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.     

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.     

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     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Molecular farming of recombinant antibodies in plants

Antibodies represent a large proportion of therapeutic drugs currently in development. In most cases, they are produced in mammalian cell lines or transgenic animals because these have been shown to fold and assemble the proteins correctly and generate authentic glycosylation patterns. However, such expression systems are expensive, difficult to scale up and there are safety concerns due to potential contamination with pathogenic organisms or oncogenic DNA sequences. Plants represent an inexpensive, efficient and safe alternative for the production of recombinant antibodies. Research over the last 10 years has shown that plants can produce a variety of functional antibodies and there is now intense interest in scaling up production to commercial levels. In this review, we discuss the advantages of plants over traditional expression systems, describe how antibody expression in plants is achieved and optimized and then consider the practical issues concerning large-scale molecular farming in plants. The first plant-produced therapeutic antibodies are already in clinical trials, and, given the economic benefits of this production system, we are likely to see many more recombinant antibodies produced in this manner in the future.     

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。     

Über den Einfluss desd,l-Methioninsulfoximins auf den Eiweißstoffwechsel bei Erbsen

Summary The influence of thed,l-Methioninesulphoximine on the nitrogen metabolism of germinating seeds has been studied on peas. It has been shown thatd,l-Methioninesulphoximine, even at low concentrations, inhibited the germination of seeds and produced a decrease of growth of the roots. The inhibited germinating seeds contained less non-protein-nitrogen and less asparagine than the control plants. On the basis of the experimental results,d,l-Methioninesulphoximine could be considered as an inhibitor of proteolysis.     

Biological interactions in the mycorrhizosphere

Summary Microbial communities in the mycorrhizosphere (i.e. in the small volume of soil immediately surrounding a mycorrhizal root) actively interact with the establishment and functioning of the mycorrhizal symbiosis. Examples of competitive and mutualistic interactions are given, and are discussed in the light of recent observations and experimental results. The significance of biological interactions in the mycorrhizosphere is considered from the standpoints of plant ecology and of practical application.     

How plants recognize pathogens and defend themselves

Plants have an innate immunity system to defend themselves against pathogens. With the primary immune system, plants recognize microbe-associated molecular patterns (MAMPs) of potential pathogens through pattern recognition receptors (PRRs) that mediate a basal defense response. Plant pathogens suppress this basal defense response by means of effectors that enable them to cause disease. With the secondary immune system, plants have gained the ability to recognize effector-induced perturbations of host targets through resistance proteins (RPs) that mediate a strong local defense response that stops pathogen growth. Both primary and secondary immune responses in plants depend on germ line-encoded PRRs and RPs. During induction of local immune responses, systemic immune responses also become activated, which predispose plants to become more resistant to subsequent pathogen attacks. This review gives an update on recent findings that have enhanced our understanding of plant innate immunity and the arms race between plants and their pathogens. Received 24 June 2007; received after revision 18 July 2007; accepted 15 August 2007     

Notizie preliminari sull'azione fitodinamica dell'acido difenililacetico sulla pianta del tabacco

Summary The application of diphenylilacetic acid has shown favorable effects on the growth of tobacco plants. Solutions containing 10 p.p.m. and 20 p.p.m. produce, in weight, an almost double growth in treated plants in comparison with the controls. Diphenylilacetic acid can be considered as a growth-promoting substance. The data are not definite.     

Inhibition of the growth of cariogenic bacteria in vitro by plant falvanones

Phytoalexins, defensive compounds produced by plants against microbial infections, were purified fromSophora exigua (Leguminosae) and their growth inhibitory effects on oral cariogenic bacteria were determined in vitro. Among three isolated compounds, 5,7,2,4-tetrahydroxy-8-lavandulylflavanone completely inhibited the growth of oral bacteria including primary cariogenic mutans streptococci, other oral streptococci, actinomycetes, and lactobacilli, at concentrations of 1.56 to 6.25 g/ml.     

植物硫苷-黑芥子酶系统对环境胁迫的响应

硫苷-黑芥子酶系统是植物中一种特有的底物酶系统,其水解产物具有高度生物活性,该系统对植物的生长发育、生长素微调以及植物的防御保护等均有重要意义.目前对植物硫苷-黑芥子酶系统对环境胁迫的响应的研究多为对硫苷的研究,而对系统相关物质研究较少.本文对硫苷-黑芥子酶系统做了简单介绍,并对植物硫苷-黑芥子酶系统对病虫害和酸雨、水、温度、盐、重金属胁迫下的响应进行了综述.     

Predicting long-term earnings growth: Comparisons of expected return models,submartingales and value line analysts

This paper derives four–five year predictions of growth rates of accounting earnings per share implicit in four expected return models commonly used in financial research. A comparison of such growth rates with those produced and reported by Value Line analysts and those generated by a submartingale model revealed the following: two expected return models—the Sharpe–Lintner–Mossin model and the Black model—were significantly more accurate than the submartingale model, though not significantly more accurate than the other return models. However, the growth rate forecasts provided by Value Line significantly outperformed all the other models tested—none of which relied on the direct input of a security analyst.     

Reciprocal biological activities of the cyclic tetrapeptides chlamydocin and HC-toxin

Chlamydocin, a potent cytostatic agent against cultured mammalian cells, and HC-toxin, a host-specific phytotoxin, are cyclic tetrapeptides containing the same epoxide alpha-amino acid. We show here that these compounds have reciprocal biological activity; HC-toxin is cytostatic against cultured mastocytoma cells, and chlamydocin has host-specific toxin activity against maize. Chlamydocin and another related cyclic peptide, Cyl-2, are less host-specific than HC-toxin because maize tolerant to HC-toxin is more sensitive to chlamydocin and Cyl-2.