Screening of Bioactive Metabolites and Biological Activities of Calli, Shoots, and Seedlings of <i>Mertensia maritima</i> (L.) Gray

<i>Mertensia maritima</i> (L.) Gray is threatened with extinction owing to climate change, poor seed germination, and ocean warming. In vitro explant-culture is used for ex situ preservation and plantlet massive production. In vitro cell and organ cultures serve as an alternative plant material source to investigate the biological activities and phytochemical profiles of rare plants. We aimed to develop an efficient callus and shoot production protocol and investigate bioactive metabolites, antioxidants, and enzyme inhibitory potential of <i>M. maritima</i> calli, shoots, and in vivo seedlings. The effects of combinations of different plant growth regulators, 6-BA (N⁶-benzyladenine), 6-KN (Kinetin), TDZ (Thidiazuron), and NAA (1-Naphthylacetic acid), in MS (Murashige and Skoog) nutrient medium were studied. The highest callus proliferation was obtained after 5-week cultivation over a 16-h photoperiod on growth medium MS enriched with 4 µM each of 6-BA and NAA. The medium with 2 µM 6-BA and 4 µM 6-KN had the best shoot induction rate (91.1%) with a mean of 13.4 shoots. The combination of two cytokinins (6-BA and 6-KN) was found to be effective in <i>M. maritima</i> shoot regeneration. The rooting frequency was 100% in ½ MS with Indole-3-butyric acid (IBA 2 µM). The number of detected compounds and chemical composition in the <i>M. maritima</i> shoots and seedlings extracts were similar. The total amount of phenolics in the shoots was 216.4% and 369.5% higher than in seedlings and calli, respectively. The total amount of flavonoids in the shoots was 241.1% and 429.3% higher than in seedlings and calli, respectively. The best antioxidant activity was obtained in the shoots, followed by seedlings and calli. However, the order was seedlings > calli > shoots regarding metal chelating ability. The strongest acetylcholinesterase inhibition properties were obtained in the calli, followed by seedlings and shoots. However, the tested samples can be ranked as seedlings > shoots > calli in butylcholinestrase inhibition assay. This study is the first report on the enzyme inhibitory effects of <i>M. maritima</i> extracts, providing valuable contributions to the scientific community.