Highway to stress: Proline dynamics in tomato plants under osmotic disturbances

Upon exposure to osmotic stress, plants accumulate proline, an osmoprotectant and antioxidant, often associated with stress tolerance. However, recent studies suggest that proline buildup does not necessarily improve the redox status or stress resilience in plant tissues, being instead perceived as a sign of sensitivity. Therefore, this study aimed to elucidate how plants coordinate proline accumulation and metabolism during osmotic stress and subsequent recovery. A bifactorial experimental design was conducted using tomato plants in a semi-hydroponic system. Tissue samples were collected in three moments: t0: after 20 days of growth under control conditions; t1: after 4 days of exposure to 8% (w/v) polyethylene glycol (PEG); t2: after one week of recovery under control conditions. PEG-induced stress triggered proline accumulation. Despite this pronounced buildup, exposure to PEG delayed growth and caused oxidative damage. Following recovery, proline decreased in roots despite persistent biosynthetic activity, whereas in shoots, proline remained high despite a decline of the biosynthetic activity, coinciding with oxidative damage. Expression and activity of P5CS and ProDH illustrate that proline metabolism shows differential transcriptional and post-transcriptional regulation. Results also indicate the involvement of the ornithine pathway in proline accumulation in shoots, and regulation of proline content via distribution among organs, besides metabolic homeostasis. Overall, proline appears to act as a compatible solute in roots with rapid but transient accumulation, while its sustained and massive accumulation in shoots does not confer effective antioxidant protection or stress tolerance. This supports previous observations that proline may function as a sign of stress severity rather than a tolerance mechanism.