Article

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: 5.53). 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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    • "In many cases, this involves symbiotic interactions with microorganisms that produce essential amino acids whose synthesis is not encoded in animal genomes (Crotti et al., 2010; Defossez et al., 2011; Nardi et al., 2002; Oh et al., 2010; Russell et al., 2009; Schloss et al., 2006; Spiteller et al., 2000). Some of the best-studied symbioses of this type are found in aphids, whiteflies and other phloem-feeding Hemiptera (Braendle et al., 2003; Douglas et al., 2006; Kikuchi et al., 2007, 2009; Wilkinson et al., 2001). Phloem-feeding insects benefit from having evolved rapid and efficient pathways for the uptake and metabolic conversion of glutamine, glutamic acid, asparagine and aspartic acid into essential amino acids. "

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    • "2004; Chau et al., 2005). However, N fertility, with increasing amino acid and nitrate level in host plant (Mengel and Kirkby, 2001) can enhance nutritional quality and attractiveness of plants for herbivorous insects; therefore, it improves performance parameters of phloem feeders (Mattson, 1980; White, 1993; Douglas, 2006; Fallahpour et al., 2015). For instance, results of a study conducted by Aqueel and Leather (2011) showed that 0.4 g plant -1 N fertilizer in the form of ammonium nitrate significantly enhanced the fecundity and longevity of Sitobion avenae (F.) and Rhopalosiphum padi (L.) in comparison to those by 0.1 g plant -1 N fertilizer level. "
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    • "-chain oligosaccharides and excrete them as honeydew (Douglas, 2006). Phloem sap-sucking insects are also able to get their amino acids from symbiotic microorganisms and amino acids synthetase enzymes (Wilkinson, 1998; Douglas, 2006). "
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