As a result of the industrial revolution, anthropogenic activities have enhanced there distribution of many toxic heavy metals from the earth's crust to different environmental compartments. Environmental pollution by toxic heavy metals is increasing worldwide, and poses a rising threat to both the environment and to human health.Plants are exposed to heavy metals from various sources: mining and refining of ores, fertilizer and pesticide applications, battery chemicals, disposal of solid wastes(including sewage sludge), irrigation with wastewater, vehicular exhaust emissions and adjacent industrial activity.Heavy metals induce various morphological, physiological, and biochemical dysfunctions in plants, either directly or indirectly, and cause various damaging effects. The most frequently documented and earliest consequence of heavy metal toxicity in plants cells is the overproduction of ROS. Unlike redox-active metals such as iron and copper, heavy metals (e.g, Pb, Cd, Ni, AI, Mn and Zn) cannot generate ROS directly by participating in biological redox reactions such as Haber Weiss/Fenton reactions. However, these metals induce ROS generation via different indirect mechanisms, such as stimulating the activity of NADPH oxidases, displacing essential cations from specific binding sites of enzymes and inhibiting enzymatic activities from their affinity for -SH groups on the enzyme.Under normal conditions, ROS play several essential roles in regulating the expression of different genes. Reactive oxygen species control numerous processes like the cell cycle, plant growth, abiotic stress responses, systemic signalling, programmed cell death, pathogen defence and development. Enhanced generation of these species from heavy metal toxicity deteriorates the intrinsic antioxidant defense system of cells, and causes oxidative stress. Cells with oxidative stress display various chemical,biological and physiological toxic symptoms as a result of the interaction between ROS and biomolecules. Heavy-metal-induced ROS cause lipid peroxidation, membrane dismantling and damage to DNA, protein and carbohydrates. Plants have very well-organized defense systems, consisting of enzymatic and non-enzymatic antioxidation processes. The primary defense mechanism for heavy metal detoxification is the reduced absorption of these metals into plants or their sequestration in root cells.Secondary heavy metal tolerance mechanisms include activation of antioxidant enzymes and the binding of heavy metals by phytochelatins, glutathione and amino acids. These defense systems work in combination to manage the cascades of oxidative stress and to defend plant cells from the toxic effects of ROS.In this review, we summarized the biochemiCal processes involved in the over production of ROS as an aftermath to heavy metal exposure. We also described the ROS scavenging process that is associated with the antioxidant defense machinery.Despite considerable progress in understanding the biochemistry of ROS overproduction and scavenging, we still lack in-depth...

Nitrate and N02-transport by roots of 8-day-old uninduced and induced intact barley (Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of N03-and N02-was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km. possibly a constitutive uptake system, and a linear system for both N03-and N02-. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both N03-and N02-uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of N03-and N02-uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of N03-uptake ranged from 34 to 36 micromolar and of N02-uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that N03-and N02-inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that N03-was a more effective inhibitor than N02-. Nitrate and N02-transport by both the low and high Km systems were greatly inhibited by phydroxymercuribenzoate, whereas the linear system was only slightly inhibited.

ABSTIRACIThe effect of NaCl and Na2SO salinity on NO3-assimilaon in yonng barley (Horden m vlgare L var Numar) seed was studied. The induction of the NO-tranporter was affected very little the major effect of the salts was on its activity. Both a-and SO42-salts severely inhibited uptake of NO-. When compared on the basis of osmolality of the uptake solutions, a-salts were more inhibitory (15-30%) than S042-salts. At equal concentrions, SO42-salts inhibited NOj-uptake 30 to 40% more tha did a-salts. The absolute concentrabons of each io seemed more important as inhibitors of NOi-uptake than did the osmolality of the uptake solutios. Both KI and Na salts inhibited NO3-uptake similarlr, hence, the process seemed more sensitive to anionic linity than to cationic salinity.Unlike NO3-uptake, NO3-reduction was not affected by salinity in short-term studies (12 honrs). The rte of reduction of endogeos NOiin leaves of seelings gron on NaCl for 8 days decrased only 25%. Nitrte redutase activity in the salt-treated laves also decrased 20% but its activity, determined either in vitro or by the 'amerobic' in vivo assay, was always greater than the actual in sits rate of NOQ reducton.When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in sits nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO-transporter, rather than that of the NO3-reduction system, may be a critical factor to plant survival during salt stress.The assimilation of NO3-, the predominant form of N available in an aerobic environment, is critical if plants are to adapt, grow, and reproduce in saline conditions. Not only is NO3 assimilation required for growth and development, but some of its metabolites accumulate during stress (1 1, 16, 30). It is well known that both proline (1 1, 16, 30) and betaine (10) accumulate during stress. Proline apparently originates from recently formed glutamate (5). Methylated quaternary ammonium compounds and possibly some amino acids accumulating in stressed plants could serve as osmotica for osmoregulation (10,16,30 Stargrass. In contrast, NaCI slinization had little effect on N uptake in winter barley but impaired its incorporation into the protein fraction (13). In leafdiscs ofNicotiana rustica, salt stress reduced both the uptake of L-leucine and its incorporation into proteins (3).The reported effects of salinity on N assimilation are controversial, because no studies were done that measured all of the processes of NO3-assimilation simultaneously. Measuring only uptake or internal reduced N does not yield a balance sheet needed to determine which processes are affected.This report describes the effects of salinity on the processes of NO3-assimilation. MATERAS AND METHODSSeedling Growth. Two varieties of barley (Hordeum vulgare L.), Numar and Arivat, were grown. Numar is a salt-tolerant and Arivat is a salt-sensitive variet...

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The influence of light, dark, and ambient CO2 on nitrate assimilation in 8-to 9-day-old barley seedlngs was studied. To develop the photosynthetic apparatus fully, the seedings were grown in nitrogen-free Hoagland solution for 5 days in darkness followed by 3 days in continuous ight.The It is well established that light energy is intimately involved in the assimilation of nitrate (1,(4)(5)(6)24). The influence of light is not well understood, however, and reports are contradictory. An obligatory requirement of light for nitrate assimilation has been reported for several species (6,18,19), whereas others report nitrate reduction in darkness in tissue slices or detached leaves of many species under both anaerobic (3,13,(17)(18)(19) and aerobic (12, 13) conditions. Sawhney et aL (18) recently proposed that reduction of nitrate in light only avoids the accumulation of toxic levels of nitrite in the dark. In contrast, Jones and Sheard (12) and placed in the dark at room temperature. On the 6th day, the seedlings were transferred to aerated one-fourth-strength Hoagland solution (9) lacking nitrogen and placed in continous light of 500 ,uE m-2 s-1 for 3 more days at 25 C and 70 to 75% RH to develop the photosynthetic apparatus (10). When carbohydratedeficient seedlings were used, they were given 3 days of light treatment and then placed in darkness at 25 C and 70 to 75% RH for the desired period (24-48 h).Nitrate and Nitrite Uptake. Uptake of nitrate and nitrite was measured as the amounts disappearing from the substrate solution with time. Ten seedlings per treatment (each treatment replicated twice and each experiment repeated two times) were placed in 140 ml of one-fourth-strength Hoagland solution containing 1.0 mm KNO3 or 1.0 mm NaNO2 and 5.0 mm CaSO4. The initial pH of the solutions was 5.8. The solutions were renewed once after a 12-h absorption period. By this time, nitrate concentration from the uptake medium was reduced to about 0.5 mm. Previous studies in this laboratory have shown that uptake rates between 0.5 and 1.0 mm nitrate were constant (7). When excised roots were used, they were excised at the scutellar node and submerged in the uptake medium for the desired periods. Two g of excised roots per treatment were used. All solutions were aerated during uptake.The addition of 50 jig/ml chloramphenicol to uptake solutions had no effect on the results showing that bacterial contamination was not a problem.To study the effect of CO2 on nitrate uptake, the seedlings were placed in a 15-liter Plexiglas chamber. The light intensity inside the chamber at the top of the seedling canopy was 450 ,uE m-2 s-'.Normal or CO2-free air was passed through the chamber at 2 liters/min, giving about eight exchanges/h, with a positive pressure inside the chamber. The treatment solutions were aerated with the same gas. The air from the Plexiglas chamber was passed through a cold bath and recirculated back into the chamber at 6 liters/min. This maintained temperature and RH at about the same level as in the growth cham...

The absorption of Si by wheat, Triticum aestivum L. 'Yecora Rojo,' followed Michaelis-Menten kinetics over a concentration range of 0.004-1.0 mM. K m and V max were determined using linear transformations and the calculated curve fitted the data closely. The absorption resulted in accumulation ratios of 200/1 or more. In keeping with that finding, this study also demonstrated that Si uptake by wheat is under metabolic control, being severely restricted by dinitrophenol (DNP) and potassium cyanide (KCN). Silicon uptake by wheat was not significantly affected by phosphate ions, but the chemical analog Ge exerted a direct competitive effect on Si uptake, and vice versa.

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Cellulose is the most abundant biomaterial in the biosphere and the major component of plant biomass.Cellulase is an enzymatic system required for conversion of renewable cellulose biomass into free sugar for subsequent use in different applications. Cellulase system mainly consists of three individual enzymes namely: endoglucanase, exoglucanase and β-glucosidases. β-Glucosidases are ubiquitous enzymes found in all living organisms with great biological significance. β-Glucosidases have also tremendous biotechnological applications such as biofuel production, beverage industry, food industry, cassava detoxification and oligosaccharides synthesis. Microbial β-glucosidases are preferred for industrial uses because of robust activity and novel properties exhibited by them. This review aims at describing the various biochemical methods used for screening and evaluating β-glucosidases activity from microbial sources. Subsequently, it generally highlights techniques used for purification of β-glucosidases. It then elaborates various biochemical and molecular properties of this valuable enzyme such as pH and temperature optima, glucose tolerance, substrate specificity, molecular weight, and multiplicity. Furthermore, it describes molecular cloning and expression of bacterial, fungal and metagenomic β-glucosidases. Finally, it highlights the potential biotechnological applications of β-glucosidases.