Agricultural soil degradation is occurring at unprecedented rates, not only as an indirect effect of climate change (CC) but also due to intensified agricultural practices which affect soil properties and biodiversity. Therefore, understanding the impacts of CC and soil degradation on plant physiology is crucial for the sustainable development of mitigation strategies to prevent crop productivity losses. The amino acid proline has long been recognized for playing distinct roles in plant cells undergoing osmotic stress. Due to its osmoprotectant and redox-buffering ability, a positive correlation between proline accumulation and plants’ tolerance to abiotic stress has been pointed out in numerous reviews. Indeed, proline quantification is used systematically by plant physiologists as an indicator of the degree of tolerance and a measurement of the antioxidant potential in plants under stressful conditions. Moreover, the exogenous application of proline has been shown to increase resilience to several stress factors, including those related to soil degradation such as salinity and exposure to metals and xenobiotics. However, recent data from several studies often refer to proline accumulation as a signal of stress sensitivity with no clear correlation with improved antioxidant activity or higher stress tolerance, including when proline is used exogenously as a stress reliever. Nevertheless, endogenous proline levels are strongly modified by these stresses, proving its involvement in plant responses. Hence, one main question arises—is proline augmentation always a sign of improved stress resilience? From this perspective, the present review aims to provide a more comprehensive understanding of the implications of proline accumulation in plants under abiotic stress induced by soil degradation factors, reinforcing the idea that proline quantification should not be employed as a sole indicator of stress sensitivity or resilience but rather complemented with further biochemical and physiological endpoints.

Wild tomato species (WTS) have long been attracting the attention of plant researchers due to their ability to tolerate harsh environmental conditions in their natural habitats. In this context, their use in grafting procedures, where they serve as rootstocks, has been proposed as a strategy to enhance the stress resilience of cultivated tomato. However, insights into the effectiveness of this approach are still needed to understand its real potential. Thus, this study evaluated whether grafting tomato plants (cv. Chico III) onto WTS rootstocks (Solanum habrochaites LA1223 and Solanum galapagense LA1403) could improve growth and physiological performance under combined drought (limited irrigation) and salinity (200 mM NaCl) stress. Biometrical and photosynthetic parameters were evaluated in grafted and non-grafted plants, before and after stress exposure, and stress-induced changes in biochemical endpoints were also assessed. Grafting onto WTS delayed scion growth and did not prevent the reductions in photosynthesis, growth, or reproductive output observed under stress conditions. Still, the use of wild rootstocks contributed to some physiological adjustments, including enhanced photosystem II efficiency under control conditions and improved antioxidant and osmotic regulation under stress, although these effects were insufficient to fully counteract the impact of combined drought and salinity. These findings indicate that, under severe yet relevant conditions of combined abiotic stress, grafting onto WTS alone is not enough to improve tomato plant resilience. Importantly, this study provides novel insights into rootstock-mediated physiological responses and highlights species-specific stress adjustments among WTS, offering valuable information for future integrated stress mitigation strategies.