ABSTRACTAmino acids play a crucial role in the growth and development of insects. Aphids cannot ingest enough amino acids in plant phloem to meet their requirements, and therefore, they are mainly dependent on the obligate symbiontBuchnera aphidicolato synthesize essential amino acids. BesidesBuchnera, aphids may harbor another facultative symbiont,Arsenophonus, which alters the requirement of the cotton-melon aphidAphis gossypiifor amino acid. However, it is unclear howArsenophonusregulates the requirement. Here, we found thatArsenophonusameliorated growth performance ofA. gossypiion an amino acid-deficient diet. A deficiency in lysine (Lys) or methionine (Met) led to changes in the abundance ofArsenophonus.Arsenophonussuppressed the abundance ofBuchnerawhen aphids were fed a normal amino acid diet, but this suppression was eliminated or reversed when aphids were on a Lys- or Met-deficient diet. The relative abundance ofArsenophonuswas positively correlated with that ofBuchnera, but neither of them was correlated with the body weight of aphids. The relative expression levels of Lys and Met synthase genes ofBuchnerawere affected by the interaction betweenArsenophonusinfections andBuchneraabundance, especially in aphids reared on a Lys- or Met-deficient diet.Arsenophonuscoexisted withBuchnerain bacteriocytes, which strengthens the interaction.IMPORTANCEThe obligate symbiontBuchneracan synthesize amino acids for aphids. In this study, we found that a facultative symbiont,Arsenophonus, can help improve aphids’ growth performance under amino acid deficiency stress by changing the relative abundance ofBuchneraand the expression levels of amino acid synthase genes. This study highlights the interaction betweenArsenophonusandBuchnerato ameliorate aphid growth under amino acid stress.