It's a gardener's worse nightmare: your newly planted seedlings become infested by a horde of aphids. With unlimited sap on tap, aphids should have a ticket to the good times, but Meena Haribal and Georg Jander from the Boyce Thompson Institute for Plant Research, USA, explain that the sugary fluid is far from nutritious. Plant roots primarily produce four amino acids – glutamine, glutamate, asparagine and aspartate – which provide the voracious insect's only source of nitrogen. To overcome the restrictions of their diet, many aphid species have struck a cooperative deal with symbiotic bacteria: providing their lodgers with food and security in return for nutritional supplements that the aphids cannot provide for themselves. Haribal and Jander were curious to find out how the pea aphid (Acyrthosiphon pisum) and its bacterial lodger (Buchnera aphidicola) convert the four non-essential amino acids into the essential amino acids that are necessary for the aphid's survival.
Feeding the aphids a series of diets laced with amino acids fortified with heavy nitrogen and carbon isotopes, the duo then embarked on a brain-teasing set of gas chromatography–mass spectrometry experiments to untangle the amino acid synthetic web linking the aphid to its symbiotic bacteria. Scrutinising hundreds of mass spectra and tracing the paths of nitrogen and carbon atoms, the duo found that the amine nitrogen of asparagine was incorporated more readily into other amino acids than the amide nitrogen. However, the duo was intrigued to see that most of the amino acids in the aphids’ bodies were composed of combinations of normal and heavy carbon, suggesting that instead of building the amino acids directly from the skeletons of the four plant-derived amino acids, the aphids were breaking down the plant-derived amino acids into small components which they then used to synthesise the essential amino acids that were not provided by their diet. The duo was also surprised that instead of directly using the amino acids that they consumed in sap, the aphids were resynthesising them too. ‘Even when there is an excess of an amino acid in the diet, it gets broken down and re-synthesised by the aphids’, says Jander.