NSCCs-mediated Potassium Transmembrane Localization Of Wheat Roots

Potassium(K⁺) is an essential nutrient and play an important role in many physiological functions of plants. But its relative element sodium is one of the main ions that cause plant damage under saline conditions. To keep a higher intracellular K⁺/Na⁺ ratio is very important for plants to survive in salt stress. Means of a relationship between K⁺, Na⁺ uptake and salt-tolerant. The studies about K⁺ uptake were focused on K⁺ vetors and K⁺ channels before. However, more and more evidences showed that nonselective cation channels (NSCCs) play an important role in many cations absorption. NSCCs mediate current of both sides, participate in K⁺ inflow and outflow. Indicate that NSCCs can regulate intracellular K⁺ content from both aspects. At the same time, NSCCs is also the main pathway for Na⁺ to get into cells. Just because the characters of NSCCs which are absent of K⁺ channels, it can be presumed that NSCCs can not only affect K⁺ uptake directely, but also accommodate intracellular K⁺/Na⁺ ratio by K⁺ and Na⁺ transport of both directions. Whereas the researches about NSCCs'role in K⁺ uptake and influential factors are deficient.The researche route of this paper is to select salt-tolerant and salt-sensitive wheat varieties firstly, and compare their K⁺ uptake conditions. The contribution rate of NSCCs-mediated K uptake of varieties and its influential factors were studied by using of specific inhibitors. Based on this, it is expected that the nutrition uptake and salt-tolerant machanism of different salt-resistant wheat varieties could be revealed from NSCCs aspect, and to provide a reference to cultivate salt-tolerant varieties. The materials were wheat, they are salt-tolerant Shijiazhuang 8, moderate salt-tolerant Yangmai 16 and salt-sensitive Suxu 2. The seedlings were grown hydroponically. K⁺ uptake speed was measured by general kinetics technique. And the plasma membrane potential (MP) was measured by glass micro-electrode. The inhibitors of K⁺ vectors and K⁺ channels were used to separate NSCCs-mediated K⁺ uptake from total K⁺ uptake of plants. The main results were as follows:(1) Salt-tolerance of wheat was concerned to its K⁺ accumulation. Better K⁺ collection led to more resistance to salt stress. At low exoteric K⁺ concentration (showed as [K⁺]ext below), NSCCs induced K⁺ outflow. K⁺ influx through NSCCs occurred at higher [K⁺]ext.(2) Environment factors can affect NSCCs-mediated K⁺ uptake of wheat roots. Contribution rate of NSCCs-mediated K⁺ uptake decreased at 40℃, indicated that higher temperature was apt to affect NSCCs more than K⁺ channels. At 20℃, the situation was converse. Lower temperature increased the contribution rate and showed preference to affect K⁺ channel.(3) Both cadmium and copper exhibited a concentration-dependent effect on NSCCs-mediated K⁺ uptake of Shijiazhuang 8. Compared with K⁺ channels, NSCCs was less sensitive to Cd²⁺ and more to Cu²⁺. Suggested that these two channel proteins were inhibited by different heavy metal ions through different mechanism.(4) The K⁺ absorption was not suppressed, even promoted by lower [Na⁺]ext. but higher [Na⁺]ext depressed the K⁺ absorption of three wheats significantly.NSCCs-mediated K⁺ uptake of three wheat varieties was affected by Na⁺ differently. NSCCs-mediated K⁺ uptake of Shijiazhuang 8, the most salt-tolerant type, was less influenced, and the contribution rate of its NSCCs up-regulated by higher salt concentration. The relevant of Suxu 2, the most salt-sensitive type, was exquisitely impressed, and the contribution rate of its NSCCs was down-regulated. NSCCs of Yangmai 16, the salt sensitivity of which lies between Shijiazhuang 8 and Suxu 2, were affected moderately. These results indicated that for the varieties with higher salt-tolerance the suppress of Na⁺ to NSCCs-mediated K⁺ influx are usually smaller.(5) Calcium could promote K⁺ uptake of all three wheats. Higher concentration Ca²⁺ had more benefit to salt-resistance. Compared with salt-sensitive wheat, the promotion of Ca²⁺ to NSCCs-mediated K⁺ uptake of salt-tolerant wheat was more significant. For salt-tolerant wheat, the enhancement of Ca²⁺ to K⁺ influx seemed to through both K⁺ channels and NSCCs. But for salt-sensitive wheat, it preferred to affect K⁺ channels.(6) Effects of K⁺ uptake on MP through K⁺ channels and NSCCs were different. K⁺ influx through K⁺ channels led to hyperpolarization, and through NSCCs led to depolarization.(7) For different salt-tolerant wheats, the diverse chang of membrane-potential was due to K⁺ transportation through NSCCs, rather than through K⁺ channels.(8) compared with salt-tolerant wheat, salt-sensitive wheat was more hyperpolarizable upon K⁺ translocation, it might because more cation outflow through NSCCs. (9) Na⁺ uptake through NSCCs led to a positive shift in the changing potential. Higher [Na⁺]ext made more remarkable shift.(10) Compared with salt-sensitive wheat, salt-tolerant wheat was insensitive to lower [Na⁺]ext , allowed little Na⁺ influx, and was good at maintaining potential gradient across plasma membrane at higher [Na⁺]ext, which was helpful to K⁺ uptake. These results meant that salt-tolerant wheat can keep more negative MP to keep K⁺ uptake and help maintaining K⁺/Na⁺ ratio of cells.(11) At low [Na⁺]ext, Na⁺ can increase the hyperpolarization level of salt-tolerant wheat, so that to keep high potential gradient in favor of K⁺ uptake. That might be the reason of impregnabe K⁺ uptake through NSCCs. But for salt-sensitive wheat, Na⁺ induced a positive shift in changing potential, which was harmful to K⁺ uptake, and then reduced K⁺ uptake through NSCCs. At high [Na⁺]ext, the depolarization level of salt-tolerant wheat caused by NSCCs-mediated K⁺ influx was lower than that of salt-sensitive wheat. It can be concluded that salt-tolerant wheat can keep higher potential gradient through NSCCs under salt stress conditions, so as to maintaining higher K⁺/Na⁺ ratio.(12) MP depolarization level of treatment with inhibitors was more significant than without inhibitors at high [Na⁺]ext condition. Suggested that there might be K⁺ out flow through K⁺ outward-rectifying channels of salt-sensitive wheat, with the purpose of reducing positive charge inside cells and keeping potential gradient.(13) Ca²⁺ could help to keep stable membrane potential by making up depolarization caused by Na⁺ and accelerating potential resumption. It seemed to be the reason of lessening of salt toxicity by Ca²⁺. For salt-tolerant wheat, Ca²⁺ could suppress NSCCs-mediated Na⁺ influx and promote K⁺ uptake through both K⁺ channels and NSCC. But for salt-sensitive wheat, Ca²⁺ preferred to promote K⁺ uptake through K⁺ channels than through NSCCs.