In plants, salt activates MAPK signalling (Droillard et al. 2002; Munnik et al. 1999). Hence, it is not surprising that in Arabidopsis the only member of classical PTP, AtPTPl is a stress-responsive gene (Fordham-Skelton et al. 1999; Xu et al. 1998) up-regulated by salt, and down-regulated by cold treatment. Similar PTP genes are also found in other plants. Dephosphorylation of phosphor-tyrosine in Arabidopsis MPK4 by AtPTP1 in vitro suggests that

AtPTPl phosphatase may function as a switch-off mechanism for salt-stress activated MAPKs (Huang et al. 2000); however, data in planta has to support this premise. Highly conserved cysteine and aspartate residues are essential for the AtPTPl activity underlining the common catalytic mechanism between plant and other eukaryotic PTPs (Xu et al. 1998). A catalytic cysteine has to be in a reduced form for phosphatase activity. Thus, reactive oxygen species (ROS) produced during various stress conditions regulate mammalian PTPs by reversible oxidation of the active site cysteine and abrogation of its nucleophilic properties, thereby inhibiting PTP activity (Barford 2004; Tonks 2005). Similarly, the recombinant AtPTPl protein can be reversibly inactivated by H2O2, suggesting that plant PTPs may also be regulated by the redox state of the cysteine, thereby serving as molecular targets during oxidative stress (Gupta and Luan 2003).

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