Less than 5% of the total pool of free amino acids in plants under stress-free conditions is provided by proline. In many plants under various forms of. stress, the concentration increases up to 80% of the amino acid pool. This observation raises the question about the molecular mechanisms, making a high proline concentration favourable under stress conditions. Therefore, the literature about the chemical properties of proline is reviewed, linking it to the plant physiological observations. In addition to its role as an osmolyte and a reservoir of carbon and nitrogen, etc. proline has been shown to protect plants against free radical-induced damage. A recent concept that proline accumulation is linked with the quenching of singlet oxygen has inspired us to look at the molecular mechanism of singlet oxygen quenching by proline. In this review, the key properties and the chemical reactivity of proline with singlet oxygen and other reactive oxygen species are discussed.
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