Plant cells express several stress calcium ATPases but apparently no sodium ATPase
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The existence of plant Na+-ATPases has been investigated in barley (Hordeum vulgare) and in the seagrass Cymodocea nodosa, by a combination of RT–PCR and flux approaches. Systematic RT–PCR amplifications were carried out in mRNA preparations of barley roots exposed to Na+ or of Cymodocea leaves, using degenerate primers that can amplify all known plant and fungal Na+- and Ca2+-ATPases and animal Na+,K+-ATPases. This allowed the amplification of fourteen different cDNAs that could encode P-type ATPases. A phylogenetic analysis showed that none of these ATPases belongs to the ENA type, in which all fungal Na+-ATPases cluster, or to the animal Na+,K+-ATPase type, and that all cluster with known plant and fungal Ca2+-ATPases. Expression analysis of the barley transcripts indicates that the expressions of all but one of the ATPases are enhanced at high Ca2+, high pH, or high Na+, and that three ATPases are only expressed under stress conditions. Genes encoding ENA- or Na+,K+-ATPases were not found in the complete genomes of Arabidopsis thaliana and rice (Oryza sativa). On the basis of these results, we discuss the probable absence of Na+-ATPases in plants, and the function of Ca2+-ATPases that are expressed only under conditions of stress.
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