Class I BASIC PENTACYSTEINE factors regulate HOMEOBOX genes involved in meristem size maintenance.

The BASIC PENTACYSTEINE (BCP) family is a poorly characterized plant transcription factor family of GAGA BINDING PROTEINS. In Arabidopsis, there are seven members (BPC1-7) that are broadly expressed, and they can potentially bind more than 3000 Arabidopsis GAGA-repeat-containing genes. To date, BPCs are known to be direct regulators of the INNER NO OUTER (INO), SEEDSTICK (STK), and LEAFY COTYLEDON 2 (LEC2) genes. Because of the high functional redundancy, neither single knockout nor double bpc mutant combinations cause aberrant phenotypes. The bpc1-2 bpc2 bpc3 triple mutant shows several pleiotropic developmental defects, including enlargement of the inflorescence meristem and flowers with supernumerary floral organs. Here, we demonstrated through expression analysis and chromatin immunoprecipitation assays that this phenotype is probably due to deregulation of the expression of the SHOOTMERISTEMLESS (STM) and BREVIPEDICELLUS/KNAT1 (BP) genes, which are both direct targets of BPCs. Moreover, we assigned a role to BPCs in the fine regulation of the cytokinin content in the meristem, as both ISOPENTENYLTRANSFERASE 7 (IPT7) and ARABIDOPSIS RESPONSE REGULATOR 7 (ARR7) genes were shown to be overexpressed in the bpc1-2 bpc2 bpc3 triple mutant.