Modulation of plant cytokinin levels in the Wolbachia-free leaf-mining species Phyllonorycter mespilella

As phytohormones lie at the very core of molecular mechanisms controlling the plant physiology and development, they have long been hypothesized to be involved in insect-induced plant manipulations. Cytokinins (CKs) are phytohormones now widely recognized to be utilized by leaf-mining and gall-inducing insects in the control of the physiology of their host plant. In some leaf-mining moth species, larvae can supply the hormones themselves, bacterial symbionts contributing to the production of CKs. Our objective was to investigate whether closely related leaf-miner species sharing the same ecological niche but differing in their Wolbachia infection status develop similar strategies to manipulate their host plant. An extensive identification and quantification of CKs has been used to elucidate physiological patterns relevant for the plant-insect interactions. Our results show that modulation of plant CK levels is impaired in the Wolbachia-free leaf-mining moth Phyllonorycter mespilella (Hubner) (Lepidoptera: Gracillariidae) contrasting with results previously observed in the closely related moth species Phyllonorycter blancardella (Fabricius) that produce and deliver CKs to the plant through an intricate interaction with Wolbachia. Our study suggests that mechanisms underlying colonization of the host plant and adaptation to fluctuating environmental conditions are different between the two leaf-miner species and that P.mespilella larvae most likely do not produce CKs. This species rather only buffers the degradation of CKs naturally occurring during the senescence of leaves leading to few active CKs being maintained at a sufficiently high level to induce and maintain a green island' phenotype (photosynthetically active green patches in otherwise senescing leaves). This study further provides converging experimental evidence pointing toward the influence of bacterial symbionts in the ability of leaf-mining moths to control the physiology of their host plant with consequences for their ecology and evolutionary diversification.