Identification of BABY BOOM downstream targets

A glucocorticoid-regulated BBM protein (35S:BBM-GR) was used in combination with microarray analysis to identify genes directly activated by BBM. We employed the system described by (Lloyd et al., 1994) in which dexamethasone (DEX) and cycloheximide (CHX) are applied together to respectively, induce nuclear localization of the BBM-GR protein and prevent translation of the primary targets mRNAs. In this way it is possible to identify direct targets of a transcriptional activator by comparing gene expression profiles between DEX+CHX-treated transgenic and wild-type tissues. The ability of the 35S:BBM GR construct to induce somatic embryogenesis in Arabidopsis seedlings was determined by phenotypic observation of 35S:BBM GR seeds germinated and grown in the presence of 10 µM dexamethasone (DEX). As in 35S:BBM seedlings, we observed somatic embryo formation on the cotyledons, first leaves and shoot meristem of DEX-treated 35S:BBM GR seedlings. We identified a set of 20 genes (including BBM itself) and our analysis indicates that BBM directly activates a signaling pathway comprising transcription factors and other signaling molecules, but which does not initially include genes known to induce somatic embryogenesis, such as LEC1, LEC2 or WUS. The functions of the BBM target genes are unknown, however a number of them have recently been identified in microarray screens for meristem-expressed genes. The identification of BBM-interacting partners and downstream targets provides new tools for unraveling pathways related to plant cell growth and organogenesis.