Host plant nitrogen

Figure 11 The different steps of nitrogen metabolism in the extraradical hyphae, ccto-mycorrhizal roots, and roots of the host plant. I, absorption 2, assimilation 3. storage 4. translocation A, extramatrical hyphae B. ectomycorrhizal sheath C, Hartig net D, root cortical cells AA amino. acids.

For an effective symbiotic state, the plant and the microsymbiont must maintain a constant metabolic flow of carbon and nitrogen. While the bacteroids function as an engine for nitrogen fixation the fuel comes from the plant. Dicarboxylic acids are the primary carbon sources fed to the bacteroids by the plant. This unidirectional flow of carbon must be controlled by the PBM. Recently, several specific carbon and amino acid transport systems have been identified in the PBM using isolated peribac-teroid units (PBU Day et al., 1990). Thus, in order for the host plant to house endosymbiotic bacteria and support their metabolic needs, a number of nodulin genes must be induced to support the ontogeny and function of the nodules. 

Sequestration of plant natural products by herbivorous insects is widespread. This is not surprising, since most insects are herbivorous and it is estimated that there are between two and six million species. While these sequestrators obviously exploit their alkaloid-rich host plants, such specialists represent a very minor component of the total herbivorous population. In essence, these nitrogenous compounds are highly effective deterrents for most of the herbivorous species with which they share their world. On the other hand, a brief discussion of some of the alkaloids and their specialist herbivores illustrates the versatiliity of these insects as exploiters of toxic compounds This alkaloidal treatment has been derived from the tables of Brown and Trigo10 and emphasizes... 

Transferring the NIF gene to other bacteria which have a broader spectrum of host plants but have no natural nitrogen-fixing capability... 

However, not all conditions that meet the technical definition of di.sease%re detrimental to plants. Nodules on the roots of peas and beans are caused by bacteria, and they do interrupt some of the normal plant functions, so they are a disease. Their net effect, however, is beneficial because these bacteria take nitrogen from the air and convert it into a form that plants can use. Similarly, the roots of almost all plants are infected with mycorrhizal fungi. Although they sap some food from plants, these beneficial fungi help their host plants garner nutrients from the soil by increasing the effective surface area of the root system. 

Obbard JP, Jones KC. Measurement of symbiotic nitrogen-fixation in leguminous host-plants grown in heavy metal-contaminated soils amended with sewage sludge. Environ Pollut 2001 111 (2) 311-20. 

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