Abstract
The formation of galls by gall-inducing insects on their host plants demonstrates the concept of 'extended phenotype', referring to traits expressed in a host organism manipulated by a parasite. Phytohormones, amino acids, and proteins have been suggested as compound types involved in gall formation, and a few specific molecules have been proposed as candidate effector molecules; however, no specific gall-inducing effector molecules have been identified. Hence, in this study, we identified the cysteine-rich secretory proteins, antigen 5, and pathogenesis-related 1 proteins (CAP) peptide from horned gall aphid Schlechtendalia chinensis transcripts through in silico screening and the Arabidopsis-based gall-forming assay, a bioassay system for analysing the molecular mechanisms of gall formation. Furthermore, we successfully generated the insect-gall-like structure in Veronica peregrina-the host plant of the gall-forming weevil Gymnaetron miyoshii Miyoshi-without insect parasitism, using three minimal effector elements: the CAP peptide, auxin, and cytokinin. Given the similarity of the gall structure to the native gall, both in gene expression profile and morphology, we experimentally demonstrated that the gall-like structure was artificially generated using effector molecules, the CAP peptide, auxin, and cytokinin.