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Abstract
ALTERED MERISTEM PROGRAM1 (AMP1) is a member of the M28 family of carboxypeptidases with a pivotal role in plant development and stress adaptation. Its most prominent mutant defect is a unique hypertrophic shoot phenotype combining a strongly increased organ formation rate with enhanced meristem size and the formation of ectopic meristem poles. However, so far the role of AMP1 in shoot development could not be assigned to a specific molecular pathway nor is its biochemical function resolved. In this work we evaluated the level of functional conservation between AMP1 and its human homolog HsGCPII, a tumor marker of medical interest. We show that HsGCPII cannot substitute AMP1 in planta and that an HsGCPII-specific inhibitor does not evoke amp1-specific phenotypes. We used a chemical genetic approach to identify the drug hyperphyllin (HP), which specifically mimics the shoot defects of amp1, including plastochron reduction and enlargement and multiplication of the shoot meristem. We assessed the structural requirements of HP activity and excluded that it is a cytokinin analog. HP-treated wild-type plants showed amp1-related tissue-specific changes of various marker genes and a significant transcriptomic overlap with the mutant. HP was ineffective in amp1 and elevated the protein levels of PHAVOLUTA, consistent with the postulated role of AMP1 in miRNA-controlled translation, further supporting an AMP1-related mode of action. Our work suggests that plant and animal members of the M28 family of proteases adopted unrelated functions. With HP we provide a tool to characterize the plant-specific functions of this important class of proteins.
Highlights
ALTERED MERISTEM PROGRAM1 (AMP1) is a member of the M28 family of carboxypeptidases with a pivotal role in plant development and stress adaptation
To analyze the level of functional conservation between AMP1 and its best-characterized homolog human Glu carboxypeptidase II (HsGCPII), we determined which of the amino-acid signatures shown to be required for HsGCPII substrate recognition and catalysis are conserved in the Arabidopsis protein
Of the residues defining the substrate binding pockets S1 and S19, only 4 out of 15 (27%) amino acids are conserved in AMP1, with a high variation in the S1 site indicating that substrate-specificities might differ between the two proteins
Summary
ALTERED MERISTEM PROGRAM1 (AMP1) is a member of the M28 family of carboxypeptidases with a pivotal role in plant development and stress adaptation. The most prominent defect of AMP1 loss-of-function mutants is hypertrophic activity of the shoot apical meristem (SAM). Additional phenotypes of amp in seemingly unrelated processes were described including constitutive photomorphogenesis, ecotype-dependent alterations in germination and flowering time (Chaudhury et al, 1993; Lee, 2009; Griffiths et al, 2011), synergid to egg cell conversion in the embryo sac (Kong et al, 2015), suspensor proliferation in the presence of an intact embryo (Vidaurre et al, 2007), increased capacity for somatic embryogenesis (Mordhorst et al, 1998), and elevated abiotic stress resistance (Shi H et al, 2013; Shi Y et al, 2013; Yao et al, 2014)
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