| Potassium is one of the most abundant cations in the cytosol of plant cells and essential for plant growth and development. Potassium plays crucial roles in many fundamental activities in living plant cells. K~+ deficiency in soil has been a globe-wide serious problem for crop production. There exist great differences among different plant species or ecotypes in their K~+ efficiency, which is known to be genetically controlled. To increase potassium-efficiency of plants or crops by genetic engineering is the most promising way to overcome the potassium deficiency for sustainable crop production.In this dissertation work, model plant Arabidopsis thanlina was used to screen potassium-nutrition mutants. By isolating seedlings showing root-bending phenotype after treated under low potassium conditions (100μM K~+) for 4 days, we obtained 9 independent lines of monogenic recessive mutants, which include mutations in 4 genes (LKS1, LKS2, LKT1 and LKT2). These mutants show various phenotype after treated under the low potassium conditions. The mutant lks1 and lks2 are low-K~+-sensitive mutants. They stopped growing after 4 days and their cotyledons and leaves showed chlorotic phenotype after 10 days, while the wild type seedlings kept their leaves green even after 10 days under the same conditions. The mutant lkt1 and lkt2 are low-K~+-tolerant mutants. They are more tolerant to low potassium stress than wild type plants in either phenotypes or K~+ contents in seedlings. It is also observed that these mutants showed differential response to signal factors such as exogenous calcium. lks1 and lkt1 are more sensitive to low potassium stress when external [Ca~2+] was lowered, while lks2 and lkt2 showed no difference to low potassium when external [Ca~2+] was changed. The results indicate that these mutated genes may be involved in different signaling pathways in plant responses to low K~+ stress.LKS1, encoded by LKS1 gene cloned from the low-K~+-sensitive Arabidopsis mutant lks1, is a member of the CBL-interacting protein kinase (CIPK) family. CIPKs form a plant-specific family of Ser/Thr kinases. The previous studies in our lab showed that LKS1 interact with AKT1 (Arabidopsis K~+ Transporter, a Shaker family potassium channel) in yeast two-hybrid experiments, and akt1 has the same phenotype as lks1 when treated under low potassium conditions. In this study, the tissue-specific expression profiles of LKS1 were analyzed by creating transgenic plants carrying β-glucuronidase (GUS) driven by the LKS1 promoter. The results show that there are large overlapping band in expression profiles of LKS1 and AKT1. lks1, aktl and LKS1 overexpressing lines were examined for potassium-nutrition characteristics such as K~+ uptake rate and the K~+ contents in both roots and shoots. The results revealed that, compared with wild type plants, the K4 uptake ability and K~+ contents in lks1 and aktl plants decreased, while LKS1 overexpressing lines have much higher K~+ uptake rate and K~+ content. In vitro phosphorylation analysis clearly demonstrated that LKS1 specifically phosphorylates AKTl. All these evidences demonstrate that LKS1 positively regulates the activity of AKTl by phosphorylation, and in turn increases K~+ uptake of plants under low K~+ stress and consequently increases plant tolerance to low K~+ stress. In conclusion, this dissertation work provides the first direct evidence for regulation of a specific K~+ channel by a specific kinase through phosphorylation in plants.
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