIFiso4F in a novel strategy to target uncapped viral RNA. It may be possible to modulate and utilize these PAP-eIFs interactions for their public health benefit; by repurposing them to selectively target PAP to depurinate uncapped viral RNA, many plant and animal diseases caused by these viruses could be alleviated.

Proteomic profiling of vetiver grass (Chrysopogon zizanioides) under 2,4,6-trinitrotoluene (TNT) stress

Vetiver grass is an ideal plant for 2,4,6-trinitrotoluene (TNT) phytoremediation, due to its ability to tolerate and metabolize TNT as previously reported. The current study is the first attempt to investigate the changes in the proteomic profile of a plant under TNT stress. Vetiver plants were grown in nutrient media with varying concentrations of TNT (0, 25, 50, and 100 mg L^-1) for 10 days. Although the plants appeared healthy, significant biomass reductions (p = 0.0008) were observed in treated plants. Total proteins in the root decreased significantly (p = 0.0003). Proteomic analysis of root proteins revealed the downregulation of functional proteins involved in key cellular mechanisms such as transcription, ribosome biogenesis, nucleo-cytoplasmic transport of proteins, protein glycosylation, and translation. Growth-related proteins were downregulated; plant defense proteins were upregulated at lower TNT concentrations but downregulated at higher concentrations. Comprehensive understanding of changes in the proteomic profile provides important clues to the mechanism of TNT stress response and tolerance in vetiver.

Hybrid weakness in a rice interspecific hybrid is nitrogen-dependent, and accompanied by changes in gene expression at both total transcript level and parental allele partitioning

Background

Hybrid weakness, a phenomenon opposite to heterosis, refers to inferior growth and development in a hybrid relative to its pure-line parents. Little attention has been paid to the phenomenological or mechanistic aspect of hybrid weakness, probably due to its rare occurrence.

Methodology/Principal findings

Here, using a set of interspecific triploid F1 hybrids between Oryza sativa, ssp. japonica (genome AA) and a tetraploid wild rice species, O. alta (genome, CCDD), we investigated the phenotypic and physiological differences between the F1 hybrids and their parents under normal and nitrogen-limiting conditions. We quantified the expression levels of 21 key genes involved in three important pathways pertinent to the assayed phenotypic and physiological traits by real-time qRT-PCR. Further, we assayed expression partitioning of parental alleles for eight genes in the F1 hybrids relative to the in silico “hybrids” (parental cDNA mixture) under both normal and N-limiting conditions by using locus-specific cDNA pyrosequencing.

Conclusions/Significance

We report that the F1 hybrids showed weakness in several phenotypic traits at the final seedling-stage compared with their corresponding mid-parent values (MPVs). Nine of the 21 studied genes showed contrasted expression levels between hybrids and parents (MPVs) under normal vs. N-limiting conditions. Interestingly, under N-limiting conditions, the overtly enhanced partitioning of maternal allele expression in the hybrids for eight assayed genes echo their attenuated hybrid weakness in phenotypes, an observation further bolstered by more resemblance of hybrids to the maternal parent under N-limiting conditions compared to normal conditions in a suite of measured physiological traits. Our observations suggest that both overall expression level and differential partitioning of parental alleles of critical genes contribute to condition-specific hybrid weakness.

Aluminum Enhances Growth and Sugar Concentration, Alters Macronutrient Status and Regulates the Expression of NAC Transcription Factors in Rice

Aluminum (Al) is a beneficial element for some plant species, especially when used at low concentrations. Though some transcription factors are induced by exposure to this element, no data indicate that Al regulates the expression of NAC genes in rice. In this study we tested the effect of applying 200 μM Al on growth, chlorophyll, amino acids, sugars, macronutrient concentration and regulation of NAC transcription factors gene expression in 24-day-old plants of four rice (Oryza sativa ssp. indica) cultivars: Cotaxtla, Tres Ríos, Huimanguillo and Temporalero, grown hydroponically under greenhouse conditions. Twenty days after treatment, we observed that Al enhanced growth in the four cultivars studied. On average, plants grown in the presence of Al produced 140% more root dry biomass and were 30% taller than control plants. Cotaxtla and Temporalero showed double the root length, while Huimanguillo and Cotaxtla had three times more root fresh biomass and 2.5 times more root dry biomass. Huimanguillo plants showed 1.5 times more shoot height, while Cotaxtla had almost double the root dry biomass. With the exception of Tres Ríos, the rest of the cultivars had almost double the chlorophyll concentration when treated with Al, whereas amino acid and proline concentrations were not affected by Al. Sugar concentration was also increased in plants treated with Al, almost 11-fold in comparison to the control. Furthermore, we observed a synergic response of Al application on P and K concentration in roots, and on Mg concentration in shoots. Twenty-four hours after Al treatment, NAC transcription factors gene expression was measured in roots by quantitative RT-PCR. Of the 57 NAC transcription factors genes primer-pairs tested, we could distinguish that 44% (25 genes) showed different expression patterns among rice cultivars, with most of the genes induced in Cotaxtla and Temporalero plants. Of the 25 transcription factors up-regulated, those showing differential expression mostly belonged to the NAM subfamily (56%). We conclude that Al improves growth, increases sugar concentration, P and K concentrations in roots, and Mg concentration in shoots, and report, for the first time, that Al differentially regulates the expression of NAC transcription factors in rice.

Extensive transcriptomic studies on the roles played by abscisic acid and auxins in the development and ripening of strawberry fruits

Strawberry is an ideal model for studying the molecular biology of the development and ripening of non-climacteric fruits. Hormonal regulation of gene expression along all these processes in strawberries is still to be fully elucidated. Although auxins and ABA have been pointed out as the major regulatory hormones, few high-throughput analyses have been carried out to date. The role for ethylene and gibberellins as regulatory hormones during the development and ripening of the strawberry fruit remain still elusive. By using a custom-made and high-quality oligo microarray platform done with over 32,000 probes including all of the genes actually described in the strawberry genome, we have analysed the expression of genes during the development and ripening in the receptacles of these fruits. We classify these genes into two major groups depending upon their temporal and developmental expression. First group are genes induced during the initial development stages. The second group encompasses genes induced during the final maturation and ripening processes. Each of these two groups has been also divided into four sub-groups according their pattern of hormonal regulation. By analyzing gene expression, we clearly show that auxins and ABA are the main and key hormones that combined or independently are responsible of the development and ripening process. Auxins are responsible for the receptacle fruit development and, at the same time¸ prevent ripening by repressing crucial genes. ABA regulates the expression of the vast majority of genes involved in the ripening. The main genes expressed under the control of these hormones are presented and their physiological rule discussed. We also conclude that ethylene and gibberellins do not seem to play a prominent role during these processes.

Proteomic analysis reveals growth inhibition of soybean roots by manganese toxicity is associated with alteration of cell wall structure and lignification

Plant roots, the hidden half of plants, play a vital role in manganese (Mn) toxicity tolerance. However, molecular mechanisms underlying root adaptation to Mn toxicity remain largely unknown. In this study, soybean (Glycine max) was used to investigate alterations of root morphology and protein profiles in response to Mn toxicity. Results showed that soybean root growth was significantly inhibited by Mn toxicity. Subsequent proteomic analysis revealed that 31 proteins were successfully identified via MALDI TOF/TOF MS analysis including 21 unique up-regulated and 6 unique down-regulated proteins, which are mainly related to cell wall metabolism, protein metabolism and signal transduction. qRT-PCR analysis revealed that corresponding gene transcription patterns were correlated with accumulation of 14 of 21 up-regulated proteins, but only 1 of 6 down-regulated proteins, suggesting that most excess Mn up-regulated proteins are controlled at the transcriptional levels, while down-regulated proteins are controlled at the post-transcriptional levels. Furthermore, changes in abundances of GTP-binding nuclear protein Ran-3, expansin-like B1-like protein, dirigent protein and peroxidase 5-like protein strongly suggested that alteration of root cell wall structure and lignification might be associated with inhibited root growth. Taken together, this study was helpful to further understandings of adaptive strategies of legume roots to Mn toxicity.

SIGNIFICANCE

This study highlighted the effects of Mn toxicity on soybean root growth and its proteome profiles. Excess Mn treatments inhibited root growth. Comparative proteomic analysis was performed to analyze the changes in protein profiles of soybean roots in response to Mn toxicity. A total of 31 root proteins with differential abundances were identified and predominantly associated with signal transduction and cell wall metabolism. Among them, the abundances of the GTP-binding nuclear protein Ran-3 and Ran-binding protein 1 were significantly increased, suggesting that the proteins could be involved in the signaling network in soybean roots responsive to Mn toxicity. Interestingly, three 14-3-3 proteins were decreased by excess Mn at protein but not mRNA levels, suggesting that these proteins could be regulated at post-transcriptional modification under Mn excess conditions. Furthermore, changes in abundances of expansin-like B1-like protein, peroxidase 5-like protein, dirigent protein 2-like protein and dirigent protein strongly suggested that Mn toxicity could influence root cell wall modification, and thus inhibit root growth. This study provided significant insights into the potential molecular mechanisms underlying soybean root adaptation to Mn toxicity, which was mainly through alteration of root cell wall structure and lignification.

Small GTP-binding protein PdRanBP regulates vascular tissue development in poplar

BACKGROUND

Previous research has demonstrated that ectopic expression of Ran-binding protein (RanBP) in Arabidopsis results in more axillary buds and reduced apical dominance compared to WT plants. However, the function of RanBP in poplar, which has very typical secondary growth, remains unclear. Here, the Populus deltoides (Marsh.) RanBP gene (PdRanBP) was isolated and functionally characterized by ectopic expression in a hybrid poplar (P. davidiana Dode × P. bolleana Lauche).

RESULTS

PdRanBP was predominantly expressed in leaf buds and tissues undergoing secondary wall expansion, including immature xylem and immature phloem in the stem. Overexpression of PdRanBP in poplar increased the number of sylleptic branches and the proportion of cells in the G2 phase of the cell cycle, retarded plant growth, consistently decreased the size of the secondary xylem and secondary phloem zones, and reduced the expression levels of cell wall biosynthesis genes. The downregulation of PdRanBP facilitated secondary wall expansion and increased stem height, the sizes of the xylem and phloem zones, and the expression levels of cell wall biosynthesis genes.

CONCLUSIONS

These results suggest that PdRanBP influences the apical and radial growth of poplar trees and that PdRanBP may regulate cell division during cell cycle progression. Taken together, our results demonstrated that PdRanBP is a nuclear, vascular tissue development-associated protein in P. deltoides.

Ras-Related Nuclear Protein Ran3B Gene Is Involved in Hormone Responses in the Embryogenic Callus of Dimocarpus longan Lour

Ras-related guanosine triphosphate (GTP)-binding nuclear protein (Ran) GTPases function as molecular switches and regulate diverse cellular events in eukaryotes. Our previous work suggested that DlRan3B is active during longan (Dimocarpus longan Lour.) somatic embryogenesis (SE) processes. Herein, subcellular localization of DlRan3B was found to be localized in the nucleus and expression profiling of DlRan3B was performed during longan SE and after exposure to plant hormones (indoleacetic acid (IAA), gibberellin A3 (GA₃), salicylic acid (SA), methyl jasmonte (MeJA), and abscisic acid (ABA)). We cloned and sequenced 1569 bp of 5′-flanking sequence of DlRan3B (GenBank: JQ279697). Bioinformatic analysis indicated that the promoter contained plant hormone-related regulatory elements. Deletion analysis and responses to hormones identified stimulative and repressive regulatory elements in the DlRan3B promoter. The key elements included those responding to auxin, gibberellin, SA, MeJA, and ABA. DlRan3B was located in the nucleus and accumulated in the late stage of longan SE. The expression of DlRan3B was significantly induced by IAA, GA₃, and ABA, but suppressed by SA and MeJA. Promoter transcription was induced by IAA and GA₃, but suppressed by SA. Thus, DlRan3B might participate in auxin, gibberellin, and ABA responses during longan late SE, and DlRan3B is involved in phytohormone responsiveness.

The Small G Protein AtRAN1 Regulates Vegetative Growth and Stress Tolerance in Arabidopsis thaliana

The evolutionarily conserved small G-protein Ran plays important role in nuclear translocation of proteins, cell cycle regulation, and nuclear envelope maintenance in mammalian cells and yeast. Arabidopsis Ran proteins are encoded by a family of four genes and are highly conserved at the protein level. However, their biological functions are poorly understood. We report here that AtRAN1 plays an important role in vegetative growth and the molecular improvement of stress tolerance in Arabidopsis. AtRAN1 overexpression promoted vegetative growth and enhanced abiotic tolerance, while the atran1 atran3 double mutant showed higher freezing sensitivity than WT. The AtRAN1 gene is ubiquitously expressed in plants, and the expression levels are higher in the buds, flowers and siliques. Subcellular localization results showed that AtRAN1 is mainly localized in the nucleus, with some present in the cytoplasm. AtRAN1 could maintain cell division and cell cycle progression and promote the formation of an intact nuclear envelope, especially under freezing conditions.

Molecular cloning, structural and expression profiling of DlRan genes during somatic embryogenesis in Dimocarpus longan Lour

To clone and examine expression profiles of DlRan genes during somatic embryogenesis in Dimocarpus longan Lour. Thirty cDNA sequences and two genomic sequences encoding DlRan proteins were isolated from longan embryogenic cultures. Structural analysis of DlRan genes revealed that the longan Ran gene family is more expanded than that of Arabidopsis. Expression analysis of DlRan genes during somatic embryogenesis uncovered a high abundance of DlRan genes in early embryogenic cultures and heart- and torpedo-shaped embryos. The expression of DlRan genes in embryogenic calli was affected by exogenous 2,4-dichlorophenoxyacetic acid treatment. DlRan is involved in 2,4-D induced somatic embryogenesis and development of somatic embryos in longan.

Phloem sap proteome studied by iTRAQ provides integrated insight into salinity response mechanisms in cucumber plants

Cucumber is an economically important crop as well as a model system for plant vascular biology. Salinity is one of the major environmental factors limiting plant growth. Here, we used an iTRAQ-based quantitative proteomics approach for comparative analysis of protein abundances in cucumber phloem sap in response to salt. A total of 745 distinct proteins were identified and 111 proteins were differentially expressed upon salinity in sensitive and tolerant cultivars, of which 69 and 65 proteins changed significantly in sensitive and tolerant cultivars, respectively. A bioinformatics analysis indicated that cucumber phloem employed a combination of induced metabolism, protein turnover, common stress response, energy and transport, signal transduction and regulation of transcription, and development proteins as protection mechanisms against salinity. The proteins that were mapped to the carbon fixation pathway decreased in abundance in sensitive cultivars and had no change in tolerant cultivars under salt stress, suggesting that this pathway may promote salt tolerance by stabilizing carbon fixation and maintaining the essential energy and carbohydrates in tolerant cultivars. This study leads to a better understanding of the salinity mechanism in cucumber phloem and provides a list of potential gene targets for the further engineering of salt tolerance in plants.

Overexpressing components of the nuclear transport apparatus causes severe growth symptoms in tobacco leaves

Regulating nucleo-cytoplasmic transport of RNA and protein is a key cellular control point. Perturbing the function of plant nuclear transport components can cause significant developmental defects and in this report we add an important line to this evidence. Overexpression of AtRAN1 or AtNUP62 in Nicotiana benthamiana causes significant damage to leaf tissue. This demonstrates that the precise control of nuclear transport is an important aspect of maintaining tissue integrity.

Overexpression of Ran gene from Lepidium latifolium L. (LlaRan) renders transgenic tobacco plants hypersensitive to cold stress

Ran is a multifunctional small GTPase involved in important cellular activities like nucleocytoplasmic transport, mitotic spindle assembly, nuclear envelope formation, etc., but is also known to be differentially expressed in response to abiotic stress, particularly low temperature. We have over-expressed Lepidium latifolium (Fam. Brassicaceae) Ran gene in tobacco to study the response of the plants to cold stress (24 h; 4 °C). Transformation of the tobacco plants was verified using PCR targeting Ran gene and co-transformed selectable marker gene nptII. Segregation in Mendelian ratios was validated in five transgenic lines by germination of T1 and T2 seeds on moist filter papers containing 150 mg/l kanamycin. Higher levels of electrolyte leakage and lipid peroxidation pointed towards hypersensitivity of plants. Similarly, lesser proline accumulation compared to wild types also indicated susceptibility of plants to death under chilling conditions. Specific activity of antioxidant enzymes superoxide dismutase and glutathione reductase was also measured under stressed and control conditions. A variation was observed across the different lines, and four out of five lines showed lesser specific activity compared to wild type plants, thus indicating reduced capability of scavenging free radicals. In totality, a strong evidence on induced hypersensitivity to cold stress has been collected which may further be helpful in designing appropriate strategies for engineering crop plants for survival under cold stress conditions.

Arabidopsis RAN1 mediates seed development through its parental ratio by affecting the onset of endosperm cellularization

Although previous studies have demonstrated that endosperm development is influenced by its parental genome constitution, the genetic basis and molecular mechanisms that control parent-of-origin effects require further elucidation. Here we show that the Ras-related nuclear protein 1 (RAN1) regulates endosperm development in Arabidopsis thaliana. Reciprocal crosses between wild-type (WT) and transgenic lines misexpressing RAN1 (msRAN1) gave rise to small F1 seeds when RAN1 down-regulated/up-regulated individuals were used as a male/female parent; in contrast, F1 seeds were aborted when RAN1 down-regulated/up-regulated plants were used as a female/male parent, suggesting that seed development is affected by the parental genome ratio of RAN1. Whereas RAN1 expression in wild-type plants is reduced before the onset of endosperm cellularization, F1 seeds from reciprocal crosses between WT and msRAN1 showed abnormal endosperm cellularization and ectopic expression of RAN1. The expression of MINISEED3 (MINI3)-a gene that also controls endosperm cellularization-was also affected in these reciprocal crosses, and the misregulation of MINI3 activity rescued F1 seeds when msRAN1 plants were used in reciprocal crosses. Taken together, our results suggest that the parental ratio of RAN1 regulates the onset of endosperm cellularization through its genetic interaction with MINI3.

RAN1 is involved in plant cold resistance and development in rice (Oryza sativa)

Of the diverse abiotic stresses, low temperature is one of the major limiting factors that lead to a series of morphological, physiological, biochemical, and molecular changes in plants. Ran, an evolutionarily conserved small G-protein family, has been shown to be essential for the nuclear translocation of proteins. It also mediates the regulation of cell cycle progression in mammalian cells. However, little is known about Ran function in rice (Oryza sativa). We report here that Ran gene OsRAN1 is essential for the molecular improvement of rice for cold tolerance. Ran also affects plant morphogenesis in transgenic Arabidopsis thaliana. OsRAN1 is ubiquitously expressed in rice tissues with the highest expression in the spike. The levels of mRNA encoding OsRAN1 were greatly increased by cold and indoleacetic acid treatment rather than by addition of salt and polyethylene glycol. Further, OsRAN1 overexpression in Arabidopsis increased tiller number, and altered root development. OsRAN1 overexpression in rice improves cold tolerance. The levels of cellular free Pro and sugar levels were highly increased in transgenic plants under cold stress. Under cold stress, OsRAN1 maintained cell division and cell cycle progression, and also promoted the formation of an intact nuclear envelope. The results suggest that OsRAN1 protein plays an important role in the regulation of cellular mitosis and the auxin signalling pathway.

OsGA2ox5, a gibberellin metabolism enzyme, is involved in plant growth, the root gravity response and salt stress

Gibberellin (GA) 2-oxidases play an important role in the GA catabolic pathway through 2β-hydroxylation. There are two classes of GA2oxs, i.e., a larger class of C₁₉-GA2oxs and a smaller class of C₂₀-GA2oxs. In this study, the gene encoding a GA 2-oxidase of rice, Oryza sativa GA 2-oxidase 5 (OsGA2ox5), was cloned and characterized. BLASTP analysis showed that OsGA2ox5 belongs to the C₂₀-GA2oxs subfamily, a subfamily of GA2oxs acting on C₂₀-GAs (GA₁₂, GA₅₃). Subcellular localization of OsGA2ox5-YFP in transiently transformed onion epidermal cells revealed the presence of this protein in both of the nucleus and cytoplasm. Real-time PCR analysis, along with GUS staining, revealed that OsGA2ox5 is expressed in the roots, culms, leaves, sheaths and panicles of rice. Rice plants overexpressing OsGA2ox5 exhibited dominant dwarf and GA-deficient phenotypes, with shorter stems and later development of reproductive organs than the wild type. The dwarfism phenotype was partially rescued by the application of exogenous GA3 at a concentration of 10 µM. Ectopic expression of OsGA2ox5 cDNA in Arabidopsis resulted in a similar phenotype. Real-time PCR assays revealed that both GA synthesis-related genes and GA signaling genes were expressed at higher levels in transgenic rice plants than in wild-type rice; OsGA3ox1, which encodes a key enzyme in the last step of the bioactive GAs synthesis pathway, was highly expressed in transgenic rice. The roots of OsGA2ox5-ox plants exhibited increased starch granule accumulation and gravity responses, revealing a role for GA in root starch granule development and gravity responses. Furthermore, rice and Arabidopsis plants overexpressing OsGA2ox5 were more resistant to high-salinity stress than wild-type plants. These results suggest that OsGA2ox5 plays important roles in GAs homeostasis, development, gravity responses and stress tolerance in rice.

Transcriptomic events involved in melon mature-fruit abscission comprise the sequential induction of cell-wall degrading genes coupled to a stimulation of endo and exocytosis

BACKGROUND

Mature-fruit abscission (MFA) in fleshy-fruit is a genetically controlled process with mechanisms that, contrary to immature-fruit abscission, has not been fully characterized. Here, we use pyrosequencing to characterize the transcriptomes of melon abscission zone (AZ) at three stages during AZ-cell separation in order to understand MFA control at an early stage of AZ-activation.

PRINCIPAL FINDINGS

The results show that by early induction of MFA, the melon AZ exhibits major gene induction, while by late induction of MFA, melon AZ shows major gene repression. Although some genes displayed similar regulation in both early and late induction of abscission, such as EXT1-EXT4, EGase1, IAA2, ERF1, AP2D15, FLC, MADS2, ERAF17, SAP5 and SCL13 genes, the majority had different expression patterns. This implies that time-specific events occur during MFA, and emphasizes the value of characterizing multiple time-specific abscission transcriptomes. Analysis of gene-expression from these AZs reveal that a sequential induction of cell-wall-degrading genes is associated with the upregulation of genes involved in endo and exocytosis, and a shift in plant-hormone metabolism and signaling genes during MFA. This is accompanied by transcriptional activity of small-GTPases and synthaxins together with tubulins, dynamins, V-type ATPases and kinesin-like proteins potentially involved in MFA signaling. Early events are potentially controlled by down-regulation of MADS-box, AP2/ERF and Aux/IAA transcription-factors, and up-regulation of homeobox, zinc finger, bZIP, and WRKY transcription-factors, while late events may be controlled by up-regulation of MYB transcription-factors.

SIGNIFICANCE

Overall, the data provide a comprehensive view on MFA in fleshy-fruit, identifying candidate genes and pathways associated with early induction of MFA. Our comprehensive gene-expression profile will be very useful for elucidating gene regulatory networks of the MFA in fleshy-fruit.

Expression of a Medicago falcata small GTPase gene, MfARL1 enhanced tolerance to salt stress in Arabidopsis thaliana

To understand the role of small GTPases in response to abiotic stress, we isolated a gene encoding a small GTPase, designated MfARL1, from a subtracted cDNA library in Medicago falcata, a native legume species in semi-arid grassland in northern China. The function of MfARL1 in response to salt stress was studied by expressing MfARL1 in Arabidopsis. Wild-type (WT) and transgenic plants constitutively expressing MfARL1 showed comparable phenotype when grown under control conditions. Germination of seeds expressing MfARL1 was less suppressed by salt stress than that of WT seeds. Transgenic seedlings had higher survival rate than WT seedlings under salt stress, suggesting that expression of MfARL1 confers tolerance to salt stress. The physiological and molecular mechanisms underlying these phenomena were elucidated. Salt stress led to a significant decrease in chlorophyll contents in WT plants, but not in transgenic plants. Transgenic plants accumulated less amounts of H(2)O(2) and malondialdehyde than their WT counterparts under salt stress, which can be accounted for by the higher catalase activities, lower activities of superoxide dismutase, and peroxidase in transgenic plants than in WT plants. Transgenic plants displayed lower Na(+)/K(+) ratio due to less accumulation of Na(+) than wild-type under salt stress conditions. The lower Na(+)/K(+) ratio may result from less accumulation of Na(+) due to reduced expression of AtHKT1 that encodes Na(+) transporter in transgenic plants under salt stress. These findings demonstrate that MfARL1 encodes a novel stress-responsive small GTPase that is involved in tolerance to salt stress.

Dwarf and deformed flower 1, encoding an F-box protein, is critical for vegetative and floral development in rice (Oryza sativa L.)

Recent studies have shown that F-box proteins constitute a large family in eukaryotes, and play pivotal roles in regulating various developmental processes in plants. However, their functions in monocots are still obscure. In this study, we characterized a recessive mutant dwarf and deformed flower 1-1 (ddf1-1) in Oryza sativa (rice). The mutant is abnormal in both vegetative and reproductive development, with significant size reduction in all organs except the spikelet. DDF1 controls organ size by regulating both cell division and cell expansion. In the ddf1-1 spikelet, the specification of floral organs in whorls 2 and 3 is altered, with most lodicules and stamens being transformed into glume-like organs and pistil-like organs, respectively, but the specification of lemma/palea and pistil in whorls 1 and 4 is not affected. DDF1 encodes an F-box protein anchored in the nucleolus, and is expressed in almost all vegetative and reproductive tissues. Consistent with the mutant floral phenotype, DDF1 positively regulates B-class genes OsMADS4 and OsMADS16, and negatively regulates pistil specification gene DL. In addition, DDF1 also negatively regulates the Arabidopsis LFY ortholog APO2, implying a functional connection between DDF1 and APO2. Collectively, these results revealed that DDF1, as a newly identified F-box gene, is a crucial genetic factor with pleiotropic functions for both vegetative growth and floral organ specification in rice. These findings provide additional insights into the molecular mechanism controlling monocot vegetative and reproductive development.

Comparative proteomic profiles of Pinus monticola needles during early compatible and incompatible interactions with Cronartium ribicola

The proteomic profiles of primary needles from Cr2-resistant and cr2-susceptible Pinus monticola seedlings were analysed post Cronartium ribicola inoculation by 2-DE. One hundred-and-five protein spots exhibiting significant differential expression were identified using LC-MS/MS. Functional classification showed that the most numerous proteins are involved in defence signalling, oxidative burst, metabolic pathways, and other physiological processes. Our results revealed that differential expression of proteins in response to C. ribicola inoculation was genotype- and infection-stage dependent. Responsive proteins in resistant seedlings with incompatible white pine blister rust (WPBR) interaction included such well-characterized proteins as heat shock proteins (HSPs), reactive oxygen species (ROS) scavenging enzymes, and intermediate factors functioning in the signal transduction pathways triggered by well-known plant R genes, as well as new candidates in plant defence like sugar epimerase, GTP-binding proteins, and chloroplastic ribonucleoproteins. Fewer proteins were regulated in susceptible seedlings; most of them were in common with resistant seedlings and related to photosynthesis among others. Quantitative RT-PCR analysis confirmed HSP- and ROS-related genes played an important role in host defence in response to C. ribicola infection. To the best of our knowledge, this is the first comparative proteomics study on WPBR interactions at the early stages of host defence, which provides a reference proteomic profile for other five-needle pines as well as resistance candidates for further understanding of host resistance in the WPBR pathosystem.

Effects of salt stress on ion balance and nitrogen metabolism of old and young leaves in rice (Oryza sativa L.)

BACKGROUND

It is well known that salt stress has different effects on old and young tissues. However, it remains largely unexplored whether old and young tissues have different regulatory mechanism during adaptation of plants to salt stress. The aim of this study was to investigate whether salt stress has different effects on the ion balance and nitrogen metabolism in the old and young leaves of rice, and to compare functions of both organs in rice salt tolerance.

RESULTS

Rice protected young leaves from ion harm via the large accumulation of Na+ and Cl- in old leaves. The up-regulation of OsHKT1;1, OsHAK10 and OsHAK16 might contribute to accumulation of Na+ in old leaves under salt stress. In addition, lower expression of OsHKT1;5 and OsSOS1 in old leaves may decrease frequency of retrieving Na+ from old leaf cells. Under salt stress, old leaves showed higher concentration of NO3- content than young leaves. Up-regulation of OsNRT1;2, a gene coding nitrate transporter, might contribute to the accumulation of NO3- in the old leaves of salt stressed-rice. Salt stress clearly up-regulated the expression of OsGDH2 and OsGDH3 in old leaves, while strongly down-regulated expression of OsGS2 and OsFd-GOGAT in old leaves.

CONCLUSIONS

The down-regulation of OsGS2 and OsFd-GOGAT in old leaves might be a harmful response to excesses of Na+ and Cl-. Under salt stress, rice might accumulate Na+ and Cl- to toxic levels in old leaves. This might influence photorespiration process, reduce NH4+ production from photorespiration, and immediately down-regulate the expression of OsGS2 and OsFd-GOGAT in old leaves of salt stressed rice. Excesses of Na+ and Cl- also might change the pathway of NH4+ assimilation in old leaves of salt stressed rice plants, weaken GOGAT/GS pathway and elevate GDH pathway.