A role for GIGANTEA

The initiation of flowering in Arabidopsis is retarded or abolished by environmental stresses. Focusing on salt stress, we provide a molecular explanation for this well-known fact. A protein complex consisting of GI, a clock component important for flowering and SOS2, a kinase activating the [Na(+)] antiporter SOS1, exists under no stress conditions. GI prevents SOS2 from activating SOS1. In the presence of NaCl, the SOS2/GI complex disintegrates and GI is degraded. SO2, together with the Ca (2+)-activated sensor of sodium ions, SOS3, activates SOS1. In gi mutants, SOS1 is constitutively activated and gi plants are more highly salt tolerant than wild type Arabidopsis. The model shows GI as a transitory regulator of SOS pathway activity whose presence or amount connects flowering to environmental conditions.