Targeted metabolomic and gene expression analysis reveals an organ-specific response to chronic ionizing radiation exposure during Pinus sylvestris seedling development

Background Conifers like Scots pine ( Pinus sylvestris ) are a potential bioindicator species for radioactive contamination as they are among the most radiosensitive plant species. Phytohormones and amino acids are central to plant growth and stress signaling, but their roles in the pine radiation response, particularly in a tissue-specific context, have not been adequately studied. Results Here, we investigated the morphological, metabolic and transcriptional consequences of chronic gamma irradiation in pine seedlings under controlled laboratory conditions. We exposed P. sylvestris seedlings to 10 weeks of low-dose external gamma irradiation (682 µGy/h) and performed targeted metabolomic analysis of phytohormones and amino acids and RT-qPCR analysis of hormone-related gene expression in the shoot tip, root tip, young needles, and cotyledons, alongside a microscopic analysis of the shoot apical meristem. Irradiation induced a significant reduction in the height and area of the meristem. This morphological change coincided with a trend towards decreased abundance of growth-promoting hormones in the shoot tip, namely cytokinins and gibberellins. Our data also indicate the potential involvement of specific cytokinin and gibberellin signals in the young needles in the response to irradiation, and hint towards a tissue-specific role for jasmonyl-ACC conjugation in the young shoot. Concurrently, the cotyledons of irradiated seedlings displayed a massive and significant increase in free amino acid concentrations and decreased cytokinin abundance, consistent with accelerated senescence. Conclusions Taken together, these results reveal a tissue-specific pattern of metabolic and transcriptional changes in irradiated pine seedlings. The data are consistent with a scenario in which apical growth is hormonally suppressed and resources may be liberated from cotyledon tissue, which could then potentially be reallocated toward a stress response in developing organs. This study thereby provides an integrated view of the organ-level metabolic response to chronic radiation.