The stem galler Eurosta solidaginis induces both localized and systemic physiological changes in the leaves of Solidago canadensis

Eurosta solidaginis oviposits in the apical bud of Solidago canadensis , leading to the development of a stem gall. Whether the presence of a stem gall affects leaf physiology and whether such effects are beneficial or detrimental to Solidago are unknown. We examined the physiological effects of Eurosta infection on Solidago leaves at two locations on the stem: close to the gall and far from the gall (i.e., near the ground). Chlorophyll levels were not impacted by Eurosta infection, but stomatal density was higher in leaves from infected plants close to the gall, suggesting elevated CO ₂ uptake capacity in infected plants. Starch concentration was lower in infected plants, but only in leaves far from the gall. Starch accumulates within the galls, which may act as such powerful carbon sinks that leaves near the ground cannot maintain their own starch reserves. Catalase activity was higher in infected plants but only close to the gall. Notwithstanding, cells within leaves close to the gall were not more resistant to membrane damage from exogenous H ₂ O ₂ . We propose that Eurosta induced higher catalase activity, but only within the chloroplasts, to protect itself from photosynthetic reactive oxygen species (ROS) from nearby leaves. Levels of salicylic caid (SA), which is involved in plant defense, were elevated in infected plants but only close to the gall, suggesting a localized defense response only. Overall, Eurosta maximizes its energy supply and minimizes its oxidative damage by modulating Solidago leaf physiology, thereby increasing its own fitness at the expense of its host.