Integrative approach reveals new insights into photosynthetic and redox protection in ex vitro tobacco plantlets acclimatization to increasing light intensity

Integrative mechanisms involving photosynthetic and antioxidant protection regulated by light intensity during in vitro and ex vitro plantlets acclimatization are poorly understood. Tobacco plantlets grown under in vitro and ex vitro environments were exposed to different light regimes to evaluate the role of photosynthesis and antioxidant protection. In vitro plantlets displayed a narrow photosynthetic capacity to cope with light as revealed by low net CO₂ assimilation (P_N), decreased actual quantum efficiency of photosystem II (ΦPSII) associated with non-induction of non-photochemical quenching (NPQ). In contrast, acclimated ex vitro plants showed strong stimulation in P_N whereas ΦPSII and NPQ remained at high levels. In vitro plantlets exposed to moderate light suffered strong oxidative stress associated with increased activities of superoxide dismutases, catalases and ascorbate peroxidases, revealing an ineffective antioxidant system. In contrast, ex vitro plants presented lower oxidative damage in parallel to unchanged enzymatic activities, indicating an efficient antioxidant steady state. The levels of reduced ascorbate (ASC) and glutathione (GSH) were also increased only in ex vitro plants in response to excess light. An integrative study based on correlation networks and principal component analyses (PCA) corroborate that the two plant groups indeed displayed contrasting acclimation processes. In conclusion, during in vitro to ex vitro transition, tobacco plantlets exposed to increasing light display physiological adjustments involving photosynthesis and improvement of the enzymatic and non-enzymatic antioxidant systems. These findings highlight the importance of integrative approaches to understand ex vitro acclimatization to environmental stimuli.