Organocatalytic Asymmetric Synthesis of Chiral Dioxazinanes and Dioxazepanes with <i>in Situ</i> Generated Nitrones via a Tandem Reaction Pathway Using a Cooperative Cation Binding Catalyst

Heterocyclic skeletons play major roles in pharmaceuticals and biological processes. Cycloaddition reactions are most suitable synthetic tools to efficiently construct chemically diverse sets of heterocycles with great structural complexity owing to the simultaneous or sequential formation of two or more bonds, often with a high degree of selectivity. Herein, we report an unprecedented formal cycloaddition of <i>N</i>-Boc-<i>N</i>-hydroxy amido sulfones as the nitrone precursors with terminal-hydroxy α,β-unsaturated carbonyls in the presence of Song’s chiral oligoethylene glycol as a cation-binding catalyst and KF as a base to afford a wide range of highly enantio- and diastereo-enriched six-membered dioxazinane and seven-membered dioxazepane heterocycles. In this process, nitrones as well as terminal-hydroxy α,β-unsaturated carbonyls serve as “amphiphilic” building units, and the reaction proceeds through a tandem pathway sequence of oxa-Mannich reaction/oxa-Michael reaction/tautomerization/protonation. The cation-binding catalysis in a densely confined chiral space <i>in situ</i> formed by the incorporation of potassium salt is the key to this successful catalysis. This strategy opens a new pathway for the asymmetric synthesis of diverse heterocyclic skeletons of great complexity

Phosphorylation of a WRKY Transcription Factor by MAPKs Is Required for Pollen Development and Function in <i>Arabidopsis</i>

<div><p>Plant male gametogenesis involves complex and dynamic changes in gene expression. At present, little is known about the transcription factors involved in this process and how their activities are regulated. Here, we show that a pollen-specific transcription factor, WRKY34, and its close homolog, WRKY2, are required for male gametogenesis in <i>Arabidopsis thaliana</i>. When overexpressed using <i>LAT52</i>, a strong pollen-specific promoter, epitope-tagged WRKY34 is temporally phosphorylated by MPK3 and MPK6, two mitogen-activated protein kinases (MAPKs, or MPKs), at early stages in pollen development. During pollen maturation, WRKY34 is dephosphorylated and degraded. Native promoter-driven WRKY34-YFP fusion also follows the same expression pattern at the protein level. WRKY34 functions redundantly with WRKY2 in pollen development, germination, and pollen tube growth. Loss of MPK3/MPK6 phosphorylation sites in WRKY34 compromises the function of WRKY34 <i>in vivo</i>. Epistasis interaction analysis confirmed that <i>MPK6</i> belongs to the same genetic pathway of <i>WRKY34</i> and <i>WRKY2</i>. Our study demonstrates the importance of temporal post-translational regulation of WRKY transcription factors in the control of developmental phase transitions in plants.</p></div

Phenotype of <i>wrky2-1 wrky34-1</i> double mutant pollen.

<p>(A) Diagram of T-DNA insertion alleles of <i>wrky2</i> and <i>wrky34</i> mutants. Arrows indicate the positions of RT-qPCR primers. Black bars = untranslated regions (UTRs); gray bars = exons; lines = introns. (B) Quantitative RT-PCR of <i>WRKY2</i> expression in wild-type, <i>wrky2-1</i>, and <i>wrky2-2</i> seedlings and pollen grains. Error bars = standard derivation. (C) Normal vegetative growth and development of <i>wrky2-1 wrky34-1</i> double mutant plants. Five-week-old plants are pictured. (D, E) Alexander staining of wild-type (D) and <i>wrky2-1 wrky34-1</i> double mutant (E) pollen. Bar = 50 µm. (F, G) Vital staining by FDA of wild type (F) and <i>wrky2-1 wrky34-1</i> double mutant (G) pollen. Bar = 50 µm. (H, I) Scanning electron microscopy (SEM) of wild type (H) and <i>wrky2-1 wrky34-1</i> double mutant (I) pollen. Bar = 20 µm. (J, K, L, M) Transmission electron microscopy (TEM) of wild type (J, L) and <i>wrky2-1 wrky34-1</i> double mutant (K, M) pollen. (J, K) Bar = 5 µm. (L, M) Bar = 1 µm. Arrows in panel K indicate the germination pore with defective intine layer. P, plastid; E, endoplasmic reticulum.</p

Expression and protein localization of WRKY34 and WRKY2.

<p>(A) Quantitative RT-PCR of <i>WRKY34</i> and <i>WRKY2</i> transcripts in various tissues. R, roots; St, stems; L, leaves; Sl, seedlings; B, buds; and Of, open flowers. Error bars = standard derivation. (B to I) Expression and localization of <i>WRKY2</i> promoter-driven WRKY2:YFP fusion protein in pollen. DAPI staining was used to locate nuclei (B to E), and YFP signal reveals the localization of WRKY2:YFP fusion at different developmental stages (F to I<b>)</b>. (B and F) UNM stage, no nucleus-localized YFP signal was detected. Vegetative nucleus localized WRKY2:YFP signal was observed at BCP stage (C and G), TCP stage (D and H), and MP stage (E and I). (J to Q) Expression and localization of <i>WRKY34</i> promoter-driven WRKY34:YFP fusion protein in pollen. (J to M) DAPI staining signal. (N to Q) YFP signal. (J and N) UNM stage, weak signal was observed in microspore nucleus. Vegetative nucleus localized WRKY34:YFP signal was observed at BCP stage (K and O), and TCP stage (L and P). No YFP signal was observed in MP (M and Q). Note that as the vegetative cell expressed genes, the WRKY2 and WRKY34 fusion YFP signals were not detectable in generative or sperm cells. MN, microspore nucleus. VN, vegetative nucleus. GN, generative nucleus/nuclei. SN, sperm cell nuclei. Bar = 50 µm.</p

Graphene Quantum Dots Embedded in Bi₂Te₃ Nanosheets To Enhance Thermoelectric Performance

Novel Bi₂Te₃/graphene quantum dots (Bi₂Te₃/GQDs) hybrid nanosheets with a unique structure that GQDs are homogeneously embedded in the Bi₂Te₃ nanosheet matrix have been synthesized by a simple solution-based synthesis strategy. A significantly reduced thermal conductivity and enhanced powder factor are observed in the Bi₂Te₃/GQDs hybrid nanosheets, which is ascribed to the optimized thermoelectric transport properties of the Bi₂Te₃/GQDs interface. Furthermore, by varying the size of the GQDs, the thermoelectric performance of Bi₂Te₃/GQDs hybrid nanostructures could be further enhanced, which could be attributed to the optimization of the density and dispersion manner of the GQDs in the Bi₂Te₃ matrix. A maximum ZT of 0.55 is obtained at 425 K for the Bi₂Te₃/GQDs-20 nm, which is higher than that of Bi₂Te₃ without hybrid nanostrucure. This work provides insights for the structural design and synthesis of Bi₂Te₃-based hybrid thermoelectric materials, which will be important for future development of broadly functional material systems

Phosphorylation of WRKY34 <i>in vivo</i> during male gametogenesis.

<p>(A) Staging of flowers/buds used for WRKY34 protein analysis. Flower at Stage 13, in which anthesis is about to occur, was designated 0. An open flower right after anthesis was designated +1. Younger flowers/buds were designated using negative numbers. (B) Immunoblot and Phos-tag assays of <i>LAT52</i> promoter-driven pollen-specific 4myc-WRKY34 protein at different stages of pollen development. The −7 to +1 flowers/buds have pollen at different developmental stages. Black bar indicates flowers or buds containing mature pollen. Dark gray bar indicates buds containing tricellular pollen (TCP). Light gray bar indicates buds containing bicellular pollen (BCP). Levels of 4myc-WRKY34 protein in immunoblot (top panel) and Phos-tag assay (middle panel) were determined using an anti-myc antibody. Protein loading control was confirmed by Coomassie blue staining (bottom panel). (C) Immunoblot (top panel) and Phos-tag assay (middle panel) of pollen-specific 4myc-WRKY34^SA protein at different developmental stages. Protein loading control was confirmed by Coomassie blue staining (bottom panel). Each sample was extracted from the same number of flowers/buds at the corresponding stage, which allows the comparison of WRKY34 protein levels in an equal number of developing/mature pollen grains.</p

Pollen germination and pollen tube growth are defective in <i>wrky2-1 wrky34-1</i>.

<p><i>In vitro</i> pollen germination of wild type (A) and <i>wrky2-1 wrky34-1</i> double mutant (B) pollen. Bars = 50 µm. Aniline blue staining of wild type pistils 8 hours after pollination with wild type (C) and <i>wrky2-1 wrky34-1</i> double mutant (D) pollen. Bar = 200 µm.</p

Transmission of <i>wrky2-1</i> and <i>wrky34-1</i> single and double mutant alleles.

<p>Crosses were performed using plants of indicated genotype. The genotypes of F1 progenies were determined by PCR genotyping, which was used to determine the transmission of pollen of different genotypes. Bold numbers indicate significant aberrant transmission ratios from the expected ratio of 1∶1.</p

<i>In vivo</i> phosphorylation of WRKY34 is dependent on MPK3 and MPK6.

<p>(A) <i>MPK3</i> expression in <i>mpk6</i> and <i>MPK3RNAi mpk6</i> pollen grains. Total RNAs were isolated from pollen grains. MPK3 transcript levels were determined using quantitative RT-PCR. Error bars = standard derivation. (B) Immunoblot (top panel) and Phos-tag (middle panel) assays of WRKY34 protein at different stages of <i>MPK3RNAi mpk6 P_LAT52:4myc-WRKY34</i> flower buds. Protein loading control was confirmed by Coomassie blue staining (bottom panel). Each sample was extracted from the same number of flowers/buds at the corresponding stage, which allows the comparison of WRKY34 protein levels in an equal number of developing/mature pollen grains.</p

<i>In vitro</i> phosphorylation of WRKY34 by MPK3 and MPK6.

<p>(A) Putative MAPK-phosphorylation sites in WRKY34 and WRKY33. Bars indicate the position of potential MAPK phosphorylation sites in the protein. Grey boxes indicate WRKY domains. Note that the clusters of phosphorylation sites at N-termini were similar between WRKY34 and WRKY33. (B) <i>In vitro</i> phosphorylation assay of WRKY34 by the activated MPK3 and MPK6 (upper panel). Reactions with various components omitted (-) were used as controls. Recombinant MKK4^DD/MKK5^DD were used to activate MPK3 and MPK6. Myelin basic protein (MBP) was used as control substrate (lower panel). (C) Adjacent sequences of putative MAPK-phosphorylation sites in WRKY34, and the loss-of phosphorylation WRKY34 mutant with all Ser mutated to Ala (WRKY34^SA). (D) Mutation of MAPK-phosphorylation sites greatly reduced the phosphorylation of WRKY34 by MPK3 and MPK6. Phosphorylated WRKY34 was visualized by autoradiography after gel electrophoresis.</p