Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE credits
The patterned assignment of different cell fates, shortly termed patterning, lies at the basis of growth, development and reproduction of multicellular organisms. The single-layered epidermal tissue of Arabidopsis thaliana is characterized by hair-like cellular protuberances, the root hairs and the leaf trichomes, respectively, emerging from a subset of cells. Specification of hair and non-hair cell fate occurs in a tightly controlled fashion by gene regulatory networks of overlapping components that, however, often have opposite functions in root and shoot. Moreover, the root epidermis displays organization in hair- and non-hair cell files while leaf trichomes are distributed in a regular spacing pattern over the leaf. In contrast to the shoot, assignment of a cell file’s fate in the root epidermis depends on the cell’s position with respect to the underlying root cortical cell layer, which defines cells atop the border of two cortical cells as hair- and cells atop a single cortical cell as non-hair.
The kreuz und quer (kuq-1) mutant was isolated in a genetic screen for mutations affecting cell and tissue polarity of root-hair positioning along the epidermal cell, and additional T-DNA insertion mutant alleles such as kuq-2 were identified. These mutants also display defects in cell division plane orientation in the root meristem. Preliminary observations suggested that kuq mutants may also be defective in epidermal patterning.
In this study, light microscopic analysis revealed significant patterning defects in the root epidermis of kuq-1 and kuq-2 mutants. Both display a more random hair formation from hair and non-hair positions and an overall increased number of root hairs compared to the wild type, largely resulting from ectopic hair formation in non-hair positions. However, no hair patterning defect could be observed in the leaf epidermis of a kuq mutant. Generation and analysis of transgenic lines expressing fluorescent markers for the tubulin cytoskeleton and reporters of the non-hair fate genes WEREWOLF (WER) and GLABRA2 (GL2) in the kuq-2 mutant revealed that defects in the orientation of the cell division plane might increase non-hair-cell file duplications in the root epidermis, but did not result in the formation of ectopic hair files. To further address the effect of kuq mutation on the specification of epidermal cell fate, promoter-reporter constructs of genes with specific expression in hair- or non-hair cells were analyzed in the kuq-2 mutant. The higher incidence of fate-switches within single cell files, revealed by changes in both none-hair fate and hair differentiation markers, suggests that loss of KUQ gene function destabilizes the root epidermal pattern. To further test the involvement of KUQ in epidermal patterning and to assess possible genetic interactions, kuq-2;caprice-1 and kuq-2;wer-1 double mutants were generated and analyzed. These revealed that cpc-1 partially suppresses ectopic hair formation in the kuq-2 mutant, further supporting genetic interactions between KUQ and components of the gene regulatory network of root hair patterning.