Regulation of cell fate and meristem maintenance in Arabidopsis root development

Asymmetric cell division is an essential and universal mechanism for generating diversity and pattern in multicellular organisms. Divisions generating daughter cells different in size, shape, identity and function are fundamental to many developmental processes including fate specification, tissue patterning and self-renewal. It is hypothesized that the angiosperm root meristem has evolved from the shoot apical meristem. Presumably, this is the outcome of the plants adaptation to changing environmental conditions. Accordingly, key gene network motifs present in the shoot have been found to be important in the development and regulation of the root meristem, such as peptide ligands and their receptors. In this thesis Arabidopsis root patterning and meristem maintenance has been investigated using forward and reverse genetics approaches. I describe the identification and analysis of SCHIZORIZA (SCZ). SCZ encodes a member of the family of heat shock transcription factors, albeit one that appears to be recruited for development instead of stress signaling. The pleiotropic effects of the scz mutation define the existence of a novel mechanism for patterning cell identity in the Arabidopsis root. SCZ acts in a parallel pathway with SHR/SCR to specify the root stem cell niche in the early embryo. Overexpression of CLE family peptides restricts the size of both the shoot and root meristem, suggesting that signaling pathways involved in shoot and root meristem maintenance are conserved. An activation tagging screen was performed on transgenic plants ectopically expressing CLE19 in the root meristem, aimed to identify new components of a root CLE signaling pathway. A recessive mutant, sol3, was isolated, which suppresses the CLE19 overexpression phenotype. SOL3 has a dual role in the root controlling growth and formative cell divisions. Using a reverse genetics approach I established and analyzed a collection of homozygous T-DNA insertion lines for 69 leucine-rich repeat receptor-like kinases (LRR RLKs) for developmental and conditional phenotypes. Possibly due to genetic redundancy the functional loss-of-function studies revealed developmental phenotypes for only one mutant line, rlk902. T-DNA insertion mutants assayed for their response after exposure to environmental, hormonal/chemical and abiotic stress, reveal several novel conditional functions for a number of RLK genes. rlk902 mutants show both reduced root growth and resistance to the oomycete pathogen Hyaloperonospora arabidopsidis. These phenotypes are not caused by RLK902 inactivation but are linked to the T-DNA insertion. Microarray analysis revealed downregulated gene expression over an 84 kb region upstream of and including RLK902 in rlk902, putatively encompassing the causal gene(s). The identification of SCZ as factor involved in tissue specification and cell fate segregation provides a basis for future research into mechanisms of asymmetric division. Work on CLE ligand signaling identified SOL3 as a factor controlling root growth and formative divisions and the LRR RLK reverse genetics indicated the existence of extensive cross talk and signal integration among different RLK signaling pathways in the Arabidopsis root. A challenge for the future will be to integrate these results with signaling networks for root patterning and growth