The regulation of plant signalling responses by Mitogen-Activated Protein Kinases (MAPKs)-mediated protein phosphorylation is well recognized. MAP kinase phosphatases (MKPs) are negative regulators of MAPKs in eukaryotes. We report here the identification and the characterization of TMKP1, the first wheat MKP (Triticum turgidum L. subsp. Durum). Expression profile analyses performed in two durum wheat cultivars showing a marked difference in salt and drought stress tolerance, revealed a differential regulation of TMKP1. Under salt and osmotic stress, TMKP1 is induced in the sensitive wheat variety and repressed in the tolerant one. A recombinant TMKP1 was shown to be an active phosphatase and capable to interact specifically with two wheat MAPKs (TMPK3 and TMPK6). In BY2 tobacco cells transiently expressing GFP::TMKP1, the fusion protein was localized into the nucleus. Interestingly, the deletion of the N-terminal non catalytic domain results in a strong accumulation of the truncated fusion protein in the cytoplasm. In addition, when expressed in BY2 cells, TMPK3 and TMPK6 fused to red fluorescent protein (RFP) were shown to be present predominantly in the nucleus. Surprisingly, when co-expressed with the N-terminal truncated TMKP1 fusion protein; both kinases are excluded from the nuclear compartment and accumulate in the cytoplasm. This strongly suggests that TMKP1 interacts in vivo with TMPK3 and TMPK6 and controls their subcellular localization. Taken together, our results show that the newly isolated wheat MKP might play an active role in modulating the plant cell responses to salt and osmotic stress responses.

The durum wheat TMKP1 gene encodes a MAP kinase phosphatase. When overexpressed in Saccharomyces cerevisiae, TMKP1 leads to salt stress tolerance (especially LiCl ), which is dependent on the phosphatase activity of the protein. The TMKP1-associated Li(+) resistance is restricted to a galactose-containing medium. Interestingly, this salt tolerance is abolished in the absence of one member of the yeast type 2C Ser/Thr protein phosphatase family (Ptc1) but not when other members such as Ptc2 or Ptc3 are lacking. Increased Li(+) tolerance is not mediated by regulation of the P-type ATPase Ena1, a major determinant for salt tolerance. In contrast, the effect of TMKP1 depends on Hal3 (a negative regulator of Ppz phosphatases) and on the presence of the high-affinity potassium transporters Trk1/Trk2. Tolerance to Li(+) is also abolished in cells lacking the aldose reductase Gre3, previously shown to be involved in the resistance to this cation. This study provides evidence that the wheat TMKP1 phosphatase is contributing to reduce the exacerbated lithium toxicity in galactose-grown cells, in a way that depends on the presence of the potassium Trk transporters.