Potassium channel α-subunit AtKC1 negatively regulates AKTl-mediated K^＋ uptake in Arabidopsis roots under Iow-K^＋ stress

Potassium transporters play crucial roles in K^＋ uptake and translocation in plants. However, so far little is known about the regulatory mechanism of potassium transporters. Here, we show that a Shaker-like potassium channel AtKC1, encoded by the AtLKT1 gene cloned from the Arabidopsis thaliana low-K^＋ （LK）-tolerant mutant Atlktl, significantly regulates AKTl-mediated K^＋ uptake under LK conditions. Under LK conditions, the Atkcl mutants maintained their root growth, whereas wild-type plants stopped their root growth. Lesion of AtKC1 significantly enhanced the tolerance of the Atkcl mutants to LK stress and markedly increased K^＋ uptake and K^＋ accumulation in the Atkclmutant roots under LK conditions. Electrophysiological results showed that AtKC1 inhibited the AKT1-mediated inward K^＋ currents and negatively shifted the voltage dependence of AKT1 channels. These results demonstrate that the ‘silent＇ K^＋ channel α-subunit AtKC1 negatively regulates the AKTl-mediated K^＋ uptake in Arabidopsis roots and consequently alters the ratio of root-to-shoot under LK stress conditions.

Potassium and phosphorus transport and signaling in plants

Nitrogen(N), potassium(K), and phosphorus(P) are essential macronutrients for plant growth and development, and their availability affects crop yield. Compared with N, the relatively low availability of K and P in soils limits crop production and thus threatens food security and agricultural sustainability. Improvement of plant nutrient utilization efficiency provides a potential route to overcome the effects of K and P deficiencies. Investigation of the molecular mechanisms underlying how plants sense, absorb, transport, and use K and P is an important prerequisite to improve crop nutrient utilization efficiency. In this review, we summarize current understanding of K and P transport and signaling in plants, mainly taking Arabidopsis thaliana and rice(Oryza sativa) as examples. We also discuss the mechanisms coordinating transport of N and K, as well as P and N.

Plant Sensing and Signaling in Response to K＋-Deficiency

Potassium （K＋） is one of the essential macronutrients for plant growth and development. However, K＋ content in soils is usually limited so that the crop yields are restricted. Plants may adapt to K＋-deficient environment by adjusting their physiological and morphological status, indicating that plants may have evolved their sensing and signaling mechanisms in response to K＋-deficiency. This short review particularly discusses some components as possible sensors or signal transducers involved in plant sensing and signaling in response to K＋-deficiency, such as K＋ channels and transporters, H＋-ATPase, some cytoplasmic enzymes, etc. Possible involvement of Ca2＋ and ROS signals in plant responses to K＋-deficiency is also discussed.

Zm HAK5 and Zm HAK1 function in K^+ uptake and distribution in maize under low K^+ conditions

Potassium (K^+) is an essential macronutrient for plant growth and development. Transporters from the KT/HAK/KUP family play crucial roles in K^+ homeostasis and cell growth in various plant species. However, their physiological roles in maize are still unknown. In this study, we cloned ZmHAKs and ZmHAKi and investigated their functions in maize (Zea mays L.). In situ hybridization showed that ZmHAKs was mainly expressed in roots, especially in the epidermis, cortex, and vascular bundle. ZmH AK5 was characterized as a high-affinity K^+ transporter. Loss of function of ZmHAK5 led to defective K^+ uptake in maize, under low K^+ conditions, whereas ZmHAK5-over-expressing plants showed increased K^+ uptake activity and improved growth. ZmHAKi was upregulated under low K^+ stress, as revealed by RT-qPCR. ZmHAKi mediated K^+ uptake when heterologously expressed in yeast, but its transport activity was weaker than that of ZmHAK5. Overexpression of ZmHAKi in maize significantly affected K^+ distribution in shoots, leading to chlorosis in older leaves. These findings indicate that ZmHAKs and ZmHAKi play distinct roles in K^+ homeostasis in maize, functioning in K^+ uptake and K^+ distribution, respectively. Genetic manipulation of ZmHAK5 may represent a feasible way to improve K^+ utilization efficiency in maize.

Potassium Transporter KUP7 Is Involved in K^＋ Acquisition and Translocation in Arabidopsis Root under K^＋-Limited Conditions

Potassium （K^＋） is one of the essential macronutrients for plant growth and development. K^＋ uptake from environment and K^＋ translocation in plants are conducted by K^＋ channels and transporters. In this study, we demonstrated that KT/HAK/KUP transporter KUP7 plays crucial roles in K^＋ uptake and translocation in Arabidopsis root. The kup7 mutant exhibited a sensitive phenotype on Iow-K^＋ medium, whose leaves showed chlorosis symptoms compared with wild-type plants. Loss of function of KUP7 led to a reduction of K^＋ uptake rate and K^＋ content in xylem sap under W-deficient conditions. Thus, the K^＋ content in kup7 shoot was significantly reduced under Iow-K^＋ conditions. Localization analysis revealed that KUP7 was predominantly targeted to the plasma membrane. The complementation assay in yeast suggested that KUP7 could mediate K^＋ transport. In addition, phosphorylation on S80, S719, and S721 was important for KUP7 activity. KUP7 was ubiquitously expressed in many organs/tissues, and showed a higher expression level inArabidopsis root. Together, our data demonstrated that KUP7 is crucial for K^＋ uptake inArabidopsis root and might be also involved in K^＋ transport into xylem sap, affecting K^＋ translocation from root toward shoot, especially under K^＋-Iimited conditions.

Involvement of a cytoplasmic glyceraldehyde-3-phosphate dehydrogenase GapC-2 in low-phosphate-induced anthocyanin accumulation in Arabidopsis

Phosphorous is one of the essential mineral elements for plant growth and development.Typically, the shoots of plant seedlings usually turn a dark-brown or purple colour under low-Pi stress. Using protein 2-D gel and peptide mass fingerprinting mapping (PMF) methods, a cytoplasmic glyceralde-hyde-3-phosphate dehydrogenase GapC-2 was identified as a low-Pi responsive protein in Arabidopsisplants. Expression of AtGapC-2 protein was significantly decreased after 4 d of low-Pi stress. Two in-dependent T-DNA insertion lines of GapC-2 gene (At1g13440) showed a hypersensitive phenotype inresponse to low-Pi stress compared with wild type plants, while the transgenic complementation linesof the mutants showed a similar phenotype to the wild type. These results indicate that AtGapC-2 mayplay an important role in Arabidopsis responses to low-Pi stress, possibly by regulation of glycoly-sis-associated "Pi-pool" and accumulation of anthocyanin pigments in plants.

Cytosolic Ca^2＋ Signals Enhance the Vacuolar Ion Conductivity of Bulging Arabidopsis Root Hair Cells

Plant cell expansion depends on the uptake of solutes across the plasma membrane and their storage within the vacuole. In contrast to the well-studied plasma membrane, little is known about the regulation of ion transport at the vacuolar membrane. We therefore established an experimental approach to study vacuolar ion transport in intact Arabidopsis root cells, with multi-barreled microelectrodes. The subcellular position of electrodes was detected by imaging current-injected fluorescent dyes. Comparison of measurements with electrodes in the cytosol and vacuole revealed an average vacuolar membrane potential of -31 inV. Voltage clamp recordings of single vacuoles resolved the activity of voltage-independent and slowly deactivating channels. In bulging root hairs that express the Ca2＋ sensor R-GECO1, rapid elevation of the cytosolic Ca^2＋ concentration was observed, after impalement with microelectrodes, or injection of the Ca^2＋ chelator BAPTA. Elevation of the cytosolic Ca^2＋ level stimulated the activity of voltage- independent channels in the vacuolar membrane. Likewise, the vacuolar ion conductance was enhanced during a sudden increase of the cytosolic Ca^2＋ level in cells injected with fluorescent Ca^2＋ indicator FURA-2. These data thus show that cytosolic Ca^2＋ signals can rapidly activate vacuolar ion channels, which may prevent rupture of the vacuolar membrane, when facing mechanical forces.

Potassium channel AKT1 is involved in the auxin-mediated root growth inhibition in Arabidopsis response to low K^+stress

The changes in external K^＋ concentration affect plant root growth. However, the molecular mechanism for perceiving a K^＋ signal to modulate root growth remains unknown. It is hypothesized that the K^＋ channel AKTI is involved in low K^＋ sensing in the Arabidopsis root and subsequent regulation of root growth. Along with the decline of external K^＋ concentration, the primary root growth of wild-type plants was gradually inhibited. However, the primary root of the akt1 mutant could still grow under low K^＋（LK） conditions. Application of NAA inhibited akt1 root growth, but promoted wild-type root growth under LK conditions. By using the ProDR5：GFP and ProPIN1：PIN1-GFP lines, we found that LK treatment reduced auxin accumulation in wild-type root tips by degrading PIN1 proteins, which did not occur in the akt1 mutant. The LK-induced PIN1 degradation may be due to the inhibition of vesicle trafficking of PIN1 proteins. In conclusion, our findings indicate that AKT1 is required for an Arabidopsis response to changes in external K^＋, and subsequent regulation of K^＋-dependent root growth by modulating PINt degradation and auxin redistribution in the root.