Long-term water deficit imposed by air drought or chilling stress modifies the regulation of proline metabolism in Nicotiana tabacum
Tobacco (Nicotiana tabacum), which originated in the tropical Americas, is thought to be rather sensitive to water deficit imposed by drought, salinity or low temperatures, especially during the rapid growth period. However, many past investigations have mainly focused on the response of tobacco to a specific stress, whereas the molecular mechanism of comprehensive tolerance formation to multiple stresses has rarely been reported. Using comparative screening and comprehensive evaluation methods, our previous researches identified a tobacco variety, MS-K326, with a relatively high tolerance to both drought and chilling stresses at the seedling stage. Considering the principle role of proline in water deficit response in plants, the present research aimed at revealing the regulatory events underlying proline metabolism in tobacco, when exposed to different water deficit for a relatively long-term treatment. Tobacco seedlings were treated with air drought or chilling (2 °C) stress, for up to 144 h (6 d). It was found that free proline content significantly increased at the late stage of drought stress, whereas chilling induced a steady proline accumulation in leaves, during the whole course of treatment. Enzymatic and transcriptional analysis of key enzymes for proline metabolism revealed that proline accumulation is mainly attributed to activation of the glutamate pathway under both stresses. The inhibitory effect of chilling stress on proline catabolism also contributes to the rapid increase in free proline during the early stages of exposure. However, the relatively unchanged activity as well as slight transcriptional response of ornithine-∆-aminotransferase (OAT) indicates that the ornithine pathway is not essential for proline accumulation under either drought or chilling stress, in our experiment system. All these findings show the novel regulatory mechanisms associated with different stresses, and may provide more information on understanding the role of proline regulation in comprehensive tolerance formation under various stress conditions.