Abstract
It is well known that nitric oxide (NO) enhances salt tolerance of glycophytes. However, the effect of NO on modulating ionic balance in halophytes is not very clear. This study focuses on the role of NO in mediating K+/Na+ balance in a mangrove species, Kandelia obovata Sheue, Liu and Yong. We first analyzed the effects of sodium nitroprusside (SNP), an NO donor, on ion content and ion flux in the roots of K. obovata under high salinity. The results showed that 100 μM SNP significantly increased K+ content and Na+ efflux, but decreased Na+ content and K+ efflux. These effects of NO were reversed by specific NO synthesis inhibitor and scavenger, which confirmed the role of NO in retaining K+ and reducing Na+ in K. obovata roots. Using western-blot analysis, we found that NO increased the protein expression of plasma membrane (PM) H+-ATPase and vacuolar Na+/H+ antiporter, which were crucial proteins for ionic balance. To further clarify the molecular mechanism of NO-modulated K+/Na+ balance, partial cDNA fragments of inward-rectifying K+ channel, PM Na+/H+ antiporter, PM H+-ATPase, vacuolar Na+/H+ antiporter and vacuolar H+-ATPase subunit c were isolated. Results of quantitative real-time PCR showed that NO increased the relative expression levels of these genes, while this increase was blocked by NO synthesis inhibitors and scavenger. Above results indicate that NO greatly contribute to K+/Na+ balance in high salinity-treated K. obovata roots, by activating AKT1-type K+ channel and Na+/H+ antiporter, which are the critical components in K+/Na+ transport system.
Highlights
Intracellular K+/Na+ balance is fundamental to the physiology of living cells and is crucial for plant normal growth [1,2]
After K. obovata seedlings were treated with 400 mM NaCl and various concentrations of sodium nitroprusside (SNP) for 15 days, the appropriate concentration of exogenous nitric oxide (NO) (100 mM SNP) significantly increased K+ content and decreased Na+ content, resulting in the increased K+/Na+ ratio compared with the solely NaCltreated seedlings (Figures 1A, C and E)
To attribute the role of NO in enhancing K+/Na+ ratio in K. obovata roots exposed to high salinity, specific NO synthesis inhibitor (L-NNA) and NO
Summary
Intracellular K+/Na+ balance is fundamental to the physiology of living cells and is crucial for plant normal growth [1,2]. Optimal K+/Na+ ratio is very important for the activities of many cytosolic enzymes, and for maintaining the ideal osmoticum and membrane potential for cell volume regulation [3]. In order to maintain the optimal cytosolic K+/Na+ balance and avoid the adverse effects of high salinity on plant growth and development, halophytes have developed different strategies to avoid excessive Na+ accumulation and to maintain osmotic balance in plants. Halophytes can elevate the Na+ extrusion from the cytosol to external medium and/or Na+ compartmentation into the vacuoles through transmembrane transport proteins like plasma membrane (PM)-located Na+/H+ antiporter (SOS1) and tonoplast-located Na+/H+ antiporter (NHX1) [5,6]. The process of Na+/H+ antiporter-mediated Na+ extrusion and Na+ compartmentation is energy-dependent, and this energy is supplied by the proton-motive force, which can be generated by H+-translocating pumps (e.g., H+-ATPase and H+-PPiase) [7,8]
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