Host-symbiont interactions

A. Dell, W. S. York, M. McNeil, A. G. Darvill, and P. Albersheim, Host-symbiont interactions 14. The cyclic structure of beta-D-(l,2)-linked D-glucans secreted by rhizobia and agrobacteria, Carbohydr. Res., 117 (1983) 185-200. 

Carlson, R.W., Sanders, R.E., Napoli, C., Albersheim, P. Host-symbiont interactions III. Purification and characterization of Rhizobium lipopolysaccharides. Plant Physiol 62 (1978) 912-917. 

Tetratrophic interactions between a host plant, a phytophagous pest (primary host), a hymenopteran parasitoid or symbiont (secondary host) and a hymenopteran hyperparasitoid (which parasitizes the secondary host) are of considerable importance, because hyperparasitism can significantly reduce populations of economically beneficial parasitoids [11]. Hyperparasitoids use host-marking (=spacing) pheromones, sex pheromones [12], and host-detection cues [42], but they also show additional chemically mediated interactions with the other partners. These include detection of the primary host s secretions by the hyperparasitoid [43], detection of plant volatiles by the hyperparasitoid [44], and detection of the hyperparasitoid s secretions by the primary host [45] or by the secondary host. The latter causes the secondary host to avoid locations where the hyperparasitoid is foraging [46]. 

It is now known that the initial interaction between plants and bacteria of the Rhizobiaceae is a chemical detection by the microbe of a susceptible host, i.e., the host produces compounds which act as signals for the microbial pathogen or symbiont. The microbe responds to these signals by expression of genes necessary in subsequent stages of the interaction. For a few of the Rhizobiaceae some signal compounds involved have been identified (1-7). 

What direct effects climate change will have on various aspects of nitrogen cycling on reefs including coral bleaching events and zooxantheUae expulsion, remains to be seen. The subsequent consequences for nitrogen conservation and nitrogen acquisition of the corals and other symbioses, on the reef will undoubtedly be affected as the balance between symbionts and host interaction is very sensitive to environmental conditions (Knowlton, 2001). 

The temi symbioses was first defined loosely by De Bary (1879) as two or more difFerendy named organisms bving together. Although symbiotic interactions are ubiquitous in nature, few of the marine planktonic systems have been well characterized, and comparatively less is known of the functional role of the symbiont for the host and vice versa. Many of the planktonic symbioses are between eukaryotic hosts and cyanobacterial symbionts, or cyanobionts. Cyanobacteria are photosynthetic, and many are capable of nitrogen (N2) fixation, thus often it is presumed... 

In addition to a distinction based on microhabitats, variation in symbiont-host relationships gives rise to different terminology. In a mutualism, both the host and the symbiont profit from their relationship. Mutualism, therefore, is defined as the cooperative interaction between different species. Virtually every higher organism is involved in mumal interactions because mitochondria and chloroplasts of eukaryotic cells are descendants of bacteria (see below). Other examples are plants associated... 

Many well-known S5mibiotic relationships are difficult to categorize into one of the three types, either because distinctions are not clear-cut or because experimental data are hard to obtain. For instance, the question of whether the relationship between humans and some types of their gut flora is commensal or mutual still is unanswered because of the complexity of the multispecies association. The African oxpecker bird is an example where categories are not discrete. The bird picks flies, ticks, and other parasites from the body of a mammal and, in this respect, is a mutualist. However, because blood is its favored food, it keeps wounds open on the body of its host (16). Thus, a mutual symbiont can be a parasite in a separate interaction. 

Dale C, Moran NA. Molecular interactions between bacterial symbionts and their hosts Review. Cell 2006 126 453-465. 

This chapter reviews secondary metabolites isolated from symbionts, organized by host taxonomy. It provides an overview of the different roles of these compounds in the chemical ecology of the interaction, if known, and discusses experimental techniques to study symbiotic systems as well as present gaps of knowledge. 

Rhizobium occurs free-living, but cannot fix nitrogen when it is existing as a natural member of the soil population. The sequence of events in the infection of roots and formation of an active, nitrogen-flxing nodule reveals a complex series of interactions between symbiont and host. 

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