Drought-induced proline is mainly synthesized in leaves and transported to roots in watermelon under water deficit

Proline accumulation has been shown to occur in plants in response to various environmental stresses. Although proline metabolism-related genes have been functionally characterized, the inter-organ transport of proline in stressed plants remains unclear. In this study, free proline was detected with significant accumulations in the roots, stems, and leaves of watermelon drought-tolerant germplasm M08 and drought-susceptible line Y34 under drought stress. Expression profiling and enzyme activity measurements revealed that ClP5CS1 gene, rather than its paralog ClP5CS2, mainly contributes to the proline synthesis in leaves via the Glu pathway. Additionally, over-expression of the ClP5CS genes significantly enhanced the drought tolerance of transgenic Arabidopsis lines. Furthermore, we confirmed that proline is mainly synthesized in leaves and transported to roots in watermelon under drought stress. Transcriptome and expression analyses revealed that the genes involved in proline metabolism exhibited different expression levels. Specifically, ClP5CS1 was upregulated in leaves and roots, while ClP5CS2 was downregulated under drought stress. Also, 415 and 362 differently expressed TFs were identified in roots and leaves, respectively, with the majority upregulated in the former. Ultimately, a model for proline metabolism was proposed. The findings of this study provided new insights into the biosynthesis, transport, and regulatory mechanism of drought-induced proline in plants.