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Polyamine (PA) metabolism and functions have started to raise attention from plant scientists in the last years. PAs have been investigated for their involvement in plant cell signalling and protection, proving that alterations in their endogenous levels can affect plant growth, development and survival. The recognised roles of PAs in metal‐stressed plants are presented and discussed on a “case‐study” basis, for each metal. Bearing in mind that the contamination of soils by heavy metals (HMs) is a growing problem worldwide, it is important to find efficient mechanisms through which agricultural productivity and food quality are safeguarded in a scenario of increased pollution. Making cultivars more tolerant to HM‐stress, capable of detoxifying or accumulating them safely, is a goal that will most certainly benefit from researching the functions and applicability of PAs. To date, plant PAs have been recognised for their roles as membrane‐, protein‐ and nucleic acid‐stabilisers, as protectors of cellular integrity and photosynthetic machinery, as direct and indirect signalling agents, and as emerging members of the non‐enzymatic antioxidant system. Moreover, PAs are not only important in normal plant developmental processes but have also been suggested to induce stress priming, to act as epigenetic regulators of gene expression and to enhance the detoxification and vacuolar compartmentalization of HMs. Although the stress‐ameliorating effects of PAs have been widely studied for several abiotic stresses, not much is known regarding their effects on metal induced‐stress, except for Cd. This review summarises the available work on the effects of PAs in plants exposed to Cu, Cd, Fe, Mn, Cr, Ni, Hg, Al, Pb and Zn.

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.

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