Malate dehydrogenase of spinach: Substrate kinetics of different forms

Sephadex G-200 gel filtration of an ammonium sulfate fraction, containing the bulk of NAD-dependent malate dehydrogenase, yields forms of differing MW. Both Mg²⁺ and NADH stabilize the 127000 daltons MW form. K⁺, or incubation with dithioerythritol, cause splitting and partial reaggregation, resulting in MWs ranging between 35000 and 180000 daltons. Chromatography in the presence of dithioerythritol and NADH results in an enzyme with a non-linear reaction rate at low substrate concentrations. Plots of initial velocity vs substrate and cofactor concentration respectively are characterized by two slopes of positive cooperativity separated by an intermediary plateau of negative cooperativity. Gel chromatography in the presence of Mg²⁺ or K⁺ or drastic dilution of the enzyme results in an enzyme with linear reaction rates also at low substrate concentration. Its kinetics are consistent with the view that the enzyme undergoes conformational changes when the substrate concentration is varied.     

铀对菠菜叶片光合作用影响的研究

为了进一步揭示铀对植物光合作用的影响机理,用不同浓度铀[0、20、50、100、150mg·kg-1]土培处理6叶期菠菜(Spinacia oleracea L.),分别于处理7、14、21、28d后分析铀对菠菜叶片光合色素含量、光合气体交换参数、叶绿素荧光参数和生长指标的影响。结果显示:(1)低浓度铀(20、50mg·kg-1)显著促进菠菜叶片的叶绿素含量和净光合速率(Pn)、气孔导度(Gs)以及蒸腾速率(Tr)等光合气体交换参数;高浓度铀(100、150mg·kg-1)则表现出显著抑制作用,且处理浓度越高,处理时间越长,光合气体交换参数的下降幅度就越大,而胞间CO2浓度(Ci)却反而上升,说明导致Pn下降的主要原因是非气孔因素。(2)高浓度铀处理显著影响菠菜叶片的叶绿素荧光动力学参数,其中光系统Ⅱ(PSⅡ)的最大荧光(Fm)、最大光化学效率(Fv/Fm)和PSⅡ潜在活性(Fv/F0)均显著降低,而初始荧光(F0)显著升高。(3)菠菜幼苗的根长、株高、生物量均随着铀浓度的增加表现出先应激性上升后下降的动态变化。研究表明,低浓度铀可以显著增加菠菜叶绿素含量,有效改善叶片光合效率,促进幼苗的生长发育,而高浓度铀则会抑制菠菜叶片的光合作用,导致光合效率显著下降,显著抑制幼苗的生长发育。     

Summer meeting 2007 - the problems with fresh produce: an overview

In Fall 2006, four separate outbreaks of foodborne illness associated with the consumption of fresh produce occurred in the United States. In follow-up investigations, spinach, lettuce, and tomatoes were identified as the vehicles of illness. Epidemiologic investigations subsequently focused on finding the specific growing regions using traceback records. While the areas most likely involved in the outbreaks have been identified, the specific mode of contamination remains unconfirmed. Suspected risk factors in these cases include: proximity of irrigation wells and surface waterways exposed to faeces from cattle and wildlife; exposure in fields to wild animals and their waste materials; and improperly composted animal manure used as fertilizer. Difficulty in deciphering these and other on-farm routes of contamination is due to the sporadic nature of these events. Hence, evidence to support these contamination modes is based largely on experimental studies in the laboratory and field. Still at issue is the relevance of internalization of pathogens, whether this occurs through the roots and plant vascular tissues of vegetables and fruits or through plant surfaces into cracks and crevices. Potential for these events, conditions under which the events occur, and pathogen survival following these events, are questions that still need to be answered. Answers to these questions will ultimately affect the type of interventions needed for application postharvest. Currently, many chemical and biological interventions can reduce surface pathogens and minimize cross-contamination, however, they are largely ineffective on internalized pathogens. In the event internalization is a significant route of contamination in the field, physical interventions (irradiation and high pressure) may be needed to minimize risk. Ultimately, risk assessment studies will be useful tools in developing risk management strategies for the produce industry.     

Structure prediction of dihydroflavonol 4- reductase and anthocyanidin synthase from spinach

Spinach is an important dietary vegetable associated with beneficial health effects. Flavonoids have various biological activities such as antioxidant, antibacterial, and anticancer effect Flavonoid including anthocyanin provides brilliant and colored pigments in different plant tissues. Anthocyanidin synthase and dihydroflavonol 4-reductase are responsible for anthocyanin biosynthesis. They contributed in plant protection against UV-B radiation, microbial and herbivore pathogens. A 3D structures of anthocyanidin synthase and dihydroflavonol 4-reductase from spinach are constructed in this study through homology modeling. The homology modeling is done by using the MODELLER 9v7 software. The energy of models was minimized by applying molecular mechanics method. The root mean square deviation (RMSD) for C atoms between the template and the homology-modeled structures was estimated by CE program. The final models were assessed by PROCHECK and WHATCHECK which showed that the final refined models are reliable.     

Survival and colonization of Escherichia coli O157:H7 on spinach leaves as affected by inoculum level and carrier, temperature and relative humidity

Aims: To determine survival and colonization of Escherichia coli O157:H7 on spinach leaves as affected by inoculum level and carrier, temperature and relative humidity (r.h.). Methods and Results: Spinach leaves were inoculated with suspensions of E. coli O157:H7 in distilled water (DW) and 0·1% peptone water (PW) and incubated at 4, 12 and 25°C and 43, 85 and 100% r.h. The number of E. coli O157:H7 on leaves (5·6 or 1·9 log CFU per leaf) inoculated using DW as a carrier medium increased significantly at 25°C and 100% r.h. within 120 h but remained constant or decreased significantly under other test conditions. E. coli O157:H7 on leaves (5·4 log CFU per leaf) inoculated using PW as a carrier increased significantly within 72 and 24 h, respectively, at 12 or 25°C and 100% r.h.; counts using a low inoculum (2·2 log CFU per leaf) increased significantly within 24 h at 25°C. Conclusions: Escherichia coli O157:H7 can colonize on spinach leaves at 12 or 25°C in a 100% r.h. environment. Organic matter in the inoculum carrier may provide protection and nutrients which enhance survival and colonization. Significance and Impact of the Study: Colonization of E. coli O157:H7 on spinach leaves as affected by organic matter in the inoculum, temperature and r.h. was determined. These observations will be useful when developing strategies to prevent growth of E. coli O157:H7 on pre‐ and postharvest spinach.     

The effect of cerium (III) on the chlorophyll formation in spinach

The effect of Ce³⁺ on the chlorophyll (chl) of spinach was studied in pot culture experiments. The results showed that Ce³⁺ could obviously stimulate the growth of spinach and increase its chlorophyll contents and photosynthetic rate. It could also improve the PSII formation and enhance its electron transport rate of PSII as well. By inductively coupled plasma-mass spectroscopy and atom absorption spectroscopy methods, it was revealed that the rare-earth-element (REE) distribution pattern in the Ce³⁺-treated spinach was leaf>root>shoot in Ce³⁺ contents. The spinach leaves easily absorbed REEs. The Ce³⁺ contents of chloroplast and chlorophyll of the Ce³⁺-treated spinach were higher than that of any other rare earth and were much higher than that of the control; it was also suggested that Ce³⁺ could enter the chloroplast and bind easily to chlorophyll and might replace magnesium to form Ce-chlorophyll. By ultraviolet-visible, Fourier transform infrared, and extended X-ray absorption fine structure (EXAFS) methods, Ce³⁺-coordinated nitrogen of porphyrin rings with eight coordination numbers and average length of the Ce-N bond of 0.251 nm.     

Molecular characterization of nucleus-localized RNA-binding proteins from higher plants

We report the isolation and characterization of genes from the higher plants Arabidopsis, spinach and tobacco which code for nucleus-localized RNA-binding proteins. Common features of these polypeptides are glycine/arginine-rich regions with several RGG repeats at their N- and C-termini, which are sufficient for RNA binding in northwestern assays. All polypeptides analysed contain two basic bipartite nuclear localization signals and translational fusions harbouring these regions with the -glucuronidase gene direct the fusion proteins into the nucleus. Nuclear localization was confirmed by cellular fractionation with a polyclonal antiserum raised against the over-expressed tobacco protein NtRGG1p. Two or three copies of related RGG genes appear to be present in the analysed organisms and the expression of some of them is regulated: a tobacco gene is light-regulated and a spinach gene is preferentially expressed in roots. Possible biological functions of this class of RNA-binding proteins as well as structure/function relationships related to the modular structure are discussed.     

Effect of Eu3+ on characterization of photosystem II particles from spinach by spectroscopy

Photosystem II (PSII) particles were purified from Eu³⁺-treated spinach and studied by spectroscopy. The results showed that electron transport rate of PS II was accelerated by Eu³⁺ treating, that violet shift of the PSII Soret band or Q-band was 6 nm or 2 nm for the ultraviolet-visible (UV-Vis) spectrum, that the violet shift of the PSII fluorescence emission peak was 9 nm for fluorescence emission spectrum, that the PSII Signal II’s of low-temperature electron paramagnetic resonance (EPR) spectrum was intensified under light, and that the PSII CD spectrum was similar to that of control. It is suggested that Eu³⁺ might bind to the PSII reaction center complex and enhance the electron transport rate of PSII CD; however, Eu³⁺ treatment does not change the configuration of the PSII reaction center complex.     

Effect of nano-TiO2 on photochemical reaction of chloroplasts of spinach

The effects of nano-TiO₂ (rutile) on the photochemical reaction of chloroplasts of spinach were studied. The results showed that when spinach was treated with 0.25% nano-TiO₂, the Hill reaction, such as the reduction rate of FeCy, and the rate of evolution oxygen of chloroplasts was accelerated and noncyclic photophosphorylation (nc-PSP) activity of chloroplasts was higher than cyclic photophosphorylation (c-PSP) activity, the chloroplast coupling was improved and activities of Mg²⁺-ATPase and chloroplast coupling factor I (CF₁)-ATPase on the thylakoid membranes were obviously activated. It suggested that photosynthesis promoted by nano-TiO₂ might be related to activation of photochemical reaction of chloroplasts of spinach.     

Influence of nano-anatase TiO2 on the nitrogen metabolism of growing spinach

Previous research showed that nano-TiO₂ could significantly promote photosynthesis and greatly improve growth of spinach, but, we also speculated that an increase of spinach growth by nano-TiO₂ treatment might be closely related to the change of nitrogen metabolism. The effects of nano-anatase TiO₂ on the nitrogen metabolism of growing spinach were studied by treating them with nano-anatase TiO₂. The results showed that, nano-anatase TiO₂ treatment could obviously increase the activities of nitrate reductase, glutamate dehydrogenase, glutamine synthase, and glutamic-pyruvic transaminase during the growing stage. Nano-anatase TiO₂ treatment could also promote spinach to absorb nitrate, accelerate, inorganic nitrogen (such as NO ₃ ^t- −N and NH ₄ ⁺ −N) to be translated into organic nitrogen (such as protein and chlorophyll), and enhance the fresh weight and dry weights.