Phloem-sap feeding by animals: problems and solutions. J Exp Bot

Department of Biology, University of York, PO Box 373, York YO10 5YW, UK.
Journal of Experimental Botany (Impact Factor: 5.79). 02/2006; 57(4):747-54. DOI: 10.1093/jxb/erj067
Source: PubMed

ABSTRACT The incidence of phloem sap feeding by animals appears paradoxical. Although phloem sap is nutrient-rich compared with many other plant products and generally lacking in toxins and feeding deterrents, it is consumed as the dominant or sole diet by a very restricted range of animals, exclusively insects of the order Hemiptera. These insects display two sets of adaptations. First, linked to the high ratio of non-essential:essential amino acids in phloem sap, these insects contain symbiotic micro-organisms which provide them with essential amino acids. For example, bacteria of the genus Buchnera contribute up to 90% of the essential amino acids required by the pea aphid Acyrthosiphon pisum feeding on Vicia faba. Second, the insect tolerance of the very high sugar content and osmotic pressure of phloem sap is promoted by their possession in the gut of sucrase-transglucosidase activity, which transforms excess ingested sugar into long-chain oligosaccharides voided via honeydew. Various other animals consume phloem sap by proxy, through feeding on the honeydew of phloem-feeding hemipterans. Honeydew is physiologically less extreme than phloem sap, with a higher essential:non-essential amino acid ratio and lower osmotic pressure. Even so, ant species strongly dependent on honeydew as food may benefit from nutrients derived from their symbiotic bacteria Blochmannia.

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    • "Thus, the expression of sucrose transporters seems to be tightly regulated in accordance with cell wall invertases in response to many environmental factors including biotic stresses (Bitterlich et al. 2014; Ferrieri et al. 2015). Sucrose taken by aphid is present in plant phloem sap at high concentrations, often in the range of 0.5–1 M and is responsible for high osmotic pressure, often up to five times that of the feeding Hemipteran body fluids (Douglas 2006). "
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    • "Aphids feed on plant phloem, an abundant source of carbon and energy, and obtain nitrogen predominantly in the form of free amino acids (Douglas, 2006). Aphids can be reared on fat-free diets for many generations (Douglas, 1998). "
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    • "Essential amino acids such as Ile and Leu are required by C. fagisuga for proper development. Growth and protein synthesis in hemipterans can be constrained by a limited concentration of just one essential amino acid (Douglas, 2006). We therefore suggest that Leu in beech bark may constrain C. fagisuga populations, as we observed a strong positive association between these. "
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