Tissue- and environmental response-specific expression of 10 PP2C transcripts in Mesembryanthemum crystallinum

Ten transcripts (Mpc1-10) homologous to protein phosphatases of the 2C fami ly have been isolated from the halophyte Mesembryanthemum crystallinum (com mon ice plant). Transcripts range in size from 1.6 to 2.6 kb, and encode pr oteins whose catalytic domains are between 24% and 62% identical to that of the Arabidopsis PP2C, ABI1. Transcript expression is tissue specific. Two isoforms are present only in roots (Mpc1 and Mpc5), three in young leaves ( Mpc6, 8 and 9), two in old leaves (Mpc6 and Mpc8), and two in post-flowerin g leaves (Mpc8 and Mpc9). Mpc2 is strongly expressed in roots and also in s eeds, meristematic tissues and mature flowers. Mpc3 is specific for leaf me ristems, and Mpc4 is found in root and leaf meristems. Mpc7 is restricted t o meristematic tissues. Mpc10 is only present in mature flowers. Mpc2 (in r oots and leaves), Mpc5 (ilo roots) and Mpc8 (weakly in leaves) are induced by salinity stress and drought conditions with different kinetics in differ ent tissues, but other Mpcs are downregulated by stress. Cold stress (4 deg rees C) leads to a decline in Mpc5 and Mpc6, but low temperature provoked a Isngterm (days) increase in Mpc2 levels in leaves and a transient increase (less than 24 h) in roots. Four full-length transcripts have been obtained . In each case, after over-expression in E. coli, the isolated proteins exh ibited (Mg2+-dependent, okadeic acid-insensitive) protein phosphatase activ ity, although activity against P-32-phosphocasein varied among different PP 2Cs. Determination of tissue developmental and stress response specificity of PP2C will facilitate functional studies of signal-transducing enzymes in this halophytic organism.