Exordium : a novel gene in Arabidopsis identified by promoter trapping
The aim of the project was to characterize the expression of a GUS promoter trap in a transgenic line of Arabidopsis thaliana, and to investigate the function of the tagged gene. GUS expression in the transgenic line, designated line EM2, was found to be localized to embryos, and to the regions of most active cell division in the seedling, notably the apical meristems and young leaves. The tagged gene was named EXORDIUM (EXO). Line EM2, which contains a single copy of the promoter trap T-DNA, was found to have no obvious phenotype when homozygous for the T-DNA insertion, and when grown under a range of nutritional and hormonal conditions in vitro. It was found that the expression of the GUS gene and of the native EXO gene, was each up-regulated by exogenous auxin and down-regulated by exogenous cytokinin. The cloned EXO promoter was introduced as a GUS fusion into transgenic plants of A. thaliana and found to be expressed ion the same tissues as EM2, and additionally in the root vascular tissues, leaves and siliques, although to different extents in different transgenic lines. It was found that the EXO mRNA abundance accumulated in seedlings treated with hydroxyurea, which induces cell-cycle arrest at the Gl-S transition. Further analysis demonstrated that EXO mRNA is preferentially abundant during M-phase of the cell cycle. Transgenic plants were produced containing sense and antisense versions of the EXO gene under the transcriptional control of the CaMV35S promoter. One antisense line exhibited an abberant phenotype, characterized by a reduced size and abnormal shoot branching pattern. EXO encodes a predicted protein of 314 amino acids, of unknown function.