Plant growth homeostasis is controlled by the Arabidopsis BON1 and BAP1 genes

Wild-type Arabidopsis plants maintain a relatively constant size over a wid e range of temperatures. Here we show that this homeostasis requires the BO NZAI1 (BON1) gene because bon1 null mutants make miniature fertile plants a t 22 degreesC but have wild-type appearance at 28 degreesC. The expression of BON1 and a BON1-associated protein (BAP1) is modulated by temperature. T hus BON1 and BAP1 may have a direct role in regulating cell expansion and c ell division at lower temperatures. BON1 contains a Ca2+-dependent phosphol ipid-binding domain and is associated with the plasma membrane. It belongs to the copine gene family, which is conserved from protozoa to humans. Our data suggest that this gene family may function in the pathway of membrane trafficking in response to external conditions.