Rhododendron thickets alter N cycling and soil extracellular enzyme activities in southern Appalachian hardwood forests

Rhododendron maximum L., a spreading understory shrub, inhibits overstory regeneration and alters forest community structure in southern Appalachian hardwood forests. Using paired plots and reciprocal litter transplants in forests with and without R. maximum cover, we examined the influence of R. maximum on litter mass and quality, N cycling and soil extracellular enzymes. Standing stocks of soil organic matter, soil N, leaf litter mass and fine root biomass were greater in forests with R. maximum than those without. Tannin extracts from R. maximum foliage, and leaf litter and fine roots collected under R. maximum had a relatively high capacity to precipitate protein compared to extracts from trees. Across the growing season, soil inorganic N availability was generally lower under R. maximum, mostly due to reduced NO₃⁻ availability. Our data suggest that R. maximum litter alters N cycling through the formation of recalcitrant polyphenol–organic N complexes. Soil extracellular enzymes indicate the potential processing rates of organic substrates. Between forest types, polyphenol oxidase activity was greatest in R. maximum O horizons, regardless of litter type, suggesting that the local microbial community can better degrade and access protein–tannin-complexed N. Protease activity did not differ between forest types, but was greater on R. maximum leaf litter than hardwood leaf litter. The alteration of the N cycle via the formation of polyphenol–organic N complexes may contribute to hardwood seedling suppression, while the enzymatic release of these complexes by ericoid mycorrhizal fungi may increase N acquisition for R. maximum and contribute to its expansion in southern Appalachian forests.     

番荔枝果肉多酚氧化酶活性影响因素的研究

以番荔枝果肉为材料,研究了番荔枝多酚氧化酶(PPO)特性;比较了不同温度处理和加入几种抑制剂对番荔枝果肉加工过程中多酚氧化酶活性的影响。结果表明,用100℃处理5min,PPO相对活性的抑制率可达61.5%,在较短的时间就可以达到理想的效果。VC,L-半胱氨酸和PVP对PPO均有一定的抑制效果,其中以VC的效果最好。常温下用0.05%的VC处理10min,PPO相对活性的抑制率即可达到81.0%,该结果为番荔枝果肉加工中固有酶的灭活处理提供了依据和方法。     

Electrophoretic patterns of acid soluble proteins and active isoforms of peroxidase and polyphenoloxidase typifying calli and somatic embryos of two reputed date palm cultivars in Morocco

Summary When subjected to micropropagation by tissue culture, the two reputed cultivars of date palm (Phoenix dactylifera L.); Bou-Sthammi noire, resistant to Bayoud disease and Bou-Feggous, of high fruit quality, give rise to three types of calli, called white and root-forming callus, hyperhydric and degenerating callus and friable and embryogenic callus. All explant sources, calli and germinated embryos were analysed by denaturing polyacrylamide gel electrophoresis (SDS-PAGE) for acid soluble protein composition. Phenol-oxidizing enzymes; peroxidase and polyphenoloxidase, were also, evaluated and the isoforms separated by polyacrylamide gel electrophoresis. When compared with the explant and germinated embryos, embryogenic calli of the two date palm cultivars could be identified by a concentrated polypeptide of molecular weight 27 500 and polypeptides of molecular weights 70 000 and 11 500. Hyperhydric and degenerating callus contained the polypeptide exhibiting the molecular weight 32 000. Embryogenic calli showed high levels of soluble, ionically and covalently bound peroxidases. The soluble acidic isoperoxidase of R _f 0.60, revealed in these calli and germinated embryos could be a marker of the two tissues. White and root-forming calli of Bou-Feggous cultivar were typified by soluble acidic isoperoxidases with high mobility (R _f 0.75) and anodic ionically wall-bound polyphenoloxidases similar to those of the explant sources. Polyphemoloxidase activities detected in calli and embryos were very low when compared with those of explants. Used as an early test to screen embryogenic calli of date palm, acid soluble proteins, peroxidase and polyphenoloxidase data could lead to introduce lightening and economy in the tissue culture technique.     

Partial purification and some properties of polyphenol oxidase extracted from litchi fruit pericarp

Litchi (Litchi chinensis Sonn.) fruit peel polyphenol oxidase (PPO) was partially purified 21 fold by ammonium sulfate fractionation and gel filtration. Pyrogallol, catechol, and 4-methylcatechol were good substrates for the enzyme; with no activity observed with chlorogenic acid, p-cresol, resorcinol, or tyrosine. The optimal pH for PPO activity was 7.0 with 4-methylcatechol, with the enzyme being most stable at pH 7.4. The enzyme was relatively temperature stable with maximum activity at 70 °C and requiring a little less than 10 min at 90 °C for 50% loss of activity. The K_m and V_max for the enzyme, with 4-methylcatechol, were 10 mM and 1.47 × 10⁴ units/min per mg protein, respectively. The enzyme was not activated by SDS. Reduced glutathione, -cysteine, tropolone, thiourea, FeSO₄, and SnCl₂ markedly inhibited PPO activity, whereas MnSO₄ and CaCl₂ enhanced PPO activity. Data obtained in this study might help to better understand and control commercially, litchi fruit peel browning.     

Genotypic differences in susceptibility to wireworm attack in potato: mechanisms and implications for plant breeding

The influence of different tuber compounds on susceptibility to wire-worms (Agriotes obscurus) was studied in experiments with 10–13 potato genotypes. Both greenhouse and field experiments showed that the susceptibility to wireworm attack was negatively correlated with the concentration of glycoalkaloids and positively correlated with the concentration of reducing sugars in the outer 2 mm of the tubers. Varietal differences in chlorogenic acid had no effect on susceptibility. Breeding for high levels of glycoalkaloids in the extreme periphery, but very low levels in the rest of the tuber, will protect the tubers from wireworms and yet result in acceptable glycoalkaloid levels for food safety.     

Degradation of oxalic acid by transgenic oilseed rape plants expressing oxalate oxidase

Summary Oxalic acid is thought to have a primary role in the pathogenicity of several plant pathogens, notably Sclerotinia selerotiorum. A gene coding for the enzyme oxalate oxidase was isolated from barley roots and introduced into oilseed rape as a means of degrading oxalic acid in vivo. This report describes the production of several transgenic plants of oilseed rape and the characterisation of these plants by Southern, Western and enzyme activity assays. Plants were shown to contain an active oxalate oxidase enzyme and were tolerant of exogenously supplied oxalic acid.     

A regulatory role for phenol oxidase during decomposition in peatlands

Unique peatland properties, such as their ability to preserve intact ancient human remains (bog bodies) and to store globally significant quantities of atmospheric CO₂, can be attributed to their low rates of enzymic decomposition. Peatland soils are normally devoid of molecular oxygen in all, but the uppermost layer, and thus enzymes such as phenol oxidase, which require molecular oxygen for their activity, are rarely active. Interestingly, even the activities of enzymes such as hydrolases that have no oxygen requirement, are also extremely limited in peatlands. Here, we show that those low hydrolase activities can be indirectly attributed to oxygen constraints on phenol oxidase. On addition of oxygen, phenol oxidase activity increased 7-fold, P<0.05, a response that allowed phenolic depletion in the peatland soil. Phenolic materials are highly inhibitory to enzymes and their lower abundance allowed higher hydrolase activities (β-glucosidase 26%, P<0.05, phosphatase 18%, P<0.05, sulphatase 47%, P<0.01, xylosidase 16%, P<0.05 and chitinase 22%, P<0.05). Thus, oxygen constraints upon phenol oxidase activity promote conditions that inhibit decomposition. This mechanism has important implications for preservation of archaeological organic materials, sequestration of atmospheric CO₂ and potentially in the preservation of food and treatment of water pollution.     

香石竹ACC氧化酶基因克隆及其反义表达载体构建

据已报道的香石竹氨基环丙烷羧酸(ACC)氧化酶基因的cDNA序列设计特异引物,以香石竹品种“Master”基因组DNA为模板,用PCR方法克隆出香石竹ACO基因的部分序列。测序结果显示,克隆的序列与已报道序列基本一致。将克隆的香石竹ACO片段反向连接到植物表达载体pCAMBIA 1301中CaMV 35S启动子的下游,构建了香石竹ACO基因的反义表达载体pCAM-ACO2,为进一步通过反义技术延长转基因香石竹的花期奠定了基础。