Functional Analyses Of Potassium Transporter KUP9 In Arabidopsis Response To Low Potassium Stress

Potassium(K)is one of the essential macronutrients for plant growth and development,and plays crucial roles in many physiological processes.The K+uptake and transport in plants are conducted by multiple K+channels and transporters that constitutes a K+transport system in plants.Because of the variation of K+concentrations in soils,plants/crops often suffer the K+-deficient stress.Thus,investigation of the mechanism of plant response to K+deficiency is important for understanding plant K+utilization efficiency.Our previous work has isolated an Arabidopsis low-K+-sensitive mutant lks26.There is a single-nucleotide mutation in the KT/HAK/KUP K+transporter KUP9 in lks26,which leads to an amino acid change of L308F.In this dissertation work,we focus on the physiological functions of KUP9 in Arabidopsis response to K+deficiency through analyzing lks26(KUP9F)mutant and kup9 null mutant.After the germinated Arabidopsis plants were transferred to the low-K+medium for 10 days,KUP9F exhibited a shoot chlorosis phenotype.Under this low-K+condition,roots of wild-type plants stopped growing,but the roots of KUP9F mutants kept growing.However,the kup9 null mutants showed no phenotype difference compared with wild-type plants under the low-K+condition,suggesting that the KUP9F is not a function-loss mutant.Potassium content measurement showed that KUP9F displayed lower K+content in shoot but higher K+ content in root compared with wild-type plants under the low-K+conditions.Besides,the xylem sap K+concentration in KUP9F mutants was much lower than that of wild-type plants under low-K+conditions.These results suggest that the K+translocation from root to shoot was impaired in the KUP9F mutant.The GUS-staining assays showed that KUP9 was ubiquitously expressed in different tissues or organs and abundant in root vascular tissues and quiescent center.In the yeast complementation assays,KUP9F showed enhanced K+transport activity than that of KUP9.The structure prediction indicated that the mutation site L308F of KUP9F might be located in the transport cave that affects K+transport activity.When plants were directly germinated on the low-K+medium,the primary root growth of kup9 null mutant was inhibited,and the structure of quiescent center in root tip kup9 was disorganized,suggesting that KUP9 plays important roles in root growth in respose to low-K+ stress.In addition,this dissertation work also analyzed the physiological roles of KUP10 and KUP11 that are homologues of KUP9.The GUS-staining assays showed that KUP10 and KUP11 were also ubiquitously expressed in various tissues and organs.However,unlike KUP9,they were not expressed in root tips.In the low-K+germination assays,neither kup10 nor kup11 single mutant showed sensitive phenotype on low-K+medium,while kup9 kup10 kup11 triple mutant exhibited significant low-K+-sensitive phenotype.The triple mutant even cannot grow in the mixed soil,and it can only survive in the medium containing very high K+concentration(20 mM).Subcellular localization analyses showed that KUP9,KUP10 and KUP11 may locate both at the plasma membrane and the endomembrane in plant cells.In summary,this dissertation work demonstrated that KUP9F is a gain-of-function mutant.The mutation of L308F in KUP9 impairs K+transport from root to shoot.KUP9 plays important roles in Arabidopsis response to low-K+stress and regulates primary root growth.KUP10 and KUP11 show functional redundancy with KUP9,but they may have different physiological functions.The data in this work provide important evidence to understand the physiological functions of K+transporters from KT/HAK/KUP family.