Glutamate dehydrogenase in higher plants

The occurrence of NADPH-dependent glutamate dehydrogenase (GDH) was studied in higher plants. All of the higher plants tested had both NADH- and NADPH-dependent GDH activities, based on the following observations: 1) NADPHdependent GDH activity was found in the extracts of corn and soybean leaves, which was free of NADPH-dehydrogenase by heat treatment or DEAE-cellulose column chromatography. 2) Radish leaves and roots grown under germ-free condition showed the same NADPH-dependent GDH activity as those grown under conventional condition. 3) No conversion of NADPH to NADH in the reaction mixture was demonstrated by measuring alcohol dehydrogenase activity. Moreover, it was ascertained that rice plant cells grown on suspension culture had 50% of NADPH-dependent GDH activity, and both activities were not affected by nitrogen sources.     

Studies on aminoacylase in higher plants

Aminoacylase which catalyzes the hydrolysis of N-acyl-L-amino acids has been reported to be present both in animal and plant tissues. BEIRNBAUM et al. (1) obtained the purified preparations from hog kidney and classified them into two different aminoacylases according to their substrate specificity : aminoacylase I hydrolyzed a variety of N-acetyl amino acid while aminoacylase I1 attacked only N-acetyl- aspartic acid. Recently aminoacylase from microbial sources has been concentrated and purified to employ active preparations in the resolution of racemic amino acids (2–4).     

Determination of glucosamine and N-acetyl glucosamine in fungal cell walls

A new method was developed to determine glucosamine (GlcN) and N-acetyl glucosamine (GlcNAc) in materials containing chitin and chitosan, such as fungal cell walls. It is based on two steps of hydrolysis with (i) concentrated sulfuric acid at low temperature and (ii) dilute sulfuric acid at high temperature, followed by one-step degradation with nitrous acid. In this process, chitin and chitosan are converted into anhydromannose and acetic acid. Anhydromannose represents the sum of GlcN and GlcNAc, whereas acetic acid is a marker for GlcNAc only. The method showed recovery of 90.1% of chitin and 85.7-92.4% of chitosan from commercial preparations. Furthermore, alkali insoluble material (AIM) from biomass of three strains of zygomycetes, Rhizopus oryzae, Mucor indicus, and Rhizomucor pusillus, was analyzed by this method. The glucosamine contents of AIM from R. oryzae and M. indicus were almost constant (41.7 +/- 2.2% and 42.0 +/- 1.7%, respectively), while in R. pusillus, it decreased from 40.0 to 30.0% during cultivation from 1 to 6 days. The GlcNAc content of AIM from R. oryzae and R. pusillus increased from 24.9 to 31.0% and from 36.3 to 50.8%, respectively, in 6 days, while it remained almost constant during the cultivation of M. indicus (23.5 +/- 0.8%).     

Experimental studies on sulfur dioxide injuries in higher plants

Photosynthesis is reversibly inhibited by exposing plants to SO₂. The formation of α-hydroxy sulfonate in the exposed plants is suggested as one of the effects, because this compound inhibits glycolic oxidase which is necessary in the glycolic acid pathway. The suppression of the glycolic acid path should affect various aspects of metabolism in plants, particularly the reduction of the biosynthesis of glycine and serine. In this paper, the biosynthesis of these amino acids in plants exposed to SO₂ was investigated to estimate the significance of the formation of α-hydroxy sulfonate in the plants. The results show that photosynthetic formation of serine was reduced in the exposed plants, and that α-hydroxy sulfonate was formed.     

Comparative studies on the photosynthesis of higher plants

The rate of sugar formation from aspartate-¹⁴C(U) and alanine-L-¹⁴C was examined under various light intensities in three C₄-plants. The results obtained were as follows.

The rates of sugar formation from aspartate-¹⁴C(U) became larger in the following order, Paspalum urvillei, Egragrostis ferrunginea, and Zoysia japonica. This order agreed well with the order of their photosynthetic rates measured by gas analyzer. In all the C₄-plants, there were three steps in the sugar formation curve from aspartate-¹⁴C(U). At first, sugar linearly increased with an increase in the light intensity up to 20 klux. Second, from 20 to 40 klux, it hardly increased with an increase in the light intensity. Third, above 40 klux, it increased linearly again. On the other hand, the plateau did not exist between 20 and 40 klux in the sugar formation curves from alanine-l-¹⁴C in any of the cases, and sugar continued to increase with an increase in the light intensity up to 80 klux.

At low light intensities, the amount of CO₂ released from aspartate-¹⁴C(U) and alanine-l-¹⁴C correlated well with the magnitude of the dark respiration in the C₄-plants. At a high light intensity, however, CO₂ release closely correlated with the thickness of mesophyll layers surrounding the vascular bundle sheath. The thicker the mesophyll layers were, the smaller the release of CO₂ became. From this evidence, we conclude that the mesophyll layers play a vital role in refixation of the internal CO₂ in the light.     

Comparative studies on the photosynthesis of higher plants

1) CO₂ compensation points of the plants tested correlate well with the leaf anatomy. Low CO₂ compensation plants had well-developed VBS containing large and specialized chloroplasts but no plant with a high CO₂ compensation point possessed chloroplasts in the VBS.

2) CO₂ Compensation Points Closely Correlated With The Major Carboxylation Pathway In Photosynthesis. Low Compensation Plants Fixed CO₂ Via The C-4 Pathway (C-4 Plants) While High Compensation Plants Carried Out CO₂ Fixation By The Calvin Cycle (C-3 Plants).

3) Close correlations could be established for the CO₂ compensation point, the major carboxylation pathway, and glycolate oxidase activity. Glycolate oxidase activity was much higher in C-3 plants than in C-4 plants. On the other hand, dark respiration in C-4 plants was higher than that in C-3 plants.

4) TCA cycle activity in detached leaves was not inhibited to any large extent by illumination.

In C-3 plants, the release of ¹⁴CO₂ from alanine-1-¹⁴C increased with an increase in the ambient O₂ concentration; whereas, radioactivity in the sugar fraction was quite small at all O₂ concentrations. In C-4 plants the release of ¹⁴CO₂ was little affected by the ambient O₂ concentration while sugar formation was stimulated at high O₂ concentrations. This indicates that in C-3 plants CO₂ fixation is blocked at a high O₂ concentration, therefore, internal ¹⁴CO₂ is released from the leaf without being refixed, but in C-4 plants internal ¹⁴CO₂ can be efficiently refixed and metabolized to sugar by a combination of active PEP carboxylase and the ‘Kranz type’ of leaf anatomy.     

Some significant functions of silicon to higher plants

Silicon may be regarded as an essential element to cereals plant from an agronomic viewpoint. It is implicated as a factor influencing the degree of susceptibility of cereals to fungal attack. Vegetation in the tropics contains much more silicon for the protection. Once the silicon dioxide has been taken up by plants, it is rapidly accumulated in insoluble form and remains in the tissues. Yield response over the control will not be obtained if available silicon exceeds 11 mg SiO₂/100 g in the soil. The addition of silicon to the culture solution, at the rate of 75 ppm Si, decreased the accumulation of Mn, Cu, Fe, Zn, N, P and transpiration rate, but increased Ca, Mg, Si and carbohydrate contents. It is concluded that addition of silicon is particularly effective when combined with a heavy rate of nitrogen and magnesium.     

Experimental studies on sulfur dioxide injuries in higher plants

Reversible decrease in CO₂ fixation has been reported in rice plants exposed to low concentrations of SO₂ (Matsuoka et al., 1969). Alpha hydroxy sulfonate is thought to form in leaves by an addition-reaction between plant aldehyde and SO₂, and to inhibit the process of the photosynthesis. However, the identification of this compound in the leaves has not been successful. This report deals with the results of the radiochemical experiments to examine the occurrence of glyoxylate bisulfite, a-hydroxy sulfonate forms of glyoxylic acid in rice plant leaves exposed to radioactive sulfur dioxide. In plants exposed to SO₂, sufficient amounts of glyoxylate bisulfite could be formed and thereby inhibit the progress of the path from glicolic to glyoxylic acid.     

植物吸收硝态氮的分子生物学进展

植物吸收NO3-分为高亲和力(HATS,high.affinitynitratetransportsystem)和低亲和力(LATS,low.affinitynitratetransportsystem)转运系统。这两个系统的编码基因均已被克隆,其中HATS由NRT2基因家族和NAR2基因家族共同编码,LATS由NRT1基因家族编码。本文比较详细地介绍了这些转运蛋白的结构和功能以及在这方面的最新进展,概要性地介绍了这些基因的表达调控;同时摘要点评了该研究领域中还没有解决的一些问题。     

Literature review on higher plants for toxicity testing

Phytotoxicity tests using higher plants in general are infrequently used as a part of ecotoxicology. Many reports assess herbicide toxicity merely on the basis of faunal species tests. This is inadequate because the herbicide impact is much greater on flora than on fauna. Environmental pollution by herbicides was likely to have been quite wide-spread during the past years (1964–1984) when the use of herbicides grew five-fold. When herbicides reach non-target areas, they can cause unacceptable harm to non-target species, plants in particular. The toxicity of herbicides to algal species is not likely to be identical to that of higher plants, so that algal species may not serve as a surrogate species for the toxicity evaluation. Currently there are two promising phytotoxicity tests. Common duckweed is an aquatic species and sensitive to toxicity. Duckweek test can be used with static, renewal, or flow-through methods. The latter two are especially useful for unstable compounds or samples. Seed germination and root elongation tests are versatile and can be tested in water, wastewater, sediment, and slurry. Many recent activities in these areas suggest that phytotoxicity tests are a valuable part of ecotoxicology.     

Effect of humic acid on invertase synthesis in roots of higher plants

Humic acid (HA) stimulated the development of invertase activity in beetroot storage tissue discs washed in water and also in pea root segments, excised from the 2–4 mm region behind the root tip, during their most active growth phase when cultured in sucrose media. In contrast HA had no effect on the incorporation of [¹⁴C]leucine into proteins in either tissue.Inhibitors of protein synthesis (d-threo-chloramphenicol, cyclohexamide and puromycin) and RNA synthesis (actinomycin d) inhibited invertase development and the incorporation of [¹⁴C]leucine into proteins in both tissues, while also abolishing the stimulation of invertase development in the presence of HA. Similar effects were produced by the amino acid analogues, p-fluorophenylalanine, cis-4-hydroxy-l-proline and azetidine-2-carboxylic acid but, with the exception of p-fluorophenylalanine in beet discs, the inhibitory effects of the analogues were prevented in the presence of the corresponding natural amino acid.The results show that HA stimulated the synthesis of de noro invertase during the period when it also enhanced cell elongation and the effect was dependent on continuous RNA and protein synthesis.     

植物钾吸收转运基因的克隆与作物遗传改良

本文从分子水平对植物吸钾的生理机制、钾吸收转运基因的分离克隆、钾基因在植物生理中的作用及应用基因工程技术改良作物钾营养性状、培育钾高效品种等方面的研究进展作了较为系统的讨论。     

Role of root-induced changes in the rhizosphere for iron acquisition in higher plants

Plants can mobilize iron (Fe) in the rhizosphere by non-specific and specific (adaptive) mechanisms. Non-specific mechanisms are, for example, rhizosphere acidification related to high cation-anion uptake ratios, or citric acid excretion. The specific mechanisms are root responses to Fe deficiency and can be classified into two different strategies. The Strategy I is typical for dicots and monocots except for grasses (graminaceous species) and is characterized by increased plasma membrane-bound reductase activity, enhanced net excretion of protons and enhanced release of reducing compounds, mainly phenolics. The reductase activity is stimulated by low pH, and with supply of Fe^III chelates, ferric reduction at the plasma membrane takes place prior to uptake. In contrast, in graminaceous species (Strategy II) these root responses are absent, but enhancement of release of Fe^III chelating compounds - phytosiderophores - takes place. These phytosiderophores are very efficient in mobilizing Fe^III from artificially prepared sparingly soluble inorganic compounds (e.g. Fe^III hydroxide) and from calcareous soils. The ferrated phytosiderophores are taken up by grasses at rates 10² to 10³ times higher than Fe supplied either as synthetic chelate or microbial siderophores (e.g. ferrioxamine B), indicating a specific membrane transport system for ferrated phytosiderophores in roots of grasses. In calcareous soils phytosiderophores not only mobilize Fe, but also Zn, Mn, and Cu by chelation. However, only the Fe^III phytosiderophores are taken up preferentially by Fe deficient grasses. The ecological advantages and disadvantages of Strategy I and Strategy II for Fe acquisition from calcareous soils are discussed.     

高等植物GS/GOGAT循环研究进展

高等植物体内 95%以上的NH4+通过GS/GOGAT(谷氨酰胺合成酶 /谷氨酸合成酶 )循环同化。GS、GOGAT在植物叶片、根瘤以及根中均有分布 ,但在不同器官中GS/GOGAT循环的作用不尽相同。在绿色组织中 ,GS/GOGAT循环的主要作用是同化光呼吸产生的NH4+以及硝酸盐在叶中还原产生的NH4+,在根瘤中则主要同化根瘤菌固N产生的NH4+,而在根中则是同化吸收到体内的NH4+以及硝酸盐被吸收后在根中还原产生的NH4+。迄今有关植物GS/GOGAT循环的研究还不太深入 ,但是随着基因工程技术、免疫组织化学技术以及现代植物生理学技术的发展 ,GS/GOGAT循环研究展示广阔前景。对该循环及其调控机制的进一步了解 ,可为合理利用氮肥、提高植物N的利用率提供理论依据。本文综述了近年来对GS/GOGAT循环的研究进展情况     

Occurrence of pipecolic acid and pipecolic acid betaine (homostachydrine) in Citrus genus plants

The presence of pipecolic acid and pipecolic acid betaine, also known as homostachydrine, is herein reported for the first time in Citrus genus plants. Homostachydrine was found in fruits, seeds, and leaves of orange, lemon, and bergamot (Citrus bergamia Risso et Poit). As homostachydrine was not commercially available, as a comparative source, extracts of alfalfa leaves ( Medicago sativa L.) were used, in which homostachydrine is present at high concentration. Then, the results where confirmed by comparison with an authentic standard synthesized and purified starting from pipecolic acid. The synthesized standard was characterized by a ESI-MS/MS study using a 3D ion-trap mass spectrometer. When subjected to MS/MS fragmentation in positive ion mode, homostachydrine, unlike its lower homologue proline betaine (also known as stachydrine), showed a pattern of numerous ionic fragments that allowed unambiguous identification of the compound. For the quantitation in the plant sources, high sensitivity and specificity were achieved by monitoring the transition (158 → 72), which is absent in the fragmentation patterns of other major osmolytes commonly used as markers for studies of abiotic stress. As for the metabolic origin of homostachydrine, the occurrence in citrus plants of pipecolic acid leads to the hypothesis that it could act as a homostachydrine precursor through direct methylation.     

Effects of nitrogen supply on water‐use efficiency of higher plants

The worldwide increase of food demand and reduced sweet‐water availability in some important food‐producing regions raised interest in more efficient water use, which has become one of the central research topics in agriculture. Improved irrigation management and reduced bare‐soil evaporation have highest priority to increase agronomic water‐use efficiency (WUE). Compared to these technical (irrigation) and basic (crop production) management options, effects of nutrient management on WUE were less frequently considered. Twenty‐nine publications on nitrogen (N) effects on biomass WUE of container‐grown plants are considered in this review. Most of them indicate positive N effects on WUE, and relevance of N effects on intrinsic WUE and unproductive water and carbon loss is discussed. A plot of 90 published data of percent decreases of WUE and dry mass under variable N supply is presented. Extrapolation of biomass WUE from leaf measurements of intrinsic WUE is critically reviewed. The positive correlation between WUE and dry‐mass formation suggests that physiological rather than stomatal effects are more important in order to explain positive N effects on WUE.     

Iron metabolism in higher plants. The influence of nutrient iron on bean leaf lipoxygenase

Bean plants (Phaseolus vulgaris L.) were cultured in nutrient solutions containing three concentrations of iron. The leaves of plants grown with a limiting supply of iron contained reduced levels of protein, chlorophyll and the iron‐containing enzyme, lipoxygenase (EC 1.13.11.12). Measurements of lipoxygenase made on leaf crude extracts did not correlate with iron supply; however, upon purification of the enzyme by affinity chromatography, a direct correlation was observed. The lack of correlation was partly due to the presence of chlorophyll, an inhibitor of lipoxygenase, in the crude extract. Lipoxygenase levels changed with age of the plants, but rose to a maximum at days 14–16. A comparison of leaf lipoxygenases grown with different iron supply showed that they were identical with the same pH profile, chlorophyll inhibition, reaction products, and electrophoretic mobility. It is suggested that lipoxygenase may be used as a probe to evaluate iron availability and to investigate iron metabolism in higher plants.     

The apoplast — its significance for the nutrition of higher plants

Since the fundamental work of the botanist Ernst Munch there has been a clear differentiation between a symplastic and an apoplastic compartment of plants, separated by the plasmalemma. In contrast to the symplast, the apoplast was considered as being dead and hence attracted little interest. It is not before the late seventies of this century that plant scientists realised that processes such as growth and differentiation as well as signal transduction may not be understood without accounting for apoplastic processes. Since then growing evidence has supported the view that apoplastic properties are of significance for such diverse processes as genotypic variation in nutrient efficiency and tolerance against adverse ion relations, for plant/microbe interaction, or for water and nutrient transport. In this contribution we review apoplastic properties and processes in relation to plant mineral nutrition. Examples are taken from work being conducted in the scope of the special research project of the German Research Foundation “The apoplast of higher plants: compartment for storage, transport and reactions” and especially from own work.