Novel roles for GIGANTEA revealed under environmental conditions that modify its expression in Arabidopsis and Medicago truncatula

GIGANTEA (GI) is a large nuclear protein which is involved in circadian-clock function, red-light signaling and photoperiodic flowering. Accumulation of GI transcript displays a strong diurnal pattern, and is under circadian-clock control, as demonstrated in several diverse species. Clock entrainment and compensation, as well as flowering time, are largely responsive to changes in the environment. We asked if part of this response is mediated through modification of GI expression. We identified a strong response of GI expression to changes in temperature and light, in both Arabidopsis and the model legume Medicago truncatula. Extreme temperatures resulted in increased GI trough levels. Light increased GI expression near dawn and the response to light appeared to be gated by the circadian clock. We provide evidence that the GI response to blue and far-red light requires CRYPTOCHROME function in Arabidopsis. Unknown roles for GI in both blue-light deetiolation and precocious flowering under warm short days were revealed. Plants seem to respond to changes in the environment partly through environmentally induced modifications of a basal clock-regulated pattern of GI transcript accumulation.