Signalling networks in plant cell differentiation

In contrast to animals where cell lineage is a major determinant in cell fate decision, plant cell identity is predominantly directed by positional cues, which necessitates the action of sophisticated intercellular communication networks. The plant hormones auxin and cytokinin have been shown to be sufficient to guide plant cell fate between un-differentiated stem cell-like callus growth and the induction of organogenesis via establishment of functional meristems. A key question in meristem biology is how these hormones establish the structural organization of meristems by carefully balancing the size and activity of the stem cell pool in the centre with the consumption of cells by organ formation. This requires cell-to-cell signalling circuits and predicts the presence of additional messenger molecules, which interact with auxin, cytokinin in the control of meristem growth.
 
 
We try to further characterize the intra-meristematic communication network that controls shoot apical meristem (SAM) activity by functional analysis of a novel pathway defined by the ALTERED MERISTEM PROGRAM1 (AMP1) locus. The Arabidopsis amp1 mutant is unique in combining strong enlargement of the stem cell pool in the SAM with a drastic increase in cytokinin levels, further indicating a close link between cytokinin and SAM function. However how these phenotypes in amp1 are functionally related is currently not known. AMP1 encodes a glutamate carboxypeptidase like protein. The best-characterized member of this protein family is the tumour marker glutamate carboxypeptidase II (GCP II) also known as prostate specific membrane antigen (PSMA). It represents an extra cellular metallopeptidase anchored by a N-terminal membrane-spanning domain to the plasma membrane. GCP II exhibits cleavage activity against N-acetyl-L-aspartyl-L-glutamate (NAAG) a neuropeptide involved in synaptic neuro-transmission as well as poly-glutamated folates, resulting in the release of a C-terminal glutamate residue. Based on its homology to GCP II the primary function of AMP1 has been proposed to be the control of an unknown peptide signal that restricts SAM size and that the elevated cytokinin levels in amp1 are a consequence of deregulation of meristem proliferation.
 
 
We aim to further characterize AMP1 and to identify additional components of this pathway by a complementary set of molecular biological and genetic approaches. Our work is intented to serve a dual purpose: I) to extend our knowledge about the control of meristem function, a fundamental aspect in plant biology and II) to contribute to a better understanding of a protein class with a central role in human diseases.