侵染细胞质膜附近小泡的来源及其作用

箭舌豌豆根瘤幼龄侵染细胞的壁和质膜比较光滑,成熟侵染细胞与此不同,不仅细胞壁厚薄均,有较多的胞间连丝,而且质膜常常内陷形成各种突起,然后离质膜形成小泡。这些位于质膜附近的小泡体积较小,多呈圆形,既可单独存在,也可多个聚在一起。在向细胞中央移动中,有的小泡靠近细胞质膜,甚至与细菌周期融合,有的小泡不民附近的小液泡融合变为较大液泡,并常用降解程度不同的细菌位于其中,在衰老侵染细胞中,细胞壁附近有小泡,     

Labelling of lupine nodule metabolites with 14CO2 assimilated from the leaves

On feeding ¹⁴CO₂ to the shoots of lupine (25 mCi per plant) 30 min was the minimal time needed to determine the incorporation of label into bacteroid compounds. The predominant incorporation, exhibited in all root, nodule and bacteroid samples after 30 min exposure, was into sucrose (45–90% of the corresponding fraction radioactivity) of the neutral fraction; into malate (30–40%) of the acid fraction; into aspartic acid and asparagine (60–80% in sum) of the basic fraction. The composition of carbon compounds containing the greatest amount of ¹⁴C in the cytosol of nodules and in bacteroids was similar. Their radioactivity after 30 min exposure was for bacteroids (nCi per g of bacteroid fr. wt): sucrose 5.73, glucose 1.00, malate 0.15, succinate 0.11; for the nodule cytosol (nCi per g of nodule fr. wt): sucrose 200.00, glucose 8.40, malate 9.34, succinate 8.50. Thus it was demonstrated that in lupine, sucrose is the main photoassimilate entering not only into nodules but also into bacteroids. The biosynthesis of aspartic acid and asparagine occurs during nitrogen fixation in bacteroids.     

Carbon metabolism and bacteroid respiration in nodules of chick-pea (Cicer arietinum L.) plants grown under saline conditions

ABSTRACT

The present work investigates the relationships between nitrogen fixation, carbon metabolism and oxygen consumption by bacteroids of Mesorhizobium ciceri in root nodules of chick-pea plants. Its aim was to establish whether some of the compounds which accumulate under salt stress may be used as respiratory substrates by bacteroids to fuel their own metabolism and nitrogenase activity. Plants were grown in a growth chamber, and salt stress was induced by adding 50 mM NaCl to the nutrient solution at sowing. The data presented here show a rise in fermentative metabolism in nodules of chick-pea plants exposed to high salinity, and suggest that proline, lactate or ethanol, may play an important role as energy-yielding substrates for bacteroids in this plant species. The bacteroids could utilize glucose as a respiratory substrate both under control and saline conditions, while malate did not appear to be the preferred substrate in the presence of salt.     

Metabolism of Rhizobium Bacteroids

Nitrogen fixation within legume nodules results from a complex metabolic exchange between bacteria of the family Rhizobiaciae and the plant host. Carbon is supplied to the differentiated bacterial cells, termed bacteroids, in the form of dicarboxylic acids to fuel nitrogen fixation. In exchange, fixed nitrogen is transferred to the plant. Both the bacteroid and the plant-derived peribacteroid membrane tightly regulate the exchange of metabolites. In the bacteroid oxidation of dicarboxylic acids via the TCA cycle occurs in an oxygenlimited environment. This restricts the TCA cycle at key points, such as the 2-oxoglutarate dehydrogenase complex, and requires that inputs of carbon and reductant are balanced with outputs from the TCA cycle. This may be achieved by metabolism through accessory pathways that can remove intermediates, reductant, or ATP from the cycle. These include synthesis of the carbon polymers PHB and glycogen and bypass pathways such as the recently identified 2-oxoglutarate decarboxylase reaction in soybean bacteroids. Recent labeling data have shown that bacteroids synthesize and secrete amino acids, which has led to controversy over the role of amino acids in nodule metabolism. Here we review bacteroid carbon metabolism in detail, evaluate the labeling studies that relate to amino acid metabolism by bacteroids, and place the work in context with the genome sequences of Mesorhizobium loti and Sinorhizobium meliloti. We also consider a wider range of metabolic pathways that are probably of great importance to rhizobia in the rhizosphere, during nodule initiation, infection thread development, and bacteroid development. Referee: Dr. Robert Ludwig, Department of Molecular, Celluar, and Developmental Biology, Sinheimer Laboratories, University of California, Santa Cruz, CA 95064     

银合欢根瘤细胞的光学显微镜和电子显微镜研究

银合欢接种根瘤菌形成根瘤后,应用光镜和电镜技术观察。银合欢根瘤由分生组织细胞、皮层组织细胞、维管束系统和侵染细胞区域四个不同部分组成。根瘤菌借助于侵染线侵染细胞,释放进入宿主细胞质中,转变成固氮类菌体。最初每个包被膜内只含单独的类菌体,随后较老的侵染细胞中,每个包被膜内含有一个以上的类菌体。因此,成熟根瘤的侵染细胞可见有2~5个类菌体群集包被膜里,并且明显地累积PHB物质,显示电子染色透明颗粒。本文还讨论了上述变化的意义与银合欢根瘤细胞结构和功能的关系。     

Primary metabolites of Phaseolus vulgaris nodules

More ethanol soluble material (carbohydrate and amino nitrogen) was found in both host cell and bacteroid components of Phaseolus vulgaris nodules from plants grown at 28 W/m² than from plants grown at 7 W/m². The range of compounds identified was similar at the two irradiances. On feeding ¹⁴CO₂ to the plant tops at either irradiance the labelling patterns of carbohydrates and organic acids in the nodule host cells and bacteroids suggested that any or all of the following substances could be donated by the host to the bacteroids for general metabolism: sucrose, fructose, glucose, an unidentified carbohydrate, malic acid and an organic acid co-chromatographing with 6-phosphogluconate. Distribution and labelling patterns of nodule amino compounds were consistent with the hypothesis that ammonia is the primary product of nitrogen fixation within bacteroids, and that this ammonia is transported to host cells for assimilation, initially into glutamine and glutamate.     

Respiration supported nitrogenase activity of isolated Rhizobium meliloti bacteroids

Bacteroids having a high level of respiration-supported nitrogenase activity were isolated from nitrogen-fixing alfalfa root nodules. Gentle maceration under anaerobic conditions in the presence of sodium succinate and a fatty acid scavenging agent were employed in this method. A large proportion of isolated bacteroids retained a triple membrane structure as shown by transmission electron microscopy. Dicarboxylic acids of the TCA cycle (malate, fumarate, succinate), but not glutamate or aspartate, supported sufficient respiratory activity to supply the nitrogenase system with ATP and reducing equivalents and to protect the nitrogenase system from inactivation by 4% oxygen over a period of 20-30 min. Sugars did not support nitrogenase activity in intact bacteroids. The properties of the isolated bacteroids were ascribed to minimal damage to the cytoplasmic membrane and peribacteroidal membrane during isolation. With succinate as substrate and oxygen as terminal electron acceptor, initial nitrogenase activity was determined at 4% oxygen in the gas phase of the assay system employed. At this oxygen concentration, the sustained rate of acetylene reduction by respiring bacteroids was linear up to 30 min. Bacteroid activity declined rapidly with time of exposure to oxygen above 4% in the gas phase. The optimum temperature range for this activity was 10-20 degrees C. Nitrogenase activity was measurable at incubation temperatures below 10 degrees C under 4% oxygen. Functionally intact bacteroids had little nitrogenase activity under anaerobic conditions in the presence of an external source of ATP and reductant. Treatment of the bacteroids with chlorpromazine eliminated respiration-supported activity and rendered the bacteroid cell membrane permeable to external ATP. Bacteroids treated with chlorpromazine had high acetylene reducing activity with external ATP and dithionite in the absence of oxygen.     

Endophytic colonisation of Bacillus subtilis in the roots of Robinia pseudoacacia L

The endophytic colonisation of Bacillus subtilis strain GXJM08, isolated from roots of Podocarpus imbricatus B1. Enum. P1. Jav., in roots of the leguminous plant Robinia pseudoacacia L. was investigated. Ultrastructure observations showed that B. subtilis caused morphological changes in the root hair and colonised the plant through infected root hairs. The structure of the infection thread was similar to that of rhizobia, but the structure of infected cells was different. B. subtilis is also different from rhizobia and plant pathogens in terms of the formation of a peribacteroid membrane and the mode of penetration through the host cell wall. Our results provide a basis for studying development of the mutualistic symbiotic relationship between B. subtilis and plants, and a basis for studying the mechanism of the B. subtilis–plant interaction.     

Conformation of cytoskeletal elements during the division of infected Lupinus albus L. nodule cells

Lupin nodule cells maintain their ability to divide for several cycles after being infected by endosymbiotic rhizobia. The conformation of the cytoskeletal elements of nodule cells was studied by fluorescence labelling, immunocytochemistry, and laser confocal and transmission electron microscopy. The dividing infected cells showed the normal microtubule and actin patterns of dividing plant cells. The clustered symbiosomes were tethered to the spindle-pole regions and moved to the cell poles during spindle elongation. In metaphase, anaphase, and early telophase, the symbiosomes were found at opposite cell poles where they did not interfere with the spindle filaments or phragmoplast. This symbiosome positioning was comparable with that of the organelles (which ensures organelle inheritance during plant cell mitosis). Tubulin microtubules and actin microfilaments appeared to be in contact with the symbiosomes. The possible presence of actin molecular motor myosin in nodules was analysed using a monoclonal antibody against the myosin light chain. The antigen was detected in protein extracts of nodule and root cytosol as bands of approximately 20 kDa (the size expected). In the nodules, an additional polypeptide of 65 kDa was found. Immunogold techniques revealed the antigen to be localized over thin microfilaments linked to the cell wall, as well as over the thicker microfilament bundles and surrounding the symbiosomes. The pattern of cytoskeleton rearrangement in dividing infected cells, along with the presence of myosin antigen, suggests that the positioning of symbiosomes in lupin nodule cells might depend on the same mechanisms used to partition genuine plant cell organelles during mitosis.     

苜蓿根瘤菌自生与共生状态下核糖体蛋白的比较

苜蓿根瘤菌在与宿主植物建立共生关系的过程中,以自生状态进入宿主植物细胞,经过分化发育转变为共生状态的类菌体(Bacteroid)。由于生存环境发生了变化,类菌体在形态、结构和功能方面都产生了很大的改变,其中最为明显的改变是类菌体获得了共生固氮的能力。此时,类菌体中许多与共生相关的基因被激活,蛋白的表达量显著增加。为了探明这种改变是否与合成蛋白质的细胞器-核糖体有关,比较分析了苜蓿根瘤菌在自生和共生状态下核糖体蛋白的表达谱。蛋白质双向电泳结果显示二者之间没有明显的差别,说明类菌体的分化发育过程中核糖体蛋白的形成没有改变。