Stable isotope studies reveal pathways for the incorporation of non-essential amino acids in Acyrthosiphon pisum (pea aphids)

Abstract

Plant roots incorporate inorganic nitrogen into the amino acids glutamine, glutamic acid, asparagine and aspartic acid, which together serve as the primary metabolites of nitrogen transport to other tissues. Given the preponderance of these four amino acids, phloem sap is a nutritionally unbalanced diet for phloem-feeding insects. Therefore, aphids and other phloem feeders typically rely on microbial symbionts for the synthesis of essential amino acids. To investigate the metabolism of the four main transport amino acids by the pea aphid (Acyrthosiphon pisum), and its Buchnera aphidicola endosymbionts, aphids were fed defined diets with stable isotope-labeled glutamine, glutamic acid, asparagine or aspartic acid (U-13C, U-15N; U-15N; α-15N; or γ-15N). The metabolic fate of the dietary 15N and 13C was traced using gas chromatography–mass spectrometry (GC-MS). Nitrogen was the major contributor to the observed amino acid isotopomers with one additional unit mass (M+1). However, there was differential incorporation, with the amine nitrogen of asparagine being incorporated into other amino acids more efficiently than the amide nitrogen. Higher isotopomers (M+2, M+3 and M+4) indicated the incorporation of varying numbers of 13C atoms into essential amino acids. GC-MS assays also showed that, even with an excess of dietary labeled glutamine, glutamic acid, asparagine or aspartic acid, the overall content of these amino acids in aphid bodies was mostly the product of catabolism of dietary amino acids and subsequent re-synthesis within the aphids. Thus, these predominant dietary amino acids are not passed directly to Buchnera endosymbionts for synthesis of essential amino acids, but are rather are produced de novo, most likely by endogenous aphid enzymes.

Intercropping impacts the host location behaviour and population growth of aphids

“…On the other hand, cereals and legumes compete for soil nutrients at early growth stages (Corre-Hellou et al, 2006;Bedoussac et al, 2015). Unfortunately, our method does not distinguish essential from non-essential amino acids which, together with the ongoing debate about the pathways of uptake and utilization of plant nutrients by aphids, muddles the link between sap quality and aphid population growth (Vogel & Moran, 2011;Haribal & Jander, 2015). Nevertheless, in our experiment, we did not find a decrease in the biomass, nor differences in the concentration of free amino acids in the leaves of wheat intercropped with clover, compared to that from wheat monocultures.…”

“…Nevertheless, in our experiment, we did not find a decrease in the biomass, nor differences in the concentration of free amino acids in the leaves of wheat intercropped with clover, compared to that from wheat monocultures. Unfortunately, our method does not distinguish essential from non-essential amino acids which, together with the ongoing debate about the pathways of uptake and utilization of plant nutrients by aphids, muddles the link between sap quality and aphid population growth (Vogel & Moran, 2011;Haribal & Jander, 2015).…”

microRNA regulation in an ancient obligate endosymbiosis

“…Unlike most animals, M. persicae and A. pisum have notably lost the ability to synthesize arginine (International Aphid Genomics Consortium, ; Mathers et al., ; Wilson et al., ) and are dependent on their endosymbiont Buchnera for compensation of shortfalls in dietary arginine supply. In contrast, glutamine, the dominant aphid haemolymph amino acid (Sasaki & Ishikawa, ), is synthesized by aphids in bacteriocyte cells (Hansen & Moran, ; Haribal & Jander, ; Poliakov et al., ) for provision to Buchnera to both meet Buchnera's dietary requirements and to serve as an amino donor for the Buchnera ‐mediated biosynthesis of histidine and arginine (Hansen & Moran, ). Our recent work has both highlighted the likely importance of arginine and glutamine to regulation of amino acid biosynthesis in aphid bacteriocytes (Price et al., ) and implicated mTORC1 in aphid/ Buchnera metabolic and developmental integration (Lu, Chang, & Wilson, ; Lu, Price, Wikramanayake, Chang, & Wilson, ).…”

Plant manipulation through gall formation constrains amino acid transporter evolution in sap-feeding insects

“…Amino acid uptake, transport, and catabolism underlie the success of herbivory as a life history strategy [29, 40]. Here we present the first multigene tree for members within the Phylloxeridae; a family with both galling and free-living herbivores.…”