Transcriptome-based identification of enzymes involved in amino acid biosynthesis in the small brown planthopper, Laodelphax striatellus

The small brown planthopper, Laodelphax striatellus, is a phloem-feeding pest in many important crop plants such as rice and wheat in China. Phloem sap is rich in simple sugars, but low in nitrogenous organic compounds and especially low in essential amino acids. It would be of interest to determine whether this unbalanced diet is compensated for by intracellular microbial symbionts. Based on the transcriptome of L. striatellus and reverse transcription-polymerase chain reaction, we identified 23 fragments that encoded 17 enzymes contributing to amino acid biosynthesis. Sequence alignment, codon usage bias, and phyloge- netic analysis indicate that three genes originate from symbiont and the remainder from insect host. It is suggested that enzymes from L. striatellus are responsible for amino group donor synthesis, ammonium assimilation, and biosynthesis of nine amino acids, including glutamate, glutamine, proline, aspartate, asparagine, alanine, serine, glycine, and tyrosine. The biosynthesis of methionine might be catalyzed by enzymes from both host insect and symbiont. Symbiont enzymes are implied to be involved in the biosynthesis of valine, leucine, isoleucine, lysine, and tryptophan. The enzymes catalyzing the syntheses of arginine, threonine, phenylalanine, histi- dine, and cysteine are not found. Our results imply that the symbionts may partially compensate for the amino acid requirements of L. striatellus.