Identification of Components Controlling Meristem Homeostasis.

Abstract

Plants maintain a pool of stem cells throughout their lives from which they draw to produce the organs of the adult plant. This strictly regulated pool of stem cells is contained within structures known as meristems. There are several signal transduction pathways known that are involved in meristem homeostasis. While a number of factors reflecting different regulatory pathways controlling meristem function are known, most of these pathways have significant gaps. This thesis research has been to attempt to identify novel components involved in meristem maintenance. Using a variety of approaches, several were identified and characterized. The first novel component, CCI1, was identified in a protein-protein interaction screen with CLAVATA pathway kinases CLV1 and BAM1. CCI1 is a previously uncharacterized protein with no known or identifiable domains or motifs. I have shown that CCI1 directly binds to the receptor proteins and has phosphoinositide-binding activity in vitro. I have also shown that CCI1 partitions into detergent-resistant membrane (DRM) microdomains with other CLV pathway components. This partitioning is essential for CCI1 interaction with some CLV components, but not others, when tested in transient expression. The second component characterized was a spontaneous, novel missense allele of BLR. BLR is a homeodomain protein of the BELL family of TALE homeodomain proteins, has previously shown to be essential for internode elongation and floral evocation. The mutation I characterized is a unique allele with dominant negative characteristics that disrupt meristem homeostasis. Genetic interaction analyses as well as protein localization and DNA binding studies suggest that this mutant of BLR disrupts function of the SHOOTMERISTEMLESS transcription factor, known to be essential for meristem maintenance. Finally, I characterized two mutants generated in an EMS enhancer screen in the poltergeist (pol) mutant background. While pol pll1 double mutants lack stem cells, the pol single mutant provides a genetically sensitized background. I mapped two different mutants to AGO10 and TONSOKU (TSK). Both genes have been shown to play important roles in meristem maintenance. The specific tsk mutation identified is previously uncharacterized; however the impact of tsk mutants on meristem homeostasis may be an indirect consequence of defects in polarized cell divisions.