UC Riverside

Leaves form at the periphery of the shoot apical meristem (SAM) and are separated from the SAM by a boundary region. Boundary cells are smaller and divide less frequently than the surrounding cells. The Arabidopsis LATERAL ORGAN BOUNDARIES (LOB) is expressed in all organ boundaries and functions to separate lateral organs. LOB encodes a transcription factor that differentially regulates the expression of target genes. This dissertation focuses on characterizing the molecular function of LOB in Arabidopsis and its role in the separation of lateral organs and responses to blue-light.

In Chapter 1, I demonstrate that LOB directly regulates the expression of genes involved in the blue-light hyponastic response, including PHOTOTROPIN1 (PHOT1), PHYTOCHROME KINASE SUBSTRATE 2 (PKS2), and NON-PHOTOTROPIC HYPOCOTYL 3 (NPH3). LOB directly binds to the DNA regions of PHOT1 and PKS2 in yeast and in planta. lob mutants have altered responses to blue-light illumination and LOB transcript is regulated by blue-light, likely through PHOT1 and PKS2. This chapter suggests that the boundary region is an important player in the blue-light hyponastic response in Arabidopsis.

In Chapter 2, I show that LOB directly regulates the expression of NAKED PINS IN YUC MUTANTS (NPY1). NPY1 transcript is altered in plants with varying levels of LOB activity and LOB binds to the promoter region of NPY1. However, npy1-1 mutants do not have altered responses to blue-light nor do they exhibit fusions of lateral organs. Further experiments are required to test the biological relevance of the regulation of NPY1 by LOB.

Work in Chapter 3 shows that LOB interacts with HISTONE DEACETYLASE (HDT3) in yeast and in onion epidermal peels. Furthermore, hdt3-2 mutants exhibit fusions of the axillary stem and cauline leaf and correlates with the decreased transcript level of BAS1 compared to wild-type plants. hdt3-1 plants also show a more sensitive blue-light hyponastic response to short-term blue-light exposure. These data suggest that LOB interacts with HDT3 for the separation of lateral organs and for proper blue-light responses.

In Chapter 4, I performed a chemical genetics screen to identify chemicals that inhibit the LOB over-expression phenotype. One chemical, LAT24D02 was identified in this screen and selected for further analysis. Although LAT24D02 inhibited the LOB over-expression phenotype, it did not suppress the induction of a direct target of LOB suggesting LAT24D02 does not directly inhibit LOB function. Therefore, LAT24D02 affects another unidentified LOB-dependent pathway and further experiments are required to understand the mode of action of LAT24D02.