Acidification of soils may release water soluble, toxic aluminium species from clay minerals. Al interferes with a wide range of physical and cellular processes. Glutamine synthetase (GS, EC 6.3.1.2) is the key enzyme of primary N assimilation, as well as ammonia reassimilation and detoxification. Plant GS requires two magnesium ions per subunit for activity, which makes GS a potential target of metal stress. The objective of this investigation was to prove that Al from an organic metal complex is able to activate GS, and Al becomes bound to the polypeptide structure of the GS molecule. Aluminium(III)-nitrilotriacetic acid complex (Al(III)NTA) activated the GS prepared from wheat (Triticum aestivum L.) leaves, as Al(3+) did in vivo, but could not functionally substitute magnesium ions, which were also necessary for the activity in the in vitro GS assay. GS2 was isolated by non-denaturing polyacrylamide gel electrophoresis, and the Al and Mg content of the enzyme was determined by inductively coupled plasma atomic emission spectroscopy. The GS octamer remained intact and contained Mg(2+) bound to its specific sites after the electrophoretic separation. Al was detected in the Al(III)NTA-treated sample bound to the structure of the enzyme protein, potentially occupying one of the specific metal-binding sites of the subunits. Our results indicate that the activatory effect of the Al(III)NTA complex is because of specific binding of aluminium to the polypeptide chain of GS2, however presence of magnesium at least on one of the metal-binding sites is essential to the active state of the enzyme.
A study was made of the effects of temperature and calcium on the properties of K+ transport in rice roots (Oryza sativa L. cv. Dunghan Shali), in cell suspension culture of rice and in callus cultures. The rates of influx and efflux of K+ were measured by using 86Rb as tracer, and the net change in K+ content was determined by atomic absorption spectrophotometry. In roots of low salt status the K+ transport mechanism exhibits a positive temperature dependence and calcium exerts a stimulation. In cell and callus cultures a transport mechanism of this kind is lacking, and the K+ fluxes are inhibited by calcium and independent of temperature. Chilling‐induced K+ leakage is similar for both types of tissue, and can be characterized by a negative temperature‐coefficient and the inhibitory effect of calcium.
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