The putative SWI/SNF complex subunit BRAHMA activates flower homeotic genes in Arabidopsis thaliana

Arabidopsis thaliana BRAHMA (BRM, also called AtBRM) is a SNF2 family protein homolog of Brahma, the ATPase of the Drosophila SWI/SNF complex involved in chromatin remodeling during transcription. Here we show that, in contrast to its Drosophila counterpart, BRM is not an essential gene. Thus, homozygous BRM loss of function mutants are viable but exhibit numerous defects including dwarfism, altered leaf and root development and several reproduction defects. The analysis of the progeny of self-fertilized heterozygous brm plants and reciprocal crosses between heterozygous and wild type plants indicated that disruption of BRM reduced both male and female gametophyte transmission. This was consistent with the presence of aborted ovules in the self-fertilized heterozygous flowers that contained arrested embryos predominantly at the two terminal cells stage. Furthermore, brm homozygous mutants were completely sterile. Flowers of brm loss-of-function mutants have several developmental abnormalities, including homeotic transformations in the second and third floral whorls. In accordance with these results, brm mutants present reduced levels of APETALA2, APETALA3, PISTILLATA and NAC-LIKE, ACTIVATED BY AP3/PI. We have previously shown that BRM strongly interacts with AtSWI3C. Now we extend our interaction studies demonstrating that BRM interacts weakly with AtSWI3B but not with AtSWI3A or AtSWI3D. In agreement with these results, the phenotype described in this study for brm plants is very similar to that previously described for the AtSWI3C mutant plants, suggesting that both proteins participate in the same genetic pathway or form a molecular complex.