UC San Diego

Topless-1 (tpl-1) is an Arabidopsis mutant that was isolated in a screen due to its defective patterning of the embryo as manifested by its seedling phenotype. It is a semi-dominant and temperature sensitive mutant, the seedling phenotype being more severe in the progeny of parental plants grown at higher temperatures. The range of phenotypes include seedlings with two cotyledons that closely resemble wild type, seedlings with only one cotyledon, tube and pin shaped seedlings, and double roots. The double root is the most severe phenotype. Whereas a wild type seedlings form a single root, a hypocotyls, and two cotyledons, the double root consists of only roots, one being an apical root. This is viewed as a complete homeotic transformation of the apical pole of the embryo into a second basal pole. The cloning of TPL revealed a gene that encodes a protein with similar domain structure and organization to the GROUCHO/TUP1 family of transcriptional corepressors. Due to this observation, the tpl-1 phenotype, and also because of the genetic interaction of the tpl-1 mutant with other mutants involved in regulation of transcription, including activators and repressors, a model was developed suggesting that TPL is a putative transcriptional corepressor that functions to repress root-fate genes in the top half of the embryo. Transcriptional corepressors do not bind directly to DNA, but rather, their target specificity is conferred by their direct interaction with DNA-sequence specific binding transcription factors. In an effort to determine these interactors, a connection to the auxin signaling pathway, a pathway involved in plant development, was made. This work describes that effort, what was discovered, the connection, and its broader significance