EXORDIUM--a gene expressed in proliferating cells and with a role in meristem function, identified by promoter trapping in Arabidopsis.

To identify new genes expressed in meristematic cells, a promoter trap insertional mutagenesis strategy was used in Arabidopsis thaliana. Transgenic line AtEM201 exhibits promoter trap GUS activity in embryos and in the regions of active cell division in the seedling, notably the apical meristems and young leaves. The tagged gene was named EXORDIUM (EXO). AtEM201 contains a single copy of the promoter trap T-DNA, located in the EXO gene promoter, resulting in a much reduced level of EXO transcription. Seedlings homozygous for the T-DNA insertion have no obvious mutant phenotype. The EXO gene, which forms part of a small gene family in Arabidopsis, is structurally related to the tobacco PHI-1 gene, which is re-activated in cultured cells following release from phosphate starvation-induced cell cycle arrest. Expression of both the EXO-GUS and the native EXO genes is downregulated by exogenous cytokinin. Expression studies using semisynchronised cells suggest that EXO mRNA is preferentially abundant during M phase of the cell cycle. Double mutant studies revealed that the exo mutation can suppress the defective root meristem phenotype of the hydra2 mutant, suggesting that EXO may be a component of a negative regulatory system for cell division.