Functional Analysis Of Potassium Transporter KUP7 In Arabidopsis Responses To Low-k~+ Stress

Potassium (K) is one of the essential macronutrients for plant growth and development. K deficiency has become a common abiotic stress for plants in natural environment. However, plants have evolved many mechanisms for K uptake and translocation in order to adapt to diverse environmental stresses. K uptake from environment and K translocation in plants are conducted by K+ channels and transporters which are mainly derived from Shaker K+ channel family and KT/HAK/KUP K+ transporter family. In Arabidopsis, there are 13 members in KT/HAK/KUP family. Some KT/HAK/KUP transporters play diverse functions in many physiological aspects, such as K uptake, cell expansion and osmotic/drought responses. In this dissertation work, we analyzed the physiological function of K+ transporter KUP7 in Arabidopsis, whose regulatory mechanism was also studied.Using reverse genetic approach, we screened the mutant plants of Arabidopsis K+ transporters on low-K (LK,100 μM K+) medium. The kup7 mutant plants showed low-K-sensitive phenotype, whose leaves displayed chlorotic symptom compared with wild-type plants after grown on LK medium for 10 days. The K+ content in kup7 shoot was lower than that of wild-type plants on LK medium. The phenotype test showed that the complementation lines could rescue the sensitive phenotype of kup7 mutant, which suggested that the low-K-sensitive phenotype of kup7 was due to the disruption of KUP7. We found that KUP7 was localized at the plasma membrane and possessed K+ transport activity. The expression pattern analysis indicated that KUP7 was ubiquitously expressed in many organs/tissues, and showed higher expression level in Arabidopsis root, especially in procambium, endodermis and epidermis/root hairs. Loss-of-function of KUP7 led to the reduction of K+ content and K+ absorption rate in kup7 mutant. These results indicated that KUP7 is crucial for K uptake in peripheral cells of Arabidopsis root. In addition, under LK conditions the Shoot/Root K+ ratio in kup7 was much lower than that in wild-type plants, and the K+ content in kup7 xylem sap was also reducted. Using the K+-selective fluorescent dye, it’s further confirmed that the K+ content in stelar tissues was reduced in kup7 under K deficient condition. All these results indicated that KUP7 may also function in K translocation from root to shoot especially under LK conditions.In this study, we also demonstrated that the expression of KUP7 was not obviously induced by LK stress. The KUP7 overexpressing lines neither showed any obvious differences compared with wild-type plants. We found that three predicted phosphorylation sites of KUP7 were crucial for the K+ transport activity of KUP7. It’s indicated that KUP7 may be regulated by the post-translational modification.All the data in this dissertation demonstrate that KUP7 is essential not only for the K uptake from external enviroment but also for the K translocation from root toward shoot, especially under the K-limited conditions. These results will benefit us to understand the K transport mechanism in plants.