Biosynthetic pathways hosts

The three key fruit quality traits, color, heat, and flavor, are independent characteristics. The color of a fruit does not predict the hotness, despite many consumers associating green peppers with no-heat bell peppers and red powder with hot cayenne chile. Hence in New Mexico, the state question is Red or Green to determine which version of the host s salsa or enchilada is hotter. From a phytochemical perspective the independence of these traits relates to the biosynthetic pathways for these compounds and the genes for both the structural and regulatory genes for those pathways. This review describes some of the recent advances in the analyses of the compounds, enzymes, and genes associated with color and flavor in Capsicum. 

In a recent study (54), we showed increased activities of two enzymes of the general phenylpropanoid pathway, PAL and 4-coumarate CoA lig-ase, as well as one enzyme of the specific pathway of lignin biosynthesis, cinnamy 1-alcohol dehydrogenase (CAD), in resistant plants at the time of the hypersensitive host cell death. On the other hand, decreased activities were observed at the same time with susceptible host plants (54). Furthermore, we showed that the well known increase in peroxidase activities, which is strong in resistant and only weak in susceptible plants (55-58), is at least partly due to the increased activity of the lignin biosynthetic pathway (54,59). 

Some of the best understood polyvalent interactions are found in immune and host defense systems as well as ligand-receptor activation. An example is the use of a polyvalent immunogen based on a synthetic peptide to elicit immune responses. The subsequent production of site-specific antibodies can then be employed to confirm the identity of proteins derived from recombinant DNA, to explore biosynthetic pathways, to define precursor-product relationships (e.g., proenzyme and preproenzyme), and to determine protein structural domains.19 ... 

In accordance with their opportunistic way of life, parasitic flatworms have limited biosynthetic capacities as described in Introduction, they obtain many simple substrates from their hosts. More complex molecules that the parasite cannot obtain directly from the host are synthesized from these simpler building blocks. Obviously, the parasite has to synthesize complex structures like proteins and DNA. In general, the biosynthetic pathways of parasitic helminths bear a close resemblance to those of their mammalian hosts. However, the enzymes of these pathways often possess different properties, and in cases where parasites produce unique end products, there may be distinct pathway components that involve unique enzymes that are absent from the host. 

Apparently, parasitic flatworms have discarded some pathways of de novo lipid synthesis, but have selectively retained several biosynthetic pathways that modify host lipids. Although lipids like fatty acids and cholesterol are obtained from the host, less abundant lipids that are more difficult to acquire because of their low concentration in the host (e.g. specific unsaturated fatty acids, eicosanoids, ecdysteroids and quinones) are synthesized de novo by the parasite, usually by the modification of more abundant substrates. In this way, lipid metabolism of parasitic flatworms is adapted to an opportunistic way of life, just like their energy metabolism. 

One fascinating aspect of hydrocarbon evolution as semiochemicals lies in the documented chemical mimicry systems between parasite and host (Chapter 14, this book). Some systems operate by camouflage and passive transport, others by loss of parasite specific compounds, some use de novo synthesis of the same host mixture or part of the mixture, and others are as yet unrevealed. To date we are mostly limited to the description of the host-parasite chemical coevolution (or arms race), but we hope in the near future to be able to associate biosynthetic pathways as well as gene expression with such evolutionary processes. 

Growth retardation in host plants has been associated with the use of a number of these EBI s. This effect results from the inhibition of a reaction in the gibberellin biosynthetic pathway that involves cytochrome P-450 enzymes (24). 

This brief survey of natural products derived from the 3-deoxy-D-oro6iTk>-heptulosonic acid 7-phosphate pathway illustrates the economy of fimda-mental biosynthetic pathways. The relative economy and simplicity of the biological degradative and energy-yielding reactions is paralleled in the biosynthetic reactions. For example, 3-deoxy-D-oroW o-heptulosonic acid 7-phosphate is a precursor of a host of aromatic products mevalonic acid is the progenitor of the terpenoids and steroids, and 5-aminolevulinic acid of the porphyrins. 

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