Molecular characterization of the Brassica rapa auxin-repressed, superfamily genes, BrARP1 and BrDRM1.

Two auxin-repressed superfamily genes, auxin-repressed protein 1 (ARP1) and dormancy-associated protein 1 (DRM1), are highly expressed in both the dormant buds and non-growing tissues of several plant species. To further identify the function of these proteins in Chinese cabbage (Brassica rapa L. ssp. pekinensis), we examined comprehensive expression patterns of BrARP1 and BrDRM1 under various developmental and stress conditions. We also examined these same genes in transgenic Arabidopsis plants. Both genes were expressed in all tissues tested, but their levels were highest in mature tissues accompanied by low levels of the growth-associated marker, B. rapa ribosomal protein 27. Expression of both genes was induced by abiotic stresses, such as chilling, heat shock, and salt treatment. Overexpression of either BrARP1 or BrDRM1 in Arabidopsis causes a reduction in vegetative growth and seed productivity, without affecting morphology. The lengths of petioles and siliques were greatly reduced. Simultaneous expression of both genes showed an additive effect on the growth suppression, resulting in significant reduction in plant size. Knock-out of Arabidopsis ARP1, DRM1, or both, neither affected growth rate nor final size. Results suggest BrARP1 and BrDRM1 are either involved in growth arrest, or stop growth, possibly from inhibition of either cell elongation or cell expansion, thereby creating a "growth brake".