Host plant consuming

Unfortunately, symbiotic N2 fixation is not without cost. The host plant must provide photosynthetically reduced carbon (photosynthate) to support the N2-fixing and NH4" -assimilating activities within the nodule, the site of N2 fixation. In many cases, the photosynthate consumed to support these activities represents from 15-35% of the net photosynthate of the host (3). More than half this amount is utilized for the assimilation of NH4". Because N2 fixation is limited by the availability of photosynthate and the amoimt of photosynthate required to support the assimilation of NH4 is substantial, it is essential to imderstand the pathways by which nitrogen is assimilated and transported to the host plant as well as the carbon metabolism associated with this process. 

The ultimate purpose of finding, examining, and consuming behaviors is, of course, to increase the probability that the insect (or its offspring) consumes hosts on which fitness, i.e., developmental rate, survivorship and/or fecundity, are maximized (Jaenike, 1978 Mitchell, 1981 Rausher, 1983). These fitness components are influenced by such plant characteristics as nutritional quality, presence or absence of toxins, and exposure to natural enemies, which collectively determine the host plant s suitability. Although documented cases are yet few, it is generally hypothesized that the acceptability of plant cues is positively correlated with suitability. 

Animals are unable to synthesize folic acid (6.62) and must consume adequate quantities in their diets. Plants and bacteria, however, are able to make folic acid. The first step of this synthesis is catalyzed by dihydropteroate synthetase and reacts dihydroptero-ate diphosphate (6.69) and para-aminobenzoic acid (PABA, 6.70) (Figure 6.25). Because this pathway is not found in humans, inhibition of the reaction is a method to ultimately stop TMP synthesis in an invading bacterium while not impacting the infected host. The sulfonamides, often called sulfa drugs, are a class of antibiotic that exploits the folic acid pathway and inhibits dihydropteroate synthetase. Sulfa drugs bind in the same fashion as PABA and act as competitive inhibitors. The active form of the first sulfa drug is sulfanilamide (6.71). Sulfamethoxazole (6.72) is a sulfa drug that is widely prescribed today.26... 

As diets become more plant product oriented and less animal product oriented several factors theoretically should contribute to a decrease in iron nutritional status, particularly, in vulnerable groups such as young women. However, it appears that host adaptative mechanisms come into play which enable individuals consuming plant oriented diets to make better use of these poor iron availability resources than would be expected. 

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