Regulation of proline accumulation and its molecular and physiological functions in stress defence

The accumulation of proline is a conserved response of plants to abiotic stress conditions. Moreover, the activation of proline metabolism takes place during the plant response to some pathogens. Although these responses are well documented, the molecular and physiological functions of proline accumulation under stress are still a matter of debate. The biochemical pathways that lead to proline accumulation and its functions in regulating development are described in the cognate chapter “Proline Metabolism and its Functions in Development”. In this chapter, we will describe the potential roles assigned to proline accumulation, dissecting the data coming from in vitro/in silico and in vivo approaches, and those coming from bacterial or unicellular eukaryotes and plants. With this, we aim to present a clear view of the evidence related to the molecular and physiological functions of proline accumulation under stress conditions in plants. In recent years, the understanding of the regulation of proline accumulation at transcriptional level under stress conditions in plants has been increased considerably, yet little is known about the possible occurrence of post-translational regulatory mechanisms. We will integrate this knowledge with the potential roles of proline accumulation to see whether it contributes to comprehending which roles might be physiologically more relevant.