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Arabidopsis amps mutants show pleiotropic phenotypes, including altered shoot apical meristems, increased cell proliferation, polycotyly, constitutive photomorphogenesis, early flowering time, increased levels of endogenous cytokinin, and increased cyclin cycD3 expression. We have isolated the AMPI gene by map-based cloning. The AMP1 cDNA encodes a 706-amino acid polypeptide with significant similarity to glutamate carboxypeptidases. The AMPi mRNA was expressed in all tissues examined, with higher expression in roots, stems, inflorescences, and siliques. Microarray analysis identified four mRNA species with altered expression in two alleles of amps, including upregulation of CYP78A5, which has been shown to mark the shoot apical meristem boundary. The similarity of the AMP1 protein to glutamate carboxypeptidases, and in particular to N-acetyl a-linked acidic dipeptidases, suggests that the AMP1 gene product modulates the level of a small signaling molecule that acts to regulate a number of aspects of plant development, in particular the size of the apical meristem.
INTRODUCTION
Plant meristems are a driving force of growth and development, with profound influences on the rate of growth, the developmental fate, and the architecture of plants. Plant hormones are defined as substances that exert regulatory effects on growth and development, and much plant hormone action is on meristematic tissue. Classically, cytokinins, auxins, gibberellins, abscisic acid, and ethylene were characterized as plant hormones. More recently, it has become clear that other signaling molecules exist in plants, including brassinosteroids (Altmann, 1998), jasmonic acid (Reymond and Farmer, 1998), and peptides (Bisseling, 1999).
The amp1-1 (altered meristem program) mutant was isolated on the basis of increased cotyledon number and an increased rate of leaf initiation (Chaudhury et al., 1993). Other alleles of ampl have been isolated, including pt (primordia timing; Mordhorst et al., 1998), cop2 (constitutively photomorphogenic; Hou et al., 1993), and hpt (hauptling; Jurgens et al., 1991). Although originally isolated as a mutant with altered embryonic and postembryonic meristem programs, amp1 was shown to have many different phenotypes, such as transformation of leaves to cotyledons (Conway and Poethig, 1997), altered flowering time and photomorphogenesis (Chaudhury et al., 1993), and an increased level of cyclin D3 (Riou-Khamlichi et al., 1999; Nogue et al., 2000a). One explanation for this developmental pleiotropy is that the level of or sensitivity to one or more plant hormones is altered in ampl....