Study of LATERAL ORGAN BOUNDARIES (LOB) Genetic Pathways in Boundary Maintenance of Arabidopsis

The meristem-organ boundary resides between the shoot apical meristem (SAM) and lateral organ primordia, not only providing a physical barrier in the shoot apex, but also actively communicating between the shoot apical meristem and emerging lateral organ primordia. LATERAL ORGAN BOUNDARIES (LOB) is specifically expressed at the boundary region and functions to suppress growth to ensure boundary integrity. This dissertation focuses on the study of the LOB genetic pathway in boundary maintenance of Arabidopsis.In Chapter 1 (together with other previous work - Bell et al., in press), I revealed that LOB suppresses brassinosteroid (BR) signaling in the boundary region via directly activating PHYB ACTIVATION TAGGED SUPPRESSOR1 (BAS1) transcription to maintain boundary integrity. LOB directly binds the BAS1 promoter and activates BAS1 transcription in the boundary, thus suppressing BR signaling. Boundary-specific overexpression of positive regulators of BR signaling, such as DWARF4 (DWF4) and BRASSINOSTEROID-INSENSITIVE 1 (BRI1), leads to organ fusion, suggesting that BR suppression is required for boundary maintenance.In Chapter 2, I identified a negative feedback pathway downstream of LOB that involves the BR receptor, BRI1-LIKE 3 (BRL3). LOB directly binds the BRL3 promoter and activates BRL3 transcription in the boundary region. Similar to BRI1, mis-expression of BRL3 in the boundary region led to a boundary fusion phenotype and locally suppressed LOB expression. Taken together, I proposed that a negative feedback loop regulates LOB expression and boundary maintenance through the LOB-BRL3 genetic pathway. Work in Chapter 3 revealed that LOB and LATERAL SUPPRESSOR (LAS) have a novel synergistic interaction. On one hand, the lob las double mutant has dramatic boundary fusion between stem and lateral cauline leaves, demonstrating the synergistic genetic interaction between LOB and LAS for boundary maintenance. Consistently, overexpressing LAS inhibits plant cell growth and reduces plant size. On the other hand, lob las double mutants also have a stronger reduction in axillary meristem formation than the las single mutant, revealing a novel role for LOB in axillary meristem formation. Moreover, LAS is required for the normal boundary expression pattern of LOB and several other boundary genes such as LATERAL ORGAN FUSION1 (LOF1), CUP-SHAPED COTYLEDON1 (CUC1), suggesting hierarchical interactions between different boundary genes