Iron metabolism acts as a bridge between photosynthesis and red coloration of bud galls induced on Nothofagus obliqua (Nothofagaceae).

Color and morphology are some of the most intriguing traits of plant galls, whose patterns resemble fruits and flowers. Many hypotheses were proposed to explain the involvement of anthocyanin accumulation with the development of red gall hues, whose mechanisms seem idiosyncratic. Anthocyanins are related to photoprotective strategies in green tissues and metal accumulation in some flowers. Despite that, the combination of such physiological phenomena has been neglected for galls, which are photosynthetic neoplasms genetically similar to reproductive organs. Here, we integrated different perspectives by measuring photosynthetic pigment and anthocyanin concentration combined with fluorescence quenching analysis, antioxidant activity assays, and histochemical elemental mapping in red and green galls induced by Espinosa nothofagi (Hymenoptera) on Nothofagus obliqua (Nothofagaceae). We found no relationship between high anthocyanin concentrations, light exposure, and red coloration in galls as anthocyanin concentrations were higher in the outermost tissues of green galls than in red galls. Red galls presented higher concentrations of total chlorophyll and lower carotenoid concentrations than green galls and leaves, which correlated with their highest photosynthetic activity and iron accumulation. The red color coincides with the accumulation of aluminum and Fe³⁺ and the lowest antioxidant capacity in the gall outer tissue. The high antioxidant capacity of N. obliqua galls and the Fe²⁺ and Fe³⁺ distribution are related to high photosynthesis, Fe-use efficiency in galls, and the supply of Fe to the inducer diet. Overall, iron metabolism connects the high photosynthesis activity to the red gall color in the presence of low anthocyanin concentrations, like some flowers.