Another gene from beech, FsPP2C2, which is related to cluster D of Arabidop-sis PP2Cs (Schweighofer et al. 2004) enhances sensitivity to ABA and abiotic stress according to overexpression studies (Reyes et al. 2006), suggesting an opposite trend of cluster D to cluster A phosphatases. The phenotypes could be reversed by gibberellic acid (GA), indicating that FsPP2C2 is a positive regulator of ABA signalling that may affect GA levels in transgenic plants and pointing towards a possible cross-talk between ABA signalling and GA biosynthesis.
It is established that ABI1, ABI2, PP2CA, HAB1, HAB2 from Arabidopsis and FsPP2C1 from beech negatively regulate ABA signal transduction and that the functions of these PP2Cs overlap, but their specific tissue- or developmental expressions confer distinct and indispensable physiological functions in the ABA response (Yoshida et al. 2006). Yet, challenging tasks for the future remain in order to uncover their biological substrates and to identify cross-talks with other signalling pathways.
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