Genetic Analysis of Axillary Meristem Development in Arabidopsis: Roles of MIR164, CUC1, CUC2, CUC3 and LAS, and identification of novel regulators.

Aerial architecture and reproductive success in higher plants is determined by the formation of secondary axes of growth which are formed by axillary meristems initiated post-embryonically in the axils of leaves. Among the genetic modulators of axillary meristem fate in Arabidopsis is LATERAL SUPPRESSOR, a putative transcription factor belonging to the GRAS family, which specifically regulates the initiation of axillary meristems during the vegetative phase of development. The aim of this work was to study the mechanism of LAS function in the meristem and to identify new regulators of axillary meristem initiation in Arabidopsis. To study the spatio-temporal specification of its function, LAS was misexpressed from promoters of meristematic genes possessing adjoining or overlapping expression domains in the SAM. Analysis of STM::LAS, KNAT1::LAS and UFO::LAS transgenic plants in las-4 background revealed partial to complete complementation of the las-4 branching phenotype, but did not lead to the formation of ectopic meristems. These results imply a function for LAS in maintaining the meristematic potential in axillary cells which can later initiate axillary meristems upon activation by other developmental cues. A potential mechanism of LAS function in axillary meristems was investigated by GA spraying experiments and complementation analysis of LAS::GAI and LAS::GAI DELLA transgenic plants in las-4 mutant background. Preliminary results indicate a role for LAS as a regulator of GA signaling in axillary meristems. To identify new regulators of axillary meristem development, two approaches were employed. Firstly, an EMS mutagenesis screen was carried out to identify supperssors of the las-4 max1-1 phenotype. Characterisation of three suppressor of las-4 (sol) candidates, sol2, sol6 and sol7, revealed three novel loci that regulate axillary meristem development. sol2, sol6 and sol7 complemented the branching defect in las-4 max1-1 to different degrees and were found to be non-allelic to each other. Their phenotypes were dependent on the las-4 mutation. Molecular mapping of two of these loci is underway. Secondly, the NAC domain transcription factors CUP-SHAPED COTYLEDON1, CUC2 and CUC3, exhibiting a characteristic expression pattern in the axils of leaf primordia, were investigated for potential roles in the development of axillary meristems. Investigation of loss-of-function mutants of these genes revealed that cuc3-2 is impaired in axillary bud formation, and that the severity of this phenotype is day length dependent. Transcripts of the other two CUC genes, CUC1 and CUC2, are targeted for degradation by miR164. Overexpression of MIR164A or MIR164B in the cuc3-2 mutant caused an almost complete block in axillary bud development. Conversely, plants harbouring miR164-resistant alleles of CUC1 and CUC2 developed accessory buds in rosette and cauline leaf axils, revealing redundant functions of CUC1 and CUC2 in axillary meristem development. Development of accessory buds was also observed in mir164 mutants. Thus, the role of CUC genes and miR164 in regulation of axillary meristem development was unveiled in this study