Cytokinins (CKs) are a group of mobile adenine derivatives that act as chemical signals regulating a variety of biological processes implicated in plant development and stress responses. Their ...synthesis, homeostasis, and signaling perception evoke complicated intracellular traffic, intercellular movement, and in short- and long-distance translocation. Over nearly two decades, subsets of membrane transporters have been recognized and implicated in the transport of CKs as well as the related adenylates. In this review, we aim to recapitulate the key progresses in exploration of the transporter proteins involved in cytokinin traffic and translocation, discuss their functional implications in the cytokinin-mediated paracrine and long-distance communication, and highlight some knowledge gaps and open issues toward comprehensively understanding the molecular mechanism of membrane transporters in controlling spatiotemporal distribution of cytokinin species.
Summary
Grass lignocelluloses feature complex compositions and structures. In addition to the presence of conventional lignin units from monolignols, acylated monolignols and flavonoid tricin also ...incorporate into lignin polymer; moreover, hydroxycinnamates, particularly ferulate, cross‐link arabinoxylan chains with each other and/or with lignin polymers. These structural complexities make grass lignocellulosics difficult to optimize for effective agro‐industrial applications. In the present study, we assess the applications of two engineered monolignol 4‐O‐methyltransferases (MOMTs) in modifying rice lignocellulosic properties. Two MOMTs confer regiospecific para‐methylation of monolignols but with different catalytic preferences. The expression of MOMTs in rice resulted in differential but drastic suppression of lignin deposition, showing more than 50% decrease in guaiacyl lignin and up to an 90% reduction in syringyl lignin in transgenic lines. Moreover, the levels of arabinoxylan‐bound ferulate were reduced by up to 50%, and the levels of tricin in lignin fraction were also substantially reduced. Concomitantly, up to 11 μmol/g of the methanol‐extractable 4‐O‐methylated ferulic acid and 5–7 μmol/g 4‐O‐methylated sinapic acid were accumulated in MOMT transgenic lines. Both MOMTs in vitro displayed discernible substrate promiscuity towards a range of phenolics in addition to the dominant substrate monolignols, which partially explains their broad effects on grass phenolic biosynthesis. The cell wall structural and compositional changes resulted in up to 30% increase in saccharification yield of the de‐starched rice straw biomass after diluted acid‐pretreatment. These results demonstrate an effective strategy to tailor complex grass cell walls to generate improved cellulosic feedstocks for the fermentable sugar‐based production of biofuel and bio‐chemicals.
The plant hormone salicylic acid (SA) plays critical roles in plant defense, stress responses, and senescence. Although SA biosynthesis is well understood, the pathways by which SA is catabolized ...remain elusive. Here we report the identification and characterization of an SA 3-hydroxylase (S3H) involved in SA catabolism during leaf senescence. S3H is associated with senescence and is inducible by SA and is thus a key part of a negative feedback regulation system of SA levels during senescence. The enzyme converts SA (with a K ₘ of 58.29 µM) to both 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,5-DHBA in vitro but only 2,3-DHBA in vivo. The s3h knockout mutants fail to produce 2,3-DHBA sugar conjugates, accumulate very high levels of SA and its sugar conjugates, and exhibit a precocious senescence phenotype. Conversely, the gain-of-function lines contain high levels of 2,3-DHBA sugar conjugates and extremely low levels of SA and its sugar conjugates and display a significantly extended leaf longevity. This research reveals an elegant SA catabolic mechanism by which plants regulate SA levels by converting it to 2,3-DHBA to prevent SA overaccumulation. The research also provides strong molecular genetic evidence for an important role of SA in regulating the onset and rate of leaf senescence.
Plant lignification is a tightly regulated complex cellular process that occurs via three sequential steps： the synthesis of monolignols within the cytosol; the transport of monomeric precursors ...across plasma membrane; and the oxidative polymerization of monolignols to form lignin macromolecules within the cell wall. Although we have a reasonable understanding of monolignol biosynthesis, many aspects of lignin assembly remain elusive. These include the pre- cursors＇ transport and oxidation, and the initiation of lignin polymerization. This review describes our current knowledge of the molecular mechanisms underlying monolignol transport and oxidation, discusses the intriguing yet least- understood aspects of lignin assembly, and highlights the technologies potentially aiding in clarifying the enigma of plant lignification.
Herein we report the synthesis, structure solution, and catalytic properties of PST‐24, a novel channel‐based medium‐pore zeolite. This zeolite was synthesized via the excess fluoride approach. ...Electron diffraction shows that its structure is built by composite cas‐zigzag (cas‐zz) building chains, which are connected by double 5‐ring (d5r) columns. While the cas‐zz building chains are ordered in the PST‐24 framework, the d5r columns adopt one of two possible arrangements; the two adjacent d5r columns are either at the same height or at different heights, denoted arrangements S and D, which can be regarded as open and closed valves that connect the channels, respectively. A framework with arrangement D only has a 2D 10‐ring channel system, whereas that with arrangement S only contains 3D channels. In actual PST‐24 crystals, the open and closed valves are almost randomly dispersed to yield a zeolite framework where the channel dimensionality varies locally from 2D to 3D.
PST‐24, a medium‐pore zeolite with a disordered arrangement of double 5‐ring (d5r) units, has been synthesized under excess fluoride conditions. Its three ordered polytypes have different channel dimensionalities, depending on the d5r arrangement order.
Solar‐energy‐driven photoreduction of CO2 is promising in alleviating environment burden, but suffers from low efficiency and over‐reliance on sacrificial agents. Herein, rhenium (Re) is atomically ...dispersed in In2O3 to fabricate a 2Re‐In2O3 photocatalyst. In sacrificial‐agent‐free photoreduction of CO2 with H2O, 2Re‐In2O3 shows a long‐term stable efficiency which is enhanced by 3.5 times than that of pure In2O3 and is also higher than those on Au‐In2O3, Ag‐In2O3, Cu‐In2O3, Ir‐In2O3, Ru‐In2O3, Rh‐In2O3 and Pt‐In2O3 photocatalysts. Moreover, carbon‐based product of the photoreduction overturns from CO on pure In2O3 to CH3OH on 2Re‐In2O3. Re promotes charge separation, H2O dissociation and CO2 activation, thus enhancing photoreduction efficiency of CO2 on 2Re‐In2O3. During the photoreduction, CO is a key intermediate. CO prefers to desorption rather than hydrogenation on pure In2O3, as CO binds to pure In2O3 very weakly. Re strengthens the interaction of CO with 2Re‐In2O3 by 5.0 times, thus limiting CO desorption but enhancing CO hydrogenation to CH3OH. This could be the origin for photoreduction product overturn from CO on pure In2O3 to CH3OH on 2Re‐In2O3. The present work opens a new way to boost sacrificial‐agent‐free photoreduction of CO2.
A 2Re‐In2O3 photocatalyst with atomically dispersed Re boosts selective CH3OH formation in sacrificial‐agent‐free photoreduction of CO2. The atomically dispersed Re is beneficial for CO2 conversion to CO which is strongly adsorbed on 2Re‐In2O3 and prefers to undergo hydrogenation to CH3OH rather than desorption from 2Re‐In2O3, thus boosting CH3OH formation. This opens a new way to improve sacrificial‐agent‐free selective photoreduction of CO2.
We confirmed here that the catalyst preparation methodologies have a significant effect on the activity and stability of Ni/SiO2 catalyst for methanation of syngas (CO + H2). Catalyst ...characterizations using X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR) and transmission electron microscope (TEM) were performed to investigate the structure and performance of the catalysts. The activity and stability of catalysts prepared by thermal decomposition and dielectric-barrier discharge (DBD) plasma decomposition of nickel precursor were compared. The plasma decomposition results in a high dispersion, an enhanced interaction between Ni and the SiO2 support, as well as less defect sites on Ni particles. Enhanced resistance to Ni sintering was also observed. In addition, the plasma prepared catalyst effectively inhibits the formation of inactive carbon species. As a result, the plasma prepared catalyst exhibits significantly improved activity with enhanced stability.
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► Nickel precursor is decomposed using dielectric-barrier discharge plasma. ► Such prepared Ni/SiO2 shows smaller size, less step and improved crystallinity. ► Significantly enhanced activity and stability for methanation is achieved.
Ni/In2O3 was confirmed to be active for CO2 hydrogenation to methanol. Herein, the In2O3-ZrO2 solid solution was prepared to support the highly dispersed nickel catalyst by chemical reduction. A CO2 ...conversion of 17.9% with a space-time yield (STY) of methanol of 0.63 gMeOH gcat−1 h−1 was achieved at 300 °C and 5 MPa on Ni/In2O3-ZrO2 of 5 wt% Ni loading. The STY of methanol has a 43.2% increase, compared to Ni/In2O3. The use of ZrO2 optimizes the oxygen vacancies for CO2 activation. The highly dispersed Ni catalyst facilitates hydrogen spillover, which improves hydrogenation ability and the formation of oxygen vacancies.
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•The chemical reduction leads to highly dispersed Ni catalyst on In2O3-ZrO2.•The highly dispersed Ni catalyst causes an excellent hydrogen spillover.•The use of ZrO2 optimizes and stabilizes the oxygen vacancies for CO2 activation.•Ni/In2O3-ZrO2 is a highly active catalyst for CO2 hydrogenation to methanol.•High methanol selectivity is achieved over Ni/In2O3-ZrO2.
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•The strong metal-support interaction (SMSI) of Pt/TiO2 is dependent on TiO2 facet.•The order of encapsulation degree is Pt/TiO2-100 > Pt/TiO2-001 > Pt/TiO2-101.•Encapsulation ...correlates well with reduction degree of TiO2 and location of oxygen vacancy.•The highest reaction rate and turnover frequency were achieved on Pt/TiO2-001.
Metal/reducible metal oxide catalysts are widely used in hydrodeoxygenation (HDO) reactions for upgrading bio-oil to produce chemicals and fuel components. Under such strong reducing condition, the strong metal-support interaction (SMSI) should take place, however, few work has been devoted to understand the effect of facet of metal oxide on the SMSI and its consequence on HDO performance. Herein, Pt supported on anatase TiO2 with varying dominative exposed facets of (101), (100) and (001) were prepared and tested in HDO of m-cresol at 350 °C and atmosphere H2 pressure. The degree of SMSI of Pt/TiO2 is strongly dependent on the facet of TiO2 when reduced at 350 °C, and the degree of encapsulation decreases following the order of Pt/TiO2-100 > Pt/TiO2-001 > Pt/TiO2-101. The encapsulation correlates well with the reduction degree of TiO2 and the location of oxygen vacancy, which are resulted from varied oxygen vacancy formation and anisotropic diffusion rates in different facets. The intermediate SMSI is achieved on TiO2-001, which creates the maximal density of Pt-Ov-Ti3+ sites at the interfacial perimeter of TiOx/Pt for deoxygenation of m-cresol, resulting in the highest reaction rate and turnover frequency (TOF) while the lowest activation energy (Ea). In contrast, the maximum SMSI on TiO2-100 leads to the lowest deoxygenation rate while the highest Ea. This work provides insight into facet dependent SMSI of Pt/TiO2 and indicates the HDO activity can be finely tailored by tuning the facet of TiO2.
Carbon‐supported platinum (Pt) and palladium (Pd) alloy catalyst has become a promising alternative electrocatalyst for oxygen reduction reaction (ORR) in proton exchange membrane fuel cells. In this ...work, the synthesis of highly active and stable carbon‐supported Pt–Pd alloy catalysts is reported with a room‐temperature electron reduction method. The alloy nanoparticles thus prepared show a particle size around 2.6 nm and a core–shell structure with Pt as the shell. With this structure, the breaking of O–O bands and desorption of OH are both promoted in electrocatalysis of ORR. In comparison with the commercial Pt/C catalyst prepared by conventional method, the mass activity of the Pt–Pd/C catalyst for ORR is shown to increase by a factor of ≈4. After 10 000‐cycle durability test, the Pt–Pd/C catalyst is shown to retain 96.5% of the mass activity, which is much more stable than that of the commercial Pt/C catalyst.
Highly active and stable Pt–Pd alloy catalysts have been developed for oxygen reduction reaction (ORR). The catalysts are synthesized via a novel “low‐temperature conversion” method. The as‐prepared catalysts exhibit a particle size around 2.6 nm and a core–shell structure with Pt as the shell. With this structure, the Pt–Pd alloy catalysts show brilliant ORR activities and stabilities.
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