A rice really interesting new gene H2‐type E3 ligase,OsSIRH2‐14, enhances salinity tolerance via ubiquitin/26S proteasome‐mediated degradation of salt‐related proteins

Salinity is a deleterious abiotic stress factor that affects growth, productivity, and physiology of crop plants. Strategies for improving salinity tolerance in plants are critical for crop breeding programmes. Here, we characterized the rice (Oryza sativa) really interesting new gene (RING) H2‐type E3 ligase, OsSIRH2‐14 (previously named OsRFPH2‐14), which plays a positive role in salinity tolerance by regulating salt‐related proteins including an HKT‐type Na⁺ transporter (OsHKT2;1). OsSIRH2‐14 expression was induced in root and shoot tissues treated with NaCl. The OsSIRH2‐14‐EYFP fusion protein was predominately expressed in the cytoplasm, Golgi, and plasma membrane of rice protoplasts. In vitro pull‐down assays and bimolecular fluorescence complementation assays revealed that OsSIRH2‐14 interacts with salt‐related proteins, including OsHKT2;1. OsSIRH2‐14 E3 ligase regulates OsHKT2;1 via the 26S proteasome system under high NaCl concentrations but not under normal conditions. Compared with wild type plants, OsSIRH2‐14‐overexpressing rice plants showed significantly enhanced salinity tolerance and reduced Na⁺ accumulation in the aerial shoot and root tissues. These results suggest that the OsSIRH2‐14 RING E3 ligase positively regulates the salinity stress response by modulating the stability of salt‐related proteins.     

水稻OsRPK2基因的克隆及功能初步鉴定

类受体蛋白激酶基因OsRPK1在水稻的抗逆信号传导中起着重要作用。本研究扩增获得与OsRPK1高度同源的OsRPK2基因,构建p1300:35S:OsRPK2过表达载体后转化拟南芥。对35S:OsRPK2纯合体拟南芥进行抗逆性分析表明,在盐、ABA、PEG胁迫下,OsRPK2过表达拟南芥萌发率都明显低于野生型拟南芥,其幼苗的根长生长和成株生长状况方面比野生型拟南芥表现出更为明显的受抑现象。生理检测表明,盐胁迫处理后,与野生型拟南芥相比,35S:OsRPK2转基因拟南芥中叶绿素含量下降更为明显,脯氨酸上升量较小,丙二醛含量上升更为明显,这些内在生理机制使得OsRPK2过表达拟南芥抗逆性明显下降。通过对35S:OsRPK2拟南芥的qRTPCR检测发现,OsRPK2的过量表达使拟南芥抗逆信号通路下游的SAD、SOS3和FRY基因表达明显受到抑制,OsRPK2基因可能通过SOS和CDPK信号通路影响拟南芥的抗逆性。     

A Novel Little Membrane Protein Confers Salt Tolerance in Rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Plasma membrane proteins play critical roles in sensing and responding abiotic and biotic stresses in plants. In the present study, we characterized a previously unknown gene stress associated little protein 1 (SALP1) encoding a plasma membrane protein. SALP1, a small and plant-specific membrane protein, contains only 74 amino acid residues. SALP1 was constitutively expressed in various rice tissues while highly expressed in roots, leaf blade, and immature panicles. Expression analysis indicated that SALP1 was induced by various abiotic stresses and abscisic acid (ABA). Subcellular localization assay indicated that SALP1 was localized on plasma membrane in rice protoplast cells. Overexpressing of SALP1 in rice improved salt tolerance through increasing free proline contents and the expression level of OsP5CS gene, and balancing ion contents under salt stress. Moreover, SALP1 transgenic rice showed reduced sensitivity to ABA treatment, and expression level of SALP1 is not altered by ABI5-like 1 protein. Conclusively, SALP1, a novel membrane protein, is involved in salt tolerance through an ABA-independent signaling pathway in rice.     

Identification of elicitor‐responsive proteins in rice leaves by a proteomic approach

Experiments were conducted to identify the differentially expressed proteins in rice (Oryza sativa L.) plants after treatment with the glycoprotein elicitor CSB I, purified from ZC₁₃, a race of the rice blast fungus Magnaporthe grisea. The interactions of two near isogenic lines of rice, C101A51 and CO39, with ZC₁₃ resulted in completely incompatible and compatible types, respectively. Proteins were extracted from rice leaves at 12 and 24 h after treatment with CSB I. Temporal changes in total proteins were examined using 2‐DE. Among more than 900 protein spots reproducibly detected on each gel, 11 were up‐regulated, three were down‐regulated and seven were newly induced during, at a minimum, one time point. Twenty‐one differentially expressed proteins were identified by linear ion trap quadrupole (LTQ)‐MS/MS. The identified proteins were classified into six categories based on their putative function reported: (i) defense proteins (PR‐10a, PR‐5 and putative salt‐induced protein), (ii) signal transduction (nucleoside diphosphate kinase and putative profilin), (iii) ROS (Mn‐SOD, Cu/Zn‐SOD, GST and CAT), (iv) programmed cell death (translationally controlled tumor protein), (v) molecule biosynthesis (putative ribosomal protein S5, putative ribosomal protein L12, putative translational elongation factor Tu and putative chaperonin 21 precursor) and (vi) metabolism (putative fructose‐bisphosphate aldolase class‐I, putative malate dehydrogenase, cytoplasmic malate dehydrogenase, putative acid phosphatase, putative transketolase1 and gamma hydroxybutyrate dehydrogenase‐like protein). All of these proteins (except Cu/Zn‐SOD, putative acid phosphatase and translationally controlled tumor protein) were induced faster and to a higher degree in C101A51 than in CO39. These data suggest that the incompatible rice line may possess a more sensitive recognition system that can identify and react to specific chemical, biological or physical triggers in a more efficient manner, thus eliciting an early and fast defense response.     

The Response of Haloferax volcanii to Salt and Temperature Stress: A Proteome Study by Label‐Free Mass Spectrometry

In‐depth proteome analysis of the haloarchaeal model organism Haloferax volcanii has been performed under standard, low/high salt, and low/high temperature conditions using label‐free mass spectrometry. Qualitative analysis of protein identification data from high‐pH/reversed‐phase fractionated samples indicates 61.1% proteome coverage (2509 proteins), which is close to the maximum recorded values in archaea. Identified proteins match to the predicted proteome in their physicochemical properties, with only a small bias against low‐molecular‐weight and membrane‐associated proteins. Cells grown under low and high salt stress as well as low and high temperature stress are quantitatively compared to standard cultures by sequential window acquisition of all theoretical mass spectra (SWATH‐MS). A total of 2244 proteins, or 54.7% of the predicted proteome, are quantified across all conditions at high reproducibility, which allowed for global analysis of protein expression changes under these stresses. Of these, 2034 are significantly regulated under at least one stress condition. KEGG pathway enrichment analysis shows that several major cellular pathways are part of H. volcanii’s universal stress response. In addition, specific pathways (purine, cobalamin, and tryptophan) are affected by temperature stress. The most strongly downregulated proteins under all stress conditions, zinc finger protein HVO_2753 and ribosomal protein S14, are found oppositely regulated to their immediate genetic neighbors from the same operon.     

Proteomics Reveals New Salt Responsive Proteins Associated with Rice Plasma Membrane

The signaling processes in plants that initiate cellular responses to biotic and abiotic factors are believed to be located in the plasma membrane (PM). A better understanding of the PM proteome response to environmental stresses might lead to new strategies for improving stress-tolerant crops. A sub-cellular proteomics approach was applied to monitor changes in abundance of PM-associated protein in response to salinity, a key abiotic stress affecting rice productivity worldwide. Proteome was extracted from a root plasma-membrane-rich fraction of a rice salt tolerant variety, IR651, grown under saline and normal conditions. Comparative two-dimensional electrophoresis revealed that 24 proteins were differentially expressed in response to salt stress. From these, eight proteins were identified by mass spectrometry analysis. Most of the proteins identified are likely to be PM-associated and are known to be involved in several important mechanisms of plant adaptation to salt stress. These include regulation of PM pumps and channels, membrane structure, oxidative stress defense, signal transduction, protein folding, and the methyl cycle. To investigate the correlation between mRNA and protein level in response to salinity, we performed quantitative Real-Time PCR analysis of three genes that were salt responsive at the protein level, including 1,4-Benzoquinone reductase, a putative remorin and a hypersensitive induced response protein. No concordance was detected between the changes in levels of gene and protein expression. Our results indicate that the proteomics approach is suitable for expression analysis of membrane associated proteins under salt stress.     

Proteomics reveals new salt responsive proteins associated with rice plasma membrane

The signaling processes in plants that initiate cellular responses to biotic and abiotic factors are believed to be located in the plasma membrane (PM). A better understanding of the PM proteome response to environmental stresses might lead to new strategies for improving stress-tolerant crops. A sub-cellular proteomics approach was applied to monitor changes in abundance of PM-associated protein in response to salinity, a key abiotic stress affecting rice productivity worldwide. Proteome was extracted from a root plasma-membrane-rich fraction of a rice salt tolerant variety, IR651, grown under saline and normal conditions. Comparative two-dimensional electrophoresis revealed that 24 proteins were differentially expressed in response to salt stress. From these, eight proteins were identified by mass spectrometry analysis. Most of the proteins identified are likely to be PM-associated and are known to be involved in several important mechanisms of plant adaptation to salt stress. These include regulation of PM pumps and channels, membrane structure, oxidative stress defense, signal transduction, protein folding, and the methyl cycle. To investigate the correlation between mRNA and protein level in response to salinity, we performed quantitative Real-Time PCR analysis of three genes that were salt responsive at the protein level, including 1,4-Benzoquinone reductase, a putative remorin and a hypersensitive induced response protein. No concordance was detected between the changes in levels of gene and protein expression. Our results indicate that the proteomics approach is suitable for expression analysis of membrane associated proteins under salt stress.     

Rice plastidial NAD‐dependent malate dehydrogenase 1 negatively regulates salt stress response by reducing the vitamin B6 content

Salinity is an important environmental factor that adversely impacts crop growth and productivity. Malate dehydrogenases (MDHs) catalyse the reversible interconversion of malate and oxaloacetate using NAD(H)/NADP(H) as a cofactor and regulate plant development and abiotic stress tolerance. Vitamin B6 functions as an essential cofactor in enzymatic reactions involved in numerous cellular processes. However, the role of plastidial MDH in rice (Oryza sativa) in salt stress response by altering vitamin B6 content remains unknown. In this study, we identified a new loss‐of‐function osmdh1 mutant displaying salt stress‐tolerant phenotype. The OsMDH1 was expressed in different tissues of rice plants including leaf, leaf sheath, panicle, glume, bud, root and stem and was induced in the presence of NaCl. Transient expression of OsMDH1‐GFP in rice protoplasts showed that OsMDH1 localizes to chloroplast. Transgenic rice plants overexpressing OsMDH1 (OsMDH1OX) displayed a salt stress‐sensitive phenotype. Liquid chromatography–mass spectrometry (LC‐MS) metabolic profiling revealed that the amount of pyridoxine was significantly reduced in OsMDH1OX lines compared with the NIP plants. Moreover, the pyridoxine content was higher in the osmdh1 mutant and lower in OsMDH1OX plants than in the NIP plants under the salt stress, indicating that OsMDH1 negatively regulates salt stress‐induced pyridoxine accumulation. Furthermore, genome‐wide RNA‐sequencing (RNA‐seq) analysis indicated that ectopic expression of OsMDH1 altered the expression level of genes encoding key enzymes of the vitamin B6 biosynthesis pathway, possibly reducing the level of pyridoxine. Together, our results establish a novel, negative regulatory role of OsMDH1 in salt stress tolerance by affecting vitamin B6 content of rice tissues.     

Proteomic analysis of rice seedlings infected by Sinorhizobium meliloti 1021

Rhizobial endophytes infect and colonize not only leguminous plants, but several non‐leguminous species as well. Using green fluorescent protein tagging technique, it has been shown that Rhizobia infect different varieties of rice species and migrate from plant roots to aerial tissues such as leaf sheaths and leaves. The interaction between them was found to promote the growth of rice. The growth promotion is the cumulative result of enhanced photosynthesis and stress resistance. In addition, indole‐3‐acetic acid also contributes to the promotion. Gel‐based comparative proteomic approaches were applied to analyze the protein profiles of three different tissues (root, leaf sheath and leaf) of Sinorhizobium meliloti 1021 inoculated rice in order to get an understanding about the molecular mechanism. Upon the inoculation of rhizobia, proteins involved in nine different functional categories were either up‐regulated or down‐regulated. Photosynthesis related proteins were up‐regulated only in leaf sheath and leaf, while the up‐regulated proteins in root were exclusively defense related. The results implied that there might have been an increase in the import and transport of proteins involved in light and dark reactions to the chloroplast as well as more efficient distribution of nutrients, hence enhanced photosynthesis. Although the initiation of defensive reactions mainly occurred in roots, some different defense mechanisms were also evoked in the aerial tissues.     

Identification and functional characterization of the Arabidopsis Snf1‐related protein kinase SnRK2.4 phosphatidic acid‐binding domain

Phosphatidic acid (PA) is an important signalling lipid involved in various stress‐induced signalling cascades. Two SnRK2 protein kinases (SnRK2.4 and SnRK2.10), previously identified as PA‐binding proteins, are shown here to prefer binding to PA over other anionic phospholipids and to associate with cellular membranes in response to salt stress in Arabidopsis roots. A 42 amino acid sequence was identified as the primary PA‐binding domain (PABD) of SnRK2.4. Unlike the full‐length SnRK2.4, neither the PABD‐YFP fusion protein nor the SnRK2.10 re‐localized into punctate structures upon salt stress treatment, showing that additional domains of the SnRK2.4 protein are required for its re‐localization during salt stress. Within the PABD, five basic amino acids, conserved in class 1 SnRK2s, were found to be necessary for PA binding. Remarkably, plants overexpressing the PABD, but not a non‐PA‐binding mutant version, showed a severe reduction in root growth. Together, this study biochemically characterizes the PA–SnRK2.4 interaction and shows that functionality of the SnRK2.4 PABD affects root development.     

Plasma membrane proteome analysis identifies a role of barley membrane steroid binding protein in root architecture response to salinity

Although the physiological consequences of plant growth under saline conditions have been well described, understanding the core mechanisms conferring plant salt adaptation has only started. We target the root plasma membrane proteomes of two barley varieties, cvs. Steptoe and Morex, with contrasting salinity tolerance. In total, 588 plasma membrane proteins were identified by mass spectrometry, of which 182 were either cultivar or salinity stress responsive. Three candidate proteins with increased abundance in the tolerant cv. Morex were involved either in sterol binding (a GTPase‐activating protein for the adenosine diphosphate ribosylation factor [ZIGA2], and a membrane steroid binding protein [MSBP]) or in phospholipid synthesis (phosphoethanolamine methyltransferase [PEAMT]). Overexpression of barley MSBP conferred salinity tolerance to yeast cells, whereas the knock‐out of the heterologous AtMSBP1 increased salt sensitivity in Arabidopsis. Atmsbp1 plants showed a reduced number of lateral roots under salinity, and root‐tip‐specific expression of barley MSBP in Atmsbp1 complemented this phenotype. In barley, an increased abundance of MSBP correlates with reduced root length and lateral root formation as well as increased levels of auxin under salinity being stronger in the tolerant cv. Morex. Hence, we concluded the involvement of MSBP in phytohormone‐directed adaptation of root architecture in response to salinity.     

A Quantitative Proteomics Study of Early Heat‐Regulated Proteins by Two‐Dimensional Difference Gel Electrophoresis Identified OsUBP21 as a Negative Regulator of Heat Stress Responses in Rice

To understand the early heat shock (HS)‐regulated cellular responses that influence the tolerance of rice plant to high environmental temperatures, two‐dimensional difference gel electrophoresis (2D‐DIGE) is performed to explore the early HS‐regulated proteome. Multiple proteins that show abundance changes after 1 and 5 min of HS treatment are identified. Of the early HS‐regulated proteins identified, the abundance of a ubiquitin‐specific protease, OsUBP21, and its Arabidopsis homolog, AtUBP13, is found to be upregulated by 5 min of HS treatment. Further, knocking the expression of OsUBP21 or AtUBP13 down or out increases the tolerance of rice and Arabidopsis plants to HS stress, suggesting that the function of these ubiquitin‐specific proteases in regulating plant HS responses is conserved between monocots and dicots. 2D‐DIGE showed a group of proteins are differentially regulated in wild‐type and ubp21 mutant after 30 min of HS treatment. Among these proteins, 11 are found to interact directly with OsUBP21; thus, they may be targets of OsUBP21. Future analyses of the roles of these OsUBP21‐interacting proteins in plant HS responses will help reveal the protein ubiquitination/deubiquitination‐regulated cellular responses induced by HS in rice.     

Proteomics of MUC1‐containing lipid rafts from plasma membranes and exosomes of human breast carcinoma cells MCF‐7

Apically expressed human MUC1 is known to become endocytosed and either to re‐enter the secretory pathway for recycling to the plasma membrane or to be exported by the cells via the formation of multi‐vesicular bodies and the release of exosomes. By using recombinant fusion‐tagged MUC1 as a bait protein we followed an anti‐myc affinity‐based approach for isolating subpopulations of lipid rafts from the plasma membranes and exosomes of MCF‐7 breast cancer cells. MUC1⁺ lipid rafts were not only found to contain genuine raft proteins (flotillin‐1, prohibitin, G protein, annexin A2), but also raft‐associated proteins linking these to the cytoskeleton (ezrin/villin‐2, profilin II, HSP27, γ‐actin, β‐actin) or proteins in complexes with raft proteins, including the bait protein (HSP60, HSP70). Major overlaps were revealed for the subproteomes of plasma membranous and exosomal lipid raft preparations, indicating that MUC1 is sorted into subpopulations of rafts for its trafficking via flotillin‐dependent pathways and export via exosomes.     

Conserved V‐ATPase c subunit plays a role in plant growth by influencing V‐ATPase‐dependent endosomal trafficking

In plant cells, the vacuolar‐type H⁺‐ATPases (V‐ATPase) are localized in the tonoplast, Golgi, trans‐Golgi network and endosome. However, little is known about how V‐ATPase influences plant growth, particularly with regard to the V‐ATPase c subunit (VHA‐c). Here, we characterized the function of a VHA‐c gene from Puccinellia tenuiflora (PutVHA‐c) in plant growth. Compared to the wild‐type, transgenic plants overexpressing PutVHA‐c in Arabidopsis thaliana exhibit better growth phenotypes in root length, fresh weight, plant height and silique number under the normal and salt stress conditions due to noticeably higher V‐ATPase activity. Consistently, the Arabidopsis atvha‐c5 mutant shows reduced V‐ATPase activity and retarded plant growth. Furthermore, confocal and immunogold electron microscopy assays demonstrate that PutVHA‐c is mainly localized to endosomal compartments. The treatment of concanamycin A (ConcA), a specific inhibitor of V‐ATPases, leads to obvious aggregation of the endosomal compartments labelled with PutVHA‐c‐GFP. Moreover, ConcA treatment results in the abnormal localization of two plasma membrane (PM) marker proteins Pinformed 1 (AtPIN1) and regulator of G protein signalling‐1 (AtRGS1). These findings suggest that the decrease in V‐ATPase activity blocks endosomal trafficking. Taken together, our results strongly suggest that the PutVHA‐c plays an important role in plant growth by influencing V‐ATPase‐dependent endosomal trafficking.     

N‐glycan maturation is crucial for cytokinin‐mediated development and cellulose synthesis in Oryza sativa

To explore the physiological significance of N‐glycan maturation in the plant Golgi apparatus, gnt1, a mutant with loss of N‐acetylglucosaminyltransferase I (GnTI) function, was isolated in Oryza sativa. gnt1 exhibited complete inhibition of N‐glycan maturation and accumulated high‐mannose N‐glycans. Phenotypic analyses revealed that gnt1 shows defective post‐seedling development and incomplete cell wall biosynthesis, leading to symptoms such as failure in tiller formation, brittle leaves, reduced cell wall thickness, and decreased cellulose content. The developmental defects of gnt1 ultimately resulted in early lethality without transition to the reproductive stage. However, callus induced from gnt1 seeds could be maintained for periods, although it exhibited a low proliferation rate, small size, and hypersensitivity to salt stress. Shoot regeneration and dark‐induced leaf senescence assays indicated that the loss of GnTI function results in reduced sensitivity to cytokinin in rice. Reduced expression of A‐type O. sativa response regulators that are rapidly induced by cytokinins in gnt1 confirmed that cytokinin signaling is impaired in the mutant. These results strongly support the proposed involvement of N‐glycan maturation in transport as well as in the function of membrane proteins that are synthesized via the endomembrane system.