Acquisition of nutrients from organic resources by mycorrhizal autotrophic plants

Summary Evidence exists to suggest that mycorrhizal fungi are capable of producing enzymes allowing them to access carbon, nitrogen and phosphorus from complex organic resources in soil. This facility is mainly demonstrated in ectomycorrhizal and ericaceous endomycorrhizal fungi associated with highly organic soils and climatically stressed environments. These data support a direct nutrient cycling hypothesis proposed for tropical ectomycorrhizal forests. In terms of forest succession, the evidence agrees with a major contribution of the mycorrhizal symbiosis in late stages of the succession, where elemental cycling becomes increasingly more conservative and process rates limited by biotic factors. Here, tree growth benefits from direct nutrient cycling mediated by their mycorrhizal symbionts.     

Biochemical approaches to the study of plant-fungal interactions in arbuscular mycorrhiza

This communication compares some biochemical methods for quantifying colonization by arbuscular mycorrhizal (AM) fungi. The degree of mycorrhizal colonization can conveniently be measured by determining fungal specific sterols. AM-colonized plants show a specific synthesis of 24-methylene cholesterol and an enhanced level of campesterol (=24-methyl cholesterol). A gene probe for nitrate reductase, the key enzyme for nitrogen assimilation, has been developed, which allows the monitoring of the distribution of this enzyme in fungi. Among the phytohormones tested, only abscisic acid (ABA) is found at a considerably higher level in AM-colonized plants than in controls. The concentration of ABA is about twenty times higher in spores and hyphae of the AM fungusGlomus than in maize roots. Other phytohormones (auxins, cytokinins) do not show such alterations after mycorrhizal colonization. The roots of gramineous plants become yellow as a result of mycorrhizal colonization. The yellow pigment(s) formed is (are) deposited in larger quantities in the vacuole(s) of the root parenchyma and endodermis cells during the development of the gramineous plants. A substance isolated from such roots has now been identified as a C-14 carotenoid with two carboxylic groups, and named mycorradicin.     

Phenols in mycorrhizal roots ofArachis hypogaea

Summary Roots of mycorrhizal groudnut plants had higher amounts of phenols compared to non-mycorrhizal roots. Histochemical study of the mycorrhizal root revealed the accumulation of phenols in hyphae and arbuscules of the fungus within the host.     

Response of maize to different inoculum densities of vesicular arbuscular mycorrhizal endophytes

Summary The effect of the inoculum density of vesicular arbuscular mycorrhizal endophytes on growth and development in maize was investigated in sterilized soil under glass-house conditions. Mycorrhizal plants grew robust and produced three times more dry weight than non-mycorrhizal plants. 40 or more endophytes per plant produced the highest mycorrhizal association and the maximum growth in maize. The uninoculated plants exhibited the symptoms of chlorosis.     

Mycorrhizas in ecosystems

Summary The results of analyses of the distribution, structure and function of ericoid, ecto and vesicular-arbuscular mycorrhizas are used to challenge the conventional view, which was based largely upon studies of isolated plants and excised plant roots under controlled conditions, that the symbiosis is primarily involved in the capture of phosphate ions. In nature, each mycorrhizal type is associated with an ecosystem and soil environment with distinctive characteristics in which selection has favoured the development of a particular range of attributes. These attributes are evaluated and their importance for the individual plant and for the ecosystems in which they occur is assessed. It is concluded that knowledge of the full range of functions of each mycorrhizal type is essential for an understanding of the distribution and dynamics of the ecosystem in which it predominates.     

Mycorrhizas and root architecture

Summary Roots function dually as a support system and as the nutrient uptake organ of plants. Root morphology changes in response to the soil environment to minimize the metabolic cost of maintaining the root system, while maximizing nutrient acquisition. In response to nutrient-limiting conditions, plants may increase root fineness or specific root length (root length per gram root weight), root/shoot ratio, or root hair length and number. Each of these adaptations involves a different metabolic cost to the plant, with root hair formation as the least costly change, buffering against more costly changes in root/shoot ratio. Mycorrhizal symbiosis is another alternative to such changes. Plants with high degrees of dependence on the symbiosis have coarser root systems, less plasticity in root/shoot ratio, and develop fewer root hairs in low-fertility soils. In nutrient-limited soils, plants highly dependent on mycorrhiza reduce metabolic cost by developing an even more coarse or magnolioid root system, which is less able to obtain nutrients and thus creates a greater dependence of the plant on the symbiosis. These subtle changes in root architecture may be induced by mycorrhizal fungi and can be quantified using topological analysis of rooting patterns. The ability of mycorrhizal fungi to elicit change in root architecture appears to be limited to plant species which are highly dependent upon mycorrhizal symbiosis.     

Influence of phosphorus supply and light intensity on mycorrhizal response inPisum-Rhizobium-Glomus symbiosis

The influence of mycorrhizal colonization withGlomus mosseae on parameters of N₂ fixation and plant growth was studied in pot experiments with pea plants (Pisum sativum L.) infected withRhizobium leguminosarum and supplied with varied levels of phosphorus (P) and nitrogen (N). Reduced light intensities were used to evaluate the dependence of the microsymbionts on assimilate supply. In plants grown with low P supply, mycorrhization increased the concentration of P in shoots, and thus N₂ fixation. Reduced light intensity significantly depressed mycorrhizal colonization and nodule growth in low-P plants. When P supply did not limit plant growth and N₂ fixation, however, the percentage of mycorrhizal colonization was reduced due to the higher P status, and the microsymbionts were not impaired by low light intensities. To maximize carbohydrate supply, another experiment was carried out at high light intensity of 900 mol m^–2s^–1 and with non-limiting P supply. Nitrogen fertilization, given as starter N, enhanced plant growth, but delayed nodule formation. Towards flowering, nodulation rapidly increased, but less so inGlomus inoculated plants. After 28 days mycorrhizal plants were lower in shoot dry weight, nodule dry weight and nitrogenase activity. The results suggest that under many, but not all, environmental conditions the host plant is able to restrict mycorrhizal colonization and, thus, to prevent impairment ofRhizobium symbiosis.deceased in May 1994     

Phytoalexin response is elicited by a pathogen (Rhizoctonia solani) but not by a mycorrhizal fungus (Glomus mosseae) in soybean roots

Summary A container system was constructed to study the response of soybean roots to infection by mycorrhizal or pathogenic fungi. The system allows a rapid and synchronous inoculation byGlomus mosseae orRhizoctonia solani. The phytoalexin glyceollin was measured in roots of inoculated and uninoculated plants for a period of 30 days. A significantly increased content of phytoalexin was found inR. solani-infected roots as compared to uninfected control roots. However, there was no difference in the glyceollin contents of the mycorrhizal and the control roots for up to 23 days after inoculation. The accumulation of glyceollin inR. solani-infected roots was not influenced by a subsequent inoculation withG. mosseae. Moreover glyceollin accumulated in mycorrhizal plants to the same extent as in control plants when they were inoculated withR. solani. The two fungi did not mutually influence the course of infection when they were inoculated together.     

Comparative anatomy of the host-fungus interface in mycorrhizas

Summary There are several types of mycorrhizal symbiosis (ectomycorrhiza, endomycorrhiza, ectendomycorrhiza), and the interfaces between the host-plant and the fungal symbiont have different organizations. The interfaces between the partners are always limited on the one side by the fungal plasmalemma and on the other side by the plasmalemma of the host plant or the perisymbiont membrane derived from it The cytoplasms of the partners are therefore separated by a mixed apoplast consisting of a fungal wall and a host wall or an apposition layer.     

Assimilation of mineral nitrogen and ion balance in the two partners of ectomycorrhizal symbiosis: Data and hypothesis

Summary Assimilation pathways of mineral nitrogen and ion balances of the two partners of ectomycorrhizal symbiosis (fungi and woody plants) are reviewed. Data are presented about the partners both in pure culture and in mycorrhizal association. The two forms of mineral nitrogen, ammonium and nitrate, differ in their mobility in the soil, their transport into the cells, their uptake rates by plants and their assimilation pathways. These metabolic differences are related to differences in adjustment of ion balances and carbon metabolism under conditions of nitrate or ammonium nutrition. The data obtained on the partners of ectomycorrhizal symbiosis are discussed from this point of view and the observations composed with those on herbaceous angiosperms.     

Regulation of mycorrhizal infection by hormonal factors produced by hosts and fungi

Summary An overview of current research on hormonal factors produced by plants and fungi in mycorrhizal associations is presented. On the one hand, growth hormones in roots and their exudates influence the metabolism and growth of fungi. On the other, fungal hormones influence root morphology, metabolic changes and the growth of the entire plant.     

The significance of mycorrhizas for protective ecosystems

Summary On the basis of the reviews presented in this issue, the ecological significance of mycorrhizal symbioses is discussed. Mycorrhizas may have some importance in the acquisition of mineral nutrients during the productive phase of ecosystems in early stages of succession, but their main role is played during the protective phase of ecosystems in the final stages of succession when most resources are incorporated into biomass. In these successional stages, mycorrhizas short-circuit nutrient cycles by directly reacquiring nutrients in organic form from plant (and fungal) litter, and they may reallocate resources between different plant individuals, preventing loss of resources from the entire ecosystem.     

Costs and benefits of mycorrhizas: Implications for functioning under natural conditions

Summary It is paradoxical that most plants under natural conditions are infected with vesicular-arbuscular mycorrhizal fungi, yet that it is often difficult to demonstrate that infected plants receive any benefit from the association. The costs and benefits of infection are analysed and a hypothesis formulated that infection only yields benefits at times during the life cycle when P demand by the plant exceeds the capacity of the root system. A simulation model is described that suggests that infection density should be more or less constant below a threshold value of root P uptake rate, but that above this value roots should be non-mycorrhizal. More extensive study of mycorrhizas under field conditions is needed to test such predictions.     

植物修复重金属与多环芳烃污染土壤强化措施研究进展

重金属和多环芳烃是土壤环境中的重要污染物,其复合污染土壤的修复已成为环境科学研究的热点问题。植物修复技术是目前修复土壤复合污染重要方法之一,但植物本身修复能力有限,需借助化学、微生物、基因工程等手段对其修复效果进行强化。本文对国内外近几年来植物修复重金属-多环芳烃复合污染强化措施研究成果进行综述,并重点讨论了植物根际生长促进菌及菌根在强化修复中的应用,在此基础上对未来该领域需要深入研究的科学问题进行了阐述。     

Nectin family of cell-adhesion molecules: structural and molecular aspects of function and specificity

Cell–cell adhesive processes are central to the physiology of multicellular organisms. A number of cell surface molecules contribute to cell–cell adhesion, and the dysfunction of adhesive processes underlies numerous developmental defects and inherited diseases. The nectins, a family of four immunoglobulin superfamily members (nectin-1 to -4), interact through their extracellular domains to support cell–cell adhesion. While both homophilic and heterophilic interactions among the nectins are implicated in cell–cell adhesion, cell-based and biochemical studies suggest heterophilic interactions are stronger than homophilic interactions and control a range of physiological processes. In addition to interactions within the nectin family, heterophilic associations with nectin-like molecules, immune receptors, and viral glycoproteins support a wide range of biological functions, including immune modulation, cancer progression, host-pathogen interactions and immune evasion. We review current structural and molecular knowledge of nectin recognition processes, with a focus on the biochemical and biophysical determinants of affinity and selectivity that drive distinct nectin associations. These proteins and interactions are discussed as potential targets for immunotherapy.     

Glycoconjugates in sperm function and gamete interactions: how much sugar does it take to sweet-talk the egg?

Glycoconjugates in the mammalian reproductive tract are critical components of the molecular mechanisms that control sperm maturation, sperm transport and gamete interactions. In the oviduct of many species, sperm transport and maturation are regulated by protein-carbohydrate interactions that form a sperm reservoir. Subsequently, gamete interactions are mediated by the binding of lectin-like sperm proteins with carbohydrate moieties on the zona pellucida. The sperm glycocalyx is extensively modified during sperm transport and maturation. Multiple functions have been proposed for this dense carbohydrate layer overlying the sperm plasmalemma, and sperm-surface carbohydrates have been implicated in immune-mediated human infertility. The structure and function of glycoconjugates in the oviductal sperm reservoir, the zona pellucida, and on the sperm surface are reviewed.     

Using artificial systems to explore the ecology and evolution of symbioses

The web of life is weaved from diverse symbiotic interactions between species. Symbioses vary from antagonistic interactions such as competition and predation to beneficial interactions such as mutualism. What are the bases for the origin and persistence of symbiosis? What affects the ecology and evolution of symbioses? How do symbiotic interactions generate ecological patterns? How do symbiotic partners evolve and coevolve? Many of these questions are difficult to address in natural systems. Artificial systems, from abstract to living, have been constructed to capture essential features of natural symbioses and to address these key questions. With reduced complexity and increased controllability, artificial systems can serve as useful models for natural systems. We review how artificial systems have contributed to our understanding of symbioses.     

Molecular pathogenesis of virus infections

Although a very wide range of viral diseases exists in vertebrates, certain generalizations can be made regarding pathogenetic pathways on the molecular level. The presentation will focus on interactions of virions and their components with target cells. Using coronaviruses as examples the changes in virulence have been traced back to single mutational events; recombination, however, is likely to be an alternative mechanism by which virus-host interactions (e.g. the cell-, organ- or animal species-spectrum) can dramatically change. Receptor molecules are essential for the early interactions during infection and some of these have been identified. Events in the target cell and the host organism are discussed, and wherever possible, aspects of virus evolution and cooperation between infectious agents are highlighted.     

Mesenchymal–epithelial interactions during digestive tract development and epithelial stem cell regeneration

The gastrointestinal tract develops from a simple and uniform tube into a complex organ with specific differentiation patterns along the anterior–posterior and dorso-ventral axes of asymmetry. It is derived from all three germ layers and their cross-talk is important for the regulated development of fetal and adult gastrointestinal structures and organs. Signals from the adjacent mesoderm are essential for the morphogenesis of the overlying epithelium. These mesenchymal–epithelial interactions govern the development and regionalization of the different gastrointestinal epithelia and involve most of the key morphogens and signaling pathways, such as the Hedgehog, BMPs, Notch, WNT, HOX, SOX and FOXF cascades. Moreover, the mechanisms underlying mesenchyme differentiation into smooth muscle cells influence the regionalization of the gastrointestinal epithelium through interactions with the enteric nervous system. In the neonatal and adult gastrointestinal tract, mesenchymal–epithelial interactions are essential for the maintenance of the epithelial regionalization and digestive epithelial homeostasis. Disruption of these interactions is also associated with bowel dysfunction potentially leading to epithelial tumor development. In this review, we will discuss various aspects of the mesenchymal–epithelial interactions observed during digestive epithelium development and differentiation and also during epithelial stem cell regeneration.