Proline: A Reliable Biochemical Marker of Plant Abiotic Stress Tolerance?

Climate change is placing global agriculture under growing pressure, as plants must withstand extreme environmental conditions such as drought and high salinity, both inducing osmotic and oxidative stress. As part of their survival strategies, plants accumulate protective molecules (osmolytes), including the amino acid proline. For decades, plant biology has largely assumed that high proline accumulation under stress signals strong stress tolerance. However, this review challenges that "proline-centric" perspective. Analyses across a wide range of plant species reveal a more complex picture. Stress-induced proline accumulation is not universal: in some species, proline levels remain relatively unchanged, with other metabolites acting as functional osmolytes, or increase only in response to artificially applied severe stress conditions. Even when proline increases, its absolute concentrations may be too low to contribute significantly to osmotic adjustment. Nevertheless, proline may still be involved in stress tolerance mechanisms through its additional roles, detoxifying reactive oxygen species (ROS), directly stabilising proteins or acting as a stress signalling molecule. Comparative analyses of genetically related taxa with varying degrees of stress tolerance sometimes show negative correlations between proline accumulation and tolerance, with higher proline concentrations measured in the most sensitive genotypes. Overall, the evidence indicates that proline's role in plant survival is highly context-dependent and strongly influenced by genetic background and must therefore be evaluated on a case-by-case basis. Distinguishing whether proline acts as an adaptive defence or merely as a biochemical marker of physiological strain under stress is essential for accurately assessing plant stress tolerance.