Juglone (5-hydroxy-1,4-naphthoquinone) is an important allelochemical in walnut trees (Juglans nigra L.). Its allelopathic potential has been reported in different plant species. We investigated the phytotoxic effects of the allelochemical juglone and the protective role of proline in tobacco seedlings. Juglone inhibited the growth of tobacco seedlings and increased reactive oxygen species content in tobacco roots. Moreover, juglone stress increased proline concentration. The expression of two proline synthesis genes, pyrroline-5-carboxylate synthetase and ornithine aminotransferase, was upregulated and that of a proline catabolism gene, proline dehydrogenase, was downregulated with juglone treatment in tobacco roots. Furthermore, plants pretreated with proline and then exposed to juglone showed attenuated toxic effects in roots. Proline was able to modulate allelochemical juglone-induced stress in tobacco. In summary, this study suggested that increased proline content in the tobacco seedlings treated with juglone may mitigate the deleterious effect of allelochemical stress in plants by inhibiting reactive oxygen species accumulation.
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